## User Project Publications

Open list in Research Information System

## 2021

M.D. Wever, A. Tornede, F. Mohr, E. Hüllermeier, IEEE Transactions on Pattern Analysis and Machine Intelligence (2021), pp. 1-1

Automated machine learning (AutoML) supports the algorithmic construction and data-specific customization of machine learning pipelines, including the selection, combination, and parametrization of machine learning algorithms as main constituents. Generally speaking, AutoML approaches comprise two major components: a search space model and an optimizer for traversing the space. Recent approaches have shown impressive results in the realm of supervised learning, most notably (single-label) classification (SLC). Moreover, first attempts at extending these approaches towards multi-label classification (MLC) have been made. While the space of candidate pipelines is already huge in SLC, the complexity of the search space is raised to an even higher power in MLC. One may wonder, therefore, whether and to what extent optimizers established for SLC can scale to this increased complexity, and how they compare to each other. This paper makes the following contributions: First, we survey existing approaches to AutoML for MLC. Second, we augment these approaches with optimizers not previously tried for MLC. Third, we propose a benchmarking framework that supports a fair and systematic comparison. Fourth, we conduct an extensive experimental study, evaluating the methods on a suite of MLC problems. We find a grammar-based best-first search to compare favorably to other optimizers.

F. Mohr, M.D. Wever, A. Tornede, E. Hüllermeier, IEEE Transactions on Pattern Analysis and Machine Intelligence (2021)

Automated Machine Learning (AutoML) seeks to automatically find so-called machine learning pipelines that maximize the prediction performance when being used to train a model on a given dataset. One of the main and yet open challenges in AutoML is an effective use of computational resources: An AutoML process involves the evaluation of many candidate pipelines, which are costly but often ineffective because they are canceled due to a timeout. In this paper, we present an approach to predict the runtime of two-step machine learning pipelines with up to one pre-processor, which can be used to anticipate whether or not a pipeline will time out. Separate runtime models are trained offline for each algorithm that may be used in a pipeline, and an overall prediction is derived from these models. We empirically show that the approach increases successful evaluations made by an AutoML tool while preserving or even improving on the previously best solutions.

J.M. Hanselle, A. Tornede, M.D. Wever, E. Hüllermeier, 2021

M. Riabinin, P. Sharapova, T. Bartley, T. Meier, Journal of Physics Communications (2021)

J. Heine, C. Wecker, E. Kenig, H. Bart. Stofftransportmessung und -visualisierung am ruhenden und bewegten Einzeltropfen. 2021.

L.M. Witschen, T. Wiersema, M. Raeisi Nafchi, A. Bockhorn, M. Platzner, in: Proceedings of International Symposium on Applied Reconfigurable Computing (ARC'21), Springer Lecture Notes in Computer Science, 2021

T.T.N. Nguyen, T. Sollfrank, C. Tegenkamp, E. Rauls, U. Gerstmann, Physical Review B (2021)

L. Claes, R.S. Chatwell, E. Baumhögger, T. Hetkämper, H. Zeipert, J. Vrabec, B. Henning, Measurement (2021)

F. Schmidt, A.L. Kozub, U. Gerstmann, W.G. Schmidt, A. Schindlmayr, Crystals (2021), 11(5)

Lithium niobate (LiNbO3), a material frequently used in optical applications, hosts different kinds of polarons that significantly affect many of its physical properties. In this study, a variety of electron polarons, namely free, bound, and bipolarons, are analyzed using first-principles calculations. We perform a full structural optimization based on density-functional theory for selected intrinsic defects with special attention to the role of symmetry-breaking distortions that lower the total energy. The cations hosting the various polarons relax to a different degree, with a larger relaxation corresponding to a larger gap between the defect level and the conduction-band edge. The projected density of states reveals that the polaron states are formerly empty Nb 4d states lowered into the band gap. Optical absorption spectra are derived within the independent-particle approximation, corrected by the GW approximation that yields a wider band gap and by including excitonic effects within the Bethe-Salpeter equation. Comparing the calculated spectra with the density of states, we find that the defect peak observed in the optical absorption stems from transitions between the defect level and a continuum of empty Nb 4d states. Signatures of polarons are further analyzed in the reflectivity and other experimentally measurable optical coefficients.

A. Magnier, T. Wu, S.R. Tinkloh, T. Tröster, B. Scholtes, T. Niendorf, Polymer Testing (2021)

J. Kossmann, D. Piankova, N. V. Tarakina, J.J. Heske, T. Kühne, J. Schmidt, M. Antonietti, N. López-Salas, Carbon (2021), 172, pp. 497-505

Simple thermal treatment of guanine at temperatures ranging from 600 to 700 °C leads to C1N1 condensates with unprecedented CO2/N2 selectivity when compared to other carbonaceous solid sorbents. Increasing the surface area of the CN condensates in the presence of ZnCl2 salt melts enhances the amount of CO2 adsorbed while preserving the high selectivity values and C1N1 structure. Results indicate that these new materials show a sorption mechanism a step closer to that of natural CO2 caption proteins and based on metal free structural cryptopores.

S.R. Tinkloh, T. Wu, T. Tröster, T. Niendorf. Development of a submodel technique for FFT-based solvers in micromechanical analysis. 2021.

C. Wecker, A. Schulz, J. Heine, H. Bart, E. Kenig. Numerische Untersuchung der Marangonikonvektion in Flüssig-Flüssig-Systemen: Von der Tropfenbildung bis zur Tropfeninteraktion. 2021.

H. Aldahhak, C. Hogan, S. Lindner, S. Appelfeller, H. Eisele, W.G. Schmidt, M. Dähne, U. Gerstmann, M. Franz, Physical Review B (2021)

M. Reichelt, H. Rose, A.N. Kosarev, S.V. Poltavtsev, M. Bayer, I.A. Akimov, C. Schneider, M. Kamp, S. Höfling, T. Meier, in: Ultrafast Phenomena and Nanophotonics XXV, 2021

D. Bauch, D.F. Heinze, J. Förstner, K.D. Jöns, S. Schumacher, Physical Review B (2021), 104, pp. 085308

Employing the ultrafast control of electronic states of a semiconductor quantum dot in a cavity, we introduce an approach to achieve on-demand emission of single photons with almost perfect indistinguishability and photon pairs with near ideal entanglement. Our scheme is based on optical excitation off resonant to a cavity mode followed by ultrafast control of the electronic states using the time-dependent quantum-confined Stark effect, which then allows for cavity-resonant emission. Our theoretical analysis considers cavity-loss mechanisms, the Stark effect, and phonon-induced dephasing, allowing realistic predictions for finite temperatures.

C. Wecker, A. Hoppe, A. Schulz, J. Heine, H. Bart, E. Kenig. Numerische Untersuchungen zu Fluiddynamik und Stofftransport binärer Tropfeninteraktion unter Berücksichtigung von Marangonikonvektion. 2021.

S. Alhaddad, J. Förstner, S. Groth, D. Grünewald, Y. Grynko, F. Hannig, T. Kenter, F. Pfreundt, C. Plessl, M. Schotte, T. Steinke, J. Teich, M. Weiser, F. Wende, in: Euro-Par 2020: Parallel Processing Workshops, 2021

Solving partial differential equations on unstructured grids is a cornerstone of engineering and scientific computing. Nowadays, heterogeneous parallel platforms with CPUs, GPUs, and FPGAs enable energy-efficient and computationally demanding simulations. We developed the HighPerMeshes C++-embedded Domain-Specific Language (DSL) for bridging the abstraction gap between the mathematical and algorithmic formulation of mesh-based algorithms for PDE problems on the one hand and an increasing number of heterogeneous platforms with their different parallel programming and runtime models on the other hand. Thus, the HighPerMeshes DSL aims at higher productivity in the code development process for multiple target platforms. We introduce the concepts as well as the basic structure of the HighPerMeshes DSL, and demonstrate its usage with three examples, a Poisson and monodomain problem, respectively, solved by the continuous finite element method, and the discontinuous Galerkin method for Maxwell’s equation. The mapping of the abstract algorithmic description onto parallel hardware, including distributed memory compute clusters, is presented. Finally, the achievable performance and scalability are demonstrated for a typical example problem on a multi-core CPU cluster.

F. Barkhausen, M. Pukrop, S. Schumacher, X. Ma, Physical Review B (2021), 103(7)

I.A. Ruiz Alvarado, M. Karmo, E. Runge, W.G. Schmidt, ACS Omega (2021), pp. 6297-6304

X. Wang, R. Kormath Madam Raghupathy, C.J. Querebillo, Z. Liao, D. Li, K. Lin, M. Hantusch, Z. Sofer, B. Li, E. Zschech, I.M. Weidinger, T.D. Kühne, H. Mirhosseini, M. Yu, X. Feng, Advanced Materials (2021), 33(20), pp. 2008752

Abstract Developing resource-abundant and sustainable metal-free bifunctional oxygen electrocatalysts is essential for the practical application of zinc–air batteries (ZABs). 2D black phosphorus (BP) with fully exposed atoms and active lone pair electrons can be promising for oxygen electrocatalysts, which, however, suffers from low catalytic activity and poor electrochemical stability. Herein, guided by density functional theory (DFT) calculations, an efficient metal-free electrocatalyst is demonstrated via covalently bonding BP nanosheets with graphitic carbon nitride (denoted BP-CN-c). The polarized PN covalent bonds in BP-CN-c can efficiently regulate the electron transfer from BP to graphitic carbon nitride and significantly promote the OOH* adsorption on phosphorus atoms. Impressively, the oxygen evolution reaction performance of BP-CN-c (overpotential of 350 mV at 10 mA cm−2, 90\% retention after 10 h operation) represents the state-of-the-art among the reported BP-based metal-free catalysts. Additionally, BP-CN-c exhibits a small half-wave overpotential of 390 mV for oxygen reduction reaction, representing the first bifunctional BP-based metal-free oxygen catalyst. Moreover, ZABs are assembled incorporating BP-CN-c cathodes, delivering a substantially higher peak power density (168.3 mW cm−2) than the Pt/C+RuO2-based ZABs (101.3 mW cm−2). The acquired insights into interfacial covalent bonds pave the way for the rational design of new and affordable metal-free catalysts.

T. Tornede, A. Tornede, M.D. Wever, E. Hüllermeier, in: Proceedings of the Genetic and Evolutionary Computation Conference, 2021

H. Rose, J. Paul, J.K. Wahlstrand, A.D. Bristow, T. Meier, in: Ultrafast Phenomena and Nanophotonics XXV, 2021

H. Farheen, T. Leuteritz, S. Linden, V. Myroshnychenko, J. Förstner, in: arXiv:2106.02468, 2021

Optical travelling wave antennas offer unique opportunities to control and selectively guide light into a specific direction which renders them as excellent candidates for optical communication and sensing. These applications require state of the art engineering to reach optimized functionalities such as high directivity and radiation efficiency, low side lobe level, broadband and tunable capabilities, and compact design. In this work we report on the numerical optimization of the directivity of optical travelling wave antennas made from low-loss dielectric materials using full-wave numerical simulations in conjunction with a particle swarm optimization algorithm. The antennas are composed of a reflector and a director deposited on a glass substrate and an emitter placed in the feed gap between them serves as an internal source of excitation. In particular, we analysed antennas with rectangular- and horn-shaped directors made of either Hafnium dioxide or Silicon. The optimized antennas produce highly directional emission due to the presence of two dominant guided TE modes in the director in addition to leaky modes. These guided modes dominate the far-field emission pattern and govern the direction of the main lobe emission which predominately originates from the end facet of the director. Our work also provides a comprehensive analysis of the modes, radiation patterns, parametric influences, and bandwidths of the antennas that highlights their robust nature.

A.L. Kozub, A. Schindlmayr, U. Gerstmann, W.G. Schmidt, 2021

Density-functional theory within a Berry-phase formulation of the dynamical polarization is used to determine the second-order susceptibility χ(2) of lithium niobate (LiNbO3). Defect trapped polarons and bipolarons are found to strongly enhance the nonlinear susceptibility of the material, in particular if localized at NbV–VLi defect pairs. This is essentially a consequence of the polaronic excitation resulting in relaxation-induced gap states. The occupation of these levels leads to strongly enhanced χ(2) coefficients and allows for the spatial and transient modification of the second-harmonic generation of macroscopic samples.

S. Alhaddad, J. Förstner, S. Groth, D. Grünewald, Y. Grynko, F. Hannig, T. Kenter, F. Pfreundt, C. Plessl, M. Schotte, T. Steinke, J. Teich, M. Weiser, F. Wende, Concurrency and Computation: Practice and Experience (2021), pp. e6616

S.R. Tinkloh, T. Wu, T. Tröster, T. Niendorf, Metals (2021)

A.A. Camberg, T. Tröster, C. Latuske, Springer, 2021

L. Claes, Universiät Paderborn, 2021

The prerequisite for a complete description of fluid dynamic and acoustic processes is that all properties of the fluid are known.While fluid parameters such as the speed of sound or the shear viscosity are known for many liquids over a wide range of thermodynamic states, only limited measurement data exist for the bulk viscosity.In this thesis, a measurement method for the selective determination of the bulk viscosity of liquids, based on the absorption of ultrasonic waves, is developed and implemented.The focus is on the simulation-driven design of algorithms for processing the measurement signals as well as the analysis and further development of a measurement set-up based on the pulse-echo method.In addition to absorption in the fluid, there are other effects (for example diffraction or incomplete reflection) that weaken or otherwise influence the acoustic signal.Therefore, the development of procedures to separate these effects from acoustic absorption is another focus of this work.The bulk viscosity is determined from the measured acoustic absorption for different fluids in different thermodynamic states. An uncertainty analysis of the measured quantities concludes this thesis.

A. Schulz, C. Wecker, E. Kenig. Mehrkomponenten-Stofftransport an bewegten Phasengrenzflächen unter Berücksichtigung von Diffusionskreuzeffekten. 2021.

S. Lange, D. Schröder, C. Hedayat, H. Kuhn, U. Hilleringmann, in: 22nd IEEE International Conference on Industrial Technology (ICIT), IEEE, 2021

In this publication important aspects for the implementation of inductive locating are explained. The miniaturized sensor platform called Sens-o-Spheres is used as an application of this locating method. The sensor platform is applied in bioreactors in order to obtain the environmental parameters, which makes a localization by magnetic fields necessary. Since the properties of magnetic fields in the localization area are very different from the wave characteristics, the principle of inductive localization is investigated in this publication and explained by using electrical equivalent circuit diagrams. Thereby, inductive localization uses the coupling or the mutual inductivities between coils, which is noticeable by an induced voltage. Therefore some properties and procedures are explained to extract the location of Sens-o-Spheres or other industrial sensor platforms from the couplings of the coils. One method calculates the location from an adapted ratio calculation and the other method uses neural networks and stochastic filters to obtain the results. In the end, these results are evaluated and compared.

M. Riabinin, P. Sharapova, T. Meier, Optics Express (2021)

R. Bertling, M. Hack, I. Ausner, B. Horschitz, E. Kenig. CFD-basierte Untersuchung von Zweiphasenströmungen in strukturierten Packungen. 2021.

K.J. Rohlfing, P. Cimiano, I. Scharlau, T. Matzner, H.M. Buhl, H. Buschmeier, E. Esposito, A. Grimminger, B. Hammer, R. Haeb-Umbach, I. Horwath, E. Huellermeier, F. Kern, S. Kopp, K. Thommes, A. Ngonga Ngomo, C. Schulte, H. Wachsmuth, P. Wagner, B. Wrede, IEEE Transactions on Cognitive and Development Systems (2021), 13(3), pp. 717-728

The recent surge of interest in explainability in artificial intelligence (XAI) is propelled by not only technological advancements in machine learning but also by regulatory initiatives to foster transparency in algorithmic decision making. In this article, we revise the current concept of explainability and identify three limitations: passive explainee, narrow view on the social process, and undifferentiated assessment of explainee’s understanding. In order to overcome these limitations, we present explanation as a social practice in which explainer and explainee co-construct understanding on the microlevel. We view the co-construction on a microlevel as embedded into a macrolevel, yielding expectations concerning, e.g., social roles or partner models: typically, the role of the explainer is to provide an explanation and to adapt it to the current level of explainee’s understanding; the explainee, in turn, is expected to provide cues that direct the explainer. Building on explanations being a social practice, we present a conceptual framework that aims to guide future research in XAI. The framework relies on the key concepts of monitoring and scaffolding to capture the development of interaction. We relate our conceptual framework and our new perspective on explaining to transparency and autonomy as objectives considered for XAI.

S. Ober-Blöbaum, S. Peitz, International Journal of Robust and Nonlinear Control (2021), 31(2), pp. 380-403

Model predictive control is a prominent approach to construct a feedback control loop for dynamical systems. Due to real-time constraints, the major challenge in MPC is to solve model-based optimal control problems in a very short amount of time. For linear-quadratic problems, Bemporad et al. have proposed an explicit formulation where the underlying optimization problems are solved a priori in an offline phase. In this article, we present an extension of this concept in two significant ways. We consider nonlinear problems and - more importantly - problems with multiple conflicting objective functions. In the offline phase, we build a library of Pareto optimal solutions from which we then obtain a valid compromise solution in the online phase according to a decision maker's preference. Since the standard multi-parametric programming approach is no longer valid in this situation, we instead use interpolation between different entries of the library. To reduce the number of problems that have to be solved in the offline phase, we exploit symmetries in the dynamical system and the corresponding multiobjective optimal control problem. The results are verified using two different examples from autonomous driving.

R. Haeb-Umbach, J. Heymann, L. Drude, S. Watanabe, M. Delcroix, T. Nakatani, Proceedings of the IEEE (2021), 109(2), pp. 124-148

The machine recognition of speech spoken at a distance from the microphones, known as far-field automatic speech recognition (ASR), has received a significant increase of attention in science and industry, which caused or was caused by an equally significant improvement in recognition accuracy. Meanwhile it has entered the consumer market with digital home assistants with a spoken language interface being its most prominent application. Speech recorded at a distance is affected by various acoustic distortions and, consequently, quite different processing pipelines have emerged compared to ASR for close-talk speech. A signal enhancement front-end for dereverberation, source separation and acoustic beamforming is employed to clean up the speech, and the back-end ASR engine is robustified by multi-condition training and adaptation. We will also describe the so-called end-to-end approach to ASR, which is a new promising architecture that has recently been extended to the far-field scenario. This tutorial article gives an account of the algorithms used to enable accurate speech recognition from a distance, and it will be seen that, although deep learning has a significant share in the technological breakthroughs, a clever combination with traditional signal processing can lead to surprisingly effective solutions.

L. Krauss-Kodytek, W. Hannes, T. Meier, C. Ruppert, M. Betz, Physical Review B (2021)

M. Franz, S. Chandola, M. Koy, R. Zielinski, H. Aldahhak, M. Das, M. Freitag, U. Gerstmann, D. Liebig, A.K. Hoffmann, M. Rosin, W.G. Schmidt, C. Hogan, F. Glorius, N. Esser, M. Dähne, Nature Chemistry (2021), pp. 828-835

## 2020

A. Schulz, C. Wecker, E. Kenig. Ein PLIC-basierter Ansatz zur Erfassung des Stoffübergangs an bewegten Phasengrenzflächen. 2020.

J. Heine, C. Wecker, E. Kenig, H. Bart. Stofftransportmessung am ruhenden und bewegten Einzeltropfen. 2020.

V. Rengaraj, M. Lass, C. Plessl, T.D. Kühne, Computation (2020), 8(2), pp. 39

C. Hielscher, J. Grenz, A.A. Camberg, N. Wingenbach, ATZ worldwide (2020), pp. 58-61

K. Bieker, S. Peitz, S.L. Brunton, J.N. Kutz, M. Dellnitz, Theoretical and Computational Fluid Dynamics (2020), 34, pp. 577–591

The control of complex systems is of critical importance in many branches of science, engineering, and industry, many of which are governed by nonlinear partial differential equations. Controlling an unsteady fluid flow is particularly important, as flow control is a key enabler for technologies in energy (e.g., wind, tidal, and combustion), transportation (e.g., planes, trains, and automobiles), security (e.g., tracking airborne contamination), and health (e.g., artificial hearts and artificial respiration). However, the high-dimensional, nonlinear, and multi-scale dynamics make real-time feedback control infeasible. Fortunately, these high- dimensional systems exhibit dominant, low-dimensional patterns of activity that can be exploited for effective control in the sense that knowledge of the entire state of a system is not required. Advances in machine learning have the potential to revolutionize flow control given its ability to extract principled, low-rank feature spaces characterizing such complex systems.We present a novel deep learning modelpredictive control framework that exploits low-rank features of the flow in order to achieve considerable improvements to control performance. Instead of predicting the entire fluid state, we use a recurrent neural network (RNN) to accurately predict the control relevant quantities of the system, which are then embedded into an MPC framework to construct a feedback loop. In order to lower the data requirements and to improve the prediction accuracy and thus the control performance, incoming sensor data are used to update the RNN online. The results are validated using varying fluid flow examples of increasing complexity.

J. Niederhausen, R.W. MacQueen, K. Lips, H. Aldahhak, W.G. Schmidt, U. Gerstmann, Langmuir (2020), pp. 9099-9113

M. Pukrop, S. Schumacher, Physical Review E (2020)

M. Sellmann, K. Tierney, in: Lecture Notes in Computer Science, 2020

D. Ojha, T.D. Kühne, Molecules (2020), 25

<jats:p>In the present work, we provide an electronic structure based method for the “on-the-fly” determination of vibrational sum frequency generation (v-SFG) spectra. The predictive power of this scheme is demonstrated at the air-water interface. While the instantaneous fluctuations in dipole moment are obtained using the maximally localized Wannier functions, the fluctuations in polarizability are approximated to be proportional to the second moment of Wannier functions. The spectrum henceforth obtained captures the signatures of hydrogen bond stretching, bending, as well as low-frequency librational modes.</jats:p>

M.A. Salem, T.D. Kühne, Molecular Physics (2020), pp. 1-6

C. Boeddeker, T. Nakatani, K. Kinoshita, R. Haeb-Umbach, in: ICASSP 2020 - 2020 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2020

S.H. Mirhosseini, R. Kormath Madam Raghupathy, S.K. Sahoo, H. Wiebeler, M. Chugh, T. Kühne, Phys. Chem. Chem. Phys. (2020), 22, pp. 26682-26701

Photovoltaics is one of the most promising and fastest-growing renewable energy technologies. Although the price-performance ratio of solar cells has improved significantly over recent years{,} further systematic investigations are needed to achieve higher performance and lower cost for future solar cells. In conjunction with experiments{,} computer simulations are powerful tools to investigate the thermodynamics and kinetics of solar cells. Over the last few years{,} we have developed and employed advanced computational techniques to gain a better understanding of solar cells based on copper indium gallium selenide (Cu(In{,}Ga)Se2). Furthermore{,} we have utilized state-of-the-art data-driven science and machine learning for the development of photovoltaic materials. In this Perspective{,} we review our results along with a survey of the field.

S.R. Tinkloh, T. Wu, T. Tröster, T. Niendorf, in: Proceedings of the 4th International Conference Hybrid 2020 Materials and Structures, 2020

A. Tornede, M.D. Wever, E. Hüllermeier, in: Discovery Science, 2020

D. Schröder, S. Lange, C. Hangmann, C. Hedayat, in: Tensorial Analysis of Networks (TAN) Modelling for PCB Signal Integrity and EMC Analysis,1st ed., The Institution of Engineering and Technology (IET), 2020, pp. 315-346 (32)

Using near-field (NF) scan data to predict the far-field (FF) behaviour of radiating electronic systems represents a novel method to accompany the whole RF design process. This approach involves so-called Huygens' box as an efficient radiation model inside an electromagnetic (EM) simulation tool and then transforms the scanned NF measured data into the FF. For this, the basic idea of the Huygens'box principle and the NF-to-FF transformation are briefly presented. The NF is measured on the Huygens' box around a device under test using anNF scanner, recording the magnitude and phase of the site-related magnetic and electric components. A comparison between a fullwave simulation and the measurement results shows a good similarity in both the NF and the simulated and transformed FF.Thus, this method is applicable to predict the FF behaviour of any electronic system by measuring the NF. With this knowledge, the RF design can be improved due to allowing a significant reduction of EM compatibility failure at the end of the development flow. In addition, the very efficient FF radiation model can be used for detailed investigations in various environments and the impact of such an equivalent radiation source on other electronic systems can be assessed.

