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We use a Feshbach resonance to tune the scattering length a of a Bose-Einstein condensate of 7Li in the |F=1,mF=1> state. Using the spatial extent of the trapped condensate, we extract a over a range spanning 7 decades from small attractive interactions to extremely strong repulsive interactions. The shallow zero crossing in the wing of the Feshbach resonance enables the determination of a as small as 0.01 Bohr radii. Evidence of the weak anisotropic magnetic dipole interaction is obtained by comparison with different trap geometries for small a.

We have measured the binding energy of 7 Li Feshbach molecules deep into the nonuniversal regime by associating atoms in a Bose-Einstein condensate with a modulated magnetic field. We extract the scattering length from these measurements, correcting for nonuniversal short-range effects using the field-dependent effective range. With this more precise determination of the Feshbach resonance parameters we reanalyze our previous data on the location of atom loss features produced by the Efimov effect [Pollack et al., Science 326, 1683]. We find the measured locations of the three-and four-body Efimov features to be consistent with universal theory at the 20%-30% level. Efimov showed more than 40 years ago that three particles interacting via resonant two-body interactions could form an infinite series of three-body bound states as the two-body s-wave scattering length a was varied [1]. In the limit of zero-range interactions, the ratios of scattering lengths corresponding to the appearance of each bound state were predicted to be a universal constant, equal to approximately 22.7. The only definitive observations of the Efimov effect have been in ultracold atoms, where the ability to tune a via a Feshbach resonance [2,3] has proven to be essential. Since the first evidence for Efimov trimers was obtained in ultracold Cs [4], experiments have revealed both three-and four-body Efimov states in several atomic species. Although the Efimov effect has now been confirmed, several open questions remain, including a full understanding of the role of nonuniversal finite-range effects. Accurate comparisons with theory require that these nonuniversal contributions be quantitatively determined and incorporated.We previously characterized the F = 1,m F = 1 Feshbach resonance in 7 Li, which is located at approximately 738 G, by extracting a from the measured size of trapped Bose-Einstein condensates (BEC) assuming a mean-field Thomas-Fermi density distribution [5,6]. These data were fit to obtain a(B), the function giving a versus magnetic field, which was used to assign values of a to Efimov features observed in the rate of inelastic three-and four-body loss of trapped atoms [5]. More recently, two groups have characterized the same Feshbach resonance by directly measuring the binding energy, E b , of the weakly bound dimers on the a > 0 side of the Feshbach resonance [7][8][9]. These measurements disagree with our previous measurements based on BEC size. The disagreement in the parameters characterizing the Feshbach parameters is sufficiently large to affect the comparison of the measured Efimov features with universal theory.In this paper, we report measurements of E b , which we fit to obtain a(B). The measurement of E b has fewer systematic uncertainties than the BEC size measurement, which is affected at large scattering length by beyond meanfield effects and by anharmonic contributions to the trapping potential. The extraction of a from E b can therefore be more accurate, and unlike the condensate size measurement, E b is r...

We investigate the effects of impurities, either correlated disorder or a single Gaussian defect, on the collective dipole motion of a Bose-Einstein condensate of 7 Li in an optical trap. We find that this motion is damped at a rate dependent on the impurity strength, condensate center-of-mass velocity, and interatomic interactions. Damping in the Thomas-Fermi regime depends universally on the disordered potential strength scaled to the condensate chemical potential and the condensate velocity scaled to the speed of sound. The damping rate is comparatively small in the weakly interacting regime, and, in this case, is accompanied by strong condensate fragmentation. In situ and time-of-flight images of the atomic cloud provide evidence that this fragmentation is driven by dark soliton formation.

We excite the lowest-lying quadrupole mode of a Bose-Einstein condensate by modulating the atomic scattering length via a Feshbach resonance. Excitation occurs at various modulation frequencies, and resonances located at the natural quadrupole frequency of the condensate and at the first harmonic are observed. We also investigate the amplitude of the excited mode as a function of modulation depth. Numerical simulations based on a variational calculation agree with our experimental results and provide insight into the observed behavior. Collective excitation of a Bose-Einstein condensate (BEC) is an essential diagnostic tool for investigating properties of the ultracold quantum state. Fundamental information about condensate dynamics can be determined from observations of collective modes [1][2][3], including the effects of temperature [4][5][6], and the dimensionality of the system [7]. In addition, the interplay between these modes and external agents, such as random potentials [8,9], lattices [10,11], as well as other atoms [12][13][14][15][16], can be investigated. Examining the excitation spectrum of the BEC allows for a detailed comparison with theoretical models [17][18][19][20] and related quantum systems such as superfluid helium [21,22] and superconductors [23,24].Generically, a collective excitation is generated by the modification of the trapping potential of the condensate [25]. One convenient method is to apply a sudden magnetic field gradient, thereby shifting the center of the trap and exciting a dipole oscillation about the trap center. One may also suddenly change the curvature of the trap to excite quadrupole modes. The lowest-lying m = 0 quadrupole mode is characterized by out-of-phase axial and radial oscillations.If the condensate is not the only occupant of the trap (i.e., there exists a thermal component or another species of atoms) then the other atoms may also be excited through these processes. The evolution of a collective excitation can therefore be complicated because the multiple components may affect damping or induce frequency shifts of the oscillation [12][13][14][15][16]. Therefore, modulating the trap, although an extremely useful tool for an isolated condensate, can be cumbersome when the system to be studied is multispecied. An alternative approach is to excite the condensate alone, leaving the other occupants of the trap untouched.In this work, we demonstrate the excitation of the lowestlying quadrupole mode in a BEC of 7 Li by modulating the atomic scattering length via a magnetic Feshbach resonance. In contrast to abruptly changing the scattering length [26], sinusoidal modulation enables the controlled excitation of a single mode at a specific frequency. In addition, by using this method, a coexisting thermal component will be minimally excited by the mean-field coupling to the normal gas [27,28]. In the case of a multispecies experiment, resonant modulation of the scattering length of one species will not necessarily excite the others, depending on the details of other F...