Observation of magnon polarons in the van der Waals itinerant ferromagnet Fe3GeTe2.

Magnon polarons, hybrid quasiparticles embodying the intrinsic coupling between spin and lattice dynamics, bridge magnetism and phononics in quantum materials. Despite extensive studies in magnetic insulators, spectroscopic evidence for magnon polarons has been scarcely reported in metallic ferromagnets. Here, we reveal magnon polarons at the nanoscale in single crystals of Fe3GeTe2 using inelastic scanning tunneling spectroscopy (ISTS) with a milli-Kelvin scanning tunneling microscope. While ISTS of phonons or magnons has been widely explored, our study focuses on two-dimensional van der Waals (vdW) itinerant ferromagnets, where momentum selection rules normally hinder magnon excitation. We show that strong magnon-phonon interaction leads to hybridization between the magnonic and phononic bands, lifting the selection rules at their avoided band crossings. This emergent momentum selection due to magnon-phonon coupling is a hallmark of magnon polarons. Our findings establish a platform for probing and engineering magnon-phonon hybrid excitations in two-dimensional materials at the nanoscale.