SUPERCONDUCTIVITY OF BA DOPED C-60 - SUSCEPTIBILITY RESULTS AND UPPERCRITICAL-FIELD

A series of BaxC60 samples were prepared with stoichiometries ranging from x = 3 to x = 6. The lattice structure was analysed by using X-ray diffraction (XRD) investigations and the superconducting properties w ere determined by applying acsusceptibility and dc-magnetization (SQUI D) techniques in external magnetic de fields up to 2 T. Beyond x = 3, XRD-results reveal the presence of more than one phase in the samples and, in contrast to the Ba3C60 sample, superconductivity around T-c = (6.8+/-0.2)K occurs. Three intrinsic BaxC60, phases (x = 3, 4, 6) are identified, and superconductivity is associated to the x = 4 compositi on which exhibits the largest superconducting volume fraction (similar to 22 %) in this series. Furthermore first estimates of the upper cri tical field of Ba4C60, are presented. The slope of the upper critical field (partial derivative B-c2/partial derivative T) near T-c is one o rder of magnitude smaller than in alkali-metal doped C-60 compounds. T he whole B-c2(T)-curve is compared with the Werthamer-Helfand-Hohenber g (WHH) theory in the dirty and clean limit. We find zero temperature values for the upper critical field B-c2(0) = (2.4+/-0.3) T and for th e corresponding Ginzburg-Landau coherence length xi(GL)(0) = (116+/-7) ) A..