ABSTRACT
Atomic Fermi gases provide an ideal platform for studying pairing and superfluid physics, using a Feshbach resonance between closed-channel molecular states and open-channel scattering states. Of particular interest is the strongly interacting regime. We show that the closed-channel fraction [Formula: see text] provides an effective probe for important many-body interacting effects, especially through its density dependence, which is absent from two-body theoretical predictions. Here we measure [Formula: see text] as a function of interaction strength and the Fermi temperature [Formula: see text] in a trapped 6Li superfluid throughout the entire Bardeen-Cooper-Schrieffer-Bose-Einstein-condensate crossover, in quantitative agreement with theory when important thermal contributions outside the superfluid core are taken into account. Away from the deep-BEC regime, the fraction [Formula: see text] is sensitive to [Formula: see text]. In particular, our data show [Formula: see text] with [Formula: see text] at unitarity, in quantitative agreement with calculations of a two-channel pairing fluctuation theory, and [Formula: see text] increases rapidly into the BCS regime, reflecting many-body interaction effects as predicted.
ABSTRACT
We have measured the heat capacity of an optically trapped, strongly interacting Fermi gas of atoms. A precise addition of energy to the gas is followed by single-parameter thermometry, which determines the empirical temperature parameter of the gas cloud. Our measurements reveal a clear transition in the heat capacity. The energy and the spatial profile of the gas are computed using a theory of the crossover from Fermi to Bose superfluids at finite temperatures. The theory calibrates the empirical temperature parameter, yields excellent agreement with the data, and predicts the onset of superfluidity at the observed transition point.
