Effect of Subgrain Structure on the Critical Current Density in HTS Films

Abstract

We have suggested a model of film–substrate interface and calculated its energy. The calculations have shown that rotational relaxation occurs due to the finite size of clusters and to the non-equilibrium effect of the film growth. We have predicted the real subgrain size and the expected rotation of domains. In some kinds of high temperature superconducting films, such as the mostly used YBCO, domains can be subdivided into twin domains. The twin structure has smaller scale since it is being formed at tetra-ortho transition during cooling and/or oxygen annealing when other defects are already formed in tetra-phase during deposition. Usually, twin boundaries have no strong influence on the critical current because they introduce almost no perturbation in the superconducting order parameter. We have shown that when a strain is applied, a difference between critical temperatures of a- and b-oriented twin domains appears and results in an additional pinning and unusual reversible strain dependence of critical current. The suggested model explains experimental strain dependences of the critical current.