[Paper Review] Large Exciton Binding Energy in the Bulk van der Waals Magnet CrSBr

Shane Smolenski, Ming Wen+14 more|arXiv (Cornell University)|Mar 20, 2024

2D Materials and ApplicationsMaterials Science3 citations

TL;DR

This study reports an exceptionally large exciton binding energy of over 480 meV in bulk CrSBr, a van der Waals antiferromagnet, via angle-resolved photoemission spectroscopy (ARPES) and self-consistent GW calculations. The large binding energy arises from strong electronic and structural anisotropy, which enhances electron-hole correlation and suppresses screening, enabling robust excitonic states in a bulk 3D system—offering a platform for tunable excitonic optoelectronics and many-body physics.

ABSTRACT

Excitons, bound electron-hole pairs, influence the optical properties in strongly interacting solid state systems. Excitons and their associated many-body physics are typically most stable and pronounced in monolayer materials. Bulk systems with large exciton binding energies, on the other hand, are rare and the mechanisms driving their stability are still relatively unexplored. Here, we report an exceptionally large exciton binding energy in single crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizing state-of-the-art angle-resolved photoemission spectroscopy and self-consistent ab-initio GW calculations, we present direct spectroscopic evidence that robust electronic and structural anisotropy can significantly amplify the exciton binding energy within bulk crystals. Furthermore, the application of a vertical electric field enables broad tunability of the optical and electronic properties. Our results indicate that CrSBr is a promising material for the study of the role of anisotropy in strongly interacting bulk systems and for the development of exciton-based optoelectronics.

Motivation & Objective

• To identify and measure large exciton binding energies in bulk van der Waals magnets, which are rare due to strong dielectric screening.
• To investigate the role of electronic and structural anisotropy in stabilizing tightly bound excitons in 3D bulk materials.
• To establish CrSBr as a candidate for studying many-body excitonic effects in bulk systems with tunable electronic and optical properties.
• To bridge the gap between monolayer TMDs (with large binding energies) and bulk materials by demonstrating comparable excitonic stability in a bulk van der Waals system.

Proposed method

• Performed angle-resolved photoemission spectroscopy (ARPES) on single-crystal CrSBr to directly probe the electronic band structure and excitonic features.
• Conducted self-consistent ab-initio GW calculations to compute quasiparticle band structures and electron-hole interaction effects.
• Used Wannier interpolation and Nevanlinna analytical continuation to reconstruct high-resolution momentum-space electronic densities from Matsubara Green’s functions.
• Performed orbital decomposition using symmetric atomic orbitals (SAO) to assign electronic character to bands near the Fermi level.
• Measured optical gap via reflection contrast (RC) on CrSBr/SiO₂/Si samples to determine exciton energy and temperature dependence.
• Applied vertical electric fields to tune the electronic and optical response, demonstrating tunability of excitonic states.

Experimental results

Research questions

• RQ1 Can large exciton binding energies persist in bulk 3D van der Waals materials despite strong dielectric screening?
• RQ2 To what extent does electronic and structural anisotropy enhance excitonic binding in bulk CrSBr?
• RQ3 How do many-body effects such as electron correlation and reduced screening contribute to exciton stability in bulk CrSBr?
• RQ4 Can the optical and electronic properties of CrSBr be electrically tuned via an external field, enabling device applications?
• RQ5 What is the relationship between the observed excitonic features in ARPES and optical measurements (e.g., RC) in terms of energy and temperature dependence?

Key findings

• An exciton binding energy exceeding 480 meV was directly measured in bulk CrSBr using ARPES and confirmed by reflection contrast (RC) measurements.
• The excitonic feature at 1.361 eV at 5 K redshifts to 1.349 eV at 140 K and disappears above 140 K, consistent with the Néel transition temperature.
• Self-consistent GW calculations revealed strong electronic anisotropy with a quasi-one-dimensional conduction band, supporting enhanced electron-hole correlation.
• Orbital decomposition showed dominant contributions from Cr 3d and S 3p orbitals near the Fermi level, with spatial charge localization enhancing excitonic binding.
• The application of a vertical electric field enabled broad tunability of the optical and electronic properties, indicating potential for field-effect control of excitons.
• The large binding energy is attributed to a combination of strong Coulomb interactions, reduced screening due to anisotropy, and charge localization in the layered structure.