12,164 research outputs found
Helicoidal magnetic order in a clean copper oxide spin chain compound
We report susceptibility, specific heat, and neutron diffraction measurements
on NaCuO, a spin-1/2 chain compound isostructural to LiCuO,
which has been extensively investigated. Below 13 K, we find a long-range
ordered, incommensurate magnetic helix state with a propagation vector similar
to that of LiCuO. In contrast to the Li analogue, substitutional
disorder is negligible in NaCuO. We can thus rule out that the helix is
induced by impurities, as was claimed on the basis of prior work on
LiCuO. A spin Hamiltonian with frustrated longer-range exchange
interactions provides a good description of both the ordered state and the
paramagnetic susceptibility.Comment: 4 pages, 4 figures Improved Fig.1 and 4. Minor rephrasing. Reference
adde
Magnetic excitations and phonons in the spin-chain compound NaCu2O2
We report an inelastic light scattering study of single-crystalline
NaCuO, a spin-chain compound known to exhibit a phase with helical
magnetic order at low temperatures. Phonon excitations were studied as a
function of temperature and light polarization, and the phonon frequencies are
compared to the results of ab-initio lattice dynamical calculations, which are
also reported here. The good agreement between the observed and calculated
modes allows an assignment of the phonon eigenvectors. Two distinct high-energy
two-magnon features as well as a sharp low-energy one-magnon peak were also
observed. These features are discussed in terms of the magnon modes expected in
a helically ordered state. Their polarization dependence provides evidence of
substantial exchange interactions between two closely spaced spin chains within
a unit cell. At high temperatures, the spectral features attributable to
magnetic excitations are replaced by a broad, quasielastic mode due to
overdamped spin excitations
Ab initio study of canted magnetism of finite atomic chains at surfaces
By using ab initio methods on different levels we study the magnetic ground
state of (finite) atomic wires deposited on metallic surfaces. A
phenomenological model based on symmetry arguments suggests that the
magnetization of a ferromagnetic wire is aligned either normal to the wire and,
generally, tilted with respect to the surface normal or parallel to the wire.
From a first principles point of view, this simple model can be best related
to the so--called magnetic force theorem calculations being often used to
explore magnetic anisotropy energies of bulk and surface systems. The second
theoretical approach we use to search for the canted magnetic ground state is
first principles adiabatic spin dynamics extended to the case of fully
relativistic electron scattering. First, for the case of two adjacent Fe atoms
an a Cu(111) surface we demonstrate that the reduction of the surface symmetry
can indeed lead to canted magnetism. The anisotropy constants and consequently
the ground state magnetization direction are very sensitive to the position of
the dimer with respect to the surface. We also performed calculations for a
seven--atom Co chain placed along a step edge of a Pt(111) surface. As far as
the ground state spin orientation is concerned we obtain excellent agreement
with experiment. Moreover, the magnetic ground state turns out to be slightly
noncollinear.Comment: 8 pages, 5 figures; presented on the International Conference on
Nanospintronics Design and Realizations, Kyoto, Japan, May 2004; to appear in
J. Phys.: Cond. Matte
The eta' meson from lattice QCD
We study the flavour singlet pseudoscalar mesons from first principles using
lattice QCD. With N_f=2 flavours of light quark, this is the so-called eta_2
meson and we discuss the phenomenological status of this. Using maximally
twisted-mass lattice QCD, we extract the mass of the eta_2 meson at two values
of the lattice spacing for lighter quarks than previously discussed in the
literature. We are able to estimate the mass value in the limit of light quarks
with their physical masses.Comment: 16 pages: version accepted for publicatio
Bond breaking in vibrationally excited methane on transition metal catalysts
The role of vibrational excitation of a single mode in the scattering of
methane is studied by wave packet simulations of oriented CH4 and CD4 molecules
from a flat surface. All nine internal vibrations are included. In the
translational energy range from 32 up to 128 kJ/mol we find that initial
vibrational excitations enhance the transfer of translational energy towards
vibrational energy and increase the accessibility of the entrance channel for
dissociation. Our simulations predict that initial vibrational excitations of
the asymmetrical stretch (nu_3) and especially the symmetrical stretch (nu_1)
modes will give the highest enhancement of the dissociation probability of
methane.Comment: 4 pages REVTeX, 2 figures (eps), to be published in Phys. Rev. B.
