1,362 research outputs found
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
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
Magnetic excitations in coupled Haldane spin chains near the quantum critical point
Two quasi-1-dimensional S=1 quantum antiferromagnetic materials, PbNi2V2O8
and SrNi2V2O8, are studied by inelastic neutron scattering on powder samples.
While magnetic interactions in the two systems are found to be very similar,
subtle differences in inter-chain interaction strengths and magnetic anisotropy
are detected. The latter are shown to be responsible for qualitatively
different ground state properties: magnetic long-range order in SrNi2V2O8 and
disordered ``spin liquid'' Haldane-gap state in PbNi2V2O8.Comment: 15 figures, Figs. 5,9, and 10 in color. Some figures in JPEG format.
Complete PostScript and PDF available from
http://papillon.phy.bnl.gov/publicat.ht
Spin correlations among the charge carriers in an ordered stripe phase
We have observed a diffuse component to the low-energy magnetic excitation
spectrum of stripe-ordered La5/3Sr1/3NiO4 probed by neutron inelastic
scattering. The diffuse scattering forms a square pattern with sides parallel
and perpendicular to the stripe directions. The signal is dispersive, with a
maximum energy of ~10 meV. Probed at 2 meV the scattering decreases in strength
with increasing temperature, and is barely visible at 100 K. We argue that the
signal originates from dynamic, quasi- one-dimensional, antiferromagnetic
correlations among the stripe electrons.Comment: 4 pages, 4 figures. To be published in Physical Review Letter
The critical behavior of frustrated spin models with noncollinear order
We study the critical behavior of frustrated spin models with noncollinear
order, including stacked triangular antiferromagnets and helimagnets. For this
purpose we compute the field-theoretic expansions at fixed dimension to six
loops and determine their large-order behavior. For the physically relevant
cases of two and three components, we show the existence of a new stable fixed
point that corresponds to the conjectured chiral universality class. This
contradicts previous three-loop field-theoretical results but is in agreement
with experiments.Comment: 4 pages, RevTe
Electronic structure and magnetic properties of the spin-1/2 Heisenberg system CuSe2O5
A microscopic magnetic model for the spin-1/2 Heisenberg chain compound
CuSe2O5 is developed based on the results of a joint experimental and
theoretical study. Magnetic susceptibility and specific heat data give evidence
for quasi-1D magnetism with leading antiferromagnetic (AFM) couplings and an
AFM ordering temperature of 17 K. For microscopic insight, full-potential DFT
calculations within the local density approximation (LDA) were performed. Using
the resulting band structure, a consistent set of transfer integrals for an
effective one-band tight-binding model was obtained. Electronic correlations
were treated on a mean-field level starting from LDA (LSDA+U method) and on a
model level (Hubbard model). In excellent agreement of experiment and theory,
we find that only two couplings in CuSe2O5 are relevant: the nearest-neighbour
intra-chain interaction of 165 K and a non-frustrated inter-chain coupling of
20 K. From a comparison with structurally related systems (Sr2Cu(PO4)2,
Bi2CuO4), general implications for a magnetic ordering in presence of
inter-chain frustration are made.Comment: 20 pages, 8 figures, 3 table
Excitations of the field-induced soliton lattice in CuGeO3
Here we report the first inelastic neutron scattering study of the magnetic
excitations in the incommensurate phase of a spin-Peierls material. The results
on CuGeO3 provide direct evidence of a finite excitation gap, two sharp
magnetic excitation branches and a very low-lying excitation which is
identified as a phason mode, the Goldstone mode of the incommensurate soliton
lattice.Comment: 5 pages, revtex, 4 figures (*.eps), win-zippe
Spin dynamics in stripe-ordered La5/3Sr1/3NiO4
Polarized and unpolarized neutron inelastic scattering has been used to
measure the spin excitations in the spin-charge-ordered stripe phase of
La5/3Sr1/3NiO4. At high energies, sharp magnetic modes are observed
characteristic of a static stripe lattice. The energy spectrum is described
well by a linear spin wave model with intra- and inter-stripe exchange
interactions between neighbouring Ni spins given by J = 15 +/- 1.5 meV and J' =
7.5 +/- 1.5 meV respectively. A pronounced broadening of the magnetic
fluctuations in a band between 10 meV and 25 meV is suggestive of coupling to
collective motions of the stripe domain walls.Comment: ReVTeX 4, 4 pages inc. 4 Fig
- …