Lower critical fields of alkali-metal-doped fullerene superconductors

We report on the lower critical field of fullerene superconductors derived from a "trapped magnetic moment" method. Superconducting ${\mathrm{K}}_3$${\mathrm{C}}_{60}$, ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$, and Rb${\mathrm{Cs}}_2$${\mathrm{C}}_{60}$ crystals and powders with shielding fractions between 3 and 100% were investigated. In contrast to all previous investigations made by other methods, the values of $H_{c1\mathrm{}}$ are much smaller and of the order of 1 mT at $T=0\mathrm{}$. We show that the smallness of $H_{c1\mathrm{}}$ is not related to Josephson junctions and granularity, but is an intrinsic property of the fullerenes.