Finite temperature phase diagram of a polarised Fermi condensate

TL;DR

This paper maps the finite temperature phase diagram of polarized Fermi gases, revealing phase separation, tricritical points, and their relevance to experimental observations in ultracold atomic systems.

Contribution

It provides the first detailed theoretical analysis of the phase diagram, highlighting the role of tricritical points and phase separation in polarized Fermi gases.

Findings

Existence of phase separation between superfluid and normal states.

Presence of a tricritical point where phase boundary changes order.

Line of tricritical points ending at zero temperature on the BEC side.

Abstract

The two-component Fermi gas is the simplest fermion system displaying superfluidity, and as such finds applications ranging from the theory of superconductivity to QCD. Ultracold atomic gases provide an exceptionally clean realization of this system, where the interatomic interaction and the atom species population are both independent, tuneable parameters. This allows one to investigate the Fermi gas with imbalanced spin populations, which had previously been experimentally elusive, and this prospect has stimulated much theoretical activity. Here we show that the finite temperature phase diagram contains a region of phase separation between the superfluid and normal states that touches the boundary of second-order superfluid transitions at a tricritical point, reminiscent of the phase diagram of $^3$He-$^4$He mixtures. A variation of interaction strength then results in a line of tricritical points that terminates at zero temperature on the molecular Bose-Einstein condensate (BEC) side. On this basis, we argue that tricritical points will play an important role in the recent experiments on polarised atomic Fermi gases.