At&t Bell Labs Murray Hill Nj 07974
Max Planck Inst Festkorperforsch W-7000 Stuttgart 80 Germany

ac susceptibility measurements of superconducting K3C60 powders in magnetic fields to 30 T yield a roughly linear H(c2)(T) curve extending up to the highest experimental fields. Samples prepared by two different methods give -dH(c2)/dT = 2.14+/-0.08 T/K, with T(c) = 18.7 K. The observed H(c2) at low temperatures exceeds the H(c2)(T) curve from the standard theory by Werthamer-Helfand-Hohenberg. We discuss the role of flux motion and evidence for enhancement of H(c2) by intrinsic mechanisms such as strong coupling, Fermi surface anisotropy, and/or granularity of the samples.
 

Physical Review B, 46 5876-5879, 1992.