Abstract：When topology is combined with strong electron correlations, exotic phenomena can arise. One of the most prominent example is the fractional quantum Hall effect, in which the constituent particles are electrons but the quasiparticles carry fractions of the electron charge. Recently, Kitaev quantum spin liquid (QSL) has aroused a great interest, because exotic quasiparticles, such as Majorana fermions and non-Abelian anyons, emerge as a result of fractionalization of electron spins. Kitaev model represents a spin-1/2 on a honeycomb lattice interacting through bond-dependent Ising ferromagnetic couplings.

Recently strongly spin-orbit coupled two-dimensional (2D) Mott insulator a-RuCl3 has taken a center stage in investigating the Kitaev spin liquid. Here we report a novel type of quantization of the thermal Hall effect in a-RuCl3 [1][2]. In this compound, the application of a parallel magnetic field destroys the long-range magnetic order, leading to a field-induced QSL ground state with massive entanglement of local spins. In the low-temperature regime of the QSL state, we show that the 2D thermal Hall conductance kxy2D exhibits a quantized plateau at half integer, kxy2D =K0/2, where K0=(p2/3)(kB2/h) T is the quantum thermal conductance [2], in analogous to the quantum electronic conductance e2/h. We also show that the half integer thermal Hall plateau is observed even when the magnetic field is applied parallel to the 2D plane. In addition, the Chern number determined by the sign of the quantized thermal Hall conductance is consistent with that expected in Kitaev QSL[3]. These results provide direct signatures of topologically protected chiral currents of charge neutral Majorana fermions at the edge and no-Abelian anyons in the bulk of the crystal.

[1] Y. Kasahara et al. Phys. Rev. Lett. 120, 217205 (2018).

[2] Y. Kasahara et al. Nature 559, 227 (2018).

[3] T. Yokoi et al. a preprint.