M. Grabo, C. Staggenborg, K.A. Philippi, E. Kenig, Frontiers in Energy Research (2020), 8

R. Bertling, M. Hack, I. Ausner, E. Kenig, 2020

A. Zibart, E. Kenig, International Journal of Heat and Mass Transfer (2020)

X. Xu, A. Port, C. Wiebeler, K. Zhao, I. Schapiro, W. Gärtner, Proceedings of the National Academy of Sciences (2020)

<jats:p>The three-dimensional (3D) crystal structures of the GAF3 domain of cyanobacteriochrome Slr1393 (<jats:italic>Synechocystis</jats:italic> PCC6803) carrying a phycocyanobilin chromophore could be solved in both 15-<jats:italic>Z</jats:italic> dark-adapted state, Pr, λ<jats:sub>max</jats:sub> = 649 nm, and 15-<jats:italic>E</jats:italic> photoproduct, Pg, λ<jats:sub>max</jats:sub> = 536 nm (resolution, 1.6 and 1.86 Å, respectively). The structural data allowed identifying the large spectral shift of the Pr-to-Pg conversion as resulting from an out-of-plane rotation of the chromophore’s peripheral rings and an outward movement of a short helix formed from a formerly unstructured loop. In addition, a third structure (2.1-Å resolution) starting from the photoproduct crystals allowed identification of elements that regulate the absorption maxima. In this peculiar form, generated during X-ray exposition, protein and chromophore conformation still resemble the photoproduct state, except for the D-ring already in 15-<jats:italic>Z</jats:italic> configuration and tilted out of plane akin the dark state. Due to its formation from the photoproduct, it might be considered an early conformational change initiating the parental state-recovering photocycle. The high quality and the distinct features of the three forms allowed for applying quantum-chemical calculations in the framework of multiscale modeling to rationalize the absorption maxima changes. A systematic analysis of the PCB chromophore in the presence and absence of the protein environment showed that the direct electrostatic effect is negligible on the spectral tuning. However, the protein forces the outer pyrrole rings of the chromophore to deviate from coplanarity, which is identified as the dominating factor for the color regulation.</jats:p>

L. Burkhardt, Y. Vukadinovic, M. Nowakowski, A. Kalinko, J. Rudolph, P. Carlsson, C.R. Jacob, M. Bauer, Inorganic Chemistry (2020), pp. 3551-3561

C. Wecker, A. Schulz, J. Heine, H. Bart, E. Kenig. CFD-basierte Untersuchung stofftransportinduzierter Marangoni-konvektion in Flüssig-Flüssig-Systemen. 2020.

T. Kühne, M. Iannuzzi, M.D. Ben, V.V. Rybkin, P. Seewald, F. Stein, T. Laino, R.Z. Khaliullin, O. Schütt, F. Schiffmann, D. Golze, J. Wilhelm, S. Chulkov, M.H.B. Mohammad Hossein Bani-Hashemian, V. Weber, U. Borstnik, M. Taillefumier, A.S. Jakobovits, A. Lazzaro, H. Pabst, T. Müller, R. Schade, M. Guidon, S. Andermatt, N. Holmberg, G.K. Schenter, A. Hehn, A. Bussy, F. Belleflamme, G. Tabacchi, A. Glöß, M. Lass, I. Bethune, C.J. Mundy, C. Plessl, M. Watkins, J. VandeVondele, M. Krack, J. Hutter, The Journal of Chemical Physics (2020), 152(19)

CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-theart ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2K to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post–Hartree–Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension.

H. Aldahhak, P. Powroźnik, P. Pander, W. Jakubik, F.B. Dias, W.G. Schmidt, U. Gerstmann, M. Krzywiecki, The Journal of Physical Chemistry C (2020)(124), pp. 6090-6102

J.M. Hanselle, A. Tornede, M.D. Wever, E. Hüllermeier, in: KI 2020: Advances in Artificial Intelligence, 2020

T. Biktagirov, W.G. Schmidt, U. Gerstmann, Physical Review Research (2020)

S. Kuhlemann, K. Tierney, Journal of Heuristics (2020)

S. Heindorf, Y. Scholten, H. Wachsmuth, A. Ngonga Ngomo, M. Potthast, in: Proceedings of the 28th ACM International Conference on Information and Knowledge Management (CIKM 2020), 2020, pp. 3023-3030

K. Kinoshita, T.C. von Neumann, M. Delcroix, T. Nakatani, R. Haeb-Umbach, in: Proc. Interspeech 2020, 2020, pp. 2652-2656

Recently, the source separation performance was greatly improved by time-domain audio source separation based on dual-path recurrent neural network (DPRNN). DPRNN is a simple but effective model for a long sequential data. While DPRNN is quite efficient in modeling a sequential data of the length of an utterance, i.e., about 5 to 10 second data, it is harder to apply it to longer sequences such as whole conversations consisting of multiple utterances. It is simply because, in such a case, the number of time steps consumed by its internal module called inter-chunk RNN becomes extremely large. To mitigate this problem, this paper proposes a multi-path RNN (MPRNN), a generalized version of DPRNN, that models the input data in a hierarchical manner. In the MPRNN framework, the input data is represented at several (>_ 3) time-resolutions, each of which is modeled by a specific RNN sub-module. For example, the RNN sub-module that deals with the finest resolution may model temporal relationship only within a phoneme, while the RNN sub-module handling the most coarse resolution may capture only the relationship between utterances such as speaker information. We perform experiments using simulated dialogue-like mixtures and show that MPRNN has greater model capacity, and it outperforms the current state-of-the-art DPRNN framework especially in online processing scenarios.

S.R. Tinkloh, T. Wu, T. Tröster, T. Niendorf, Composite Structures (2020), 238

P. Schöppe, S. Schönherr, M. Chugh, H. Mirhosseini, P. Jackson, R. Wuerz, M. Ritzer, A. Johannes, G. Martínez-Criado, W. Wisniewski, T. Schwarz, C. T. Plass, M. Hafermann, T. Kühne, C. S. Schnohr, C. Ronning, Nano Energy (2020), 71, pp. 104622

M. Yu, N. Chandrasekhar, R. Kormath Madam Raghupathy, K.H. Ly, H. Zhang, E. Dmitrieva, C. Liang, X. Lu, T. Kühne, S.H. Mirhosseini, I.M. Weidinger, X. Feng, Journal of the American Chemical Society (2020), 142(46), pp. 19570-19578

A. El Mesaoudi-Paul, D. Weiß, V. Bengs, E. Hüllermeier, K. Tierney, in: Learning and Intelligent Optimization. LION 2020., Springer, 2020, pp. 216 - 232

V. Bengs, E. Hüllermeier, in: arXiv:2011.00813, 2020

We consider a resource-aware variant of the classical multi-armed bandit problem: In each round, the learner selects an arm and determines a resource limit. It then observes a corresponding (random) reward, provided the (random) amount of consumed resources remains below the limit. Otherwise, the observation is censored, i.e., no reward is obtained. For this problem setting, we introduce a measure of regret, which incorporates the actual amount of allocated resources of each learning round as well as the optimality of realizable rewards. Thus, to minimize regret, the learner needs to set a resource limit and choose an arm in such a way that the chance to realize a high reward within the predefined resource limit is high, while the resource limit itself should be kept as low as possible. We derive the theoretical lower bound on the cumulative regret and propose a learning algorithm having a regret upper bound that matches the lower bound. In a simulation study, we show that our learning algorithm outperforms straightforward extensions of standard multi-armed bandit algorithms.

M. Meyer, T. Kenter, C. Plessl, in: 2020 IEEE/ACM International Workshop on Heterogeneous High-performance Reconfigurable Computing (H2RC), 2020

FPGAs have found increasing adoption in data center applications since a new generation of high-level tools have become available which noticeably reduce development time for FPGA accelerators and still provide high-quality results. There is, however, no high-level benchmark suite available, which specifically enables a comparison of FPGA architectures, programming tools, and libraries for HPC applications. To fill this gap, we have developed an OpenCL-based open-source implementation of the HPCC benchmark suite for Xilinx and Intel FPGAs. This benchmark can serve to analyze the current capabilities of FPGA devices, cards, and development tool flows, track progress over time, and point out specific difficulties for FPGA acceleration in the HPC domain. Additionally, the benchmark documents proven performance optimization patterns. We will continue optimizing and porting the benchmark for new generations of FPGAs and design tools and encourage active participation to create a valuable tool for the community. To fill this gap, we have developed an OpenCL-based open-source implementation of the HPCC benchmark suite for Xilinx and Intel FPGAs. This benchmark can serve to analyze the current capabilities of FPGA devices, cards, and development tool flows, track progress over time, and point out specific difficulties for FPGA acceleration in the HPC domain. Additionally, the benchmark documents proven performance optimization patterns. We will continue optimizing and porting the benchmark for new generations of FPGAs and design tools and encourage active participation to create a valuable tool for the community.

S.A. Bernemann, Master's thesis, 2020

A. Olenberg, E. Kenig, Chemical Engineering Science (2020), 219, pp. 115559

M.D. Wever, A. Tornede, F. Mohr, E. Hüllermeier, Springer, 2020

In multi-label classification (MLC), each instance is associated with a set of class labels, in contrast to standard classification where an instance is assigned a single label. Binary relevance (BR) learning, which reduces a multi-label to a set of binary classification problems, one per label, is arguably the most straight-forward approach to MLC. In spite of its simplicity, BR proved to be competitive to more sophisticated MLC methods, and still achieves state-of-the-art performance for many loss functions. Somewhat surprisingly, the optimal choice of the base learner for tackling the binary classification problems has received very little attention so far. Taking advantage of the label independence assumption inherent to BR, we propose a label-wise base learner selection method optimizing label-wise macro averaged performance measures. In an extensive experimental evaluation, we find that or approach, called LiBRe, can significantly improve generalization performance.

R.S. Chatwell, J. Vrabec, The Journal of Chemical Physics (2020)

E. Speiser, N. Esser, B. Halbig, J. Geurts, W.G. Schmidt, S. Sanna, Surface Science Reports (2020), 75(1)

B. Zhao, U. Manthe, The Journal of Chemical Physics (2020)

A. Tornede, M.D. Wever, E. Hüllermeier, in: Workshop MetaLearn 2020 @ NeurIPS 2020, 2020

M. Lass, R. Schade, T. Kühne, C. Plessl, in: Proc. International Conference for High Performance Computing, Networking, Storage and Analysis (SC), IEEE Computer Society, 2020, pp. 1127-1140

Electronic structure calculations based on density-functional theory (DFT) represent a significant part of today's HPC workloads and pose high demands on high-performance computing resources. To perform these quantum-mechanical DFT calculations on complex large-scale systems, so-called linear scaling methods instead of conventional cubic scaling methods are required. In this work, we take up the idea of the submatrix method and apply it to the DFT computations in the software package CP2K. For that purpose, we transform the underlying numeric operations on distributed, large, sparse matrices into computations on local, much smaller and nearly dense matrices. This allows us to exploit the full floating-point performance of modern CPUs and to make use of dedicated accelerator hardware, where performance has been limited by memory bandwidth before. We demonstrate both functionality and performance of our implementation and show how it can be accelerated with GPUs and FPGAs.

Y. Grynko, Y. Shkuratov, J. Förstner, Journal of Quantitative Spectroscopy and Radiative Transfer (2020), 255, pp. 107234

We numerically simulate multiple light scattering in discrete disordered media represented by large clusters of irregular non-absorbing particles. The packing density of clusters is 0.5. With such conditions diffuse scattering is significantly reduced and light transport follows propagation channels that are determined by the particle size and topology of the medium. This kind of localization produces coherent backscattering intensity surge and enhanced negative polarization branch if compared to lower density samples.

C. Boeddeker, T. Cord-Landwehr, J. Heitkaemper, C. Zorila, D. Hayakawa, M. Li, M. Liu, R. Doddipatla, R. Haeb-Umbach, in: Proc. CHiME 2020 Workshop on Speech Processing in Everyday Environments, 2020

T.C. von Neumann, K. Kinoshita, L. Drude, C. Boeddeker, M. Delcroix, T. Nakatani, R. Haeb-Umbach, in: ICASSP 2020 - 2020 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2020, pp. 7004-7008

The rising interest in single-channel multi-speaker speech separation sparked development of End-to-End (E2E) approaches to multispeaker speech recognition. However, up until now, state-of-theart neural network–based time domain source separation has not yet been combined with E2E speech recognition. We here demonstrate how to combine a separation module based on a Convolutional Time domain Audio Separation Network (Conv-TasNet) with an E2E speech recognizer and how to train such a model jointly by distributing it over multiple GPUs or by approximating truncated back-propagation for the convolutional front-end. To put this work into perspective and illustrate the complexity of the design space, we provide a compact overview of single-channel multi-speaker recognition systems. Our experiments show a word error rate of 11.0% on WSJ0-2mix and indicate that our joint time domain model can yield substantial improvements over cascade DNN-HMM and monolithic E2E frequency domain systems proposed so far.

A.A. Camberg, T. Erhart, T. Tröster, 2020

A. Elizabeth, H. Conradi, S. K. Sahoo, T. Kodalle, C. A. Kaufmann, T. Kühne, H. Mirhosseini, D. Abou-Ras, H. Mönig, Acta Materialia (2020), 200

Individual grains of chalcopyrite solar cell absorbers can facet in different crystallographic directions at their surfaces. To gain a deeper understanding of the junction formation in these devices, we correlate variations in the surface facet orientation with the defect electronic properties. We use a combined analytical approach based on scanning tunneling spectroscopy (STS), scanning electron microscopy, and electron back scatter diffraction (EBSD), where we perform these experiments on identical surface areas as small as 2 × 2 µm2 with a lateral resolution well below 50 nm. The topography of the absorber surfaces indicates two main morphological features: micro-faceted, long basalt-like columns and their short nano-faceted terminations. Our STS results reveal that the long columns exhibit spectral signatures typical for the presence of pronounced oxidation-induced surface dipoles in conjunction with an increased density of electronic defect levels. In contrast, the nano-faceted terminations of the basalt-like columns are largely passivated in terms of electronic defect levels within the band gap region. Corresponding crystallographic data based on EBSD experiments show that the surface of the basalt-like columns can be assigned to intrinsically polar facet orientations, while the passivated terminations are assigned to non-polar planes. Ab-initio calculations suggest that the polar surfaces are more prone to oxidation and resulting O-induced defects, in comparison to non-polar planes. Our results emphasize the correlation between morphology, surface facet orientations and surface electronic properties. Furthermore, this work aids in gaining a fundamental understanding of oxidation induced lateral inhomogeneities in view of the p-n junction formation in chalcopyrite thin-film solar cells.

S.K. Sahoo, J.J. Heske, M. Antonietti, Q. Qin, M. Oschatz, T. Kühne, ACS Applied Energy Materials (2020), 3(10), pp. 10061-10069

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH3) is a promising alternative route for an NH3 synthesis at ambient conditions to the conventional high temperature and pressure Haber--Bosch process without the need for hydrogen gas. Single metal ions or atoms are attractive candidates for the catalytic activation of non-reactive nitrogen (N2), and for future targeted improvement of NRR catalysts, it is of utmost importance to get detailed insights into structure-performance relationships and mechanisms of N2 activation in such structures. Here, we report density functional theory studies on the NRR catalyzed by single Au and Fe atoms supported in graphitic C2N materials. Our results show that the metal atoms present in the structure of C2N are the reactive sites, which catalyze the aforesaid reaction by strong adsorption and activation of N2. We further demonstrate that a lower onset electrode potential is required for Fe--C2N than for Au--C2N. Thus, Fe--C2N is theoretically predicted to be a potentially better NRR catalyst at ambient conditions than Au--C2N owing to the larger adsorption energy of N2 molecules. Furthermore, we have experimentally shown that single sites of Au and Fe supported on nitrogen-doped porous carbon are indeed active NRR catalysts. However, in contrast to our theoretical results, the Au-based catalyst performed slightly better with a Faradaic efficiency (FE) of 10.1{\%} than the Fe-based catalyst with an FE of 8.4{\%} at −0.2 V vs. RHE. The DFT calculations suggest that this difference is due to the competitive hydrogen evolution reaction and higher desorption energy of ammonia.

L. Meier, C. Braun, T. Hannappel, W.G. Schmidt, physica status solidi (b) (2020)

A. Mukherjee, A. Widhalm, D. Siebert, S. Krehs, N. Sharma, A. Thiede, D. Reuter, J. Förstner, A. Zrenner, Applied Physics Letters (2020), 116, pp. 251103

R. Zuo, X. Song, T. Meier, W. Yang, Journal of Physics: Conference Series (2020)

R. Bertling, M. Hack, I. Ausner, E. Kenig. CFD-basierte Untersuchung von Zweiphasenströmungen in strukturierten Packungen. 2020.

F. Schmidt, A.L. Kozub, T. Biktagirov, C. Eigner, C. Silberhorn, A. Schindlmayr, W.G. Schmidt, U. Gerstmann, Physical Review Research (2020), 2(4)

Polarons in dielectric crystals play a crucial role for applications in integrated electronics and optoelectronics. In this work, we use density-functional theory and Green's function methods to explore the microscopic structure and spectroscopic signatures of electron polarons in lithium niobate (LiNbO3). Total-energy calculations and the comparison of calculated electron paramagnetic resonance data with available measurements reveal the formation of bound polarons at Nb_Li antisite defects with a quasi-Jahn-Teller distorted, tilted configuration. The defect-formation energies further indicate that (bi)polarons may form not only at Nb_Li antisites but also at structures where the antisite Nb atom moves into a neighboring empty oxygen octahedron. Based on these structure models, and on the calculated charge-transition levels and potential-energy barriers, we propose two mechanisms for the optical and thermal splitting of bipolarons, which provide a natural explanation for the reported two-path recombination of bipolarons. Optical-response calculations based on the Bethe-Salpeter equation, in combination with available experimental data and new measurements of the optical absorption spectrum, further corroborate the geometries proposed here for free and defect-bound (bi)polarons.

V. Rengaraj, M. Lass, C. Plessl, T. Kühne, Computation (2020), 8(2)

In scientific computing, the acceleration of atomistic computer simulations by means of custom hardware is finding ever-growing application. A major limitation, however, is that the high efficiency in terms of performance and low power consumption entails the massive usage of low precision computing units. Here, based on the approximate computing paradigm, we present an algorithmic method to compensate for numerical inaccuracies due to low accuracy arithmetic operations rigorously, yet still obtaining exact expectation values using a properly modified Langevin-type equation.

M.A. Fodor, Z. Ható, T. Kristóf, M. Pósfai, Chemical Geology (2020)

M. Navickas, L. Giriūnas, V. Kalendra, T. Biktagirov, U. Gerstmann, W.G. Schmidt, M. Mączka, A. Pöppl, J. Banys, M. Šimėnas, Physical Chemistry Chemical Physics (2020), 22, pp. 8513-8521

C. Braun, S. Neufeld, U. Gerstmann, S. Sanna, J. Plaickner, E. Speiser, N. Esser, W.G. Schmidt, Physical Review Letters (2020), 124(14)

S. Johannesmann, S. Becker, M. Webersen, B. Henning, in: SMSI 2020 - Measurement Science, 2020

S.V. Badalov, R. Wilhelm, W.G. Schmidt, Journal of Computational Chemistry (2020), pp. 1921-1930

F. Barkhausen, S. Schumacher, X. Ma, Optics Letters (2020)

M. Streiter, T.G. Fischer, C. Wiebeler, S. Reichert, J. Langenickel, K. Zeitler, C. Deibel, The Journal of Physical Chemistry C (2020), pp. 15007-15014

W. Hannes, T. Meier, in: Ultrafast Phenomena and Nanophotonics XXIV, 2020

T. Tornede, A. Tornede, M.D. Wever, F. Mohr, E. Hüllermeier, in: Proceedings of the ECMLPKDD 2020, 2020

K. Chandrakar, Master's thesis, 2020

K. Pfannschmidt, E. Hüllermeier, in: Lecture Notes in Computer Science, 2020

R. Bertling, M. Hack, I. Ausner, E. Kenig. CFD-Simulationen von Film- und Rinnsalströmungen auf mikrostrukturierten Oberflächen. 2020.

C. Wecker, A. Hoppe, A. Schulz, J. Heine, H. Bart, E. Kenig. Numerische Untersuchungen des Stofftransports in Flüssig-Flüssig-Systemen unter Berücksichtigung der Marangonikonvektion. 2020.

J. Joswig, J. Anders, H. Zhang, C. Rademacher, B.G. Keller, in: bioRxiv, 2020

<jats:title>Abstract</jats:title><jats:p>The C-type lectin receptor langerin plays a vital role in the mammalian defense against invading pathogens. Its function hinges on the affinity to its co-factor Ca<jats:sup>2+</jats:sup> which in turn is regulated by the pH. We studied the structural consequences of pro-tonating the allosteric pH-sensor histidine H294 by molecular dynamics simulations (total simulation time: about 120 μs) and Markov models. We discovered a mechanism in which the signal that the pH has dropped is transferred to the Ca<jats:sup>2+</jats:sup>-binding site without transferring the initial proton. Instead, protonation of H294 unlocks a conformation in which a protonated lysine side-chain forms a hydrogen bond with a Ca<jats:sup>2+</jats:sup>-coordinating aspartic acid. This destabilizes Ca<jats:sup>2+</jats:sup> in the binding pocket, which we probed by steered molecular dynamics. After Ca<jats:sup>2+</jats:sup>-release, the proton is likely transferred to the aspartic acid and stabilized by a dyad with a nearby glutamic acid, triggering a conformational transition and thus preventing Ca<jats:sup>2+</jats:sup>-rebinding.</jats:p>

T. Biktagirov, W.G. Schmidt, U. Gerstmann, Physical Review Research (2020), 2(2)

T.D. Kühne, M. Iannuzzi, M. Del Ben, V.V. Rybkin, P. Seewald, F. Stein, T. Laino, R.Z. Khaliullin, O. Schütt, F. Schiffmann, e. al., The Journal of Chemical Physics (2020), 152(19), pp. 194103

C. Hielscher, J. Grenz, A.A. Camberg, N. Wingenbach, ATZ - Automobiltechnische Zeitschrift (2020), pp. 60-65

X. Ma, B. Berger, M. Aßmann, R. Driben, T. Meier, C. Schneider, S. Höfling, S. Schumacher, Nature Communications (2020)

M. Eckhoff, P.E. Blöchl, J. Behler, Physical Review B (2020)

C. Parreño-Torres, R. Alvarez-Valdes, R. Ruiz, K. Tierney, Transportation Research Part E: Logistics and Transportation Review (2020)

M. Krenz, U. Gerstmann, W.G. Schmidt, ACS Omega (2020), pp. 24057-24063

J. Ibaceta-Jaña, R. Muydinov, P. Rosado, H. Mirhosseini, M. Chugh, O. Nazarenko, D.N. Dirin, D. Heinrich, M.R. Wagner, T.D. Kühne, B. Szyszka, M.V. Kovalenko, A. Hoffmann, Phys. Chem. Chem. Phys. (2020), 22, pp. 5604-5614

H. Elgabarty, T. Kampfrath, D.J. Bonthuis, V. Balos, N.K. Kaliannan, P. Loche, R.R. Netz, M. Wolf, T.D. K{\, M. Sajadi, Science Advances (2020), 6(17)

A. Tornede, M.D. Wever, S. Werner, F. Mohr, E. Hüllermeier, in: ACML 2020, 2020

Algorithm selection (AS) deals with the automatic selection of an algorithm from a fixed set of candidate algorithms most suitable for a specific instance of an algorithmic problem class, where "suitability" often refers to an algorithm's runtime. Due to possibly extremely long runtimes of candidate algorithms, training data for algorithm selection models is usually generated under time constraints in the sense that not all algorithms are run to completion on all instances. Thus, training data usually comprises censored information, as the true runtime of algorithms timed out remains unknown. However, many standard AS approaches are not able to handle such information in a proper way. On the other side, survival analysis (SA) naturally supports censored data and offers appropriate ways to use such data for learning distributional models of algorithm runtime, as we demonstrate in this work. We leverage such models as a basis of a sophisticated decision-theoretic approach to algorithm selection, which we dub Run2Survive. Moreover, taking advantage of a framework of this kind, we advocate a risk-averse approach to algorithm selection, in which the avoidance of a timeout is given high priority. In an extensive experimental study with the standard benchmark ASlib, our approach is shown to be highly competitive and in many cases even superior to state-of-the-art AS approaches.

