We report a 35 Cl nuclear magnetic resonance study in the honeycomb lattice α-RuCl3, a material that has been suggested to potentially realize a Kitaev quantum spin liquid (QSL) ground state. Our results provide direct evidence that α-RuCl3 exhibits a magnetic-field-induced QSL. For fields larger than ∼ 10 T, a spin gap opens up while resonance lines remain sharp, evidencing that spins are quantum disordered and locally fluctuating. The spin gap increases linearly with an increasing magnetic field, reaching ∼ 50 K at 15 T, and is nearly isotropic with respect to the field direction. The unusual rapid increase of the spin gap with increasing field and its isotropic nature are incompatible with conventional magnetic ordering and, in particular, exclude that the ground state is a fully polarized ferromagnet. The presence of such a field-induced gapped QSL phase has indeed been predicted in the Kitaev model.When the interactions between magnetic spins are strongly frustrated, quantum fluctuations can cause spins to remain disordered even at very low temperatures [1]. The quantum spin liquid (QSL) state that ensues is conceptually very interesting -for instance, new fractionalized excitations appear that are very different from the ordinary spin-wave excitations in ordered magnets [2][3][4][5]. A QSL appears in the so-called Kitaev honeycomb model -a prototypical and mathematically wellunderstood model of strongly frustrated interacting spins [6,7]. In an external magnetic field the topological QSL state acquires a gap that, in the generic case grows linearly with field strength [8].This observation has motivated the search for the experimental realization of the Kitaev honeycomb model and its topological QSL phases. The quest was centered, until recently, mainly on honeycomb iridate materials [9, 10] of the type A 2 IrO 3 (A = Na or Li). However, in these iridates long-range magnetic order develops at low temperatures for all known different crystallographic phases [11][12][13][14][15]. Their QSL regime is most likely preempted by the presence of significant residual Heisenberg-type interactions, by longer-range interactions between the spins or by crystallographically distinct Ir-Ir bonds, if not by a combination of these factors [16][17][18][19]. More promising in this respect is ruthenium trichloride α-RuCl 3 in its honeycomb crystal phase, as numerous experimental and theoretical studies pointed the significance of the anisotropic Kitaev exchange in the material [20][21][22][23][24][25][26][27]. Neutron scattering studies have shown that the magnetic interactions in this material are closer to the Kitaev limit [28], although at low temperatures also this quasi-2D material exhibits long-range magnetic order.In this Letter, we show by means of nuclear magnetic resonance (NMR) that in α-RuCl 3 large magnetic fields larger than ∼ 10 T melt the magnetic order, and a spingap opens that scales linearly with the magnetic field, implying that the detrimental effects of residual magnetic interactions between the Ru moment...

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We report on terahertz spectroscopy of quantum spin dynamics in α-RuCl_{3}, a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. We follow the evolution of an extended magnetic continuum below the structural phase transition at T_{s2}=62  K. With the onset of a long-range magnetic order at T_{N}=6.5  K, spectral weight is transferred to a well-defined magnetic excitation at ℏω_{1}=2.48  meV, which is accompanied by a higher-energy band at ℏω_{2}=6.48  meV. Both excitations soften in a magnetic field, signaling a quantum phase transition close to B_{c}=7  T, where a broad continuum dominates the dynamical response. Above B_{c}, the long-range order is suppressed, and on top of the continuum, emergent magnetic excitations evolve. These excitations follow clear selection rules and exhibit distinct field dependencies, characterizing the dynamical properties of a possibly field-induced quantum spin liquid.

We report on the unusual behavior of the in-plane thermal conductivity κ and torque τ response in the Kitaev-Heisenberg material α-RuCl_{3}. κ shows a striking enhancement with linear growth beyond H=7  T, where magnetic order disappears, while τ for both of the in-plane symmetry directions shows an anomaly at the same field. The temperature and field dependence of κ are far more complex than conventional phonon and magnon contributions, and require us to invoke the presence of unconventional spin excitations whose properties are characteristic of a field-induced spin-liquid phase related to the enigmatic physics of the Kitaev model in an applied magnetic field.

Raman scattering has been employed to investigate lattice and magnetic excitations of the honeycomb Kitaev material α-RuCl3 and its Heisenberg counterpart CrCl3. Our phonon Raman spectra give evidence for a first-order structural transition from a monoclinic to a rhombohedral structure for both compounds. Significantly, only α-RuCl3 features a large thermal hysteresis, consistent with the formation of a wide phase of coexistence. In the related temperature interval of 70 − 170 K, we observe a hysteretic behavior of magnetic excitations as well. The stronger magnetic response in the rhombohedral compared to the monoclinic phase evidences a coupling between the crystallographic structure and low-energy magnetic response. Our results demonstrate that the Kitaev magnetism concomitant with fractionalized excitations is susceptible to small variations of bonding geometry.