467 research outputs found
Non-stationary heat conduction in one-dimensional chains with conserved momentum
The Letter addresses the relationship between hyperbolic equations of heat
conduction and microscopic models of dielectrics. Effects of the non-stationary
heat conduction are investigated in two one-dimensional models with conserved
momentum: Fermi-Pasta-Ulam (FPU) chain and chain of rotators (CR). These models
belong to different universality classes with respect to stationary heat
conduction. Direct numeric simulations reveal in both models a crossover from
oscillatory decay of short-wave perturbations of the temperature field to
smooth diffusive decay of the long-wave perturbations. Such behavior is
inconsistent with parabolic Fourier equation of the heat conduction. The
crossover wavelength decreases with increase of average temperature in both
models. For the FPU model the lowest order hyperbolic Cattaneo-Vernotte
equation for the non-stationary heat conduction is not applicable, since no
unique relaxation time can be determined.Comment: 4 pages, 5 figure
Energy exchange and localization in essentially nonlinear oscillatory systems: canonical formalism
Over recent years, a lot of progress has been achieved in understanding of
the relationship between localization and transport of energy in essentially
nonlinear oscillatory systems. In this paper we are going to demonstrate that
the structure of the resonance manifold can be conveniently described in terms
of canonical action-angle variables. Such formalism has important theoretical
advantages: all resonance manifolds may be described at the same level of
complexity, appearance of additional conservation laws on these manifolds is
easily proven both in autonomous and non-autonomous settings. The harmonic
balance - based complexification approach, used in many previous studies on the
subject, is shown to be a particular case of the canonical formalism. Moreover,
application of the canonic averaging allows treatment of much broader variety
of dynamical models. As an example, energy exchanges in systems of coupled
trigonometrical and vibro-impact oscillators are considered
A simple one-dimensional model of heat conduction which obeys Fourier's law
We present the computer simulation results of a chain of hard point particles
with alternating masses interacting on its extremes with two thermal baths at
different temperatures. We found that the system obeys Fourier's law at the
thermodynamic limit. This result is against the actual belief that one
dimensional systems with momentum conservative dynamics and nonzero pressure
have infinite thermal conductivity. It seems that thermal resistivity occurs in
our system due to a cooperative behavior in which light particles tend to
absorb much more energy than the heavier ones.Comment: 5 pages, 4 figures, to be published in PR
Heat conduction in 1D lattices with on-site potential
The process of heat conduction in one-dimensional lattice with on-site
potential is studied by means of numerical simulation. Using discrete
Frenkel-Kontorova, --4 and sinh-Gordon we demonstrate that contrary to
previously expressed opinions the sole anharmonicity of the on-site potential
is insufficient to ensure the normal heat conductivity in these systems. The
character of the heat conduction is determined by the spectrum of nonlinear
excitations peculiar for every given model and therefore depends on the
concrete potential shape and temperature of the lattice. The reason is that the
peculiarities of the nonlinear excitations and their interactions prescribe the
energy scattering mechanism in each model. For models sin-Gordon and --4
phonons are scattered at thermalized lattice of topological solitons; for
sinh-Gordon and --4 - models the phonons are scattered at localized
high-frequency breathers (in the case of --4 the scattering mechanism
switches with the growth of the temperature).Comment: 26 pages, 18 figure
Nonstationary heat conduction in one-dimensional models with substrate potential
The paper investigates non-stationary heat conduction in one-dimensional
models with substrate potential. In order to establish universal characteristic
properties of the process, we explore three different models ---
Frenkel-Kontorova (FK), phi4+ (+) and phi4- (). Direct numeric
simulations reveal in all these models a crossover from oscillatory decay of
short-wave perturbations of the temperature field to smooth diffusive decay of
the long-wave perturbations. Such behavior is inconsistent with parabolic
Fourier equation of the heat conduction and clearly demonstrates the necessity
of hyperbolic models. The crossover wavelength decreases with increase of
average temperature. The decay patterns of the temperature field almost do not
depend on the amplitude of the perturbations, so the use of linear evolution
equations for temperature field is justified. In all model investigated, the
relaxation of thermal perturbations is exponential -- contrary to linear chain,
where it follows a power law. However, the most popular lowest-order hyperbolic
generalization of the Fourier law, known as Cattaneo-Vernotte (CV) or telegraph
equation (TE) is not valid for description of the observed behavior of the
models with on-site potential. In part of the models a characteristic
relaxation times exhibit peculiar scaling with respect to the temperature
perturbation wavelength. Quite surprisingly, such behavior is similar to that
of well-known model with divergent heat conduction (Fermi-Pasta-Ulam chain) and
rather different from the model with normal heat conduction (chain of
rotators). Thus, the data on the non-stationary heat conduction in the systems
with on-site potential do not fit commonly accepted concept of universality
classes for heat conduction in one-dimensional models.Comment: 9 pages, 7 figures, corrected versio
On the universality of anomalous one-dimensional heat conductivity
In one and two dimensions, transport coefficients may diverge in the
thermodynamic limit due to long--time correlation of the corresponding
currents. The effective asymptotic behaviour is addressed with reference to the
problem of heat transport in 1d crystals, modeled by chains of classical
nonlinear oscillators. Extensive accurate equilibrium and nonequilibrium
numerical simulations confirm that the finite-size thermal conductivity
diverges with the system size as . However, the
exponent deviates systematically from the theoretical prediction
proposed in a recent paper [O. Narayan, S. Ramaswamy, Phys. Rev.
Lett. {\bf 89}, 200601 (2002)].Comment: 4 pages, submitted to Phys.Rev.
