Angle-dependent thermodynamics of $\alpha$-RuCl$_3$

TL;DR

This study investigates the thermodynamic behavior of $ ext{α-RuCl}_3$ under various in-plane magnetic field directions, revealing critical fields, phase transitions, and entropy effects, thereby challenging the existence of a field-induced Kitaev spin liquid.

Contribution

It provides the first detailed angle-dependent thermodynamic measurements of $ ext{α-RuCl}_3$, identifying critical fields and phase boundaries, and discusses implications for the Kitaev spin liquid state.

Findings

Identification of two critical fields $B_c^{AF1}$ and $B_c^{AF2}$

Observation of inverse melting near $B_c^{AF2}$

Detection of a shoulder anomaly at 8-10 T

Abstract

Thermodynamics of the Kitaev honeycomb magnet $\alpha$-RuCl$_3$ is studied for different directions of in-plane magnetic field using measurements of the magnetic Gr\"uneisen parameter $\Gamma_B$ and specific heat $C$. We identify two critical fields $B_c^{\rm AF1}$ and $B_c^{\rm AF2}$ corresponding, respectively, to a transition between two magnetically ordered states and the loss of magnetic order toward a quantum paramagnetic state. The $B_c^{AF2}$ phase boundary reveals a narrow region of magnetic fields where inverse melting of the ordered phase may occur. No additional transitions are detected above $B_c^{\rm AF2}$ for any direction of the in-plane field, although a shoulder anomaly in $\Gamma_B$ is observed systematically at $8-10$ T. Large field-induced entropy effects imply additional low-energy excitations at low fields and/or strongly field-dependent phonon entropies. Our results establish universal features of $\alpha$-RuCl$_3$ in high magnetic fields and challenge the presence of a field-induced Kitaev spin liquid in this material.