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Superconductivity of Ba doped C60 — susceptibility results and upper critical field

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Baenitz, M., Heinze, M., Lüders, K., Werner, H., Schlögl, R., Weiden, M., et al. (1995). Superconductivity of Ba doped C60 — susceptibility results and upper critical field. Solid State Communications, 96(8), 539-544. doi:10.1016/0038-1098(95)00498-X.

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 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 composition which exhibits the largest superconducting volume fraction (similar to 22 %) in this series. Furthermore first estimates of the upper critical field of Ba4C60, are presented. The slope of the upper critical field (partial derivative B-c2/partial derivative T) near T-c is one order of magnitude smaller than in alkali-metal doped C-60 compounds. The whole B-c2(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 B-c2(0) = (2.4+/-0.3) T and for the corresponding Ginzburg-Landau coherence length xi(GL)(0) = (116+/-7)) A..