Characterisation of YBCO surfaces and STO/YBCO interfaces using DFT

Abstract

The focus of this project was to investigate the surface structure of the high temperature superconductor YBa2Cu3O7 (YBCO) as well as determining the interface structure when YBCO is capped with a thin film of SrTiO3 (STO). The motivation for STO capping is to see if it can protect YBCO from degradation losing its superconductivity. Here surface and interface terminations were studied with a literature review as well as calculations based on density functional theory (DFT). Together with this, a method of determining the surface energy contribution of a slab structure was used which found the CuO2(Y) surface and the BaO(CuO) surface as most probable surface terminations of YBCO. The change in electron density was also studied that showed a rearrangement of charges at the interface in the STO/YBCO systems. The STO capping itself together with electronic redistribution at the interface made the top YBCO layers become more bulk-like, and this can be a part of the explanation how STO can protect the YBCO surface.