Phase diagram of a two-component Fermi gas with resonant interactions

TL;DR

This paper maps the phase diagram of a resonantly interacting two-component Fermi gas, revealing the nature of superfluidity stability and phase transitions under spin imbalance at various temperatures.

Contribution

It provides the first detailed experimental phase diagram of a spin-polarized Fermi gas at unitarity, including the identification of a tricritical point and quantum phase transition.

Findings

Observation of a first-order superfluid-to-normal phase transition.

Identification of a tricritical point where the transition changes from first to second order.

Detection of a quantum phase transition from superfluid to normal gas at zero temperature.

Abstract

The pairing of fermions is at the heart of superconductivity and superfluidity. The recent experimental realization of strongly interacting atomic Fermi gases has opened a new, controllable way to study novel forms of pairing and superfluidity. A major controversial issue has been the stability of superfluidity against an imbalance between the two spin components when the fermions interact resonantly. Here we present the phase diagram of a spin-polarized Fermi gas of $^6$Li atoms at unitarity, mapping out the superfluid phase versus temperature and density imbalance. Using tomographic techniques, we reveal spatial discontinuities in the spin polarization, the signature of a first-order superfluid-to-normal phase transition, which disappears at a tricritical point where the nature of the phase transition changes from first-order to second-order. At zero temperature, there is a quantum phase transition from a fully-paired superfluid to a partially-polarized normal gas. These observations and the implementation of an in situ ideal gas thermometer provide quantitative tests of theoretical calculations on the stability of resonant superfluidity.