Symmetry Classification for Alternating Excitons in Two-Dimensional Altermagnets

TL;DR

This paper develops a theoretical framework using spin space group symmetry to classify and analyze excitons in two-dimensional altermagnets, revealing optical properties and tunability for potential valleytronic applications.

Contribution

It introduces a novel symmetry-based classification scheme for excitons in 2D altermagnets, linking band representations to optical and valley polarization features.

Findings

Classified excitons into s-like and p-like based on symmetry.

Identified optical selection rules and bright exciton characteristics.

Demonstrated tunability of excitonic properties via strain and valley splitting.

Abstract

Excitons, bound electron-holes states, often dominate the optical response of two-dimensional (2D) materials and reflect their inherent properties, including spin-orbit coupling, magnetic ordering, or band topology. By focusing on a growing class of collinear antiferromagnets with a nonrelativistic spin splitting, referred to also as altermagnets (AM), we propose a theoretical framework based on the spin space group (SSG) to elucidate their resulting excitons. Our approach is illustrated on 2D AM with spin-polarized valleys, where we classify the combination of conduction and valence bands by the SSG representations into two cases that hosts bright $s$-like and $p$-like excitons, respectively. This analysis is further supported by effective Hamiltonians and the Bethe-Salpeter equation. We identify the excitonic optical selection rules from the calculated absorption spectra and the symmetry of bright excitons from their momentum space envelope functions. Our framework provides optical fingerprints for various cases of AM, while their tunability, such as the strain-induced valley splitting, is also transferred to excitons allowing, additionally, valley-polarized photocurrent generation.