Spin Susceptibility Above the Superuid Onset in Ultracold Fermi Gases

The high-T_c cuprates display a suppression of the density of states at the transition temperature, known as pseudogap, which has a manifestation in reducing spin susceptibility with temperature. Motivated by such effect and possible analogies in cold gases, we want to study the spin susceptibility of an ultracold fermionic system in the BEC-BCS crossover. Towards this end, we prepared a quantum degenerate Fermi gas in a mixture of its second lowest two hyperfine states, using an all-optical method. We developed and deployed a novel spin susceptibility measurement method using radiofrequency (RF) dressing. By exposing the Fermi gas mixtures to RF radiation along with a magnetic gradient, we succeeded in mapping the mixture to an RF-dressed basis where we can reach thermal equilibrium with a small chemical potential difference between the spin states. By measuring the spin number difference, we are able to extract the spin susceptibility and perform analysis. In this talk, I will elaborate on the details of the method we adopted to measure spin susceptibility and discuss the results of such measurements over the interaction strength-temperature phase diagram. I will compare our result for gases in the strongly interacting regions, to a mean-field model (which is suprisingly accurate), to the ideal Fermi gas model, and to experimental results from several other publications. Certain highlights on the design of our apparatus will also be mentioned.