Investigations of the zero-field (a,b)-plane conductivity of YBa2Cu3O7-delta near the critical temperature

The resistive transition R(T) and the current-voltage characteristics (I-V) in the range 10(-9)-10(-6) V and close to the critical temperature (T-c) w ere measured in a single-grain of melt-textured YBa2Cu3O7-delta (YBCO) and in a detwinned YBCO single-crystal, with the transport current parallel to the (a,b) planes and in zero external magnetic field using a SQUID picovolt meter. In the case of the melt-textured sample, we measured the I-V curves also at higher levels of dissipation. For T < T-c, we interpreted the lower part of the I-V curves in the framework of the Jensen-Minnhagen model of c urrent-induced unbinding of thermally-created vortex-antivortex pairs, whic h leads to V proportional to I(I-I-c1)(a-1). Our results confirm a recently proposed current-temperature (I-T) phase diagram by S.W. Pierson. In addit ion, we determined for the first time the dependence of the characteristic (unbinding) current I-c1 with t = 1 - T/T-c for temperatures very close to T-c and low transport currents. (C) 1998 Published by Elsevier Science B.V. All rights reserved.