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The field of thermodynamics is one of the crown jewels of classical physics. Extending its ideas and concepts to the non-equilibrium setting is a challenging topic of perennial interest. Here, we study perhaps the simplest non-equilibrium class of quantum problems, namely Floquet systems, i.e. systems whose Hamiltonians depend on time periodically, H(t+T) = H(t). For these, there is no energy conservation, and hence not even a natural definition of temperature. We find that it is nonetheless possible to identify three fundamentally distinct thermodynamic ensembles. We also ask if there exists a sharp notion of a phase in such driven, interacting quantum systems. Disorder turns out to play a crucial role, enabling the existence of states which are straightforward analogues of equilibrium states with broken symmetries and topological order, while others - genuinely new to the Floquet problem - are characterized by a combination of order and non-trivial periodic dynamics.

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Roderich Moessner, MPI for Physics of Complex Systems, Dresden
TP seminar room 0.03
Contact: Martin Zirnbauer