Collective modes and superflow instabilities of strongly correlated Fermi superfluids

Abstract

We study the superfluid phase of the one-band attractive Hubbard model of fermions as a prototype of a strongly correlated s-wave fermion superfluid on a lattice. We show that the collective mode spectrum of this superfluid exhibits, in addition to the long wavelength sound mode, a sharp roton mode over a wide range of densities and interaction strengths. We compute the sound velocity and the roton gap within a generalized random phase approximation (GRPA) and show that the GRPA results are in good agreement, at strong coupling, with a spin wave analysis of the appropriate strong-coupling pseudospin model. We also investigate, using this two-pronged approach, the breakdown of superfluidity in the presence of a supercurrent. We find that the superflow can break down at a critical flow momentum via several distinct mechanisms - depairing, Landau instabilities or dynamical instabilities - depending on the dimensionality, the interaction strength and the fermion density. The most interesting of these instabilities is a charge modulation dynamical instability which is distinct from previously studied dynamical instabilities of Bose superfluids. The charge order associated with this instability can be of two types: (i) a commensurate checkerboard modulation driven by softening of the roton mode at the Brillouin zone corner, or, (ii) an incommensurate density modulation arising from superflow-induced finite momentum pairing of Bogoliubov quasiparticles. We elucidate the dynamical phase diagram showing the critical flow momentum of the leading instability over a wide range of fermion densities and interaction strengths and point out implications of our results for experiments on cold atom fermion superfluids in an optical lattice.

14 pages, 10 figures. Corrected 3d phase diagram. References added. Minor changes in text

References in corpus (9)

• Many-Body Physics with Ultracold Gases
• Quantum Fluctuations in the Superfluid State of the BCS-BEC Crossover
• Critical velocity for superfluid flow across the BEC-BCS crossover
• BCS-BEC crossover on the two-dimensional honeycomb lattice
• Phase diagram for a Bose-Einstein condensate moving in an optical lattice
• Decay of a superfluid currents in a moving system of strongly interacting bosons
• Superfluid-insulator transition in a moving system of interacting bosons
• Landau damping: instability mechanism of superfluid Bose gases moving in optical lattices
• Stability of Superflow for Ultracold Fermions in Optical Lattices

Cited by in corpus (9)

• Critical velocity of superfluid flow through single barrier and periodic potentials
• Effects of periodic potentials on the critical velocity of superfluid Fermi gases in the BCS-BEC crossover
• Flow-Induced Charge Modulation in Superfluid Atomic Fermions Loaded into an Optical Kagome Lattice
• Impurity induced phase competition and supersolidity
• Collective excitation and stability of flow-induced gapless Fermi superfluids
• A universal pairing gap measurement proposal by dynamical excitations in 2D doped attractive Fermi-Hubbard model with spin-orbit coupling
• Spectrum of density, spin and pairing fluctuations of an attractive two-dimensional Fermi gas
• Dynamical structure factor and a new method to measure the pairing gap in two-dimensional attractive Fermi-Hubbard model
• Novel dynamical excitations and roton-based measurement of Cooper-pair momentum in a two-dimensional Fulde-Ferrell-Larkin-Ovchinnikov superfluid on optical lattices