FLUX DYNAMICS IN RB3C60 CRYSTALS

We report the results of ac susceptibility measurements on Rb3C60 full erene superconductor prepared with C-60 crystals. The temperature, T-i rr, of the maximum in chi'' shifts to high temperature with increasing ac magnetic-field frequency f. We fit the irreversibility line (IL) T -irr (H-dc) with the scaling relation H-dC = H-irr(0)(1-T-irr/T-o)(n) and obtain n = 1.5 and T-0 = 30.3 K for the 10 kHz IL near T-c. We fin d a roughly lin ear dependence on logarithmic f for 1/T-irr. By compar ing the results to the expected frequency responses for flux pinning a nd flux flow, the observed frequency dependence of T-irr is interprete d by thermally activated flux creep in the fullerene.