My research interest is focused on the collective behavior in quantum many-body systems out of equilibrium. This is a broad field of research and includes several branches of the modern physics from the strongly correlated solid-state systems, nano-scale system and the quantum optics to the basic questions of quantum statistical physics.
My research is motivated by an increased experimental ability to manipulate and probe quantum materials out of equilibrium. On one side, my aim is to bring a more general description of the basic principles governing the non-equilibrium dynamics of correlated system and on the other hand provide useful (numerical) tools for a realistic description of the experimental setups. Not so surprisingly, these two aims are complementary.
Strongly-correlated system out of equilibrium
The dynamics of strongly-correlated quantum systems out of equilibrium is a fascinating and a rather nonintuitive field of research. I will not try to give an overview on it, but rather show an example how powerful can techniques based on many-body Greens function be. The attached movie illustrates the possibility to obtain the electronic structure out of equilibrium using the time and angular-resolved photo-emission spectroscopy (tr-ARPES) in a correlated material. The system under study is an excitonic insulator, which is macroscopically quantum coherent state of matter, excited with an electric field pulse E(t). The ARPES spectrum is recorded with a probe pulse at different delays with respect to the pump (upper panel). A careful analysis of the spectrum reveals multiple dynamical processes: A filling of the conduction bands is followed by a broadening of the bands due to a change of the quasiparticle lifetime, internal relaxation, a closing of the gap (insulator-to-metal transition) due to a non-thermal melting of the exciton condensate, and thermalization. The real advantage of this numerical procedure is that these processes can be directly compared with experiments on the candidate materials.
Based on article: Denis Golež, Philipp Werner, and Martin Eckstein, Photo-induced gap closure in an excitonic insulator. Phys. Rev. B 94, 035121 (2016).
Nonequilibrium Greens function library
I’ve developed a broad range of theoretical methods ranging from the time-dependent exact diagonalization to the nonequilibrium field theory (Keldysh-Schwinger formalism). The advances on realistic description of the nonequilibrium quantum materials are mostly based on the latter techniques and I am an active developer of the open-source numerical library called ”NESSi” which allows the manipulation of nonequilibrium Green’s functions defined on the Kadanoff-Baym contour. These methodological development has opened two main directions in my research: the ability to simulate realistic materials under consideration and extraction of the minimal theoretical mechanisms involved.