Superfluidity and pairing in a two-dimensional Fermi gas

Dimensionality plays a fundamental role in the phenomenology of many-body systems. In this talk, I will present an overview of our research on strongly interacting two-dimensional Fermi gases, with particular focus on the observations of superfluidity and high-temperature pairing in this system. To probe superfluidity, we measure the momentum distribution of fermion pairs in the BEC-BCS crossover using a matter wave focusing scheme. Below a critical temperature, we observe an enhanced occupation of low momentum modes accompanied by a change in behaviour of the first order coherence function from exponential to algebraic decay. This suggests a transition to a superfluid phase described by the Berezinskii-Kosterlitz-Thouless mechanism. In our recent work, we study pair formation in the normal phase of the BEC-BCS crossover using spatially resolved radio-frequency spectroscopy, which enables the measurement of the local pairing energy in the system. While the pairing energy in the weakly interacting BEC regime is consistent with two-body physics, it significantly exceeds the two-body expectation in the strongly interacting crossover regime and shows a clear dependence on the local density. Our results show that many-body correlations play a significant role in pairing phenomena at remarkably high temperatures far above the critical temperature for superfluidity.