Thickness dependence mechanisms of the critical current density in high-Tccuprate superconductor films

The obtained electron backscattering diffraction data and high-resolution transmission electron microscopy have shown that the nanostructure of epitaxial HTS YBCO films evolves essentially with the film thickness. This happens due to a high deposition temperature and a high dislocation mobility that induce polygonization, dislocation rearrangement and a remarkable reduction of the out-of-plane dislocation density. The analysis of experimental thickness dependences of the critical current in the framework of existing pinning models leads to the conclusion that the c -axis-correlated pinning by out-of-plane edge dislocations plays a dominant role. Thus, the evolution of the dislocation nanostructure is responsible for the critical current density reduction. Embedding of nanoparticles into HTS films should be useful only if they are coherently coupled with the matrix and extra dislocations are being formed during the film growth. The nanoparticles just preserve a high density of dislocations. Another effect is self-assembling of nanoparticles within dislocation cores forming a 'bamboo structure'. This phenomenon may result in an enlargement of the dislocation normal core and an essential increase of the elementary pinning force.