Two-particle current from Superfluid Fermi Gases in the BCS-BEC Crossover

In recent years, the crossover from the BCS-type superfluid to the Bose-Einstein condensation (BEC) of tightly-bound molecules has been realized in ultracold atomic Fermi gases using a tunable pairing interaction associated with a Feshbach resonance.In the BCS-BEC crossover it will be important to reveal the nature of fermion pairs. In this paper, we propose that two-particle (double photoemission) current (DPE current) is a powerful technics to provide direct insight into the pair-correlations. The DPE from superconductors has been studied both theoretically and experimentally, where a pair of electrons is emitted from the system upon the absorption of one photon. In this study, we consider an analogous situation in ultracold atomic gases, and derive a general expression for DPE current from superfluid Fermi gases in the BCS-BEC crossover.We show DPE current as a function of energy and momentum transfers, and identify the contributions of the condensed pair components and uncorrelated pair states, and discuss the possibility of distinguishing between weakly-bound Cooper pairs and tightly-bound molecules.