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

We present a comprehensive experimental study of Tan's universal contact parameter 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/(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.