Voltage-current characteristics in c-axis-oriented Bi_1.8Pb_0.4Sr₂Ca₂Cu₃O/Ag tapes

Detailed measurements of the voltage-current (V-I) characteristics for c-axis-oriented Bi_1.8Pb_0.4Sr₂Ca₂Cu₃O_y (Bi(2:2:2:3))/Ag tapes have been performed in a low-magnetic field range. The results show that the V-I curves are determined by a thermally activated flux creep process and possess a power-law behaviour. The activation energy U_J derived from the V-I curves was found to follow a (1-T/T_c)ⁿ/H^0.15 functional relationship, n being 0.37 for H//ab and 0.57 for H//c. In contrast with the (1-T/T_c)³2//H theoretical prediction, this result shows very weak field and temperature dependences of U_J in the low-field region. A small negative curvature of log V versus log I plots near the temperature T_m at which the V-I curves change from being strictly linear to being nonlinear can also be found. This observation was tested using vortex glass (VG) or collective pinning (CP) models. The results showed that the VG or CP descriptions fail probably because of the high current density used in the measurement. We think that the negative curvature can be interpreted from the possible existence of the crossover from flux creep to flux flow.