We perform a systematic study of the time evolution of the transport critical current in polycrystalline samples of the high temperature superconducting system $\left({\mathrm{Hg}}_{1-x}{\mathrm{Re}}_x\right){\mathrm{Ba}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+\delta }$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ after application and removal of an external magnetic field $H_m.$ Within our time, temperature, and remanent field windows, the transport critical current increases logarithmically in time. The relaxation rates in the range 80–115 K decrease with increasing temperature at a fixed $H_m,$ while temperature-dependent maxima are observed in the relaxation rate versus $H_m$ plots. These experimental results are reproduced by a phenomenological model applicable to any high-$T_c$ polycrystals. In the model, the time increase of the transport current is determined by the effective field at the intergrain junctions, which relaxes in time due to the flux creep of the intragrain magnetization.

• Received 3 December 1998

DOI:https://doi.org/10.1103/PhysRevB.60.3673