(See also arXiv:physics.chem-ph/0003031). Journal version at
http://publish.aps.org/abstract/PRB/v61/p1565
Perturbative calculation of improvement coefficients to O(g^2a) for bilinear quark operators in lattice QCD
We calculate the O(g^2 a) mixing coefficients of bilinear quark operators in
lattice QCD using a standard perturbative evaluation of on-shell Green's
functions. Our results for the plaquette gluon action are in agreement with
those previously obtained with the Schr\"odinger functional method. The
coefficients are also calculated for a class of improved gluon actions having
six-link terms.Comment: 14 pages, REVTe
Reliability of Clinician Rated Physical Effort Determination During Functional Capacity Evaluation in Patients with Chronic Musculoskeletal Pain
Introduction Functional capacity evaluation (FCE) can be used to make clinical decisions regarding fitness-for-work. During FCE the evaluator attempts to assess the amount of physical effort of the patient. The aim of this study is to analyze the reliability of physical effort determination using observational criteria during FCE. Methods Twenty-one raters assessed physical effort in 18 video-recorded FCE tests independently on two occasions, 10 months apart. Physical effort was rated on a categorical four-point physical effort determination scale (P-ED) based on the Isernhagen criteria, and a dichotomous submaximal effort determination scale (S-ED). Cohen's Kappa, squared weighted Kappa and % agreement were calculated. Results Kappa values for intra-rater reliability of P-ED and S-ED for all FCE tests were 0.49 and 0.68 respectively. Kappa values for inter-rater reliability of P-ED for all FCE tests in the first and the second session were 0.51, and 0.72, and for S-ED Kappa values were 0.68 and 0.77 respectively. The inter-rater reliability of P-ED ranged from kappa = 0.02 to kappa = 0.99 between FCE tests. Acceptable reliability scores (kappa > 0.60, agreement a parts per thousand yen80 %) for each FCE test were observed in 38 % of scores for P-ED and 67 % for S-ED. On average material handling tests had a higher reliability than postural tolerance and ambulatory tests. Conclusion Dichotomous ratings of submaximal effort are more reliable than categorical criteria to determine physical effort in FCE tests. Regular education and training may improve the reliability of observational criteria for effort determination
Magnetic and Metal-Insulator Transitions in beta-Na0.5CoO2 and gamma-K0.5CoO2 -NMR and Neutron Diffraction Studies-
Co-oxides beta-Na0.5CoO2 and gamma-K0.5CoO2 have been prepared by the Na
de-intercalation from alpha-NaCoO2 and by the floating-zone method,
respectively. It has been found that successive phase transitions take place at
temperatures Tc1 and Tc2 in both systems. The appearance of the internal
magnetic field at Tc1 with decreasing temperature T indicates that the
antiferromagnetic order exists at T < Tc1, as in gamma-Na0.5CoO2. For
beta-Na0.5CoO2, the transition temperatures and the NMR parameters determined
from the data taken for magnetically ordered state are similar to those of
gamma-Na0.5CoO2, indicating that the difference of the stacking ways of the
CoO2 layers between these systems do not significantly affect their physical
properties. For gamma-K0.5CoO2, the quantitative difference of the physical
quantities are found from those of beta- and gamma-Na0.5CoO2. The difference
between the values of Tci (i = 1 and 2) of these systems might be explained by
considering the distance between CoO2 layers.Comment: 8 pages, 14 figures, 1 Tabl
High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter CO 4-3 emission in the
Magellanic Clouds. The survey is comprised of 15 6'x6' maps obtained using the
AST/RO telescope toward the molecular peaks of the Large and Small Magellanic
Clouds. We have used these data to constrain the physical conditions in these
objects, in particular their molecular gas density and temperature. We find
that there are significant amounts of molecular gas associated with most of
these molecular peaks, and that high molecular gas temperatures are pervasive
throughout our sample. We discuss whether this may be due to the low
metallicities and the associated dearth of gas coolants in the Clouds, and
conclude that the present sample is insufficient to assert this effect.Comment: 18 pages, 3 figures, 5 tables. To appear in Ap
- …