The Impact of Magnons, Defects, and Rapid Energy Migration on the Optical Properties of the 2D Magnet CrPS4

Abstract: Strong coupling between optical and magnetic excitations could enable contactless, spatially resolved, or ultrafast interrogation and control of magnetism two-dimensional (2D) materials and devices. The layered 2D A-type antiferromagnet CrPS4 stands out among van der Waals (vdW) magnets for its rich optical fine structure, but its spectroscopy is not yet understood and has so far been interpreted without consideration of magnetic exchange. Here, we show that this fine structure comes primarily from exchange-mediated coupling between on-site optical "spin-flip" transitions of Cr3+ and low-energy spin transitions involving the surrounding lattice. Well-resolved magnon sidebands to optical 4A2 <--> 2E transitions are observed in photoluminescence (PL) and PL excitation spectra, as well as a pronounced PL sideband due to short-range exchange splitting. Energy migration is probed using Yb3+ dopants as traps, revealing sub-picosecond inter-site excitation hopping. Formation of dispersive Frenkel excitons of coupled on-site d-d transitions due to inter-site exchange is discussed. In addition to impacting how optical fine structure is interpreted in this and potentially other vdW magnets, these findings may have ramifications for future applications of layered 2D magnets by revealing new opportunities to drive mode-specific spin-wave excitations using light.