The study of superfluid fermion pairs in a periodic potential has important ramifications for understanding superconductivity in crystalline materials. By using cold atomic gases, various models of condensed matter can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could undergo d-wave pairing at low temperatures, a possible mechanism for high temperature superconductivity in the copper oxides. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental advances in bulk atomic gases include the observation of fermion-pair condensates and high-temperature superfluidity. Experiments with fermions and bosonic bound pairs in optical lattices have been reported but have not yet addressed superfluid behaviour. Here we report the observation of distinct interference peaks when a condensate of fermionic atom pairs is released from an optical lattice, implying long-range order (a property of a superfluid). Conceptually, this means that s-wave pairing and coherence of fermion pairs have now been established in a lattice potential, in which the transport of atoms occurs by quantum mechanical tunnelling and not by simple propagation. These observations were made for interactions on both sides of a Feshbach resonance. For larger lattice depths, the coherence was lost in a reversible manner, possibly as a result of a transition from superfluid to insulator. Such strongly interacting fermions in an optical lattice can be used to study a new class of hamiltonians with interband and atom-molecule couplings.

Critical velocities have been observed in an ultracold superfluid Fermi gas throughout the BEC-BCS crossover. A pronounced peak of the critical velocity at unitarity demonstrates that superfluidity is most robust for resonant atomic interactions. Critical velocities were determined from the abrupt onset of dissipation when the velocity of a moving one dimensional optical lattice was varied. The dependence of the critical velocity on lattice depth and on the inhomogeneous density profile was studied.PACS numbers: 03.75. Kk, 03.75.Lm, 03.75.Ss The recent realization of the BEC-BCS crossover in ultracold atomic gases [1] allows one to study how bosonic superfluidity transforms into fermionic superfluidity. The critical velocity for superfluid flow is determined by the low-lying excitations of the superfluid. For weakly bound fermions, the (Landau) critical velocity is proportional to the binding energy of the pairs, which increases monotonically along the crossover into the BEC regime. However, the speed of sound, which sets the critical velocity for phonon excitations, is almost constant in the BCS regime, but then decreases monotonically on the BEC side, since the strongly bound molecules are weakly interacting. At the BEC-BCS crossover, one expects a rather narrow transition from a region where excitation of sound limits superfluid flow, to a region where pair breaking dominates. In this transition region, the critical velocity is predicted to reach a maximum [2,3,4]. This makes the critical velocity one of the few quantities which show a pronounced peak across the BEC-BCS crossover in contrast to the chemical potential, the transition temperature [5], the speed of sound [6,7] and the frequencies of shape oscillations [8], which all vary monotonically.In this paper, we report the first study of critical velocities across the BEC-BCS crossover, where a Feshbach resonance allows the magnetic tuning of the atomic interactions, and find that superfluid flow is most robust near the resonance. Our observation of a pronounced maximum of the critical velocity is in agreement with the predicted crossover between the two different mechanisms for dissipation.Critical velocities have been determined before in atomic BECs perturbed by a stirring beam [9,10,11] as well as by a 1D moving optical lattice [12]. In both cases, the inhomogeneous density of the harmonically trapped sample had to be carefully accounted for in order to make quantitative comparisons to theory. Here * Present address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138 † Website: cua.mit.edu/ketterle group we mitigate this problem by probing only the central region of our sample with a tightly focused moving lattice formed from two intersecting laser beams. For decreasing lattice depths, the critical velocity increases and, at very small depths, approaches a value which is in agreement with theoretical predictions.In our experiments, we first create a superfluid of 6 Li pairs according to the procedure de...

The phase transition from a superfluid to a Mott insulator has been observed in a 23 Na BoseEinstein condensate. A dye laser detuned ≈ 5 nm red of the Na 3 2 S→ 3 2 P 1/2 transition was used to form the three dimensional optical lattice. The heating effects of the small detuning as well as the three-body decay processes constrained the timescale of the experiment. Certain lattice detunings were found to induce a large loss of atoms. These loss features were shown to be due to photoassociation of atoms to vibrational levels in the Na2 (1) 3 Σ + g state.

This paper reports results from a search for nucleon decay through invisible modes, where no visible energy is directly deposited during the decay itself, during the initial water phase of SNOþ. However, such decays within the oxygen nucleus would produce an excited daughter that would subsequently deexcite, often emitting detectable gamma rays. A search for such gamma rays yields limits of 2.5 × 10 29 y at 90% Bayesian credibility level (with a prior uniform in rate) for the partial lifetime of the neutron, and 3.6 × 10 29 y for the partial lifetime of the proton, the latter a 70% improvement on the previous limit from SNO. We also present partial lifetime limits for invisible dinucleon modes of 1.3 × 10 28 y for nn, 2.6 × 10 28 y for pn and 4.7 × 10 28 y for pp, an improvement over existing limits by close to 3 orders of magnitude for the latter two.