Quantized thermal Hall effect and topological phase transition in the Kitaev spin liquid candidate α-RuCl₃

  Kitaev quantum spin liquid (QSL) hosts Majorana fermions and non-Abelian anyons in magnetid fields as a result of the fractionalization of quantum spins. The emergence of chiral Majorana edge modes leads to a half-integer quantized thermal Hall conductance, which has been reported for the Kitaev spin liquid candidate α-RuCl₃ [1-4]. The remarkable feature is that the half-integer thermal Hall plateau appeas even in a planar Hall geometry, i.e., in magnetic fields with no out-of-plane components. This is in marked contrast to the quantum Hall effect in two-dimensional electron gas. Moreover, the measured field-angular variation of the quantized plateau has the same sign structure as the topological Chern number proposed in the original Kitaev model [4]. 

  An important question is the fate of topologically nontrivial state at intense fields. At high magnetic fields, the half-integer quantization disappears and the thermal Hall conductance κ_xy goes to zero rapidly, indicating a possible topological phase transition from the Kitaev QSL to a state without chiral Majorana edge modes. We find that the heat capacity exhibits a distinct anomaly at μ₀H* ~ 11 T. Interestingly, H* is closed to the field where κ_xy vanishes. Along with the fact that the longitudinal thermal conductivity shows a discontinuous jump at H ~ H* [5], our results provide thermodynamic evidence for a first-order topological phase transition in the spin liquid state of α-RuCl₃. 

This work is in collaboration with T. Onishi, S. Ma, T. Yokoi, Y. Matsuda (Kyoto Univ.), S. Kasahara (Okayama Univ.), Y. Mizukami, O. Tanaka, T. Shibauchi, Y. Motome (Univ. of Tokyo), N. Kurita, H. Tanaka (Tokyo Institute of Technology), C. Hickey, S. Trebst (Univ. of Cologne).