Electronic structure of epitaxial perovskite films in the two-dimensional limit: Role of the surface termination

An often-overlooked property of transition metal oxide thin films is their microscopic surface structure and its effect on the electronic properties in the ultrathin limit. Contrary to the expected conservation of the perovskite stacking order in the (001) direction, heteroepitaxially grown SrIrO₃ films on TiO₂-terminated SrTiO₃ are found to exhibit a terminating SrO surface layer. The proposed mechanism for the self-organized conversion involves the adsorption of excess oxygen ions at the apical sites of the IrO₂-terminated surface and the subsequent decomposition of the IrO₆ octahedra into gaseous molecular IrO₃ and the remaining SrO-terminated surface. Whereas the ab initio calculated electronic structure of SrO-terminated SrIrO₃ in the monolayer limit exhibits a striking similarity to bulk $Sr2IrO4$⁠, the broken octahedral symmetry at the IrO₂-terminated surface would mix the otherwise crystal field split e_g and $t2g$ states, resulting in distinctly different low-energy electronic states.