Pairing Phenomena and Superfluidity of Atomic Fermi Gases in a Two-Dimensional Optical Lattice: Unusual Effects of Lattice-Continuum Mixing

The superfluid behavior of ultracold atomic Fermi gases with a short range attractive interaction in a two-dimensional optical lattice (2DOL) is studied using a pairing fluctuation theory, within the context of BCS-BEC crossover. It is found 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 Tc$T_c$ exhibits a remarkable reentrant behavior as a function of the interaction strength, and leads to a pair density wave ground state, where Tc$T_c$ 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 Bose-Einstein condensation 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.