Atomic and electronic structure of the YBa₂ Cu₃ O₇ / SrTiO₃ interface from first principles

The atomic structure, adhesion energy, and bonding nature of the YBa ₂ Cu₃ O₇ (001) / SrTiO₃ (001) interface are systematically investigated from first principles. A total of 48 candidate interface geometries, which involve two SrTiO₃ terminations, six YBa₂ Cu₃ O₇ terminations, and four highly symmetric stacking sequences, were considered. Although the SrTiO₃ substrate has a negligible effect on the atomic structure of optimal TiO₂-terminated interface, its electronic influence is significant: the electronic states of interfacial O of YBa₂ Cu ₃ O₇ resemble those of O in bulk SrTiO₃ rather than bulk YBa₂ Cu₃ O₇. Consequently, the interfacial O of YBa₂ Cu₃ O₇ can be reasonably regarded as a natural extension of substrate layer across the interface. However, the effect of substrate on the optimal SrO-terminated interface is twofold characterized atomically by straightening the originally zigzag interfacial YBa₂ Cu₃ O₇ layer and electronically by altering noticeably the density of states projected on this layer. In addition, the substrate is found to affect the interface chemistry via tuning effectively the first monolayer of the YBa₂ Cu₃ O₇ film.The preferred geometries (i.e., having the largest adhesion energy) are those that maintain the substrate perovskite stacking across the interface. By applying several methods of analysis, we thoroughly characterized electronic structure and determined that interfacial bonding is mainly ionic, yet maintains a small degree of covalency.