Multi‐Center Magnon Excitations Open the Entire Brillouin Zone to Terahertz Magnetometry of Quantum Magnets

Abstract Due to the small photon momentum, optical spectroscopy commonly probes magnetic excitations only at the center of the Brillouin zone; however, there are ways to override this restriction. In case of the distorted kagome quantum magnet Y‐kapellasite, Y 3 Cu 9 (OH) 19 Cl 8 , under scrutiny here, the spin (magnon) density of states (SDOS) can be accessed over the entire Brillouin zone through three‐center magnon excitations. This mechanism is aided by the three different magnetic sublattices and strong short‐range correlations in the distorted kagome lattice. The results of THz time‐domain experiments agree remarkably well with linear spin‐wave theory (LSWT). Relaxing the conventional zone‐center constraint of photons gives a new aspect to probe magnetism in matter.