Stability of Superfluid Fermi Gases in Optical Lattices

Abstract

Critical velocities of superfluid Fermi gases in optical lattices are theoretically investigated across the BCS-BEC crossover. We calculate the excitation spectra in the presence of a superfluid flow in one-and two-dimensional optical lattices. It is found that the spectrum of low-lying Anderson-Bogoliubov (AB) mode exhibits a roton-like structure in the short-wavelength region due to the strong charge density wave fluctuations, and with increasing the superfluid velocity one of the roton-like minima reaches zero before the single-particle spectrum does. This means that superfluid Fermi gases in optical lattices are destabilized due to spontaneous emission of the roton-like AB mode instead of due to Cooper pair breaking.

Collective modes and superflow instabilities of strongly correlated Fermi superfluids

“…We observe Landau instabilities of the collective mode at incommensurate wavevectors as has also been reported in Ref. [22]. More importantly, we discover dynamical instabilities involving checkerboard or incommensurate stripe-like density modulations which are distinct from previously studied dynamical instabilities of Bose superfluids.…”

“…The existence of such a roton minimum has been pointed out in earlier work. 20,21,22 We present our results for the values of sound velocity and roton gap as functions of fermion filling and interaction strength. These results of the collective mode spectrum could potentially be verified in studies of collective modes in the superfluid phase of cold Fermi gases in an optical lattice.…”