|
Résumé |
Isolated systems consisting of many interacting particles are generally assumed
to relax to a stationary state, whose macroscopic properties are described by
the laws of thermodynamics and statistical physics. In this seminar we will
explore whether quantum systems can avoid relaxation and if stationarity itself
can be unstable, meaning that even the slightest disturbance of the thermal
state leads to perpetually changing physical properties. Analogous behaviour
was experimentally observed in driven quantum systems (Floquet time crystals),
in which the response of the system does not follow the period of the driving.
We will show that time translation symmetry breaking is possible even in
perfectly isolated quantum systems without any driving, and provide rigorous
bounds on it in terms of dynamical symmetries. Finally, we will see how time
translation symmetry breaking occurs in the Heisenberg XXZ spin chain, and show
that its properties are a no-where continuous (fractal) function of the system
parameters.
[1] M. Medenjak, B. Buča, and D. Jaksch, Phys. Rev. B 102, 041117(R) (2020).
[2] M. Medenjak, T. Prosen, and L. Zadnik, SciPost Phys. 9, 3 (2020). |
