Peculiarities of REBCO Films Growth on Single Crystalline Substrates
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Peculiarities of REBCO Films Growth on Single Crystalline Substrates Constantin G. Tretiatchenko, Victor S. Flis, Vassily L. Svetchnikov, and Volodymyr M. Pan G. V. Kurdyumov Institute for Metal Physics, 36 Vernadsky Blvd., Kiev, 03142, Ukraine ABSTRACT We suggest a model of mismatched interface and calculate its energy in order to describe formation of threading edge dislocations by the mechanism of rotational relaxation of interface stresses. The model takes into account strongly layered perovskite structure of high-temperature superconductors. We have shown that rotational relaxation occurs due to finite size of clusters and to non-equilibrium effect of the film growth. We have predicted the subgrain size and the expected rotation of domains depending on the lattice mismatch. The computed values are consistent with the observed YBCO film nanostructure. INTRODUCTION Out-of-plane edge dislocations in low-angle boundaries between subgrains (domains) are known to be the strongest pins in c-oriented epitaxial YBCO films [1-3]. This was proven by a model of perturbed vortex line lattice collective depinning in transverse magnetic field from such a statistical ensemble of linear defects. The model well quantitatively describes dependences of the critical current density on applied field and temperature Jc(H, T). The most convincing is that the size distribution and the mean misalignment angle of adjacent single-crystal subgrains can be extracted from measured Jc(H, T) by fitting to theoretical dependences calculated in this model. The results were confirmed by X-ray analysis. Structural investigations show that YBCO films usually consist of columnar domains separated with low-angle dislocation boundaries irrespective of a deposition technique and a substrate. This is due to high anisotropy of perovskites, in particular, highly anisotropic growth rate in different crystallographic directions. This results in that formation of first atomic layers does not determine a nanostructure of the whole film. However, many imperfections once formed reproduce themselves in upper layers of the film. Mismatch between the film and the substrate crystal lattices is the main source of defects. There are two essentially different ways to reduce the free energy of mismatched interface – by formation of misfit dislocations and by rotational relaxation of strains with formation of threading dislocations. In the first case atoms of the film are displaced longitudinally along the principal crystallographic direction. Just spacing between them becomes non-uniform. However, when chemical binding within the forming layer is much stronger than interaction between adatoms and the substrate, the film atoms may displace transversally. As a result, the film lattice rotates around the c-axis and divides into domains to reduce the free energy. We developed an analytical model in order to determine conditions, under which one or another mechanism takes place and to estimate characteristic size of the domains and angles between them, We suggested a hypoth
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