Superconducting properties of mixed phases of nominal stoichiometry Rb2NaC60

When attempting to prepare Rb2NaCo60, one obtains a phase-separated materia l, consisting of the energetically more stable superconducting Rb3C60 and n onsuperconducting NaxC60. This sample 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 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, impl ying high phase purity of the Rb3C60. The zero temperature values of the up per 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 magnetiz ation as a function of temperature. The coherence length and penetration de pth are 44 Angstrom and 3400 Angstrom, significantly higher than earlier li terature estimates. (C) 1999 Elsevier Science B.V. All rights reserved.