Twinning studies on YBCO thin films.

摘要

The tetragonal to orthorhombic phase transition in the high temperature superconductor YBa2Cu3O7-delta (YBCO) gives rise to the formation of twins. In the thin film form, these twin structures are highly influenced by the confining effects of the substrate. As a result, the choice of substrate can determine the degree of microtwinning in these thin films. This thesis examines the entire spectrum of twinning phenomenon as it pertains to thin films. There is heavy microtwinning in YBCO deposited on substrates that present a cubic deposition surface such as LaAlO3 , LaSrGaO4, SrTiO3, MgO, and (1 1 0) NdGaO 3.* There is preferential twinning in YBCO deposited on the (0 0 1) NdGaO3 substrate, which is the result of a non-orthogonal pseudo-cubic deposition surface. Finally, we present a method for altering the twinning structures of YBCO thin films using air bridge structures, where a portion of the YBCO film is removed from the confining effects of underlying substrate. Using this method, YBCO thin films can be produced that exhibit macrotwins or that are mostly detwinned along the length of the bridge. The degree of detwinning is largely determined by the differential thermal expansion between the film and the underlying substrate. Taken together, the films studied in this work span the entire range of twinning phenomena extending from a heavily microtwinned state to a state where the material is almost completely detwinned. Covering this entire range has enabled a study of the effect of twins upon the local transport properties including those associated with an isolated twin boundary. These results, obtained using laser scanning microscopy and conventional transport measurements, indicate that the twins play a relatively minor role in the transport properties. They do not significantly influence the level of dissipation in a film, nor do they act as pinning sites to the extent that they are able to significantly increase the superconductor's critical current density. Furthermore, the laser scanning microscopy measurements reveal that, not only can YBCO air bridges be produced that are nearly twin free, but the detwinning procedure also removes the dense network of grain boundaries, thus yielding a more structurally perfect YBCO thin film.; *(1 1 0) NdGaO3 presents an almost cubic deposition surface.