Magnetic properties and strong-coupling corrections in an ultracold Fermi gas with population imbalance

TL;DR

This paper improves the theoretical understanding of magnetic properties in imbalanced ultracold Fermi gases by extending the T-matrix approach to include higher order fluctuations, aligning theory with recent experimental results.

Contribution

It introduces an extended T-matrix theory that accurately accounts for pseudogap effects and many-body corrections, resolving unphysical results in the BCS-BEC crossover.

Findings

Extended theory matches experimental spin susceptibility data

Identifies limitations of the standard NSR theory in population-imbalanced gases

Provides a more accurate description of pairing fluctuations in ultracold Fermi gases

Abstract

We investigate magnetic properties of an ultracold Fermi gas with population imbalance. In the presence of population imbalance, the strong-coupling theory developed by Nozieres and Schmitt-Rink (which is frequently referred to as the NSR theory, or Gaussian fluctuation theory) is known to give unphysical results in the BCS-BEC crossover region. We point out that this problem comes from how to treat pseudogap effects originating from pairing fluctuations and many-body corrections to the spin susceptibility. We also clarify how to overcome this problem by including higher order fluctuations beyond the ordinary T-matrix theory. Calculated spin susceptibility based on our extended T-matrix theory agrees well with the recent experiment on a 6Li Fermi gas.