Enhancement effect of mass imbalance on Fulde-Ferrell-Larkin-Ovchinnikov type of pairing in Fermi-Fermi mixtures of ultracold quantum gases

Abstract

Ultracold two-component Fermi gases with a tunable population imbalance have provided an excellent opportunity for studying the exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, which have been of great interest in condensed matter physics. However, the FFLO states have not been observed experimentally in Fermi gases in three dimensions (3D), possibly due to their small phase space volume and extremely low temperature required for an equal-mass Fermi gas. Here we explore possible effects of mass imbalance, mainly in a ⁶Li-⁴⁰K mixture, on the one-plane-wave FFLO phases for a 3D homogeneous case at the mean-field level. We present various phase diagrams related to the FFLO states at both zero and finite temperatures, throughout the BCS-BEC crossover, and show that a large mass ratio may enhance substantially FFLO type of pairing.

Figure 1. T–p phase diagram of a homogeneous ⁶Li–⁴⁰K mixture at unitarity.

Here “PG” and “PS” indicate pseudogapped normal state and phase separation, respectively, and LP labels a Lifshitz point. An FFLO superfluid (gray shaded) phase exists in the high p regime when ⁴⁰K dominates, while it becomes unstable in the dotted region. A Sarma superfluid lives in the intermediate T and low p regime (brown shaded region).

Figure 2. T–p phase diagram of a homogeneous ⁶Li–⁴⁰K mixture at 1/k_Fa = −1, similar to Fig. 1.

Here the FFLO phase (gray shaded regions) exists for both p > 0 and p < 0.

Figure 3. Typical behavior of the order parameter amplitude Δ in the FFLO phase as a function of T, for a homogeneous ⁶Li–⁴⁰K mixture.

Plotted in (a) are the cases at (1/k_Fa, p) = (0,0.68) (black solid) and (−1, 0.23) (red dashed), and in (b) are at unitarity with p = 0.55 (green solid) and 0.6 (blue dashed line).

Figure 4. T–p phase diagram of stable FFLO superfluid in Fermi-Fermi mixtures with different mass ratios (as labeled) at unitarity.

Large mass ratio enhances FFLO type of pairing.

Figure 5

Phase diagram of ⁶Li–⁴⁰K in the p − 1/k_Fa plane at T = 0 for (a) p > 0 and (b) p < 0. Stable FFLO phase lives in the narrow (yellow) shaded regions. Here “PS” labels phase separation (against unstable FFLO and Sarma superfluids), divided by the (red) dashed q = 0 line.