Ground-state thermodynamic quantities of homogeneous spin-$1/2$ fermions   from the BCS region to the unitarity limit
  We experimentally determined various thermodynamic quantities of interacting two-component fermions at the zero-temperature limit from the Bardeen-Cooper-Schrieffer (BCS) region to the unitarity limit. The obtained results are very accurate in the sense that the systematic error is within 4% around the unitarity limit. Using this advantage, we can compare our data with various many-body theories. We found that an extended ${\it T}$-matrix approximation, which is a strong-coupling theory involving fluctuations in the Cooper channel, well reproduces our experimental results. We also found that the superfluid order parameter ${\it \Delta}$ calculated by solving the ordinary BCS gap equation with the chemical potential of interacting fermions is close to the binding energy of the paired fermions directly observed in a spectroscopic experiment and that obtained using a quantum Monte Carlo method. Since understanding the strong-coupling properties of a superfluid Fermi gas in the BCS-BEC (Bose-Einstein condensation) crossover region is a crucial issue in condensed matter physics and nuclear physics, the results of the present study are expected to be useful in the further development of these fields. 
Munekazu Horikoshi, Masato Koashi, Hiroyuki Tajima, Yoji Ohashi,   Makoto Kuwata-Gonokami
cond-mat.quant-gas