3,280 research outputs found
First-principles calculation of the temperature dependence of the optical response of bulk GaAs
A novel approach has been developed to calculate the temperature dependence
of the optical response of a semiconductor. The dielectric function is averaged
over several thermally perturbed configurations that are extracted from
molecular dynamic simulations. The calculated temperature dependence of the
imaginary part of the dielectric function of GaAs is presented in the range
from 0 to 700 K. This approach that explicitly takes into account lattice
vibrations describes well the observed thermally-induced energy shifts and
broadening of the dielectric function.Comment: 6 pages, 3 figure
Density of states of a layered S/N d-wave superconductor
We calculate the density of states of a layered superconductor in which there
are two layers per unit cell. One of the layers contains a d-wave pairing
interaction while the other is a normal metal. The goal of this article is to
understand how the d-wave behaviour of the system is modified by the coupling
between the layer-types. This coupling takes the form of coherent, single
particle tunneling along the c-axis. We find that there are two physically
different limits of behaviour, which depend on the relative locations of the
Fermi surfaces of the two layer-types. We also discuss the interference between
the interlayer coupling and pairing interaction and we find that this
interference leads to features in the density of states.Comment: 33 pages and 11 PostScript figure
Positivity Constraints on Anomalies in Supersymmetric Gauge Theories
The relation between the trace and R-current anomalies in supersymmetric
theories implies that the U, U and U anomalies which
are matched in studies of N=1 Seiberg duality satisfy positivity constraints.
Some constraints are rigorous and others conjectured as four-dimensional
generalizations of the Zamolodchikov -theorem. These constraints are tested
in a large number of N=1 supersymmetric gauge theories in the non-Abelian
Coulomb phase, and they are satisfied in all renormalizable models with unique
anomaly-free R-current, including those with accidental symmetry. Most striking
is the fact that the flow of the Euler anomaly coefficient, , is
always positive, as conjectured by Cardy.Comment: latex, 36 page
Photoelasticity of crystalline and amorphous silica from first principles
Based on density-functional perturbation theory we have computed from first
principles the photoelastic tensor of few crystalline phases of silica at
normal conditions and high pressure (quartz, -cristobalite,
-cristobalite) and of models of amorphous silica (containig up to 162
atoms), obtained by quenching from the melt in combined classical and
Car-Parrinello molecular dynamics simulations. The computational framework has
also been checked on the photoelastic tensor of crystalline silicon and MgO as
prototypes of covalent and ionic systems. The agreement with available
experimental data is good.
A phenomenological model suitable to describe the photoelastic properties of
different silica polymorphs is devised by fitting on the ab-initio data.Comment: ten figure
Second harmonic generation in SiC polytypes
LMTO calculations are presented for the frequency dependent second harmonic
generation (SHG) in the polytypes 2H, 4H, 6H, 15R and 3C of SiC. All
independent tensor components are calculated. The spectral features and the
ratios of the 333 to 311 tensorial components are studied as a function of the
degree of hexagonality. The relationship to the linear optical response and the
underlying band structure are investigated. SHG is suggested to be a sensitive
tool for investigating the near band edge interband excitations.Comment: 12 pages, 10 figure
Clinical, genetic, and functional characterization of the glycine receptor β-subunit A455P variant in a family affected by hyperekplexia syndrome
Hyperekplexia is a rare neurological disorder characterized by exaggerated startle response affecting newborns with the hallmark characteristics of hypertonia, apnea, and noise or touch-induced non-epileptic seizures. The genetic causes of the disease can vary and several associated genes and mutations have been reported to affect glycine receptors (GlyRs); however, the mechanistic links between GlyRs and hyperekplexia are not yet understood. Here, we describe a patient with hyperekplexia from a consanguineous family. Extensive genetic screening using exome sequencing coupled with autozygome analysis and iterative filtering supplemented by in silico prediction identified that the patient carries the homozygous missense mutation A455P in GLRB, which encodes the GlyR β-subunit. To unravel the physiological and molecular effects of A455P on GlyRs, we used electrophysiology in a heterologous system as well as immunocytochemistry, confocal microscopy, and cellular biochemistry. We found a reduction in glycine-evoked currents in N2A cells expressing the mutation compared to wild type cells. Western blot analysis also revealed a reduced amount of GlyR β protein both in cell lysates and isolated membrane fractions. In line with the above observations, co-immunoprecipitation assays suggested that the GlyR α1-subunit retained co-assembly with βA455P to form membrane-bound heteromeric receptors. Finally, structural modelling showed that the A455P mutation affected the interaction between the GlyR β-subunit transmembrane domain 4 and the other helices of the subunit. Taken together, our study identifies and validates a novel loss-of-function mutation in GlyRs whose pathogenicity is likely to cause hyperekplexia in affected individuals
Physical, Mechanical, and Morphological Properties of Woven Kenaf/Polymer Composites Produced Using a Vacuum Infusion Technique
