Superconducting Properties of Alkali Doped C60 Prepared by Precipitation from Liquid Methylamine or Ammonia

Abstract An extremely convenient method of preparing alkali doped face-centered cubic phases of C60 consists of the partial solubilization of the constituent materials in liquid ammonia or monomethylamine followed by precipitation and annealing. The preparation and properties of Rb3C60, K3C60 and Rb2CsC60 prepared in this manner from monomethylamine are decribed. When attempting to prepare Rb2NaC60 from liquid ammonia, one obtains a phase separated material, consisting of the energetically more stable superconducting Rb3C60 and nonsuperconducting NaxC60. This sample exhibits a very sharp superconducting transition temperature at 26.3 K. The decrease in Tc with respect to that of pure-phase Rb3C60 is explained as the result of the decrease in density of states in Josephson-like junctions formed between the two phases. This composition displays a large volume fraction of superconductivity, and all measurements of superconducting parameters are extremely well defined, implying high phase purity of the Rb3C60...