1,898 research outputs found
Contactless electroreflectance, in the range of 20 K \u3c T \u3c 300 K, of freestanding wurtzite GaN prepared by hydride-vapor-phase epitaxy
We have performed a detailed contactless electroreflectance study of the interband excitonic transitions on both the Ga and N faces of a 200-μm-thick freestanding hydride-vapor-phase-epitaxy grown wurtzite GaN sample with low defect concentration in the temperature range between 20 and 300 K. The transition energies of the A, B, and C excitons and broadening parameters of the A and B excitons have been determined by least-square fits to the first derivative of a Lorentzian line shape. The energy positions and separations of the excitonic transitions in the sample reveal the existence of residual strain. At 20 K the broadening parameter of A exciton deduced for the Ga (5×105 dislocation cm−2) and N (1×107 dislocation cm−2) faces are 3 and 7 meV, respectively, indicating a lower defect concentration on the former face. The parameters that describe the temperature dependence of the interband transition energies of the A, B, and C excitons as well as the broadening function of the A and B features are evaluated. The results from an analysis of the temperature dependence of the broadening function of excitons A and B indicate that GaN exhibits a very large exciton-phonon coupling
Electroreflectance spectroscopy in self-assembled quantum dots: lens symmetry
Modulated electroreflectance spectroscopy of semiconductor
self-assembled quantum dots is investigated. The structure is modeled as dots
with lens shape geometry and circular cross section. A microscopic description
of the electroreflectance spectrum and optical response in terms of an external
electric field () and lens geometry have been considered. The field
and lens symmetry dependence of all experimental parameters involved in the
spectrum have been considered. Using the effective mass formalism
the energies and the electronic states as a function of and dot
parameters are calculated. Also, in the framework of the strongly confined
regime general expressions for the excitonic binding energies are reported.
Optical selection rules are derived in the cases of the light wave vector
perpendicular and parallel to . Detailed calculation of the Seraphin
coefficients and electroreflectance spectrum are performed for the InAs and
CdSe nanostructures. Calculations show good agreement with measurements
recently performed on CdSe/ZnSe when statistical distribution on size is
considered, explaining the main observed characteristic in the
electroreflectance spectra
Persistent X-Ray Photoconductivity and Percolation of Metallic Clusters in Charge-Ordered Manganites
Charge-ordered manganites of composition exhibit persistent photoconductivity upon
exposure to x-rays. This is not always accompanied by a significant increase in
the {\it number} of conduction electrons as predicted by conventional models of
persistent photoconductivity. An analysis of the x-ray diffraction patterns and
current-voltage characteristics shows that x-ray illumination results in a
microscopically phase separated state in which charge-ordered insulating
regions provide barriers against charge transport between metallic clusters.
The dominant effect of x-ray illumination is to enhance the electron {\it
mobility} by lowering or removing these barriers. A mechanism based on magnetic
degrees of freedom is proposed.Comment: 8 pages, 4 figure
An Exact Formula for the Average Run Length to False Alarm of the Generalized Shiryaev-Roberts Procedure for Change-Point Detection under Exponential Observations
We derive analytically an exact closed-form formula for the standard minimax
Average Run Length (ARL) to false alarm delivered by the Generalized
Shiryaev-Roberts (GSR) change-point detection procedure devised to detect a
shift in the baseline mean of a sequence of independent exponentially
distributed observations. Specifically, the formula is found through direct
solution of the respective integral (renewal) equation, and is a general result
in that the GSR procedure's headstart is not restricted to a bounded range, nor
is there a "ceiling" value for the detection threshold. Apart from the
theoretical significance (in change-point detection, exact closed-form
performance formulae are typically either difficult or impossible to get,
especially for the GSR procedure), the obtained formula is also useful to a
practitioner: in cases of practical interest, the formula is a function linear
in both the detection threshold and the headstart, and, therefore, the ARL to
false alarm of the GSR procedure can be easily computed.Comment: 9 pages; Accepted for publication in Proceedings of the 12-th
German-Polish Workshop on Stochastic Models, Statistics and Their
Application
Thermal and Gelling Properties of Maize Mutants from the OH43 Inbred Line
Starches were isolated from the maize (Zea mays) inbred line Oh43, from its single mutants (amylose extender [ae], brittle-1 [bt1], brittle-2 [bt1], dull-1 [dul], floury-2 [fl2], horny [h], shrunken-2 [sh2], sugary- 1 [sul], and waxy [wx]), and from the double-mutant combinations within Oh43. Differential scanning calorimetry was used to determine the onset temperature (To), range, and enthalpy (DeltaH) of gelatinization and retrogradation, and percentage of retrogradation. The gel strength was measured by using a Voland-Stevens texture analyzer. For gelatinization, the starches of wx dul and sh2 dul had the highest To. Double-mutants ae bt2 and ae dul had the highest To of retrogradation. The highest DeltaH of gelatinization was observed for h wx. The gelatinization enthalpy peak for bt1 starch had a characteristic low temperature shoulder and wide range. Compared with the respective single mutants, most double-mutant combinations had higher To and DeltaH for gelatiniztion and lower To for retrogradation. For gel strength, the dul starch gave the lowest values for firmness and stickiness among the samples. Double mutants generally had gel strength measurements lower than those of the single mutants bt1, bt2, fl2, h, and sh2 but higher than those of dul
Cross-link governed dynamics of biopolymer networks
Cytoskeletal networks of biopolymers are cross-linked by a variety of
proteins. Experiments have shown that dynamic cross-linking with physiological
linker proteins leads to complex stress relaxation and enables network flow at
long times. We present a model for the mechanical properties of transient
networks. By a combination of simulations and analytical techniques we show
that a single microscopic timescale for cross-linker unbinding leads to a broad
spectrum of macroscopic relaxation times, resulting in a weak power-law
dependence of the shear modulus on frequency. By performing rheological
experiments, we demonstrate that our model quantitatively describes the
frequency behavior of actin network cross-linked with -Actinin- over
four decades in frequency.Comment: 4 page
Physicochemical Properties of Starches from Mutant Genotypes of the Oh43 Inbred Line.
The physicochemical properties of 17 mutant genotypes of the Oh43 inbred line were investigated to clarify the relationship between structural characteristics and physicochemical properties and among the properties themselves. These physicochemical properties included blue value (BV), maximum absorbance wavelength (lambda[max]), limiting viscosity number ([nu]), swelling power and solubility at 85 C, and percent light transmittance (%T) of starch paste at 650 nm. Pasting properties were determined by means of Brabender viscoamylography, gel strength by texture analysis, and thermal properties by differential scanning calorimetry. Amylose content was the most important structural characteristic affecting the physicochemical properties of starch. Amylose content was significantly (P less than 0.01) correlated with BV (r = 0.96) and lambda(max) (r = 0.81) and was negatively correlated with [nu] (r = -0.83), %T (r = -0.88), swelling power (r = -0.86), and peak viscosity (r = -0.97). Other structural characteristics, including intermediate material content, average chain length of debranched amylopectin, and ratio of long B chains to short B chains plus A chains of amylopectin, were weakly correlated with properties. Some significant correlations were found among properties, including BV, %T, swelling power, and peak viscosity
Universal Crossover between Efros-Shklovskii and Mott Variable-Range-Hopping Regimes
A universal scaling function, describing the crossover between the Mott and
the Efros-Shklovskii hopping regimes, is derived, using the percolation picture
of transport in strongly localized systems. This function is agrees very well
with experimental data. Quantitative comparison with experiment allows for the
possible determination of the role played by polarons in the transport.Comment: 7 pages + 1 figure, Revte
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