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      Atomic-scale observation of structural and electronic orders in the layered compound α-RuCl 3

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          Abstract

          A pseudospin-1/2 Mott phase on a honeycomb lattice is proposed to host the celebrated two-dimensional Kitaev model which has an elusive quantum spin liquid ground state, and fascinating physics relevant to the development of future templates towards topological quantum bits. Here we report a comprehensive, atomically resolved real-space study by scanning transmission electron and scanning tunnelling microscopies on a novel layered material displaying Kitaev physics, α-RuCl 3. Our local crystallography analysis reveals considerable variations in the geometry of the ligand sublattice in thin films of α-RuCl 3 that opens a way to realization of a spatially inhomogeneous magnetic ground state at the nanometre length scale. Using scanning tunnelling techniques, we observe the electronic energy gap of ≈0.25 eV and intra-unit cell symmetry breaking of charge distribution in individual α-RuCl 3 surface layer. The corresponding charge-ordered pattern has a fine structure associated with two different types of charge disproportionation at Cl-terminated surface.

          Abstract

          The two-dimensional Kitaev model is a quantum spin liquid state that theory predicts should appear in some materials with a honeycomb lattice. Here, the authors use atom-resolution scanning transmission electron and scanning tunnelling microscopies to characterize one such candidate material, α-RuCl 3.

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          Generalized Gradient Approximation Made Simple.

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            Mott Insulators in the Strong Spin-Orbit Coupling Limit: From Heisenberg to a Quantum Compass and Kitaev Models

            We study the magnetic interactions in Mott-Hubbard systems with partially filled \(t_{2g}\)-levels and with strong spin-orbit coupling. The latter entangles the spin and orbital spaces, and leads to a rich variety of the low energy Hamiltonians that extrapolate from the Heisenberg to a quantum compass model depending on the lattice geometry. This gives way to "engineer" in such Mott insulators an exactly solvable spin model by Kitaev relevant for quantum computation. We, finally, explain "weak" ferromagnetism, with an anomalously large ferromagnetic moment, in Sr\(_2\)IrO\(_4\).
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              Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet

              Quantum spin liquids (QSLs) are topological states of matter exhibiting remarkable properties such as the capacity to protect quantum information from decoherence. Whereas their featureless ground states have precluded their straightforward experimental identification, excited states are more revealing and particularly interesting owing to the emergence of fundamentally new excitations such as Majorana fermions. Ideal probes of these excitations are inelastic neutron scattering experiments. These we report here for a ruthenium-based material, α-RuCl3, continuing a major search (so far concentrated on iridium materials) for realizations of the celebrated Kitaev honeycomb topological QSL. Our measurements confirm the requisite strong spin-orbit coupling and low-temperature magnetic order matching predictions proximate to the QSL. We find stacking faults, inherent to the highly two-dimensional nature of the material, resolve an outstanding puzzle. Crucially, dynamical response measurements above interlayer energy scales are naturally accounted for in terms of deconfinement physics expected for QSLs. Comparing these with recent dynamical calculations involving gauge flux excitations and Majorana fermions of the pure Kitaev model, we propose the excitation spectrum of α-RuCl3 as a prime candidate for fractionalized Kitaev physics.
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                Author and article information

                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Publishing Group
                2041-1723
                12 December 2016
                2016
                : 7
                : 13774
                Affiliations
                [1 ]Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, USA
                [2 ]Institute for Functional Imaging of Materials, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, USA
                [3 ]Quantum Condensed Matter Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, USA
                [4 ]Bredesen Center for Interdisciplinary Research, University of Tennessee , Knoxville, Tennessee 37996, USA
                [5 ]Computer Science and Mathematics Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, USA
                [6 ]Material Science & Technology Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, USA
                [7 ]Department of Material Science and Engineering, University of Tennessee , Knoxville, Tennessee 37996, USA
                [8 ]Chemical Sciences Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, USA
                Author notes
                Article
                ncomms13774
                10.1038/ncomms13774
                5159869
                27941761
                7b7ff1d6-3900-4793-8dfa-d435074dabc7
                Copyright © 2016, The Author(s)

                This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

                History
                : 01 July 2016
                : 01 November 2016
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