Understanding nonequilibrium dynamics of strongly interacting quantum systems represents one of the most challenging problems in many-body physics. Here, quantum thermalization dynamics are explored in real-time in an ultracold Fermi gas suddenly quenched to the BEC-BCS crossover. When quenched to unitarity, it is observed that the cloud size remains unchanged in the early evolution while the momentum distribution emerges two prethermal states with a lifetime difference of two orders of magnitude in the early and intermediate stage before very slowly evolving to the final stationary state. It is revealed that a crossover momentum, at which the momentum distribution remains nearly unchanged, is determined by the thermal wavelength at high temperatures and the Fermi momentum while at low temperatures. It is identified that the universal prethermal dynamics scaling where momentum distributions with different temperatures collapse onto one curve. When quenched to the BEC side, the thermalization rapidly relaxes into a prethermal state and exhibits the low energy oscillation related to the molecular bound states. This work provides benchmarks for the study of quantum thermalization in strongly interacting fermionic many-body systems.
The coherence and tunability of Feshbach-resonant gasses enable the experimental studies of highly nonequiblirum quantum many-body states, with a particular focus on the dynamics following a quantum Hamiltonian quench Hi to Hf. These have raised numerous fundamental questions on thermalization under unitary time evolution of a closed quantum system. In my dissertation, I studied the dynamics of two specific systems following a quantum quench: 3d Rb85 resonant Bose gas and 1d spin-imbalanced Fermi-Hubbard model. Using self-consistent Bogoliubov theory and Bosonization formalism, I identify rich post quench dynamics, which lead to thermal or prethermal states that depend strongly on initial state properties. My finding helps better understand the intriguing physics of unitary Bose gas as well as the elusive inhomogeneous superconductivity.
We study the nonequilibrium quench dynamics of a mixed Sachdev-Ye-Kitaev model, with competing two-body random interactions leading to maximally chaotic non-Fermi liquid dynamics and a single body term which dominates at low temperatures and leads to Fermi liquid behavior. For different quench protocols, including sudden switching of two-body interaction and double quench protocols, we solve the large
N real-time Dyson equation on the Keldysh contour and compute the dynamics of Green's functions from which we obtain effective temperature and relaxation rates. We show that the model thermalizes to a finite temperature equilibrium and that depending on the value of the quench parameters, the effective temperature can be below or above the Fermi-liquid to non-Fermi liquid crossover scale, which can then be accessed through the nonequilibrium dynamics. We identify quench protocols for which the heating dynamics slow down significantly---an effect that we interpret as a signature of prethermalization.