1,261 research outputs found
Anemia Prevalence among Pregnant Women and Birth Weight in Five Areas in China
Objectives: To investigate the current prevalence of anemia among pregnant women in different areas of China and the association with birth weight and educational level. Methods: A total of 6,413 women aged 24-37 in the third trimester of pregnancy from five areas were randomly selected from all gravidas who gave birth in the hospitals from 1999 to 2003. Blood hemoglobin concentration (Hb) was measured by the cyanomethemoglobin method; Hb <110 g/l was considered as anemia. Results: The overall prevalence of anemia was 58.6%, ranging from 48.1 to 70.5% in the five areas. There was a significant difference in the prevalence of anemia between women who have mental jobs and those who have physical jobs (52.3 vs. 61.1%, p <0.01). The prevalence of anemia depended on the level of education: with 52.9, 62.4 and 66.5%, for college, secondary school and primary education, respectively, and the difference was statistically significant (p = 0.005). Results showed that higher birth weight was associated with Hb concentrations ranging from 90 to 140 g/l, whereas lower birth weight occurred below 80 g/l and above 140 g/l Hb. Conclusions: The prevalence of anemia in Chinese pregnant women was high both in rural areas and towns. Area of residence, education level and type of job influenced the prevalence of anemia. Low maternal Hb concentrations influenced birth weight
A very convenient setup to generate intense VUV coherent light at 125 nm with use of nonlinear effects in mercury vapor at room temperature
Using only one dye laser, efficient generation of VUV radiation (10^13 photons/pulse) is demonstrated at 125.140 nm and 125.053 nm by four-wave sum-frequency mixing in a room-temperature mercury vapor. The emission at 125.053 nm, which is out of two-photon resonance but near three-photon resonance, has been observed for the first time and is carefully analyzed. In particular, numerical calculations have been carried out for gaussian pump beams taking into account absorption of VUV photons and optical Kerr effect. The results of the calculations reproduce the VUV emission lineshape and power saturation effects measured in the experiment
Zeta Function Zeros, Powers of Primes, and Quantum Chaos
We present a numerical study of Riemann's formula for the oscillating part of
the density of the primes and their powers. The formula is comprised of an
infinite series of oscillatory terms, one for each zero of the zeta function on
the critical line and was derived by Riemann in his paper on primes assuming
the Riemann hypothesis. We show that high resolution spectral lines can be
generated by the truncated series at all powers of primes and demonstrate
explicitly that the relative line intensities are correct. We then derive a
Gaussian sum rule for Riemann's formula. This is used to analyze the numerical
convergence of the truncated series. The connections to quantum chaos and
semiclassical physics are discussed
Spin effects in the magneto-drag between double quantum wells
We report on the selectivity to spin in a drag measurement. This selectivity
to spin causes deep minima in the magneto-drag at odd fillingfactors for
matched electron densities at magnetic fields and temperatures at which the
bare spin energy is only one tenth of the temperature. For mismatched densities
the selectivity causes a novel 1/B-periodic oscillation, such that negative
minima in the drag are observed whenever the majority spins at the Fermi
energies of the two-dimensional electron gasses (2DEGs) are anti-parallel, and
positive maxima whenever the majority spins at the Fermi energies are parallel.Comment: 4 pages, 3 figure
Sign-reversal of drag in bilayer systems with in-plane periodic potential modulation
We develop a theory for describing frictional drag in bilayer systems with
in-plane periodic potential modulations, and use it to investigate the drag
between bilayer systems in which one of the layers is modulated in one
direction. At low temperatures, as the density of carriers in the modulated
layer is changed, we show that the transresistivity component in the direction
of modulation can change its sign. We also give a physical explanation for this
behavior.Comment: 4 pages, 4 figure
Modelling the effect of non-planarity on luminescence energy of conjugated polymers
We present theoretical investigations of structural and electronic properties of
ground-state and low-lying excited singlet states in isolated chains of conjugated
polymers using a self-consistent quantum molecular dynamics method. With this
approach, we have determined the energy of both states as function of the twist angle
between two planar segments of the same polymer chain, for polymer chains with
variable length. The conjugated polymers investigated here are poly(para-phenylene
vinylene) (PPV) and polydiacetylene (PDA). Our results show that the energy of the
excited-state increases more than that of the ground-state, as the twist angle increases up
to 90Âş degrees. The change in the twist angle of both polymers leads to a blueshift in
luminescence transition energy, the effect being stronger in PPV when the planar
segments have similar sizes. The predicted blueshift in both polymers is dependent on the
chain length, the effect being more pronounced for shorter-chains.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)Fundação para a Ciência e a Tecnologia (FCT) - Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/2001 e SFRH/BD/11231/200
Metal-insulator transition in the one-dimensional Holstein model at half filling
We study the one-dimensional Holstein model with spin-1/2 electrons at
half-filling. Ground state properties are calculated for long chains with great
accuracy using the density matrix renormalization group method and extrapolated
to the thermodynamic limit. We show that for small electron-phonon coupling or
large phonon frequency, the insulating Peierls ground state predicted by
mean-field theory is destroyed by quantum lattice fluctuations and that the
system remains in a metallic phase with a non-degenerate ground state and
power-law electronic and phononic correlations. When the electron-phonon
coupling becomes large or the phonon frequency small, the system undergoes a
transition to an insulating Peierls phase with a two-fold degenerate ground
state, long-range charge-density-wave order, a dimerized lattice structure, and
a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure
Dynamical Structure Factor for the Alternating Heisenberg Chain: A Linked Cluster Calculation
We develop a linked cluster method to calculate the spectral weights of
many-particle excitations at zero temperature. The dynamical structure factor
is expressed as a sum of exclusive structure factors, each representing
contributions from a given set of excited states. A linked cluster technique to
obtain high order series expansions for these quantities is discussed. We apply
these methods to the alternating Heisenberg chain around the dimerized limit
(), where complete wavevector and frequency dependent spectral
weights for one and two-particle excitations (continuum and bound-states) are
obtained. For small to moderate values of the inter-dimer coupling parameter
, these lead to extremely accurate calculations of the dynamical
structure factors. We also examine the variation of the relative spectral
weights of one and two-particle states with bond alternation all the way up to
the limit of the uniform chain (). In agreement with Schmidt and
Uhrig, we find that the spectral weight is dominated by 2-triplet states even
at , which implies that a description in terms of triplet-pair
excitations remains a good quantitative description of the system even for the
uniform chain.Comment: 26 pages, 17 figure
Frictional drag between non-equilibrium charged gases
The frictional drag force between separated but coupled two-dimensional
electron gases of different temperatures is studied using the non-equilibrium
Green function method based on the separation of center-of-mass and relative
dynamics of electrons. As the mechanisms of producing the frictional force we
include the direct Coulomb interaction, the interaction mediated via virtual
and real TA and LA phonons, optic phonons, plasmons, and TA and LA
phonon-electron collective modes. We found that, when the distance between the
two electron gases is large, and at intermediate temperature where plasmons and
collective modes play the most important role in the frictional drag, the
possibility of having a temperature difference between two subsystems modifies
greatly the transresistivity.Comment: 8figure
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