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

Abstract

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, is well approximated at 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 given form is also applicable to the results of other authors. Although the value of jc(H=0,τ) itself, like the values of H*, are substantially different for films investigated by different authors, the values of α obtained when their results are approximated by a logarithmic dependence are extremely close to one another and to the values found in our studies. A model is proposed which qualitatively explains the nature of the observed behavior of jc(Hz,T), and the basic properties of this model are discussed. It is shown that the approximately logarithmic dependence is due to a mechanism involving the depinning of the ensemble of Abrikosov vortices pinned at edge dislocations in the low-angle grain boundaries that exist in epitaxial thin films of HTSCs. A sharp transition from the plateau to the logarithmic segment is realized if at H≈Hm there is a change of the mechanism governing the critical current density, specifically, in the case when for H<Hm the value of jc is limited more strongly by the transparency of the grain boundaries to the superconducting current than by the depinning of vortices in low fields.