376 research outputs found
Intensity correlations in electronic wave propagation in a disordered medium: the influence of spin-orbit scattering
We obtain explicit expressions for the correlation functions of transmission
and reflection coefficients of coherent electronic waves propagating through a
disordered quasi-one-dimensional medium with purely elastic diffusive
scattering in the presence of spin-orbit interactions. We find in the metallic
regime both large local intensity fluctuations and long-range correlations
which ultimately lead to universal conductance fluctuations. We show that the
main effect of spin-orbit scattering is to suppress both local and long-range
intensity fluctuations by a universal symmetry factor 4. We use a scattering
approach based on random transfer matrices.Comment: 15 pages, written in plain TeX, Preprint OUTP-93-42S (University of
Oxford), to appear in Phys. Rev.
Anomalous Coherent Backscattering of Light from Opal Photonic Crystals
We studied coherent backscattering (CBS) of light from opal photonic crystals
in air at different incident inclination angles, wavelengths and along various
[hkl] directions inside the opals. Similar to previously obtained CBS cones
from various random media, we found that when Bragg condition with the incident
light beam is not met then the CBS cones from opals show a triangular line
shape in excellent agreement with light diffusion theory. At Bragg condition,
however, we observed a dramatic broadening of the opal CBS cones that depends
on the incident angle and [hkl] direction. This broadening is explained as due
to the light intensity decay in course of propagation along the Bragg direction
{\em before the first} and {\em after the last} scattering events. We modified
the CBS theory to incorporate the attenuation that results from the photonic
band structure of the medium. Using the modified theory we extract from our CBS
data the light mean free path and Bragg attenuation length at different [hkl].
Our study shows that CBS measurements are a unique experimental technique to
explore photonic crystals with disorder, when other spectroscopical methods
become ambiguous due to disorder-induced broadening.Comment: 10 pages, 5 figure
Multiple light scattering in anisotropic random media
In the last decade Diffusing Wave Spectroscopy (DWS) has emerged as a
powerful tool to study turbid media. In this article we develop the formalism
to describe light diffusion in general anisotropic turbid media. We give
explicit formulas to calculate the diffusion tensor and the dynamic absorption
coefficient, measured in DWS experiments. We apply our theory to uniaxial
systems, namely nematic liquid crystals, where light is scattered from thermal
fluctuations of the local optical axis, called director. We perform a detailed
analysis of the two essential diffusion constants, parallel and perpendicular
to the director, in terms of Frank elastic constants, dielectric anisotropy,
and applied magnetic field. We also point out the relevance of our results to
different liquid crystalline systems, such as discotic nematics, smectic-A
phases, and polymer liquid crystals. Finally, we show that the dynamic
absorption coefficient is the angular average over the inverse viscosity, which
governs the dynamics of director fluctuations.Comment: 23 pages, 12 ps figures, to be published in Phys. Rev.
Free-breathing Pulmonary (1)H and Hyperpolarized (3)He MRI: Comparison in COPD and Bronchiectasis.
RATIONALE AND OBJECTIVES: In this proof-of-concept demonstration, we aimed to quantitatively and qualitatively compare pulmonary ventilation abnormalities derived from Fourier decomposition of free-breathing (1)H magnetic resonance imaging (FDMRI) to hyperpolarized (3)He MRI in subjects with chronic obstructive pulmonary disease (COPD) and bronchiectasis.
MATERIALS AND METHODS: All subjects provided written informed consent to a protocol approved by a local research ethics board and Health, Canada, and they underwent MRI, computed tomography (CT), spirometry, and plethysmography during a single 2-hour visit. Semiautomated segmentation was used to generate ventilation defect measurements derived from FDMRI and (3)He MRI, and these were compared using analysis of variance and Pearson correlations.
RESULTS: Twenty-six subjects were evaluated including 12 COPD subjects (67 ± 9 years) and 14 bronchiectasis subjects (70 ± 11 years). For COPD subjects, FDMRI and (3)He MRI ventilation defect percent (VDP) was 7 ± 6% and 24 ± 14%, respectively (P \u3c .001; bias = -16 ± 9%). In COPD subjects, FDMRI was significantly correlated with (3)He MRI VDP (r = .88; P = .0001), (3)He MRI apparent diffusion coefficient (r = .71; P \u3c .05), airways resistance (r = .60; P \u3c .05), and RA950 (r = .80; P \u3c .01). In subjects with bronchiectasis, FDMRI VDP (5 ± 3%) and (3)He MRI VDP (18 ± 9%) were significantly different (P \u3c .001) and not correlated (P \u3e .05). The Dice similarity coefficient (DSC) for FDMRI and (3)He MRI ventilation was 86 ± 7% for COPD and 86 ± 4% for bronchiectasis subjects (P \u3e .05); the DSC for FDMRI ventilation defects and CT RA950 was 19 ± 20% in COPD and 2 ± 3% in bronchiectasis subjects (P \u3c .01).
