BCS-BEC crossover in a quasi-two-dimensional Fermi superfluid

TL;DR

This paper investigates the BCS-BEC crossover in a quasi-two-dimensional ultracold Fermi gas, revealing universal relations on the BCS side and the impact of reduced dimensionality on pairing physics, with results aligning with recent experiments.

Contribution

It introduces a theoretical framework for the BCS-BEC crossover in quasi-2D systems using a Gaussian pair fluctuation approach, highlighting the effects of reduced dimensionality on superfluid properties.

Findings

Universal relations between order parameter and chemical potential on BCS side

Breakdown of universality approaching BEC limit with stronger interactions

Good agreement with recent experimental data on ultracold gases and layered nitrides

Abstract

We study the crossover from the Bardeen-Cooper-Shrieffer (BCS) regime to the Bose-Einstein-condensation (BEC) regime in a quasi-two-dimensional quantum gas of ultracold fermionic atoms. Using an effective two-dimensional Hamiltonian with renormalized interactions between atoms and dressed molecules within a Gaussian pair fluctuation theory, we investigate how Fermi superfluidity is affected by reduced dimensionality at zero temperature in a wide range of crossover. We observe that the order parameter and pair size show universal relations with the chemical potential on the BCS side, irrespective of dimensionality. However, such universal dependences break down towards the BEC limit with increasing interaction strength. This results reveal the notable effect of reduced dimenionality on pairing physics, which can also be observed in the sound velocity and convexity parameter of the Goldstone mode. We compare our results with the latest experiments in both ${}^{6}$Li atomic gases and layered nitrides LixZrNCl and find good agreements.