Showing papers by "Chris J. Vale published in 2011"

Crossover From 2D to 3D in a Weakly Interacting Fermi Gas

Abstract: We have studied the transition from two to three dimensions in a low temperature weakly interacting $^{6}\mathrm{Li}$ Fermi gas. Below a critical atom number ${N}_{2\mathrm{D}}$ only the lowest transverse vibrational state of a highly anisotropic oblate trapping potential is occupied and the gas is two dimensional. Above ${N}_{2\mathrm{D}}$ the Fermi gas enters the quasi-2D regime where shell structure associated with the filling of individual transverse oscillator states is apparent. This dimensional crossover is demonstrated through measurements of the cloud size and aspect ratio versus atom number.

Temperature dependence of the universal contact parameter in a unitary Fermi gas.

Abstract: The contact $\mathcal{I}$, introduced by Tan, has emerged as a key parameter characterizing universal properties of strongly interacting Fermi gases. For ultracold Fermi gases near a Feshbach resonance, the contact depends upon two quantities: the interaction parameter $1/({k}_{F}a)$, where ${k}_{F}$ is the Fermi wave vector and $a$ is the $s$-wave scattering length, and the temperature $T/{T}_{F}$, where ${T}_{F}$ is the Fermi temperature. We present the first measurements of the temperature dependence of the contact in a unitary Fermi gas using Bragg spectroscopy. The contact is seen to follow the predicted decay with temperature and shows how pair-correlations at high momentum persist well above the superfluid transition temperature.

Density distribution of a trapped two-dimensional strongly interacting Fermi gas

Abstract: We calculated and measured the density distribution and cloud size of a trapped two-dimensional (2D) 6Li Fermi gas near a Feshbach resonance at low temperatures. Density distributions and cloud sizes were calculated for a wide range of interaction parameters using a local density approximation (LDA) and a zero-temperature equation of state obtained from quantum Monte Carlo simulations reported by Bertaina and Giorgini (2011 Phys. Rev. Lett. 106 110403). We found that LDA predictions agree well with experimental measurements across a Feshbach resonance. Theoretical results for Tan's contact parameter in a trapped gas are reported here along with predictions for the static structure factor at large momentum which could be measured in future Bragg spectroscopy experiments on 2D Fermi gases.

Growth dynamics of a Bose-Einstein condensate in a dimple trap without cooling

Abstract: We study the formation of a Bose-Einstein condensate in a cigar-shaped three-dimensional harmonic trap, induced by the controlled addition of an attractive ``dimple'' potential along the weak axis. In this manner we are able to induce condensation without cooling due to a localized increase in the phase-space density. We perform a quantitative analysis of the thermodynamic transformation in both the sudden and adiabatic regimes for a range of dimple widths and depths. We find good agreement with equilibrium calculations based on self-consistent semiclassical Hartree-Fock theory describing the condensate and thermal cloud. We observe that there is an optimal dimple depth that results in a maximum in the condensate fraction. We also study the nonequilibrium dynamics of condensate formation in the sudden turn-on regime, finding good agreement for the observed time dependence of the condensate fraction with calculations based on quantum kinetic theory.

Studies of the universal contact in a strongly interacting Fermi gas using Bragg spectroscopy

Abstract: We present a comprehensive experimental study of Tan's universal contact parameter I in a two-component ultracold Fermi gas, using Bragg spectroscopy. The contact uniquely parameterizes a number of universal properties of Fermi gases in the strongly interacting regime. It is linked to the spin-antiparallel component of the static structure factor S"#(k)at high momenta, which can readily be obtained via Bragg scattering. Contact depends upon the relative interaction strength 1/(kFa)and temperature T/TF, wherekF is the Fermi wave vector, a is the s-wave scattering length and TF is the Fermi temperature. We present measurements of both of these dependencies in a cloud of 6 Li atoms and compare our findings to theoretical predictions. We also compare Bragg spectroscopic methods based on measuring the energy and momentum transferred to the cloud and examine the conditions under which the energy transfer method provides improved accuracy. Our measurements of the dynamic structure factor and contact are found to be in good agreement with theoretical predictions based on the quantum virial expansion.

Universal structure of a strongly interacting Fermi gas

Abstract: This paper presents studies of the universal properties of strongly interacting Fermi gases using Bragg spectroscopy. We focus on pair-correlations, their relationship to the contact C introduced by Tan, and their dependence on both the momentum and temperature. We show that short-range pair correlations obey a universal law, first derived by Tan through measurements of the static structure factor, which displays a universal scaling with the ratio of the contact to the momentum C/q. Bragg spectroscopy of ultracold 6Li atoms is employed to measure the structure factor for a wide range of momenta and interaction strengths, providing broad confirmation of this universal law. We show that calibrating our Bragg spectra using the f-sum rule leads to a dramatic improvement in the accuracy of the structure factor measurement. We also measure the temperature dependence of the contact in a unitary gas and compare our results to calculations based on a virial expansion.