876 research outputs found
Group delay in Bragg grating with linear chirp
An analytic solution for Bragg grating with linear chirp in the form of
confluent hypergeometric functions is analyzed in the asymptotic limit of long
grating. Simple formulas for reflection coefficient and group delay are
derived. The simplification makes it possible to analyze irregularities of the
curves and suggest the ways of their suppression. It is shown that the increase
in chirp at fixed other parameters decreases the oscillations in the group
delay, but gains the oscillations in the reflection spectrum. The conclusions
are in agreement with numerical calculations.Comment: 16 pages, 8 figures, submitted to Opt. Com
Nonuniversal correlations in multiple scattering
We show that intensity of a wave created by a source embedded inside a
three-dimensional disordered medium exhibits a non-universal space-time
correlation which depends explicitly on the short-distance properties of
disorder, source size, and dynamics of disorder in the immediate neighborhood
of the source. This correlation has an infinite spatial range and is
long-ranged in time. We suggest that a technique of "diffuse microscopy" might
be developed employing spatially-selective sensitivity of the considered
correlation to the disorder properties.Comment: 15 pages, 3 postscript figures, accepted to Phys. Rev.
Linearly Polarized Modes of a Corrugated Metallic Waveguide
A linearly polarized (LP[subscript mn]) mode basis set for oversized, corrugated, metallic waveguides is derived for the special case of quarter-wavelength-depth circumferential corrugations. The relationship between the LPmn modes and the conventional modes (HEmn, EHmn, TE0n, TM0n) of the corrugated guide is shown. The loss in a gap or equivalent miter bend in the waveguide is calculated for single-mode and multimode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal (m) indexes that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results describe the propagation of high-power linearly polarized radiation in overmoded corrugated waveguides.United States. Dept. of Energy (Office of Fusion Energy Sciences)United States. Dept. of Energy (Virtual Laboratory for Technology)United States. Dept. of Energy (Office of Science, US ITER Project
Low-Power Testing of Losses in Millimeter-Wave Transmission Lines for High-Power Applications
We report the measurement of small losses in transmission line (TL) components intended for high-power millimeter-wave applications. Measurements were made using two different low-power techniques: a coherent technique using a vector network analyzer (VNA) and an incoherent technique using a radiometer. The measured loss in a 140 GHz 12.7 mm diameter TL system, consisting of 1.7 m of circular corrugated waveguide and three miter bends, is dominated by the miter bend loss. The measured loss was 0.3±0.1 dB per miter bend using a VNA; and 0.22±0.1 dB per miter bend using a radiometer. Good agreement between the two measurement techniques implies that both are useful for measuring small losses. To verify the methodology, the VNA technique was employed to measure the extremely small transmission loss in a 170 GHz ITER prototype TL system consisting of three lengths of 1 m, 63.5 mm diameter, circular corrugated waveguide and two miter bends. The measured loss of 0.05±0.02 dB per miter bend may be compared with the theoretical loss of 0.027 dB per miter bend. These results suggest that low-power testing of TL losses, utilizing a small, simple TL system and a VNA, is a reliable method for evaluating performance of low-loss millimeter-wave TL components intended for use in high-power applications
Introduction to Arithmetic Mirror Symmetry
We describe how to find period integrals and Picard-Fuchs differential
equations for certain one-parameter families of Calabi-Yau manifolds. These
families can be seen as varieties over a finite field, in which case we show in
an explicit example that the number of points of a generic element can be given
in terms of p-adic period integrals. We also discuss several approaches to
finding zeta functions of mirror manifolds and their factorizations. These
notes are based on lectures given at the Fields Institute during the thematic
program on Calabi-Yau Varieties: Arithmetic, Geometry, and Physics
The Future Evolution of White Dwarf Stars Through Baryon Decay and Time Varying Gravitational Constant
Motivated by the possibility that the fundamental ``constants'' of nature
could vary with time, this paper considers the long term evolution of white
