Altermagnetic boosting of chiral phonons

TL;DR

This paper demonstrates that helical spin order in chiral materials can significantly enhance chiral phonons via an altermagnetic effect, revealing a new method to amplify phonon chirality through magnetic symmetry breaking.

Contribution

It introduces the concept of altermagnetic boosting of chiral phonons in helical spin states, linking magnetic chirality with phonon behavior in a novel way.

Findings

Tenfold enhancement of chiral-phonon coupling in helical spin state

Identification of altermagnetic effect as a mechanism for boosting chiral phonons

Connection between magnetic chirality and phonon amplification

Abstract

Chirality characterizes the asymmetry between a structure and its mirror image and underlies a wide range of chiral functionalities. In crystallographically chiral materials, phonons with non-zero linear momentum $\textbf{k}$ can acquire a $k$-induced longitudinal magnetization, giving rise to chiral phonons. Helical spin order, with its proper screw-type configuration, breaks all mirror symmetries and therefore carries magnetic chirality. Such helical spins also generate non-relativistic spin splitting for any quasiparticle excitations propagating along the screw axis. To explore the possible connection between chiral phonons and magnetic chirality, we investigated the crystallographically polar and chiral compound (Mn,Ni)$_3$TeO$_6$, which hosts three distinct states: a paramagnetic state, a helical spin state with magnetic chirality, and a collinear spin state without magnetic chirality. We find an approximately tenfold enhancement of chiral-phonon coupling in the helical spin state along the screw axis, compared with both the paramagnetic and collinear spin states. These results identify a new route to amplify chiral phonons through an altermagnetic effect arising from the broken parity-time symmetry in helical spins. %from non-relativistic spin splitting.