1,406 research outputs found
Properties of Pt Schottky Type Contacts On High-Resistivity CdZnTe Detectors
In this paper we present studies of the I-V characteristics of CdZnTe
detectors with Pt contacts fabricated from high-resistivity single crystals
grown by the high-pressure Brigman process. We have analyzed the experimental
I-V curves using a model that approximates the CZT detector as a system
consisting of a reversed Schottky contact in series with the bulk resistance.
Least square fits to the experimental data yield 0.78-0.79 eV for the Pt-CZT
Schottky barrier height, and <20 V for the voltage required to deplete a 2 mm
thick CZT detector. We demonstrate that at high bias the thermionic current
over the Schottky barrier, the height of which is reduced due to an interfacial
layer between the contact and CZT material, controls the leakage current of the
detectors. In many cases the dark current is not determined by the resistivity
of the bulk material, but rather the properties of the contacts; namely by the
interfacial layer between the contact and CZT material.Comment: 12 pages, 11 figure
The link between galaxy and black hole growth in the eagle simulation
We investigate the connection between the star formation rate (SFR) of galaxies and their central black hole accretion rate (BHAR) using the EAGLE cosmological hydrodynamical simulation. We find, in striking concurrence with recent observational studies, that the ăSFRăâBHAR relation for an active galactic nucleus (AGN)-selected sample produces a relatively flat trend, whilst the ăBHARăâSFR relation for an SFR-selected sample yields an approximately linear trend. These trends remain consistent with their instantaneous equivalents even when both SFR and BHAR are time averaged over a period of 100 Myr. There is no universal relationship between the two growth rates. Instead, SFR and BHAR evolve through distinct paths that depend strongly on the mass of the host dark matter halo. The galaxies hosted by haloes of mass M200 âČ 1011.5âMâ grow steadily, yet black holes (BHs) in these systems hardly grow, yielding a lack of correlation between SFR and BHAR. As haloes grow through the mass range 1011.5 âČ M200 âČ 1012.5 Mâ BHs undergo a rapid phase of non-linear growth. These systems yield a highly non-linear correlation between the SFR and BHAR, which are non-causally connected via the mass of the host halo. In massive haloes (M200 âł 1012.5 Mâ), both SFR and BHAR decline on average with a roughly constant scaling of SFR/BHAR ⌠103. Given the complexity of the full SFRâBHAR plane built from multiple behaviours, and from the large dynamic range of BHARs, we find the primary driver of the different observed trends in the ăSFRăâBHAR and ăBHARăâSFR relationships are due to sampling considerably different regions of this plane
Coupled-mode equations and gap solitons in a two-dimensional nonlinear elliptic problem with a separable periodic potential
We address a two-dimensional nonlinear elliptic problem with a
finite-amplitude periodic potential. For a class of separable symmetric
potentials, we study the bifurcation of the first band gap in the spectrum of
the linear Schr\"{o}dinger operator and the relevant coupled-mode equations to
describe this bifurcation. The coupled-mode equations are derived by the
rigorous analysis based on the Fourier--Bloch decomposition and the Implicit
Function Theorem in the space of bounded continuous functions vanishing at
infinity. Persistence of reversible localized solutions, called gap solitons,
beyond the coupled-mode equations is proved under a non-degeneracy assumption
on the kernel of the linearization operator. Various branches of reversible
localized solutions are classified numerically in the framework of the
coupled-mode equations and convergence of the approximation error is verified.
Error estimates on the time-dependent solutions of the Gross--Pitaevskii
equation and the coupled-mode equations are obtained for a finite-time
interval.Comment: 32 pages, 16 figure
The KMOS Deep Survey (KDS) â I. Dynamical measurements of typical star-forming galaxies at z â 3.5
We present dynamical measurements from the KMOS (K-band Multi-Object Spectrograph) Deep Survey (KDS), which is comprised of 77 typical star-forming galaxies at z z â 3.5 in the mass range 9.0 1), with the sample average VC/Ïint value much smaller than at lower redshift. After carefully selecting comparable star-forming samples at multiple epochs, we find that the rotation-dominated fraction evolves with redshift with a zâ0.2 dependence. The rotation-dominated KDS galaxies show no clear offset from the local rotation velocity-stellar mass (i.e. VC â Mâ) relation, although a smaller fraction of the galaxies are on the relation due to the increase in the dispersion-dominated fraction. These observations are consistent with a simple equilibrium model picture, in which random motions are boosted in high-redshift galaxies by a combination of the increasing gas fractions, accretion efficiency, specific star-formation rate and stellar feedback and which may provide significant pressure support against gravity on the galactic disk scale
Incompatibility Systems in Switchgrass
Switchgrass (Panicum virgatum L.), a cross-pollinated perennial, produces very little or no seed when self-pollinated, indicating the presence of self-incompatibility mechanisms. Knowledge of self-incompatibility mechanisms is required to use germplasm effectively in a breeding program. The objective of this study was to characterize features of the incompatibility systems in switchgrass. Seed set and seed characteristics of reciprocal matings of tetraploid, octaploid, and tetraploid x octaploid plants were used as measures of incompatibility. Both bagged mutual pollination and manual emasculation and pollination methods were used to make crosses. The percentages of self-compatibility in the tetraploid and octaploid parent plants were 0.35 and 1.39%, respectively. Prefertilization incompatibility in switchgrass is apparently under gametophytic control, since there were significant differences in percentage of compatible pollen as measured by percentage of total seed set between reciprocal matings within ploidy levels. Results indicated that the prefertilization incompatibility system in switchgrass is similar to the S-Z incompatibility system found in other members of the Poaceae. A postfertilization incompatibility system also exists that inhibits intermatings among octaploid and tetraploid plants. In these interploidy crosses, two very distinctive types of abnormal seed were found. When the female parent was the tetraploid plant, the resulting seed was small and shriveled, while when the female parent was the octaploid, small seed with floury endosperm was obtained. These results are similar to those obtained for endosperm incompatibility due to the endosperm balance number system found in other species
