691 research outputs found
Optical conductivity of a granular metal at not very low temperatures
We study the finite-temperature optical conductivity, sigma(omega,T), of a
granular metal using a simple model consisting of a array of spherical metallic
grains. It is necessary to include quantum tunneling and Coulomb blockade
effects to obtain the correct temperature dependence of sigma(omega, T), and to
consider polarization oscillations to obtain the correct frequency dependence.
We have therefore generalized the Ambegaokar-Eckern-Schoen (AES) model for
granular metals to obtain an effective field theory incorporating the
polarization fluctuations of the individual metallic grains. In contrast to the
DC conductivity, which is determined by inter-grain charge transfer and obeys
an Arrhenius law at low temperature, the AC conductivity is dominated by a
resonance peak for intra-grain polarization oscillations, which has a power-law
tail at low frequencies. More importantly, although the resonance frequency
agrees with the classical prediction, the resonance width depends on intergrain
quantum tunneling and Coulomb blockade parameters, in addition to the classical
Drude relaxation within the grain. This additional damping is due to inelastic
cotunneling of polarization fluctuations to neighbouring grains and it
qualitatively differs from the DC conductivity in its temperature dependence
quite unlike the expectation from Drude theory.Comment: Added figures, published version, 16 pages, REVTe
Design Study of CANGAROO-III, Stereoscopic Imaging Atmospheric Cherenkov Telescopes for sub-TeV Gamma-ray
CANGAROO-III is an Imaging Atmospheric Cherenkov Telescope (IACT) array of
four 10 m telescopes for very high energy (sub-TeV) gamma-ray astronomy. A
design study of the CANGAROO-III telescope system was carried out using the
Monte Carlo technique in order to optimize the pixel size and the telescope
spacing. Studies were also made of observations at low elevation angles.Comment: 17pages, 13 figures, elsart, to appear in Astro. Part. Phy
Mie-resonances, infrared emission and band gap of InN
Mie resonances due to scattering/absorption of light in InN containing
clusters of metallic In may have been erroneously interpreted as the infrared
band gap absorption in tens of papers. Here we show by direct thermally
detected optical absorption measurements that the true band gap of InN is
markedly wider than currently accepted 0.7 eV. Micro-cathodoluminescence
studies complemented by imaging of metallic In have shown that bright infrared
emission at 0.7-0.8 eV arises from In aggregates, and is likely associated with
surface states at the metal/InN interfaces.Comment: 4 pages, 5 figures, submitted to PR
HENA, heterogeneous network-based data set for Alzheimer's disease.
Alzheimer's disease and other types of dementia are the top cause for disabilities in later life and various types of experiments have been performed to understand the underlying mechanisms of the disease with the aim of coming up with potential drug targets. These experiments have been carried out by scientists working in different domains such as proteomics, molecular biology, clinical diagnostics and genomics. The results of such experiments are stored in the databases designed for collecting data of similar types. However, in order to get a systematic view of the disease from these independent but complementary data sets, it is necessary to combine them. In this study we describe a heterogeneous network-based data set for Alzheimer's disease (HENA). Additionally, we demonstrate the application of state-of-the-art graph convolutional networks, i.e. deep learning methods for the analysis of such large heterogeneous biological data sets. We expect HENA to allow scientists to explore and analyze their own results in the broader context of Alzheimer's disease research
Collective versus single-particle effects in the optical spectra of finite electronic quantum systems
We study optical spectra of finite electronic quantum systems at frequencies
smaller than the plasma frequency using a quasi-classical approach. This
approach includes collective effects and enables us to analyze how the nature
of the (single-particle) electron dynamics influences the optical spectra in
finite electronic quantum systems. We derive an analytical expression for the
low-frequency absorption coefficient of electro-magnetic radiation in a finite
quantum system with ballistic electron dynamics and specular reflection at the
boundaries: a two-dimensional electron gas confined to a strip of width a (the
approach can be applied to systems of any shape and electron dynamics --
diffusive or ballistic, regular or irregular motion). By comparing with results
of numerical computations using the random-phase approximation we show that our
analytical approach provides a qualitative and quantitative understanding of
the optical spectrum.Comment: 4 pages, 3 figure
Resonant Visible Light Modulation with Graphene
Fast modulation and switching of light at visible and near-infrared (vis-NIR)
frequencies is of utmost importance for optical signal processing and sensing
technologies. No fundamental limit appears to prevent us from designing
wavelength-sized devices capable of controlling the light phase and intensity
at gigaherts (and even terahertz) speeds in those spectral ranges. However,
this problem remains largely unsolved, despite recent advances in the use of
quantum wells and phase-change materials for that purpose. Here, we explore an
alternative solution based upon the remarkable electro-optical properties of
graphene. In particular, we predict unity-order changes in the transmission and
absorption of vis-NIR light produced upon electrical doping of graphene sheets
coupled to realistically engineered optical cavities. The light intensity is
enhanced at the graphene plane, and so is its absorption, which can be switched
and modulated via Pauli blocking through varying the level of doping.
