Fermionic Superfluidity with Imbalanced Spin Populations and the Quantum Phase Transition to the Normal State

Whether it occurs in superconductors, helium-3 or inside a neutron star, fermionic superfluidity requires pairing of fermions, particles with half-integer spin. For an equal mixture of two states of fermions ("spin up" and "spin down"), pairing can be complete and the entire system will become superfluid. When the two populations of fermions are unequal, not every particle can find a partner. Will the system nevertheless stay superfluid? Here we study this intriguing question in an unequal mixture of strongly interacting ultracold fermionic atoms. The superfluid region vs population imbalance is mapped out by employing two complementary indicators: The presence or absence of vortices in a rotating mixture, as well as the fraction of condensed fermion pairs in the gas. Due to the strong interactions near a Feshbach resonance, the superfluid state is remarkably stable in response to population imbalance. The final breakdown of superfluidity marks a new quantum phase transition, the Pauli limit of superfluidity.Comment: 15 pages, 5 figure

Photo-induced two-body loss of ultracold molecules

The lifetime of nonreactive ultracold bialkali gases was conjectured to be limited by sticky collisions amplifying three-body loss. We show that the sticking times were previously overestimated and do not support this hypothesis. We find that electronic excitation of NaK+NaK collision complexes by the trapping laser leads to the experimentally observed two-body loss. We calculate the excitation rate with a quasiclassical, statistical model employing ab initio potentials and transition dipole moments. Using longer laser wavelengths or repulsive box potentials may suppress the losses

Phase diagram of a dilute fermion gas with density imbalance

We map out the phase diagram of a dilute two-component atomic fermion gas with unequal populations and masses under a Feshbach resonance. As in the case of equal masses, no uniform phase is stable for an intermediate coupling regime. For majority component heavier, the unstable region moves towards the BEC side. When the coupling strength is increased from the normal phase, there is an increased parameter space where the transition is into the FFLO state. The converse is true if the majority is light.Comment: Proceeding for M$^2$S-HTSC VIII meeting, July 9-14 2006, Dresden; To appear in Physica

Coherence and clock shifts in ultracold Fermi gases with resonant interactions

Using arguments based on sum rules, we derive a general result for the average shifts of rf lines in Fermi gases in terms of interatomic interaction strengths and two-particle correlation functions. We show that near an interaction resonance shifts vary inversely with the atomic scattering length, rather than linearly as in dilute gases, thus accounting for the experimental observation that clock shifts remain finite at Feshbach resonances.Comment: 4 pages, 2 figures. Nordita preprint NORDITA-2007-2

Pairing without Superfluidity: The Ground State of an Imbalanced Fermi Mixture

Radio-frequency spectroscopy is used to study pairing in the normal and superfluid phases of a strongly interacting Fermi gas with imbalanced spin populations. At high spin imbalances the system does not become superfluid even at zero temperature. In this normal phase full pairing of the minority atoms is observed. This demonstrates that mismatched Fermi surfaces do not prevent pairing but can quench the superfluid state, thus realizing a system of fermion pairs that do not condense even at the lowest temperature

Observation of Feshbach resonances between two different atomic species

We have observed three Feshbach resonances in collisions between lithium-6 and sodium-23 atoms. The resonances were identified as narrow loss features when the magnetic field was varied. The molecular states causing these resonances have been identified, and additional lithium-sodium resonances are predicted. These resonances will allow the study of degenerate Bose-Fermi mixtures with adjustable interactions, and could be used to generate ultracold heteronuclear molecules

Formation Time of a Fermion Pair Condensate

The formation time of a condensate of fermionic atom pairs close to a Feshbach resonance was studied. This was done using a phase-shift method in which the delayed response of the many-body system to a modulation of the interaction strength was recorded. The observable was the fraction of condensed molecules in the cloud after a rapid magnetic field ramp across the Feshbach resonance. The measured response time was slow compared to the rapid ramp, which provides final proof that the molecular condensates reflect the presence of fermion pair condensates before the ramp.Comment: 5 pages, 4 figure

Observation of Bose-Einstein Condensation of Molecules

We have observed Bose-Einstein condensation of molecules. When a spin mixture of fermionic Li-6 atoms was evaporatively cooled in an optical dipole trap near a Feshbach resonance, the atomic gas was converted into Li_2 molecules. Below 600 nK, a Bose-Einstein condensate of up to 900,000 molecules was identified by the sudden onset of a bimodal density distribution. This condensate realizes the limit of tightly bound fermion pairs in the crossover between BCS superfluidity and Bose-Einstein condensation.Comment: 4 pages, 3 figure

Development of an apparatus for cooling 6Li-87Rb Fermi-Bose mixtures in a light-assisted magnetic trap

We describe an experimental setup designed to produce ultracold trapped gas clouds of fermionic 6Li and bosonic 87Rb. This combination of alkali metals has the potential to reach deeper Fermi degeneracy with respect to other mixtures since it allows for improved heat capacity matching which optimizes sympathetic cooling efficiency. Atomic beams of the two species are independently produced and then decelerated by Zeeman slowers. The slowed atoms are collected into a magneto-optical trap and then transferred into a quadrupole magnetic trap. An ultracold Fermi gas with temperature in the 10^-3 T_F range should be attainable through selective confinement of the two species via a properly detuned laser beam focused in the center of the magnetic trap.Comment: Presented at LPHYS'06, 8 figure

Fifty-fold improvement in the number of quantum degenerate fermionic atoms

We have produced a quantum degenerate Li-6 Fermi gas with up to 7 x 10^7 atoms, an improvement by a factor of fifty over all previous experiments with degenerate Fermi gases. This was achieved by sympathetic cooling with bosonic Na-23 in the F=2, upper hyperfine ground state. We have also achieved Bose-Einstein condensation of F=2 sodium atoms by direct evaporation