Two-dimensional spin-imbalanced Fermi gases at nonzero temperature: Phase separation of a noncondensate

We study a trapped two-dimensional (2D) spin-imbalanced Fermi gas over a range of temperatures. In the moderate-temperature regime, associated with current experiments, we find reasonable semiquantitative agreement with the measured density profiles as functions of varying spin imbalance and interaction strength. Our calculations show that, in contrast to the three-dimensional case, the phase separation which appears as a spin-balanced core can be associated with noncondensed fermion pairs. To shed light on the nature of 2D quasicondensation we compute the momentum, $q$, distribution of pairs, called ${n}_{B}(\mathbf{q})$; a pronounced low momentum peak is found, but importantly there is no macroscopic condensate. Following the protocols of Jochim and collaborators, we compute the characteristic temperature at which this peak disappears, thus providing a phase diagram for a quasicondensation onset temperature in the polarized case. Additional information about ${n}_{B}(\mathbf{q})$ is reflected in the behavior of the density profiles when the trap is removed. We show how quasicondensed bosons have a distinctive signature; they evolve relatively slowly under time of flight.