Thermodynamic functions of Fermi gas with quadruple BCS-type binding potential

A gas of spin 1/2 fermions with an interaction V + W = -2 gamma Sigma((k) under bar)chi((k) under bar)b((k) under bar)*b((k) under bar)+ - \Lambda \(-1) g Sigma((k) under bar<(k')under bar>)chi((k) under bar)chi(<(k')under bar>)b((k) under bar)*b((k) under bar)*b(<(k')under bar>)b(-<(k')under bar>), where b((k) under bar)= a((k) under bar+) a((k) under bar-) and a((k) under bar sigma), a(<(k')under bar>sigma') satisfy Fermi anticommutation relations, is investigated by the method of Muhlschlegel. W + V is nonzero only within a thin layer of single-fermion energies around the chemical potential mu and chi((k) under bar) denotes the characteristic function of the corresponding range of momenta. Two cases are studied: 1(0)gamma = 0, 2(0)gamma = 0.10025 eV. In the first case, the system exhibits a first-order transition, in the second the transition is second order. The temperature dependence of the system's thermodynamic functions is examined and compared with that of the BCS model. (C) 2000 Elsevier Science B.V. All rights reserved.