Pseudogap phenomena near the BKT transition of a two-dimensional ultracold Fermi gas in the crossover region

TL;DR

This paper improves the theoretical understanding of pseudogap phenomena in two-dimensional ultracold Fermi gases near the BKT transition by developing a self-consistent T-matrix approximation, aligning theory with experimental observations.

Contribution

The authors introduce a self-consistent T-matrix approximation to accurately describe pseudogap phenomena in 2D Fermi gases, overcoming limitations of previous non-self-consistent methods.

Findings

Self-consistent T-matrix approximation better matches experimental data.

Improved theory reduces unphysical pseudogap sizes in the crossover region.

Results are relevant for understanding BKT transition in ultracold gases.

Abstract

We investigate strong-coupling properties of a two-dimensional ultracold Fermi gas in the normal phase. In the three-dimensional case, it has been shown that the so-called pseudogap phenomena can be well described by a (non-self-consistent) $T$-matrix approximation (TMA). In the two-dimensional case, while this strong coupling theory can explain the pseudogap phenomenon in the strong-coupling regime, it unphysically gives large pseudogap size in the crossover region, as well as in the weak-coupling regime. We show that this difficulty can be overcome when one improve TMA to include higher order pairing fluctuations within the framework of a self-consistent $T$-matrix approximation (SCTMA). The essence of this improvement is also explained. Since the observation of the BKT transition has recently been reported in a two-dimensional $^6$Li Fermi gas, our results would be useful for the study of strong-coupling physics associated with this quasi-long-range order.