Pseudogap phenomena in a two-dimensional ultracold Fermi gas near the Berezinskii-Kosterlitz-Thouless transition

TL;DR

This paper studies how pairing fluctuations cause a pseudogap in the density of states of a 2D ultracold Fermi gas near the BKT transition, providing insights into pre-superfluid phenomena.

Contribution

It demonstrates the emergence of a pseudogap due to superfluid fluctuations in a 2D Fermi gas near the BKT transition using Gaussian fluctuation theory.

Findings

Pseudogap appears in the density of states near T_BKT

Pseudogap resembles BCS superfluid density of states

Results are relevant for recent 2D Fermi gas experiments

Abstract

We investigate single-particle excitations and strong-coupling effects in a two-dimensional Fermi gas. Including pairing fluctuations within a Gaussian fluctuation theory, we calculate the density of states $\rho(\omega)$ near the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature $T_{\rm BKT}$. Near $T_{\rm BKT}$, we show that superfluid fluctuations induce a pseudogap in $\rho(\omega)$. The pseudogap structure is very similar to the BCS superfluid density of states, although the superfluid order parameter is absent in the present two-dimensional case. Since a two-dimensional $^{40}$K Fermi gas has recently been realized, our results would contribute to the understanding of single-particle properties near the BKT instability.