Direct observation of topological magnon edge states

Abstract Magnons, the quanta of spin waves, can serve as information carriers with promising applications in ultra-low-power spintronic devices.1,2 Magnon Chern insulators (MCIs) exhibiting unique topological magnon band structures featuring chiral edge states which can be utilized to construct novel spintronic devices.3,4 Though the existence of the MCIs are supported by the magnon related thermal Hall effect,5 direct observations of the topologically protected magnon edge states is still been pursued. Here, we report the spatially resolved detection of magnon edge states in a two-dimensional ferromagnet with honeycomb lattice (single-layer chromium triiodide). Using scanning tunneling microscopy, we observed magnon-assisted inelastic tunneling conductance and revealed the gapped magnon spectra with enhanced signals at the van Hove singularities. Extra tunneling conductance contributed from the magnon edge states was detected at three different edge configurations. Our work provided direct evidence proving the existence of MCI states down to the single-layer limit, initiating spatially-resolved explorations on exotic properties arising from topological edge states of MCIs.