A new many-body theory for the superfluid Fermi gas in the molecular Bose-Einstein condensed state

Markus Greiner, Cindy A. Regal, Deborah Jin

Григорий Ефимович Воловик, V. P. Mineev, Isaak M. Khalatnikov

Shina Tan

We demonstrate how solutions to quantum few-fermion scattering problems can be the point-of-departure of a new treatment of a generalized many-body wave function. Our focus is on a particular ansatz for the ground state wave function of a superfluid Fermi gas introduced earlier (cond-mat/0506293). Our method is perturbative in the sense that the probability amplitudes for few-fermion scattering processes are treated as small quantities; it is also NONPERTURBATIVE in the sense that whenever such scattering events occur, nonperturbative quantum few-fermion scattering physics dominates. This approach can be viewed as a new diagrammatic methodology, based on a wave function as distinct from a perturbation series in the interparticle interactions. Some generic properties of the wave function are studied, and its parameters in the Bose-Einstein condensed limit are computed beyond mean-field. These results enable us to predict many observable properties of this Fermi gas with well-controlled accuracy, such as the superfluid pairing function, the four-fermion and six-fermion correlation functions, the momentum distribution, and the two-body reduced density matrix, etc.

L. E. Reichl

Markus Greiner, Cindy A. Regal, Deborah Jin

The realization of fermionic superfluidity in a dilute gas of atoms, analogous to superconductivity in metals, is a long-standing goal of ultracold gas research. Beyond being a new example of this fascinating quantum phenomenon, fermionic superfluidity in an atomic gas holds the promise of adjustable interactions and the ability to tune continuously from BCS-type superfluidity to Bose-Einstein condensation (BEC). This crossover between BCS superfluidity of correlated atom pairs in momentum space and BEC of local pairs has long been of theoretical interest, motivated in part by the discovery of high Tc superconductors. In atomic Fermi gas experiments superfluidity has not yet been demonstrated; however recent experiments have made remarkable progress toward this goal. Starting from an ultracold Fermi gas experimenters have used Feshbach resonances to reversibly create molecules, i.e. composite bosons consisting of local fermion pairs. Furthermore, the experiments have shown that the resulting diatomic molecules can have surprisingly long lifetimes. Here we report the conversion of a Fermi sea of atoms into a molecular BEC. In addition to being the first molecular condensate in thermal equilibrium, this BEC represents one extreme of the predicted BCS-BEC continuum.

M. Bartenstein, A. Altmeyer, S. Riedl +4 more

We demonstrate a reversible conversion of a 6Li2 molecular Bose-Einstein condensate to a degenerate Fermi gas of atoms by adiabatically crossing a Feshbach resonance. By optical in situ imaging, we observe a smooth change of the cloud size in the crossover regime. On the Feshbach resonance, the ensemble is strongly interacting and the measured cloud size is 75(7)% of the one of a noninteracting zero-temperature Fermi gas. The high condensate fraction of more than 90% and the adiabatic crossover suggest our Fermi gas to be cold enough to form a superfluid.