Multiband superfluidity and superfluid to band-insulator transition of strongly interacting fermions in an optical lattice

TL;DR

This paper investigates the superfluid to band insulator transition in strongly interacting fermionic atoms in optical lattices, proposing a simplified intraband pairing model that aligns with experimental observations and predicts new features.

Contribution

It introduces a mean field approach based on intraband pairing for cold atom superfluids, providing insights into the SF-BI transition and unique properties of cold Fermi gases.

Findings

Accurately predicts critical lattice depth for SF-BI transition

Describes the condensate density modulation and quasiparticle gap dependence

Highlights unique features like uniform pair field and vanishing Hartree field

Abstract

We study the multiband superfluid and the superfluid (SF) to band insulator (BI) transition of strongly interacting fermionic atoms in an optical lattice at a filling of two fermions per well. We present physical arguments to show that a consistent mean field description of this problem is obtained by retaining only intraband pairing between the fermions. Using this approach we obtain a reasonable account of the experimentally observed critical lattice depth for the SF-BI transition and the modulated components of the condensate density, and make predictions for the lattice depth dependence of the quasiparticle gap which can be tested in future experiments. We also highlight some interesting features unique to cold atom superfluids within this intraband pairing approximation; for instance, the pair field is forced to be uniform in space and the Hartree field vanishes identically. These arise as a result of the fact that while the pairing interaction is cut off at the scale of the Debye frequency in conventional superconductors, or at the lattice scale in tight binding model Hamiltonians, such a cutoff is absent for cold Fermi gases.