Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers

TL;DR

This paper demonstrates how interlayer charge imbalance and ligand distortions can be used to control the electronic level sequence in layered transition-metal oxides, revealing unexpected $d$-orbital configurations in Sr$_2$IrO$_4$.

Contribution

It provides experimental and theoretical evidence of $d$-level inversion in Sr$_2$IrO$_4$, challenging canonical ligand-field predictions and highlighting the potential for orbital engineering in nonpolar oxide layers.

Findings

Electron spin resonance shows $g_{ ext{parallel}}>2$ in Sr$_2$IrO$_4$.

Electronic-structure calculations confirm $d$-level inversion in Sr$_2$IrO$_4$.

Ba$_2$IrO$_4$ exhibits normally ordered $d$-levels.

Abstract

A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the $d$-level structure of layered Sr$_2$IrO$_4$ by electron spin resonance. While canonical ligand-field theory predicts $g_{\parallel}$-factors $\!<\!2$ for positive tetragonal distortions as present in Sr$_2$IrO$_4$, the experiment indicates $g_{\parallel}\!>\!2$. This implies that the iridium $d$ levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr$_2$IrO$_4$, whereas we find them in Ba$_2$IrO$_4$ to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore $d$-orbital reconstruction in the context of oxide electronics.