Tc 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 families: Na₂Cs_xC₆₀ (0<x<1) and M_3-yBa_yC₆₀ (0.2<y<2, M = K, Rb, or Cs). These are isostructural with known Pa3^¯ and Fm3^¯m trivalent superconductors, respectively, but with variable valence 2<n<5. Tc is peaked at or very near n = 3, decreasing rapidly 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 models to explain the dependence of Tc on band filling.