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...

Quantum phase transitions take place between distinct phases of matter at zero temperature. Near the transition point, exotic quantum symmetries can emerge that govern the excitation spectrum of the system. A symmetry described by the E8 Lie group with a spectrum of 8 particles was long predicted to appear near the critical point of an Ising chain. We realize this system experimentally by tuning the quasi-one-dimensional Ising ferromagnet CoNb 2 O 6 through its critical point using strong transverse magnetic fields. The spin excitations are observed to change character from pairs of kinks in the ordered phase to spin-flips in the paramagnetic phase. Just below the critical field, the spin dynamics shows a fine structure with two sharp modes at low energies, in a ratio that approaches the golden mean as predicted for the first two meson particles of the E8 spectrum. Our results demonstrate the power of symmetry to describe complex quantum behaviours.

The Kitaev quantum spin liquid (KQSL) is an exotic emergent state of matter exhibiting Majorana fermion and gauge flux excitations. The magnetic insulator α-RuCl is thought to realize a proximate KQSL. We used neutron scattering on single crystals of α-RuCl to reconstruct dynamical correlations in energy-momentum space. We discovered highly unusual signals, including a column of scattering over a large energy interval around the Brillouin zone center, which is very stable with temperature. This finding is consistent with scattering from the Majorana excitations of a KQSL. Other, more delicate experimental features can be transparently associated with perturbations to an ideal model. Our results encourage further study of this prototypical material and may open a window into investigating emergent magnetic Majorana fermions in correlated materials.

The ground-state ordering and dynamics of the two-dimensional (2D) S=1/2 frustrated Heisenberg antiferromagnet Cs2CuCl4 is explored using neutron scattering in high magnetic fields. We find that the dynamic correlations show a highly dispersive continuum of excited states, characteristic of the RVB state, arising from pairs of S=1/2 spinons. Quantum renormalization factors for the excitation energies (1.65) and incommensuration (0.56) are large.The concept of fractional quantum states is central to the modern theory of strongly correlated systems. In magnetism, the most famous example is the spin S=1/2 1D Heisenberg antiferromagnetic chain (HAFC) where pairs of S=1/2 spinons are deconfined from locally allowed S=1 states; a phenomenon that is now well established both theoretically [1] and experimentally [2]. These spinons are topological excitations identified with quantum domain walls. Experimentally, such fractionalization is manifest as a highly dispersive continuum in the dynamical magnetic susceptibility measured by e.g. neutron scattering [2], and for the HAFC identified as creation of pairs of spinons.In 1973 Anderson [3] suggested that a 2D fractional quantum spin liquid may take the form of a "resonating valence bond" (RVB) state comprising singlet spin pairings in the ground state, and with pairs of excited S=1/2 spinons separating via rearrangement of those bonds. The dominant feature of the RVB state, present in all its theoretical descriptions [4][5][6] is an extended, highlydispersive, continuum. To date this feature remains unobserved in any 2D magnet; in the case of the S=1/2 Heisenberg square lattice (HSL) mean field confining effects lead to S=1 magnons and a renormalized classical picture of fluctuations around local Néel order emerges [7,8]. One may think, however, that because frustrating interactions can counteract the staggered fields responsible for confinement [8,9], they may provide a route to generating fractional phases in 2D.We explore such a scenario by making neutron scattering studies on Cs 2 CuCl 4 . By exploiting its unique experimental properties as a low-exchange quantum magnet [10] we reveal an unexpectedly strong two-dimensionality in the form of a triangular antiferromagnet with partially released frustration. The simplicity of the couplings in Cs 2 CuCl 4 makes it a model system to investigate generic features of 2D frustrated quantum antiferromagnets.The structure of Cs 2 CuCl 4 is orthorhombic (Pnma) with lattice parameters a=9.65Å, b=7.48Å and c=12.35Å at 0.3 K. Magnetic interactions are mostly restricted between Cu 2+ S=1/2 spin-sites in the (b, c) plane, see Fig. 1(a), with coupling J along b ("chains") and zigzag "interchain" coupling J ′ along the c-axis [11]. A small interlayer coupling J ′′ < 10 −2 J (along a) stabilizes 3D order below T N = 0.62 K into an incommensurate structure along b due to the frustrated couplings; weak anisotropies confine the ordered moments to rotate in cycloids near coincident with the (b, c) plane, see Fig. 1(d) but with a small tilt...

In principle, a complex assembly of strongly interacting electrons can self-organize into a wide variety of collective states, but relatively few such states have been identified in practice. We report that, in the close vicinity of a metamagnetic quantum critical point, high-purity strontium ruthenate Sr3Ru2O7 possesses a large magnetoresistive anisotropy, consistent with the existence of an electronic nematic fluid. We discuss a striking phenomenological similarity between our observations and those made in high-purity two-dimensional electron fluids in gallium arsenide devices.