206 research outputs found
Universal Conductance Fluctuations in Mesoscopic Systems with Superconducting Leads: Beyond the Andreev Approximation
We report our investigation of the sample to sample fluctuation in transport
properties of phase coherent normal metal-superconductor hybrid systems.
Extensive numerical simulations were carried out for quasi-one dimensional and
two dimensional systems in both square lattice (Fermi electron) as well as
honeycomb lattice (Dirac electron). Our results show that when the Fermi energy
is within the superconducting energy gap , the Andreev conductance
fluctuation exhibits a universal value (UCF) which is approximately two times
larger than that in the normal systems. According to the random matrix theory,
the electron-hole degeneracy (ehD) in the Andreev reflections (AR) plays an
important role in classifying UCF. Our results confirm this. We found that in
the diffusive regime there are two UCF plateaus, one corresponds to the
complete electron-hole symmetry (with ehD) class and the other to conventional
electron-hole conversion (ehD broken). In addition, we have studied the Andreev
conductance distribution and found that for the fixed average conductance
the Andreev conductance distribution is a universal function that depends only
on the ehD. In the localized regime, our results show that ehD continues to
serve as an indicator for different universal classes. Finally, if normal
transport is present, i.e., Fermi energy is beyond energy gap , the AR
is suppressed drastically in the localized regime by the disorder and the ehD
becomes irrelevant. As a result, the conductance distribution is that same as
that of normal systems
Response time of a normal-superconductor hybrid system under the step-like pulse bias
The response of a quantum dot coupled with one normal lead and a
superconductor lead driven by a step-like pulse bias is studied using the
non-equilibrium Green function method. In the linear pulse bias regime, the
responses of the upwards and downwards bias are symmetric. In this regime the
turn-on time and turn-off time are much slower than that of the normal system
due to the Andreev reflection. On the other hand, for the large pulse bias
, the instantaneous current exhibits oscillatory behaviors with the
frequency . The turn on/off times are in (or shorter than)
the scale of , so they are faster for the larger bias . In
addition, the responses for the upwards and downwards bias are asymmetric at
large . The turn-on time is larger than the turn-off time but the
relaxation time \cite{note1} depends only on the coupling strength and
it is much smaller than the turn-on/off times for the large bias .Comment: 8 pages, 4 figures, accepted for publication in Phys. Rev.
Alternative method for the identification of critical nodes leading to voltage instability in a power system
Abstract: Introduction of new operation enhancement technologies plus increasing application of power electronics coupled with the continuous increase in load demand has increased the risk of power networks to voltage instability and susceptibility to voltage collapse. This frequent occurrence of voltage collapse in modern power system has been a growing concern to power system utilities. This paper proposes alternative techniques for the identification of critical nodes that are liable to voltage instability in a power system. The first method is based on the critical mode corresponding to the smallest eigenvalues, while the second technique is based on the centrality measure to identify the influential node of the networks. The eigenvector centrality measure is formulated from the response matrices of both the load and generator nodes of the networks. The effectiveness of the suggested approaches is tested using the IEEE 30 bus and the Southern Indian 10 bus power networks. The results are compared to the techniques based on the traditional power flow. The whole procedure of the results involved in the identification of critical nodes through the proposed methods is totally non-iterative and thereby save time and require less computational burden
Analysis of Voltage Collapse in a Power System Using Voltage Stability Indices
Voltage collapse phenomenon and its incessant occurrence in a modern power system has of recent been a great concern to power system utilities. The aftermath of which could be very devastating and detrimental to the optimum operation of a power system. Thus, it becomes imperative to identify the most vulnerable bus to voltage collapse by power system operator to prevent outages and blackout that may result. Presented in this paper are techniques based on the use of voltage stability index (L-index) and voltage collapse proximity index (VCPI) to identify a voltage collapse bus. First, the basecase values of the L-index and VCPI for all the load nodes are evaluated. The system is then subjected to a gradual reactive power load change to determine the voltage collapse bus. The bus with the maximum value of L-index and VCPI is taken as the critical bus liable to voltage collapse. The effectiveness of the methodology is tested on the Southern Indian 10-bus and IEEE 14 bus power systems. Result obtained shows that the use of the techniques of L-index and VCPI in the identification of critical node in a power system could be of great benefit to the power system operation and planning. Keywords: Voltage stability, Voltage collapse, L-Index, VCPI, power flow, power syste
Spectral Properties of Populations Behind the Coherence in Spitzer Near-infrared and Chandra X-Ray Backgrounds
We study the coherence of the near-infrared and X-ray background fluctuations
and the X-ray spectral properties of the sources producing it. We use data from
multiple Spitzer and Chandra surveys, including the UDS/SXDF surveys, the
Hubble Deep Field North, the EGS/AEGIS field, the Chandra Deep Field South and
the COSMOS surveys, comprising 2275 Spitzer/IRAC hours and ~16 Ms
of Chandra data collected over a total area of ~1~deg. We report an
overall 5 detection of a cross-power signal on large angular
scales 20 between the 3.6 and 4.5\mum\ and the X-ray bands, with the IR
vs [1-2] keV signal detected at 5.2. The [0.5-1] and [2-4] keV bands
are correlated with the infrared wavelengths at a 13
significance level. The hardest X-ray band ([4-7] keV) alone is not
