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

TL;DR

This paper demonstrates that three-center magnon excitations in a distorted kagome quantum magnet enable terahertz magnetometry across the entire Brillouin zone, overcoming the usual photon momentum limitations.

Contribution

It introduces a novel method using three-center magnon excitations to access the full Brillouin zone in terahertz magnetometry of quantum magnets.

Findings

THz experiments match linear spin-wave theory

Access to entire Brillouin zone via three-center magnons

Overcoming photon momentum restrictions in optical spectroscopy

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 the case of the distorted kagome quantum magnet Y-kapellasite, Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$, under scrutiny here, the magnon density of states 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. Relaxing the conventional zone-center constraint of photons gives a new aspect to probe magnetism in matter.