Coexistence of insulator-like paramagnon and metallic spin-orbit exciton modes in SrIrO$_3$

TL;DR

This study reveals the coexistence of insulator-like paramagnon and metallic spin-orbit exciton modes in SrIrO$_3$, showing persistent antiferromagnetic fluctuations and charge excitations in a semimetallic state through advanced RIXS measurements.

Contribution

It demonstrates the simultaneous presence of contrasting magnetic and charge excitations in SrIrO$_3$, highlighting their intrinsic nature in the ground state of this spin-orbit-coupled semimetal.

Findings

Antiferromagnetic paramagnon persists in the metallic regime.

Spin-orbit exciton shows inverted dispersion and doubled periodicity.

Both excitations are intrinsic to SrIrO$_3$'s ground state.

Abstract

We probe the spectrum of elementary excitations in SrIrO$_3$ by using heterostructured [(SrIrO$_3$)$_m$/(SrTiO$_3$)$_l$] samples to approach the bulk limit. Our resonant inelastic x-ray scattering (RIXS) measurements at the Ir $L_3$-edge reveal a robust low-lying collective magnetic mode with an antiferromagnetic (AF) dispersion similar to the insulators Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$, albeit with a large gap and much larger linewidth. At higher energies we find the spin-orbit exciton, also strongly broadened, but with an inverted dispersion and doubled periodicity that are controlled by the charge hopping. These results demonstrate that the AF paramagnon persists, somewhat counterintuitively, far into the metallic regime of the insulator-metal transition driven by the degree of confinement in the heterostructure. We conclude that these two excitations, which are contrasting but coexisting hallmarks of strong AF pseudospin and charge fluctuations in a spin-orbit-coupled Mott-Slater material, are properties intrinsic to the ground state of semimetallic perovskite SrIrO$_3$.