Spin Susceptibility and Strong Coupling Effects in an Ultracold Fermi Gas

TL;DR

This paper explores how spin susceptibility behaves in an ultracold Fermi gas across the BCS-BEC crossover, revealing non-monotonic temperature dependence due to preformed Cooper pairs, which is relevant for understanding pairing fluctuations.

Contribution

It introduces an extended T-matrix theoretical framework to analyze spin susceptibility and identifies the spin-gap temperature in the BCS-BEC crossover regime.

Findings

Non-monotonic temperature dependence of spin susceptibility due to preformed pairs

Identification of the spin-gap temperature where susceptibility peaks

Relevance to pairing fluctuations in ultracold Fermi gases

Abstract

We investigate magnetic properties and strong coupling corrections in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Within the framework of an extended $T$-matrix theory, we calculate the spin susceptibility $\chi$ above the superfluid phase transition temperature $T_{\rm c}$. In the crossover region, the formation of preformed Cooper pairs is shown to cause a non-monotonic temperature dependence of $\chi$, which is similar to the so-called spin-gap phenomenon observed in the under-doped regime of high-$T_{c}$ cuprates. From this behavior of $\chi$, we determine the spin-gap temperature as the temperature at which $\chi$ takes a maximum value, in the BCS-BEC crossover region. Since the spin susceptibility is sensitive to the formation of singlet Cooper pairs, our results would be useful in considering the temperature region where pairing fluctuations are important in the BCS-BEC crossover regime of an ultracold Fermi gas.