The effects of space charge, dopants, and strain fields on surfaces and grain boundaries inYBCO compounds

Abstract

Statistical thermodynamical and kinetically-limited models are applied to study the origin andevolution of space charges and band-bending effects at low-angle [001] tilt grain boundaries inYBa2Cu3O7 and the effects of Ca doping upon them. Atomistic simulations, using shell models ofinteratomic forces, are used to calculate the energetics of various relevant point defects. Theintrinsic space charge profiles at ideal surfaces are calculated for two limits of oxygen contents,i.e. YBa2Cu3O6 andYBa2Cu3O7. At one limit,O6, the system is aninsulator, while at O7 it is a metal. This is analogous to the intrinsic and doping cases of semiconductors. The siteselections for doping calcium and creating holes are also investigated by calculating theheat of solution. In a continuum treatment, the volume of formation of doping calcium atY-sites is computed. It is then applied to study the segregation of calcium ions to grainboundaries in the Y-123 compound. The influences of the segregation of calcium ions onspace charge profiles are finally studied to provide one guide for understanding theimprovement of transport properties by doping calcium at grain boundaries in the Y-123compound.