BEC-BCS crossover, phase transitions and phase separation in polarized resonantly-paired superfluids

TL;DR

This paper explores the complex phase diagram of polarized resonantly-paired superfluids, revealing phase separation, FFLO states, and polarized gases across the BEC-BCS crossover, with implications for experimental detection.

Contribution

It provides the first detailed zero-temperature phase diagram of polarized superfluids across the BEC-BCS crossover, including novel phases and transition signatures.

Findings

Identification of phase separation regions

Prediction of FFLO superfluid states

Characterization of polarized normal and molecular superfluids

Abstract

We study resonantly-paired s-wave superfluidity in a degenerate gas of two species (hyperfine states labeled by $\uparrow,\downarrow$) of fermionic atoms when the numbers $N_{\uparrow}$ and $N_{\downarrow}$ of the two species are {\it unequal}, i.e., the system is "polarized". We find that the continuous crossover from the Bose-Einstein condensate (BEC) limit of tightly-bound diatomic molecules to the Bardeen-Cooper-Schrieffer (BCS) limit of weakly correlated Cooper pairs, studied extensively at equal populations, is interrupted by a variety of distinct phenomena under an imposed population difference $\Delta N \equiv N_\uparrow - N_\downarrow$. Our findings are summarized by a `` polarization\rq\rq ($\Delta N$) versus Feshbach-resonance detuning ($\delta$) zero-temperature phase diagram, which exhibits regions of phase separation, a periodic FFLO superfluid, a polarized normal Fermi gas and a polarized molecular superfluid consisting of a molecular condensate and a fully polarized Fermi gas. We describe numerous experimental signatures of such phases and the transitions between them, in particular focusing on their spatial structure in the inhomogeneous environment of an atomic trap.