A. Posazhennikova, M. Trujillo-Martinez, and J. Kroha:
Annalen der Physik, 530, 1700124 (2017)
đ arXiv:1705.01754 (2017)
If and how an isolated quantum system thermalizes despite its unitary time evolution is a longâstanding, open problem of manyâbody physics. The eigenstate thermalization hypothesis (ETH) postulates that thermalization happens at the level of individual eigenstates of a system's Hamiltonian. However, the ETH requires stringent conditions to be validated, and it does not address how the thermal state is reached dynamically from an initial nonâequilibrium state. We consider a BoseâEinstein condensate (BEC) trapped in a doubleâwell potential with an initial population imbalance. We find that the system thermalizes although the initial conditions violate the ETH requirements. We identify three dynamical regimes. After an initial regime of undamped Josephson oscillations, the subsystem of incoherent excitations or quasiparticles (QP) becomes strongly coupled to the BEC subsystem by means of a dynamically generated, parametric resonance. When the energy stored in the QP system reaches its maximum, the number of QPs becomes effectively constant, and the system enters a quasiâhydrodynamic regime where the two subsystems are weakly coupled. In this final regime the BEC acts as a grandâcanonical heat reservoir for the QP system (and vice versa), resulting in thermalization. We term this mechanism dynamical bath generation (DBG).