Superconductivity of Ba doped C60 — susceptibility results and upper critical 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 were determined by applying acsusceptibility and dc-magnetization (SQUID) techniques in external magnetic dc 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 Tc = (6.8±0.2) K occurs. Three intrinsic BaxC60 phases (x = 3, 4, 6) are identified, and superconductivity is associated to the x = 4 composition which exhibits the largest superconducting volume fraction (∼ 22 %) in this series. Furthermore first estimates of the upper critical field of Ba4C60 are presented. The slope of the upper critical field ( ∂ B c2 ∂ T ) near Tc is one order of magnitude smaller than in alkali-metal doped C60 compounds. The whole Bc2(T)-curve is compared with the Werthamer-Helfand-Hohenberg (WHH) theory in the dirty and clean limit. We find zero temperature values for the upper critical field Bc2(0) = (2.4±0.3) T and for the corresponding Ginzburg-Landau coherence length ξ GL (0) = (116±7) A .