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.
Categories: cond-mat