MAGNON EXCITATIONS STUDIED OVER THE ENTIRE BRILLOUIN ZONE BY THz MAGNETOMETRY

Denis Gorbunov, HLD Dresden.

Optical excitations in solids require conservation of momentum and energy. This applies also to magnetic excitations, when a magnon with a given momentum and energy is created / annihilated by absorbing / emitting a photon. The small photon momentum typically confines possible excitations close to the center of the Brillouin zone. For example, THz spectroscopy provides access to the magnetic transitions by focusing on the zone-center magnons. This constraint, however, can be overcome. In a recent work, scientists from Germany, China, Switzerland, the USA, and Austria performed THz time-domain experiments of the kagome quantum magnet Y₃Cu₉(OH)₁₉Cl₈(Y-kapellasite) that provided access to the entire Brillouin zone of this material through three-center magnon excitations. This mechanism is possible due to the three distinct magnetic sublattices and strong short-range correlations in the distorted kagome lattice. Y-kapellasite shows plateaus at 1/6 and 1/3 of the full magnetization as evidenced by pulsed high-field magnetization measurements at HLD. The imaginary part of the magnetic susceptibility unveils an asymmetric broad feature with a maximum at 12 cm^-1 at 1.6 K, which reflects multi-center magnon excitations that can expand over the entire Brillouin zone (Figure, panel a). The calculated spin density of states is gapless in zero field and becomes gapped in field (panels b and c). The simultaneous magnon excitations in the three distinct magnetic sublattices of Y-kapellasite conserve the total wavevector (panel d). These findings establish THz spectroscopy as a method to directly probe the spin density of states over the entire Brillouin zone of systems with low symmetry.

Figure: (a) Frequency-dependent imaginary part of the magnetic susceptibility; (b) Calculated spin magnon density of states; (c) Spin-wave dispersion of Y-kapellasite in zero field; (d) Three-center magnon process in momentum space.

Multi-Center Magnon Excitations Open the Entire Brillouin Zone to Terahertz Magnetometry of Quantum Magnets, T. Biesner, S. Roh, A. Razpopov, J. Willwater, S. Süllow, Y. Li, K. M. Zoch, M. Medarde, J. Nuss, D. Gorbunov, Y. Skourski, A. Pustogow, S. E. Brown, C. Krellner, R. Valentí, P. Puphal, M. Dressel, Adv. Quantum Technol. 5, 2200023 (2022).

https://onlinelibrary.wiley.com/doi/full/10.1002/qute.202200023

Contact: d.gorbunov@hzdr.de