Abstract: Amorphous silicon nitrides are deposited on crystalline silicon as antireflection and passivating coatings. Up to date detailed knowledge about the interfaces is largely lacking. We have investigated the electronic and structural properties of c-Si/a-Si₃N_xH_y interfaces obtained by large scale ab initio molecular dynamics simulations. Over 500 independent samples have been generated for each considered stoichiometry to perform a reliable defect analysis. While the classes of dominant defect states coincided with previous bulk calculations, we found a considerably increased defect density at the interface. By applying an energy and spatially resolved defect analysis, we observed that most of the defect states originate from the first silicon nitride layer at the interface. Additionally, we examined passivation effects of hydrogen at the interface which play an importantrole to increase the efficiency of modern solar cells.

Abstract: Quantum mechanical embedding methods allow for the theoretical description of problems that cannot be tackled with one single ab-initio method. One prominent example is the interaction of a gas molecule with a metal surface. Density functional theory is perfectly suited to describe extended metal surfaces, but fails to correctly account for subtle details of the molecule-surface interaction, e.g., charge transfer processes. More expensive correlated wavefunction methods, in turn, can very accurately describe the molecule itself, but become prohibitively expensive even for moderately-sized surface clusters. Embedding methods combine the advantages of different methods: a system of interest is partitioned into different parts (e.g. the approaching molecule interacting with a small surface cluster, and the remainder of the surface), which are each described by a suitable method. Their interaction is mediated by a global embedding potential. We present different approaches to determine this embedding potential, and discuss two applications: the dissociation of oxygen molecules by an aluminum surface, and the hot-electron mediated dissociation of hydrogen on a gold surface. For both cases we compare to experimental results.

Abstract: Many methods in correlated electronic structure calculation estimate the dominant set of interaction processes among all possible ones to improve precision while maintaining computational efficiency. We approximate the second order exchange (SOEX) contribution to the polarizability of materials by the respective SOEX contribution in the uniform electron gas. This can be computed much faster while we still believe to capture the dominant part of this contribution for many materials. Monte Carlo integration with importance sampling and symmetrization considerations is used to integrate the divergent kernels of the SOEX contribution to the polarizability numerically as a function of momentum and frequency. Correlated electronic structure calculation methods such as the Random Phase Approximation (RPA) can then be amended using this polarizability.

Abstract: One of the most fundamental processes in the interaction of light with matter is the photoelectric effect, i.e. the emission of an electron after the absorption of a photon. With the advent of ultrashort and well-controlled laser pulses photoemission can now be studied with unprecedented temporal resolution. The challenge lies in interpreting the timing information obtained by attosecond pump-probe experiments. One fundamental question is whether an electron is emitted instantaneously upon absorption of a photon or with a certain time shift. In this talk, we will discuss how such a time delay can be defined quantum mechanically and classically. For an accurate theoretical description we numerically solve the time-dependent Schrödinger equation for one- and two-electron atoms and can show that, as long as all measurement-induced distortions can be quantified, the intrinsic (Eisenbud-Wigner-Smith) time delay of photoemission becomes accessible by attosecond pump-probe setups.

Abstract: An important aspect of computational condensed matter physics is the computation of phase diagrams. The thermodynamically stable phase is the one with the lowest Gibbs free energy. "Interface pinning" is a method where the Gibbs free energy differences between phases is computed directly in a single equilibrium simulation. This is done by applying an artificial external field that biases the system towards two-phase configurations. The Gibbs free energy difference is then determined from the average force that the field exert on the system. In addition, the method gives information about the interface between the phases.

Published in J. Chem. Phys. 139, 104102 (20123)

Abstract: One of the main problem studied these days in the field of mathematical general relativity is a proof of the stability of the slowly rotating black holes, known as Kerr black holes. The understanding of this stability mostly relies on establishing good decay/scattering properties of higher spin fields (such as Maxwell fields) evolving on the Kerr background. After a short presentation on the meaning of the stability of the Einstein equation in the context of the Cauchy problem, the decay properties of the most simple toy model, the scalar wave equation (spin-0 field) on the flat background, will be described and the obstructions to the generalization of these scattering properties of solutions of the scalar wave on black holes will be explained. Finally, a reduction process of higher spin fields to the scalar wave equation, introduced by Penrose, will be given. 

Abstract: Using the Chern-Simons formulation of gravity in three dimensions we study the asymptotic analysis of 3D pure/conformal gravity with Minkowskian (flat) and AdS boundary conditions.

Abstract: In the 1950's, when computational methods were established as new way to study and understand physical phenomena, hard spheres were among the first systems studied. As early as 1957 Alder and Wainwright  showed that hard spheres exhibit a first order ordering phase transition. Now, 50 years later, hard spheres and their experimental realizations, colloidal particles with nearly hard interactions are still a very attractive object of research. Nowadays, new real-space imagining techniques enable the experimental study of nucleation of hard colloidal spheres  and, by that, testing theoretical findings and numerical results. Besides hard colloidal spheres, experimentalists can now,  due to the advent of new fabrication techniques, produce colloidal particles with various shapes or interactions.

Besides giving an introduction to the soft matter physics in general, my talk discusses new theoretical and experimental results for the freezing and self assembly of anisotropic hard particles.

