Enhanced orbital anisotropy through the proximity to a SrTi O 3 layer in the perovskite iridate superlattices

We have used angle-dependent soft x-ray absorption spectroscopy (XAS) at the O $K$ edge and first-principles calculations to investigate the electronic structures of iridate-based superlattices ${(\mathrm{SrIr}{\mathrm{O}}_{3})}_{m}/(\mathrm{SrTi}{\mathrm{O}}_{3})$ ($m=1$, 2, 3, and \ensuremath{\infty}). We focus on the pre-edge Ir $5d {t}_{2g}--\mathrm{O} 2p$ orbital hybridization feature in the XAS spectra. By varying the measurement geometry relative to the incident photon polarization, we are able to extract the dichroic contrast and observe the systematic increase in the anisotropy of Ir $5d$ orbitals as $m$ decreases. First-principles calculations elucidate the orbital anisotropy coming mainly from the enhanced out-of-plane compression of $\mathrm{Ir}{\mathrm{O}}_{6}$ octahedra in the $\mathrm{SrIr}{\mathrm{O}}_{3}$ layers that are adjacent to the inserted $\mathrm{SrTi}{\mathrm{O}}_{3}$ layers. As $m$ decreases, the increased volume fraction of these interfacial $\mathrm{SrIr}{\mathrm{O}}_{3}$ layers and their contact with the $\mathrm{SrTi}{\mathrm{O}}_{3}$ layers within the ${(\mathrm{SrIr}{\mathrm{O}}_{3})}_{m}/(\mathrm{SrTi}{\mathrm{O}}_{3})$ supercell lead to enhanced orbital anisotropy. Furthermore, the tilt and rotation of $\mathrm{Ir}{\mathrm{O}}_{6}$ octahedra are shown to be essential to understand the subtle orbital anisotropy in these superlattices, and constraining these degrees of freedom will give an incorrect trend. Our results demonstrate that the structural constraint from the inserted $\mathrm{SrTi}{\mathrm{O}}_{3}$ layers, in addition to other electronic means such as polar interface and charge transfer, can serve as a knob to control the orbital degree of freedom in these iridate-based superlattices.