Fermi Gas with 4-Fermion BCS-type Interaction

Abstract

bb-k replac- ing αk+α*_ k_, but because the excitation energies are not simply additive, the trial density matrix ρ0 differs from the BCS one. The expectation values (G|H|G)and Tr(Hp 0) are minimized, revealing the presence of a second-order phase transition, with T, > Tc(BOS) for appropriately adjusted Yk. It is shown that the minimization procedure applied leads to an expression for the free energy density of H, which is asymptotically exact in the infinite-volume limit. Comparison with experimental data on high-temperature supercon- ductors is made and for a particular choice of γk qualitative agreement is found with the temperature dependence of. the order parameter of the BSCCO superconductor.