Magnon topology driven by altermagnetism

TL;DR

This paper explores how altermagnetism induces topologically nontrivial magnon bands with finite thermal Hall effects, revealing new magnetic topological phases driven by unique symmetries.

Contribution

It introduces a two-dimensional altermagnetic spin model demonstrating how symmetry-driven magnon band splitting leads to topological magnons with observable thermal Hall effects.

Findings

Magnon bands become fully gapped and topologically nontrivial due to altermagnetism.

Finite thermal Hall conductivity with a characteristic T^4 scaling at low temperatures.

Presence of topologically protected chiral edge modes.

Abstract

Altermagnets present a class of fully compensated collinear magnetic order, where the two sublattices are not related merely by time-reversal combined with lattice translation or inversion, but require an additional lattice rotation. This distinctive symmetry leads to a characteristic splitting of the magnon bands; however the splitting is only partial -- residual degeneracies persist along certain lines in the Brillouin zone as a consequence of the underlying altermagnetic rotation. We consider a two-dimensional $d$-wave altermagnetic spin model on the checkerboard lattice and introduce additional interactions such as an external magnetic field and Dzyaloshinskii-Moriya interactions, that lift these degeneracies. The resulting magnon bands become fully gapped and acquire non-trivial topology, characterized by nonzero Chern numbers. We demonstrate the crucial role of altermagnetism for the generation of the Berry curvature. As a direct consequence of the topological magnons, we find finite thermal Hall conductivity $\kappa_{xy}$, which exhibits a characteristic low-temperature scaling, $\kappa_{xy}\propto T^4$. Moreover, $\kappa_{xy}$ changes sign under reversal of the magnetic field, exhibiting a sharp jump across zero field at low temperatures. We also demonstrate topologically protected chiral edge modes in a finite strip geometry.