2,361 research outputs found
Spin dynamics of counterrotating Kitaev spirals via duality
Incommensurate spiral order is a common occurrence in frustrated magnetic
insulators. Typically, all magnetic moments rotate uniformly, through the same
wavevector. However the honeycomb iridates family Li2IrO3 shows an
incommensurate order where spirals on neighboring sublattices are
counter-rotating, giving each moment a different local environment.
Theoretically describing its spin dynamics has remained a challenge: the Kitaev
interactions proposed to stabilize this state, which arise from strong
spin-orbit effects, induce magnon umklapp scattering processes in spin-wave
theory. Here we propose an approach via a (Klein) duality transformation into a
conventional spiral of a frustrated Heisenberg model, allowing a direct
derivation of the dynamical structure factor. We analyze both Kitaev and
Dzyaloshinskii-Moriya based models, both of which can stabilize counterrotating
spirals, but with different spin dynamics, and we propose experimental tests to
identify the origin of counterrotation.Comment: 4 pages, 3 figures; appendix 5 pages, 2 figure
Valence Bonds in Random Quantum Magnets: Theory and Application to YbMgGaO4
We analyze the effect of quenched disorder on spin-1/2 quantum magnets in
which magnetic frustration promotes the formation of local singlets. Our
results include a theory for 2d valence-bond solids subject to weak bond
randomness, as well as extensions to stronger disorder regimes where we make
connections with quantum spin liquids. We find, on various lattices, that the
destruction of a valence-bond solid phase by weak quenched disorder leads
inevitably to the nucleation of topological defects carrying spin-1/2 moments.
This renormalizes the lattice into a strongly random spin network with
interesting low-energy excitations. Similarly when short-ranged valence bonds
would be pinned by stronger disorder, we find that this putative glass is
unstable to defects that carry spin-1/2 magnetic moments, and whose residual
interactions decide the ultimate low energy fate. Motivated by these results we
conjecture Lieb-Schultz-Mattis-like restrictions on ground states for
disordered magnets with spin-1/2 per statistical unit cell. These conjectures
are supported by an argument for 1d spin chains. We apply insights from this
study to the phenomenology of YbMgGaO, a recently discovered triangular
lattice spin-1/2 insulator which was proposed to be a quantum spin liquid. We
instead explore a description based on the present theory. Experimental
signatures, including unusual specific heat, thermal conductivity, and
dynamical structure factor, and their behavior in a magnetic field, are
predicted from the theory, and compare favorably with existing measurements on
YbMgGaO and related materials.Comment: v2: Stylistic revisions to improve clarity. 22 pages, 8 figures, 2
tables main text; 13 pages, 3 figures appendice
Minimal models for topological Weyl semimetals
Topological Weyl semimetals (TWS) can be classified as type-I TWS, in which
the density of states vanishes at the Weyl nodes, and type-II TWS where an
electron and a hole pocket meet with finite density of states at the nodal
energy. The dispersions of type-II Weyl nodes are tilted and break Lorentz
invariance, allowing for physical properties distinct from those in a type-I
TWS. We present minimal lattice models for both time-reversal-breaking and
inversion-breaking type-II Weyl semimetals, and investigate their bulk
properties and topological surface states. These lattice models capture the
extended Fermi pockets and the connectivities of Fermi arcs. In addition to the
Fermi arcs, which are topologically protected, we identify surface "track
states" that arise out of the topological Fermi arc states at the transition
from type-I to type-II with multiple Weyl nodes, and persist in the type-II
TWS.Comment: 13 pages, 9 figure
Topological Crystalline Bose Insulator in Two Dimensions via Entanglement Spectrum
Strongly correlated analogues of topological insulators have been explored in
systems with purely on-site symmetries, such as time-reversal or charge
conservation. Here, we use recently developed tensor network tools to study a
quantum state of interacting bosons which is featureless in the bulk, but
distinguished from an atomic insulator in that it exhibits entanglement which
is protected by its spatial symmetries. These properties are encoded in a model
many-body wavefunction that describes a fully symmetric insulator of bosons on
the honeycomb lattice at half filling per site. While the resulting integer
unit cell filling allows the state to bypass `no-go' theorems that trigger
fractionalization at fractional filling, it nevertheless has nontrivial
entanglement, protected by symmetry. We demonstrate this by computing the
boundary entanglement spectra, finding a gapless entanglement edge described by
a conformal field theory as well as degeneracies protected by the non-trivial
action of combined charge-conservation and spatial symmetries on the edge.
Here, the tight-binding representation of the space group symmetries plays a
particular role in allowing certain entanglement cuts that are not allowed on
other lattices of the same symmetry, suggesting that the lattice representation
can serve as an additional symmetry ingredient in protecting an interacting
topological phase. Our results extend to a related insulating state of
electrons, with short-ranged entanglement and no band insulator analogue.Comment: 18 pages, 13 figures Added additional reference
Robust non-Abelian spin liquid and possible intermediate phase in antiferromagnetic Kitaev model with magnetic field
We investigate the non-Abelian topological chiral spin liquid phase in the
two-dimensional (2D) Kitaev honeycomb model subject to a magnetic field. By
combining density matrix renormalization group (DMRG) and exact diagonalization
(ED) we study the energy spectra, entanglement, topological degeneracy, and
expectation values of Wilson loop operators, allowing for robust
characterization. While the ferromagnetic (FM) Kitaev spin liquid is already
destroyed by a weak magnetic field with Zeeman energy , the antiferromagnetic (AFM) spin liquid remains robust up to a magnetic
field that is an order of magnitude larger, .
Interestingly, for larger fields , an
intermediate gapless phase is observed, before a second transition to the
high-field partially-polarized paramagnet. We attribute this rich phase
diagram, and the remarkable stability of the chiral topological phase in the
AFM Kitaev model, to the interplay of strong spin-orbit coupling and
frustration enhanced by the magnetic field. Our findings suggest relevance to
recent experiments on RuCl under magnetic fields.Comment: 8 pages, 8 figure
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