Optical signatures of spin symmetries in unconventional magnets

TL;DR

This paper explores how spin symmetries influence optical responses in unconventional magnets, demonstrating that spin group symmetry determines the dominant photoresponse in weak spin-orbit coupling materials, exemplified by Mn$_5$Si$_3$.

Contribution

It reveals that spin group symmetry, rather than magnetic group symmetry, governs optical responses in certain magnetic materials, supported by ab-initio calculations on Mn$_5$Si$_3$.

Findings

Non-coplanar spin structure produces shift currents consistent with spin symmetry.

Coplanar structure's shift current is suppressed by spin symmetry constraints.

Ab-initio calculations confirm the dominance of non-coplanar configuration in optical response.

Abstract

The concept of spin symmetries has gained renewed interest as a valuable tool for classifying unconventional magnetic phases, including altermagnets and recently identified p-wave magnets. In this work, we show that in compounds with weak spin-orbit coupling, the dominant spin and charge photoresponse is determined by spin group rather than the conventional magnetic group symmetry. As a concrete realization we consider the nonlinear shift photocurrent in Mn$_5$Si$_3$, a material that features the two possible classes of unconventional p-wave magnetism in the form of two competing spin structures, a coplanar and non-coplanar one. While both are predicted to generate shift currents based on magnetic symmetry considerations, only the non-coplanar configuration survives the spin symmetry requirements. This is numerically confirmed by our $\textit{ab-initio}$ calculations, providing a protocol to experimentally identify the spin configuration of this promising material in photogalvanic or transport measurements.