Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material α-RuCl 3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p ∼ 0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin-liquid phase in α-RuCl 3 strongly competes with the crystallization of spin singlets into a valence bond solid. DOI: 10.1103/PhysRevB.97.241108 The Kitaev model on a honeycomb lattice has grown into a hot topic in the last decade due to its exact solubility and its quantum spin-liquid ground state, which would be relevant for, e.g., quantum computing [1,2]. It implies a bonddependent compass-type coupling K and strong intrinsic spin frustration [3]. A crucial ingredient for realizing the Kitaev model in real materials is a strong spin-orbit coupling together with a honeycomb structure. Recently, Kitaev interactions were identified in α-RuCl 3 , from its unusual magnetic excitation spectrum [4,5], its strong magnetic anisotropy [6], and electronic-structure calculations [7,8], which render this material an ideal platform for exploring Kitaev magnetism experimentally.α-RuCl 3 is a j eff = 1/2 Mott insulator with a twodimensional (2D) layered structure of edge-sharing RuCl 6 octahedra forming a honeycomb lattice. At ambient pressure, * g.bastien@ifw-dresden.de Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.the honeycomb layers are arranged in a monoclinic (C2/m) structure at room temperature with one of the three nearestneighbor (NN) Ru-Ru bonds slightly shorter than the other two [9]. A structural phase transition was reported at T S 60 K under cooling and T S 166 K upon warming, but the low-temperature crystal structure is still under debate and could be either rhombohedral (R3) [10,11] or monoclinic (C2/m) [12,13]. The onset of long-range magnetic order at T N 7 K [9] in α-RuCl 3 implies that other magnetic interactions have to be considered in addition to the Kitaev interaction K: a NN Heisenberg J , an off-diagonal coupling , as well as next-NN interactions J 2 and J 3 [7,8,14,15]. While electronic-structure calculations indicate that K is ferromagnetic in α-RuCl 3 and indeed defines the largest exchange energy scale [7,8,14,15], the debate on the minimal effective spin model and precise magnitude of the different couplings is not fully settled yet. By applying a magnetic field in the basal plane, the magnetic zigzag ground sta...

The honeycomb Mott insulator α-RuCl3 loses its low-temperature magnetic order by pressure. We report clear evidence for a dimerized structure at P > 1 GPa and observe the breakdown of the relativistic j eff picture in this regime strongly affecting the electronic properties. A pressure-induced Kitaev quantum spin liquid cannot occur in this broken symmetry state. We shed light on the new phase by broad-band infrared spectroscopy of the low-temperature properties of α-RuCl3 and ab initio density functional theory calculations, both under hydrostatic pressure.

The honeycomb Kitaev model in a magnetic field is a source of a topological quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. We present experimental results for the thermal Hall effect of the material α-RuCl3 which recently emerged as a prime candidate for realizing such physics. At temperatures above long-range magnetic ordering T TN ≈ 8 K, we observe with an applied magnetic field B perpendicular to the honeycomb layers a sizeable positive transversal heat conductivity κxy which increases linearly with B. Upon raising the temperature, κxy(T ) increases strongly, exhibits a broad maximum at around 30 K, and eventually becomes negligible at T 125 K. Remarkably, the longitudinal heat conductivity κxx(T ) exhibits a sizeable positive thermal magnetoresistance effect. Thus, our findings provide clear-cut evidence for longitudinal and transverse magnetic heat transport and underpin the unconventional nature of the quasiparticles in the paramagnetic phase of α-RuCl3.About ten years ago, Kitaev proposed a new type of quantum spin model whose ground state has been exactly shown to be a realization of a gapless quantum spin-liquid (QSL) [1]. This is a peculiar state of matter where magnetic long-range order is suppressed due to substantial quantum fluctuations even at zero temperature [2][3][4]. In the presence of certain time-reversalsymmetry breaking perturbations, e.g. external magnetic fields, this spin liquid opens a gap, leading to a topologically non-trivial spin liquid (TQSL). The striking feature of Kitaev's TQSL is, that in addition to its unconventional bulk excitations, which are due to fractionalization of spins into localized Z 2 gauge fluxes and itinerant Majorana fermions [1,5,6], and which are present already in the gapless state, a chiral Majorana edge mode arises in the field induced gap and the Z 2 vortices acquire non-Abelian anyonic statistics [1].When investigating possible candidate materials for realizing such exotic physics, heat conductivity experiments are considered one of the few probes to study the TQSL quasiparticle fingerprints because information on the quasiparticles' specific heat, their velocity, and their scattering is provided [7]. The magnetically frustrated honeycomb compound α-RuCl 3 has been intensively studied recently as it has been suggested to host a proximate Kitaev TQSL [8,9]. A profound understanding of how putative fractional magnetic excitations contribute to the heat transport in α-RuCl 3 is thus highly desirable.Several heat conductivity studies on α-RuCl 3 yield an inconsistent picture on possible heat transport by emergent quasiparticles of the spin system. Heat transport by itinerant spin excitation has been inferred from an anomaly in the in-plane longitudinal heat conductivity κ xx at around 100 K [10] and from a magnetic fieldinduced low-temperature enhancement of κ xx for fields B 8 T parallel to the material's honeycomb planes [11]. These interpretations however, have recently been ruled out by results for the...

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Topologically ordered states of matter are generically characterized by excitations with quantum number fractionalization. A prime example is the spin liquid realized in Kitaev's honeycomb-lattice compass model where spin-flip excitations fractionalize into Majorana fermions and Ising gauge fluxes. While numerous compounds have been proposed to be proximate to such a spin-liquid phase, clear-cut evidence for fractionalized excitations is lacking. Here we employ microwave absorption measurements to study the low-energy excitations in α-RuCl3 over a wide range of frequencies, magnetic fields, and temperatures, covering in particular the vicinity of the field-driven quantum phase transition where long-range magnetic order disappears. In addition to conventional gapped magnon modes we find a highly unusual broad continuum characteristic of fractionalization whichmost remarkably -extends to energies below the lowest sharp mode and to temperatures significantly higher than the ordering temperature, and develops a gap of a nontrivial origin in strong magnetic fields. Our results unravel the signatures of fractionalized excitations in α-RuCl3 and pave the way to a more complete understanding of the Kitaev spin liquid and its instabilities.