Competition between the Fulde–Ferrell–Larkin–Ovchinnikov phase and the Bardeen–Cooper– Schrieffer phase in the presence of an optical potential

Abstract

In three-dimensional Fermi gases with spin imbalance, a competition exists between Cooper pairing with zero and finite momentum. The latter gives rise to the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superfluid phase, which only exists in a restricted area of the phase diagram as a function of chemical potential imbalance and interaction strength. Applying an optical potential along one direction enhances the FFLO region in this phase diagram. In this paper, we construct the phase diagram as a function of polarization and interaction strength in order to study the competition between the FFLO phase and the spin balanced Bardeen–Cooper–Schrieffer (BCS) phase. This allows us to take into account the region of phase separation, and provides a more direct connection with experiment. Subsequently, we investigate the effects of the wavelength and the depth of the optical potential, which is applied along one direction, on the FFLO state. It is shown that the FFLO state can exist up to a higher level of spin imbalance if the wavelength of the optical potential becomes smaller. Our results give rise to an interesting effect: the maximal polarization at which the FFLO state can exist decreases when the interaction strength exceeds a certain critical value. This counterintuitive phenomenon is discussed and the connection to the optical potential is explained.