Circular dichroism in resonant inelastic x-ray scattering from birefringence in CuO

TL;DR

This paper demonstrates how birefringence influences circular dichroism signals in resonant inelastic x-ray scattering from CuO, highlighting the importance of accounting for optical anisotropies in analyzing symmetry-broken quasiparticles.

Contribution

It introduces a model incorporating birefringence effects into RIXS scattering amplitudes, enabling accurate interpretation of dichroic signals in anisotropic materials like CuO.

Findings

Significant CD observed in CuO RIXS spectra.

Birefringence effects can be modeled with single-ion calculations.

Proper treatment is essential for identifying symmetry-broken quasiparticles.

Abstract

Resonant inelastic x-ray scattering (RIXS) has become a prominent technique to study quasiparticle excitations. With advances in polarization analysis capabilities at different facilities, RIXS offers exceptional potential for investigating symmetry-broken quasiparticles like chiral phonons and magnons. At optical wavelengths birefringence can severely affect polarization states in low-symmetry systems. Here we show its importance for soft x-ray resonances. Given the growing interest in Circular Dichroism (CD) in RIXS, it is important to evaluate how birefringence may affect the RIXS spectra of anisotropic systems. We investigate CuO, a well-known anisotropic material, using Cu $L_3$-edge RIXS and detect significant CD in both magnetic and orbital excitations in the collinear antiferromagnetic phase. We demonstrate that the CD can be modeled by a proper treatment of RIXS scattering amplitudes derived from single-ion calculations with birefringence. Recognizing these effects is crucial for unambiguous identification of subtle dichroic effects induced by symmetry-broken quasiparticles. Furthermore, the combined sensitivity of RIXS and birefringence to local symmetry presents an opportunity to study microscopic changes driven by external perturbations.