Testing Topological Phase Transitions in Kitaev Materials Under In-Plane Magnetic Fields*

Material realization of the non-Abelian Kitaev spin liquid phase - an example of Ising topological order (ITO) - has been the subject of intense research in recent years. The 4d honeycomb Mott insulator α-RuCl₃ has emerged as a leading candidate as it enters a field-induced magnetically disordered state where a half-integer quantized thermal Hall conductivity κ_xy was reported. Further, a recent report of a sign change in the quantized κ_xy across a certain crystallographic direction is strong evidence for a topological phase transition between two ITOs with opposite chirality. Although this is fascinating, independent verification remains elusive, and one may ask if there is a thermodynamic quantity sensitive to the transition. Here we propose that the magnetotropic coefficient k would serve such a purpose. We report a singular feature in k that indicates a topological phase transition across the b-axis where ITO is prohibited by a C₂ symmetry. If the transition in α-RuCl₃ is indeed a direct transition between ITOs, or a more broad realization of a topological phase transition, then this feature in k should be observable.