Evidence of upper-critical-field enhancement inK3C60powders

Ac susceptibility measurements of superconducting ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ powders in magnetic fields to 30 T yield a roughly linear ${\mathit{H}}_{\mathit{c}2}$(T) curve extending up to the highest experimental fields. Samples prepared by two different methods give -${\mathit{dH}}_{\mathit{c}2}$/dT=2.14±0.08 T/K, with ${\mathit{T}}_{\mathit{c}}$=18.7 K. The observed ${\mathit{H}}_{\mathit{c}2}$ at low temperatures exceeds the ${\mathit{H}}_{\mathit{c}2}$(T) curve from the standard theory by Werthamer-Helfand-Hohenberg. We discuss the role of flux motion and evidence for enhancement of ${\mathit{H}}_{\mathit{c}2}$ by intrinsic mechanisms such as strong coupling, Fermi surface anisotropy, and/or granularity of the samples.