59 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
"Quasi-particle breakdown" in the quasi-one-dimensional Ising ferromagnet CoNbO
We present experimental and theoretical evidence that an interesting quantum
many-body effect -- quasi-particle breakdown -- occurs in the
quasi-one-dimensional spin-1/2 Ising-like ferromagnet CoNbO in its
paramagnetic phase at high transverse field as a result of explicit breaking of
spin inversion symmetry. We propose a quantum spin Hamiltonian capturing the
essential one-dimensional physics of CoNbO and determine the exchange
parameters of this model by fitting the calculated single particle dispersion
to the one observed experimentally in applied transverse magnetic fields. We
present high-resolution inelastic neutron scattering measurements of the single
particle dispersion which observe "anomalous broadening" effects over a narrow
energy range at intermediate energies. We propose that this effect originates
from the decay of the one particle mode into two-particle states. This decay
arises from (i) a finite overlap between the one-particle dispersion and the
two-particle continuum in a narrow energy-momentum range and (ii) a small
misalignment of the applied field away from the direction perpendicular to the
Ising axis in the experiments, which allows for non-zero matrix elements for
decay by breaking the spin inversion symmetry of the
Hamiltonian.Comment: v1: 15 pages, 10 figures. v2: 16 pages, 10 figures, minor changes, as
accepted to PR
Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnet Cs2CuCl4
We report magnetization and specific heat measurements in the 2D frustrated
spin-1/2 Heisenberg antiferromagnet Cs2CuCl4 at temperatures down to 0.05 K and
high magnetic fields up to 11.5 T applied along a, b and c-axes. The low-field
susceptibility chi (T) M/B shows a broad maximum around 2.8 K characteristic of
short-range antiferromagnetic correlations and the overall temperature
dependence is well described by high temperature series expansion calculations
for the partially frustrated triangular lattice with J=4.46 K and J'/J=1/3. At
much lower temperatures (< 0.4 K) and in in-plane field (along b and c-axes)
several new intermediate-field ordered phases are observed in-between the
low-field incommensurate spiral and the high-field saturated ferromagnetic
state. The ground state energy extracted from the magnetization curve shows
strong zero-point quantum fluctuations in the ground state at low and
intermediate fields
Anomalous Excitation Spectra of Frustrated Quantum Antiferromagnets
We use series expansions to study the excitation spectra of spin-1/2
antiferromagnets on anisotropic triangular lattices. For the isotropic
triangular lattice model (TLM) the high-energy spectra show several anomalous
features that differ strongly from linear spin-wave theory (LSWT). Even in the
Neel phase, the deviations from LSWT increase sharply with frustration, leading
to roton-like minima at special wavevectors. We argue that these results can be
interpreted naturally in a spinon language, and provide an explanation for the
previously observed anomalous finite-temperature properties of the TLM. In the
coupled-chains limit, quantum renormalizations strongly enhance the
one-dimensionality of the spectra, in agreement with experiments on Cs_2CuCl_4.Comment: 4 pages, 5 figures. New Fig. 3 with higher-order series data, paper
shortened, references updated, one added (Ref. 28), minor changes otherwise.
Published versio
Excitations of quantum Ising chain CoNb2O6 in low transverse field: quantitative description of bound states stabilized by off-diagonal exchange and applied field
We present experimental and theoretical evidence of novel bound state
formation in the low transverse field ordered phase of the
quasi-one-dimensional Ising-like material CoNbO. High resolution single
crystal inelastic neutron scattering measurements observe that small transverse
fields lead to a breakup of the spectrum into three parts, each evolving very
differently upon increasing field. We show that this can be naturally
understood starting from the excitations of the ordered phase of the transverse
field Ising model, domain wall quasiparticles (solitons). The transverse field
and a staggered off-diagonal exchange create one-soliton hopping terms with
opposite signs. This leads to a rich spectrum and a special field, when the
strengths of the off-diagonal exchange and transverse field match, at which
solitons become localized; the highest field investigated is very close to this
special regime. We solve this case analytically and find three two-soliton
continua, along with three novel bound states. We also present calculations
using exact diagonalization of a recently refined Hamiltonian model for
CoNbO and using diagonalization of the two-soliton subspace, both of
which provide a quantitative agreement with the observed spectrum. The
theoretical two-soliton model qualitatively and quantitatively captures a
variety of non-trivial features in the observed spectrum, providing insight
into the underlying physics of bound state formation.Comment: 17 pages, 5 figure
Spin dynamics of coupled spin ladders near quantum criticality in Ba2CuTeO6
We report inelastic neutron scattering measurements of the magnetic
excitations in Ba2CuTeO6, proposed by ab initio calculations to magnetically
realize weakly coupled antiferromagnetic two-leg spin-1/2 ladders. Isolated
ladders are expected to have a singlet ground state protected by a spin gap.
Ba2CuTeO6 orders magnetically, but with a small Neel temperature relative to
the exchange strength, suggesting that the interladder couplings are relatively
small and only just able to stabilize magnetic order, placing Ba2CuTeO6 close
in parameter space to the critical point separating the gapped phase and Neel
order. Through comparison of the observed spin dynamics with linear spin wave
theory and quantum Monte Carlo calculations, we propose values for all relevant
intra- and interladder exchange parameters, which place the system on the
ordered side of the phase diagram in proximity to the critical point. We also
compare high field magnetization data with quantum Monte Carlo predictions for
the proposed model of coupled ladders.Comment: 14 pages, 12 figure
Excitation spectra and ground state properties of the layered spin-1/2 frustrated antiferromagnets Cs_2CuCl_4 and Cs_2CuBr_4
We use series expansion methods to study ground- and excited-state properties
in the helically ordered phase of spin-1/2 frustrated antiferromagnets on an
anisotropic triangular lattice. We calculate the ground state energy, ordering
wavevector, sublattice magnetization and one-magnon excitation spectrum for
parameters relevant to Cs_2CuCl_4 and Cs_2CuBr_4. Both materials are modeled in
terms of a Heisenberg model with spatially anisotropic exchange constants; for
Cs_2CuCl_4 we also take into account the additional Dzyaloshinskii-Moriya (DM)
interaction. We compare our results for Cs_2CuCl_4 with unpolarized neutron
scattering experiments and find good agreement. In particular, the large
quantum renormalizations of the one-magnon dispersion are well accounted for in
our analysis, and inclusion of the DM interaction brings the theoretical
predictions for the ordering wavevector and the magnon dispersion closer to the
experimental results.Comment: 10 pages, 8 figure
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