MAGNETIC-PROPERTIES OF SUPERCONDUCTING K3C60 AND RB3C60 SYNTHESIZED FROM LARGE SINGLE-CRYSTAL FULLERENES

Single crystals of C-60 were doped with alkali metal (K and Rb), chara cterized, and measured magnetically in fields up to 5 T. Detailed expe riments show the samples to be of high quality with superconducting tr ansition widths on the order of 1 K. Data on the field and temperature dependences of the critical current density J(C) and H-Cl are present ed. We find that while the general behavior is similar to that of powd er specimens, there are significant differences when comparing magnitu des. We attribute these differences to the fact that powder samples ha ve much more surface area per volume than single crystals which can af fect the determination of superconducting parameters. The critical cur rent density also shows clear differences when compared to polycrystal line samples. In addition, comparisons between K3C60 and Rb3C60 indica te that defects may be more prevalent in the latter compound due to mo re sensitive synthesis requirements.