Symmetry classification of magnetic octupole current based on multipole representation theory

TL;DR

This paper develops a symmetry-based framework to classify magnetic octupole currents, distinguishing them from spin currents, and demonstrates how symmetry breaking activates these currents through theoretical and microscopic calculations.

Contribution

It introduces a multipole representation of the magnetic octupole conductivity tensor and classifies its symmetry-allowed components across all crystallographic point groups.

Findings

Magnetic octupole currents are classified by their symmetry properties.

Symmetry lowering activates magnetic octupole conductivities.

Distinct symmetry signatures differentiate MO currents from spin currents.

Abstract

Magnetic octupole (MO) currents have recently attracted significant attention as a driving force for the Neel vector dynamics in d-wave altermagnets, a new class of antiferromagnets that exhibit nonrelativistic spin-split band structures. From a symmetry perspective, the MO includes an axial-dipole component analogous to that of the spin, making it essential to clarify how MO currents differ from spin currents. We here investigate the correspondence between MO conductivities and electronic multipoles, which provide a unified and powerful framework for symmetry analysis. We derive the multipole representation of the rank-five MO conductivity tensor and classify its symmetry-allowed components for all crystallographic point groups, in direct comparison with spin conductivity. We show that time-reversal-even electric-type multipoles give rise to the dissipationless MO current, whereas time-reversal-odd magnetic-type multipoles generate dissipative MO current under an applied electric field. Complementing this macroscopic analysis, the linear-response calculations for a microscopic tight-binding model demonstrate how MO conductivities are activated by symmetry lowering, exemplified by the symmetry reduction from Oh to Th. Our results elucidate the symmetry distinctions between MO currents and spin currents, and provide insights into their experimental identification.