1,517 research outputs found
Ferroelectricity from spin supercurrents in LiCuVO4
We have studied the magnetic structure of the ferroelectric frustrated
spin-1/2 chain material LiCuVO4 in applied electric and magnetic fields using
polarized neutrons. A symmetry and mean-field analysis of the data rules out
the presence of static Dzyaloshinskii-Moriya interaction, while exchange
striction is shown to be negligible by our specific-heat measurements. The
experimentally observed magnetoelectric coupling is in excellent agreement with
the predictions of a purely electronic mechanism based on spin supercurrents.Comment: 4 pages, 3 figures, final versio
Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain
Assemblies of interacting quantum particles often surprise us with properties
that are difficult to predict. One of the simplest quantum many-body systems is
the spin 1/2 Heisenberg antiferromagnetic chain, a linear array of interacting
magnetic moments. Its exact ground state is a macroscopic singlet entangling
all spins in the chain. Its elementary excitations, called spinons, are
fractional spin 1/2 quasiparticles; they are created and detected in pairs by
neutron scattering. Theoretical predictions show that two-spinon states exhaust
only 71% of the spectral weight while higher-order spinon states, yet to be
experimentally located, are predicted to participate in the remaining. Here, by
accurate absolute normalization of our inelastic neutron scattering data on a
compound realizing the model, we account for the full spectral weight to within
99(8)%. Our data thus establish and quantify the existence of higher-order
spinon states. The observation that within error bars, the entire weight is
confined within the boundaries of the two-spinon continuum, and that the
lineshape resembles a rescaled two-spinon one, allow us to develop a simple
physical picture for understanding multi-spinon excitations.Comment: 22 pages, 4 figures, Supplementary material
Quenched chirality in RbNiCl
The critical behaviour of stacked-triangular antiferromagnets has been
intensely studied since Kawamura predicted new universality classes for
triangular and helical antiferromagnets. The new universality classes are
linked to an additional discrete degree of freedom, chirality, which is not
present on rectangular lattices, nor in ferromagnets. However, the theoretical
as well as experimental situation is discussed controversially, and generic
scaling without universality has been proposed as an alternative scenario. Here
we present a careful investigation of the zero-field critical behaviour of
RbNiCl, a stacked-triangular Heisenberg antiferromagnet with very small
Ising anisotropy. From linear birefringence experiments we determine the
specific heat exponent as well as the critical amplitude ratio
. Our high-resolution measurements point to a single second order
phase transition with standard Heisenberg critical behaviour, contrary to all
theoretical predictions. From a supplementary neutron diffraction study we can
exclude a structural phase transition at T. We discuss our results in the
context of other available experimental results on RbNiCl and related
compounds. We arrive at a simple intuitive explanation which may be relevant
for other discrepancies observed in the critical behaviour of
stacked-triangular antiferromagnets. In RbNiCl the ordering of the
chirality is suppressed by strong spin fluctuations, yielding to a different
phase diagram, as compared to e.g.\@ CsNiCl, where the Ising anisotropy
prevents these fluctuations
Haydeeite: a spin-1/2 kagome ferromagnet
The mineral haydeeite, alpha-MgCu3(OD)6Cl2, is a S=1/2 kagome ferromagnet
that displays long-range magnetic order below TC=4.2 K with a strongly reduced
moment. Our inelastic neutron scattering data show clear spin-wave excitations
that are well described by a Heisenberg Hamiltonian with ferromagnetic
nearest-neighbor exchange J1=-38 K and antiferromagnetic exchange Jd=+11 K
across the hexagons of the kagome lattice. These values place haydeeite very
close to the quantum phase transition between ferromagnetic order and
non-coplanar twelve-sublattice cuboc2 antiferromagnetic order. Diffuse dynamic
short-range ferromagnetic correlations observed above TC persist well into the
ferromagnetically ordered phase with a behavior distinct from critical
scattering
The food safety impact of salt and sodium reduction initiatives
Excessive or high salt or sodium intake is known to cause hypertension and other diseases. Within the United Kingdom voluntary targets for salt reduction have been set and laid out in the Secretary of State responsibility deal. This review considers the options available to food manufacturers to enable them to reduce salt and the potential food safety risks associated with those options. Gaps in research and knowledge within the areas of information supplied to food manufacturers, alternative solutions for salt replacement and the food safety impact of salt reduction are discussed
Low dimensional ordering and fluctuations in methanol--hydroquinone-clathrate studied by X-ray and neutron diffraction
Methanol--hydroquinone-clathrate has been established as a model
system for dielectric ordering and fluctuations and is conceptually close to
magnetic spin systems. In X-ray and neutron diffraction experiments, we
investigated the ordered structure, the one-dimensional (1D) and the
three-dimensional (3D) critical scattering in the paraelectric phase, and the
temperature dependence of the lattice constants. Our results can be explained
by microscopic models of the methanol pseudospin in the hydroquinone cage
network, in consistency with previous dielectric investigations
Evidence of a bond-nematic phase in LiCuVO4
Polarized and unpolarized neutron scattering experiments on the frustrated
ferromagnetic spin-1/2 chain LiCuVO4 show that the phase transition at HQ of 8
Tesla is driven by quadrupolar fluctuations and that dipolar correlations are
short-range with moments parallel to the applied magnetic field in the
high-field phase. Heat-capacity measurements evidence a phase transition into
this high-field phase, with an anomaly clearly different from that at low
magnetic fields. Our experimental data are consistent with a picture where the
ground state above HQ has a next-nearest neighbour bond-nematic order along the
chains with a fluid-like coherence between weakly coupled chains.Comment: 5 pages, 4 figures. To appear in Phys. Rev. Let
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