Intra- and inter-orbital correlated electron spin dynamics in $\rm Sr_2 Ir O_4$: spin-wave gap and spin-orbit exciton

TL;DR

This paper investigates the spin dynamics and excitations in Sr2IrO4, revealing how spin-orbit coupling and Hund's interactions influence magnetic anisotropy, spin-wave gaps, and spin-orbit excitons, aligning with experimental RIXS data.

Contribution

The study introduces a pseudo-orbital basis formalism to analyze spin dynamics in Sr2IrO4, explicitly demonstrating the effects of Hund's coupling and spin-orbit interactions on magnetic excitations.

Findings

Hund's coupling causes easy-plane anisotropy with distinct gapless and gapped modes.

Two propagating spin-orbit exciton modes are identified with energies matching RIXS measurements.

The formalism accurately reproduces experimental spin-wave and exciton dispersions.

Abstract

Transformation of Coulomb interaction terms to the pseudo-orbital basis constituted by $J=1/2$ and $3/2$ states arising from spin-orbit coupling provides a versatile tool. This formalism is applied to investigate magnetic anisotropy effects on low-energy spin-wave excitations as well as high-energy spin-orbit exciton modes in $\rm Sr_2 Ir O_4$. The Hund's coupling term explictly yields easy-plane anisotropy, resulting in gapless (in-plane) and gapped (out-of-plane) modes, in agreement with recent resonant inelastic x-ray scattering (RIXS) measurements. The collective mode of inter-orbital, spin-flip, particle-hole excitations with appropriate interaction strengths and renormalized spin-orbit gap yields two well-defined propagating spin-orbit exciton modes, with energy scale and dispersion in good agreement with RIXS studies.