Specifically, we explore dielectric planar cavities operating under either
tunneling or Fabry-Perot resonant transmission conditions, as well as Mie modes
in silicon nanospheres and lattice resonances in metal particle arrays. Our
simulations reveal absolute variations in transmission exceeding 90% as well as
an extinction ratio >15 dB with small insertion losses using feasible material
parameters, thus supporting the application of graphene in fast electro-optics
at vis-NIR frequencies.Comment: 17 pages, 13 figures, 54 reference
Determining the extragalactic extinction law with SALT
We present CCD imaging observations of early-type galaxies with dark lanes
obtained with the Southern African Large Telescope (SALT) during its
performance-verification phase. We derive the extinction law by the
extragalactic dust in the dark lanes in the spectral range 1.11mu m^{-1} <
lambda^{-1} < 2.94 mu m^{-1} by fitting model galaxies to the unextinguished
parts of the image, and subtracting from these the actual images. We find that
the extinction curves run parallel to the Galactic extinction curve, which
implies that the properties of dust in the extragalactic enviroment are similar
to those of the Milky Way. The ratio of the total V band extinction to the
selective extinction between the V and B bands is derived for each galaxy with
an average of 2.82+-0.38, compared to a canonical value of 3.1 for the Milky
Way. The similar values imply that galaxies with well-defined dark lanes have
characteristic dust grain sizes similar to those of Galactic dust.Comment: 20 pages, 15 figures and 4 tables. Accepted for publication in MNRA
Supersymmetry and the relationship between a class of singular potentials in arbitrary dimensions
The eigenvalues of the potentials
and
, and of the special cases of these potentials such as the Kratzer and
Goldman-Krivchenkov potentials, are obtained in N-dimensional space. The
explicit dependence of these potentials in higher-dimensional space is
discussed, which have not been previously covered.Comment: 13 pages article in LaTEX (uses standard article.sty). Please check
"http://www1.gantep.edu.tr/~ozer" for other studies of Nuclear Physics Group
at University of Gaziante
Nonlinear Magneto-Optics of Fe Monolayers from first principles: Structural dependence and spin-orbit coupling strength
We calculate the nonlinear magneto-optical response of free-standing fcc
(001), (110) and (111) oriented Fe monolayers. The bandstructures are
determined from first principles using a full-potential LAPW method with the
additional implementation of spin-orbit coupling. The variation of the
spin-orbit coupling strength and the nonlinear magneto-optical spectra upon
layer orientation are investigated. We find characteristic differences which
indicate an enhanced sensitivity of nonlinear magneto-optics to surface
orientation and variation of the in-plane lattice constants. In particular the
crossover from onedimensional stripe structures to twodimensional films of
(111) layers exhibits a clean signature in the nonlinear Kerr-spectra and
demonstrates the versatility of nonlinear magneto-optics as a tool for in situ
thin-film analysis.Comment: 28 pages, RevTeX, psfig, submitted to PR
Angiographic and Optical Coherence Tomography Insights into Bioresorbable Scaffold Thrombosis: Single-Center Experience
Background - As bioresorbable vascular scaffolds (BVSs) are being increasingly used in complex real-world lesions and populations, BVS thrombosis cases have been repo
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