DIFFERENTIAL RESISTANCE AND CRITICAL-CURRENT DISTRIBUTION IN YBA2CU3O7-X CERAMICS

V-I curves of several Y-Ba-Cu-O samples have been measured in the temperature range 7791 K in weak magnetic fields (H<150 Oe). For the well prepared low-resistivity samples Rf==dV/dI reaches a value of about 0.1Rn (Rn is the normal resistance just above transition) in the linear V-I regime. This is consistent with a picture of decoupled superconducting grains and does not yield any direct information about the flux-flow effects. Because of self-field effects, critical-current density (Jc) obtained from transport (and magnetization) measurements does not provide clear insight on the nature of intergrain coupling. Information on the distribution of critical currents, obtained from d2V/dI2, supports the tunneling and/or proximity-effect coupling between the grains and seems to rule out large enhancements of Jc in sintered samples with nonoriented grains. © 1990 The American Physical Society.