Effects of particle-hole fluctuations on the superfluid transition in two-dimensional atomic Fermi gases

TL;DR

This paper studies how particle-hole fluctuations influence the superfluid transition in two-dimensional Fermi gases, revealing a continuous screening effect that shifts the transition temperature and aligns well with experimental and simulation data.

Contribution

It introduces a self-consistent pairing fluctuation theory that includes particle-hole fluctuations, providing a more accurate description of the BKT transition across the BCS-BEC crossover.

Findings

Screening reduces the pairing gap and transition temperature.

The transition temperature shifts towards the BEC regime due to particle-hole effects.

Results agree with experimental data and quantum Monte Carlo simulations.

Abstract

Proper treatment of the many-body interactions is of paramount importance in our understanding of strongly correlated systems. Here we investigate the effects of particle-hole fluctuations on the Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional Fermi gases throughout the entire BCS-BEC crossover. We include self-consistently in the self energy treatment the entire particle-hole $T$ matrix, which constitutes a renormalization of the bare interaction that appears in the particle-particle scattering $T$ matrix, leading to a screening of the pairing interaction and hence a dramatic reduction of the pairing gap and the transition temperature. The BKT transition temperature $T_\text{BKT}$ is determined by the critical phase space density, for which the pair density and pair mass are determined using a pairing fluctuation theory, which accommodates self-consistently the important self-energy feedback in the treatment of finite-momentum pairing fluctuations. The screening strength varies continuously from its maximum in the BCS limit to essentially zero in BEC limit. In the unitary regime, it leads to an interaction-dependent shift of $T_\text{BKT}$ towards the BEC regime. This shift is crucial in an attempt to explain experimental data quantitatively, which often depends on the interaction strength. Our findings are consistent with available experimental results in the unitary and BEC regimes and with quantum Monte Carlo simulations in the BCS and unitary regimes.