RECENTLY there has been significant effort directed towards exploring the physical and chemical properties of C60 and other large carbon clusters1-11. Particularly intriguing are reports of superconductivity in potassium- and rubidium-doped C60 crystals and films5-7. The transition temperatures (Tc) 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 CsxC60 (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 Tc.
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