Ligand-field helical luminescence in a 2D ferromagnetic insulator

• Main Authors: Seyler, Kyle L. (Author), Zhong, Ding (Author), Klein, Dahlia Rivka (Author), Gao, Shiyuan (Author), Zhang, Xiaoou (Author), Huang, Bevin (Author), Navarro Moratalla, Efren Adolfo (Author), Yang, Li (Author), Cobden, David H. (Author), McGuire, Michael A. (Author), Yao, Wang (Author), Xiao, Di (Author), Xu, Xiaodong (Author), Jarillo-Herrero, Pablo (Author)
• Other Authors: Massachusetts Institute of Technology. Department of Physics (Contributor), MIT Materials Research Laboratory (Contributor)
• Format: Article
• Language: English
• Published: Springer Nature, 2019-06-21T16:38:13Z.
• Subjects: Article
• Online Access: Get fulltext

 Bulk chromium tri-iodide (CrI₃) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI₃ under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI₃ photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in Crl₃ bilayers. Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors , our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI₃. We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr³⁺ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.