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.

arXiv comment14 pages, 10 figures. Corrected 3d phase diagram. References added. Minor changes in text

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Cited by in corpus (27)

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• Rotonlike instability and pattern formation in spinor Bose-Einstein condensates
• Critical velocity of flowing supersolids of dipolar Bose gases in optical lattices
• Instability of superfluid Fermi gases induced by a roton-like density mode in optical lattices
• s-wave Cooper pair insulators and theory of correlated superconductors
• Detecting the superfluid critical momentum of Bose gases in optical lattices through dipole oscillations
• Effects of periodic potentials on the critical velocity of superfluid Fermi gases in the BCS-BEC crossover
• Superfluidity of Dirac Fermions in a Tunable Honeycomb Lattice: Cooper Pairing, Collective Modes, and Critical Currents
• Quenching the Superconducting State of Cuprate Compounds with Electric Currents: A Variational Study
• Dynamical structure factor of a fermionic supersolid on an optical lattice
• Vortex core order and field-driven phase coexistence in the attractive Hubbard model
• Intervortex forces in competing-order superconductors
• Flow-Induced Charge Modulation in Superfluid Atomic Fermions Loaded into an Optical Kagome Lattice
• Excitonic collective modes in Weyl semi-metals
• Higgs modes in two-dimensional spatially-indirect exciton condensates
• Aspects of Superfluid Cold Atomic Gases in Optical Lattices
• Impurity induced phase competition and supersolidity
• A universal pairing gap measurement proposal by dynamical excitations in 2D doped attractive Fermi-Hubbard model with spin-orbit coupling
• Collective excitation and stability of flow-induced gapless Fermi superfluids
• 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
• Boosted one dimensional superfluids on a lattice
• Vortex core ordering in Abrikosov lattices
• Novel dynamical excitations and roton-based measurement of Cooper-pair momentum in a two-dimensional Fulde-Ferrell-Larkin-Ovchinnikov superfluid on optical lattices
• The attractive Hubbard model as an $SO(3)$ system of competing phases: supersolid order and its thermal melting