Optical signatures of periodic magnetization: the moir\'e Zeeman effect

TL;DR

This paper explores how periodic magnetic order in moiré materials affects exciton energy structures, proposing optical detection methods for various magnetic phases in TMD-based and ferromagnetic twisted bilayer systems.

Contribution

It introduces a novel optical probe based on exciton band structure analysis to detect and distinguish different magnetic orders in moiré magnets.

Findings

Sizable splitting in exciton resonances indicates magnetic order.

Spin-valley interactions enable probing of electron spin arrangements.

Proposed detection schemes for moiré magnetism in heterostructures.

Abstract

Detecting magnetic order at the nanoscale is of central interest for the study of quantum magnetism in general, and the emerging field of moir\'e magnets in particular. Here, we analyze the exciton band structure that arises from a periodic modulation of the valley Zeeman effect. Despite long-range electron-hole exchange interactions, we find a sizable splitting in the energy of the bright circularly-polarized exciton Umklapp resonances, which serves as a direct optical probe of magnetic order. We first analyze quantum moir\'e magnets realized by periodic ordering of electron spins in Mott-Wigner states of transition metal dichalcogenide (TMD) monolayers or twisted bilayers: we show that spin-valley dependent exciton-electron interactions allow for probing the spin-valley order of electrons and demonstrate that it is possible to observe unique signatures of ferromagnetic order in a triangular lattice and both ferromagnetic and N\'eel order in a honeycomb lattice. We then focus on semiclassical moir\'e magnets realized in twisted bilayers of ferromagnetic materials: we propose a detection scheme for moir\'e magnetism which is based on inter-layer exchange coupling between spins in a moir\'e magnet and excitons in a TMD monolayer.