Testing topological phase transitions in Kitaev materials under in-plane magnetic fields: Application to α-RuCl3

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 alpha-RuCl3 has emerged as a leading candidate, as it enters a field-induced magnetically disordered state where a half-integer quantized thermal Hall conductivity kappa(xy) was reported. Further, a recent report of a sign change in the quantized kappa(xy) across a certain crystallographic direction is strong evidence for a topological phase transition between two ITOs with opposite Chern numbers. Although this is a fascinating result, independent verification remains elusive, and one may ask if there is a thermodynamic quantity sensitive to the phase 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) over cap -axis where ITO is prohibited by a C-2 symmetry. If the transition in alpha-RuCl3 is indeed a direct transition between ITOs, or is a more broad realization of a topological phase transition, then this feature in k should be observable. Implications of C-2 symmetry on other experimental quantities are also discussed.

Automated summary

Research on the material realization of the non-Abelian Kitaev spin liquid phase, specifically the Ising topological order (ITO), has focused on the 4d honeycomb Mott insulator alpha-RuCl3. The observation of field-induced magnetically disordered state and half-integer quantized thermal Hall conductivity in alpha-RuCl3 provides evidence for the topological phase transition between two ITOs with opposite Chern numbers. The proposal that the magnetotropic coefficient k could serve as a thermodynamic quantity sensitive to the phase transition highlights the potential for further understanding of the transition in alpha-RuCl3.