SUPERCONDUCTING PROPERTIES OF ALKALI DOPED C-60 PREPARED BY PRECIPITATION FROM LIQUID METHYLAMINE OR AMMONIA

An extremely convenient method of preparing alkali doped face-centered cubic phases of C-60 consists of the partial solubilization of the co nstituent materials in liquid ammonia or monomethylamine followed by p recipitation and annealing. The preparation and properties of Rb3C60, K3C60 and Rb2CsC60 prepared in this manner from monomethylamine are de scribed. When attempting to prepare Rb2NaC60 from liquid ammonia, one obtains a phase separated material, consisting of the energetically mo re stable superconducting Rb3C60 and nonsuperconducting NaxC60. This s ample exhibits a very sharp superconducting transition temperature at 26.3 K. The decrease in T-c with respect to that of pure-phase Rb3C60 is explained as the result of the decrease in density of states in Jos ephson-like junctions formed between the two phases. This composition displays a large volume fraction of superconductivity, and all measure ments of superconducting parameters are extremely well defined, implyi ng high phase purity of the Rb3C60. The zero temperature values of the upper and lower critical magnetic fields are H-C2(0)=165 KOe and H-C1 (0)=62 Oe, calculated from measurements of the field dependence of the magnetization as a function of temperature. The coherence length and penetration depth are 44 Angstrom and 3400 Angstrom, significantly hig her than earlier literature estimates. The critical current density wa s found to be J(c)=7x10(6) A/cm(2), larger by a factor of 5 than previ ously reported values.