We report magnetic and thermodynamic properties of single crystal α-RuCl3, in which the Ru 3+ (4d 5 ) ion is in its low spin state and forms a honeycomb lattice. Two features are observed in both magnetic susceptibility and specific heat data; a sharp peak at 7 K and a broad hump near 10-15K. In addition, we observe a metamagnetic transition between 5 T and 10 T. Our neutron diffraction study of single crystal samples confirms that the low temperature peak in the specific heat is associated with a magnetic order with unit cell doubling along the honeycomb (100) direction, which is consistent with zigzag order, one of the types of magnetic order predicted within the framework of the Kitaev-Heisenberg model.

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The honeycomb Kitaev-Heisenberg model is a source of a quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. Here we unveil the highly unusual low-temperature heat conductivity κ of α-RuCl_{3}, a prime candidate for realizing such physics: beyond a magnetic field of B_{c}≈7.5  T, κ increases by about one order of magnitude, both for in-plane as well as out-of-plane transport. This clarifies the unusual magnetic field dependence unambiguously to be the result of severe scattering of phonons off putative Kitaev-Heisenberg excitations in combination with a drastic field-induced change of the magnetic excitation spectrum. In particular, an unexpected, large energy gap arises, which increases linearly with the magnetic field, reaching remarkable ℏω_{0}/k_{B}≈50  K at 18 T.

The low-temperature magnetic phases in the layered honeycomb lattice material α-RuCl3 have been studied as a function of in-plane magnetic field. In zero field this material orders magnetically below 7 K with so-called zigzag order within the honeycomb planes. Neutron diffraction data show that a relatively small applied field of 2 T is sufficient to suppress the population of the magnetic domain in which the zigzag chains run along the field direction. We found that the intensity of the magnetic peaks due to zigzag order is continuously suppressed with increasing field until their disappearance at µoHc=8 T. At still higher fields (above 8 T) the zigzag order is destroyed, while bulk magnetization and heat capacity measurements suggest that the material enters a state with gapped magnetic excitations. We discuss the magnetic phase diagram obtained in our study in the context of a quantum phase transition.

α-RuCl 3 is drawing much attention as a promising candidate Kitaev quantum spin liquid [1][2][3][4][5][6][7][8]. However, despite intensive research efforts, controversy remains about the form of the basic interactions governing the physics of this material. Even the sign of the Kitaev interaction (the bonddependent anisotropic interaction responsible for Kitaev physics) is still under debate, with conflicting results from theoretical and experimental studies [5,6,[9][10][11][12][13][14][15]. The significance of the symmetric off-diagonal exchange interaction (referred to as the Γ term) is another contentious question [16][17][18]. Here, we present resonant elastic x-ray scattering data that provides unambiguous experimental constraints to the two leading terms in the magnetic interaction Hamiltonian. We show that the Kitaev interaction (K) is ferromagnetic, and that the Γ term is antiferromagnetic and comparable in size to the Kitaev interaction. Our findings also provide a natural explanation for the large anisotropy of the magnetic susceptibility in α-RuCl 3 as arising from the large Γ term. We therefore provide a crucial foundation for understanding the interactions underpinning the exotic magnetic behaviours observed in α-RuCl 3 .