CONCLUSIONS: FDMRI and (3)He MRI VDP were strongly related in COPD but not in bronchiectasis subjects. In COPD only, FDMRI ventilation defects were spatially related with (3)He ventilation defects and emphysema
Analytical solutions to the third-harmonic generation in trans-polyacetylene: Application of dipole-dipole correlation on the single electron models
The analytical solutions for the third-harmonic generation (THG) on infinite
chains in both Su-Shrieffer-Heeger (SSH) and Takayama-Lin-Liu-Maki (TLM) models
of trans-polyacetylene are obtained through the scheme of dipole-dipole ()
correlation. They are not equivalent to the results obtained through static
current-current () correlation or under polarization operator
. The van Hove singularity disappears exactly in the analytical forms,
showing that the experimentally observed two-photon absorption peak (TPA) in
THG may not be directly explained by the single electron models.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
Nonadiabatic approach to dimerization gap and optical absorption coefficient of the Su-Schrieffer-Heeger model
An analytical nonadiabatic approach has been developed to study the
dimerization gap and the optical absorption coefficient of the
Su-Schrieffer-Heeger model where the electrons interact with dispersive quantum
phonons. By investigating quantitatively the effects of quantum phonon
fluctuations on the gap order and the optical responses in this system, we show
that the dimerization gap is much more reduced by the quantum lattice
fluctuations than the optical absorption coefficient is. The calculated optical
absorption coefficient and the density of states do not have the
inverse-square-root singularity, but have a peak above the gap edge and there
exist a significant tail below the peak. The peak of optical absorption
spectrum is not directly corresponding to the dimerized gap. Our results of the
optical absorption coefficient agree well with those of the experiments in both
the shape and the peak position of the optical absorption spectrum.Comment: 14 pages, 7 figures. to be published in PR
Asymmetry of Early Endosome Distribution in C. elegans Embryos
development, we examined the distribution and dynamics of early endosomes (EEs) in embryos.EEs are primarily found at the cell periphery with an initially uniform distribution after fertilization. Strikingly, we find that during the first cell cycle, EEA-1 positive EEs become enriched at the anterior cortex. In contrast, the Golgi compartment shows no asymmetry in distribution. Asymmetric enrichment of EEs depends on acto-myosin contractility and embryonic PAR polarity. In addition to their localization at the cortex, EEs are also found around the centrosome. These EEs move rapidly (1.3um/s) from the cortex directly to the centrosome, a speed comparable to that of the minus end directed motor dynein.We speculate that the asymmetry of early endosomes might play a role in cell asymmetries or fate decisions
Single amino acid change in gp41 region of HIV-1 alters bystander apoptosis and CD4 decline in humanized mice
<p>Abstract</p> <p>Background</p> <p>The mechanism by which HIV infection leads to a selective depletion of CD4 cells leading to immunodeficiency remains highly debated. Whether the loss of CD4 cells is a direct consequence of virus infection or bystander apoptosis of uninfected cells is also uncertain.</p> <p>Results</p> <p>We have addressed this issue in the humanized mouse model of HIV infection using a HIV variant with a point mutation in the gp41 region of the Env glycoprotein that alters its fusogenic activity. We demonstrate here that a single amino acid change (V38E) altering the cell-to-cell fusion activity of the Env minimizes CD4 loss in humanized mice without altering viral replication. This differential pathogenesis was associated with a lack of bystander apoptosis induction by V38E virus even in the presence of similar levels of infected cells. Interestingly, immune activation was observed with both WT and V38E infection suggesting that the two phenomena are likely not interdependent in the mouse model.</p> <p>Conclusions</p> <p>We conclude that Env fusion activity is one of the determinants of HIV pathogenesis and it may be possible to attenuate HIV by targeting gp41.</p
Largest ancient fortress of South-West Asia and the western world?:Recent fieldwork at Sasanian Qaleh Iraj at Pishva, Iran
The article presents recent works at Qale Iraj, near Varamin, Iran. My short contribution is on the Middle Persian ostraka found at the site
Economic analysis including long-term risks and costs of alternative diagnostic strategies to evaluate patients with chest pain
Background: Diagnosis costs for cardiovascular disease waste a large amount of healthcare
resources. The aim of the study is to evaluate the clinical and economic outcomes of alternative
diagnostic strategies in low risk chest pain patients.
Methods: We evaluated direct and indirect downstream costs of 6 strategies: coronary
angiography (CA) after positive troponin I or T (cTn-I or cTnT) (strategy 1); after positive exercise
electrocardiography (ex-ECG) (strategy 2); after positive exercise echocardiography (ex-Echo)
(strategy 3); after positive pharmacologic stress echocardiography (PhSE) (strategy 4); after positive
myocardial exercise stress single-photon emission computed tomography with technetium Tc 99m
sestamibi (ex-SPECT-Tc) (strategy 5) and direct CA (strategy 6).
Results: The predictive accuracy in correctly identifying the patients was 83,1% for cTn-I, 87% for
cTn-T, 85,1% for ex-ECG, 93,4% for ex-Echo, 98,5% for PhSE, 89,4% for ex-SPECT-Tc and 18,7%
for CA. The cost per patient correctly identified results 2.086 for cTn-T, 803 for ex-Echo, 1.521 for ex-SPECT-Tc (29.673 for CA ($29.999 including cost of extra risk of cancer). The
average relative cost-effectiveness of cardiac imaging compared with the PhSE equal to 1 (as a cost
comparator), the relative cost of ex-Echo is 1.5×, of a ex-SPECT-Tc is 3.1×, of a ex-ECG is 3.5×,
of cTnI is ×3.8, of cTnT is ×3.9 and of a CA is 56.3×.
Conclusion: Stress echocardiography based strategies are cost-effective versus alternative
imaging strategies and the risk and cost of radiation exposure is void
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