509 research outputs found
Structural relaxation of E' gamma centers in amorphous silica
We report experimental evidence of the existence of two variants of the E'
gamma centers induced in silica by gamma rays at room temperature. The two
variants are distinguishable by the fine features of their line shapes in
paramagnetic resonance spectra. These features suggest that the two E' gamma
differ for their topology. We find a thermally induced interconversion between
the centers with an activation energy of about 34 meV. Hints are also found for
the existence of a structural configuration of minimum energy and of a
metastable state.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Pre-pregnancy predictors of hypertension in pregnancy among Aboriginal and Torres Strait Islander women in north Queensland, Australia; a prospective cohort study
BACKGROUND Compared to other Australian women, Indigenous women are frequently at greater risk for hypertensive disorders of pregnancy. We examined pre-pregnancy factors that may predict hypertension in pregnancy in a cohort of Aboriginal and Torres Strait Islander women in north Queensland. METHODS Data on a cohort of 1009 Indigenous women of childbearing age (15–44 years) who participated in a 1998–2000 health screening program in north Queensland were combined with 1998–2008 Queensland hospitalisations data using probabilistic data linkage. Data on the women in the cohort who were hospitalised for birth (n = 220) were further combined with Queensland perinatal data which identified those diagnosed with hypertension in pregnancy. RESULTS Of 220 women who gave birth, 22 had hypertension in the pregnancy after their health check. The mean age of women with and without hypertension was similar (23.7 years and 23.9 years respectively) however Aboriginal women were more affected compared to Torres Strait Islanders. Pre-pregnancy adiposity and elevated blood pressure at the health screening program were predictors of a pregnancy affected by hypertension. After adjusting for age and ethnicity, each 1 cm increase in waist circumference showed a 4% increased risk for hypertension in pregnancy (PR 1.04; 95% CI; 1.02-1.06); each 1 point increase in BMI showed a 9% adjusted increase in risk (1.09; 1.04-1.14). For each 1 mmHg increase in baseline systolic blood pressure there was an age and ethnicity adjusted 6% increase in risk and each 1 mmHg increase in diastolic blood pressure showed a 7% increase in risk (1.06; 1.03-1.09 and 1.07; 1.03-1.11 respectively). Among those free of diabetes at baseline, the presence of the metabolic syndrome (International Diabetes Federation criteria) predicted over a three-fold increase in age-ethnicity-adjusted risk (3.5; 1.50-8.17). CONCLUSIONS Pre-pregnancy adiposity and features of the metabolic syndrome among these young Aboriginal and Torres Strait Islander women track strongly to increased risk of hypertension in pregnancy with associated risks to the health of babies.Sandra K Campbell, John Lynch, Adrian Esterman and Robyn McDermot
Proof of the thermodynamical stability of the E' center in SiO2
The E' center is a paradigmatic radiation-induced defect in SiO2 whose
peculiar EPR and hyperfine activity has been known since over 40 years. This
center has been traditionally identified with a distorted, positively-charged
oxygen vacancy V_O+. However, no direct proof of the stability of this defect
has ever been provided, so that its identification is still strongly
incomplete. Here we prove directly that distorted V_O+ is metastable and that
it satisfies the key requirements for its identification as E', such as thermal
and optical response, and activation-deactivation mechanisms.Comment: RevTeX 4 pages, 2 figure
Evaluation of low-energy effective Hamiltonian techniques for coupled spin triangles
Motivated by recent work on Heisenberg antiferromagnetic spin systems on
various lattices made up of triangles, we examine the low-energy properties of
a chain of antiferromagnetically coupled triangles of half-odd-integer spins.
We derive the low-energy effective Hamiltonian to second order in the ratio of
the coupling J_2 between triangles to the coupling J_1 within each triangle.
The effective Hamiltonian contains four states for each triangle which are
given by the products of spin-1/2 states with the states of a pseudospin-1/2.
