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...

Geometrical constraints to the electronic degrees of freedom within condensed-matter systems often give rise to topological quantum states of matter such as fractional quantum Hall states, topological insulators, and Weyl semimetals 1-3 . In magnetism, theoretical studies predict an entangled magnetic quantum state with topological ordering and fractionalized spin excitations, the quantum spin liquid 4 . In particular, the so-called Kitaev spin model 5 , consisting of a network of spins on a honeycomb lattice, is predicted to host Majorana fermions as its excitations. By means of a combination of specific heat measurements and inelastic neutron scattering experiments, we demonstrate the emergence of Majorana fermions in single crystals of α-RuCl 3 , an experimental realization of the Kitaev spin lattice. The specific heat data unveils a two-stage release of magnetic entropy that is characteristic of localized and itinerant Majorana fermions. The neutron scattering results corroborate this picture by revealing quasielastic excitations at low energies around the Brillouin zone centre and an hour-glass-like magnetic continuum at high energies. Our results confirm the presence of Majorana fermions in the Kitaev quantum spin liquid and provide an opportunity to build a unified conceptual framework for investigating fractionalized excitations in condensed matter 1,6-8 .Quantum spin liquids (QSLs) are an unconventional electronic phase of matter characterized by an absence of magnetic longrange order down to zero temperature. They are typically predicted to occur in geometrically frustrated magnets such as triangular, kagome, and pyrochlore lattices 4 , and typically display a macroscopic degeneracy that stabilizes a topologically ordered ground state. The Kitaev QSL state arises as an exact solution of the ideal two-dimensional (2D) honeycomb lattice with bond-directional Ising-type interactions (H = J γ K S γ i S γ j ; γ = x, y, z) on the three dis-

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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.