Magnetic Excitations in Square Lattice Iridates: Contrast between Ba${}_2$IrO${}_4$ and Sr${}_2$IrO${}_4$

Abstract

We report a resonant inelastic x-ray scattering (RIXS) investigation of ultra-thin epitaxial films of Ba${}_2$IrO${}_4$, and compare their low energy magnetic and spin-orbit excitations to those of their sister compound Sr${}_2$IrO${}_4$. Due to the 180${}^{\circ }$ Ir-O-Ir bond, the bandwidth of the magnon and spin-orbiton is significantly larger in Ba${}_2$IrO${}_4$, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion observed in Sr${}_2$IrO${}_4$ is absent in Ba${}_2$IrO${}_4$, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba${}_2$IrO${}_4$ are substantially different from those of Sr${}_2$IrO${}_4$, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.

7 pages, 4 figures