Non-equilibrium dynamics and atom-pair coherence in strongly interacting Bose-Fermi mixtures

TL;DR

This paper develops a cumulant expansion approach to study non-equilibrium dynamics in strongly interacting Bose-Fermi mixtures, capturing non-Gaussian correlations and analyzing atom-pair coherence, condensate depletion, and molecule formation.

Contribution

It introduces a new dynamical theory using cumulant expansion to include non-Gaussian correlations in Bose-Fermi mixtures near Feshbach resonances.

Findings

Atom-pair coherence is enhanced by Fermi sea and BEC effects.

Depletion of BEC follows quadratic scaling at early times.

Population dynamics are characterized by a timescale related to mixture parameters.

Abstract

Theoretical treatments of non-equilibrium dynamics in strongly interacting Bose-Fermi mixtures are complicated by the inherent non-Gaussian nature of the vacuum two-body physics, invalidating the typical Hartree-Fock-Bogoliubov approximation. Here, we apply the cumulant expansion to study non-equilibrium Bose-Fermi mixtures, which allows us to explicitly include the missing non-Gaussian quantum correlations, leading to a consistent dynamical theory of a Bose-Fermi mixture near an interspecies Feshbach resonance. We first apply our theory to a study of atom-pair coherence in the gas, which is significantly enhanced by the competing influences of the Fermi sea and Bose-Einstein condensation, in agreement with analytical calculations. Then, we study the depletion of a degenerate Bose-Fermi mixture following a quench to the unitary regime, characterizing the resulting depletion of the Bose-Einstein condensate, the deformation of the Fermi surface, and the production of molecules. We find that at early times, the population dynamics scale quadratically with the hold time, and define an associated characteristic timescale set by the parameters of the mixture and the width of the Feshbach resonance.