Topological states of matter such as quantum spin liquids (QSLs) are of great interest because of their remarkable predicted properties including protection of quantum information and the emergence of Majorana fermions. Such QSLs, however, have proven difficult to identify experimentally. The most promising approach is to study their exotic nature via the wavevector and intensity dependence of their dynamical response in neutron scattering. A major search has centered on iridate materials which are proposed to realize the celebrated Kitaev model on a honeycomb lattice -a prototypical topological QSL system in two dimensions (2D). The difficulties of iridium for neutron measurements have, however, impeded progress significantly. Here we provide experimental evidence that a material based on ruthenium, α-RuCl 3 realizes the same Kitaev physics but is highly amenable to neutron investigation. Our measurements confirm the requisite strong spin-orbit coupling, and a low temperature 2 magnetic order that matches the predicted phase proximate to the QSL. We also show that stacking faults, inherent to the highly 2D nature of the material, readily explain some puzzling results to date. Measurements of the dynamical response functions, especially at energies and temperatures above that where interlayer effects are manifest, are naturally accounted for in terms of deconfinement physics expected for QSLs. Via a comparison to the recently calculated dynamics from gauge flux excitations and Majorana fermions of the pure Kitaev model we propose α-RuCl 3 as the prime candidate for experimental realization of fractionalized Kitaev physics.Exotic physics associated with frustrated quantum magnets is an enduring theme in condensed matter research. The formation of quantum spin liquids (QSL) The Kitaev model consists of a set of spin-1/2 moments � ���⃗ � arrayed on a honeycomb lattice. The Kitaev couplings, of strength K in eqn.(1) are highly anisotropic with a different spin component interacting for each of the three bonds of the honeycomb lattice. In actual materials a Heisenberg interaction (J) is also generally expected to be present, giving rise to the Heisenberg-Kitaev (H-K) Hamiltonian given by 11 .where, for example, m is the component of the spin directed along the bond connecting spins (i,j). The QSL phase of the pure Kitaev model (J=0), for both ferro and antiferromagnetic K, is stable for relatively small Heisenberg perturbations.Remarkably the Hamiltonian (1) has been proposed to accurately describe octahedrallycoordinated magnetic systems, Fig. 1 21 -27 . Whilst these studies lend support to the material as a potential Kitaev material, conflicting results centering on the low temperature magnetic properties have hindered progress. To resolve this we undertake a comprehensive evaluation of the magnetic and spin orbit properties of α-RuCl 3 , and further measure the dynamical response establishing this as a material proximate to the widely searched for quantum spin liquid.We begin by investigating the crystal and m...

2 The Kitaev model on a honeycomb lattice predicts a paradigmatic quantum spin liquid (QSL) exhibiting Majorana Fermion excitations. The insight that Kitaev physics might be realized in practice has stimulated investigations of candidate materials, recently including α-RuCl3. In all the systems studied to date, significant non-Kitaev interactions induce magnetic order at low temperature. However, inplane magnetic fields of roughly 8 Tesla suppress the long-range magnetic order in α-RuCl3 raising the intriguing possibility of a field-induced QSL exhibiting non-Abelian quasiparticle excitations. Here we present inelastic neutron scattering in α-RuCl3 in an applied magnetic field. At a field of 8 Tesla the spin waves characteristic of the ordered state vanish throughout the Brillouin zone. The remaining single dominant feature of the response is a broad continuum centered at the Γ point, previously identified as a signature of fractionalized excitations. This provides compelling evidence that a field-induced QSL state has been achieved. 3 The Kitaev model on a honeycomb lattice [1] has been exactly solved to reveal a unique quantum spin liquid (QSL) exhibiting itinerant Majorana Fermion and gauge-flux excitations. The Kitaev candidate system α-RuCl3 is an insulating magnetic material comprised of van der Waals coupled honeycomb layers of 4d 5 Ru 3+ cations nearly centered in edge-sharing RuCl6 octahedra. A strong cubic crystal field combined with spin-orbit coupling leads to a Kramer's doublet, nearly perfect J = 1/2 ground state [2][3][4], thus satisfying the conditions necessary for producing Kitaev couplings in the low energy Hamiltonian [5]. Similar to the widely studied honeycomb [6] and hyper-honeycomb [7] Iridates, at low temperatures α-RuCl3 exhibits small-moment antiferromagnetic zigzag order [3,[8][9][10][11] with TN ≈ 7 K for crystals with minimal stacking faults. In the zigzag state the magnetic excitation spectrum shows well-defined low-energy spin waves with minima at the M points (See Supplementary Materials (SM) Fig. S1 for the Brillouin Zone (BZ) definition) as well as a broad continuum that extends to much higher energies centered at the Γ points [12,13]. Above TN the spin waves disappear but the continuum remains, essentially unchanged until high temperatures of the order of 100 K [3,12,13]. In analogy with the situation for coupled spin-½ antiferromagnetic Heisenberg chains [14], the high energy part of the continuum has been interpreted as a signature of fractionalized excitations [3,12,13]. The overall features of the inelastic neutron scattering (INS) response resemble those of the Kitaev QSL [15][16][17] and are consistent with an unusual response seen in Raman scattering [16,18,19], suggesting that the system is proximate to a QSL state exhibiting magnetic Majorana fermion excitations [3,12,13]. Magnetic field offers a clean quantum tuning parameter for Kitaev materials [7][8][9]20] and can be applied on large single crystals facilitating INS studies. It is known to suppress the magnetic ord...

