Spin Susceptibility Above the Superfluid Onset in Ultracold Fermi Gases

TL;DR

This study measures spin susceptibility in ultracold Fermi gases across various interaction strengths and temperatures, revealing persistent reductions consistent with mean-field models, thus providing insights into the normal state above the superfluid transition.

Contribution

The paper introduces a novel radiofrequency technique to measure spin susceptibility in ultracold Fermi gases, extending understanding of their normal state properties at higher temperatures.

Findings

Spin susceptibility is reduced compared to non-interacting gases across all conditions.

The reduction persists at higher temperatures, aligning with mean-field predictions.

At unitarity, the uniform gas susceptibility matches mean-field models from transition to Fermi temperature.

Abstract

Ultracold atomic Fermi gases can be tuned to interact strongly, where they display spectroscopic signatures above the superfluid transition reminiscent of the pseudogap in cuprates. However, the extent of the analogy can be questioned, since thermodynamic quantities in the low temperature spin-imbalanced normal state can be described successfully using Fermi liquid theory. Here we present spin susceptibility measurements across the interaction strength-temperature phase diagram using a novel radiofrequency technique with ultracold $^6\textrm{Li}$ gases. For all significant interaction strengths and at all temperatures we find the spin susceptibility is reduced compared with the equivalent value for a non-interacting Fermi gas, with the low temperature results consistent with previous studies. However, our measurements extend to higher temperatures, where we find that the reduction persists consistently with a mean-field scenario. At unitarity, we can use the local density approximation to extract the spin susceptibility for the uniform gas, which is well described by mean-field models at temperatures from the superfluid transition to the Fermi temperature.