Sjjmf Servaas Kokkelmans

Experimental study of the BEC-BCS crossover region in lithium 6

We report Bose-Einstein condensation of weakly bound 6Li2 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.

Production of long-lived ultracold Li/sub 2/ molecules from a Fermi gas

We create weakly-bound Li2 molecules from a degenerate two component Fermi gas by sweeping a magnetic field across a Feshbach resonance. The atom-molecule transfer efficiency can reach 85% and is studied as a function of magnetic field and initial temperature. The bosonic molecules remain trapped for 0.5 s and their temperature is within a factor of 2 from the Bose-Einstein condensation temperature. A thermodynamical model reproduces qualitatively the experimental findings.

Interisotope Determination of Ultracold Rubidium Interactions from Three High-Precision Experiments

Combining the measured binding energies of four of the most weakly bound rovibrational levels of the 87Rb2 molecule with results of two other recent high-precision experiments, we obtain exceptionally strong constraints on the atomic interaction parameters in a highly model independent analysis. The comparison of (85)Rb and (87)Rb data, where the two isotopes are related by a mass scaling procedure, plays a crucial role. We predict scattering lengths, clock shifts, and Feshbach resonances with an unprecedented level of accuracy. Two of the Feshbach resonances occur at easily accessible magnetic fields in mixed-spin channels. One is related to a d-wave shape resonance.

Measurement of the interaction energy near a Feshbach resonance in a /sup 6/Li Fermi gas

We investigate the strongly interacting regime in an optically trapped 6Li Fermi mixture near a Feshbach resonance. The resonance is found at 800(40) G in good agreement with theory. Anisotropic expansion of the gas is interpreted by collisional hydrodynamics. We observe an unexpected and large shift (80 G) between the resonance peak and both the maximum of atom loss and the change of sign of the interaction energy.

Exploring an ultracold fermi-fermi mixture: Interspecies feshbach resonances and scattering properties of 6Li and 40K

We report on the observation of Feshbach resonances in an ultracold mixture of two fermionic species, (6)Li and (40)K. The experimental data are interpreted using a simple asymptotic bound state model and full coupled channels calculations. This unambiguously assigns the observed resonances in terms of various s- and p-wave molecular states and fully characterizes the ground-state scattering properties in any combination of spin states.

P-wave Feshbach resonances of ultracold ⁶Li

We report the observation of three p-wave Feshbach resonances of 6 Li atoms in the lowest hyperfine state f =1/2. Thepositions of the resonances are in good agreement with theory. We study the lifetime of the cloud in the vicinity of the Feshbach resonances and show that, depending on the spin states, two- or three-body mechanisms are at play. In the case of dipolar losses, we observe a nontrivial temperature dependence that is well explained by a simple model. DOI: 10.1103/PhysRevA.70.030702 In the presence of a magnetic field, it is possible to obtain a quasidegeneracy between the relative energy of two colliding atoms and that of a weakly bound molecular state. This effect, known as a Feshbach resonance, is usually associated with the divergence of the scattering length and is the key ingredient that led to the recent observation of superfluids from fermion atom pairs of 6 Li [1‐4] and 40 K [5] .U p to now these pairs were formed in s-wave channels but it is known from condensed matter physics that fermionic superfluidity can arise through higher angular momentum pairing: p-wave Cooper pairs have been observed in 3 He [6] and d-wave cooper pairs in high-Tc superconductivity [7]. Although Feshbach resonances involving p or higher partial waves have been found in cold atom systems [8‐10], p-wave atom pairs have never been directly observed. In this paper we report the observation of three narrow p-wave Feshbach resonances of 6 Li in the lowest hyperfine state f =1/2. We measure the position of the resonance as well as the lifetime of the atomic sample for all combinations uf =1/2, mfl + uf =1/2, m fl, henceforth denoted smf , m fd .W e show that the position of the resonances are in good agreement with theory. In the case of atoms polarized in the ground state s 1/2,1/2 d, the atom losses are due to threebody processes. We show that the temperature dependence of the losses at resonance cannot be described by the threshold law predicted by [11] on the basis of the symmetrization principle for identical particles. In the case of atoms polarized in s ˛1/2,˛1/2 d or that of a mixture s 1/2,˛1/2 d, the losses are mainly due to two-body dipolar losses. These losses show a nontrivial temperature dependence that can nevertheless be understood by a simple theoretical model with only one adjustable parameter. In the s 1/2,˛1/2 d channel, we take advantage of a sharp decrease of the two-body loss rate below the Feshbach resonance to present a first evidence for the generation of p-wave molecules.

Resonance superfluidity : renormalization of resonance scattering theory

We derive a theory of superfluidity for a dilute Fermi gas that is valid when scattering resonances are present. The treatment of a resonance in many-body atomic physics requires a novel mean-field approach starting from an unconventional microscopic Hamiltonian. The mean-field equations incorporate the microscopic scattering physics, and the solutions to these equations reproduce the energy-dependent scattering properties. This theory describes the high-T/sub c/ behavior of the system, and predicts a value of T/sub c/ that is a significant fraction of the Fermi temperature. It is shown that this mean-field approach does not break down for typical experimental circumstances, even at detunings close to resonance. As an example of the application of our theory, we investigate the feasibility for achieving superfluidity in an ultracold gas of fermionic /sup 6/Li.

Ramsey fringes in a Bose-Einstein condensate between atoms and molecules

In a recent experiment, a Feshbach scattering resonance was exploited to observe Ramsey fringes in a 85Rb Bose-Einstein condensate. The oscillation frequency corresponded to the binding energy of the molecular state. We show that the observations are remarkably consistent with predictions of a resonance field theory in which the fringes arise from oscillations between atoms and molecules.

Very-high-precision bound-state spectroscopy near a 85Rb Feshbach resonance

We precisely measured the binding energy ɛbind) of a molecular stale near the Feshbach resonance In a 85Rb Bose-Einstein condensate (BEC). Rapid magnetic-field pulses induced coherent atom-molecule oscillations in the BEC. We measured the oscillation frequency as a function B tield and fit the data TO a coupled-channel model. Our analysis constrained the Feshbach resonance position [155.041(18) Gl, width 10.71(2; G] and background scattering length [-443[3]a0] and yielded new values fur the Rb interaction parameters. These results improved our estimate lor the stability condition of an attractive BEC. We also found evidence for a mean-field shift to ɛbind.

Signatures of resonance superfluidity in a quantum Fermi gas

We predict a direct and observable signature of the superfluid phase in a quantum Fermi gas, in a temperature regime already accessible in current experiments. We apply the theory of resonance superfluidity to a gas confined in a harmonic potential and demonstrate that a significant increase in density will be observed in the vicinity of the trap center.