V. Myroshnychenko, S. Smirnov, P.M.M. Jose, C. Brosseau, J. Förstner, Acta Materialia (2020), 203, pp. 116432

The challenge of designing new tunable nonlinear dielectric materials with tailored properties has attracted an increasing amount of interest recently. Herein, we study the effective nonlinear dielectric response of a stochastic paraelectric-dielectric composite consisting of equilibrium distributions of circular and partially penetrable disks (or parallel, infinitely long, identical, partially penetrable, circular cylinders) of a dielectric phase randomly dispersed in a continuous matrix of a paraelectric phase. The random microstructures were generated using the Metropolis Monte Carlo algorithm. The evaluation of the effective permittivity and tunability were carried out by employing either a Landau thermodynamic model or its Johnson’s approximation to describe the field-dependent permittivity of the paraelectric phase and solving continuum-electrostatics equations using finite element calculations. We reveal that the percolation threshold in this composite governs the critical behavior of the effective permittivity and tunability. For microstructures below the percolation threshold, our simulations demonstrate a strong nonlinear behaviour of the field-dependent effective permittivity and very high tunability that increases as a function of dielectric phase concentration. Above the percolation threshold, the effective permittivity shows the tendency to linearization and the tunability dramatically drops down. The highly reduced permittivity and extraordinarily high tunability are obtained for the composites with dielectric impenetrable disks at high concentrations, in which the triggering of the percolation transition is avoided. The reported results cast light on distinct nonlinear behaviour of 2D and 3D stochastic composites and can guide the design of novel composites with the controlled morphology and tailored permittivity and tunability.

L. Ebers, M. Hammer, J. Förstner, Optics Express (2020), 28(24), pp. 36361

A stepwise angular spectrum method (SASM) for curved interfaces is presented to calculate the wave propagation in planar lens-like integrated optical structures based on photonic slab waveguides. The method is derived and illustrated for an effective 2D setup first and then for 3D slab waveguide lenses. We employ slab waveguides of different thicknesses connected by curved surfaces to realize a lens-like structure. To simulate the wave propagation in 3D including reflection and scattering losses, the stepwise angular spectrum method is combined with full vectorial finite element computations for subproblems with lower complexity. Our SASM results show excellent agreement with rigorous numerical simulations of the full structures with a substantially lower computational effort and can be utilized for the simulation-based design and optimization of complex and large scale setups.

X. Ma, B. Berger, M. Aßmann, R. Driben, T. Meier, C. Schneider, S. Höfling, S. Schumacher, Nature Communications (2020), 11

W. Hannes, A. Trautmann, M. Stein, F. Schäfer, M. Koch, T. Meier, Physical Review B (2020), 101(7)

J. Heitkaemper, J. Schmalenströer, R. Haeb-Umbach, in: INTERSPEECH 2020 Virtual Shanghai China, 2020

Speech activity detection (SAD), which often rests on the fact that the noise is "more'' stationary than speech, is particularly challenging in non-stationary environments, because the time variance of the acoustic scene makes it difficult to discriminate speech from noise. We propose two approaches to SAD, where one is based on statistical signal processing, while the other utilizes neural networks. The former employs sophisticated signal processing to track the noise and speech energies and is meant to support the case for a resource efficient, unsupervised signal processing approach. The latter introduces a recurrent network layer that operates on short segments of the input speech to do temporal smoothing in the presence of non-stationary noise. The systems are tested on the Fearless Steps challenge database, which consists of the transmission data from the Apollo-11 space mission. The statistical SAD achieves comparable detection performance to earlier proposed neural network based SADs, while the neural network based approach leads to a decision cost function of 1.07% on the evaluation set of the 2020 Fearless Steps Challenge, which sets a new state of the art.

T.C. von Neumann, C. Boeddeker, L. Drude, K. Kinoshita, M. Delcroix, T. Nakatani, R. Haeb-Umbach, T. von Neuann, in: Proc. Interspeech 2020, 2020, pp. 3097-3101

Most approaches to multi-talker overlapped speech separation and recognition assume that the number of simultaneously active speakers is given, but in realistic situations, it is typically unknown. To cope with this, we extend an iterative speech extraction system with mechanisms to count the number of sources and combine it with a single-talker speech recognizer to form the first end-to-end multi-talker automatic speech recognition system for an unknown number of active speakers. Our experiments show very promising performance in counting accuracy, source separation and speech recognition on simulated clean mixtures from WSJ0-2mix and WSJ0-3mix. Among others, we set a new state-of-the-art word error rate on the WSJ0-2mix database. Furthermore, our system generalizes well to a larger number of speakers than it ever saw during training, as shown in experiments with the WSJ0-4mix database.

S.. Kumar Sahoo, J.J. Heske, S. Azadi, Z.. Zhang, .N.. V Tarakina, M.. Oschatz, R.. Z. Khaliullin, .M.. Antonietti, T. Kühne, Scientific Reports (2020), 10(1)

T. Kühne, J.J. Heske, E. Prodan, Annals of Physics (2020), 421, pp. 168290

This is the second part of a project on the foundations of first-principle calculations of the electron transport in crystals at finite temperatures, aiming at a predictive first-principles platform that combines ab-initio molecular dynamics (AIMD) and a finite-temperature Kubo-formula with dissipation for thermally disordered crystalline phases. The latter are encoded in an ergodic dynamical system (Ω,G,dP), where Ω is the configuration space of the atomic degrees of freedom, G is the space group acting on Ω and dP is the ergodic Gibbs measure relative to the G-action. We first demonstrate how to pass from the continuum Kohn–Sham theory to a discrete atomic-orbitals based formalism without breaking the covariance of the physical observables w.r.t. (Ω,G,dP). Then we show how to implement the Kubo-formula, investigate its self-averaging property and derive an optimal finite-volume approximation for it. We also describe a numerical innovation that made possible AIMD simulations with longer orbits and elaborate on the details of our simulations. Lastly, we present numerical results on the transport coefficients of crystal silicon at different temperatures.

A. Elizabeth, S.K. Sahoo, D. Lockhorn, A. Timmer, N. Aghdassi, H. Zacharias, T. Kühne, S. Siebentritt, H. Mirhosseini, H. M\, Phys. Rev. Materials (2020), 4, pp. 063401

A. El Mesaoudi-Paul, V. Bengs, E. Hüllermeier, in: arXiv:2002.04275, 2020

We consider an extension of the contextual multi-armed bandit problem, in which, instead of selecting a single alternative (arm), a learner is supposed to make a preselection in the form of a subset of alternatives. More specifically, in each iteration, the learner is presented a set of arms and a context, both described in terms of feature vectors. The task of the learner is to preselect $k$ of these arms, among which a final choice is made in a second step. In our setup, we assume that each arm has a latent (context-dependent) utility, and that feedback on a preselection is produced according to a Plackett-Luce model. We propose the CPPL algorithm, which is inspired by the well-known UCB algorithm, and evaluate this algorithm on synthetic and real data. In particular, we consider an online algorithm selection scenario, which served as a main motivation of our problem setting. Here, an instance (which defines the context) from a certain problem class (such as SAT) can be solved by different algorithms (the arms), but only $k$ of these algorithms can actually be run.

V. Bengs, E. Hüllermeier, in: International Conference on Machine Learning, 2020, pp. 778-787

S. Lange, D. Schroder, C. Hedayat, C. Hangmann, T. Otto, U. Hilleringmann, in: 2020 International Symposium on Electromagnetic Compatibility - EMC EUROPE, IEEE, 2020

In this publication, the near-field to far-field transformation using the self-built near-field scanner NFS3000 is examined with regard to its geometry. This device allows to measure electric and magnetic fields in small distances to the DUT (Device under Test) with high geometric precision and high sensitivity. Leading to a fast examination of EMC (Electromagnetic Compatibility) problems, because the electromagnetic properties are better understandable and therefore easier to solve than e.g. measurements in a far-field chamber. In addition, it is possible to extrapolate the near-fields into the far-field and to determine the radiation pattern of antennas and emitting objects. For this purpose, this paper deals with the basis of this transformation, the so-called surface equivalence theorem. This principle is then adapted to the measurement of near-field scanners and implemented accordingly. Due to the non-ideal design of the near-field scanner, the effects on a far-field transformation are finally presented and discussed.

## 2019

C. Wecker, A. Schulz, J. Heine, H. Bart, E. Kenig. Stofftransport und Fluidmechanik bei der Tropfenbildung unter Berücksichtigung von Marangonikonvektion mittels CFD. 2019.

A. Bocchini, S. Neufeld, U. Gerstmann, W.G. Schmidt, Journal of Physics: Condensed Matter (2019)

A.A. Camberg, I. Stratmann, T. Tröster, in: Technologies for economical and functional lightweight design, 2019

A. Redder, A. Ramaswamy, D. Quevedo, in: Proceedings of the 8th IFAC Workshop on Distributed Estimation and Control in Networked Systems, 2019

This work considers the problem of control and resource allocation in networked systems. To this end, we present DIRA a Deep reinforcement learning based Iterative Resource Allocation algorithm, which is scalable and control-aware. Our algorithm is tailored towards large-scale problems where control and scheduling need to act jointly to optimize performance. DIRA can be used to schedule general time-domain optimization based controllers. In the present work, we focus on control designs based on suitably adapted linear quadratic regulators. We apply our algorithm to networked systems with correlated fading communication channels. Our simulations show that DIRA scales well to large scheduling problems.

W. Hannes, T. Meier, Physical Review B (2019), 99(12)

M. Pukrop, S. Schumacher, X. Ma, in: arXiv:1907.10974, 2019

Vortices are topological objects carrying quantized orbital angular momentum, also known as topological charge, and have been widely studied in many physical systems for their applicability in information storage and processing. Here we focus on vortices in semiconductor microcavity polariton condensates. In systems with spin degree of freedom the elementary excitations are so called half-vortices. A half-vortex carries a quantum circulation only in one of the two spin components. It has lower energy in comparison with a full-vortex and, importantly, has a circularly polarized density peak in the vortex core region, while elsewhere the condensate is linearly polarized. We demonstrate the spontaneous formation of localized half-vortices in spinor polariton condensates, non-resonantly excited by a linearly polarized ring pump. The pseudospin structure of the condensate includes a TE-TM splitting leading to effective spin-orbit coupling, resulting in solutions with broken cylindrical symmetry. The cross-interaction between different spin components provides an efficient method to realize optical vortex core switching between left- and right-circularly polarized states. This switching can be easily detected by measuring the polarization resolved intensity in the vortex core region and it can also be applied to higher order half-vortex states.

C. Dong, S. Schumacher, 2019

<jats:p><div> <div> <div> <p>Molecular doping in conjugated polymers is a crucial process for their application in organic photovoltaics and optoelectronics. In the present work we theoretically investigate p-type molecu- lar doping in a series of (poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b”]dithiophene)-alt- 4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT) conjugated oligomers with different lengths and three widely-used dopants with different electron affinities, namely F4TCNQ, F6TCNNQ, and CN6-CP. We study in detail the molecular geometry of possible oligomer-dopant complexes and its influence on the doping mechanisms and electronic system properties. We find that the mechanisms of dop- ing and charge transfer observed sensitively depend on the specific geometry of the oligomer-dopant complexes. For a given complex different geometries may exist, some of which show transfer of an entire electron from the oligomer chain onto the dopant molecule resulting in an integer-charge transfer complex, leaving the system in a ground state with broken spin symmetry. In other ge- ometries merely hybridization of oligomer and dopant frontier orbitals occurs with partial charge transfer but spin-symmetric ground state. Considering the resulting electronic density of states both cases may well contribute to an increased electrical conductivity of corresponding film samples while the underlying physical mechanisms are entirely different. </p> </div> </div> </div></jats:p>

J. Heymann, L. Drude, R. Haeb-Umbach, K. Kinoshita, T. Nakatani, in: ICASSP 2019, Brighton, UK, 2019

Signal dereverberation using the Weighted Prediction Error (WPE) method has been proven to be an effective means to raise the accuracy of far-field speech recognition. First proposed as an iterative algorithm, follow-up works have reformulated it as a recursive least squares algorithm and therefore enabled its use in online applications. For this algorithm, the estimation of the power spectral density (PSD) of the anechoic signal plays an important role and strongly influences its performance. Recently, we showed that using a neural network PSD estimator leads to improved performance for online automatic speech recognition. This, however, comes at a price. To train the network, we require parallel data, i.e., utterances simultaneously available in clean and reverberated form. Here we propose to overcome this limitation by training the network jointly with the acoustic model of the speech recognizer. To be specific, the gradients computed from the cross-entropy loss between the target senone sequence and the acoustic model network output is backpropagated through the complex-valued dereverberation filter estimation to the neural network for PSD estimation. Evaluation on two databases demonstrates improved performance for on-line processing scenarios while imposing fewer requirements on the available training data and thus widening the range of applications.

X. Song, R. Zuo, S. Yang, P. Li, T. Meier, W. Yang, Optics Express (2019), 27

N. Kanda, C. Boeddeker, J. Heitkaemper, Y. Fujita, S. Horiguchi, R. Haeb-Umbach, in: INTERSPEECH 2019, Graz, Austria, 2019

In this paper, we present Hitachi and Paderborn University’s joint effort for automatic speech recognition (ASR) in a dinner party scenario. The main challenges of ASR systems for dinner party recordings obtained by multiple microphone arrays are (1) heavy speech overlaps, (2) severe noise and reverberation, (3) very natural onversational content, and possibly (4) insufficient training data. As an example of a dinner party scenario, we have chosen the data presented during the CHiME-5 speech recognition challenge, where the baseline ASR had a 73.3% word error rate (WER), and even the best performing system at the CHiME-5 challenge had a 46.1% WER. We extensively investigated a combination of the guided source separation-based speech enhancement technique and an already proposed strong ASR backend and found that a tight combination of these techniques provided substantial accuracy improvements. Our final system achieved WERs of 39.94% and 41.64% for the development and evaluation data, respectively, both of which are the best published results for the dataset. We also investigated with additional training data on the official small data in the CHiME-5 corpus to assess the intrinsic difficulty of this ASR task.

A. Schoch, L. Burkhardt, R. Schoch, K. Stührenberg, M. Bauer, Faraday Discussions (2019), pp. 113-132

A. Nelus, J. Ebbers, R. Haeb-Umbach, R. Martin, in: INTERSPEECH 2019, Graz, Austria, 2019

In this paper we highlight the privacy risks entailed in deep neural network feature extraction for domestic activity monitoring. We employ the baseline system proposed in the Task 5 of the DCASE 2018 challenge and simulate a feature interception attack by an eavesdropper who wants to perform speaker identification. We then propose to reduce the aforementioned privacy risks by introducing a variational information feature extraction scheme that allows for good activity monitoring performance while at the same time minimizing the information of the feature representation, thus restricting speaker identification attempts. We analyze the resulting model’s composite loss function and the budget scaling factor used to control the balance between the performance of the trusted and attacker tasks. It is empirically demonstrated that the proposed method reduces speaker identification privacy risks without significantly deprecating the performance of domestic activity monitoring tasks.

J. Heymann, B.L. Khe Chai Sim, in: ICASSP 2019, Brighton, UK, 2019

Connectionist temporal classification (CTC) is a sequence-level loss that has been successfully applied to train recurrent neural network (RNN) models for automatic speech recognition. However, one major weakness of CTC is the conditional independence assumption that makes it difficult for the model to learn label dependencies. In this paper, we propose stimulated CTC, which uses stimulated learning to help CTC models learn label dependencies implicitly by using an auxiliary RNN to generate the appropriate stimuli. This stimuli comes in the form of an additional stimulation loss term which encourages the model to learn said label dependencies. The auxiliary network is only used during training and the inference model has the same structure as a standard CTC model. The proposed stimulated CTC model achieves about 35% relative character error rate improvements on a synthetic gesture keyboard recognition task and over 30% relative word error rate improvements on the Librispeech automatic speech recognition tasks over a baseline model trained with CTC only.

L.M. Witschen, H. Ghasemzadeh Mohammadi, M. Artmann, M. Platzner, in: Fourth Workshop on Approximate Computing (AxC 2019), 2019

State-of-the-art frameworks for generating approximate circuits usually rely on information gained through circuit synthesis and/or verification to explore the search space and to find an optimal solution. Throughout the process, a large number of circuits may be subject to processing, leading to considerable runtimes. In this work, we propose a search which takes error bounds and pre-computed impact factors into account to reduce the number of invoked synthesis and verification processes. In our experimental results, we achieved speed-ups of up to 76x while area savings remain comparable to the reference search method, simulated annealing.

A.A. Camberg, T. Tröster, in: 26. Sächsische Fachtagung Umformtechnik, 2019

L.M. Witschen, T. Wiersema, H. Ghasemzadeh Mohammadi, M. Awais, M. Platzner, Microelectronics Reliability (2019), 99, pp. 277-290

Existing approaches and tools for the generation of approximate circuits often lack generality and are restricted to certain circuit types, approximation techniques, and quality assurance methods. Moreover, only few tools are publicly available. This hinders the development and evaluation of new techniques for approximating circuits and their comparison to previous approaches. In this paper, we ﬁrst analyze and classify related approaches and then present CIRCA, our ﬂexible framework for search-based approximate circuit generation. CIRCA is developed with a focus on modularity and extensibility. We present the architecture of CIRCA with its clear separation into stages and functional blocks, report on the current prototype, and show initial experiments.

A. Sprenger, S. Hellebrand, Journal of Circuits, Systems and Computers (2019), 28(1), pp. 1-23

A. Ferreri, P. Sharapova, K.H. Luo, H. Herrmann, C. Silberhorn, in: Quantum Information and Measurement (QIM) V: Quantum Technologies, 2019

C. Wiebeler, I. Schapiro, Molecules (2019)

<jats:p>Cyanobacteriochromes are compact and spectrally diverse photoreceptor proteins that are promising candidates for biotechnological applications. Computational studies can contribute to an understanding at a molecular level of their wide spectral tuning and diversity. In this contribution, we benchmark methods to model a 110 nm shift in the UV/Vis absorption spectrum from a red- to a green-absorbing form of the cyanobacteriochrome Slr1393g3. Based on an assessment of semiempirical methods to describe the chromophore geometries of both forms in vacuo, we find that DFTB2+D leads to structures that are the closest to the reference method. The benchmark of the excited state calculations is based on snapshots from quantum mechanics/molecular mechanics molecular dynamics simulations. In our case, the methods RI-ADC(2) and sTD-DFT based on CAM-B3LYP ground state calculations perform the best, whereas no functional can be recommended to simulate the absorption spectra of both forms with time-dependent density functional theory. Furthermore, the difference in absorption for the lowest energy absorption maxima of both forms can already be modelled with optimized structures, but sampling is required to improve the shape of the absorption bands of both forms, in particular for the second band. This benchmark study can guide further computational studies, as it assesses essential components of a protocol to model the spectral tuning of both cyanobacteriochromes and the related phytochromes.</jats:p>

F. Schmidt, A. Riefer, W.G. Schmidt, A. Schindlmayr, M. Imlau, F. Dobener, N. Mengel, S. Chatterjee, S. Sanna, Physical Review Materials (2019), 3(5)

The cubic, tetragonal, and orthorhombic phase of potassium niobate (KNbO3) are studied based on density-functional theory. Starting from the relaxed atomic geometries, we analyze the influence of self-energy corrections on the electronic band structure within the GW approximation. We find that quasiparticle shifts widen the direct (indirect) band gap by 1.21 (1.44), 1.58 (1.55), and 1.67 (1.64) eV for the cubic, tetragonal, and orthorhombic phase, respectively. By solving the Bethe-Salpeter equation, we obtain the linear dielectric function with excitonic and local-field effects, which turn out to be essential for good agreement with experimental data. From our results, we extract an exciton binding energy of 0.6, 0.5, and 0.5 eV for the cubic, tetragonal, and orthorhombic phase, respectively. Furthermore, we investigate the nonlinear second-harmonic generation (SHG) both theoretically and experimentally. The frequency-dependent second-order polarization tensor of orthorhombic KNbO3 is measured for incoming photon energies between 1.2 and 1.6 eV. In addition, calculations within the independent-(quasi)particle approximation are performed for the tetragonal and orthorhombic phase. The novel experimental data are in excellent agreement with the quasiparticle calculations and resolve persistent discrepancies between earlier experimental measurements and ab initio results reported in the literature.

C. Dong, S. Schumacher, The Journal of Physical Chemistry C (2019), pp. 30863-30870

L. Drude, J. Heitkaemper, C. Boeddeker, R. Haeb-Umbach, ArXiv e-prints (2019)

We present a multi-channel database of overlapping speech for training, evaluation, and detailed analysis of source separation and extraction algorithms: SMS-WSJ -- Spatialized Multi-Speaker Wall Street Journal. It consists of artificially mixed speech taken from the WSJ database, but unlike earlier databases we consider all WSJ0+1 utterances and take care of strictly separating the speaker sets present in the training, validation and test sets. When spatializing the data we ensure a high degree of randomness w.r.t. room size, array center and rotation, as well as speaker position. Furthermore, this paper offers a critical assessment of recently proposed measures of source separation performance. Alongside the code to generate the database we provide a source separation baseline and a Kaldi recipe with competitive word error rates to provide common ground for evaluation.

K. Pfannschmidt, P. Gupta, E. Hüllermeier, in: arXiv:1901.10860, 2019

We study the problem of learning choice functions, which play an important role in various domains of application, most notably in the field of economics. Formally, a choice function is a mapping from sets to sets: Given a set of choice alternatives as input, a choice function identifies a subset of most preferred elements. Learning choice functions from suitable training data comes with a number of challenges. For example, the sets provided as input and the subsets produced as output can be of any size. Moreover, since the order in which alternatives are presented is irrelevant, a choice function should be symmetric. Perhaps most importantly, choice functions are naturally context-dependent, in the sense that the preference in favor of an alternative may depend on what other options are available. We formalize the problem of learning choice functions and present two general approaches based on two representations of context-dependent utility functions. Both approaches are instantiated by means of appropriate neural network architectures, and their performance is demonstrated on suitable benchmark tasks.

