  Ultracold atom gases provide model systems in which many-body quantum physics
phenomena can be studied. Recent experiments on Fermi gases have realized a
phase transition to a Fermi superfluid state with strong interparticle
interactions. This system is a realization of the BCS-BEC crossover connecting
the physics of BCS superconductivity and that of Bose-Einstein condensation
(BEC). While many aspects of this system have been investigated, it has not yet
been possible to measure the single-particle excitation spectrum, which is a
fundamental property directly predicted by many-body theories. Here we show
that the single-particle spectral function of the strongly interacting Fermi
gas at T ~ Tc is dramatically altered in a way that is consistent with a large
pairing gap. We use photoemission spectroscopy to directly probe the elementary
excitations and energy dispersion in the Fermi gas of atoms. In these
photoemission experiments, an rf photon ejects an atom from our strongly
interacting system via a spin-flip transition to a weakly interacting state. We
measure the occupied single-particle density of states for an ultracold Fermi
gas of 40-potassium atoms at the cusp of the BCS-BEC crossover and on the BEC
side of the crossover, and compare these results to that for a nearly ideal
Fermi gas. Our results probe the many-body physics in a way that could be
compared to data for high-Tc superconductors. This new measurement technique
for ultracold atom gases, like photoemission spectroscopy for electronic
materials, directly probes low energy excitations and thus can reveal
excitation gaps and/or pseudogaps. Furthermore, this technique can provide an
analog to angle-resolved photoemission spectroscopy (ARPES) for probing
anisotropic systems, such as atoms in optical lattice potentials.
