Ultracold Fermi Gases with Resonant Dipole-Dipole Interaction

Abstract

The superfluid phases in the resonant dipolar Fermi gases are investigated by the standard meanfield theory. In contrast to the crossover from Bose-Einstein condensation (BEC) to Bardeen-CooperSchrieffer (BCS) superfluid in the Fermi gases with the isotropic interactions, the resonant dipolar interaction leads to two completely different BEC phases of the tight-binding Fermi molecules on both sides of the resonance, which are characterized by two order parameters with the distinct internal symmetries. We point that near the resonance, the two competitive phases can coexist, and an emergent relative phase between the two order parameters spontaneously breaks the time-reversal symmetry, which could be observed in the momentum resolved rf-spectroscopy. Introduction. -The unprecedented experimental progresses [1][2][3][4][5] in creating quantum gases of fermions with large dipole moment has stimulated extensive investigations of dipolar Fermi gases. Owing to the long-range and anisotropic nature of the dipole-dipole interaction (DDI), new quantum phenomena emerge in the dipolar Fermi gases, e.g., the ferro-nematic phase [6], the novel static and dynamical properties in the normal phase [7], and the p-wave dominated BCS superfluids induced by the partially attractive DDI [8][9][10][11]. Of particular interest, the recent studies demonstrated that in the two-species dipolar Fermi gases the competition between the shortrange contact interaction and DDI led to the coexistence of singlet-and triplet-paired superfluids [12][13][14].

Leggett mode in a two-component Fermi gas with dipolar interactions

“…We see the non-trivial behavior of the order parameters, depending on the sign of a 02 and the associated two-body bound state [38]. The change of the dominant order parameter implies that the condensate can have two different symmetries and therefore there exists a quantum phase transition, as discussed in the previous work [38]. Near (k F a 02 ) −1 0, the two order parameters become comparable.…”

“…We see in Fig. 3(a) that the dipolar superfluid supports only the phonon mode when the channel coupling a 02 is weak, which becomes damped once it enters the twoparticle continuum as we increase the momentum q/k F , indicating that for this interaction regime the system is BCS like [38,51]. Looking at Fig.…”

“…The momentum dependence in the different interaction regimes is non-trivial due to the mixing of order parameters and angular dependence of the dipolar interaction. This has already been investigated in the previous work, by considering the quasiparticle spectral function [38].…”

“…For the positive scattering length, k F a 02 = 5, in Fig. 2(d) the relative phase of order parameters becomes non-trivial, and the time reversal symmetry has been broken due to the order parameter mixing [38]. In this interaction regime we expect there to be no Leggett mode as the order parameters are not out-of-phase.…”

“…where the dipolar interaction is U (k) = 4πd 2 cos 2 θ k − 1 /3, with d being the dipole moment of the two dipoles polarized along the z-axis and θ k the angle between k and the z-axis. We can write the interaction in the following separable form [38,[46][47][48],…”