C. Dues, W.G. Schmidt, S. Sanna, ACS Omega (2019), pp. 3850-3859

L.M. Witschen, H. Ghasemzadeh Mohammadi, M. Artmann, M. Platzner, in: Proceedings of the 2019 on Great Lakes Symposium on VLSI - GLSVLSI '19, ACM, 2019

State-of-the-art frameworks for generating approximate circuits automatically explore the search space in an iterative process - often greedily. Synthesis and verification processes are invoked in each iteration to evaluate the found solutions and to guide the search algorithm. As a result, a large number of approximate circuits is subjected to analysis - leading to long runtimes - but only a few approximate circuits might form an acceptable solution. In this paper, we present our Jump Search (JS) method which seeks to reduce the runtime of an approximation process by reducing the number of expensive synthesis and verification steps. To reduce the runtime, JS computes impact factors for each approximation candidate in the circuit to create a selection of approximate circuits without invoking synthesis or verification processes. We denote the selection as path from which JS determines the final solution. In our experimental results, JS achieved speed-ups of up to 57x while area savings remain comparable to the reference search method, Simulated Annealing.

S. Tanaka, K. Tierney, C. Parreño-Torres, R. Alvarez-Valdes, R. Ruiz, European Journal of Operational Research (2019), pp. 211-225

R.S. Chatwell, M. Heinen, J. Vrabec, International Journal of Heat and Mass Transfer (2019), 132, pp. 1296-1305

W. Hannes, L. Krauß-Kodytek, C. Ruppert, M. Betz, T. Meier, in: Ultrafast Phenomena and Nanophotonics XXIII, 2019

J. Vollbrecht, C. Wiebeler, H. Bock, S. Schumacher, H. Kitzerow, The Journal of Physical Chemistry C (2019), 123(7), pp. 4483-4492

S.K. Peter, C. Kaulen, A. Hoffmann, W. Ogieglo, S. Karthäuser, M. Homberger, S. Herres-Pawlis, U. Simon, The Journal of Physical Chemistry C (2019), 123(11), pp. 6537-6548

A. Schulz, C. Wecker, E. Kenig. Ein Finite-Volumen Ansatz für den Stoffübergang an bewegten Phasengrenzflächen. 2019.

J. Heine, C. Wecker, E. Kenig, H. Bart. Stofftransport bei der Tropfenbildung. 2019.

C.W. Nicholson, M. Puppin, A. Lücke, U. Gerstmann, M. Krenz, W.G. Schmidt, L. Rettig, R. Ernstorfer, M. Wolf, Physical Review B (2019), 99(15)

L. Drude, R. Haeb-Umbach, IEEE Journal of Selected Topics in Signal Processing (2019)

We formulate a generic framework for blind source separation (BSS), which allows integrating data-driven spectro-temporal methods, such as deep clustering and deep attractor networks, with physically motivated probabilistic spatial methods, such as complex angular central Gaussian mixture models. The integrated model exploits the complementary strengths of the two approaches to BSS: the strong modeling power of neural networks, which, however, is based on supervised learning, and the ease of unsupervised learning of the spatial mixture models whose few parameters can be estimated on as little as a single segment of a real mixture of speech. Experiments are carried out on both artificially mixed speech and true recordings of speech mixtures. The experiments verify that the integrated models consistently outperform the individual components. We further extend the models to cope with noisy, reverberant speech and introduce a cross-domain teacher–student training where the mixture model serves as the teacher to provide training targets for the student neural network.

D. Ojha, N.K. Kaliannan, T.D. Kühne, Communications Chemistry (2019), 2(1), pp. 116

J. Heitkaemper, T. Feher, M. Freitag, R. Haeb-Umbach, in: International Conference on Statistical Language and Speech Processing 2019, Ljubljana, Slovenia, 2019

Multi-talker speech and moving speakers still pose a significant challenge to automatic speech recognition systems. Assuming an enrollment utterance of the target speakeris available, the so-called SpeakerBeam concept has been recently proposed to extract the target speaker from a speech mixture. If multi-channel input is available, spatial properties of the speaker can be exploited to support the source extraction. In this contribution we investigate different approaches to exploit such spatial information. In particular, we are interested in the question, how useful this information is if the target speaker changes his/her position. To this end, we present a SpeakerBeam-based source extraction network that is adapted to work on moving speakers by recursively updating the beamformer coefficients. Experimental results are presented on two data sets, one with articially created room impulse responses, and one with real room impulse responses and noise recorded in a conference room. Interestingly, spatial features turn out to be advantageous even if the speaker position changes.

A.A. Camberg, T. Tröster, F. Bohner, J. Tölle, IOP Conference Series: Materials Science and Engineering (2019), 651, pp. 012057

A.A. Desouki, M. Röder, A. Ngonga Ngomo, in: Proceedings of the 30th ACM Conference on Hypertext and Social Media - HT '19, ACM, 2019, pp. 163-171

Ranking plays a central role in a large number of applications driven by RDF knowledge graphs. Over the last years, many popular RDF knowledge graphs have grown so large that rankings for the facts they contain cannot be computed directly using the currently common 64-bit platforms. In this paper, we tackle two problems: Computing ranks on such large knowledge bases efficiently and incrementally. First, we present D-HARE, a distributed approach for computing ranks on very large knowledge graphs. D-HARE assumes the random surfer model and relies on data partitioning to compute matrix multiplications and transpositions on disk for matrices of arbitrary size. Moreover, the data partitioning underlying D-HARE allows the execution of most of its steps in parallel. As very large knowledge graphs are often updated periodically, we tackle the incremental computation of ranks on large knowledge bases as a second problem. We address this problem by presenting I-HARE, an approximation technique for calculating the overall ranking scores of a knowledge without the need to recalculate the ranking from scratch at each new revision. We evaluate our approaches by calculating ranks on the 3 × 10^9 and 2.4 × 10^9 triples from Wikidata resp. LinkedGeoData. Our evaluation demonstrates that D-HARE is the first holistic approach for computing ranks on very large RDF knowledge graphs. In addition, our incremental approach achieves a root mean squared error of less than 10E−7 in the best case. Both D-HARE and I-HARE are open-source and are available at: https://github.com/dice-group/incrementalHARE.

X. Ma, Y.Y. Kartashov, T. Gao, S. Schumacher, New Journal of Physics (2019)

E.F. Akbulut Irmak, J. Hanses, S. Schweizer, T. Tröster, 2019

M. Heinen, J. Vrabec, The Journal of Chemical Physics (2019)

T. Kodalle, R. Kormath Madam Raghupathy, T. Bertram, N. Maticiuc, H.A. Yetkin, R. Gunder, R. Schlatmann, T.D. Kühne, C.A. Kaufmann, H. Mirhosseini, physica status solidi (RRL)--Rapid Research Letters (2019), 13(3), pp. 1800564

M. Chugh, .T.D. Kühne, H. Mirhosseini, ACS Applied Materials & Interfaces (2019), 11(16), pp. 14821−14829

S. Lange, D. Schröder, C. Hedayat, T. Otto, U. Hilleringmann, in: 2019 17th IEEE International New Circuits and Systems Conference (NEWCAS), 2019

For the measurement of process data in bioreactors, very small wireless sensors are currently under development to replace the conventional rod probes. The so-called Sens-o-Spheres measure the temperature and in future the oxygen content and the pH of fluids. In order to evaluate the distribution of the measured values within the process, it is necessary to locate the wireless sensors. Because of the small size of the sphere (diameter 8 mm), inhomogeneous ambient media and the size of the reactor (less than 2 m), an inductive locating by magnetic fields with a frequency of f = 13.56 MHz is necessary. Since the behaviour of the magnetic field is very different from that of the electromagnetic wave, new locating methods are required, which are presented in this paper.

M.D. Wever, F. Mohr, A. Tornede, E. Hüllermeier, 2019

Existing tools for automated machine learning, such as Auto-WEKA, TPOT, auto-sklearn, and more recently ML-Plan, have shown impressive results for the tasks of single-label classification and regression. Yet, there is only little work on other types of machine learning problems so far. In particular, there is almost no work on automating the engineering of machine learning solutions for multi-label classification (MLC). We show how the scope of ML-Plan, an AutoML-tool for multi-class classification, can be extended towards MLC using MEKA, which is a multi-label extension of the well-known Java library WEKA. The resulting approach recursively refines MEKA's multi-label classifiers, nesting other multi-label classifiers for meta algorithms and single-label classifiers provided by WEKA as base learners. In our evaluation, we find that the proposed approach yields strong results and performs significantly better than a set of baselines we compare with.

C. Ansótegui, B. Heymann, J. Pon, M. Sellmann, K. Tierney, in: Learning and Intelligent Optimization, Springer International Publishing, 2019, pp. 309-325

R. Fingerhut, J. Vrabec, Fluid Phase Equilibria (2019), pp. 270-281

X. Ma, B. Berger, M. Assmann, R. Driben, T. Meier, C. Schneider, S. Höfling, S. Schumacher, in: arXiv:1907.03171, 2019

Vortices are topological objects representing the circular motion of a fluid. With their additional degree of freedom, the 'vorticity', they have been widely investigated in many physical systems and different materials for fundamental interest and for applications in data storage and information processing. Vortices have also been observed in non-equilibrium exciton-polariton condensates in planar semiconductor microcavities. There they appear spontaneously or can be created and pinned in space using ring-shaped optical excitation profiles. However, using the vortex state for information processing not only requires creation of a vortex but also efficient control over the vortex after its creation. Here we demonstrate a simple approach to control and switch a localized polariton vortex between opposite states. In our scheme, both the optical control of vorticity and its detection through the orbital angular momentum of the emitted light are implemented in a robust and practical manner.

R. Driben, X. Ma, S. Schumacher, T. Meier, Optics Letters (2019), 44(6)

R.D. Rittinghaus, P.M. Schäfer, P. Albrecht, C. Conrads, A. Hoffmann, A.N. Ksiazkiewicz, O. Bienemann, A. Pich, S. Herres-Pawlis, ChemSusChem (2019), 12(10), pp. 2161-2165

Abstract Polylactide is a biodegradable versatile material based on annually renewable resources and thus CO2-neutral in its lifecycle. Until now, tin(II)octanoate [Sn(Oct2)] was used as catalyst for the industrial ring-opening polymerization of lactide in spite of its cytotoxicity. On the way towards a sustainable catalyst, three iron(II) hybrid guanidine complexes were investigated concerning their molecular structure and applied to the ring-opening polymerization of lactide. The complexes could polymerize unpurified technical-grade rac-lactide as well as recrystallized l-lactide to long-chain polylactide in bulk with monomer/initiator ratios of more than 5000:1 in a controlled manner following the coordination–insertion mechanism. For the first time, a biocompatible complex has surpassed Sn(Oct)2 in its polymerization activity under industrially relevant conditions.

C. Wecker, A. Schulz, J. Heine, H. Bart, E. Kenig. Numerische Untersuchungen zum Stofftransport und Fluidmechanik bei der Tropfenbildung. 2019.

A. Zibart, E. Kenig. Reduktion von parasitären Strömungen in Mehrphasensimulationen durch Verwendung der Height-Function Methode. 2019.

S. Herres-Pawlis, R.D. Rittinghaus, J. Tremmel, A. Ruzicka, C. Conrads, P. Albrecht, A. Hoffmann, A. Ksiazkiewicz, A. Pich, R. Jambor, Chemistry – A European Journal (2019)

L. Claes, S. Johannesmann, E. Baumhögger, B. Henning, in: 2019 International Congress on Ultrasonics, 2019

J. Ebbers, R. Haeb-Umbach, in: DCASE2019 Workshop, New York, USA, 2019

In this paper we present our audio tagging system for the DCASE 2019 Challenge Task 2. We propose a model consisting of a convolutional front end using log-mel-energies as input features, a recurrent neural network sequence encoder and a fully connected classifier network outputting an activity probability for each of the 80 considered event classes. Due to the recurrent neural network, which encodes a whole sequence into a single vector, our model is able to process sequences of varying lengths. The model is trained with only little manually labeled training data and a larger amount of automatically labeled web data, which hence suffers from label noise. To efficiently train the model with the provided data we use various data augmentation to prevent overfitting and improve generalization. Our best submitted system achieves a label-weighted label-ranking average precision (lwlrap) of 75.5% on the private test set which is an absolute improvement of 21.7% over the baseline. This system scored the second place in the teams ranking of the DCASE 2019 Challenge Task 2 and the fifth place in the Kaggle competition “Freesound Audio Tagging 2019” with more than 400 participants. After the challenge ended we further improved performance to 76.5% lwlrap setting a new state-of-the-art on this dataset.

M. Grabo, S. Christoph, E. Kenig. Modellierung und Optimierung von makroverkapselten Latentwärmespeicherelementen. 2019.

T. Hetkämper, L. Claes, B. Henning, in: 2019 International Congress on Ultrasonics, 2019

S. Johannesmann, M. Webersen, J. Düchting, L. Claes, B. Henning, in: 45th Annual Review of Progress in Quantitative Nondestructive Evaluation , 2019

A. Tornede, M.D. Wever, E. Hüllermeier, in: Proceedings - 29. Workshop Computational Intelligence, Dortmund, 28. - 29. November 2019, KIT Scientific Publishing, Karlsruhe, 2019, pp. 135-146

R. Haeb-Umbach, S. Watanabe, T. Nakatani, M. Bacchiani, B. Hoffmeister, M.L. Seltzer, H. Zen, M. Souden, IEEE Signal Processing Magazine (2019), 36(6), pp. 111-124

Once a popular theme of futuristic science fiction or far-fetched technology forecasts, digital home assistants with a spoken language interface have become a ubiquitous commodity today. This success has been made possible by major advancements in signal processing and machine learning for so-called far-field speech recognition, where the commands are spoken at a distance from the sound capturing device. The challenges encountered are quite unique and different from many other use cases of automatic speech recognition. The purpose of this tutorial article is to describe, in a way amenable to the non-specialist, the key speech processing algorithms that enable reliable fully hands-free speech interaction with digital home assistants. These technologies include multi-channel acoustic echo cancellation, microphone array processing and dereverberation techniques for signal enhancement, reliable wake-up word and end-of-interaction detection, high-quality speech synthesis, as well as sophisticated statistical models for speech and language, learned from large amounts of heterogeneous training data. In all these fields, deep learning has occupied a critical role.

R. Haeb-Umbach, DFG forschung 1/2019 (2019), pp. 12-15

Wenn akustische Signalverarbeitung mit automatisiertem Lernen verknüpft wird: Nachrichtentechniker arbeiten mit mehreren Mikrofonen und tiefen neuronalen Netzen an besserer Spracherkennung unter widrigsten Bedingungen. Von solchen Sensornetzwerken könnten langfristig auch digitale Sprachassistenten profitieren.

X. Ma, Y.Y. Kartashov, T. Gao, S. Schumacher, New Journal of Physics (2019)

S. Peitz, S. Ober-Blöbaum, M. Dellnitz, Acta Applicandae Mathematicae (2019), 161(1), pp. 171–199

In a wide range of applications it is desirable to optimally control a dynamical system with respect to concurrent, potentially competing goals. This gives rise to a multiobjective optimal control problem where, instead of computing a single optimal solution, the set of optimal compromises, the so-called Pareto set, has to be approximated. When the problem under consideration is described by a partial differential equation (PDE), as is the case for fluid flow, the computational cost rapidly increases and makes its direct treatment infeasible. Reduced order modeling is a very popular method to reduce the computational cost, in particular in a multi query context such as uncertainty quantification, parameter estimation or optimization. In this article, we show how to combine reduced order modeling and multiobjective optimal control techniques in order to efficiently solve multiobjective optimal control problems constrained by PDEs. We consider a global, derivative free optimization method as well as a local, gradient-based approach for which the optimality system is derived in two different ways. The methods are compared with regard to the solution quality as well as the computational effort and they are illustrated using the example of the flow around a cylinder and a backward-facing-step channel flow.

S.R. Tinkloh, T. Wu, T. Tröster, T. Niendorf, 2019

R. Walczak, A. Savateev, J.J. Heske, N.V. Tarakina, S. Sahoo, J.D. Epping, T. Kühne, B. Kurpil, M. Antonietti, M. Oschatz, Sustainable Energy Fuels (2019), pp. -

Thermal treatment of hexaazatriphenylene-hexacarbonitrile (HAT-CN) in the temperature range from 500 °C to 700 °C leads to precise control over the degree of condensation{,} and thus atomic construction and porosity of the resulting C2N-type materials. Depending on the condensation temperature of HAT-CN{,} nitrogen contents of more than 30 at% can be reached. In general{,} these carbons show adsorption properties which are comparable to those known for zeolites but their pore size can be adjusted over a wider range. At condensation temperatures of 525 °C and below{,} the uptake of nitrogen gas remains negligible due to size exclusion{,} but the internal pores are large and polarizing enough that CO2 can still adsorb on part of the internal surface. This leads to surprisingly high CO2 adsorption capacities and isosteric heat of adsorption of up to 52 kJ mol−1. Theoretical calculations show that this high binding enthalpy arises from collective stabilization effects from the nitrogen atoms in the C2N layers surrounding the carbon atom in the CO2 molecule and from the electron acceptor properties of the carbon atoms from C2N which are in close proximity to the oxygen atoms in CO2. A true CO2 molecular sieving effect is achieved for the first time in such a metal-free organic material with zeolite-like properties{,} showing an IAST CO2/N2 selectivity of up to 121 at 298 K and a N2/CO2 ratio of 90/10 without notable changes in the CO2 adsorption properities over 80 cycles.

L. Claes, L.M. Hülskämper, E. Baumhögger, N. Feldmann, R.S. Chatwell, J. Vrabec, B. Henning, tm - Technisches Messen (2019), pp. 2-6

N.K.. Kaliannan, A. Henao Aristizabal, H. Wiebeler, F. Zysk, T. Ohto, Y. Nagata, T. D. Kühne, Molecular Physics (2019), pp. 1-10

H. Elgabarty, N.K. Kaliannan, T.D. Kühne, Scientific Reports (2019), 9, pp. 10002

A. Schulz, C. Wecker, E. Kenig, Chemie Ingenieur Technik (2019)

J. Heine, C. Wecker, E. Kenig, H. Bart. Visualization of Marangoni Phenomena during Droplet Formation. 2019.

G. Guevara-Carrion, S. Ancherbak, A. Mialdun, J. Vrabec, V. Shevtsova, Scientific Reports (2019), 9

F.H. Cho, Z. Peng, T. Biktagirov, U. Gerstmann, S. Takahashi, The Journal of Chemical Physics (2019), 150(13), pp. 134702

M. Pukrop, S. Schumacher, in: arXiv:1903.12534, 2019

Spontaneous formation of transverse patterns is ubiquitous in nonlinear dynamical systems of all kinds. An aspect of particular interest is the active control of such patterns. In nonlinear optical systems this can be used for all-optical switching with transistor-like performance, for example realized with polaritons in a planar quantum-well semiconductor microcavity. Here we focus on a specific configuration which takes advantage of the intricate polarization dependencies in the interacting optically driven polariton system. Besides detailed numerical simulations of the coupled light-field exciton dynamics, in the present paper we focus on the derivation of a simplified population competition model giving detailed insight into the underlying mechanisms from a nonlinear dynamical systems perspective. We show that such a model takes the form of a generalized Lotka-Volterra system for two competing populations explicitly including a source term that enables external control. We present a comprehensive analysis both of the existence and stability of stationary states in the parameter space spanned by spatial anisotropy and external control strength. We also construct phase boundaries in non-trivial regions and characterize emerging bifurcations. The population competition model reproduces all key features of the switching observed in full numerical simulations of the rather complex semiconductor system and at the same time is simple enough for a fully analytical understanding of the system dynamics.

S.K. Peter, C. Kaulen, A. Hoffmann, W. Ogieglo, S. Karthäuser, M. Homberger, S. Herres-Pawlis, U. Simon, The Journal of Physical Chemistry C (2019), 123(11), pp. 6537-6548

A. Klümper, W. Nuding, A. Sedrakyan, Physical Review B (2019), 100, pp. 140201

J.M. Martin-Donas, J. Heitkaemper, R. Haeb-Umbach, A.M. Gomez, A.M. Peinado, in: INTERSPEECH 2019, Graz, Austria, 2019

This paper deals with multi-channel speech recognition in scenarios with multiple speakers. Recently, the spectral characteristics of a target speaker, extracted from an adaptation utterance, have been used to guide a neural network mask estimator to focus on that speaker. In this work we present two variants of speakeraware neural networks, which exploit both spectral and spatial information to allow better discrimination between target and interfering speakers. Thus, we introduce either a spatial preprocessing prior to the mask estimation or a spatial plus spectral speaker characterization block whose output is directly fed into the neural mask estimator. The target speaker’s spectral and spatial signature is extracted from an adaptation utterance recorded at the beginning of a session. We further adapt the architecture for low-latency processing by means of block-online beamforming that recursively updates the signal statistics. Experimental results show that the additional spatial information clearly improves source extraction, in particular in the same-gender case, and that our proposal achieves state-of-the-art performance in terms of distortion reduction and recognition accuracy.

P. Müller, A. Neuba, U. Flörke, G. Henkel, T.D. Kühne, M. Bauer, The Journal of Physical Chemistry A (2019), pp. 3575-3581

D. Richters, M. Lass, A. Walther, C. Plessl, T. Kühne, Communications in Computational Physics (2019), 25(2), pp. 564-585

We address the general mathematical problem of computing the inverse p-th root of a given matrix in an efficient way. A new method to construct iteration functions that allow calculating arbitrary p-th roots and their inverses of symmetric positive definite matrices is presented. We show that the order of convergence is at least quadratic and that adaptively adjusting a parameter q always leads to an even faster convergence. In this way, a better performance than with previously known iteration schemes is achieved. The efficiency of the iterative functions is demonstrated for various matrices with different densities, condition numbers and spectral radii.

M. Riabinin, P. Sharapova, T. Bartley, T. Meier, 2019

M. Guc, T. Kodalle, R. Kormath Madam Raghupathy, H. Mirhosseini, T.D. Kühne, I. Becerril-Romero, A. Pérez-Rodríguez, C.A. Kaufmann, V. Izquierdo-Roca, The Journal of Physical Chemistry C (2019), 124, pp. 1285-1291

T. Clark, J.J. Heske, T. Kühne, ChemPhysChem (2019), 20, pp. 1-6

Abstract The effect of extending the O−H bond length(s) in water on the hydrogen-bonding strength has been investigated using static ab initio molecular orbital calculations. The “polar flattening” effect that causes a slight σ-hole to form on hydrogen atoms is strengthened when the bond is stretched, so that the σ-hole becomes more positive and hydrogen bonding stronger. In opposition to this electronic effect, path-integral ab initio molecular-dynamics simulations show that the nuclear quantum effect weakens the hydrogen bond in the water dimer. Thus, static electronic effects strengthen the hydrogen bond in H2O relative to D2O, whereas nuclear quantum effects weaken it. These quantum fluctuations are stronger for the water dimer than in bulk water.

A. Zibart, E. Kenig. Numerische Untersuchung des Eisbildungsprozesses auf der welligen Oberfläche von Kissenplatten. 2019.

P. Müller, A. Neuba, U. Flörke, G. Henkel, T.D. Kühne, M. Bauer, The Journal of Physical Chemistry A (2019), 123(16), pp. 3575-3581

L. Drude, J. Heymann, R. Haeb-Umbach, in: INTERSPEECH 2019, Graz, Austria, 2019

We present an unsupervised training approach for a neural network-based mask estimator in an acoustic beamforming application. The network is trained to maximize a likelihood criterion derived from a spatial mixture model of the observations. It is trained from scratch without requiring any parallel data consisting of degraded input and clean training targets. Thus, training can be carried out on real recordings of noisy speech rather than simulated ones. In contrast to previous work on unsupervised training of neural mask estimators, our approach avoids the need for a possibly pre-trained teacher model entirely. We demonstrate the effectiveness of our approach by speech recognition experiments on two different datasets: one mainly deteriorated by noise (CHiME 4) and one by reverberation (REVERB). The results show that the performance of the proposed system is on par with a supervised system using oracle target masks for training and with a system trained using a model-based teacher.

