715 research outputs found
Nonequilibrium Green's Function Approach to Phonon Transport in Defective Carbon Nanotubes
We have developed a new theoretical formalism for phonon transport in
nanostructures using the nonequilibrium phonon Green's function technique and
have applied it to thermal conduction in defective carbon nanotubes. The
universal quantization of low-temperature thermal conductance in carbon
nanotubes can be observed even in the presence of local structural defects such
as vacancies and Stone-Wales defects, since the long wavelength acoustic
phonons are not scattered by local defects. At room temperature, however,
thermal conductance is critically affected by defect scattering since incident
phonons are scattered by localized phonons around the defects. We find a
remarkable change from quantum to classical features for the thermal transport
through defective CNTs with increasing temperature.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Electronic Transport in Fullerene C20 Bridge Assisted by Molecular Vibrations
The effect of molecular vibrations on electronic transport is investigated
with the smallest fullerene C20 bridge, utilizing the Keldysh nonequilibrium
Green's function techniques combined with the tight-binding molecular-dynamics
method. Large discontinuous steps appear in the differential conductance when
the applied bias-voltage matches particular vibrational energies. The magnitude
of the step is found to vary considerably with the vibrational mode and to
depend on the local electronic states besides the strength of
electron-vibration coupling. On the basis of this finding, a novel way to
control the molecular motion by adjusting the gate voltage is proposed.Comment: 9 pages, 4 figures, accepted for publication in Phys. Rev. Let
Sparse-firing regularization methods for spiking neural networks with time-to-first spike coding
The training of multilayer spiking neural networks (SNNs) using the error
backpropagation algorithm has made significant progress in recent years. Among
the various training schemes, the error backpropagation method that directly
uses the firing time of neurons has attracted considerable attention because it
can realize ideal temporal coding. This method uses time-to-first spike (TTFS)
coding, in which each neuron fires at most once, and this restriction on the
number of firings enables information to be processed at a very low firing
frequency. This low firing frequency increases the energy efficiency of
information processing in SNNs, which is important not only because of its
similarity with information processing in the brain, but also from an
engineering point of view. However, only an upper limit has been provided for
TTFS-coded SNNs, and the information-processing capability of SNNs at lower
firing frequencies has not been fully investigated. In this paper, we propose
two spike timing-based sparse-firing (SSR) regularization methods to further
reduce the firing frequency of TTFS-coded SNNs. The first is the membrane
potential-aware SSR (M-SSR) method, which has been derived as an extreme form
of the loss function of the membrane potential value. The second is the firing
condition-aware SSR (F-SSR) method, which is a regularization function obtained
from the firing conditions. Both methods are characterized by the fact that
they only require information about the firing timing and associated weights.
The effects of these regularization methods were investigated on the MNIST,
Fashion-MNIST, and CIFAR-10 datasets using multilayer perceptron networks and
convolutional neural network structures
Earthquake-Induced Potentiation of Acute Risk Factors in Hypertensive Elderly Patients: Possible Triggering of Cardiovascular Events After a Major Earthquake
AbstractObjectives. We sought to investigate the potentiation of acute risk factors after the Hanshin-Awaji earthquake (7.2 on the Richter scale).Background. The frequency of cardiovascular events increases just after a major earthquake, but the causative factors have not been fully investigated.Methods. We studied the changes in cardiovascular risk factors in 42 elderly outpatients with well-controlled hypertension living near the epicenter (Awaji-Hokudan districts) 7 to 14 days after the earthquake when the major felt-aftershocks persisted. They all experienced the highest stress grading of 6 (catastrophic stress) according to the DSM-III-R. To study the hemostatic profile and endothelial cell state, we measured the blood pressure (BP), hematocrit and lipid profiles as well as fibrinogen, a marker of fibrin turnover (d-dimer), fibrinolytic factors (plasmin-alpha2–plasmin inhibitor complex [PIC], tissue-type plasminogen activator [t-PA] antigen and t-PA inhibitor [PAI] activity) and an endothelial cell-derived marker (von Willebrand factor [vWF]).Results. Systolic and diastolic blood pressures and other variables increased after the earthquake. Before and after the earthquake, the median (25th to 75th percentiles) systolic BP was 152 (range 142 to 164) and 170 mm Hg (range 161 to 178), respectively (p < 0.0001), and the diastolic BP was 83 (range 79 to 88) and 91 mm Hg (range 84 to 96), respectively (p < 0.0001). Of blood viscosity determinants, hematocrit was 38.1% (range 40.7% to 35.9%) and 39.7% (range 42.9% to 38.3%), respectively (p < 0.001), and fibrinogen 316 (range 272 to 360) and 335 mg/dl (range 307 to 391), respectively (p < 0.05). Von Willebrand factor was 128% (range 74% to 148%) and 148% (range 100% to 178%), respectively (p < 0.01); d-dimer was 410 (range 285 to 633) and 560 ng/ml (range 391 to 888), respectively (p < 0.0001); and PIC was 0.74 (range 0.58 to 0.91) and 0.75 μg/ml (range 0.58 to 1.1), respectively (p < 0.05). In contrast, lipid profiles did not change after the quake. When the patients were classified into the high stress and moderate stress groups according to the degrees of damage to their house and injury to family members, the levels of fibrinogen, vWF, PIC and t-PA antigen were increased only in the former group, whereas BP, hematocrit and d-dimer levels were increased in both groups. These abnormalities of acute risk factors, except for vWF, were transient and decreased to prequake levels by 4 to 6 months after the quake.Conclusions. Earthquake-induced stress seems to induce transient increases in BP, blood viscosity determinants and fibrin turnover and to prolong endothelial cell stimulation. The potentiation of these acute risk factors might contribute to the occurrence of cardiovascular events just after a major earthquake in elderly subjects with hypertension.(J Am Coll Cardiol 1997;29:926–33)© 1997 by the American College of Cardiolog
Universal Features of Quantized Thermal Conductance of Carbon Nanotubes
The universal features of quantized thermal conductance of carbon nanotubes
(CNTs) are revealed through theoretical analysis based on the Landauer theory
of heat transport. The phonon-derived thermal conductance of semiconducting
CNTs exhibits a universal quantization in the low temperature limit,
independent of the radius or atomic geometry. The temperature dependence
follows a single curve given in terms of temperature scaled by the phonon
energy gap. The thermal conductance of metallic CNTs has an additional
contribution from electronic states, which also exhibits quantized behavior up
to room temperature.Comment: 4 pages, 5 figures. accepted for publication in Phys. Rev. Let
An Efficient Synthesis of 2'-O-(β-D-Ribofuranosyl)biopterin
N(2)-(N,N-Dimethylaminomethylene)-3-[2-(4-nitrophenyl)ethyl]-1',2'-di-O (trimethylsilyl)biopterin (4) was prepared
from biopterin (1a, 86% overall yield) in 5 steps. Glycosylation of 4 with 1,2,3,5-tetra-O-acetyl-β D-ribofuranose (5a) and its 2,3,5-tri-O-benzoyl analog (5b) respectively afforded the corresponding 2'-O-(2,3,5-tri-Oacetyl-
and 2,3,5-tri-O-benzoyl-β-D ribofuranosyl)biopterin derivatives (6a, 42% and 6b, 60%) as major products. Removal of the protecting groups of 6b provided 2'-O-(β-D-ribofuranosyl)biopterin (1c, 87% overall yield) in 3 steps
- …