Anisotropic Pairing on a Three-Band Fermi Surface for $A_3{\rm C}_{60}$

Abstract

We study a model for anisotropic singlet pairing in $A_3{\rm C}_{60}$, using a realistic model for the Fermi surface in a hypothesized orientationally-ordered doped crystal. Anisotropic solutions are studied by combining numerical solutions to the gap equation in the low-temperature phase with a Landau expansion near the mean field critical temperature. We focus on a class of three-dimensional nodeless $d$-wave solutions to the model, which exhibit a fully developed gap everywhere on the Fermi surface, but non-BCS temperature dependence of the order parameter in the condensed state, a relatively large value of the low-temperature gap, $2\Delta/kT_{\rm c}$ , and non-BCS structure in the quasiparticle spectrum near the gap.