Excitation Spectrum and Superfluid Gap of an Ultracold Fermi Gas

Ultracold Fermi gases with tunable interactions have enabled the realization of 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. Using Bragg spectroscopy, we measure the full momentum-resolved low-energy excitation spectrum of strongly interacting Fermi gases and directly observe the smooth transformation from a bosonic to a fermionic superfluid that takes place in the BEC-BCS crossover. Moreover, we determine the evolution of the superfluid gap from these spectra 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.