Condensed Matter Theory

The theory group studies the interplay of spin and charge degrees of freedom in correlated materials and their coherent control in equilibrium and nonequilibrium. We are concerned with the influence of dynamical screening of local moments via the so-called Kondo effect on current-voltage characteristics or with inelastic contributions to scanning tunneling spectroscopy on molecules. The modeling of the emergence of free magnetic degrees of freedom in multi-perturbation problems also belongs to our range of topics, as does the theoretical simulation of the effect of laser pulses on electron and nuclear spins in ensembles of quantum dots, where spin noise leads to the decoherence of a pulse-driven spin dynamics. The latter activities are located in the Collaborative Research Center TRR 160, and are studied on the supercomputers at Forschungszentrum Jülich.

The research group deals with soft matter theory and biological physics. We study statistical physics, mechanics and hydrodynamics of soft and biological matter using analytical and numerical methods. In the soft matter field, the main interests are polymer physics, rheology of elastic materials such as capsules or shells, active self-propelled systems, and microswimmers. We are also actively developing new Monte Carlo simulation techniques (event-chain Monte Carlo) and new applications of machine learning in soft matter physics. In biological physics, cytoskeletal filaments (actin and microtubules) are the main topic. They convert chemical energy (ATP or GTP) during their polymerization to generate forces and motion at the cellular level. We study the physics of single filaments to bundles and networks of many filaments, and from equilibrium fluctuation to chemical drive by polymerization or molecular motors.

Modern technology makes it possible today to produce structures in the size range of a few nanometers, i.e. thousands of these nanostructures fit into the diameter of a hair. For the movement of the particles in these structures, a trillionth of a second, i.e. a picosecond, is already a small eternity. On these extreme scales, completely new ef­fects arise compared to what we know from our everyday life, because quantum physics dominates there. These ef­fects can be studied with laser pulses and even the dynamics in the nanostructures can be con­trolled. Understanding them requires theoretical description and computer-aided simulations, which is the focus of our work. We consider structures made of semiconductor materials, especially so-called quantum dots, as well as metallic structures that act like antennas for visible light. The simulations allow us to predict how to control the dynamics, which is of crucial importance for applications in quantum in­for­mation technology.

The re­search group employs molecular simulation techniques to tackle re­le­vant problems in the field of biophysics and related molecular fields within a multidisciplinary and inter­dis­ci­plin­ary re­search environment (e.g., the Resolv excellence cluster). Particularly, we focus on the development of enhanced sampling and multiscale methods to study phenomena occurring at fluid (e.g., biological lipid membranes) and solid interfaces (e.g., metal interfaces and graphene). An im­por­tant current focuses is on a subject coined "facial recognition of fluid interfaces", i.e., how can (bio)mol­ecules optimally recognize and bind fluid interfaces despite their highly disordered and dynamic nature. To this aim, we develop evolutionary molecular dynamics simulations (evo-MD) methods which couple evolutionary algorithms (sampling of chemical space) to coarse-grained molecular dynamic simulations (sampling of phase-space) to resolve the global optima in chemical space. In essence, evo-MD enables mol­ecules to adapt themselves to optimally recognize fluid interfaces in the course of a simulated evolution. Our re­search may yield unique insights on how biomolecules such as peptides and proteins recognize distinct fluid features in biological lipid membranes with interesting applications for the fields of drug de­sign, surfactant de­sign, and the de­sign of (bio)sensors.

My research group deals with the quantum theory of condensed matter systems. We study solid-state systems in which strong correlations prevail, so that metals become Mott insulators. Understanding their magnetic properties and elementary excitations, as measured by inelastic scattering or absorption, is our goal. Binding phenomena are particularly fascinating. Since recently, we also focus on the topological properties of electronic and magnetic excitations that lead to edge modes and are important for magnonics. Physics far from normal equilibrium is a second core theme where we tackle questions such as the follwing. Can open quantum systems be manipulated so that they acquire special quantum states? How can coherence be maintained which is essential for quantum information processing? Which methods of detection are suitable to detect non-equilibrium states?