We report the production of matter-wave solitons in an ultracold lithium-7 gas. The effective interaction between atoms in a Bose-Einstein condensate is tuned with a Feshbach resonance from repulsive to attractive before release in a one-dimensional optical waveguide. Propagation of the soliton without dispersion over a macroscopic distance of 1.1 millimeter is observed. A simple theoretical model explains the stability region of the soliton. These matter-wave solitons open possibilities for future applications in coherent atom optics, atom interferometry, and atom transport.

One of the greatest challenges in modern physics is to understand the behaviour of an ensemble of strongly interacting particles. A class of quantum many-body systems (such as neutron star matter and cold Fermi gases) share the same universal thermodynamic properties when interactions reach the maximum effective value allowed by quantum mechanics, the so-called unitary limit. This makes it possible in principle to simulate some astrophysical phenomena inside the highly controlled environment of an atomic physics laboratory. Previous work on the thermodynamics of a two-component Fermi gas led to thermodynamic quantities averaged over the trap, making comparisons with many-body theories developed for uniform gases difficult. Here we develop a general experimental method that yields the equation of state of a uniform gas, as well as enabling a detailed comparison with existing theories. The precision of our equation of state leads to new physical insights into the unitary gas. For the unpolarized gas, we show that the low-temperature thermodynamics of the strongly interacting normal phase is well described by Fermi liquid theory, and we localize the superfluid transition. For a spin-polarized system, our equation of state at zero temperature has a 2 per cent accuracy and extends work on the phase diagram to a new regime of precision. We show in particular that, despite strong interactions, the normal phase behaves as a mixture of two ideal gases: a Fermi gas of bare majority atoms and a non-interacting gas of dressed quasi-particles, the fermionic polarons.

We discuss the behavior of weakly bound bosonic dimers formed in a two-component cold Fermi gas at a large positive scattering length a for the interspecies interaction. We find the exact solution for the dimer-dimer elastic scattering and obtain a strong decrease of their collisional relaxation and decay with increasing a. The large ratio of the elastic to inelastic rate is promising for achieving Bose-Einstein condensation of the dimers and cooling the condensed gas to very low temperatures.

Ultracold cesium atoms are prepared in the ground energy band of the potential induced by an optical standing wave. We observe Bloch oscillations of the atoms driven by a constant inertial force. We measure the momentum distribution of Bloch states and effective masses different from the mass of the free atom. [S0031-9007 (96)00366-3] PACS numbers: 32.80.Pj, 03.75.-b 45080031-9007͞96͞76(24)͞4508(4)$10.00

We report Bose-Einstein condensation of weakly bound 6 Li2 molecules in a crossed optical trap near a Feshbach resonance. We measure a molecule-molecule scattering length of 170 +100−60 nm at 770 G, in good agreement with theory. We study the 2D expansion of the cloud and show deviation from hydrodynamic behavior in the BEC-BCS crossover region.PACS numbers: 03.75. Ss, 05.30.Fk, 32.80.Pj, By applying a magnetic field to a gas of ultra-cold atoms, it is possible to tune the strength and the sign of the effective interaction between particles. This phenomenon, known as Feshbach resonance, offers in the case of fermions the unique possibility to study the crossover between situations governed by Bose-Einstein and FermiDirac statistics. Indeed, when the scattering length a characterizing the 2-body interaction at low temperature is positive, the atoms are known to pair in a bound molecular state. When the temperature is low enough, these bosonic dimers can form a Bose-Einstein condensate (BEC) as observed very recently in 40 K [1] and 6 Li [2,3]. On the side of the resonance where a is negative, one expects the well known Bardeen-Cooper-Schrieffer (BCS) model for superconductivity to be valid. However, this simple picture of a BEC phase on one side of the resonance and a BCS phase on the other is valid only for small atom density n. When n|a| 3 > ∼ 1 the system enters a strongly interacting regime that represents a challenge for many-body theories [4,5,6] and that now begins to be accessible to experiments [7,8,9].In this letter, we report on Bose-Einstein condensation of 6 Li dimers in a crossed optical dipole trap and a study of the BEC-BCS crossover region. Unlike all previous observations of molecular BEC made in single beam dipole traps with very elongated geometries, our condensates are formed in nearly isotropic traps. Analyzing free expansions of pure condensates with up to 4×10 4 molecules, we measure the molecule-molecule scattering length a m = 170 +100 −60 nm at a magnetic field of 770 gauss. This measurement is in good agreement with the value deduced from the resonance position [9] and the relation a m = 0.6 a of ref. [10]. Combined with tight confinement, these large scattering lengths lead to a regime of strong interactions where the chemical potential µ is on the order of k B T C where T C ≃ 1.5 µK is the condensation temperature. As a consequence, we find an important modification of the thermal cloud time of flight expansion induced by the large condensate mean field. Moreover, the gas parameter n m a 3 m is no longer small but on the order of 0.3. In this regime, the validity of mean field theory becomes questionable [11,12,13]. We show, in particular, that the anisotropy and gas energy released during expansion varies monotonically across the Feshbach resonance.Our experimental setup has been described previously [14,15]. A gas of 6 Li atoms is prepared in the absolute ground state |1/2, 1/2 in a Nd-YAG crossed beam optical dipole trap. The horizontal beam (resp. vertical) propagates along x (y)...

We report the observation of coexisting Bose-Einstein condensate (BEC) and Fermi gas in a magnetic trap. With a very small fraction of thermal atoms, the 7Li condensate is quasipure and in thermal contact with a 6Li Fermi gas. The lowest common temperature is 0.28 microK approximately 0.2(1)T(C) = 0.2(1)T(F) where T(C) is the BEC critical temperature and T(F) the Fermi temperature. The 7Li condensate has a one-dimensional character.

Interacting fermions are ubiquitous in nature, and understanding their thermodynamics is an important problem. We measured the equation of state of a two-component ultracold Fermi gas for a wide range of interaction strengths at low temperature. A detailed comparison with theories including Monte-Carlo calculations and the Lee-Huang-Yang corrections for low-density bosonic and fermionic superfluids is presented. The low-temperature phase diagram of the spin-imbalanced gas reveals Fermi liquid behavior of the partially polarized normal phase for all but the weakest interactions. Our results provide a benchmark for many-body theories and are relevant to other fermionic systems such as the crust of neutron stars.

We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasiparticle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find m*/m = 1.17(10), in agreement with the most recent theoretical predictions.