Abstract: In statistical mechanics, many quantities of interest, like a system's energy as a function of temperature, are given as a high dimensional integral in the configuration space of the system which cannot be evaluated directly. However, it is possible to approximate this integral by a sequence of random samples, distributed according to the underlying probability distribution, e. g. the Boltzmann distribution in the case of a system with fixed temperature, volume and particle number. One way to generate such a sequence is the Metropolis algorithm. In my talk, I will present the theoretical background of this algorithm and give a few examples of its applications in the field of soft and condensed matter simulations. This applications range from comparatively easy systems like the simulation of a Lennard-Jones fluid to more complex cases like the sampling of typical folding pathways in a model protein

Abstract: Vector-like quarks are currently studied in increasing detail at the LHC, due to their importance in numerous models of New Physics. The current experimental null-results for their on-shell production have pushed the lower bounds on their masses near a TeV, however valid only under certain assumptions. Embedding vector-like quarks into models addressing the electroweak hierarchy problem by treating the Higgs as a pseudo-goldstone boson of a global symmetry changes current conclusions from flavour and electroweak precision tests and can have interesting implications for direct searches. We have studied the flavour phenomenology of such new states and calculated the implications for the electroweak hierarchy problem in light of recent Higgs data.
Abstract: 

Abstract: Bulk electronic correlated systems are often well described by dynam- ical mean field theory (DMFT)[1]. Amongst several successes, DMFT is able to properly describe the Mott-Hubbard metal-to-insulator transition (MIT), which is an intrinsic non-perturbative phenomenon. In this talk, after a short introduction to DMFT and the MIT at the one-particle-level, the notion of two-particle vertex functions and their necessity for exten- sions of DMFT are discussed[2]. Furthermore, within this two-particle level framework, hallmarks of the MIT are already identified well inside the metallic phase in terms of divergences of the (frequency resolved) ir- reducible vertex [2,3]. Specifically, the strong enhancements and the sign changes of the irreducible vertex functions, which mark this precursor of the MIT, stem from enhanced local scattering processes and can be traced in the high temperature regime up to the atomic limit.

[1] A. Georges et al., Rev. Mod. Phys. 68, 13-125 (1996)
[2] G. Rohringer, A. Valli and A. Toschi, Phys. Rev. B 86, 125114 (2012)
[3] T. Schaefer et al., Phys. Rev. Lett. 110 246405 (2013)

Abstract: I will argue that classical strings, both bosonic and supersymmetric, can have finite energy and momentum at their endpoints. I will also show that, in a general curved background, the string endpoints must propagate along null geodesics as long as their energy remains finite. Finite endpoint momentum allows strings with a fixed energy to travel a greater distance in an AdS5-Schwarzschild background than has been possible for classical solutions considered previously. I will also review the relevance to heavy ion phenomenology of the dependence of this distance on energy and propose a scheme for determining the instantaneous rate of energy loss.

Abstract: Modern physical theories provide explanations for processes in both the small dimensions of the quantum world and on cosmologically large scales, but unfortunately a unifying theory valid on all scales does still not exist. In this seminar, we will - in the framework of existing theories - introduce scenarios where quantum effects can however still have consequences on astrophysical or cosmological scales.
In one of these examples we will develop a model for dark energy, i.e. the cause of the accelerated expansion of the universe, by calculating the vacuum fluctuations of quantum fields. Via the cancellation of the opposite-sign contributions of bosons and fermions the vacuum energy can explain the observed expansion behavior of the universe.
Experimentally, the magnitude of the cosmic acceleration can be obtained through the investigation of observational data like the luminosity of supernova events in the universe. Through the technique of cosmography, it is possible to extract kinematical constraints on the cosmic acceleration and therefore on the specific properties of its origin, without assuming a particular model of cosmology to be valid a priori. We confirm the validity of the vacuum energy of quantum fields as a possible candidate to explain the behaviour of the cosmic expansion.
Ultimately, we turn our attention to a quantum phenomenon in astrophysics, i.e. the occurrence of a Bose-Einstein condensed phase of the matter within compact objects such as neutron stars and white dwarfs. Conditions in these environments allow for the formation of Bose-Einstein condensates due to a favourable combination of temperature and density, and thus it is of interest to study the condensation of bosonic particles under the influence of gravity in the framework of a Hartree-Fock theory.Abstract: 

Abstract: Nonlinear evolution equations play a pivotal role in many areas of physics ranging from fluid dynamics, plasma physics, nonlinear optics, quantum field theory, to approximative descriptions of the many-body quantum problem such as the Gross-Pitaevskii equation, the time-dependent Hartree Fock equations, or time-dependent density functional theory. In some cases nonlinear evolution equations are exactly solvable via the inverse scattering transform - a fact closely related to the existence of solitons. In this talk a short introduction to the inverse scattering transform will be given. As an example we will discuss the Korteweg-de Vries equation which describes propagation of water waves.

Abstract: Glueballs, bound states of gluonic degrees of freedom, are a prediction of quantum chromodynamics. Due to lack of theoretical input, they have not yet been established experimentally. We discuss how this problem can be solved within the framework of the AdS/CFT correspondence, a holographic duality between a strongly coupled gauge theory in four dimensions and a string theory in a five-dimensional Anti-de Sitter geometry. We explain how in a particular realization of holographic QCD, the Sakai-Sugimoto model, one can describe glueball interactions and explicitly calculate decay rates relevant for theexperimental search.