Pairing and superfluidity in strongly interacting Fermi gases

Abstract

This thesis describes experiments with superfluid spin mixtures of ultracold fermionic 6Li atoms. The properties of the strongly interacting gas are studied in the crossover regime between Bose-Einstein condensation (BEC) of two-body bound molecules and a Bardeen-Cooper-Schrieffer (BCS) superfluid of pairs bound by many-body interactions. We obtain the homogeneous phase diagram of the two -component gas with resonant interactions. As a function of temperature and spin polarization the phase diagram shows first and second order phase transitions that merge at a tricritical point. At zero temperature a first order phase transition from a superfluid with equal spin populations to a mixed normal phase is observed at a critical spin polarization known as the Chandrasekhar-Clogston limit of superfluidity. Pairing correlations in the superfluid and normal phase are studied with radio-frequency (rf) spectroscopy. A signature of strong correlations is observed above the critical temperature but also at spin polarizations where superfluidity is quenched even at zero temperature. Significant limitations for the interpretation of these experiments due to final state interactions are overcome by the creation of new superfluid spin mixtures. The asymmetric rf dissociation spectra of the new mixture allow us to determine the spectroscopic pair size in the crossover regime. The size of the resonantly interacting pairs is found to be on the order of, but smaller than the interparticle spacing. Rf spectra of the majority component in an imbalanced system show a signature of thermally excited quasiparticles and by comparison to the minority spectra reveal changes in the nature of the binding as a function of spin polarization.