Thermodynamic evidence for a topological phase transition in a spin liquid state of α-RuCl₃

Kitaev quantum spin liquid (QSL) displays the fractionalization of quantum spins into Majorana fermions. In magnetic field, the emergence of chiral Majorana edge modes leads to a half-integer quantized thermal Hall conductance, which has been reported in the Kitaev candidate material α-RuCl₃ [1-4]. At high magnetic fields, the 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. However, this phase transition, including its presence, is still elusive because an anomaly associated with the transition has not been clearly detected by thermodynamic quantities. Here we present the results of high-resolution heat capacity measurements on α-RuCl₃ down to 200 mK. We find that the heat capacity exhibits a distinct anomaly at H ~ H*. 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.

[1] Y. Kasahara et al., Nature 559, 227-231 (2018). 

[2] M. Yamashita et al., Phys. Rev. B 102, 220404(R) (2020). 

[3] J. A. N. Bruin et al., arXiv:2104.12184.

[4] Y. Yokoi et al., Science 373, 568-572 (2021). 

[5] S. Suetsugu et al., in preparation.