Strong Momentum-Dependent Electron-Magnon Renormalization of a Surface Resonance on Iron

The coupling of fermionic quasiparticles to magnons is essential for a wide range of processes, from ultrafast magnetization dynamics in ferromagnets to Cooper pairing in superconductors. Although magnon energies are generally much larger than phonon energies, up to now their electronic band renormalization effect in ferromagnetic metals suggests a significantly weaker quasiparticle interaction. Here, using spin- and angle-resolved photoemission, we show an extraordinarily strong renormalization leading to replica-band formation of an iron surface resonance at ~200 meV. Its strong magnetic linear dichroism unveils the magnetic nature and momentum dependence of the energy renormalization. By determining the frequency- and momentum-dependent self-energy due to generic electron-boson interaction to compute the resultant electron spectral function, we show that the surface-state replica formation is consistent with strong coupling to an optical spin wave in a Fe thin film.