Recent high-pressure studies found that superconductivity can be achieved under very low pressure in the parent iron arsenide compound CaFe2As2, although details of the sharpness and temperature of transitions vary between liquid medium and gas medium measurements. To better understand this issue, we performed high-pressure susceptibility and transport studies on CaFe2As2, using helium as the pressure medium. The signatures of the transitions to the low-temperature orthorhombic and collapsed tetragonal phase remained exceptionally sharp and no signature of bulk superconductivity was found under our hydrostatic conditions. Our results suggest that phase separation and superconductivity in CaFe2As2 are induced by non-hydrostatic conditions associated with the frozen liquid media. PACS numbers: 74.20.Mn, 74.25.Fy, 74.25.Dw, 74.62.Fj, 64.70.Tg The recent discovery of superconductivity in doped iron arsenide compounds[1, 2, 3] and the later improvement of the superconducting transition temperature T c in both the pnictide oxides such as ROFeAs (R111) [4,5,6,7] and the ThCr 2 Si 2 -structure compounds such as (Ba,K)Fe 2 As 2 (R122) [3] have caused extensive experimental and theoretical studies in this new class of materials with layered FeAs planes. Similar to the high-T c cuprates, the parent compounds exhibit structural transitions from a high-temperature tetragonal phase to a lowtemperature orthorhombic phase, and the orthorhombic phase is usually antiferromagnetically (AF) ordered [8]. Upon doping, both the orthorhombic structure and the AF phase are suppressed and superconductivity is induced.Several unique properties have been found in the iron arsenide superconductors. For example, these materials are semimetals and therefore metallic even without doping, in contrast to the cuprates. In BaFe 2 As 2 , doping Co into the FeAs-plane also induces superconductivity [9], which differs from the suppression of superconductivity and formation of local moments by any doping into the cuprate CuO-planes. Superconductivity has been reported under hydrostatic pressure in the parent compounds CaFe 2 As 2 [10, 11, 12], SrFe 2 As 2 [13, 14, 15], and BaFe 2 As 2 [14]. In particular, for CaFe 2 As 2 , T c as high as 10K has been found in a moderate 0.4GPa pressure [10,11,12], while for SrFe 2 As 2 and BaFe 2 As 2 , superconductivity is achieved at about 28K at P=3.2 GPa and 4.5 GPa respectively [14].In CaFe 2 As 2 in ambient pressure, a structural phase transition (from tetragonal to orthorhombic) is seen at T S1 = 170 K[16], accompanied by the appearance of magnetic order [17]; this transition is seen as a sharp upwards anomaly in resistivity. Hydrostatic pressure causes a reduction of T S1 . The signature in resistivity becomes a broad upturn, rather than the sharp discontinuous change seen in ambient pressure [10,12]. Above 0.5GPa, a collapsed tetragonal structure is identified below a separate structural transition temperature (T S2 ) [10,18]. The collapsed tetragonal phase has the same crystal symmetry as the high-temperat...

