# Prof. Dr. Matthias Krüger

## Non-equilibrium Statistical Physics

(Quantum-)thermal fluctuations are a fundamental part of our world, as they influence our daily observed surroundings in many ways. In equilibrium, such fluctuations are well understood in the framework of statistical physics and thermodynamics. In non-equilibrium, however, many of the helpful tools and theorems, employed for equilibrium systems, are not applicable. This makes non-equilibrium on the one hand harder to understand and treat theoretically, but on the other hand often richer in phenomenology and possibilities. Our group aims at improving both the theoretical description and understanding of non-equilibrium fluctuations, mostly in the following fields.In quantum electrodynamics, we investigate effects which are mediated by the fluctuating electromagnetic field, namely radiative heat transfer and Casimir interactions. We try to give concrete predictions in interesting scenarios, but also aim at further developing the principle concepts and assumptions involved when describing such non-equilibrium systems.

In soft matter we investigate properties of non-equilibrium fluids or suspensions. We thereby employ and advance microscopic theories like density functional theory or stochastic field theoretic approaches, which act on a coarse grained level. One goal is to understand in how far important properties of non-equilibrium fluids can be obtained by coarse graining, i.e., which of the degrees of freedom can be integrated out.

Last, we are also interested in nonlinear response theory, which tries to provide fundamental relations and properties of far from equilibrium systems. One aim is at connecting nonlinear response functions to the fluctuations of the unperturbed system, which are often easier to measure or compute.