Quantum fluctuations in the BCS-BEC crossover of two-dimensional Fermi gases

TL;DR

This paper investigates the ground state of 2D Fermi gases across the BCS-BEC crossover, showing that quantum fluctuations are essential for accurate descriptions, and provides a comparison with experiments and simulations.

Contribution

The study demonstrates that including Gaussian quantum fluctuations in 2D Fermi gases recovers key features of the BCS-BEC crossover absent in mean-field theory.

Findings

Quantum fluctuations significantly affect the equation of state.

The boson scattering length ratio is approximately 0.56.

Results agree well with quantum Monte Carlo simulations and experiments.

Abstract

We present a theoretical study of the ground state of the BCS-BEC crossover in dilute two-dimensional Fermi gases. While the mean-field theory provides a simple and analytical equation of state, the pressure is equal to that of a noninteracting Fermi gas in the entire BCS-BEC crossover, which is not consistent with the features of a weakly interacting Bose condensate in the BEC limit and a weakly interacting Fermi liquid in the BCS limit. The inadequacy of the 2D mean-field theory indicates that the quantum fluctuations are much more pronounced than those in 3D. In this work, we show that the inclusion of the Gaussian quantum fluctuations naturally recovers the above features in both the BEC and the BCS limits. In the BEC limit, the missing logarithmic dependence on the boson chemical potential is recovered by the quantum fluctuations. Near the quantum phase transition from the vacuum to the BEC phase, we compare our equation of state with the known grand canonical equation of state of 2D Bose gases and determine the ratio of the composite boson scattering length $a_{\rm B}$ to the fermion scattering length $a_{\rm 2D}$. We find $a_{\rm B}\simeq 0.56 a_{\rm 2D}$, in good agreement with the exact four-body calculation. We compare our equation of state in the BCS-BEC crossover with recent results from the quantum Monte Carlo simulations and the experimental measurements and find good agreements.