Stability of Superflow for Ultracold Fermions in Optical Lattices

TL;DR

This paper studies the stability of superfluid flow in ultracold fermions within optical lattices, revealing different dynamical instabilities and a transition between depairing and density wave instabilities depending on filling and interaction strength.

Contribution

It uncovers the conditions under which superfluid flow becomes unstable, identifying a transition between depairing and density wave instabilities in fermionic optical lattices.

Findings

Superflow breaks down via dynamical instability near one fermion per site.

At lower fillings, superfluid stiffness becomes negative, indicating instability.

A transition from fermion depairing to density wave instability occurs with increasing pairing interaction.

Abstract

Motivated by recent observations of superfluidity of ultracold fermions in optical lattices, we investigate the stability of superfluid flow of paired fermions in the lowest band of a strong optical lattice. For fillings close to one fermion per site, we show that superflow breaks down via a dynamical instability leading to a transient density wave. At lower fillings, there is a distinct dynamical instability, where the superfluid stiffness becomes negative; this evolves, with increasing pairing interaction, from the fermion pair breaking instability to the well-known dynamical instability of lattice bosons. Our most interesting finding is the existence of a transition, over a range of fillings close to one fermion per site, from the fermion depairing instability to the density wave instability as the strength of the pairing interaction is increased.