Analyzing thermodynamic signatures of field-induced phase transitions in α-RuCl₃

In α-RuCl₃ the observed approximately quantized thermal Hall conductivity at intermediate magnetic field strengths has alluded to a potentially field-induced Kitaev spin liquid (KSL). As a topologically nontrivial state, it cannot be adiabatically connected to the trivial high-field polarized phase and must be bounded to that by a phase transition. We therefore theoretically analyze recently measured^1,2 Grüneisen Parameters (GP), which are sensitive probes of quantum phase transitions.

We find that the magnetic GP data contradicts a field-induced KSL scenario. A high-field shoulder-anomaly is instead identified as a signature of crossings in the lowest-energy excitations without a phase transition. We compare to generalized Kitaev models using exact diagonalization methods, obtaining qualitative agreement. The shoulder-anomaly appears in models that have close proximity to competing phases without entering them².

Unlike the magnetic GP, the structural GP couples pseudospin and lattice physics. We tackle it theoretically with ab-initio magnetoelastic modelling³.

[1] Gass et al., PRB 101, 245158 (2020)
[2] Bachus et al., PRL 125, 097203 (2020)
[3] Kaib et al., arXiv:2008.08616