Critical magnetic fields of superconducting ${\mathrm{Ba}}_6$${\mathrm{C}}_{60}$

We have measured the upper and lower critical fields of superconducting ${\mathrm{Ba}}_6$${\mathrm{C}}_{60}$. Near ${\mathit{T}}_{\mathit{c}}$, from the slope of the measured upper critical field, ${\mathit{dH}}_{\mathit{c}2}$/dT=0.45 T/K, we obtain the zero-temperature Ginzburg-Landau coherence length as ${\xi }_{\mathrm{GL}}$=120 Å. Using the lower critical-field data, the penetration depth at T=0 is estimated to be λ(0)=1800 Å. Combining λ(0) with the value of ${\xi }_{\mathrm{GL}}$, we estimate the Fermi energy ${\mathit{E}}_{\mathit{F}}$ to be =0.15 eV. Thus, in comparison with the alkali-metal-doped fullerenes, the superconducting parameters ${\mathit{H}}_{\mathit{c}2}$, ${\xi }_{\mathrm{GL}}$, and the resulting Ginzburg-Landau parameter κ, are significantly different in the case of the superconducting Ba-doped fullerene.