Spin susceptibility and effects of inhomogeneous strong pairing fluctuations in a trapped ultracold Fermi gas

TL;DR

This paper theoretically examines how strong pairing fluctuations and trap inhomogeneity influence the magnetic properties, specifically spin susceptibility, of a unitary Fermi gas near the superfluid transition, revealing suppression effects and methods to evaluate spin-gap temperature.

Contribution

It introduces a theoretical framework combining extended T-matrix approximation and local density approximation to analyze spin susceptibility in trapped ultracold Fermi gases with strong pairing fluctuations.

Findings

Preformed singlet Cooper pairs suppress local spin susceptibility near T_c

Spatial variation of susceptibility reveals spin-gap temperature in a trap

Results aid understanding of thermodynamic properties in BCS-BEC crossover

Abstract

We theoretically investigate magnetic properties of a unitary Fermi gas in a harmonic trap. Including strong pairing fluctuations within the framework of an extended $T$-matrix approximation (ETMA), as well as effects of a trap potential within the local density approximation (LDA), we calculate the local spin susceptibility $\chi(T,r)$ above the superfluid phase transition temperature $T_{\rm c}$. We show that the formation of preformed singlet Cooper pairs anomalously suppresses $\chi(T,r)$ in the trap center near $T_{\rm c}$. We also point out that, in the unitarity limit, the spin-gap temperature in a uniform Fermi gas can be evaluated from the observation of the spatial variation of $\chi(T,r)$. Since a real ultracold Fermi gas is always in a trap potential, our results would be useful for the study of how this spatial inhomogeneity affects thermodynamic properties of an ultracold Fermi gas in the BCS-BEC crossover region.