Alteraxial Phonons in Collinear Magnets

TL;DR

This paper classifies axial phonons in collinear magnets based on symmetry, revealing new alteraxial phonons with higher-order angular momentum patterns and predicting their presence in many magnetic materials.

Contribution

It introduces a symmetry-based classification of axial phonons, including the novel alteraxial category with higher-order wave PAM, expanding understanding of phonon-magnetic interactions.

Findings

Classified axial phonons into ferroaxial, antiferro-nonaxial, and alteraxial categories.

Discovered alteraxial phonons with p- to j-wave PAM patterns.

Predicted hundreds of materials hosting alteraxial phonons.

Abstract

Axial phonons, carrying angular momentum through rotational lattice vibrations, offer a promising platform for exploring phonon-magnetic coupling effects. However, how the interplay of lattice and magnetism determine the phonon angular momentum (PAM) of axial phonons remains elusive. Here, based on magnetic point group theory, we establish a symmetry framework to classify phonons in collinear magnets (e.g. ferromagnets, antiferromangets, altermagnets) into three distinct categories: ferroaxial, antiferro-nonaxial, and alteraxial phonons, which are distinguished by their different PAM patterns. Beyond the ferroaxial phonons featuring $s$-wave PAM, we reveal a complete series of alteraxial phonons, characterized by higher-order-wave PAM patterns ranging from $p$- to $j$-wave. Notably, alteraxial phonons are not limited to altermagnets, but also emerge in ferromagnets and antiferromagets. Our high-throughput search predicts hundreds of candidate magnetic materials hosting alteraxial phonons. Ab initio calculations on representative magnets further confirm the existence and distinct symmetry-enforced nodal structures of PAM in alteraxial phonons. Our work provides a complete classification for axial phonons in collinear magnetic systems and paves the way for engineering magneto-phononic phenomena.