Spin-Imbalanced Quasi-Two-Dimensional Fermi Gases

TL;DR

This study investigates spin-imbalanced quasi-two-dimensional Fermi gases of lithium-6, revealing deviations from mean-field theory, supporting a polaron model, and observing a phase transition to a spin-balanced core, providing key insights into their phase structure.

Contribution

It presents experimental measurements of density profiles in spin-imbalanced quasi-2D Fermi gases and compares them with theoretical models, highlighting a phase transition not predicted by existing theories.

Findings

Disagreement with mean-field BCS theory for 2D systems.

Normal-fluid mixtures fit well with a polaron model.

Observation of a phase transition to a spin-balanced core.

Abstract

We measure the density profiles for a Fermi gas of $^6$Li containing $N_1$ spin-up atoms and $N_2$ spin-down atoms, confined in a quasi-two-dimensional geometry. The spatial profiles are measured as a function of spin-imbalance $N_2/N_1$ and interaction strength, which is controlled by means of a collisional (Feshbach) resonance. The measured cloud radii and central densities are in disagreement with mean-field Bardeen-Cooper-Schrieffer theory for a true two-dimensional system. We find that the data for normal-fluid mixtures are reasonably well fit by a simple two-dimensional polaron model of the free energy. Not predicted by the model is a phase transition to a spin-balanced central core, which is observed above a critical value of $N_2/N_1$. Our observations provide important benchmarks for predictions of the phase structure of quasi-two-dimensional Fermi gases.