Direct Observation of the Superfluid Phase Transition in Ultracold Fermi Gases

TL;DR

This paper reports the direct observation of the superfluid phase transition in ultracold Fermi gases through changes in cloud shape, providing a clear experimental signature of the transition in strongly interacting fermionic systems.

Contribution

The study demonstrates a method to directly observe the superfluid transition in fermionic gases by analyzing cloud shape changes, bypassing the need for magnetic field sweeps.

Findings

Superfluid transition observed via shape changes in atomic clouds.

Critical population imbalance for transition is approximately 70%.

Enhanced contrast achieved by preparing unequal spin mixtures.

Abstract

Water freezes into ice, atomic spins spontaneously align in a magnet, liquid helium becomes superfluid: Phase transitions are dramatic phenomena. However, despite the drastic change in the system's behaviour, observing the transition can sometimes be subtle. The hallmark of Bose-Einstein condensation (BEC) and superfluidity in trapped, weakly interacting Bose gases is the sudden appearance of a dense central core inside a thermal cloud. In strongly interacting gases, such as the recently observed fermionic superfluids, this clear separation between the superfluid and the normal parts of the cloud is no longer given. Condensates of fermion pairs could be detected only using magnetic field sweeps into the weakly interacting regime. The quantitative description of these sweeps presents a major theoretical challenge. Here we demonstrate that the superfluid phase transition can be directly observed by sudden changes in the shape of the clouds, in complete analogy to the case of weakly interacting Bose gases. By preparing unequal mixtures of the two spin components involved in the pairing, we greatly enhance the contrast between the superfluid core and the normal component. Furthermore, the non-interacting wings of excess atoms serve as a direct and reliable thermometer. Even in the normal state, strong interactions significantly deform the density profile of the majority spin component. We show that it is these interactions which drive the normal-to-superfluid transition at the critical population imbalance of 70(5)%.