Spin Responses Of A Superfluid Fermi Gas Of Atoms At Finite Temperatures

The experimental observations of Bose-Einstein condensation of resonance moleculesfor positive scattering lengths and of BCS transition of atoms for negative scattering lengthsin an ultracold Fermi gas of atoms have stimulated a great interest in the BEC-BCS crossoverphenomenon. The crossover regime in an ultracold Fermi gas of atoms can be easily accessedand controlled experimentally through the Feshbach resonance. The knowledge gained fromthe study of the crossover regime will certainly enhance our understanding of the properties ofquantum fluids and of the interplay between Bose and Fermi fluids, and in particular it will beof great value to the construction of a microscopic mechanism for high transition-temperaturesuperconductivity in many aspects.In connection with the experimental identification of the nature of condensation, i.e.,whether Bose-Einstein condensation of resonance molecules or BCS transition of atoms hasoccurred when an ultracold Fermi gas of atoms is found to be in the superfluid phase, thedensity and spin correlation functions have been proposed as probes of the onset ofsuperfluidity, and the density and spin response functions at zero temperature have beencalculated.In this paper, we study the dynamic spin structure factor of a superfluid Fermi gas ofatoms in the BEC-BCS crossover regime and its dependence on the frequency, wavevector,temperature, and two-body interaction. It has been found that the dynamic spin structurefactor possesses a peak at a fixed frequency below the superfluid gap for wavevectors in therange from 0.05k_F to k_F with k_F the Fermi wavevector. The peak is attributed to the spinexcitons below the superfluid gap. To verify and further understand the peak we found in ourcalculations of the dynamic spin structure factor in the superfluid phase of Fermi gases ofatoms, the experimental investigation of this peak in trapped Fermi gases of atoms is calledfor. A thorough understanding of the spin structure in the superfluid phase of trapped Fermigases of atoms will certainly shed light on the resonance peaks observed in thesuperconducting phase of high temperature superconductors.