Strong-coupling corrections to spin susceptibility in the BCS-BEC crossover regime of a superfluid Fermi gas

TL;DR

This paper theoretically examines how strong pairing fluctuations influence the spin susceptibility in a superfluid Fermi gas across the BCS-BEC crossover, especially below the transition temperature, revealing a spin-gap regime linked to pseudogap phenomena.

Contribution

It extends previous work to the superfluid phase, analyzing the impact of pairing fluctuations on spin susceptibility below T_c using an extended T-matrix approximation.

Findings

Identification of a spin-gap regime below T_c influenced by pairing fluctuations.

Consistency of the spin-gap regime with the pseudogap regime from density of states.

Comparison with experiments on ultracold Li-6 Fermi gases.

Abstract

We theoretically investigate the uniform spin susceptibility $\chi$ in the superfluid phase of an ultracold Fermi gas in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. In our previous paper [H. Tajima, {\it et. al.}, Phys. Rev. A {\bf 89}, 033617 (2014)], including pairing fluctuations within an extended $T$-matrix approximation (ETMA), we showed that strong pairing fluctuations cause the so-called spin-gap phenomenon, where $\chi$ is anomalously suppressed even in the normal state near the superfluid phase transition temperature $T_{\rm c}$. In this paper, we extend this work to the superfluid phase below $T_{\rm c}$, to clarify how this many-body phenomenon is affected by the superfluid order. From the comparison of the ETMA $\chi$ with the Yosida function describing the spin susceptibility in a weak-coupling BCS superfluid, we identify the region where pairing fluctuations crucially affect this magnetic quantity below $T_{\rm c}$ in the phase diagram with respect to the strength of a pairing interaction and the temperature. This spin-gap regime is found to be consistent with the previous pseudogap regime determined from the pseudogapped density of states. We also compare our results with a recent experiment on a $^6$Li Fermi gas. Since the spin susceptibility is sensitive to the formation of spin-singlet preformed pairs, our results would be useful for the study of pseudogap physics in an ultracold Fermi gas on the viewpoint of the spin degrees of freedom.