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Repulsive interaction helps superconductivity in fullerides

A repulsive interaction model of superconductivity (SC) is studied for tight-binding models with three-fold degenerate molecular orbitals. Taking a weak-coupling approach, we derive dimensionless coupling constants for various symmetries of SC pairs. In addition to anisotropic SC pairs, the s-wave pairing ($A_g$) can also be formed. With the purely repulsive interaction, however, the $A_g$ pair is not the most stable in both bcc and fcc lattices. The most stable SC pair for the bcc lattice has the $T_g$ symmetry, which is favored by a strongly nesting Fermi surface. In the fcc lattice, various SC symmetries have comparable coupling strengths. With the electron-phonon interaction combined, it is likely that the $A_g$ pair becomes the most stable.