Two B-site ordered double perovskites, La 2 LiReO 6 and Ba 2 YReO 6 , with S = 1 were investigated as geometrically frustrated antiferromagnets, using x-ray and neutron diffraction, superconducting quantum interference device magnetometry, heat capacity, muon spin relaxation ͑SR͒, and 89 Y magic-angle spinning ͑MAS͒ NMR. La 2 LiReO 6 has a monoclinic structure ͑P2 1 / n͒ with cell parameters at room temperature; a = 5.58262͑22͒ Å, b = 5.67582͑20͒ Å, c = 7.88586͑27͒ Å, and ␤ = 90.240͑4͒°. A zero-field cooled/field cooled ͑ZFC/FC͒ divergence at 50 K was observed in the susceptibility. The ZFC susceptibility is zero below ϳ5 K for polycrystalline samples, suggesting a cooperative singlet ground state but weak moments are induced by cooling in very small fields ϳ1 mT. No evidence of long-range ordering is evident in heat capacity, neutrondiffraction, or SR data. The ZF spin dynamics from SR are anomalous and can be fitted to a stretched exponential rather than the Kubo-Toyabe form expected for random frozen spins but the muon spins are decoupled in longitudinal fields ͑LF͒, consistent with spin freezing of the fraction of spins relaxing within the muon time scale. The internal fields sensed by the muons are anomalously small, consistent with an electronic spin-singlet state. Ba 2 YReO 6 is found to be cubic ͑Fm3m͒ with cell parameter a = 8.36278͑2͒ Å at 300 K with no change in symmetry at 3.8 K, at variance with the Jahn-Teller theorem for a t 2g 2 configuration for Re 5+ . 89 Y MAS NMR shows a single peak indicating that Y/Re site disorder is at most 0.5%. The susceptibility shows two broad peaks around 50 and 25 K but no evidence for long-range order from heat capacity, neutron diffraction, or SR. The ZF SR result shows a two-component ground state with both slow and fast relaxations and decoupling results in a 1 kG LF, indicating spin freezing. These results are in sharp contrast to the long-range AF order found in the S =3/ 2 isostructural materials, La 2 LiRuO 6 and Ba 2 YRuO 6 , indicating that the reduction to S = 1 plays a major role in ground state determination.

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In a prototypical ferromagnet (Ga,mn)As based on a III-V semiconductor, substitution of divalent mn atoms into trivalent Ga sites leads to severely limited chemical solubility and metastable specimens available only as thin films. The doping of hole carriers via (Ga,mn) substitution also prohibits electron doping. To overcome these difficulties, masek et al. theoretically proposed systems based on a I-II-V semiconductor LiZnAs, where isovalent (Zn,mn) substitution is decoupled from carrier doping with excess/deficient Li concentrations. Here we show successful synthesis of Li 1 + y (Zn 1 − x mn x )As in bulk materials. Ferromagnetism with a critical temperature of up to 50 K is observed in nominally Li-excess (y = 0.05-0.2) compounds with mn concentrations of x = 0.02-0.15, which have p-type metallic carriers. This is presumably due to excess Li in substitutional Zn sites. semiconducting LiZnAs, ferromagnetic Li(Zn,mn)As, antiferromagnetic LimnAs, and superconducting LiFeAs systems share square lattice As layers, which may enable development of novel junction devices in the future.

Intrinsic, two-dimensional ferromagnetic semiconductors are an important class of materials for overcoming the limitations of dilute magnetic semiconductors for spintronics applications. CrSiTe3 is a particularly interesting member of this class, since it can likely be exfoliated down to single layers, where TC is predicted to increase dramatically. Establishing the nature of the magnetism in the bulk is a necessary precursor to understanding the magnetic behavior in thin film samples and the possible applications of this material. In this work, we use elastic and inelastic neutron scattering to measure the magnetic properties of single crystalline CrSiTe3. We find that there is a very small single ion anisotropy favoring magnetic ordering along the c-axis and that the measured spin waves fit well to a model where the moments are only weakly coupled along that direction. Finally, we find that both static and dynamic correlations persist within the ab-plane up to at least 300 K, strong evidence of this material's two-dimensional characteristics that are relevant for future studies on thin film and monolayer samples.

We present a neutron diffraction study of FeV2O4, which is rare in exhibiting spin and orbital degrees of freedom on both cation sublattices of the spinel structure. Our data confirm the existence of three structural phase transitions previously identified with x-ray powder diffraction, and reveal that the lower two transitions are associated with sequential collinear and canted ferrimagnetic transitions involving both cation sites. Through consideration of local crystal and spin symmetry, we further conclude that Fe2+ cations are ferro-orbitally ordered below 135K and V3+ orbitals order at 60K in accordance with predictions for vanadium spinels with large trigonal distortions and strong spin-orbit coupling. Intriguingly, the direction of ordered vanadium spins at low temperatures obey `ice rules' more commonly associated with the frustrated rare-earth pyrochlore systems.Comment: 10 pages, 9 figures, including Supplemental Materia