Pairing phenomena and superfluidity of atomic Fermi gases in a two-dimensional optical lattice: Unusual effects of lattice-continuum mixing

TL;DR

This paper investigates the superfluid properties of ultracold atomic Fermi gases in a 2D optical lattice, revealing exotic phenomena like reentrant superfluid transition temperatures and pair density wave states due to lattice-continuum mixing.

Contribution

It introduces a theoretical analysis of BCS-BEC crossover in 2D optical lattices, highlighting unusual effects caused by lattice-continuum interplay, including reentrant behavior and nonmonotonic chemical potential dependence.

Findings

Reentrant superfluid transition temperature as a function of interaction strength.

Emergence of pair density wave ground state at intermediate coupling.

Distinct power law behaviors in BEC regime compared to pure 3D and 1D systems.

Abstract

We study the superfluid behavior of ultracold atomic Fermi gases with a short range attractive interaction in a two-dimensional optical lattice (2DOL) using a pairing fluctuation theory, within the context of BCS-BEC crossover. We find that the mixing of lattice and continuum dimensions leads to exotic phenomena. For relatively large lattice constant $d$ and small hopping integral $t$, the superfluid transition temperature $T_c$ exhibits a remarkable reentrant behavior as a function of the interaction strength, and leads to a pair density wave ground state, where $T_c$ vanishes, for a range of intermediate coupling strength. In the unitary and BCS regimes, the nature of the in-plane and overall pairing changes from particle-like to hole-like, with an unexpected nonmonotonic dependence of the chemical potential on the pairing strength. The BEC asymptotic behaviors exhibit distinct power law dependencies on the interaction strength compared to cases of pure 3D lattice, 3D continuum, and 1DOL. These predictions can be tested in future experiments.