Strain-engineering of the charge and spin-orbital interactions in Sr2IrO4

TL;DR

This study demonstrates how epitaxial strain can manipulate the magnetic and charge excitations in Sr2IrO4, revealing the crucial role of lattice distortions in controlling its ground state properties.

Contribution

It provides experimental evidence of strain-induced tuning of magnetic interactions and charge gaps in Sr2IrO4 using RIXS, highlighting the impact of lattice distortions on spin-orbital physics.

Findings

Strain causes large softening and hardening of pseudospin-wave dispersion.

Charge gap size varies with compressive and tensile strain.

Lattice distortions significantly influence charge and spin excitations.

Abstract

In the high spin-orbit coupled Sr2IrO4, the high sensitivity of the ground state to the details of the local lattice structure shows a large potential for the manipulation of the functional properties by inducing local lattice distortions. We use epitaxial strain to modify the Ir-O bond geometry in Sr2IrO4 and perform momentum-dependent Resonant Inelastic X-ray Scattering (RIXS) at the metal and at the ligand sites to unveil the response of the low energy elementary excitations. We observe that the pseudospin-wave dispersion for tensile-strained Sr2IrO4 films displays large softening along the [h,0] direction, while along the [h,h] direction it shows hardening. This evolution reveals a renormalization of the magnetic interactions caused by a strain-driven crossover from anisotropic to isotropic interactions between the magnetic moments. Moreover, we detect dispersive electron-hole pair excitations which shift to lower (higher) energies upon compressive (tensile) strain, manifesting a reduction (increase) in the size of the charge gap. This behavior shows an intimate coupling between charge excitations and lattice distortions in Sr2IrO4, originating from the modified hopping elements between the t2g orbitals. Our work highlights the central role played by the lattice degrees of freedom in determining both the pseudospin and charge excitations of Sr2IrO4 and provides valuable information towards the control of the ground state of complex oxides in the presence of high spin-orbit coupling.