Momentum Distribution of Cooper Pairs and Strong-Coupling Effects in a Two-Dimensional Fermi Gas Near the Berezinskii-Kosterlitz-Thouless Transition.

We investigate strong-coupling properties of a two-dimensional ultracold Fermi gas in the normal state. Including pairing fluctuations within the framework of a T-matrix approximation, we calculate the distribution function $$n({\varvec{Q}})$$ of Cooper pairs in terms of the center of mass momentum $${\varvec{Q}}$$ . In the strong-coupling regime, $$n({\varvec{Q}}=0)$$ is shown to exhibit a remarkable increase with decreasing the temperature in the low temperature region, which agrees well with the recent experiment on a two-dimensional $$^6$$ Li Fermi gas (Ries et al. in Phys Rev Lett 114:230401, ). Our result indicates that the observed remarkable increase of the number of Cooper pairs with zero center of mass momentum can be explained without assuming the Berezinskii-Kosterlitz-Thouless (BKT) transition, when one properly includes pairing fluctuations that are enhanced by the low-dimensionality of the system. Since the BKT transition is a crucial topic in two-dimensional Fermi systems, our results would be useful for the study toward the realization of this quasi-long-range order in an ultracold Fermi gas. [ABSTRACT FROM AUTHOR]