The quest for quantum spin liquids has triggered intense investigations on frustrated magnetic systems, resulting in the synthesis of various layered materials with triangular, honeycomb or kagome lattices. Among the
latter, herbertsmithite and its analogues were studied in minute detail by various magnetic probes. While optical measurements are primarily sensitive to the charge degrees of freedom, valuable information can be obtained on magnetoelastic coupling and low-energy spin excitations. Here, we performed comprehensive infrared and THz studies on several paradigmatic kagome compounds. Our work on ZnCu3(OH)6Cl2 – a copper hydroxide system with a charge transfer gap of 3.3 eV [1,2] – revealed pronounced nonthermal redshifts
and broadening specifically for phonon modes that deform the kagome layer or affect the Cu-O-Cu bond angles [3]. Via this strong spin-lattice coupling, sketched in Fig. 1(a), we utilize lattice vibrations as a probe of the magnetic
ground state. Similar effects are observed in the closely related Y3Cu9(OH)19Cl8, where our time-domain THz experiments (Fig. 1b) access the spin density of states (SDOS) 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, in excellent agreement with linear spin-wave theory. Relaxing the
conventional zone-center constraint of photons provides a new aspect to probe magnetism in matter. Lately, we have also observed magnetic THz resonances in the paramagnetic state of the newly synthesized averievite Cu5−xZnxV2O10(CsCl), where Cu2+ kagome layers are sandwiched between honeycomb planes consisting of V and Cu [4]. This comparison allows a direct probe of the different contributions from magnetic order, frustration, and structural properties in the phase diagram of averievite. Overall, our results illustrate the
effect of magnetic interactions in THz spectra of various frustrated magnets.
[1] P. Puphal, M. Bolte, D. Sheptyakov, A. Pustogow, K. Kliemt, M. Dressel, M. Baenitz, and C. Krellner, J. Mater. Chem. C 5, 2629 (2017).
[2] A. Pustogow, Ying Li, I. Voloshenko, P. Puphal, C. Krellner, I. I. Mazin, M. Dressel, and R. Valentí, Phys. Rev. B 96, 241114(R) (2017).
[3] Ying Li, A. Pustogow, M. Bories, P. Puphal, C. Krellner, M. Dressel, and R. Valentí, Phys. Rev. B 101, 161115(R) (2020). [Editors’ Suggestion]
[4] 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, and M. Dressel, Adv. Quantum Technol. 2022, 2200023 (2022).
[5] T. Biesner, S. Roh, A. Pustogow, H. Zheng, J. F. Mitchell, and M. Dressel, Phys. Rev. B 105, L060410 (2022).
