7,282 research outputs found
Studies of surface two-dimensional photonic band-gap structures
Two-dimensional (2D) surface photonic band-gap (SPBG) structures can be obtained by providing a shallow corrugation of the inner surface of a waveguide wall. It can be used as a distributed mirror, a cavity, or a filter in integrated optics or microwave electronics. These structures can also be an alternative to conventional 2D PBG or 1D Bragg structures. In this article, we present the results of theoretical and experimental studies of 2D SPBG structures. Data obtained from experiments are compared with theoretical results and good agreement between theory and experiment is demonstrated. Comparison of a coaxial 2D SPBG structure with a conventional 1D Bragg structure is also presented
Spread of Infectious Diseases with a Latent Period
Infectious diseases spread through human networks.
Susceptible-Infected-Removed (SIR) model is one of the epidemic models to
describe infection dynamics on a complex network connecting individuals. In the
metapopulation SIR model, each node represents a population (group) which has
many individuals. In this paper, we propose a modified metapopulation SIR model
in which a latent period is taken into account. We call it SIIR model. We
divide the infection period into two stages: an infected stage, which is the
same as the previous model, and a seriously ill stage, in which individuals are
infected and cannot move to the other populations. The two infectious stages in
our modified metapopulation SIR model produce a discontinuous final size
distribution. Individuals in the infected stage spread the disease like
individuals in the seriously ill stage and never recover directly, which makes
an effective recovery rate smaller than the given recovery rate.Comment: 6 pages, 3 figure
Cylindrical, periodic surface lattice — theory, dispersion analysis, and experiment
A two-dimensional surface lattice of cylindrical topology obtained via perturbing the inner surface of a cylinder is considered. Periodic perturbations of the surface lead to observation of high-impedance, dielectric-like media and resonant coupling of surface and non-propagating volume fields. This allows synthesis of tailored-for-purpose "coating" material with dispersion suitable, for instance, to mediate a Cherenkov type interaction. An analytical model of the lattice is discussed and coupled-wave equations are derived. Variations of the lattice dispersive properties with variation of parameters are shown, illustrating the tailoring of the structure's electromagnetic properties. Experimental results are presented showing agreement with the theoretical model
Barkhausen Noise in a Relaxor Ferroelectric
Barkhausen noise, including both periodic and aperiodic components, is found
in and near the relaxor regime of a familiar relaxor ferroelectric,
PbMgNbO, driven by a periodic electric field. The
temperature dependences of both the amplitude and spectral form show that the
size of the coherent dipole moment changes shrink as the relaxor regime is
entered, contrary to expectations based on some simple models.Comment: 4 pages RevTeX4, 5 figures; submitted to Phys Rev Let
Phase stability and structural temperature dependence in sodium niobate: A high resolution powder neutron diffraction study
We report investigation of structural phase transitions in technologically
important material sodium niobate as a function of temperature on heating over
300-1075 K. Our high resolution powder neutron diffraction data show variety of
structural phase transitions ranging from non-polar antiferrodistortive to
ferroelectric and antiferroelectric in nature. Discontinuous jump in lattice
parameters is found only at 633 K that indicates that the transition of
orthorhombic antiferroelectric P (space group Pbcm) to R (space group Pbnm)
phase is first order in nature, while other successive phase transitions are of
second order. New superlattice reflections appear at 680 K (R phase) and 770 K
(S phase) that could be indexed using an intermediate long-period modulated
orthorhombic structure whose lattice parameter along direction is 3 and 6
times that of the CaTiO3-like Pbnm structure respectively. The correlation of
superlattice reflections with the phonon instability is discussed. The critical
exponent ({\beta}) for the second order tetragonal to cubic phase transition at
950 K, corresponds to a value {\beta}, as obtained from the
temperature variation of order parameters (tilt angle and intensity of
superlattice reflections). It is argued that this exponent is due to a second
order phase transition close to a tricritical point. Based on our detailed
temperature dependent neutron diffraction studies, the phase diagram of sodium
niobate is presented that resolves existing ambiguities in the literature.Comment: 21 Pages, 8 Figure
Bistability of Slow and Fast Traveling Waves in Fluid Mixtures
The appearence of a new type of fast nonlinear traveling wave states in
binary fluid convection with increasing Soret effect is elucidated and the
parameter range of their bistability with the common slower ones is evaluated
numerically. The bifurcation behavior and the significantly different
spatiotemporal properties of the different wave states - e.g. frequency, flow
structure, and concentration distribution - are determined and related to each
other and to a convenient measure of their nonlinearity. This allows to derive
a limit for the applicability of small amplitude expansions. Additionally an
universal scaling behavior of frequencies and mixing properties is found.
