Regensburg 2025 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 33: Correlated Magnetism – Spin Liquids

TT 33.5: Talk

Wednesday, March 19, 2025, 16:00–16:15, H33

What is carrying the heat in the thermal Hall effect of honeycomb magnets? — •Ralf Claus, Jan Bruin, Yosuke Matsumoto, Pascal Reiss, Akmal Hossain, Lichen Wang, Pascal Puphal, Bernhard Keimer, and Hidenori Takagi — Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart

The observation of a half-integer quantized thermal Hall effect in the honeycomb magnet α-RuCl₃ was interpreted as an experimental hallmark for Kitaev majorana fermions. However, follow-up studies only partly reproduced this result and have offered alternative explanations such as phonons or topological magnons. To narrow down the nature of the heat carrying quasiparticles, we conducted a comparative study of the longitudinal (κ_xx) and transversal (κ_xy) heat transport on α-RuCl₃ and Na₃Co₂SbO₆ (NCSO). Both share the same crystal symmetries and have comparable magnetic phase diagrams. However, one key difference is that for applied in-plane magnetic fields B>3 T NCSO is in a fully spin-polarized phase convincingly excluding the presence of any majorana fermions. Remarkably, we observed a finite κ_xy in NCSO up to B≈ 10 T, which displays striking similarities in shape, angle-dependence, and magnitude to that of α-RuCl₃. Furthermore, the field dependences of κ_xx and of the thermal Hall angle (κ_xy/κ_xx) across all α-RuCl₃ and NCSO samples suggest a substantial phononic contribution to κ_xy. Ultimately, we propose that topological magnons are responsible for generating the Hall temperature gradient which in turn is significantly enhanced by phonon-magnon interaction.

Keywords: Kitaev; honeycomb magnet; quantum spin liquid; thermal conductivity; thermal Hall effect