Ultrafast Spontaneous Localization of a Jahn-Teller Exciton Polaron in Two-Dimensional Semiconducting CrI<sub>3</sub> by Symmetry Breaking
Xufeng Li, Aolei Wang, Hailong Chen, Weijian Tao, Zeng Chen, Chi Zhang, Yujie Li, Yiran Zhang, Honghui Shang, Yuxiang Weng, Jin Zhao, Haiming Zhu
Abstract
The excited state species and properties in low-dimensional semiconductors can be completely redefined by electron-lattice coupling or a polaronic effect. Here, by combining ultrafast broadband pump–probe spectroscopy and first-principles GW and Bethe-Salpeter equation calculations, we show semiconducting CrI3 as a prototypical 2D polaronic system with characteristic Jahn-Teller exciton polaron induced by symmetry breaking. A photogenerated electron and hole in CrI3 localize spontaneously in ∼0.9 ps and pair geminately to a Jahn-Teller exciton polaron with elongated Cr–I octahedra, large binding energy, and an unprecedentedly small exciton-exciton annihilation rate constant (∼10–20 cm3 s–1). Coherent phonon dynamics indicates the localization is mainly triggered by the coherent nuclear vibration of the I–Cr–I out-of-plane stretch mode at 128.5 ± 0.1 cm–1. The excited state Jahn-Teller exciton polaron in CrI3 broadens the realm of 2D polaron systems and reveals the decisive role of coupled electron-lattice motion on excited state properties and exciton physics in 2D semiconductors.