J. Heine, C. Wecker, E. Kenig, H. Bart. In-situ Messung des Stofftransports bei der Tropfenbildung. 2019.

S.A. Manavi, E. Kenig, in: Computer Aided Chemical Engineering, 29th European Symposium on Computer Aided Process Engineering, 2019

H.T. Duc, C. Ngo, T. Meier, Physical Review B (2019), 100(4)

H.J. von Bardeleben, S. Zhou, U. Gerstmann, D. Skachkov, W.R.L. Lambrecht, Q.D. Ho, P. Deák, APL Materials (2019), 7

Y. Xue, I. Chestnov, E. Sedov, S. Schumacher, X. Ma, A. Kavokin, in: arXiv:1907.00383, 2019

Superposition states of circular currents of exciton-polaritons mimic the superconducting flux qubits. The current states are formed by a macroscopic number of bosonic quasiparticles that compose a single quantum state of a many-body condensate. The essential difference between a polariton fluid and a superconducting current comes from the fact that in contrast to Cooper pairs polaritons are electrically neutral, and the magnetic field would not have a significant effect on a polariton flow. Nevertheless, the phase of a polariton condensate must change by an integer number of 2$\pi$, when going around the ring. If one introduces a $\pi$-phase delay line in the ring, the system is obliged to propagate a clockwise or anticlockwise circular current to reduce the total phase gained over one round-trip to zero or to build it up to $2\pi$. We show that such a $\pi$-delay line can be provided by a dark soliton embedded into a ring condensate and pinned to a potential well created by a C-shape non-resonant pump-spot. The physics of resulting split-ring polariton condensates is essentially similar to the physics of flux qubits. In particular, they exhibit pronounced coherent oscillations passing periodically through clockwise and anticlockwise current states. We predict that these oscillations may persist far beyond the coherence time of polariton condensates. As a consequence, the qubits based on split-ring polariton condensates are expected to possess very high figures of merit that makes them a valuable alternative to superconducting qubits.

L. Drude, D. Hasenklever, R. Haeb-Umbach, in: ICASSP 2019, Brighton, UK, 2019

We propose a training scheme to train neural network-based source separation algorithms from scratch when parallel clean data is unavailable. In particular, we demonstrate that an unsupervised spatial clustering algorithm is sufficient to guide the training of a deep clustering system. We argue that previous work on deep clustering requires strong supervision and elaborate on why this is a limitation. We demonstrate that (a) the single-channel deep clustering system trained according to the proposed scheme alone is able to achieve a similar performance as the multi-channel teacher in terms of word error rates and (b) initializing the spatial clustering approach with the deep clustering result yields a relative word error rate reduction of 26% over the unsupervised teacher.

E.F. Akbulut Irmak, T. Tröster, Procedia Structural Integrity (2019), pp. 190-197

S. Azadi, T.D. Kühne, Physical Review B (2019), 100, pp. 155103-5

J. Ebbers, L. Drude, R. Haeb-Umbach, A. Brendel, W. Kellermann, in: CAMSAP 2019, Guadeloupe, West Indies, 2019

In this paper we consider human daily activity recognition using an acoustic sensor network (ASN) which consists of nodes distributed in a home environment. Assuming that the ASN is permanently recording, the vast majority of recordings is silence. Therefore, we propose to employ a computationally efficient two-stage sound recognition system, consisting of an initial sound activity detection (SAD) and a subsequent sound event classification (SEC), which is only activated once sound activity has been detected. We show how a low-latency activity detector with high temporal resolution can be trained from weak labels with low temporal resolution. We further demonstrate the advantage of using spatial features for the subsequent event classification task.

C. Zorila, C. Boeddeker, R. Doddipatla, R. Haeb-Umbach, in: ASRU 2019, Sentosa, Singapore, 2019

Despite the strong modeling power of neural network acoustic models, speech enhancement has been shown to deliver additional word error rate improvements if multi-channel data is available. However, there has been a longstanding debate whether enhancement should also be carried out on the ASR training data. In an extensive experimental evaluation on the acoustically very challenging CHiME-5 dinner party data we show that: (i) cleaning up the training data can lead to substantial error rate reductions, and (ii) enhancement in training is advisable as long as enhancement in test is at least as strong as in training. This approach stands in contrast and delivers larger gains than the common strategy reported in the literature to augment the training database with additional artificially degraded speech. Together with an acoustic model topology consisting of initial CNN layers followed by factorized TDNN layers we achieve with 41.6% and 43.2% WER on the DEV and EVAL test sets, respectively, a new single-system state-of-the-art result on the CHiME-5 data. This is a 8% relative improvement compared to the best word error rate published so far for a speech recognizer without system combination.

A.A. Camberg, C. Hielscher, in: Aachen Body Engineering Days 2019, 2019

M.D. Wever, F. Mohr, E. Hüllermeier, A. Hetzer. Towards Automated Machine Learning for Multi-Label Classification. 2019.

M. Mennicken, S.K. Peter, C. Kaulen, U. Simon, S. Karthäuser, The Journal of Physical Chemistry C (2019), pp. 21367-21375

R. Fingerhut, G. Herres, J. Vrabec, Molecular Physics (2019)

R. Driben, X. Ma, S. Schumacher, T. Meier, Optics Letters (2019)

M.D. Wever, L. van Rooijen, H. Hamann, Evolutionary Computation (2019)

In software engineering, the imprecise requirements of a user are transformed to a formal requirements specification during the requirements elicitation process. This process is usually guided by requirements engineers interviewing the user. We want to partially automate this first step of the software engineering process in order to enable users to specify a desired software system on their own. With our approach, users are only asked to provide exemplary behavioral descriptions. The problem of synthesizing a requirements specification from examples can partially be reduced to the problem of grammatical inference, to which we apply an active coevolutionary learning approach. However, this approach would usually require many feedback queries to be sent to the user. In this work, we extend and generalize our active learning approach to receive knowledge from multiple oracles, also known as proactive learning. The ‘user oracle’ represents input received from the user and the ‘knowledge oracle’ represents available, formalized domain knowledge. We call our two-oracle approach the ‘first apply knowledge then query’ (FAKT/Q) algorithm. We compare FAKT/Q to the active learning approach and provide an extensive benchmark evaluation. As result we find that the number of required user queries is reduced and the inference process is sped up significantly. Finally, with so-called On-The-Fly Markets, we present a motivation and an application of our approach where such knowledge is available.

S. Neufeld, A. Bocchini, U. Gerstmann, A. Schindlmayr, W.G. Schmidt, Journal of Physics: Materials (2019), 2(4)

The KTiOPO4 (KTP) band structure and dielectric function are calculated on various levels of theory starting from density-functional calculations. Within the independent-particle approximation an electronic transport gap of 2.97 eV is obtained that widens to about 5.23 eV when quasiparticle effects are included using the GW approximation. The optical response is shown to be strongly anisotropic due to (i) the slight asymmetry of the TiO6 octahedra in the (001) plane and (ii) their anisotropic distribution along the [001] and [100] directions. In addition, excitonic effects are very important: The solution of the Bethe–Salpeter equation indicates exciton binding energies of the order of 1.5 eV. Calculations that include both quasiparticle and excitonic effects are in good agreement with the measured reflectivity.

## 2018

H. Aldahhak, M. Paszkiewicz, E. Rauls, F. Allegretti, S. Tebi, A.C. Papageorgiou, Y. Zhang, L. Zhang, T. Lin, T. Paintner, R. Koch, W.G. Schmidt, J.V. Barth, W. Schöfberger, S. Müllegger, F. Klappenberger, U. Gerstmann, Chemistry - A European Journal (2018), pp. 6787-6797

L. Drude, C. Boeddeker, J. Heymann, K. Kinoshita, M. Delcroix, T. Nakatani, R. Haeb-Umbach, in: INTERSPEECH 2018, Hyderabad, India, 2018

The weighted prediction error (WPE) algorithm has proven to be a very successful dereverberation method for the REVERB challenge. Likewise, neural network based mask estimation for beamforming demonstrated very good noise suppression in the CHiME 3 and CHiME 4 challenges. Recently, it has been shown that this estimator can also be trained to perform dereverberation and denoising jointly. However, up to now a comparison of a neural beamformer and WPE is still missing, so is an investigation into a combination of the two. Therefore, we here provide an extensive evaluation of both and consequently propose variants to integrate deep neural network based beamforming with WPE. For these integrated variants we identify a consistent word error rate (WER) reduction on two distinct databases. In particular, our study shows that deep learning based beamforming benefits from a model-based dereverberation technique (i.e. WPE) and vice versa. Our key findings are: (a) Neural beamforming yields the lower WERs in comparison to WPE the more channels and noise are present. (b) Integration of WPE and a neural beamformer consistently outperforms all stand-alone systems.

P. Müller, K. Karhan, M. Krack, U. Gerstmann, W.G. Schmidt, M. Bauer, T.D. Kühne, Journal of Computational Chemistry (2018), pp. 712-716

S.A. Zargaleh, H.J. von Bardeleben, J.L. Cantin, U. Gerstmann, S. Hameau, B. Ebl\'e, W. Gao, Phys. Rev. B (2018), 98(21), pp. 214113

M. Webersen, S. Johannesmann, J. Düchting, L. Claes, B. Henning, Ultrasonics (2018), 84, pp. 53-62

N. Esser, W.G. Schmidt, physica status solidi (b) (2018)(256)

C. Boeddeker, J. Heitkaemper, J. Schmalenstroeer, L. Drude, J. Heymann, R. Haeb-Umbach, in: Proc. CHiME 2018 Workshop on Speech Processing in Everyday Environments, Hyderabad, India, 2018

This contribution presents a speech enhancement system for the CHiME-5 Dinner Party Scenario. The front-end employs multi-channel linear time-variant filtering and achieves its gains without the use of a neural network. We present an adaptation of blind source separation techniques to the CHiME-5 database which we call Guided Source Separation (GSS). Using the baseline acoustic and language model, the combination of Weighted Prediction Error based dereverberation, guided source separation, and beamforming reduces the WER by 10:54% (relative) for the single array track and by 21:12% (relative) on the multiple array track.

R. Kormath Madam Raghupathy, H. Wiebeler, T. Kühne, C. Felser, H. Mirhosseini, Chemistry of Materials (2018), 30(19), pp. 6794-6800

M. Webersen, S. Johannesmann, T. Brockschmidt, F. Rump, L. Claes, B. Henning, 2018

X. Ma, S. Schumacher, Physical Review Letters (2018), 121(22)

L. Drude, J. Heymann, C. Boeddeker, R. Haeb-Umbach, in: ITG 2018, Oldenburg, Germany, 2018

NARA-WPE is a Python software package providing implementations of the weighted prediction error (WPE) dereverberation algorithm. WPE has been shown to be a highly effective tool for speech dereverberation, thus improving the perceptual quality of the signal and improving the recognition performance of downstream automatic speech recognition (ASR). It is suitable both for single-channel and multi-channel applications. The package consist of (1) a Numpy implementation which can easily be integrated into a custom Python toolchain, and (2) a TensorFlow implementation which allows integration into larger computational graphs and enables backpropagation through WPE to train more advanced front-ends. This package comprises of an iterative offline (batch) version, a block-online version, and a frame-online version which can be used in moderately low latency applications, e.g. digital speech assistants.

Z. Mamiyev, T. Lichtenstein, C. Tegenkamp, C. Braun, W.G. Schmidt, S. Sanna, H. Pfnür, Physical Review Materials (2018), 2(6)

K. Seino, S. Sanna, W.G. Schmidt, Surface Science (2018), 667, pp. 101-104

P. Zimmer, L. Burkhardt, R. Schepper, K. Zheng, D. Gosztola, A. Neuba, U. Flörke, C. Wölper, R. Schoch, W. Gawelda, S.E. Canton, M. Bauer, European Journal of Inorganic Chemistry (2018), pp. 5203-5214

J. Schmalenstroeer, A. Chinaev, G. Enzner, in: Speech Communication; 13th ITG-Symposium, 2018, pp. 1-5

Arbitrary sampling rate conversion has already received considerable attention in the past, but still lacks an equivalent representation of the effective time-dilation process in the block frequency domain. Good sampling rate converters in the time domain have been known, for instance, in terms of time-varying 'Sinc' or fixed 'Farrow' polynomial filters. The former can deliver nearly exact conversion at high complexity, while the latter has pronounced computational efficiency with limited accuracy. Only recently, it was shown that a composite 'polyphase Farrow' form with high resampling precision can be implemented with quasi-fixed filters that operate at the input sampling rate. We therefore propose to capitalize from that fixed-filter architecture in that we translate the polyphase-Farrow filters into an equivalent FFT-based overlap-save form. Experimental evaluation and comparison with other state-of-the art frequency-domain approaches then proves currently the best price-performance ratio of the proposed algorithm. It is thus an ideal candidate for the new framework of acoustic sensor networks that critically rests upon fast and accurate alignment of autonomous sampling processes.

L.M. Witschen, T. Wiersema, H. Ghasemzadeh Mohammadi, M. Awais, M. Platzner, in: Third Workshop on Approximate Computing (AxC 2018), 2018

Existing approaches and tools for the generation of approximate circuits often lack generality and are restricted to certain circuit types, approximation techniques, and quality assurance methods. Moreover, only few tools are publicly available. This hinders the development and evaluation of new techniques for approximating circuits and their comparison to previous approaches. In this paper, we ﬁrst analyze and classify related approaches and then present CIRCA, our ﬂexible framework for search-based approximate circuit generation. CIRCA is developed with a focus on modularity and extensibility. We present the architecture of CIRCA with its clear separation into stages and functional blocks, report on the current prototype, and show initial experiments.

M. Webersen, S. Johannesmann, J. Düchting, L. Claes, B. Henning, in: Fortschritte der Akustik - DAGA 2018, 2018, pp. 1263-1266

D. Beermann, M. Dellnitz, S. Peitz, S. Volkwein, in: PAMM, 2018, pp. 51-54

A framework for set‐oriented multiobjective optimal control of partial differential equations using reduced order modeling has recently been developed [1]. Following concepts from localized reduced bases methods, error estimators for the reduced cost functionals are utilized to construct a library of locally valid reduced order models. This way, a superset of the Pareto set can efficiently be computed while maintaining a prescribed error bound. In this article, this algorithm is applied to a problem with non‐smooth objective functionals. Using an academic example, we show that the extension to non‐smooth problems can be realized in a straightforward manner. We then discuss the implications on the numerical results.

C. Wiebeler, A.G. Rao, W. Gärtner, I. Schapiro, Angewandte Chemie International Edition (2018), pp. 1934-1938

M.D. Wever, F. Mohr, E. Hüllermeier, 2018

S. Sahoo, R. Kormath Madam Raghupathy, T.D. Kühne, H. Mirhosseini, J. Phys. Chem. C (2018), 122(37), pp. 21202-21209

K. Pfannschmidt, P. Gupta, E. Hüllermeier, in: arXiv:1803.05796, 2018

Object ranking is an important problem in the realm of preference learning. On the basis of training data in the form of a set of rankings of objects, which are typically represented as feature vectors, the goal is to learn a ranking function that predicts a linear order of any new set of objects. Current approaches commonly focus on ranking by scoring, i.e., on learning an underlying latent utility function that seeks to capture the inherent utility of each object. These approaches, however, are not able to take possible effects of context-dependence into account, where context-dependence means that the utility or usefulness of an object may also depend on what other objects are available as alternatives. In this paper, we formalize the problem of context-dependent ranking and present two general approaches based on two natural representations of context-dependent ranking functions. Both approaches are instantiated by means of appropriate neural network architectures, which are evaluated on suitable benchmark task.

M. Paszkiewicz, T. Biktagirov, H. Aldahhak, F. Allegretti, E. Rauls, W. Schöfberger, W.G. Schmidt, J.V. Barth, U. Gerstmann, F. Klappenberger, The Journal of Physical Chemistry Letters (2018), pp. 6412-6420

K. Tierney, J.F. Ehmke, A.M. Campbell, D. Müller, Flexible Services and Manufacturing Journal (2018), pp. 620-652

Z.A. Geiger, K.M. Fujiwara, K. Singh, R. Senaratne, S.V. Rajagopal, M. Lipatov, T. Shimasaki, R. Driben, V.V. Konotop, T. Meier, D.M. Weld, Physical Review Letters (2018), 120(21)

D. Breddermann, T. Praschan, D.F. Heinze, R. Binder, S. Schumacher, Physical Review B (2018), 97(12)

M. Naumova, D. Khakhulin, M. Rebarz, M. Rohrmüller, B. Dicke, M. Biednov, A. Britz, S. Espinoza, B. Grimm-Lebsanft, M. Kloz, N. Kretzschmar, A. Neuba, J. Ortmeyer, R. Schoch, J. Andreasson, M. Bauer, C. Bressler, W.G. Schmidt, G. Henkel, M. Rübhausen, Physical Chemistry Chemical Physics (2018), pp. 6274-6286

A.A. Camberg, K. Engelkemeier, J. Dietrich, T. Heggemann, Lightweight Design worldwide (2018), 11(2), pp. 24-29

M. Lass, T. Kühne, C. Plessl, Embedded Systems Letters (2018), 10(2), pp. 33-36

Approximate computing has shown to provide new ways to improve performance and power consumption of error-resilient applications. While many of these applications can be found in image processing, data classification or machine learning, we demonstrate its suitability to a problem from scientific computing. Utilizing the self-correcting behavior of iterative algorithms, we show that approximate computing can be applied to the calculation of inverse matrix p-th roots which are required in many applications in scientific computing. Results show great opportunities to reduce the computational effort and bandwidth required for the execution of the discussed algorithm, especially when targeting special accelerator hardware.

M.D. Wever, F. Mohr, E. Hüllermeier, in: Proceedings of the Genetic and Evolutionary Computation Conference, GECCO 2018, Kyoto, Japan, July 15-19, 2018, ACM, 2018

In multinomial classification, reduction techniques are commonly used to decompose the original learning problem into several simpler problems. For example, by recursively bisecting the original set of classes, so-called nested dichotomies define a set of binary classification problems that are organized in the structure of a binary tree. In contrast to the existing one-shot heuristics for constructing nested dichotomies and motivated by recent work on algorithm configuration, we propose a genetic algorithm for optimizing the structure of such dichotomies. A key component of this approach is the proposed genetic representation that facilitates the application of standard genetic operators, while still supporting the exchange of partial solutions under recombination. We evaluate the approach in an extensive experimental study, showing that it yields classifiers with superior generalization performance.

S. Johannesmann, J. Düchting, M. Webersen, L. Claes, B. Henning, tm - Technisches Messen (2018), 2018(85), pp. 478-486

C. Wiebeler, A.G. Rao, W. Gärtner, I. Schapiro, Angewandte Chemie (2018), pp. 1952-1957

F. Mohr, M.D. Wever, E. Hüllermeier, 2018

A. Zibart, E. Kenig. FALLING LIQUID FILM FLOW OVER THE WAVY SURFACE OF VERTICAL PILLOW PLATES − A NUMERICAL INVESTIGATION. 2018.

N. Esser, W.G. Schmidt, physica status solidi (b) (2018)

S. Tanaka, K. Tierney, European Journal of Operational Research (2018), 264(1), pp. 165 - 180

T. Rösener, A. Hoffmann, S. Herres-Pawlis, European Journal of Inorganic Chemistry (2018), 2018(27), pp. 3164-3175

Ligands DMEG6etqu, TMG6etqu, DMEG6buqu, and TMG6buqu were developed on the basis of guanidine quinoline (GUAqu) ligands 1,3-dimethyl-N-(quinolin-8-yl)imidazolidin-2-imine (DMEGqu) and 1,1,3,3-tetramethyl-2-(quinolin-8-yl)guanidine (TMGqu). These ligands feature an alkyl substituent at the C6 of the quinoline backbone. The synthetic strategy developed here enables inexpensive syntheses of any kind of C6-substituted GUAqu ligands. On one hand, the alkylation increases the solubility of corresponding copper complexes in apolar atom transfer radical polymerization (ATRP) monomers like styrene. On the other hand, it has a significant electronic influence and thus an effect on the donor properties of the new ligands. Seven CuI and CuII complexes of DMEG6etqu and TMG6etqu have been crystallized and were studied with regard to their structural and electrochemical properties. CuI and CuII complexes of DMEG6buqu and TMG6buqu turned out to be perfectly soluble in pure styrene even at room temperature, which makes them excellent catalysts in the ATRP of apolar monomers. The key characteristics of the ATRP equilibrium, KATRP and kact, were determined for the new complexes. In addition, we used our recently developed DFT methodology, NBO analysis, and isodesmic reactions to predict the influence of the introduced alkyl substituents. It turned out that high conformational freedom in the complex structures leads to a significant uncertainty in prediction of the thermodynamic properties.

T. Biktagirov, W.G. Schmidt, U. Gerstmann, B. Yavkin, S. Orlinskii, P. Baranov, V. Dyakonov, V. Soltamov, Physical Review B (2018), 98(19)

T. Lichtenstein, Z. Mamiyev, C. Braun, S. Sanna, W.G. Schmidt, C. Tegenkamp, H. Pfnür, Physical Review B (2018), 97(16)

A.A. Camberg, F. Bohner, J. Tölle, A. Schneidt, S. Meiners, T. Tröster, IOP Conference Series: Materials Science and Engineering (2018)

C. Ansotegui, M. Sellmann, K. Tierney, in: Principles and Practice of Constraint Programming, Springer International Publishing, 2018, pp. 524-534

We revisit algorithm selection for declarative programming solvers. We introduce two main ideas to improve cost-sensitive hierarchical clustering: First, to augment the portfolio builder with a self-configuration component. And second, we propose that the algorithm selector assesses the confidence level of its own prediction, so that a more defensive recourse action can be used to overturn the original recommendation.

P. Mausbach, A. Köster, J. Vrabec, Phys. Rev. E (2018), 97(5), pp. 052149

R. Driben, V.V. Konotop, B.A. Malomed, T. Meier, A.V. Yulin, Physical Review E (2018), 97(6)

S. Klus, S. Peitz, I. Schuster, in: arXiv:1805.10118, 2018

Kernel transfer operators, which can be regarded as approximations of transfer operators such as the Perron-Frobenius or Koopman operator in reproducing kernel Hilbert spaces, are defined in terms of covariance and cross-covariance operators and have been shown to be closely related to the conditional mean embedding framework developed by the machine learning community. The goal of this paper is to show how the dominant eigenfunctions of these operators in combination with gradient-based optimization techniques can be used to detect long-lived coherent patterns in high-dimensional time-series data. The results will be illustrated using video data and a fluid flow example.