dwarf stars under the combined action of proton decay and variations in the
gravitational constant. White dwarfs are thus used as a theoretical laboratory
to study the effects of possible time variations, especially their implications
for the future history of the universe. More specifically, we consider the
gravitational constant to vary according to the parametric relation , where the time scale is the same order as
the proton lifetime. We then study the long term fate and evolution of white
dwarf stars. This treatment begins when proton decay dominates the stellar
luminosity, and ends when the star becomes optically thin to its internal
radiation.Comment: 12 pages, 10 figures, accepted to Astrophysics and Space Scienc
Kohn Anomalies in Superconductors
I present the detailed behavior of phonon dispersion curves near momenta
which span the electronic Fermi sea in a superconductor. I demonstrate that an
anomaly, similar to the metallic Kohn anomaly, exists in a superconductor's
dispersion curves when the frequency of the phonon spanning the Fermi sea
exceeds twice the superconducting energy gap. This anomaly occurs at
approximately the same momentum but is {\it stronger} than the normal-state
Kohn anomaly. It also survives at finite temperature, unlike the metallic
anomaly. Determination of Fermi surface diameters from the location of these
anomalies, therefore, may be more successful in the superconducting phase than
in the normal state. However, the superconductor's anomaly fades rapidly with
increased phonon frequency and becomes unobservable when the phonon frequency
greatly exceeds the gap. This constraint makes these anomalies useful only in
high-temperature superconductors such as .Comment: 18 pages (revtex) + 11 figures (upon request), NSF-ITP-93-7
Evidence for a model of agonist-induced activation of 5-hydroxytryptamine 2A serotonin receptors that involves the disruption of a strong ionic interaction between helices 3 and 6
5-Hydroxytryptamine 2A (5-HT2A) receptors are essential for the actions of serotonin (5-hydroxytryptamine (5-HT)) on physiological processes as diverse as vascular smooth muscle contraction, platelet aggregation, perception, and emotion. In this study, we investigated the molecular mechanism(s) by which 5-HT activates 5-HT2A receptors using a combination of approaches including site-directed mutagenesis, molecular modeling, and pharmacological analysis using the sensitive, cell-based functional assay R-SAT. Alanine-scanning mutagenesis of residues close to the intracellular end of H6 of the 5-HT2A receptor implicated glutamate Glu-318(6.30) in receptor activation, as also predicted by a newly constructed molecular model of the 5-HT2A receptor, which was based on the x-ray structure of bovine rhodopsin. Close examination of the molecular model suggested that Glu-318(6.30) could form a strong ionic interaction with Arg-173(3.50) of the highly conserved "(D/E)RY motif" located at the interface between the third transmembrane segment and the second intracellular loop (i2). A direct prediction of this hypothesis, that disrupting this ionic interaction by an E318(6.30)R mutation would lead to a highly constitutively active receptor with enhanced affinity for agonist, was confirmed using R-SAT. Taken together, these results predict that the disruption of a strong ionic interaction between transmembrane helices 3 and 6 of 5-HT2A receptors is essential for agonist-induced receptor activation and, as recently predicted by ourselves (B. L. Roth and D. A. Shapiro (2001) Expert Opin. Ther. Targets 5, 685-695) and others, that this may represent a general mechanism of activation for many, but not all, G-protein-coupled receptors
Interaction of Low - Energy Induced Gravity with Quantized Matter and Phase Transition Induced by Curvature
At high energy scale the only quantum effect of any asymptotic free and
asymptotically conformal invariant GUT is the trace anomaly of the
energy-momentum tensor. Anomaly generates the new degree of freedom, that is
propagating conformal factor. At lower energies conformal factor starts to
interact with scalar field because of the violation of conformal invariance. We
estimate the effect of such an interaction and find the running of the
nonminimal coupling from conformal value to . Then we discuss
the possibility of the first order phase transition induced by curvature in a
region close to the stable fixed point and calculate the induced values of
Newtonian and cosmological constants.Comment: 11 pages, LaTex, KEK-TH-397-KEK Preprint 94-3
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