We compare the results obtained by exact diagonalization of the effective
Hamiltonian with those obtained for the full Hamiltonian using exact
diagonalization and the density-matrix renormalization group method. It is
found that the effective Hamiltonian is accurate only for the ground state for
rather low values of the ratio J_2 / J_1 and that too for the spin-1/2 case
with linear topology. The chain of spin-1/2 triangles shows interesting
properties like spontaneous dimerization and several singlet and triplet
excited states lying close to the ground state.Comment: Revtex, 11 pages, 11 eps figure
Density waves and density fluctuations in granular flow
We simulate the granular flow in a narrow pipe with a lattice-gas automaton
model. We find that the density in the system is characterized by two features.
One is that spontaneous density waves propagate through the system with
well-defined shapes and velocities. The other is that density waves are so
distributed to make the power spectra of density fluctuations as
noise. Three important parameters make these features observable and they are
energy dissipation, average density and the rougness of the pipe walls.Comment: Latex (with ps files appended
Model Exact Low-Lying States and Spin Dynamics in Ferric Wheels; Fe to Fe
Using an efficient numerical scheme that exploits spatial symmetries and
spin-parity, we have obtained the exact low-lying eigenstates of exchange
Hamiltonians for ferric wheels up to Fe. The largest calculation
involves the Fe ring which spans a Hilbert space dimension of about 145
million for M=0 subspace. Our calculated gaps from the singlet ground state
to the excited triplet state agrees well with the experimentally measured
values. Study of the static structure factor shows that the ground state is
spontaneously dimerized for ferric wheels. Spin states of ferric wheels can be
viewed as quantized states of a rigid rotor with the gap between the ground and
the first excited state defining the inverse of moment of inertia. We have
studied the quantum dynamics of Fe as a representative of ferric wheels.
We use the low-lying states of Fe to solve exactly the time-dependent
Schr\"odinger equation and find the magnetization of the molecule in the
presence of an alternating magnetic field at zero temperature. We observe a
nontrivial oscillation of magnetization which is dependent on the amplitude of
the {\it ac} field. We have also studied the torque response of Fe as a
function of magnetic field, which clearly shows spin-state crossover.Comment: Revtex, 24 pages, 8 eps figure
Velocity and density profiles of granular flow in channels using lattice gas automaton
We have performed two-dimensional lattice-gas-automaton simulations of
granular flow between two parallel planes. We find that the velocity profiles
have non-parabolic distributions while simultaneously the density profiles are
non-uniform. Under non-slip boundary conditions, deviation of velocity profiles
from the parabolic form of newtonian fluids is found to be characterized solely
by ratio of maximal velocity at the center to the average velocity, though the
ratio depends on the model parameters in a complex manner. We also find that
the maximal velocity () at the center is a linear function of the
driving force (g) as with non-zero in
contrast with newtonian fluids. Regarding density profiles, we observe that
densities near the boundaries are higher than those in the center. The width of
higher densities (above the average density) relative to the channel width is a
decreasing function of a variable which scales with the driving force (g),
energy dissipation parameter () and the width of the system (L) as
with exponents and . A phenomenological theory based on a scaling argument is presented to
interpret these findings.Comment: Latex, 15 figures, to appear in PR
Calculating the energy spectra of magnetic molecules: application of real- and spin-space symmetries
The determination of the energy spectra of small spin systems as for instance
given by magnetic molecules is a demanding numerical problem. In this work we
review numerical approaches to diagonalize the Heisenberg Hamiltonian that
employ symmetries; in particular we focus on the spin-rotational symmetry SU(2)
in combination with point-group symmetries. With these methods one is able to
block-diagonalize the Hamiltonian and thus to treat spin systems of
unprecedented size. In addition it provides a spectroscopic labeling by
irreducible representations that is helpful when interpreting transitions
induced by Electron Paramagnetic Resonance (EPR), Nuclear Magnetic Resonance
(NMR) or Inelastic Neutron Scattering (INS). It is our aim to provide the
reader with detailed knowledge on how to set up such a diagonalization scheme.Comment: 29 pages, many figure
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