What carries the thermal Hall signal in $\alpha$-RuCl3?
In plain words
Heat flowing through $\alpha$-RuCl3 in a magnetic field is deflected sideways, an effect called the thermal Hall effect. It is unclear whether the sideways heat is carried by Majorana fermions of a spin liquid, by magnons (spin waves) or by phonons (lattice vibrations).
Precise statement
In alpha-RuCl3 with field H of 60 to 120 kOe in the honeycomb plane and T of 2 to 20 K, identify the carriers responsible for $\kappa_{xy}(T,H)$: chiral Majorana edge modes, topological magnons, or phonons with field-induced Berry curvature or Hall viscosity. An answer is a microscopic model that reproduces $\kappa_{xy}(T,H)$ and its dependence on field angle, sample quality and $\kappa_{xx}$ across published samples.
What would settle it
Measurements of $\kappa_{xy}$ on many samples with characterized stacking, combined with phonon and magnon calculations from the measured spin Hamiltonian, that isolate one dominant channel.
Status in the literature
Unverified note
Phonon (Lefrancois et al., PRX 2022) and magnon (Czajka et al., Nature Materials 2023) explanations compete with the Majorana interpretation; Shragai et al. (Nature 2026) measured a phonon Hall viscosity that accounts for a substantial fraction of $\kappa_{xy}$.