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

• Published in: Advanced quantum technologies (Online) 2022-06, Vol.5 (6), p.n/a
• Main Authors: Biesner, Tobias, Roh, Seulki, Razpopov, Aleksandar, Willwater, Jannis, Süllow, Stefan, Li, Ying, Zoch, Katharina M., Medarde, Marisa, Nuss, Jürgen, Gorbunov, Denis, Skourski, Yurii, Pustogow, Andrej, Brown, Stuart E., Krellner, Cornelius, Valentí, Roser, Puphal, Pascal, Dressel, Martin
• Format: Article
• Language: English
• Subjects: kagome lattices; multi‐center magnons; terahertz magnetometry; terahertz photons
• ISSN: 2511-9044; 2511-9044
• DOI: 10.1002/qute.202200023

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, Y3Cu9(OH)19Cl8, 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. When three magnetic sublattices arranged like a mercedes star are simultaneously excited by terahertz light pulses, distinct time traces are observed that allow access to the spin (magnon) density of states (SDOS) over the full Brillouin zone. Three‐center magnons are a novel approach to remove the zone‐center constraint commonly imposed to photons.