MICROWAVE STUDY OF SUPERCONDUCTIVITY IN ALKALI-METAL-DOPED C-60 FILMS

We have studied the doping of films of C60 and C70 by low-temperature alkali-metal vaporization. These films develop visible phase boundaries during the doping process. By using Raman, ESR, and microwave loss (superconductivity) spectroscopies to characterize the doped films in situ, we have established the compositions which occur under these conditions. We report the first preparation of a K3C60 film in which the superconducting transition temperature approaches that of the bulk materials (T(c) = 19 K) and the first observation of superconductivity in an Rb3C60 film (T(c) = 25 K). With C60 films, all of the alkali metals studied (A = Na, K, Rb, Cs) eventually form the insulating A6C60 phase after prolonged doping. The A3C60 phase occurs for A = Na, K, and Rb, but only the K and Rb compounds superconduct above 4 K. Large frequency shifts accompanying the superconducting transition indicate macroscopic microwave flux exclusion volumes and set critical field limits. In low magnetic fields, superconductivity is dominated by weak link behavior, resembling that of polycrystalline high-T(c) materials. Superconductivity was not observed with alkali-metal-doped C70 films.