A wide search for obscured active galactic nuclei using XMM-Newton and WISE
Heavily obscured and Compton-thick active galactic nuclei (AGNs) are missing even in the deepest X-ray surveys, and indirect methods are required to detect them. Here we use a combination of the XMMâNewton serendipitous X-ray survey with the optical Sloan Digital Sky Survey (SDSS), and the infrared WISE all-sky survey in order to check the efficiency of the low X-ray-to-infrared luminosity selection method in finding heavily obscured AGNs. We select the sources which are detected in the hard X-ray band (2â8âkeV), and also have a redshift determination (photometric or spectroscopic) in the SDSS catalogue. We match this sample with the WISE catalogue, and fit the spectral energy distributions of the 2844 sources which have three, or more, photometric data points in the infrared. We then select the heavily obscured AGN candidates by comparing their 12âÎŒm luminosity to the observed 2â10âkeV X-ray luminosity and the intrinsic relation between the X-ray and the mid-infrared luminosities. With this approach, we find 20 candidate heavily obscured AGNs and we then examine their X-ray and optical spectra. Of the 20 initial candidates, we find nine (64âperâcent; out of the 14, for which X-ray spectra could be fitted) based on the X-ray spectra, and seven (78âperâcent; out of the nine detected spectroscopically in the SDSS) based on the [OâIII] line fluxes. Combining all criteria, we determine the final number of heavily obscured AGNs to be 12â19, and the number of Compton-thick AGNs to be 2â5, showing that the method is reliable in finding obscured AGNs, but not Compton thick. However, those numbers are smaller than what would be expected from X-ray background population synthesis models, which demonstrates how the opticalâinfrared selection and the scatter of the Lx-LMIR relation limit the efficiency of the method. Finally, we test popular obscured AGN selection methods based on mid-infrared colours, and find that the probability of an AGN to be selected by its mid-infrared colours increases with the X-ray luminosity. The (observed) X-ray luminosities of heavily obscured AGNs are relatively low (L2â10keV<1044ergsâ1), even though most of them are located in the âquasi stellar object (QSO) locusâ. However, a selection scheme based on a relatively low X-ray luminosity and mid-infrared colours characteristic of QSOs would not select âŒ25âperâcent of the heavily obscured AGNs of our sample
PT-symmetric Solutions of Schrodinger Equation with position-dependent mass via Point Canonical Transformation
PT-symmetric solutions of Schrodinger equation are obtained for the Scarf and
generalized harmonic oscillator potentials with the position-dependent mass. A
general point canonical transformation is applied by using a free parameter.
Three different forms of mass distributions are used. A set of the energy
eigenvalues of the bound states and corresponding wave functions for target
potentials are obtained as a function of the free parameter.Comment: 13 page
Dark energy, non-minimal couplings and the origin of cosmic magnetic fields
In this work we consider the most general electromagnetic theory in curved
space-time leading to linear second order differential equations, including
non-minimal couplings to the space-time curvature. We assume the presence of a
temporal electromagnetic background whose energy density plays the role of dark
energy, as has been recently suggested. Imposing the consistency of the theory
in the weak-field limit, we show that it reduces to standard electromagnetism
in the presence of an effective electromagnetic current which is generated by
the momentum density of the matter/energy distribution, even for neutral
sources. This implies that in the presence of dark energy, the motion of
large-scale structures generates magnetic fields. Estimates of the present
amplitude of the generated seed fields for typical spiral galaxies could reach
G without any amplification. In the case of compact rotating objects,
the theory predicts their magnetic moments to be related to their angular
momenta in the way suggested by the so called Schuster-Blackett conjecture.Comment: 5 pages, no figure
Is late-life dependency increasing or not? A comparison of the Cognitive Function and Ageing Studies (CFAS)
Background: Little is known about how dependency levels have changed between generational cohorts of older people. We estimated years lived in different care states at age 65 in 1991 and 2011 and new projections of future demand for care. Methods: Two population-based studies of older people in defined geographical areas conducted two decades apart (the Cognitive Function and Ageing Studies) provided prevalence estimates of dependency in four states: high (24-hour care); medium (daily care); low (less than daily); independent. Years in each dependency state were calculated by Sullivanâs method. To project future demand, the proportions in each dependency state (by age group and sex) were applied to the 2014 England population projections. Findings: Between 1991 and 2011 there were significant increases in years lived from age 65 with low (men:1·7 years, 95%CI 1·0-2·4; women:2·4 years, 95%CI 1·8-3·1) and high dependency (men:0·9 years, 95%CI 0·2-1·7; women:1·3 years, 95%CI 0·5-2·1). The majority of menâs extra years of life were independent (36%) or with low dependency (36%) whilst for women the majority were spent with low dependency (58%), only 5% being independent. There were substantial reductions in the proportions with medium and high dependency who lived in care homes, although, if these dependency and care home proportions remain constant in the future, further population ageing will require an extra 71,000 care home places by 2025. Interpretation: On average older men now spend 2.4 years and women 3.0 years with substantial care needs (medium or high dependency), and most will live in the community. These findings have considerable implications for older peopleâs families who provide the majority of unpaid care, but the findings also supply valuable new information for governments and care providers planning the resources and funding required for the care of their future ageing populations
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