# Dresden 2020 – wissenschaftliches Programm

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# TT: Fachverband Tiefe Temperaturen

## TT 48: Poster Session: Many Body Systems, Quantum Critical Phenomena

### TT 48.11: Poster

### Mittwoch, 18. März 2020, 15:00–19:00, P2/4OG

Irreversible real-time dynamics of quantum, classical and quantum-classical spin systems — •Leon Mixa and Michael Potthoff — I. Institute of Theoretical Physics, Department of Physics, Universität Hamburg

Macroscopic irreversibility of the time evolution of interacting many-body systems is a necessary condition for thermalisation. For classical systems, the corresponding loss of information on the initial state is due to non-linear dynamics with exponential sensitivity to small perturbations resulting in chaotic behaviour. A closed quantum system, on the other hand, is described by linear unitary dynamics. Recent ideas for quantum irreversibility conceptually build on the eigenstate-thermalisation-hypothesis and on specific measures such as out-of-time order correlators or observable echos, see e.g. [1].

Here, we consider a one-dimensional nonintegrable quantum-spin model, i.e., the Kondo necklace model with anisotropic (XY) local “Kondo” coupling to local quantum spins, and, in a first step, confirm the results of Ref. [1]. We then study the corresponding model with quantum spins replaced by classical spins and find thermalisation driven by classically chaotic motion. Our main goal is to classify the intermediate situation, namely the quantum-classical hybrid model where only the local spins are treated classically. We systematically study observable echos, Lyapunov exponents and other measures.

[1] M. Schmitt and S. Kehrein, Phys. Rev. B 98, 180301(R) (2018)