Fermi Gases in the Two-Dimensional to Quasi-Two-Dimensional Crossover

TL;DR

This study explores the transition of strongly-interacting $^6$Li Fermi gases from two-dimensional to quasi-two-dimensional regimes by tuning confinement, revealing deviations from 2D-BCS theory and aligning with Bethe-Goldstone based predictions.

Contribution

It experimentally investigates the dimensional crossover in Fermi gases and compares observed spectra and cloud radii with theoretical models, highlighting the limitations of 2D-BCS theory.

Findings

Radio frequency spectra match 2D-BCS in the 2D regime.

Spectra deviate from 2D-BCS in the quasi-2D regime.

Cloud radii agree with Bethe-Goldstone based predictions.

Abstract

We tune the dimensionality of pancake-shaped strongly-interacting $^6$Li Fermi gas clouds from two-dimensional (2D) to quasi-2D, by controlling the ratio of the radial Fermi energy $E_F$ to the harmonic oscillator energy $h\nu_z$ in the tightly confined direction. In the 2D regime, where $E_F<<h\nu_z$, the measured radio frequency resonance spectra are in agreement with 2D-BCS theory. In the quasi-2D regime, where $E_F\simeq h\nu_z$, the measured spectra deviate significantly from 2D-BCS theory. For both regimes, the measured cloud radii disagree with 2D-BCS mean field theory, but agree approximately with predictions using a free energy derived from the Bethe-Goldstone equation.