Polaronic character of excitons in a two-dimensional ferromagnet CrI₃*

Studies of exciton dynamics in two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have led to discoveries of a variety of fascinating properties for optoelectronic applications. It has been known that exciton dynamics can be strongly affected by lattice vibrations through electron-phonon (e-ph) coupling. The recently discovered 2D ferromagnetic semiconductor, CrI₃, provides a new platform to explore exciton physics beyond the well-studied 2D TMDC semiconductors because of its localized orbitals, intrinsic long-range ferromagnetic order, and strong e-ph coupling. Focusing on bilayer CrI₃, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We further show the temperature and magnetic field dependence of these periodic broad modes. Our study opens opportunities for tailoring light-matter interactions in 2D magnetic semiconductors.