Unusual Thermal Hall Effect in a Kitaev Spin Liquid Candidate α -RuCl 3

The Kitaev quantum spin liquid displays the fractionalization of quantum spins into Majorana fermions. The emergent Majorana edge current is predicted to manifest itself in the form of a finite thermal Hall effect, a feature commonly discussed in topological superconductors. Here we report on thermal Hall conductivity ${\\ensuremath{\\kappa}}_{xy}$ measurements in $\\ensuremath{\\alpha}\\text{\\ensuremath{-}}{\\mathrm{RuCl}}_{3}$, a candidate Kitaev magnet with the two-dimensional honeycomb lattice. In a spin-liquid (Kitaev paramagnetic) state below the temperature characterized by the Kitaev interaction ${J}_{K}/{k}_{B}\\ensuremath{\\sim}80\\text{ }\\text{ }\\mathrm{K}$, positive ${\\ensuremath{\\kappa}}_{xy}$ develops gradually upon cooling, demonstrating the presence of highly unusual itinerant excitations. Although the zero-temperature property is masked by the magnetic ordering at ${T}_{N}=7\\text{ }\\text{ }\\mathrm{K}$, the sign, magnitude, and $T$ dependence of ${\\ensuremath{\\kappa}}_{xy}/T$ at intermediate temperatures follows the predicted trend of the itinerant Majorana excitations.