Small polaron dynamics in a two-dimensional magnetic material

The polaron in magnetic materials can carry not only charge, but also spin information. Therefore, the polaron dynamics in magnetic material is directly correlated with the spin dynamics. In this work, taking two-dimensional (2D) ${\mathrm{CoCl}}₂$ as a prototypical system, it is found that the 2D transition metal dihalide is an ideal platform for small polaron formation. At zero temperature, three distinct structures of small polarons, with their charges mainly localized on one, two, and three Co atoms, have been identified and designated as polarons I, II, and III, respectively. Polarons II and III are stable at 300 K, and their site-to-site hopping involves the transition to each other. Notable in-plane spin canting is observed when polaron II is formed. Moreover, the photoexcited charge carrier lifetime for the spin-minority channel is distinctly reduced by the spin-polarized polaron state in the band gap, making it four orders of magnitude shorter than the spin-majority channel. The existence of a small polaron in 2D magnetic materials provides a different stratagem to design 2D spintronic and polaronic devices.