Current issues in fullerene-based superconductors

Superconductivity in the fullerides has been exciting and controversial problem. The first superconducting fullerides to be discovered, the cubic trivalent close-packed M3C60's(1), are the best understood in terms of charge transfer(2) and the relationship between T-c and lattice parameter(3,4). There are many other conducting and superconducting compounds: polymer phases of M1C605, alkaline earth fullerides(6-9) and ammonia-alkali metal ternary phases(10). These all differ from M3C60 in important ways (crystal structure, degree of hybridization, molecular valence, solvation effects, filling of higher bands), such that all the metallic/superconducting compounds cannot be fit into a general picture. Consequently, despite much effort, we still lack predictive power for the occurence of metallic or superconducting behavior in doped fullerides. To address this global concern, here we present a brief review of the most recent experimental and theoretical studies.