  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.
