Magnetic field and pressure induced magnetism of the Kitaev system α-RuCl₃

The Kitaev model on a honeycomb lattice has caused an abiding fascination due to its quantum spin liquid ground state. Here, α-RuCl₃ is believed to be the prime material to-date to harbor such a quantum spin liquid phase. Despite its antiferromagnetic ground state below T_N = 7 K, spin-liquid-like behavior may be observed if the magnetic order is suppressed under various external conditions, such as under the application of a magnetic field or pressure. We thoroughly investigated changes of the magnetic ground state of α-RuCl₃ under applied magnetic fields by means of different experimental techniques [1,2]. For the in-plane direction, we find a suppression of the zero-field antiferromagnetic order with increasing field up to μ₀H_c ≈ 7 T. In the disordered state above H_c, the magnetic excitations are strongly suppressed, implying the opening of a spin-excitation gap. Remarkably, our low-temperature specific heat data point toward a field-induced quantum critical point (QCP) at H_c; this is supported by universal scaling behavior near H_c. We relate the data to theoretical calculations based on a J₁–K₁–Γ₁–J₃ honeycomb model. As a second route towards the realization of a quantum spin-liquid state, we tuned the magnetic properties of α-RuCl₃ by the application of hydrostatic pressure. Its influence on the Kitaev-like interactions, and on the magnetic ground state was studied by means of magnetization studies under hydrostatic pressure. Surprisingly, a new high-pressure phase emerges above ~ 0.5 GPa with a collapse of the magnetic susceptibility. Via both experimental and theoretical investigations of pressure-induced changes of the crystallographic structure of α-RuCl₃ we resolve the origin and the nature of this new unconventional magnetic state.

[1] S.-H. Baek et al., Phys. Rev. Lett. 119, 037201 (2017).
[2] A. U. B. Wolter et al., Phys. Rev. B 96, 041405(R) (2017).