Resistive transition of crystalline Y₁Ba₂Cu₃O_7-x in magnetic fields

Transport and a.c. susceptibility data on a crystalline sample of Y1Ba2Cu3O7-x in fields up to 9 T along the c-axis are presented. Careful study of the I-V curves and the frequency dependent a.c. susceptibility indicate that the critical current develops suddenly at a line in the H-T phase diagram that coincides with the irreversibility line as measured by a.c. susceptibility. Above this line, which is interpreted as the melting of a pinned vortex regime, the I-V curves are linear and the resistance has a flux flow like character. However, the R versus T behaviour in the melted region supports the interpretation of two different regions of flux flow viscosity. © 1990.