PACS: 47.20.-k, 47.10.+g, 47.20.KyComment: 4 pages including 5 Postscript figure
A Quantum Non-demolition measurement of Fock states of mesoscopic mechanical oscillators
We investigate a scheme that makes a quantum non-demolition measurement of
the excitation level of a mesoscopic mechanical oscillator by utilizing the
anharmonic coupling between two elastic beam bending modes. The non-linear
coupling between the two modes shifts the resonant frequency of the readout
oscillator proportionate to the excitation of the system oscillator. This
frequency shift may be detected as a phase shift of the readout oscillation
when driven on resonance. We show that in an appropriate regime this
measurement approaches a quantum non-demolition measurement of the phonon
number of the system oscillator. As phonon energies in micromechanical
oscillators become comparable to or greater than the thermal energy, the
individual phonon dynamics within each mode can be resolved. As a result it
should be possible to monitor jumps between Fock states caused by the coupling
of the system to the thermal reservoirs.Comment: revised, 21 pages, 9 figure
Poly-arginine peptides reduce infarct volume in a permanent middle cerebral artery rat stroke model
Background: We recently reported that poly-arginine peptides have neuroprotective properties both in vitro and in vivo. In cultured cortical neurons exposed to glutamic acid excitotoxicity, we demonstrated that neuroprotective potency increases with polymer length plateauing at R15 to R18 (R = arginine resides). In an in vivo study in rats, we also demonstrated that R9D (R9 peptide synthesised with D-isoform amino acids) administered intravenously at a dose of 1000 nmol/kg 30 min after permanent middle cerebral artery occlusion (MCAO) reduces infarct volume. Based on these positive in vitro and in vivo findings, we decided to examine the neuroprotective efficacy of the L-isoform poly-arginine peptides, R12, R15 and R18 when administered at a dose of 1000 nmol/kg 30 min after permanent MCAO in the rat.
Results: At 24 h post-MCAO, there was reduced total infarct volume for R12 (12.8 % reduction) and R18 (20.5 % reduction), but this reduction only reached statistical significance for R18. Brain slice analysis revealed significantly reduced injury in coronal slices 4 and 5 for R18, and slice 5 for R12. The R15 peptide had no effect on infarct volume. Peptide treatment did not reveal any statistical significant improvement in functional outcomes.
Conclusion: While these findings confirm the in vivo neuroprotective properties of poly-arginine peptides, additional dose studies are required particularly in less severe transient MCAO models so as to further assess the potential of these agents as a stroke therapy
Existence and Stability of Steady Fronts in Bistable CML
We prove the existence and we study the stability of the kink-like fixed
points in a simple Coupled Map Lattice for which the local dynamics has two
stable fixed points. The condition for the existence allows us to define a
critical value of the coupling parameter where a (multi) generalized
saddle-node bifurcation occurs and destroys these solutions. An extension of
the results to other CML's in the same class is also displayed. Finally, we
emphasize the property of spatial chaos for small coupling.Comment: 18 pages, uuencoded PostScript file, J. Stat. Phys. (In press
Logarithmically Slow Expansion of Hot Bubbles in Gases
We report logarithmically slow expansion of hot bubbles in gases in the
process of cooling. A model problem first solved, when the temperature has
compact support. Then temperature profile decaying exponentially at large
distances is considered. The periphery of the bubble is shown to remain
essentially static ("glassy") in the process of cooling until it is taken over
by a logarithmically slowly expanding "core". An analytical solution to the
problem is obtained by matched asymptotic expansion. This problem gives an
example of how logarithmic corrections enter dynamic scaling.Comment: 4 pages, 1 figur
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