Observation of anisotropic dispersive dark exciton dynamics in CrSBr

TL;DR

This study uses resonant inelastic x-ray scattering to reveal that dark excitons in CrSBr can propagate in two dimensions, contrasting with the material's electrical and bright exciton behaviors, indicating complex anisotropic excitonic dynamics.

Contribution

It provides the first direct momentum-resolved observation of propagating dark excitons in CrSBr, revealing their two-dimensional dispersion and contrasting with other electronic behaviors.

Findings

Dark excitons propagate along both a and b crystal directions.

Most excitations are localized with some exhibiting spin-orbital diversity.

Dark exciton dispersion contrasts with electrical conductivity and bright exciton behavior.

Abstract

Many-body excitons in CrSBr are attracting intense interest in view of their highly anisotropic magneto-optical coupling and their potential for novel optical interfaces within spintronic and magnonic devices. Characterizing the orbital character and propagation of these electronic excitations is crucial for understanding and controlling their behavior; however, this information is challenging to access. Ultra-high resolution resonant inelastic x-ray scattering is a momentum-resolved technique that can address these crucial questions. We present measurements collected at the Cr $L_3$-edge which show a rich spectrum of excitations with a variety of spin-orbital characters. While most of these excitations appear to be localized, the dispersion of the lowest energy dark exciton indicates that it is able to propagate along both the $a$ and $b$ directions within the planes of the crystal. This two-dimensional character is surprising as it contrasts with electrical conductivity and the behavior of the bright exciton, both of which are strongly one-dimensional. The discovery of this propagating dark exciton highlights an unusual coexistence of one- and two-dimensional electronic behaviors in CrSBr.