Superconductivity at 33 K in Cs_xRb_yC₆₀

THE synthesis of macroscopic quantities^1,2 of the fullerenes C₆₀ and C₇₀ has led to discoveries of several unusual properties^3-10, in particular the high conductivity³ and superconductivity^4-6 of alkali-metal-doped phases. Here we report a superconducting phase of C₆₀ doped with caesium and rubidium, which has the highest transition temperature T_c and the largest diamagnetic shielding found so far for the alkali-metal-doped compounds. CS_xRb_yC₆₀ (x = 2 and y = 1 in the dopant feed) exhibits a T_c of 33 K and a diamagnetic shielding of over 60%. This is also the highest T_c yet observed in a molecular superconductor. The vari-ation of T_c with dopant, T_c (K_xC₆₀) [18 K]< T_c (Rb_xC₆₀) [∼29 K]< T_c (Cs_xRb_yC₆₀) [33 K], supports the interpretation that the transi-tion temperature of these fullerides is determined mainly by the density of states at the Fermi level.