New Trends in Nonequilibrium Many-Body Systems: Methods and Concepts

Workshop Report

Scope and Focus.  NEQMBS22 aimed at bringing together experts working on nonequilibrium physics in quantum-many body systems and materials, with a focus on newly developed theoretical methods and concepts. The workshop covered Quantum Monte Carlo techniques, Green’s function methods, matrix-product state methods, techniques for electron-phonon coupled systems, and Floquet systems. The theoretical talks were complemented by five experimental talks on recent progress in nonequilibrium studies of 2D materials, magnetic systems, 3D Dirac materials, and quantum simulators.

Participants. Due to Covid related restrictions, the participation was limited to at maximum 60 participants. We admitted 56 participants (incl. 3 organizers) from Austria, Colombia, Germany, India, Japan, Switzerland, the US etc., just to name a few. The workshop program featured 41 talks, including a colloquium, and 11 poster presentations. The colloquium given by Monika Aidelsburger, LMU, attracted an additional virtual attendance of 25 persons.
Outstanding participants in the workshop were, on the one hand, the senior experimentalists U. Bovensiepen and R. Shimano as well as our colloquium speaker, M. Aidelsburger, and on the other hand, key theory speakers including M. Eckstein (electon-phonon problems in DMFT), A. Polkovnikov (closed quantum systems), E. Gull (improved techniques for spectral functions), F. Pollmann (constrained dynamics), M. Sentef (light-induced superconductivity), T. Morimoto (nonlinear optical effects) and G. Stefanucci (nonequilibrium Green’s functions). The more junior experimentalists (D. Bossini and M. Reutzel) gave excellent and well-received talks on ultrafast amplification and nonlinear coupling of coherent magnon nodes and excitons in Moire systems, respectively.
The workshop featured a large number of 22 contributed talks with many early-career scientists presenting. The work on a matrix-product states representation of the influence functional and potential applications to quantum impurity problems by Lerose triggered significant discussions. Golez spoke about promising new ideas to compress nonequilibrium Green’s functions. The poster session was well attended and triggered very active discussions. Overall, the decision to hold the workshop in a purely onsite format and to leave enough time for coffee breaks and informal discussions resulted in very active and sustained exchanges among the participants. MPI PKS’s excellent support infrastructure made the participation of two scientists with small children possible and the overall organization was perfect, as always, which is highly appreciated.

Scientific results. The workshop highlighted both impressive progress as well as significant challenges in the theoretical description of quantum many-body systems in the nonequilibrium regime. Pushing accessible time scales forward at manageable computational cost is a main theme and progress was reported for nonequilibrium Green’s functions, DMFT methods, and MPS techniques. Moreover, improved techniques to obtain spectral functions from real-time data as discussed in the workshop may facilitate the link to experiments. We expect that the lively discussions during the workshop have stimulated new ideas and collaborations, so that this meeting at MPIPKS will contribute to the future progress in the field of nonequilibrium many-body systems.