Nowadays, due to renewable issues, environmental concerns, and the financial problems of synthetic fibres, the development of high-performance engineering products made from natural resources is increasing all over the world. Lately, kenaf fibre has been used among many different types of natural resources in various shapes. Unidirectional long fibres or randomly oriented short fibre shapes are the most common type of kenaf fibres that have been investigated in previous works. This work characterises and evaluates the physical, mechanical, and morphological properties of plain woven kenaf fabric and its composites with three types of thermoset resin at 0°/90° and 45°/−45° orientation, in order to assess their suitability as lignocellulosic reinforced polymer composites. A vacuum infusion manufacturing technique was used to prepare the specimens with fibre weight content of 35% ± 2%. Eight specimens were prepared for each test, and five replications were adopted. A total of 78 samples were tested in this study. The results show that the composites with 0°/90° had the highest tensile, flexural strengths, and modulus. The morphological properties of composite samples were analysed through scanning electron microscopy (SEM) images and these clearly demonstrated the better interfacial adhesion between the woven kenaf and the epoxy matrix
DC-electric-field-induced and low-frequency electromodulation second-harmonic generation spectroscopy of Si(001)-SiO interfaces
The mechanism of DC-Electric-Field-Induced Second-Harmonic (EFISH) generation
at weakly nonlinear buried Si(001)-SiO interfaces is studied experimentally
in planar Si(001)-SiO-Cr MOS structures by optical second-harmonic
generation (SHG) spectroscopy with a tunable Ti:sapphire femtosecond laser. The
spectral dependence of the EFISH contribution near the direct two-photon
transition of silicon is extracted. A systematic phenomenological model of the
EFISH phenomenon, including a detailed description of the space charge region
(SCR) at the semiconductor-dielectric interface in accumulation, depletion, and
inversion regimes, has been developed. The influence of surface quantization
effects, interface states, charge traps in the oxide layer, doping
concentration and oxide thickness on nonlocal screening of the DC-electric
field and on breaking of inversion symmetry in the SCR is considered. The model
describes EFISH generation in the SCR using a Green function formalism which
takes into account all retardation and absorption effects of the fundamental
and second harmonic (SH) waves, optical interference between field-dependent
and field-independent contributions to the SH field and multiple reflection
interference in the SiO layer. Good agreement between the phenomenological
model and our recent and new EFISH spectroscopic results is demonstrated.
Finally, low-frequency electromodulated EFISH is demonstrated as a useful
differential spectroscopic technique for studies of the Si-SiO interface in
silicon-based MOS structures.Comment: 31 pages, 14 figures, 1 table, figures are also available at
http://kali.ilc.msu.su/articles/50/efish.ht
ARPES: A probe of electronic correlations
Angle-resolved photoemission spectroscopy (ARPES) is one of the most direct
methods of studying the electronic structure of solids. By measuring the
kinetic energy and angular distribution of the electrons photoemitted from a
sample illuminated with sufficiently high-energy radiation, one can gain
information on both the energy and momentum of the electrons propagating inside
a material. This is of vital importance in elucidating the connection between
electronic, magnetic, and chemical structure of solids, in particular for those
complex systems which cannot be appropriately described within the
independent-particle picture. Among the various classes of complex systems, of
great interest are the transition metal oxides, which have been at the center
stage in condensed matter physics for the last four decades. Following a
general introduction to the topic, we will lay the theoretical basis needed to
understand the pivotal role of ARPES in the study of such systems. After a
brief overview on the state-of-the-art capabilities of the technique, we will
review some of the most interesting and relevant case studies of the novel
physics revealed by ARPES in 3d-, 4d- and 5d-based oxides.Comment: Chapter to appear in "Strongly Correlated Systems: Experimental
Techniques", edited by A. Avella and F. Mancini, Springer Series in
Solid-State Sciences (2013). A high-resolution version can be found at:
http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Reviews/ARPES_Springer.pdf.
arXiv admin note: text overlap with arXiv:cond-mat/0307085,
arXiv:cond-mat/020850
Management model for the logistics and competitiveness of SMEs in the city of Barranquilla
In Colombia, small and medium-sized enterprises (SMEs) are the most powerful engines of economic development, since they generate a high volume of jobs in the national territory, while diversifying productive, commercial, and service activities. In order to propose a model for logistics management as a component for the competitiveness of SMEs, a qualitative study is proposed that will allow us to collect from a structured review of recent literature, identifying the elements to be taken into account for logistics management in order to provide timely support and open space for continuous improvement. The proposed model is structured in six main blocks: characterization of processes according to the activity of the SME, external factors of influence, internal factors of influence, feeding of information, monitoring and control of operations, and feedback between support areas. The above elements have been analyzed and suggested taking into account the specific aspects of SMEs in the country, and taking into account the peculiarities of these small productive cells
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