X-ray magnetic circular dichroism originating from the $T_{z}$ term in collinear altermagnets under trigonal crystal field

TL;DR

This paper explores how XMCD signals can originate from the $T_{z}$ term in collinear altermagnets with trigonal crystal fields, despite zero net magnetization, highlighting the role of orbital symmetry and anisotropy.

Contribution

It identifies the microscopic origin of XMCD in altermagnets, emphasizing the $T_{z}$ operator and symmetry conditions under trigonal distortion, with theoretical spectral calculations.

Findings

XMCD can originate from the $T_{z}$ operator in altermagnets.

Symmetry analysis reveals conditions for finite XMCD response.

Theoretical spectra are provided for various $d^n$ configurations.

Abstract

We investigate the microscopic origin and spectral features of X-ray magnetic circular dichroism (XMCD) in collinear antiferromagnets with trigonal crystal fields, using $\alpha$-MnTe as a prototypical example. Although such systems exhibit zero net magnetization, we demonstrate that XMCD can emerge from the anisotropic magnetic dipole operator $T_{z}$, arising from quadrupolar spin distributions. By constructing a complete multipole basis and analyzing the symmetry conditions under trigonal distortion, we identify specific spin and orbital configurations that enable a finite XMCD response. Further, we employ both one-electron and multi-electron models, including spin-orbit coupling and Coulomb interactions, to calculate the XMCD spectra for various $d^n$ configurations. Our findings provide theoretical benchmarks for XMCD in altermagnets and highlight the key role of orbital symmetry and magnetic anisotropy in realizing observable dichroic effects.