L. Burkhardt, C. Mueller, O.A. Groß, Y. Sun, H. Sitzmann, M. Bauer, Inorganic Chemistry (2018), pp. 6609-6618

A. Afzal, C. Schmitt, S. Alhaddad, Y. Grynko, J. Teich, J. Förstner, F. Hannig, in: Proceedings of the 29th Annual IEEE International Conference on Application-specific Systems, Architectures and Processors (ASAP), 2018, pp. 49-56

In scientific computing, unstructured meshes are a crucial foundation for the simulation of real-world physical phenomena. Compared to regular grids, they allow resembling the computational domain with a much higher accuracy, which in turn leads to more efficient computations.<br />There exists a wealth of supporting libraries and frameworks that aid programmers with the implementation of applications working on such grids, each built on top of existing parallelization technologies. However, many approaches require the programmer to introduce a different programming paradigm into their application or provide different variants of the code. SYCL is a new programming standard providing a remedy to this dilemma by building on standard C ++17 with its so-called single-source approach: Programmers write standard C ++ code and expose parallelism using C++17 keywords. The application is<br />then transformed into a concrete implementation by the SYCL implementation. By encapsulating the OpenCL ecosystem, different SYCL implementations enable not only the programming of CPUs but also of heterogeneous platforms such as GPUs or other devices. For the first time, this paper showcases a SYCL-<br />based solver for the nodal Discontinuous Galerkin method for Maxwell’s equations on unstructured meshes. We compare our solution to a previous C-based implementation with respect to programmability and performance on heterogeneous platforms.<br

Y. Grynko, J. Förstner, in: 2018 IEEE 17th International Conference on Mathematical Methods in Electromagnetic Theory (MMET), IEEE, 2018

S. Johannesmann, T. Brockschmidt, F. Rump, M. Webersen, L. Claes, B. Henning, in: Sensoren und Messsysteme, VDE Verlag GmbH, 2018, pp. 231-234

D. Beermann, M. Dellnitz, S. Peitz, S. Volkwein, in: Reduced-Order Modeling (ROM) for Simulation and Optimization, 2018, pp. 47-72

In this chapter, we combine a global, derivative-free subdivision algorithm for multiobjective optimization problems with a posteriori error estimates for reduced-order models based on Proper Orthogonal Decomposition in order to efficiently solve multiobjective optimization problems governed by partial differential equations. An error bound for a semilinear heat equation is developed in such a way that the errors in the conflicting objectives can be estimated individually. The resulting algorithm constructs a library of locally valid reduced-order models online using a Greedy (worst-first) search. Using this approach, the number of evaluations of the full-order model can be reduced by a factor of more than 1000.

M. Friedrich, W.G. Schmidt, A. Schindlmayr, S. Sanna, Physical Review Materials (2018), 2(1)

A. Zibart, E. Kenig. Numerische Untersuchung der gravitationsgetriebenen Filmströmung auf der welligen Oberfläche von Kissenplattenwärmeübertragern. 2018.

C.W. Nicholson, A. Lücke, W.G. Schmidt, M. Puppin, L. Rettig, R. Ernstorfer, M. Wolf, Science (2018), pp. 821-825

<jats:p>Ultrafast nonequilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photoinduced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a nonequilibrium population of excited electronic states. Using femtosecond photoemission, we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photoholes in shaping the potential energy landscape and enables a combined momentum- and real-space description of PIPTs, including the ultrafast formation of chemical bonds.</jats:p>

L.M. Witschen, T. Wiersema, M. Platzner, in: 4th Workshop On Approximate Computing (WAPCO 2018), 2018

C. Braun, U. Gerstmann, W.G. Schmidt, Physical Review B (2018), 98(12)

T. Biktagirov, W.G. Schmidt, U. Gerstmann, Physical Review B (2018), 97(11)

B. Halbig, M. Liebhaber, U. Bass, J. Geurts, E. Speiser, J. Räthel, S. Chandola, N. Esser, M. Krenz, S. Neufeld, W.G. Schmidt, S. Sanna, Physical Review B (2018), 97(3)

T. Lichtenstein, Z. Mamiyev, C. Braun, S. Sanna, W.G. Schmidt, C. Tegenkamp, H. Pfnür, Physical Review B (2018), 97(16)

P. Müller, K. Karhan, M. Krack, U. Gerstmann, W.G. Schmidt, M. Bauer, T.D. Kühne, Journal of Computational Chemistry (2018), pp. 712-716

M. Lass, S. Mohr, H. Wiebeler, T. Kühne, C. Plessl, in: Proc. Platform for Advanced Scientific Computing (PASC) Conference, ACM, 2018

We present the submatrix method, a highly parallelizable method for the approximate calculation of inverse p-th roots of large sparse symmetric matrices which are required in different scientific applications. Following the idea of Approximate Computing, we allow imprecision in the final result in order to utilize the sparsity of the input matrix and to allow massively parallel execution. For an n x n matrix, the proposed algorithm allows to distribute the calculations over n nodes with only little communication overhead. The result matrix exhibits the same sparsity pattern as the input matrix, allowing for efficient reuse of allocated data structures. We evaluate the algorithm with respect to the error that it introduces into calculated results, as well as its performance and scalability. We demonstrate that the error is relatively limited for well-conditioned matrices and that results are still valuable for error-resilient applications like preconditioning even for ill-conditioned matrices. We discuss the execution time and scaling of the algorithm on a theoretical level and present a distributed implementation of the algorithm using MPI and OpenMP. We demonstrate the scalability of this implementation by running it on a high-performance compute cluster comprised of 1024 CPU cores, showing a speedup of 665x compared to single-threaded execution.

F. Mohr, M.D. Wever, E. Hüllermeier, Machine Learning (2018), pp. 1495-1515

Automated machine learning (AutoML) seeks to automatically select, compose, and parametrize machine learning algorithms, so as to achieve optimal performance on a given task (dataset). Although current approaches to AutoML have already produced impressive results, the field is still far from mature, and new techniques are still being developed. In this paper, we present ML-Plan, a new approach to AutoML based on hierarchical planning. To highlight the potential of this approach, we compare ML-Plan to the state-of-the-art frameworks Auto-WEKA, auto-sklearn, and TPOT. In an extensive series of experiments, we show that ML-Plan is highly competitive and often outperforms existing approaches.

A. Sprenger, S. Hellebrand, in: 2018 IEEE 21st International Symposium on Design and Diagnostics of Electronic Circuits & Systems (DDECS), IEEE, 2018

S. Klus, P. Gelß, S. Peitz, C. Schütte, Nonlinearity (2018), 31(7), pp. 3359-3380

Dynamic mode decomposition (DMD) is a recently developed tool for the analysis of the behavior of complex dynamical systems. In this paper, we will propose an extension of DMD that exploits low-rank tensor decompositions of potentially high-dimensional data sets to compute the corresponding DMD modes and eigenvalues. The goal is to reduce the computational complexity and also the amount of memory required to store the data in order to mitigate the curse of dimensionality. The efficiency of these tensor-based methods will be illustrated with the aid of several different fluid dynamics problems such as the von Kármán vortex street and the simulation of two merging vortices.

A.A. Camberg, T. Tröster, N. Sotirov, J. Tölle, F. Bohner. Investigation of ductility and damage characteristics of EN AW-5182 H18 at non-isothermal forming conditions. 2018.

C. Schmidt, J. Bühler, A. Heinrich, J. Allerbeck, R. Podzimski, D. Berghoff, T. Meier, W.G. Schmidt, C. Reichl, W. Wegscheider, D. Brida, A. Leitenstorfer, Nature Communications (2018), 9, pp. 2890

## 2017

C. Ansotegui, J. Pon, M. Sellmann, K. Tierney, in: AAAI, 2017, pp. 765-772

.S.. Azadi, T.D. Kühne, The Journal of Chemical Physics (2017), 146(8), pp. 084503

X. Ma, O.A. Egorov, S. Schumacher, Physical Review Letters (2017), 118(15)

O. Lafont, S.M.H. Luk, P. Lewandowski, N.H. Kwong, P.T. Leung, E. Galopin, A. Lemaitre, J. Tignon, S. Schumacher, E. Baudin, R. Binder, Applied Physics Letters (2017)

D.D. Konieczna, H. Biller, M. Witte, W.G. Schmidt, A. Neuba, R. Wilhelm, Tetrahedron (2017), pp. 142-149

A. Lücke, U. Gerstmann, T.D. Kühne, W.G. Schmidt, Journal of Computational Chemistry (2017), pp. 2276-2282

H. Aldahhak, M. Paszkiewicz, F. Allegretti, D.A. Duncan, S. Tebi, P.S. Deimel, P. Casado Aguilar, Y. Zhang, A.C. Papageorgiou, R. Koch, J.V. Barth, W.G. Schmidt, S. Müllegger, W. Schöfberger, F. Klappenberger, E. Rauls, U. Gerstmann, The Journal of Physical Chemistry C (2017), 121, pp. 2192-2200

A. Köster, P. Mausbach, J. Vrabec, The Journal of Chemical Physics (2017), 147(14), pp. 144502

A. Yulin, R. Driben, T. Meier, Physical Review A (2017), 96(3)

S. Luk, N. Kwong, P. Lewandowski, S. Schumacher, R. Binder, Physical Review Letters (2017), 119(11)

M. Friedrich, W.G. Schmidt, A. Schindlmayr, S. Sanna, Physical Review Materials (2017), 1(3)

The optical properties of pristine and titanium-doped LiNbO3 are modeled from first principles. The dielectric functions are calculated within time-dependent density-functional theory, and a model long-range contribution is employed for the exchange-correlation kernel in order to account for the electron-hole binding. Our study focuses on the influence of substitutional titanium atoms on lithium sites. We show that an increasing titanium concentration enhances the values of the refractive indices and the reflectivity.

A. Riefer, N. Weber, J. Mund, D.R. Yakovlev, M. Bayer, A. Schindlmayr, C. Meier, W.G. Schmidt, Journal of Physics: Condensed Matter (2017), 29(21)

The electronic band structures of hexagonal ZnO and cubic ZnS, ZnSe, and ZnTe compounds are determined within hybrid-density-functional theory and quasiparticle calculations. It is found that the band-edge energies calculated on the G0W0 (Zn chalcogenides) or GW (ZnO) level of theory agree well with experiment, while fully self-consistent QSGW calculations are required for the correct description of the Zn 3d bands. The quasiparticle band structures are used to calculate the linear response and second-harmonic-generation (SHG) spectra of the Zn–VI compounds. Excitonic effects in the optical absorption are accounted for within the Bethe–Salpeter approach. The calculated spectra are discussed in the context of previous experimental data and present SHG measurements for ZnO.

L. Claes, N. Feldmann, B. Henning, 2017

J.A. Striewe, C. Reuter, K. Sauerland, T. Tröster, Thin-Walled Structures (2017), pp. 501-508

M. Piper, A. Zibart, E. Kenig, International Journal of Thermal Sciences (2017), pp. 459-468

J. Heymann, L. Drude, C. Boeddeker, P. Hanebrink, R. Haeb-Umbach, in: Proc. IEEE Intl. Conf. on Acoustics, Speech and Signal Processing (ICASSP), 2017

This paper presents an end-to-end training approach for a beamformer-supported multi-channel ASR system. A neural network which estimates masks for a statistically optimum beamformer is jointly trained with a network for acoustic modeling. To update its parameters, we propagate the gradients from the acoustic model all the way through feature extraction and the complex valued beamforming operation. Besides avoiding a mismatch between the front-end and the back-end, this approach also eliminates the need for stereo data, i.e., the parallel availability of clean and noisy versions of the signals. Instead, it can be trained with real noisy multichannel data only. Also, relying on the signal statistics for beamforming, the approach makes no assumptions on the configuration of the microphone array. We further observe a performance gain through joint training in terms of word error rate in an evaluation of the system on the CHiME 4 dataset.

R. Fingerhut, W. Chen, A. Schedemann, W. Cordes, J. Rarey, C. Hsieh, J. Vrabec, S. Lin, Industrial & Engineering Chemistry Research (2017), 56(35), pp. 9868-9884

Y.M. Muñoz-Muñoz, C. Hsieh, J. Vrabec, The Journal of Physical Chemistry B (2017), 121(21), pp. 5374-5384

S.M.H. Luk, P. Lewandowski, N.H. Kwong, E. Baudin, O. Lafont, J. Tignon, P.T. Leung, C.K.P. Chan, M. Babilon, S. Schumacher, R. Binder, Journal of the Optical Society of America B (2017), 35(1)

R. Ellerbrock, U. Manthe, Chemical Physics (2017), 482, pp. 106 - 112

Initial state-selected reaction probabilities for the H+CH4→H2+CH3 reaction on a recently developed potential energy surface which employs neutral network fitting based on permutational invariant polynomials are reported. The quantum dynamics calculations use the quantum transition state concept and the multi-layer multi-configurational time-dependent Hartree approach and study the reaction process in full-dimensionality for vanishing total angular momentum. A detailed comparison with previous results obtained on other high-level potential energy surfaces is given. The connection between the level of quantum state resolution and the sensitivity of the results on differences in the potential energy surfaces is highlighted. Employing a decomposition of the total reactivity into contributions of the different vibrational states of the activated complex, it is found that differences between the potential energy surfaces are mainly related to the umbrella motion of the methyl group.

S. Sanna, W.G. Schmidt, Journal of Physics: Condensed Matter (2017)

D. Nozaki, W.G. Schmidt, Journal of Computational Chemistry (2017), 38, pp. 1685-1692

M. Rohrmüller, W.G. Schmidt, U. Gerstmann, Physical Review B (2017), 95(12)

T. Kenter, J. Förstner, C. Plessl, in: Proc. Int. Conf. on Field Programmable Logic and Applications (FPL), IEEE, 2017

Compared to classical HDL designs, generating FPGA with high-level synthesis from an OpenCL specification promises easier exploration of different design alternatives and, through ready-to-use infrastructure and common abstractions for host and memory interfaces, easier portability between different FPGA families. In this work, we evaluate the extent of this promise. To this end, we present a parameterized FDTD implementation for photonic microcavity simulations. Our design can trade-off different forms of parallelism and works for two independent OpenCL-based FPGA design flows. Hence, we can target FPGAs from different vendors and different FPGA families. We describe how we used pre-processor macros to achieve this flexibility and to work around different shortcomings of the current tools. Choosing the right design configurations, we are able to present two extremely competitive solutions for very different FPGA targets, reaching up to 172 GFLOPS sustained performance. With the portability and flexibility demonstrated, code developers not only avoid vendor lock-in, but can even make best use of real trade-offs between different architectures.

S. Peitz, M. Dellnitz, in: NEO 2016, 2017, pp. 159-182

In this article we develop a gradient-based algorithm for the solution of multiobjective optimization problems with uncertainties. To this end, an additional condition is derived for the descent direction in order to account for inaccuracies in the gradients and then incorporated into a subdivision algorithm for the computation of global solutions to multiobjective optimization problems. Convergence to a superset of the Pareto set is proved and an upper bound for the maximal distance to the set of substationary points is given. Besides the applicability to problems with uncertainties, the algorithm is developed with the intention to use it in combination with model order reduction techniques in order to efficiently solve PDE-constrained multiobjective optimization problems.

M. Schappals, A. Mecklenfeld, L. Kröger, V. Botan, A. Köster, S. Stephan, E.J. García, G. Rutkai, G. Raabe, P. Klein, K. Leonhard, C.W. Glass, J. Lenhard, J. Vrabec, H. Hasse, Journal of Chemical Theory and Computation (2017), 13(9), pp. 4270-4280

G. Rutkai, M. Thol, R. Span, J. Vrabec, Molecular Physics (2017), 115(9-12), pp. 1104-1121

R. Podzimski, H.T. Duc, T. Meier, Physical Review B (2017), 96(20)

I. Gruzberg, A. Klümper, W. Nuding, A. Sedrakyan, Phys. Rev. B (2017), 95, pp. 125414

X. Ma, S. Schumacher, Physical Review B (2017), 95(23)

A. Riefer, W.G. Schmidt, Physical Review B (2017), 96(23)

T. Frigge, B. Hafke, T. Witte, B. Krenzer, C. Streubühr, A. Samad Syed, V. Mikšić Trontl, I. Avigo, P. Zhou, M. Ligges, D. von der Linde, U. Bovensiepen, M. Horn-von Hoegen, S. Wippermann, A. Lücke, S. Sanna, U. Gerstmann, W.G. Schmidt, Nature (2017), 544, pp. 207-211

M. Landmann, E. Rauls, W.G. Schmidt, Physical Review B (2017), 95(15)

F. Edler, I. Miccoli, J.P. Stöckmann, H. Pfnür, C. Braun, S. Neufeld, S. Sanna, W.G. Schmidt, C. Tegenkamp, Physical Review B (2017), 95(12)

P. Zimmer, L. Burkhardt, A. Friedrich, J. Steube, A. Neuba, R. Schepper, P. Müller, U. Flörke, M. Huber, S. Lochbrunner, M. Bauer, Inorganic Chemistry (2017), pp. 360-373

C. Reuter, K. Sauerland, T. Tröster, Composite Structures (2017), pp. 33-44

H. Riebler, M. Lass, R. Mittendorf, T. Löcke, C. Plessl, ACM Transactions on Reconfigurable Technology and Systems (TRETS) (2017), 10(3), pp. 24:1-24:23

Branch and bound (B&B) algorithms structure the search space as a tree and eliminate infeasible solutions early by pruning subtrees that cannot lead to a valid or optimal solution. Custom hardware designs significantly accelerate the execution of these algorithms. In this article, we demonstrate a high-performance B&B implementation on FPGAs. First, we identify general elements of B&B algorithms and describe their implementation as a finite state machine. Then, we introduce workers that autonomously cooperate using work stealing to allow parallel execution and full utilization of the target FPGA. Finally, we explore advantages of instance-specific designs that target a specific problem instance to improve performance. We evaluate our concepts by applying them to a branch and bound problem, the reconstruction of corrupted AES keys obtained from cold-boot attacks. The evaluation shows that our work stealing approach is scalable with the available resources and provides speedups proportional to the number of workers. Instance-specific designs allow us to achieve an overall speedup of 47 × compared to the fastest implementation of AES key reconstruction so far. Finally, we demonstrate how instance-specific designs can be generated just-in-time such that the provided speedups outweigh the additional time required for design synthesis.

T. Isenberg, M. Platzner, H. Wehrheim, T. Wiersema, ACM Transactions on Design Automation of Electronic Systems (2017)(4), pp. 61:1--61:23

Proof-carrying hardware (PCH) is a principle for achieving safety for dynamically reconfigurable hardware systems. The producer of a hardware module spends huge effort when creating a proof for a safety policy. The proof is then transferred as a certificate together with the configuration bitstream to the consumer of the hardware module, who can quickly verify the given proof. Previous work utilized SAT solvers and resolution traces to set up a PCH technology and corresponding tool flows. In this article, we present a novel technology for PCH based on inductive invariants. For sequential circuits, our approach is fundamentally stronger than the previous SAT-based one since we avoid the limitations of bounded unrolling. We contrast our technology to existing ones and show that it fits into previously proposed tool flows. We conduct experiments with four categories of benchmark circuits and report consumer and producer runtime and peak memory consumption, as well as the size of the certificates and the distribution of the workload between producer and consumer. Experiments clearly show that our new induction-based technology is superior for sequential circuits, whereas the previous SAT-based technology is the better choice for combinational circuits.

F. Schmidt, M. Landmann, E. Rauls, N. Argiolas, S. Sanna, W.G. Schmidt, A. Schindlmayr, Advances in Materials Science and Engineering (2017), 2017

We perform a comprehensive theoretical study of the structural and electronic properties of potassium niobate (KNbO3) in the cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral phase, based on density-functional theory. The influence of different parametrizations of the exchange-correlation functional on the investigated properties is analyzed in detail, and the results are compared to available experimental data. We argue that the PBEsol and AM05 generalized gradient approximations as well as the RTPSS meta-generalized gradient approximation yield consistently accurate structural data for both the external and internal degrees of freedom and are overall superior to the local-density approximation or other conventional generalized gradient approximations for the structural characterization of KNbO3. Band-structure calculations using a HSE-type hybrid functional further indicate significant near degeneracies of band-edge states in all phases which are expected to be relevant for the optical response of the material.

A. Zibart, E. Kenig. Numerical Investigation of Liquid Falling Film Flow on the Wavy Surface of Vertical Pillow Plates. 2017.

N.H. Kwong, C.Y. Tsang, S.M.H. Luk, Y.C. Tse, C.K.P. Chan, P. Lewandowski, P.T. Leung, S. Schumacher, R. Binder, Physica Scripta (2017)

C. Braun, C. Hogan, S. Chandola, N. Esser, S. Sanna, W.G. Schmidt, Physical Review Materials (2017), 1(5)

M. Witte, M. Rohrmüller, U. Gerstmann, G. Henkel, W.G. Schmidt, S. Herres-Pawlis, Journal of Computational Chemistry (2017), pp. 1752-1761

D. Nozaki, A. Lücke, W.G. Schmidt, The Journal of Physical Chemistry Letters (2017), pp. 727-732

C. Reuter, T. Tröster, Thin-Walled Structures (2017), 117, pp. 1-9

F. Bause, L. Claes, M. Webersen, B. Henning, in: PROCEEDINGS -- AMA Conferences 2017, 2017, pp. 414

L.M. Witschen, Master's thesis, Universität Paderborn, 2017

M. Dellnitz, J. Eckstein, K. Flaßkamp, P. Friedel, C. Horenkamp, U. Köhler, S. Ober-Blöbaum, S. Peitz, S. Tiemeyer, in: Progress in Industrial Mathematics at ECMI 2014 , Springer International Publishing, 2017, pp. 633-641

During the last years, alternative drive technologies, for example electrically powered vehicles (EV), have gained more and more attention, mainly caused by an increasing awareness of the impact of CO2 emissions on climate change and by the limitation of fossil fuels. However, these technologies currently come with new challenges due to limited lithium ion battery storage density and high battery costs which lead to a considerably reduced range in comparison to conventional internal combustion engine powered vehicles. For this reason, it is desirable to increase the vehicle range without enlarging the battery. When the route and the road slope are known in advance, it is possible to vary the vehicles velocity within certain limits in order to reduce the overall drivetrain energy consumption. This may either result in an increased range or, alternatively, in larger energy reserves for comfort functions such as air conditioning. In this presentation, we formulate the challenge of range extension as a multiobjective optimal control problem. We then apply different numerical methods to calculate the so-called Pareto set of optimal compromises for the drivetrain power profile with respect to the two concurrent objectives battery state of charge and mean velocity. In order to numerically solve the optimal control problem by means of a direct method, a time discretization of the drivetrain power profile is necessary. In combination with a vehicle dynamics simulation model, the optimal control problem is transformed into a high dimensional nonlinear optimization problem. For the approximation of the Pareto set, two different optimization algorithms implemented in the software package GAIO are used. The first one yields a global optimal solution by applying a set-oriented subdivision technique to parameter space. By construction, this technique is limited to coarse discretizations of the drivetrain power profile. In contrast, the second technique, which is based on an image space continuation method, is more suitable when the number of parameters is large while the number of objectives is less than five. We compare the solutions of the two algorithms and study the influence of different discretizations on the quality of the solutions. A MATLAB/Simulink model is used to describe the dynamics of an EV. It is based on a drivetrain efficiency map and considers vehicle properties such as rolling friction and air drag, as well as environmental conditions like slope and ambient temperature. The vehicle model takes into account the traction battery too, enabling an exact prediction of the batterys response to power requests of drivetrain and auxiliary loads, including state of charge.

Y. Grynko, J. Förstner, in: Recent Trends in Computational Photonics, Springer International Publishing, 2017, pp. 261-284

We apply the Discontinuous Galerkin Time Domain (DGTD) method for numerical simulations of the second harmonic generation from various metallic nanostructures. A Maxwell–Vlasov hydrodynamic model is used to describe the nonlinear effects in the motion of the excited free electrons in a metal. The results are compared with the corresponding experimental measurements for split-ring resonators and plasmonic gap antennas.

M. Landmann, E. Rauls, W.G. Schmidt, Physical Review B (2017)

D. Müller, S. Guericke, K. Tierney, in: Computational Logistics, Springer International Publishing, 2017, pp. 306-320

Liner carriers change their network on a regular basis, and they are therefore interested in a practical evaluation of the impact these changes have on the cargo flows in their networks. Despite great advancements in the practical applicability of network evaluators in recent years, vessel deployment continues to be considered as an input into the problem, rather than a decision. In this paper, we propose an extension of a state-of-the-art mixed integer programming model for the LSCAP that incorporates the optimization of vessel count and vessel classes for each service. We perform a computational analysis on liner shipping networks of different sizes and compare our optimized results to fixed deployment scenarios. By integrating fleet deployment decisions into the cargo allocation problem, liner carriers can increase the profitability of their networks by at least 2.8 to 16.9{\%} and greatly enhance their decision making.

