CRITICAL-CURRENT DENSITIES IN K3C60 RB3C60 POWDERS DETERMINED FROM AC/DC SUSCEPTIBILITY MEASUREMENTS/

We report the magnetic J(c)(T) in K3C60 and Rb3C60 powder samples for temperatures from 5 K to T-c. J(c) near T-c was obtained from AC measu rements whereas J(c) at low temperatures was obtained from DC measurem ents. In the AC measurements, two chi '' peaks were observed on some o f the samples. This is attributed to weak links in granular supercondu ctors. The intragranular J(c) is much larger than the intergranular J( c). The J(c)(T) data are fitted by J(c)(T) = J(c)(0)(1 - T/T-c)(gamma) , with J(c)(0) and gamma as fitting parameters. In the DC measurements on Rb3C60, J(c) depends approximately linearly on temperature. Extrap olating the zero-field DC data to 0 K yields J(c)(0)= 3.1 x 10(6) A/cm (2). This is a rather large J(c) for a powdered superconductor, sugges ting strong pinning in these materials. Finally, the values of gamma f or the intergranular and intragranular components are compared with th e predictions of theories.