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

Excitation Spectrum and Superfluid Gap of an Ultracold Fermi Gas