Magnetic-field and temperature dependence of the critical current in thin epitaxial films of the high-temperature superconductor YBa2Cu3O7−δ

The magnetic-field dependence (in a field H applied along the normal to the film plane z) and temperature dependence of the critical current density jc are investigated in high-quality biaxially oriented epitaxial films of YBa2Cu3O7−δ, with maximum values of jc reaching 2×106 A/cm2 in zero magnetic field at T=77 K. It is found that the value of jc(Hz,T), which is independent of Hz in the low-field region Hz Hm by the function jc(Hz,T)/jc(0,T)=α ln(H*/Hz) over a rather wide region 0.95>jc(Hz,T)/jc(0,T)>0.3. Here H*=Hme1/α is proportional to τ=1−T/Tc, at least in the temperature region where the measurements are made, and the parameter α is nearly independent of temperature. For the sample with the highest value of the critical current density for Hz→0 an anomalously sharp transition from the low-field plateau to the logarithmic behavior is observed. Some published data on jc(Hz,T) in epitaxial thin films of HTSCs are analyzed, and it is shown that an approximation of the give...