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Abstract:
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..
