Upper-critical-field–temperature phase diagram of alkali-metal-intercalatedC60superconductors

The temperature dependences of the upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) were measured over a broad range for both ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ superconductors. We found ${\mathit{H}}_{\mathit{c}2}$(0) to be 28 T for ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and 38 T for ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ corresponding to the Ginzburg-Landau coherence length of 34 and 30 Å. The H-T phase diagrams do not show qualitative difference from the behavior of a conventional type-II superconductor.