[en] The contact 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_Fa), 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.

[en] 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_u↓(k) at high momenta, which can readily be obtained via Bragg scattering. Contact depends upon the relative interaction strength 1/(k_Fa) and temperature T/T_F, where k_F is the Fermi wave vector, a is the s-wave scattering length and T_F is the Fermi temperature. We present measurements of both of these dependencies in a cloud of ⁶Li 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.

[en] We have used Bragg spectroscopy to make precise measurements of a number of properties of a strongly interacting Fermi gas. These include the dynamic spin and density response functions, the static structure factors and Tan's universal contact parameter. We also present a technique for obtaining the homogeneous response functions and contact using measurements on a trapped (inhomogeneous) system. (authors)

[en] We have studied the transition from two to three dimensions in a low temperature weakly interacting ⁶Li Fermi gas. Below a critical atom number N_2D only the lowest transverse vibrational state of a highly anisotropic oblate trapping potential is occupied and the gas is two dimensional. Above N_2D 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.