INTERGRAIN AND INTRAGRAIN AC RESPONSE OF THE GRANULAR SUPERCONDUCTORS K3C60 AND RB3C60

In the present AC susceptibility study we report chi(T) measurements of several granular fullerides as a function of temperature and applied alternating and static magnetic field. Clear experimental evidence for the existence of inter- and intragrain dissipation mechanisms in these compounds is found and the remarkable influence of the preparation conditions is demonstrated. The results are interpreted within the framework of Bean's critical state model and critical current densities are estimated. Effects due to the finite size of the crystallites (compared to the penetration depth) are also discussed in connection with the superconducting volume fraction. In order to investigate B(c2)(T) of the single grains and of the coupling components, the chi(T) curves are analyzed in external static magnetic fields. B(c2)(0) and the Ginzburg-Landau coherence length xi(GL) are also evaluated by use of the WHH theory.