Orbital magnetic susceptibility of the attractive Hubbard model

Our analysis of the orbital magnetic susceptibility chi of the attractive Hubbard model, above the superconducting transition temperature T-c, is based on the usual T-matrix approximation to the electronic self-energy. In this framework the one-electron Green's function is coupled to virtual pair fluctuations. The linear response of the electronic current to an external vector potential A, which yields chi, obeys a linear integral equation. The attraction between electrons has three effects: (a) a renormalized group velocity appears in the coupling of the electrons to A; (b) the electrons interact with each other by exchanging bosonic pair excitations; (c) there is a direct coupling of the virtual pairs to A. The pair contribution to chi has the same structure as the one obtained in a Landau-Ginzburg approach in Gaussian approximation, but differs in some details from the latter. (C) 1999 Elsevier Science B.V, All rights reserved.