T-C VS CARRIER CONCENTRATION IN CUBIC FULLERIDE SUPERCONDUCTORS

The effect of carrier concentration on fulleride superconductivity is studied by x ray, Raman, magnetization, and ESR of two new cubic famil ies: Na2CsxC60 (0 < x < 1) and M(3-y)Ba(y)C(60) (0.2 < y < 2, M = K, R b, or Cs). These are isostructural with known Pa(3) over bar and <Fm(3 )over bar m> trivalent superconductors, respectively, but with variabl e valence 2 < n < 5. T-c is peaked at or very near n = 3, decreasing r apidly in either direction to < 0.5 K for n = 2.5 and 5. These results underscore the failure of both one-electron and strong correlation mo dels to explain the dependence of T-c on band filling.