Observation of magnon polarons in the van der Waals itinerant ferromagnet FeGeTe

Source: PubMed Central Open Access, NCBI / U.S. National Library of Medicine

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 FeGeTeusing 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. Abs1 Two-dimensional magnetic materials have magnetic properties that are sensitive to strain, implying a strong coupling between the magnetic and lattice degrees of freedom. Here, using inelastic scanning tunnelling spectroscopy Li, Bansal, and coauthors find the telltale signatures of magnon-polarons in the two-dimensional ferromagnet, FeGeTe. Abs2 web-summary