Critical velocity for superfluid flow across the BEC-BCS crossover.

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.

Evidence for superfluidity of ultracold fermions in an optical lattice.

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.

Observation of strong quantum depletion in a gaseous Bose-Einstein condensate.

We studied quantum depletion in a gaseous Bose-Einstein condensate. An optical lattice enhanced the atomic interactions and modified the dispersion relation resulting in strong quantum depletion. The depleted fraction was directly observed as a diffuse background in the time-of-flight images. Bogoliubov theory provides a semiquantitative description for our observations of depleted fractions in excess of 50%.

Comparison of MicroScan MICroSTREP, PASCO, and Sensititre MIC panels for determining antimicrobial susceptibilities of Streptococcus pneumoniae.

The MicroScan MICroSTREP MIC panel was compared with PASCO and Sensititre systems against 157 isolates of Streptococcus pneumoniae chosen to include penicillin-susceptible, intermediate, and resistant strains. Arbitration testing was performed by microbroth dilution using National Committee for Clinical Laboratory Standards guidelines. Overall essential agreement of 94-97% and categorical agreement of 91-94% with the reference method was achieved for the three systems. There were 8 very major errors (false susceptibility) for PASCO, 10 for Sensititre, and 9 for MICroSTREP; 4 major errors (false resistance) each for PASCO and MICroSTREP, and 6 for Sensititre. Most of these errors occurred with trimethoprim/sulfamethoxazole. Minor errors (susceptible or resistant versus intermediate) totaled 47 for PASCO, 69 for Sensititre, and 53 for MICroSTREP. Minor interpretive errors were most common with penicillin and ceftriaxone. This study showed that all three MIC panels provided interpretive results comparable to one another and to the reference method.