Measurement of the Homogeneous Contact of a Unitary Fermi gas

TL;DR

This study measures the local contact of a unitary Fermi gas across various temperatures, revealing a gradual decrease and a sharp drop near the superfluid transition, challenging existing theoretical predictions.

Contribution

It provides the first experimental measurement of the homogeneous contact as a function of temperature, highlighting discrepancies with theoretical models near the superfluid transition.

Findings

Gradual decrease of contact with increasing temperature for T/T_F>0.4

Sharp decrease in contact at T/T_F=0.16 possibly indicating superfluid transition

Existing theories do not fully explain the observed behavior of contact near Tc

Abstract

By selectively probing the center of a trapped gas, we measure the local, or homogeneous, contact of a unitary Fermi gas as a function of temperature. Tan's contact, C, is proportional to the derivative of the energy with respect to the interaction strength, and is thus an essential thermodynamic quantity for a gas with short-range correlations. Theoretical predictions for the temperature dependence of C differ substantially, especially near the superfluid transition, Tc, where C is predicted to either sharply decrease, sharply increase, or change very little. For T/T_F>0.4, our measurements of the homogeneous gas contact show a gradual decrease of C with increasing temperature, as predicted by theory. We observe a sharp decrease in C at T/T_F=0.16, which may be due to the superfluid phase transition. While a sharp decrease in C below Tc is predicted by some many-body theories, we find that none of the predictions fully accounts for the data.