A. Oppermann, L. Laurini, F. Etscheidt, K. Hollmann, F. Strassl, A. Hoffmann, D. Schurr, R. Dittmeyer, G. Rinke, S. Herres-Pawlis, Chemical Engineering \& Technology (2017), 40(8), pp. 1475-1483

Abstract The reaction of Cu(I) bisguanidine complexes with nitric oxide and the formation of intermediate species were monitored via UV-vis spectroscopy at low temperature, with the occurrence of characteristic absorption bands. The origin of the emerging species and their character were substantiated by electron paramagnetic resonance (EPR) measurements and density functional theory (DFT) studies showing a delocalized {CuNO}11 radical species. Furthermore, this system was transferred to the SuperFocus mixer setup, which allows rapid mixing and the determination of decay constants at ambient temperatures of the thermally sensitive species. However, these experiments demonstrated the limits of these systems, such as the NO saturation in organic solvents and a preferably precise temperature control within the SuperFocus mixer, which should be addressed in the future.

A. Lücke, U. Gerstmann, T.D. Kühne, W.G. Schmidt, Journal of Computational Chemistry (2017), 38(26), pp. 2276-2282

A numerically efficient yet highly accurate implementation of the crystal orbital Hamilton population (COHP) scheme for plane-wave calculations is presented. It is based on the projector-augmented wave (PAW) formalism in combination with norm-conserving pseudopotentials and allows to extract chemical interactions between atoms from band-structure calculations even for large and complex systems. The potential of the present COHP implementation is demonstrated by an in-depth analysis of the intensively investigated metal-insulator transition in atomic-scale indium wires self-assembled on the Si(111) surface. Thereby bond formation between In atoms of adjacent zigzag chains is found to be instrumental for the phase change. © 2017 Wiley Periodicals, Inc.

J. Vollbrecht, C. Wiebeler, S. Schumacher, H. Bock, H. Kitzerow, Molecular Crystals and Liquid Crystals (2017), pp. 66-73

C. Wiebeler, F. Plasser, G.J. Hedley, A. Ruseckas, I.D.W. Samuel, S. Schumacher, The Journal of Physical Chemistry Letters (2017), pp. 1086-1092

S. Tebi, M. Paszkiewicz, H. Aldahhak, F. Allegretti, S. Gonglach, M. Haas, M. Waser, P.S. Deimel, P.C. Aguilar, Y. Zhang, A.C. Papageorgiou, D.A. Duncan, J.V. Barth, W.G. Schmidt, R. Koch, U. Gerstmann, E. Rauls, F. Klappenberger, W. Schöfberger, S. Müllegger, ACS Nano (2017), pp. 3383-3391

G. Rutkai, A. Köster, G. Guevara-Carrion, T. Janzen, M. Schappals, C.W. Glass, M. Bernreuther, A. Wafai, S. Stephan, M. Kohns, S. Reiser, S. Deublein, M. Horsch, H. Hasse, J. Vrabec, Computer Physics Communications (2017), 221, pp. 343-351

R. Driben, V.V. Konotop, T. Meier, A.V. Yulin, Scientific Reports (2017)

N. Bouldi, N.J. Vollmers, C.G. Delpy-Laplanche, Y. Joly, A. Juhin, P. Sainctavit, C. Brouder, M. Calandra, L. Paulatto, F. Mauri, U. Gerstmann, Physical Review B (2017), 96(8), pp. 085123

R. Podzimski, H.T. Duc, T. Meier, in: Ultrafast Phenomena and Nanophotonics XXI, 2017

P. Lewandowski, S.M.H. Luk, C.K.P. Chan, P.T. Leung, N.H. Kwong, R. Binder, S. Schumacher, Optics Express (2017), 25(25)

P. Giannozzi, O. Andreussi, T. Brumme, O. Bunau, M. Buongiorno Nardelli, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, M. Cococcioni, N. Colonna, I. Carnimeo, A. Dal Corso, S. de Gironcoli, P. Delugas, R.A. DiStasio, A. Ferretti, A. Floris, G. Fratesi, G. Fugallo, R. Gebauer, U. Gerstmann, F. Giustino, T. Gorni, J. Jia, M. Kawamura, H. Ko, A. Kokalj, E. Küçükbenli, M. Lazzeri, M. Marsili, N. Marzari, F. Mauri, N.L. Nguyen, H. Nguyen, A. Otero-de-la-Roza, L. Paulatto, S. Poncé, D. Rocca, R. Sabatini, B. Santra, M. Schlipf, A.P. Seitsonen, A. Smogunov, I. Timrov, T. Thonhauser, P. Umari, N. Vast, X. Wu, S. Baroni, Journal of Physics: Condensed Matter (2017), 29(46)

F. Bause, L. Claes, M. Webersen, S. Johannesmann, B. Henning, tm - Technisches Messen (2017), 84(3)

M. Friedrich, W.G. Schmidt, A. Schindlmayr, S. Sanna, Physical Review Materials (2017), 1(5)

The optical properties of congruent lithium niobate are analyzed from first principles. The dielectric function of the material is calculated within time-dependent density-functional theory. The effects of isolated intrinsic defects and defect pairs, including the NbLi4+ antisite and the NbLi4+−NbNb4+ pair, commonly addressed as a bound polaron and bipolaron, respectively, are discussed in detail. In addition, we present further possible realizations of polaronic and bipolaronic systems. The absorption feature around 1.64 eV, ascribed to small bound polarons [O. F. Schirmer et al., J. Phys.: Condens. Matter 21, 123201 (2009)], is nicely reproduced within these models. Among the investigated defects, we find that the presence of bipolarons at bound interstitial-vacancy pairs NbV−VLi can best explain the experimentally observed broad absorption band at 2.5 eV. Our results provide a microscopic model for the observed optical spectra and suggest that, besides NbLi antisites and Nb and Li vacancies, Nb interstitials are also formed in congruent lithium-niobate samples.

S. Peitz, 2017

Multiobjective optimization plays an increasingly important role in modern applications, where several criteria are often of equal importance. The task in multiobjective optimization and multiobjective optimal control is therefore to compute the set of optimal compromises (the Pareto set) between the conflicting objectives. Since – in contrast to the solution of a single objective optimization problem – the Pareto set generally consists of an infinite number of solutions, the computational effort can quickly become challenging. This is even more the case when many problems have to be solved, when the number of objectives is high, or when the objectives are costly to evaluate. Consequently, this thesis is devoted to the identification and exploitation of structure both in the Pareto set and the dynamics of the underlying model as well as to the development of efficient algorithms for solving problems with additional parameters, with a high number of objectives or with PDE-constraints. These three challenges are addressed in three respective parts. In the first part, predictor-corrector methods are extended to entire Pareto sets. When certain smoothness assumptions are satisfied, then the set of parameter dependent Pareto sets possesses additional structure, i.e. it is a manifold. The tangent space can be approximated numerically which yields a direction for the predictor step. In the corrector step, the predicted set converges to the Pareto set at a new parameter value. The resulting algorithm is applied to an example from autonomous driving. In the second part, the hierarchical structure of Pareto sets is investigated. When considering a subset of the objectives, the resulting solution is a subset of the Pareto set of the original problem. Under additional smoothness assumptions, the respective subsets are located on the boundary of the Pareto set of the full problem. This way, the “skeleton” of a Pareto set can be computed and due to the exponential increase in computing time with the number of objectives, the computations of these subsets are significantly faster which is demonstrated using an example from industrial laundries. In the third part, PDE-constrained multiobjective optimal control problems are addressed by reduced order modeling methods. Reduced order models exploit the structure in the system dynamics, for example by describing the dynamics of only the most energetic modes. The model reduction introduces an error in both the function values and their gradients, which has to be taken into account in the development of algorithms. Both scalarization and set-oriented approaches are coupled with reduced order modeling. Convergence results are presented and the numerical benefit is investigated. The algorithms are applied to semi-linear heat flow problems as well as to the Navier-Stokes equations.

## 2016

M. Rüsing, S. Sanna, S. Neufeld, G. Berth, W.G. Schmidt, A. Zrenner, H. Yu, Y. Wang, H. Zhang, Physical Review B (2016)

T. Graf, M. Platzner, in: IEEE Computational Intelligence and Games, 2016

A. Köster, T. Spura, G. Rutkai, J. Kessler, H. Wiebeler, J. Vrabec, T.D. Kühne, Journal of Computational Chemistry (2016), 37(19), pp. 1828-1838

The accuracy of water models derived from ab initio molecular dynamics simulations by means on an improved force-matching scheme is assessed for various thermodynamic, transport, and structural properties. It is found that although the resulting force-matched water models are typically less accurate than fully empirical force fields in predicting thermodynamic properties, they are nevertheless much more accurate than generally appreciated in reproducing the structure of liquid water and in fact superseding most of the commonly used empirical water models. This development demonstrates the feasibility to routinely parametrize computationally efficient yet predictive potential energy functions based on accurate ab initio molecular dynamics simulations for a large variety of different systems. © 2016 Wiley Periodicals, Inc.

S. Amrehn, D. Berghoff, A. Nikitin, M. Reichelt, X. Wu, T. Meier, T. Wagner, Photonics and Nanostructures - Fundamentals and Applications (2016), 19, pp. 55-63

A. Lücke, F. Ortmann, M. Panhans, S. Sanna, E. Rauls, U. Gerstmann, W.G. Schmidt, The Journal of Physical Chemistry B (2016), 120, pp. 5572-5580

E. Jeckelmann, S. Sanna, W.G. Schmidt, E. Speiser, N. Esser, Physical Review B (2016), 93(24)

H.W. Yeom, D.M. Oh, S. Wippermann, W.G. Schmidt, ACS Nano (2016), 10, pp. 810-814

N.H. Kwong, C.Y. Tsang, M.H. Luk, Y.C. Tse, P. Lewandowski, C.K.P. Chan, P.T. Leung, S. Schumacher, R. Binder, Journal of the Optical Society of America B (2016)

J. Hanske, S. Aleksić, M. Ballaschk, M. Jurk, E. Shanina, M. Beerbaum, P. Schmieder, B.G. Keller, C. Rademacher, Journal of the American Chemical Society (2016), pp. 12176-12186

R. Podzimski, H.T. Duc, S. Priyadarshi, C. Schmidt, M. Bieler, T. Meier, in: Ultrafast Phenomena and Nanophotonics XX, 2016

J. Lohrenz, S. Melzer, C. Ruppert, I.A. Akimov, H. Mariette, M. Reichelt, A. Trautmann, T. Meier, M. Betz, Physical Review B (2016), 93(7)

F. Timmer, R. Oelke, C. Dues, S. Sanna, W.G. Schmidt, M. Franz, S. Appelfeller, M. Dähne, J. Wollschläger, Physical Review B (2016), 94(20)

M. Witte, U. Gerstmann, A. Neuba, G. Henkel, W.G. Schmidt, Journal of Computational Chemistry (2016), 37, pp. 1005-1018

N.J. Vollmers, P. Müller, A. Hoffmann, S. Herres-Pawlis, M. Rohrmüller, W.G. Schmidt, U. Gerstmann, M. Bauer, Inorganic Chemistry (2016), pp. 11694-11706

J. Vollbrecht, C. Wiebeler, A. Neuba, H. Bock, S. Schumacher, H. Kitzerow, The Journal of Physical Chemistry C (2016), pp. 7839-7848

A. Konoshonkin, A. Borovoi, N. Kustova, H. Okamoto, H. Ishimoto, Y. Grynko, J. Förstner, Journal of Quantitative Spectroscopy and Radiative Transfer (2016), 195, pp. 132-140

The problem of light scattering by ice crystals of cirrus clouds is considered in the case of a hexagonal ice plate with different distributions over crystal orientations. The physical-optics approximation based on (E, M)-diffraction theory is compared with two exact numerical methods: the finite difference time domain (FDTD) and the discontinuous Galerkin time domain (DGTD) in order to estimate its accuracy and limits of applicability. It is shown that the accuracy of the physical-optics approximation is estimated as 95% for the averaged backscattering Mueller matrix for particles with size parameter more than 120. Furthermore, the simple expression that allows one to estimate the minimal number of particle orientations required for appropriate spatial averaging has been derived.

N.J. Vollmers, P. Müller, A. Hoffmann, S. Herres-Pawlis, M. Rohrmüller, W.G. Schmidt, U. Gerstmann, M. Bauer, Inorganic Chemistry (2016), 55, pp. 11694-11706

I. Miccoli, F. Edler, H. Pfnür, S. Appelfeller, M. Dähne, K. Holtgrewe, S. Sanna, W.G. Schmidt, C. Tegenkamp, Physical Review B (2016)

D. Niemietz, J. Schmutzler, P. Lewandowski, K. Winkler, M. Aßmann, S. Schumacher, S. Brodbeck, M. Kamp, C. Schneider, S. Höfling, M. Bayer, Physical Review B (2016)

M. Lass, T. Kühne, C. Plessl, in: Workshop on Approximate Computing (AC), 2016

L. Claes, T. Meyer, F. Bause, J. Rautenberg, B. Henning, Journal of Sensors and Sensor Systems (2016), 5(1), pp. 187-196

A. Zibart, R. Cherkezova, H.. Figge, E. Kenig. Eine neue Rippengeometrie zur Verbesserung des Wärmeübergangs in flüssigkeitsdurchströmten Kanälen. 2016.

M. Liebhaber, B. Halbig, U. Bass, J. Geurts, S. Neufeld, S. Sanna, W.G. Schmidt, E. Speiser, J. Räthel, S. Chandola, N. Esser, Physical Review B (2016), 94(23)

X. Ma, R. Driben, B.A. Malomed, T. Meier, S. Schumacher, Scientific Reports (2016), 6, pp. 34847

M. Witte, B. Grimm-Lebsanft, A. Goos, S. Binder, M. Rübhausen, M. Bernard, A. Neuba, S. Gorelsky, U. Gerstmann, G. Henkel, W.G. Schmidt, S. Herres-Pawlis, Journal of Computational Chemistry (2016), 37(23-24), pp. 2181-2192

A.V. Yulin, I.Y. Chestnov, X. Ma, S. Schumacher, U. Peschel, O.A. Egorov, Physical Review B (2016)

J. Alberti, H. Linnenbank, S. Linden, Y. Grynko, J. Förstner, Applied Physics B (2016), 122(2), pp. 45-50

We report on second harmonic generation spectroscopy on a series of rectangular arrays of split-ring resonators. Within the sample series, the lattice constants are varied, but the area of the unit cell is kept ﬁxed. The SHG signal intensity of the different arrays upon resonant excitation of the fundamental plasmonic mode strongly depends on the respective arrangement of the split-ring resonators. This ﬁnding can be explained by variations of the electromagnetic interactions between the split-ring resonators in the different arrays. The experimental results are in agreement with numerical calculations based on the discontinuous Galerkin time-domain method. (PDF) The role of electromagnetic interactions.... Available from: https://www.researchgate.net/publication/297612326_The_role_of_electromagnetic_interactions_in_second_harmonic_generation_from_plasmonic_metamaterials [accessed Aug 13 2018].

F. Bause, J. Rautenberg, N. Feldmann, M. Webersen, L. Claes, H. Gravenkamp, B. Henning, Measurement Science and Technology (2016), 27(10)

A. Riefer, M. Friedrich, S. Sanna, U. Gerstmann, A. Schindlmayr, W.G. Schmidt, Physical Review B (2016), 93(7)

The influence of electronic many-body interactions, spin-orbit coupling, and thermal lattice vibrations on the electronic structure of lithium niobate is calculated from first principles. Self-energy calculations in the GW approximation show that the inclusion of self-consistency in the Green function G and the screened Coulomb potential W opens the band gap far stronger than found in previous G0W0 calculations but slightly overestimates its actual value due to the neglect of excitonic effects in W. A realistic frozen-lattice band gap of about 5.9 eV is obtained by combining hybrid density functional theory with the QSGW0 scheme. The renormalization of the band gap due to electron-phonon coupling, derived here using molecular dynamics as well as density functional perturbation theory, reduces this value by about 0.5 eV at room temperature. Spin-orbit coupling does not noticeably modify the fundamental gap but gives rise to a Rashba-like spin texture in the conduction band.

Y. Grynko, Y. Shkuratov, J. Förstner, Optics Letters (2016), 41(15), pp. 3491-3493

We simulate light scattering by random irregular particles that have dimensions much larger than the wavelength of incident light at the size parameter of 𝑋=200 using the discontinuous Galerkin time domain method. A comparison of the DGTD solution for smoothly faceted particles with that obtained with a geometric optics model shows good agreement for the scattering angle curves of intensity and polarization. If a wavelength-scale surface roughness is introduced, diffuse scattering at rough interface results in smooth and featureless curves for all scattering matrix elements which is consistent with the laboratory measurements of real samples.

H.T. Duc, R. Podzimski, S. Priyadarshi, M. Bieler, T. Meier, Physical Review B (2016)

T. Graf, M. Platzner, in: Computer and Games, 2016

P. Partovi-Azar, M. Berg, S. Sanna, T.D. Kühne, International Journal of Quantum Chemistry (2016), 116(15), pp. 1160-1165

Recently, the quantum harmonic oscillator model has been combined with maximally localized Wannier functions to account for long-range dispersion interactions in density functional theory calculations (Silvestrelli, J. Chem. Phys. 2013, 139, 054106). Here, we present a new, improved set of values for the three parameters involved in this scheme. To test the new parameter set we have computed the potential energy curves for various systems, including an isolated Ar2 dimer, two N2 dimers interacting within different configurations, and a water molecule physisorbed on pristine graphene. While the original set of parameters generally overestimates the interaction energies and underestimates the equilibrium distances, the new parameterization substantially improves the agreement with experimental and theoretical reference values. © 2016 Wiley Periodicals, Inc.

K. Tierney, D. Pacino, S. Voß, Flexible Services and Manufacturing Journal (2016), pp. 223-259

E. Speiser, N. Esser, S. Wippermann, W.G. Schmidt, Physical Review B (2016), 94(7)

A. Paulheim, C. Marquardt, H. Aldahhak, E. Rauls, W.G. Schmidt, M. Sokolowski, The Journal of Physical Chemistry C (2016), 10, pp. 11926-11937

S. Sanna, C. Dues, W.G. Schmidt, F. Timmer, J. Wollschläger, M. Franz, S. Appelfeller, M. Dähne, Physical Review B (2016), 93(19)

S. Tebi, H. Aldahhak, G. Serrano, W. Schöfberger, E. Rauls, W.G. Schmidt, R. Koch, S. Müllegger, Nanotechnology (2016), 27

A. Paulheim, C. Marquardt, M. Sokolowski, M. Hochheim, T. Bredow, H. Aldahhak, E. Rauls, W.G. Schmidt, Physical Chemistry Chemical Physics (2016), 18, pp. 32891-32902

P. Lewandowski, O. Lafont, E. Baudin, C.K.P. Chan, P.T. Leung, S.M.H. Luk, E. Galopin, A. Lemaître, J. Bloch, J. Tignon, P. Roussignol, N.H. Kwong, R. Binder, S. Schumacher, Physical Review B (2016)

A.V. Konoshonkin, N.V. Kustova, A.G. Borovoi, Y. Grynko, J. Förstner, Journal of Quantitative Spectroscopy and Radiative Transfer (2016), 182, pp. 12-23

The physical optics approximations are derived from the Maxwell equations. The scattered field equations by Kirchhoff, Stratton-Chu, Kottler and Franz are compared and discussed. It is shown that in the case of faceted particles, these equations reduce to a sum of the diffraction integrals, where every diffraction integral is associated with one plane–parallel optical beam leaving a particle facet. In the far zone, these diffraction integrals correspond to the Fraunhofer diffraction patterns. The paper discusses the E-, M- and (E, M)-diffraction theories as applied to ice crystals of cirrus clouds. The comparison to the exact solution obtained by the discontinuous Galerkin time domain method shows that the Kirchhoff diffraction theory is preferable.

M. Webersen, S. Johannesmann, L. Claes, B. Henning, in: 2016 IEEE IUS~Proceedings, 2016

M. Friedrich, A. Schindlmayr, W.G. Schmidt, S. Sanna, Physica Status Solidi B (2016), 253(4), pp. 683-689

The phonon dispersions of the ferro‐ and paraelectric phase of LiTaO3 are calculated within density‐functional perturbation theory. The longitudinal optical phonon modes are theoretically derived and compared with available experimental data. Our results confirm the recent phonon assignment proposed by Margueron et al. [J. Appl. Phys. 111, 104105 (2012)] on the basis of spectroscopical studies. A comparison with the phonon band structure of the related material LiNbO3 shows minor differences that can be traced to the atomic‐mass difference between Ta and Nb. The presence of phonons with imaginary frequencies for the paraelectric phase suggests that it does not correspond to a minimum energy structure, and is compatible with an order‐disorder type phase transition.

X. Ma, R. Driben, B.A. Malomed, T. Meier, S. Schumacher, Scientific Reports (2016)

## 2015

S. Sanna, C. Dues, W.G. Schmidt, Computational Materials Science (2015), pp. 145-150

S. Müllegger, E. Rauls, U. Gerstmann, S. Tebi, G. Serrano, S. Wiespointner-Baumgarthuber, W.G. Schmidt, R. Koch, Physical Review B (2015), 92(22)

M. Rohrmüller, A. Hoffmann, C. Thierfelder, S. Herres-Pawlis, W.G. Schmidt, Journal of Computational Chemistry (2015), 36(21-22), pp. 1672-1685

M. Landmann, E. Rauls, W.G. Schmidt, M. Neumann, E. Speiser, N. Esser, Physical Review B (2015)

T. Graf, M. Platzner, in: Advances in Computer Games: 14th International Conference, ACG 2015, Leiden, The Netherlands, July 1-3, 2015, Revised Selected Papers, Springer International Publishing, 2015, pp. 1-11

H. Aldahhak, W.G. Schmidt, E. Rauls, Surface Science (2015), pp. 278-281

C. Klein, N.J. Vollmers, U. Gerstmann, P. Zahl, D. Lükermann, G. Jnawali, H. Pfnür, C. Tegenkamp, P. Sutter, W.G. Schmidt, M. Horn-von Hoegen, Physical Review B (2015), 91(19)

M. Landmann, E. Rauls, W.G. Schmidt, M. Neumann, E. Speiser, N. Esser, Physical Review B (2015), 91

A. Zibart, R. Cherkezova, H.. Figge, E. Kenig. Optimierung der Kühlung eines On-Board-Ladegeräts für elektrisch betriebene Kraftfahrzeuge mithilfe der CFD-Methoden. 2015.

H. Aldahhak, E. Rauls, W.G. Schmidt, Surface Science (2015), pp. 260-265

A. Neuba, M. Rohrmüller, R. Hölscher, W.G. Schmidt, G. Henkel, Inorganica Chimica Acta (2015), 430, pp. 225-238

C. Wilfer, P. Liebhäuser, A. Hoffmann, H. Erdmann, O. Grossmann, L. Runtsch, E. Paffenholz, R. Schepper, R. Dick, M. Bauer, M. Dürr, I. Ivanović-Burmazović, S. Herres-Pawlis, Chemistry - A European Journal (2015), pp. 17639-17649

D. Di Nuzzo, C. Fontanesi, R. Jones, S. Allard, I. Dumsch, U. Scherf, E. von Hauff, S. Schumacher, E. Da Como, Nature Communications (2015)

M. Friedrich, A. Riefer, S. Sanna, W.G. Schmidt, A. Schindlmayr, Journal of Physics: Condensed Matter (2015), 27(38)

The vibrational properties of stoichiometric LiNbO3 are analyzed within density-functional perturbation theory in order to obtain the complete phonon dispersion of the material. The phonon density of states of the ferroelectric (paraelectric) phase shows two (one) distinct band gaps separating the high-frequency (~800 cm−1) optical branches from the continuum of acoustic and lower optical phonon states. This result leads to specific heat capacites in close agreement with experimental measurements in the range 0–350 K and a Debye temperature of 574 K. The calculated zero-point renormalization of the electronic Kohn–Sham eigenvalues reveals a strong dependence on the phonon wave vectors, especially near Γ. Integrated over all phonon modes, our results indicate a vibrational correction of the electronic band gap of 0.41 eV at 0 K, which is in excellent agreement with the extrapolated temperature-dependent measurements.