Identification of Differentially Expressed Proteins in Murine Embryonic and Postnatal Cortical Neural Progenitors
BACKGROUND: The central nervous system (CNS) develops from a heterogeneous pool of neural stem and progenitor cells (NSPC), the underlying differences among which are poorly understood. The study of NSPC would be greatly facilitated by the identification of additional proteins that mediate their function and that would distinguish amongst different progenitor populations. METHODOLOGY/PRINCIPAL FINDINGS: To identify membrane and membrane-associated proteins expressed by NSPC, we used a proteomics approach to profile NSPC cultured as neurospheres (NS) isolated from the murine cortex during a period of neurogenesis (embryonic day 11.5, E11.5), as compared to NSPC isolated at a peak of gliogenesis (postnatal day 1, P0) and to differentiated E11.5 NS. 54 proteins were identified with high expression in E11.5 NS, including the TrkC receptor, several heterotrimeric G proteins, and the Neogenin receptor. 24 proteins were identified with similar expression in E11.5 and P0 NS over differentiated E11.5 NS, and 13 proteins were identified with high expression specifically in P0 NS compared to E11.5 NS. To illustrate the potential relevance of these identified proteins to neural stem cell biology, the function of Neogenin was further studied. Using Fluorescence Activated Cell Sorting (FACS) analysis, expression of Neogenin was associated with a self-renewing population present in both E11.5 and adult subventricular zone (SVZ) NS but not in P0 NS. E11.5 NS expressed a putative Neogenin ligand, RGMa, and underwent apoptosis when exposed to a ligand-blocking antibody. CONCLUSIONS/SIGNIFICANCE: There are fundamental differences between the continuously self-renewing and more limited progenitors of the developing cortex. We identified a subset of differentially expressed proteins that serve not only as a set of functionally important proteins, but as a useful set of markers for the subsequent analysis of NSPC. Neogenin is associated with the continuously self-renewing and neurogenic cells present in E11.5 cortical and adult SVZ NS, and the Neogenin/RGMa receptor/ligand pair may regulate cell survival during development
Finite thermal conductivity in 1D models having zero Lyapunov exponents
Heat conduction in three types of 1D channels are studied. The channels
consist of two parallel walls, right triangles as scattering obstacles, and
noninteracting particles. The triangles are placed along the walls in three
different ways: (a) periodic, (b) disordered in height, and (c) disordered in
position. The Lyapunov exponents in all three models are zero because of the
flatness of triangle sides. It is found numerically that the temperature
gradient can be formed in all three channels, but the Fourier heat law is
observed only in two disordered ones. The results show that there might be no
direct connection between chaos (in the sense of positive Lyapunov exponent)
and the normal thermal conduction.Comment: 4 PRL page
Regional Brain Stem Atrophy in Idiopathic Parkinson's Disease Detected by Anatomical MRI
Idiopathic Parkinson's disease (PD) is a neurodegenerative disorder characterized by the dysfunction of dopaminergic dependent cortico-basal ganglia loops and diagnosed on the basis of motor symptoms (tremors and/or rigidity and bradykinesia). Post-mortem studies tend to show that the destruction of dopaminergic neurons in the substantia nigra constitutes an intermediate step in a broader neurodegenerative process rather than a unique feature of Parkinson's disease, as a consistent pattern of progression would exist, originating from the medulla oblongata/pontine tegmentum. To date, neuroimaging techniques have been unable to characterize the pre-symptomatic stages of PD. However, if such a regular neurodegenerative pattern were to exist, consistent damages would be found in the brain stem, even at early stages of the disease. We recruited 23 PD patients at Hoenn and Yahr stages I to II of the disease and 18 healthy controls (HC) matched for age. T1-weighted anatomical scans were acquired (MPRAGE, 1 mm3 resolution) and analyzed using an optimized VBM protocol to detect white and grey matter volume reduction without spatial a priori. When the HC group was compared to the PD group, a single cluster exhibited statistical difference (p<0.05 corrected for false detection rate, 4287 mm3) in the brain stem, between the pons and the medulla oblongata. The present study provides in-vivo evidence that brain stem damage may be the first identifiable stage of PD neuropathology, and that the identification of this consistent damage along with other factors could help with earlier diagnosis in the future. This damage could also explain some non-motor symptoms in PD that often precede diagnosis, such as autonomic dysfunction and sleep disorders
To Reach the Light: The Monumental Byzantine Stairs of Caesarea, a Conservation and Restoration Project
Ancient Caesarea has founded in the years 25-10 BC and named after Emperor Augustus. Throughout history, from the early Roman until the Byzantine period, Caesarea was a major city and one of the largest and most important port cities in the Mediterranean. During the Byzantine period, the city encompassed an area three times larger than that delimited by the Herodian wall and became an important center of Christianity. The monumental stairs led to the Byzantine Octagonal Church built upon giant arch above the remains of the enormous Roman stairs of Augustus temple. Stairs led a large number of people from the vast harbor, to the Temple platform. The Byzantine arch located 17 meters from the ancient quay, is 8 meters width and 4-meter long, built with specific technology from local sandstone named Kurkar. The arch fall after the Byzantine period and the staircases severely damaged due to the long exposure of almost 1500 years and environmental conditions such as capillary rise, daily winds carrying sand, high temperature, moisture, salts, and deliberate destruction, for instance, stones robber and collapse parts from the wall. The characteristics of the Kurkar with sustained deterioration and this environmental condition have led to different conservation problems, at various levels of severity erosion, the disintegration in both bonding materials and stones. The conservation measures' purpose is to stop the ongoing weathering process and prevent a deterioration state of the staircases, to restore the arch and stabilize the structure of the stairs to carry 48 tons of the restore arch. The findings of the project show that a suitable solution to ensure effective and sustainable protection of complicated staircases structure from destruction and various weathering condition to carry new massive arch depends on understanding the ancient application of building technologies and techniques, the use of original bonding material, integrated monitoring, and ongoing maintenance
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