significantly correlated with any infrared wavelengths due to poor photon and
sampling statistics. We study the X-ray SED of the cross-power signal. We find
that its shape is consistent with a variety of source populations of accreting
compact objects, such as local unabsorbed AGNs or high-z absorbed sources. We
cannot exclude that the excess fluctuations are produced by more than one
population. Because of poor statistics, the current relatively broad
photometric bands employed here do not allow distinguishing the exact nature of
these compact objects or if a fraction of the fluctuations have instead a local
origin.Comment: 14 Pages, 4 figures, Accepted by ApJ, for inquires please contact
author N. Cappellut
The SPLASH and Chandra COSMOS Legacy Survey: the Cross Power Between Near-Infrared and X-Ray Background Fluctuations
We study the source-subtracted near-infrared and X-ray background
fluctuations of the COSMOS field using data from the Spitzer SPLASH program
(1272 hours) and Chandra COSMOS Legacy Survey (4.6 Ms). The new auto
power spectra of the cosmic infrared and X-ray background fluctuations reach
maximum angular scales of 3000 and 5000, respectively. We
measure the cross power spectra between each infrared and X-ray band and
calculate the mean power above 20. We find that the soft X-ray band is
correlated with 3.6 and 4.5m at 4 significance level. The
significance between hard X-ray and the 3.6m (4.5m) band is 2.2
(3.8). The combined infrared (3.6 + 4.5m) data are
correlated with the X-ray data in soft ([0.5-2] keV), hard ([2-7] keV) and
broad ([0.5-7] keV) bands at 5.6, 4.4 and
6.6 level, respectively. We compare the new measurements with
existing models for the contributions from known populations at 7, which
are not subtracted. The model predictions are consistent with the measurements
but we cannot rule out contributions from other components, such as Direct
Collapse Black Holes (DCBH). However, the stacked cross-power spectra,
combining other available data, show excess fluctuations about an order of
magnitude on average at 4 confidence at scales within
300. By studying the X-ray SED of the cross-power signal, assuming no
significant variation from the infrared, we find that its shape is consistent
with DCBHs.Comment: 20 pages, 20 figures, 4 table
Precision microcomb design and fabrication for x-ray optics assembly
Silicon microcombs developed at our laboratory for the precision alignment and assembly of large-area foil optics have previously been demonstrated to achieve submicron-level assembly repeatability with submillimeter-thick flat substrates. In this article we report on a double-side deep reactive-ion etch fabrication process using silicon-on-insulator wafers which was developed to improve the microcombs' manufacturing accuracy
Systemic and metabolic signature of sarcopenia in community-dwelling older adults
Background
Evidence suggests the pivotal contribution of nutrition as a modifiable risk factor for sarcopenia. The present cross-sectional study characterized the nutritional and metabolic profile of sarcopenia through an extensive exploration of a wide array of blood biomarkers related to muscle protein metabolism and transcriptomic signatures in community-dwelling elderly adults.
Methods
Among 189 older individuals with a mean age of 73.2 years, sarcopenia was diagnosed according to the Asian Working Group for Sarcopenia criteria based on appendicular lean mass measured by dual-energy X-ray absorptiometry scan, muscle strength, and gait speed. Nutritional status was evaluated using the mini-nutritional assessment (MNA). In addition, we assessed specific blood biomarkers of nutritional status (plasma essential amino acids [EAAs], vitamins), nicotine-derived metabolites, and an extensive microarray analysis from peripheral blood mononuclear cells.
Results
Malnutrition defined by low MNA score was independently associated with sarcopenia (p < .001). Sarcopenic elderly showed lower body mass index and leptin and higher adiponectin and high-density lipoproteins. Levels of EAAs including lysine, methionine, phenylalanine, threonine, as well as branched-chain AAs and choline, were inversely associated with sarcopenia. Furthermore, nicotine metabolites (cotinine and trans-3′-hydroxycotine) and vitamin B6 status were linked to one or more clinical and functional measures of sarcopenia. Differentially expressed genes and ingenuity pathway analysis supported the association of nutrition with sarcopenia.
Conclusions
Herein, the characterization of a nutritional and metabolic signature of sarcopenia provides a firm basis and potential identification of specific targets and directions for the nutritional approach to the prevention and treatment of sarcopenia in aging populations
Ultra-broadband Light Absorption by a Sawtooth Anisotropic Metamaterial Slab
We present an ultra broadband thin-film infrared absorber made of saw-toothed
anisotropic metamaterial. Absorbtivity of higher than 95% at normal incidence
is supported in a wide range of frequencies, where the full absorption width at
half maximum is about 86%. Such property is retained well at a very wide range
of incident angles too. Light of shorter wavelengths are harvested at upper
parts of the sawteeth of smaller widths, while light of longer wavelengths are
trapped at lower parts of larger tooth widths. This phenomenon is explained by
the slowlight modes in anisotropic metamaterial waveguide. Our study can be
applied in the field of designing photovoltaic devices and thermal emitters.Comment: 12 pages, 4 picture
A hyperchaotic system without equilibrium
Abstract: This article introduces a new chaotic system of 4-D autonomous ordinary differential equations, which has no equilibrium. This system shows a hyper-chaotic attractor. There is no sink in this system as there is no equilibrium. The proposed system is investigated through numerical simulations and analyses including time phase portraits, Lyapunov exponents, and Poincaré maps. There is little difference between this chaotic system and other chaotic systems with one or several equilibria shown by phase portraits, Lyapunov exponents and time series methods, but the Poincaré maps show this system is a chaotic system with more complicated dynamics. Moreover, the circuit realization is also presented
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