Magnetic Properties and Strong-Coupling Corrections in an Ultracold Fermi Gas with Population Imbalance

We investigate magnetic properties of an ultracold Fermi gas with population imbalance. In the presence of population imbalance, the strong-coupling theory developed by NoziSres and Schmitt-Rink (which is frequently referred to as the NSR theory, or Gaussian fluctuation theory) is known to give unphysical results in the BCS-BEC crossover region. We point out that this problem comes from how to treat pseudogap effects originating from pairing fluctuations and many-body corrections to the spin susceptibility. We also clarify how to overcome this problem by including higher order fluctuations beyond the ordinary T-matrix approximation. Calculated spin susceptibility based on our extended T-matrix approximation agrees well with the recent experiment on a Li-6 Fermi gas.