Unusual Thermal Hall Effect in a Kitaev Spin Liquid Candidate alpha-RuCl3

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 kappa(xy) measurements in alpha-RuCl3, 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) similar to 80 K, positive 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 K, the sign, magnitude, and T dependence of kappa(xy)/T at intermediate temperatures follows the predicted trend of the itinerant Majorana excitations.