Effects of static screening on correlation-induced superconductivity in M3C60

A recurrent theme among several recently proposed mechanisms for superconductivity in M3C60 (M= alkali metal) suggests that electron pairs bind together because of a subtle intramolecular correlation effect. We have calculated the dependence of such a mechanism on the electron-electron interaction within the framework of perturbation theory. Using a static phenomenological model of screening, we determine the phase diagram for pair binding as a function of an on-site interaction U (in units of the intrafullerene hopping parameter t) and screening length. We find the binding energy to be very sensitive to the range of the interaction. When U=4t, the screening length must be at most one-half of a double-bond length, or approximately 0.7 for pair binding to occur. This indicates that a reasonable incorporation of electron-electron correlation strongly suppresses the weak pair-binding effect found when such interactions are neglected.