Superconductivity at 30 K in caesium-doped C₆₀

RECENTLY there has been significant effort directed towards exploring the physical and chemical properties of C₆₀ and other large carbon clusters^1-11. Particularly intriguing are reports of superconductivity in potassium- and rubidium-doped C₆₀ crystals and films^5-7. The transition temperatures (T_c) for K-doped C₆₀ (18 K) and Rb-doped C₆₀ (∼28 K) are significantly higher than those reported previously for other molecular superconductors¹². Earlier attempts to prepare a caesium-doped superconducting phase⁷ have proved unsuccessful. Here we report that a Cs-doped superconductor can be prepared by using as the dopant binary alloys of the type CsM (where M is Hg, Tl or Bi). We observe a reproducible superconducting transition in Cs_xC₆₀ (x = 1.2-3) at 30 K, demonstrated by flux expulsion (the Meissner effect) and flux exclusion (shielding) d.c. magnetization measurements. The low superconducting volume fraction (∼1%) suggests that further studies will be needed to determine the optimal doping concentra-tion and to place tighter bounds on T_c.