Ac susceptibility of Rb3C60 fine powder

The temperature dependence of ac susceptibility χ(T) has been measured at different values of dc magnetic field for Rb3C60 fine powder. The diamagnetic moment of the superconducting powder arises due to both screening currents in separate grains (intragrain contribution) and screening currents in clusters formed by several grains with Josephson junctions between them (intergrain contribution). The intergrain contribution of clusters is proportional to the critical current of the Josephson junctions, jc, and decreases rapidly in a dc magnetic field H, while the intragrain contribution remains unchanged up to much higher values of H. The χ(T) measurements at different dc magnetic fields allow us to distinguish between these two contributions and to obtain precisely the temperature dependence of the London penetration depth λ(T). We have found that the experimentally obtained λ(T) is perfectly described by the weak coupling BCS dependence in the dirty limit. copyright 1996 The American Physical Society