Magnetic properties of superconducting ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ synthesized from large single-crystal fullerenes

Single crystals of ${\mathrm{C}}_{60}$ were doped with alkali metal (K and Rb), characterized, and measured magnetically in fields up to 5 T. Detailed experiments show the samples to be of high quality with superconducting transition widths on the order of 1 K. Data on the field and temperature dependences of the critical current density $J_C$ and $H_{C1\mathrm{}}$ are presented. We find that while the general behavior is similar to that of powder specimens, there are significant differences when comparing magnitudes. We attribute these differences to the fact that powder samples have much more surface area per volume than single crystals which can affect the determination of superconducting parameters. The critical current density also shows clear differences when compared to polycrystalline samples. In addition, comparisons between ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ indicate that defects may be more prevalent in the latter compound due to more sensitive synthesis requirements.