H. Liu, D.F. Heinze, H. Thanh Duc, S. Schumacher, T. Meier, Journal of Physics: Condensed Matter (2015)

C. Braun, S. Sanna, W.G. Schmidt, The Journal of Physical Chemistry C (2015), pp. 9342-9346

Y. Li, W.G. Schmidt, S. Sanna, Physical Review B (2015)

H. Liu, D.F. Heinze, H. Thanh Duc, S. Schumacher, T. Meier, Journal of Physics: Condensed Matter (2015), 27

F. Edler, I. Miccoli, S. Demuth, H. Pfnür, S. Wippermann, A. Lücke, W.G. Schmidt, C. Tegenkamp, Physical Review B (2015), 92(8)

S. Sanna, C. Dues, W.G. Schmidt, Computational Materials Science (2015), 103, pp. 145-150

Y.C. Tse, C.K.P. Chan, M.H. Luk, N.H. Kwong, P.T. Leung, R. Binder, S. Schumacher, New Journal of Physics (2015)

J. Denis, S. Schumacher, G.J. Hedley, A. Ruseckas, P.O. Morawska, Y. Wang, S. Allard, U. Scherf, G.A. Turnbull, I.D.W. Samuel, I. Galbraith, The Journal of Physical Chemistry C (2015), pp. 9734-9744

S. Sanna, S. Neufeld, M. Rüsing, G. Berth, A. Zrenner, W.G. Schmidt, Physical Review B (2015)

W. Nuding, A. Klümper, A. Sedrakyan, Physical Review B (2015), 91, pp. 115107

A. Baghbanpourasl, W.G. Schmidt, M. Denk, C. Cobet, M. Hohage, P. Zeppenfeld, K. Hingerl, Surface Science (2015), 641, pp. 231-236

A. Lücke, W.G. Schmidt, E. Rauls, F. Ortmann, U. Gerstmann, The Journal of Physical Chemistry B (2015), 119, pp. 6481-6491

R. Driben, T. Meier, B.A. Malomed, Scientific Reports (2015), 5, pp. 9420

J. Schmutzler, P. Lewandowski, M. Aßmann, D. Niemietz, S. Schumacher, M. Kamp, C. Schneider, S. Höfling, M. Bayer, Physical Review B (2015)

C. Reuter, T. Tröster, C. Lauter, Verlag Stahleisen GmbH, 2015, pp. 127-132

R. Podzimski, H.T. Duc, T. Meier, in: Ultrafast Phenomena and Nanophotonics XIX, 2015

M. Piper, A. Zibart, J. Tran, E. Kenig, International Journal of Heat and Mass Transfer (2015), pp. 516-527

## 2014

U. Gerstmann, N.J. Vollmers, A. Lücke, M. Babilon, W.G. Schmidt, Physical Review B (2014), 89(16)

C. Wiebeler, S. Schumacher, The Journal of Physical Chemistry A (2014), pp. 7816-7823

J. Vollbrecht, H. Bock, C. Wiebeler, S. Schumacher, H. Kitzerow, Chemistry - A European Journal (2014), pp. 12026-12031

Y. Li, S. Sanna, W.G. Schmidt, The Journal of Chemical Physics (2014)

T. Graf, M. Platzner, in: 2014 IEEE Conference on Computational Intelligence and Games, 2014, pp. 1-8

Q. Guo, A. Paulheim, M. Sokolowski, H. Aldahhak, E. Rauls, W.G. Schmidt, The Journal of Physical Chemistry C (2014), 118, pp. 29911-29918

A. Hoffmann, M. Rohrmüller, A. Jesser, I. dos Santos Vieira, W.G. Schmidt, S. Herres-Pawlis, Journal of Computational Chemistry (2014), 35(29-30), pp. 2146-2161

S. Sanna, R. Hölscher, W.G. Schmidt, Applied Surface Science (2014), pp. 70-78

H. Liu, S. Schumacher, T. Meier, Physical Review B (2014)

R. Driben, T. Meier, Physical Review A (2014)

R. Hölscher, W.G. Schmidt, S. Sanna, The Journal of Physical Chemistry C (2014), pp. 10213-10220

D.M. Oh, S. Wippermann, W.G. Schmidt, H.W. Yeom, Physical Review B (2014), 90(15)

M. Landmann, T. Köhler, E. Rauls, T. Frauenheim, W.G. Schmidt, Journal of Physics: Condensed Matter (2014), 26

S. Sanna, W.G. Schmidt, S. Rode, S. Klassen, A. Kühnle, Physical Review B (2014), 89(7)

P. Lewandowski, V. Ardizzone, Y.C. Tse, N.H. Kwong, M.H. Luk, A. Lücke, M. Abbarchi, J. Bloch, E. Baudin, E. Galopin, A. Lemaître, P.T. Leung, P. Roussignol, R. Binder, J. Tignon, S. Schumacher, in: Ultrafast Phenomena and Nanophotonics XVIII, 2014

H. Riesen, C. Wiebeler, S. Schumacher, The Journal of Physical Chemistry A (2014), pp. 5189-5195

R. Driben, T. Meier, Optics Letters (2014)

## 2013

S. Sanna, A. Riefer, S. Neufeld, W.G. Schmidt, G. Berth, M. Rüsing, A. Widhalm, A. Zrenner, Ferroelectrics (2013), 447, pp. 63-68

R. Tautz, E. Da Como, C. Wiebeler, G. Soavi, I. Dumsch, N. Fröhlich, G. Grancini, S. Allard, U. Scherf, G. Cerullo, S. Schumacher, J. Feldmann, Journal of the American Chemical Society (2013), pp. 4282-4290

A. Riefer, S. Sanna, A. Schindlmayr, W.G. Schmidt, Physical Review B (2013), 87(19)

The frequency-dependent dielectric function and the second-order polarizability tensor of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic structure obtained from density-functional theory. The subsequent application of the GW approximation to account for quasiparticle effects and the solution of the Bethe-Salpeter equation for the stoichiometric material yield a dielectric function that slightly overestimates the absorption onset and the oscillator strength in comparison with experimental measurements. Calculations at the level of the independent-particle approximation indicate that these deficiencies are, at least, partially related to the neglect of intrinsic defects typical for the congruent material. The second-order polarizability calculated within the independent-particle approximation predicts strong nonlinear coefficients for photon energies above 1.5 eV. The comparison with measured data suggests that the inclusion of self-energy effects in the nonlinear optical response leads to a better agreement with experiments. The intrinsic defects of congruent samples reduce the optical nonlinearities, in particular, for the 21 and 31 tensor components, further improving the agreement between experiments and theory.

A. Riefer, S. Sanna, W.G. Schmidt, Ferroelectrics (2013), 447, pp. 78-85

S. Sanna, S. Rode, R. Hölscher, S. Klassen, C. Marutschke, K. Kobayashi, H. Yamada, W.G. Schmidt, A. Kühnle, Physical Review B (2013), 88

M.H. Luk, Y.C. Tse, N.H. Kwong, P.T. Leung, P. Lewandowski, R. Binder, S. Schumacher, Physical Review B (2013)

S. Ling, S. Schumacher, I. Galbraith, M.J. Paterson, The Journal of Physical Chemistry C (2013), pp. 6889-6895

A. Riefer, M. Rohrmüller, M. Landmann, S. Sanna, E. Rauls, N.J. Vollmers, R. Hölscher, M. Witte, Y. Li, U. Gerstmann, A. Schindlmayr, W.G. Schmidt, in: High Performance Computing in Science and Engineering ‘13, Springer, 2013, pp. 93-104

The frequency-dependent dielectric function and the second-order polarizability tensor of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic structure obtained from density-functional theory. The subsequent application of the GW approximation to account for quasiparticle effects and the solution of the Bethe–Salpeter equation yield a dielectric function for the stoichiometric material that slightly overestimates the absorption onset and the oscillator strength in comparison with experimental measurements. Calculations at the level of the independent-particle approximation indicate that these deficiencies are at least partially related to the neglect of intrinsic defects typical for the congruent material. The second-order polarizability calculated within the independent-particle approximation predicts strong nonlinear coefficients for photon energies above 1.5 eV. The comparison with measured data suggests that self-energy effects improve the agreement between experiment and theory. The intrinsic defects of congruent samples reduce the optical nonlinearities, in particular for the 21 and 31 tensor components, further improving the agreement with measured data.

H. Aldahhak, W.G. Schmidt, E. Rauls, Surface Science (2013), 617, pp. 242-248

M. Rohrmüller, S. Herres-Pawlis, M. Witte, W.G. Schmidt, Journal of Computational Chemistry (2013), 34, pp. 1035-1045

H. Liu, S. Schumacher, T. Meier, Physical Review B (2013)

B.M. George, J. Behrends, A. Schnegg, T.F. Schulze, M. Fehr, L. Korte, B. Rech, K. Lips, M. Rohrmüller, E. Rauls, W.G. Schmidt, U. Gerstmann, Physical Review Letters (2013), 110(13)

A. Riefer, S. Sanna, W.G. Schmidt, Ferroelectrics (2013), 447, pp. 78-85

A. Jesser, M. Rohrmüller, W.G. Schmidt, S. Herres-Pawlis, Journal of Computational Chemistry (2013), 35(1-2), pp. 1-17

V. Ardizzone, P. Lewandowski, M.H. Luk, Y.C. Tse, N.H. Kwong, A. Lücke, M. Abbarchi, E. Baudin, E. Galopin, J. Bloch, A. Lemaitre, P.T. Leung, P. Roussignol, R. Binder, J. Tignon, S. Schumacher, Scientific Reports (2013)

## 2012

C. Lauter, M. Frantz, J.P. Kohler, T. Tröster, 2012

A. Riefer, S. Sanna, W.G. Schmidt, Physical Review B (2012), 86(12)

M. Landmann, E. Rauls, W.G. Schmidt, Journal of Physics: Condensed Matter (2012), 24

S. Wall, B. Krenzer, S. Wippermann, S. Sanna, F. Klasing, A. Hanisch-Blicharski, M. Kammler, W.G. Schmidt, M. Horn-von Hoegen, Physical Review Letters (2012), 109(18)

S. Rode, R. Hölscher, S. Sanna, S. Klassen, K. Kobayashi, H. Yamada, W.G. Schmidt, A. Kühnle, Physical Review B (2012), 86(7)

M. Landmann, T. Köhler, S. Köppen, E. Rauls, T. Frauenheim, W.G. Schmidt, Physical Review B (2012), 86(6)

A. Riefer, E. Rauls, W.G. Schmidt, J. Eberhard, I. Stoll, J. Mattay, Physical Review B (2012), 85(16)

R. Hölscher, S. Sanna, W.G. Schmidt, physica status solidi (c) (2012), 9(6), pp. 1361-1365

W.G. Schmidt, S. Wippermann, S. Sanna, M. Babilon, N.J. Vollmers, U. Gerstmann, physica status solidi (b) (2012), 249(2), pp. 343-359

C. Wiebeler, R. Tautz, J. Feldmann, E. von Hauff, E. Da Como, S. Schumacher, The Journal of Physical Chemistry B (2012), pp. 4454-4460

M. Reichelt, A. Walther, T. Meier, Journal of the Optical Society of America B (2012)

S. Linden, F.B.P. Niesler, J. Förstner, Y. Grynko, T. Meier, M. Wegener, Physical Review Letters (2012)

S. Sanna, W.G. Schmidt, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control (2012), 59(9), pp. 1925-1928

C. Reuter, M. Frantz, C. Lauter, H. Block, T. Tröster, 2012

N.A. Montgomery, G.J. Hedley, A. Ruseckas, J. Denis, S. Schumacher, A.L. Kanibolotsky, P.J. Skabara, I. Galbraith, G.A. Turnbull, I.D.W. Samuel, Physical Chemistry Chemical Physics (2012)

C. Schmidt, T. Breuer, S. Wippermann, W.G. Schmidt, G. Witte, The Journal of Physical Chemistry C (2012), 116, pp. 24098-24106

A. Riefer, S. Sanna, A.V. Gavrilenko, W.G. Schmidt, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control (2012), 59(9), pp. 1929-1933

E. Rauls, W.G. Schmidt, T. Pertram, K. Wandelt, Surface Science (2012), 606, pp. 1120-1125

P. Thissen, V. Thissen, S. Wippermann, Y.J. Chabal, G. Grundmeier, W.G. Schmidt, Surface Science (2012), 606, pp. 902-907

B. Gorny, F. Hankeln, C. Lauter, H.C. Schmidt, U. Damerow, R. Mahnken, H.J. Maier, T. Tröster, W. Homberg, 2012

## 2011

S. Müllegger, M. Rashidi, T. Lengauer, E. Rauls, W.G. Schmidt, G. Knör, W. Schöfberger, R. Koch, Physical Review B (2011), 83(16)

W.G. Schmidt, M. Babilon, C. Thierfelder, S. Sanna, S. Wippermann, Physical Review B (2011), 84(11)

C. Mietze, M. Landmann, E. Rauls, H. Machhadani, S. Sakr, M. Tchernycheva, F.H. Julien, W.G. Schmidt, K. Lischka, D.J. As, Physical Review B (2011), 83(19)

S. Sanna, G. Berth, W. Hahn, A. Widhalm, A. Zrenner, W.G. Schmidt, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control (2011), 58(9), pp. 1751-1756

L.S. dos Santos, W.G. Schmidt, E. Rauls, Physical Review B (2011), 84(11)

C. Thierfelder, M. Witte, S. Blankenburg, E. Rauls, W.G. Schmidt, Surface Science (2011), 605, pp. 746-749

S. Müllegger, W. Schöfberger, M. Rashidi, T. Lengauer, F. Klappenberger, K. Diller, K. Kara, J.V. Barth, E. Rauls, W.G. Schmidt, R. Koch, ACS Nano (2011), 5(8), pp. 6480-6486

S. Sanna, C. Thierfelder, S. Wippermann, T.P. Sinha, W.G. Schmidt, Physical Review B (2011), 83(5)

S. Sanna, G. Berth, W. Hahn, A. Widhalm, A. Zrenner, W.G. Schmidt, Ferroelectrics (2011), 419, pp. 1-8

## 2010

E. Rauls, J. Wiebe, W.G. Schmidt, Journal of Crystal Growth (2010), 312, pp. 2892-2895

S. Sanna, A.V. Gavrilenko, W.G. Schmidt, physica status solidi (c) (2010), 7(2), pp. 145-148

K.A. Piegdon, S. Declair, J. Förstner, T. Meier, H. Matthias, M. Urbanski, H. Kitzerow, D. Reuter, A.D. Wieck, A. Lorke, C. Meier, Optics Express (2010), 18(8)

Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4’-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator.

A. Krivosheeva, S. Sanna, W.G. Schmidt, Computational Materials Science (2010), 49(4), pp. 895-898

E. Rauls, S. Blankenburg, W.G. Schmidt, Physical Review B (2010), 81(12)

S. Wippermann, W.G. Schmidt, P. Thissen, G. Grundmeier, physica status solidi (c) (2010), 7(2), pp. 137-140

C. Thierfelder, W.G. Schmidt, Physical Review B (2010), 82(11)

S. Blankenburg, E. Rauls, W.G. Schmidt, The Journal of Physical Chemistry Letters (2010), 1, pp. 3266-3270

S. Sanna, W.G. Schmidt, Physical Review B (2010), 81(21)

S. Sanna, W.G. Schmidt, physica status solidi (c) (2010), 7(7-8), pp. 2272-2274

S. Schumacher, I. Galbraith, A. Ruseckas, G.A. Turnbull, I.D.W. Samuel, Physical Review B (2010)

C. Thierfelder, S. Sanna, A. Schindlmayr, W.G. Schmidt, Physica Status Solidi C (2010), 7(2), pp. 362-365

Given the vast range of lithium niobate (LiNbO3) applications, the knowledge about its electronic and optical properties is surprisingly limited. The direct band gap of 3.7 eV for the ferroelectric phase – frequently cited in the literature – is concluded from optical experiments. Recent theoretical investigations show that the electronic band‐structure and optical properties are very sensitive to quasiparticle and electron‐hole attraction effects, which were included using the GW approximation for the electron self‐energy and the Bethe‐Salpeter equation respectively, both based on a model screening function. The calculated fundamental gap was found to be at least 1 eV larger than the experimental value. To resolve this discrepancy we performed first‐principles GW calculations for lithium niobate using the full‐potential linearized augmented plane‐wave (FLAPW) method. Thereby we use the parameter‐free random phase approximation for a realistic description of the nonlocal and energydependent screening. This leads to a band gap of about 4.7 (4.2) eV for ferro(para)‐electric lithium niobate.

S. Wippermann, W.G. Schmidt, Physical Review Letters (2010), 105(12)

S. Sanna, W.G. Schmidt, Applied Surface Science (2010), 256, pp. 5740-5743

S. Blankenburg, W.G. Schmidt, physica status solidi (c) (2010), 7(2), pp. 415-417

S. Blankenburg, E. Rauls, W.G. Schmidt, physica status solidi (c) (2010), 7(2), pp. 153-156

U. Gerstmann, M. Rohrmüller, F. Mauri, W.G. Schmidt, physica status solidi (c) (2010), 7(2), pp. 157-160

S. Wippermann, W.G. Schmidt, F. Bechstedt, S. Chandola, K. Hinrichs, M. Gensch, N. Esser, K. Fleischer, J.F. McGilp, physica status solidi (c) (2010), 7(2), pp. 133-136

S. Priyadarshi, A.M. Racu, K. Pierz, U. Siegner, M. Bieler, H.T. Duc, J. Förstner, T. Meier, Physical Review Letters (2010), 104(21)

It is demonstrated that valence-band mixing in GaAs quantum wells tremendously modifies electronic transport. A coherent control scheme in which ultrafast currents are optically injected into undoped GaAs quantum wells upon excitation with femtosecond laser pulses is employed. An oscillatory dependence of the injection current amplitude and direction on the excitation photon energy is observed. A microscopic theoretical analysis shows that this current reversal is caused by the coupling of the light- and heavy-hole bands and that the hole currents dominate the overall current response. These surprising consequences of band mixing illuminate fundamental physics as they are unique for experiments which are able to monitor electronic transport resulting from carriers with relatively large momenta.

## 2009

S. Sanna, W.G. Schmidt, T. Frauenheim, U. Gerstmann, Physical Review B (2009), 80(10)

B. Lange, R. Posner, K. Pohl, C. Thierfelder, G. Grundmeier, S. Blankenburg, W.G. Schmidt, Surface Science (2009), 603, pp. 60-64

A. Scholle, S. Greulich-Weber, E. Rauls, W.G. Schmidt, U. Gerstmann, Physica B: Condensed Matter (2009), 404, pp. 4742-4744

S. Chandola, K. Hinrichs, M. Gensch, N. Esser, S. Wippermann, W.G. Schmidt, F. Bechstedt, K. Fleischer, J.F. McGilp, Physical Review Letters (2009), 102(22)

M. Landmann, E. Rauls, W.G. Schmidt, Physical Review B (2009), 79(4)

S. Blankenburg, E. Rauls, W.G. Schmidt, physica status solidi (c) (2009), pp. 153-156

S. Blankenburg, E. Rauls, W.G. Schmidt, physica status solidi (c) (2009), 7(2), pp. 153-156

S. Blankenburg, E. Rauls, W.G. Schmidt, The Journal of Physical Chemistry C (2009), 113, pp. 12653-12657

P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G.L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A.P. Seitsonen, A. Smogunov, P. Umari, R.M. Wentzcovitch, Journal of Physics: Condensed Matter (2009), 21(39)

S. Wippermann, W.G. Schmidt, P. Thissen, G. Grundmeier, physica status solidi (c) (2009), 7(2), pp. 137-140

P. Thissen, G. Grundmeier, S. Wippermann, W.G. Schmidt, Physical Review B (2009), 80(24)

S. Blankenburg, W.G. Schmidt, Journal of Physics: Condensed Matter (2009), 21

S. Sanna, A.V. Gavrilenko, W.G. Schmidt, physica status solidi (c) (2009), pp. 145-148

S. Sanna, A.V. Gavrilenko, W.G. Schmidt, physica status solidi (c) (2009), 7(2), pp. 145-148

N.H. Kwong, S. Schumacher, R. Binder, Physical Review Letters (2009)

## 2008

E. Rauls, S.J. Dijkstra, W.G. Schmidt, Physical Review B (2008), 78(11)

A. Hermann, W.G. Schmidt, P. Schwerdtfeger, Physical Review Letters (2008), 100(20)

W.G. Schmidt, M. Albrecht, S. Wippermann, S. Blankenburg, E. Rauls, F. Fuchs, C. Rödl, J. Furthmüller, A. Hermann, Physical Review B (2008), 77

A. Hermann, P. Schwerdtfeger, W.G. Schmidt, Journal of Physics: Condensed Matter (2008), 20

S. Wippermann, W.G. Schmidt, Physical Review B (2008), 78(23)

S. Blankenburg, W.G. Schmidt, Physical Review B (2008), 78

S. Wippermann, N. Koch, W.G. Schmidt, Physical Review Letters (2008), 100(10)

S. Wippermann, W.G. Schmidt, Surface Science (2008), 603, pp. 247-250

E. Rauls, W.G. Schmidt, Journal of Physical Chemistry C (2008), 112, pp. 11490-11494

E. Rauls, S. Blankenburg, W.G. Schmidt, Surface Science (2008), 602, pp. 2170-2174

B. Lange, W.G. Schmidt, Surface Science (2008), 602, pp. 1207-1211

## 2007

A.A. Stekolnikov, K. Seino, F. Bechstedt, S. Wippermann, W.G. Schmidt, A. Calzolari, M.B. Nardelli, Physical Review Letters (2007), 98(2)

C. Thierfelder, W.G. Schmidt, Physical Review B (2007), 76(19)

S. Blankenburg, W.G. Schmidt, Physical Review Letters (2007), 99(19)

S. Blankenburg, W.G. Schmidt, Nanotechnology (2007), 18

S. Wippermann, W.G. Schmidt, A. Calzolari, M.B. Nardelli, A. Stekolnikov, K. Seino, F. Bechstedt, Surface Science (2007), pp. 4045-4047

## 2006

S. Blankenburg, W.G. Schmidt, Physical Review B (2006), 74(15)

C. Thierfelder, A. Hermann, P. Schwerdtfeger, W.G. Schmidt, Physical Review B (2006), 74(4)

S. Schulz, S. Schumacher, G. Czycholl, Physical Review B (2006)

T. Letzig, F. Willig, P.H. Hahn, W.G. Schmidt, Physical Review B (2006), 74(24)

S. Biering, A. Hermann, W.G. Schmidt, Physical Review B (2006), 73(23)

## 2005

S. Schumacher, G. Czycholl, F. Jahnke, I. Kudyk, L. Wischmeier, I. Rückmann, T. Voss, J. Gutowski, A. Gust, D. Hommel, Physical Review B (2005)

S. Schumacher, G. Czycholl, F. Jahnke, I. Kudyk, H.I. Rückmann, J. Gutowski, A. Gust, G. Alexe, D. Hommel, Physical Review B (2005)

## 2002

W.G. Schmidt, F. Bechstedt, W. Lu, J. Bernholc, Physical Review B (2002), 66, pp. 0855334

## 1999

J. Simon, A. Reinefeld, O. Heinz, in: SCI: Scalable Coherent Interface. Architecture and Software for High-Performance Compute Clusters, Springer, 1999, pp. 367-381