Abstract
AC susceptibility measurements were performed on Rb3C60 crystals. Two peaks are observed in the imaginary part of the susceptibility χ′′(T) and the peak near the transition temperature T c is more pronounced. This is interpreted in terms of weak links in the fullerene superconductors. The temperature associated with the intragranular χ′′ maximum T m exhibits extremely weak dependence on ac frequency f; T m shifts to higher temperature with increasing f and the effect is enhanced by increasing dc magnetic-field amplitude H dc or ac amplitude h ac. The inverse temperature 1/T m is found to depend roughly linearly on logarithmic frequency. This result is consistent with the prediction of the thermally activated flux creep model. This implies that the ac power dissipation in the Rb3C60 superconductors is predominantly due to hysteretic losses and therefore that magnetic flux is in the irreversible regime of the magnetic phase diagram. The thermal activation energy for flux lines U0 is estimated to be ≈6.1 eV at H dc=1 kOe and h ac=1 Oe. Finally, (1-T m/T c)3/2 is found to depend linearly on ln f.
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