Excitation Spectrum and Superfluid Gap of an Ultracold Fermi Gas

TL;DR

This study uses Bragg spectroscopy to map the excitation spectrum of ultracold Fermi gases across the BEC-BCS crossover, revealing the evolution of the superfluid gap and confirming theoretical models with improved accuracy near the BCS regime.

Contribution

It provides the first comprehensive momentum-resolved measurement of the excitation spectrum across the entire BEC-BCS crossover in ultracold Fermi gases.

Findings

Observed smooth transition from bosonic to fermionic superfluid

Measured superfluid gap evolution consistent with previous experiments

Improved theoretical agreement near the BCS regime with particle-hole correlations

Abstract

Ultracold atomic gases are a powerful tool to experimentally study strongly correlated quantum many-body systems. In particular, ultracold Fermi gases with tunable interactions have allowed to realize the famous BEC-BCS crossover from a Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer (BCS) superfluid of weakly bound Cooper pairs. However, large parts of the excitation spectrum of fermionic superfluids in the BEC-BCS crossover are still unexplored. In this work, we use Bragg spectroscopy to measure the full momentum-resolved low-energy excitation spectrum of strongly interacting ultracold Fermi gases. This enables us to directly observe the smooth transformation from a bosonic to a fermionic superfluid that takes place in the BEC-BCS crossover. We also use our spectra to determine the evolution of the superfluid gap and find excellent agreement with previous experiments and self-consistent T-matrix calculations both in the BEC and crossover regime. However, towards the BCS regime a calculation that includes the effects of particle-hole correlations shows better agreement with our data.