Topologically Protected Surface Altermagnetism on Antiferromagnets

TL;DR

This paper demonstrates that surface altermagnetism in antiferromagnets can be topologically protected, leading to robust surface magnetic effects detectable via spin-resolved spectroscopy, expanding the understanding of unconventional magnetism.

Contribution

The authors establish symmetry conditions for topologically protected surface altermagnetism and provide models, including an example with CuMnAs, highlighting a new platform for robust surface magnetic phenomena.

Findings

Surface altermagnetism can be topologically protected.

Spin spectral density shows d-wave-like character at surfaces.

CuMnAs exemplifies the theoretical model.

Abstract

Altermagnetism (AM) and its associated spin-transport phenomena are typically linked to spin-split electronic band structures in bulk materials. However, the crystal surface has a reduced symmetry with respect to the bulk, which can induce AM at the surface of conventional antiferromagnets (AFMs) $\unicode{x2013}$ a local effect which cannot be detected using bulk properties. In this work we define the symmetry conditions necessary for surface AM and show how it can be topologically protected, rendering it a robust effect. We provide a minimal model for one trivial and two topological examples of surface AM. We show that the spin spectral density, accessible by spin- and angle-resolved photoemission spectroscopy, can exhibit a $d$-wave-like altermagnetic character at the surface, even when the full band structure is completely spin degenerate. Our topological model describes the Dirac semimetal CuMnAs, which provides an existing realization of our theory. Our results identify crystal surfaces as a platform to realize robust, topology- and symmetry-driven unconventional magnetism beyond the bulk classification of magnetic materials.