420 research outputs found
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Impact of disorder on dynamics and ordering in the honeycomb-lattice iridate Na2IrO3
Kitaev's honeycomb spin-liquid model and its proposed realization in materials such as α-RuCl3, Li2IrO3, and Na2IrO3 continue to present open questions about how the dynamics of a spin liquid are modified in the presence of non-Kitaev interactions as well as the presence of inhomogeneities. Here we use Na23 nuclear magnetic resonance to probe both static and dynamical magnetic properties in single-crystal Na2IrO3. We find that the NMR shift follows the bulk susceptibility above 30 K but deviates from it below; moreover below TN the spectra show a broad distribution of internal magnetic fields. Both of these results provide evidence for inequivalent magnetic sites at low temperature, suggesting inhomogeneities are important for the magnetism. The spin-lattice relaxation rate is isotropic and diverges at TN, suggesting that the Kitaev cubic axes may control the critical quantum spin fluctuations. In the ordered state, we observe gapless excitations, which may arise from site substitution, emergent defects from milder disorder, or possibly be associated with nearby quantum paramagnetic states distinct from the Kitaev spin liquid
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High-temperature magnetic anomaly in the Kitaev hyperhoneycomb compound β-Li2IrO3
We report the existence of a high-temperature magnetic anomaly in the three-dimensional Kitaev candidate material, β-Li2IrO3. Signatures of the anomaly appear in magnetization, heat capacity, and muon spin relaxation measurements. The onset coincides with a reordering of the principal axes of magnetization, which is thought to be connected to the onset of Kitaev-like correlations in the system. The anomaly also shows magnetic hysteresis with a spatially anisotropic magnitude that follows the spin-anisotropic exchange anisotropy of the underlying Kitaev Hamiltonian. We discuss possible scenarios for a bulk and impurity origin
Featureless and non-fractionalized Mott insulators on the honeycomb lattice at 1/2 site filling
Within the Landau paradigm, phases of matter are distinguished by spontaneous
symmetry breaking. Implicit here is the assumption that a completely symmetric
state exists: a paramagnet. At zero temperature such quantum featureless
insulators may be forbidden, triggering either conventional order or
topological order with fractionalized excitations. Such is the case for
interacting particles when the particle number per unit cell, f, is not an
integer. But, can lattice symmetries forbid featureless insulators even at
integer f? An especially relevant case is the honeycomb (graphene) lattice ---
where free spinless fermions at f=1 (the two sites per unit cell mean f=1 is
half filling per site) are always metallic. Here we present wave functions for
bosons, and a related spin-singlet wave function for spinful electrons, on the
f=1 honeycomb, and demonstrate via quantum to classical mappings that they do
form featureless Mott insulators. The construction generalizes to symmorphic
lattices at integer f in any dimension. Our results explicitly demonstrate that
in this case, despite the absence of a non-interacting insulator at the same
filling, lack of order at zero temperature does not imply fractionalization.Comment: v2: major revision including new result on SU(2) spinful electron
state and additional author. v3: PNAS published version. 7 pages, 5 figures;
appendix 5 pages, 3 figure
Exact Chiral Spin Liquids and Mean-Field Perturbations of Gamma Matrix Models on the Ruby Lattice
We theoretically study an exactly solvable Gamma matrix generalization of the
Kitaev spin model on the ruby lattice, which is a honeycomb lattice with
"expanded" vertices and links. We find this model displays an exceptionally
rich phase diagram that includes: (i) gapless phases with stable spin fermi
surfaces, (ii) gapless phases with low-energy Dirac cones and quadratic band
touching points, and (iii) gapped phases with finite Chern numbers possessing
the values {\pm}4,{\pm}3,{\pm}2 and {\pm}1. The model is then generalized to
include Ising-like interactions that break the exact solvability of the model
in a controlled manner. When these terms are dominant, they lead to a trivial
Ising ordered phase which is shown to be adiabatically connected to a large
coupling limit of the exactly solvable phase. In the limit when these
interactions are weak, we treat them within mean-field theory and present the
resulting phase diagrams. We discuss the nature of the transitions between
various phases. Our results highlight the richness of possible ground states in
closely related magnetic systems.Comment: 9 pages, 9 figure
Non-coplanar and counter-rotating incommensurate magnetic order stabilized by Kitaev interactions in -Li2IrO3
Materials that realize Kitaev spin models with bond-dependent anisotropic
interactions have long been searched for, as the resulting frustration effects
are predicted to stabilize novel forms of magnetic order or quantum spin
liquids. Here we explore the magnetism of -LiIrO, which has the
topology of a 3D Kitaev lattice of inter-connected Ir honeycombs. Using
resonant magnetic x-ray diffraction we find a complex, yet highly-symmetric
incommensurate magnetic structure with non-coplanar and counter-rotating Ir
moments. We propose a minimal Kitaev-Heisenberg Hamiltonian that naturally
accounts for all key features of the observed magnetic structure. Our results
provide strong evidence that -LiIrO realizes a spin Hamiltonian
with dominant Kitaev interactions.Comment: 10 pages, 7 figure
Direct Evidence for Dominant Bond-directional Interactions in a Honeycomb Lattice Iridate Na2IrO3
Heisenberg interactions are ubiquitous in magnetic materials and have been
prevailing in modeling and designing quantum magnets. Bond-directional
interactions offer a novel alternative to Heisenberg exchange and provide the
building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as
its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential
realizations of the Kitaev model, and their reported magnetic behaviors may be
interpreted within the Kitaev framework. However, the extent of their relevance
to the Kitaev model remains unclear, as evidence for bond-directional
interactions remains indirect or conjectural. Here, we present direct evidence
for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and
show that they lead to strong magnetic frustration. Diffuse magnetic x-ray
scattering reveals broken spin-rotational symmetry even above Neel temperature,
with the three spin components exhibiting nano-scale correlations along
distinct crystallographic directions. This spin-space and real-space
entanglement directly manifests the bond-directional interactions, provides the
missing link to Kitaev physics in honeycomb iridates, and establishes a new
design strategy toward frustrated magnetism.Comment: Nature Physics, accepted (2015
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