Ultrafast Formation and Annihilation of Strongly Bound, Anisotropic Excitons

TL;DR

This study reveals the ultrafast formation, anisotropic nature, and dynamics of strongly bound excitons in CrSBr, a layered magnetic semiconductor, using advanced spectroscopic techniques, with implications for optoelectronic and spintronic devices.

Contribution

It provides the first direct measurement of large exciton binding energy and anisotropic exciton distribution in CrSBr, elucidating their formation and relaxation mechanisms.

Findings

Exciton binding energy is approximately 800 meV.

Excitons exhibit highly anisotropic momentum space distribution.

Interconversion between bound excitons and free carriers occurs within a few picoseconds.

Abstract

Van der Waals (vdW) layered materials with long-range magnetic order have the potential to enable novel optoelectronic and spintronic applications. Among these, CrSBr is an air-stable, direct band gap semiconductor that hosts interlayer antiferromagnetic order, a highly anisotropic electronic structure, and strongly bound excitons. In particular, excitons in CrSBr have been shown to inherit the quasi-one-dimensional nature of the material and also couple to the underlying spinorder. However, mechanisms of exciton formation, dissociation, and interaction with free carriers remain largely unexplored, despite being crucial for spintronic and optoelectronic applications. Here, we employ time- and angle-resolved photoemission spectroscopy to map the electronic structure and excited state dynamics in CrSBr. We directly resolve an exceptionally large exciton binding energy (~800 meV) and a highly anisotropic momentum space distribution of the exciton, revealing its quasi-1D real-space character. We observe an excitation-density-dependent interconversion between bound excitons and quasi-free carriers on sub- to few-picosecond timescales, indicating that many-body effects govern the excited-state dynamics and optical properties during the initial stages of relaxation. Our work highlights the strongly bound, anisotropic character of excitons in CrSBr, as well as the microscopic interactions steering relaxation pathways after photoexcitation in elevated density regimes relevant for future device applications.