Microwave studies of the superconducting state in Rb3C60

Methods of electron-spin resonance (ESR) and direct, non-linear field-modulated microwave absorption (FMMA) were applied for the measurements in low- and high-purity samples of rubidium-doped fullerene, Rb3C60. The coexistence of the normal strong ESR signals and regular series of weak absorption lines similar to those seen in systems of Josephson junctions was observed in the high-purity sample. The possible influence of the vortex lattice on the ESR signals was also studied. We determined from FMMA investigations using the Portis model the critical field mu(0)H* = 40 mu T, the depinning current density J(c)*(mu(0)H(0) = 1 mT) approximate to 4 x 10(8) A/m(2) in low magnetic field and J(c)*(mu(0)H(0) > 100 mT) approximate to 1.6 x 10(8) A/m(2) in higher fields. These values were generally one order of magnitude higher than the highest corresponding values previously observed in high-temperature superconductors (HTS's).