Expansion of a Fermi gas interacting with a Bose-Einstein condensate

We study the expansion of an atomic Fermi gas interacting attractively with a Bose-Einstein condensate. We find that the interspecies interaction affects dramatically both the expansion of the Fermi gas and the spatial distribution of the cloud in trap. We observe indeed a slower evolution of the radial-to-axial aspect ratio which reveals the importance of the mutual attraction between the two samples during the first phase of the expansion. For large atom numbers, we also observe a bimodal momentum distribution of the Fermi gas, which reflects directly the distribution of the mixture in trap. This effect allows us to extract information on the dynamics of the system at the collapse.Comment: 4 pages, 4 figure

Radio Frequency Selective Addressing of Localized Particles in a Periodic Potential

We study the localization and addressability of ultra cold atoms in a combined parabolic and periodic potential. Such a potential supports the existence of localized stationary states and we show that using a radio frequency field allows to selectively address the atoms in these states. This method is used to measure the energy and momentum distribution of the atoms in the localized states. We also discuss possible extensions of this scheme to address and manipulate particles in single lattice sites.Comment: 4 pages, 4 figure

Atom interferometry with trapped Fermi gases

We realize an interferometer with an atomic Fermi gas trapped in an optical lattice under the influence of gravity. The single-particle interference between the eigenstates of the lattice results in macroscopic Bloch oscillations of the sample. The absence of interactions between fermions allows a time-resolved study of many periods of the oscillations, leading to a sensitive determination of the acceleration of gravity. The experiment proves the superiorness of non interacting fermions with respect to bosons for precision interferometry, and offers a way for the measurement of forces with microscopic spatial resolution.Comment: 4 pages, 4 figure

Insulating Behavior of a Trapped Ideal Fermi Gas

We investigate theoretically and experimentally the center-of-mass motion of an ideal Fermi gas in a combined periodic and harmonic potential. We find a crossover from a conducting to an insulating regime as the Fermi energy moves from the first Bloch band into the bandgap of the lattice. The conducting regime is characterized by an oscillation of the cloud about the potential minimum, while in the insulating case the center of mass remains on one side of the potential.Comment: 4 pages, 4 figure

Bosons and Fermions near Feshbach resonances

Near Feshbach resonances, $n|a|^3\gg 1$, systems of Bose and Fermi particles become strongly interacting/dense. In this unitary limit both bosons and fermions have very different properties than in a dilute gas, e.g., the energy per particle approach a value $\hbar^2n^{2/3}/m$ times an universal many-body constant. Calculations based upon an approximate Jastrow wave function can quantitatively describe recent measurements of trapped Bose and Fermi atoms near Feshbach resonances. The pairing gap between attractive fermions also scales as $\Delta\sim\hbar^2n^{2/3}/m$ near Feshbach resonances and is a large fraction of the Fermi energy - promising for observing BCS superfluidity in traps. Pairing undergoes several transitions depending on interaction strength and the number of particles in the trap and can also be compared to pairing in nuclei.Comment: Revised version extended to include recent molecular BEC-BCS result

Quasi-2D Bose-Fermi mixtures in an optical lattice

We give an overview on our current experiments with Bose-Fermi degenerate mixtures in a one dimensional (1D) optical lattice. Our system consists of a Fermi gas of $^{40}$K atoms and a Bose-Einstein condensate of $^{87}$Rb. The two species are simultaneously trapped in a combined magnetic and optical lattice potential. We have investigated the thermodynamical properties of the mixture in a tight lattice and observed a change in the density of states. This is a signature of a quasi two dimensional confinement of the atoms in the lattice sites. We also discuss the mechanical stability of the mixture in presence of the 1D lattice