Elastic scattering of light from quantum-well exciton-polarization fluctuations in a microcavity

A model of exciton polarization fluctuations in a quantum well of a randomly variable lateral width is proposed. The stochastic part of the nonlocal susceptibility of quasi-two-dimensional excitons is expressed through random functions of the shape of quantum well boundaries. A theory of elastic light scattering from a quantum well placed in a Fabry-Perot cavity or a semiconductor microcavity is constructed in the lowest (Born) approximation in interface roughness height. The scattering cross section is calculated for an arbitrary statistics of interface roughness. The spectral and angular dependences of the intensity of light scattered by a quantum well have been studied using Gaussian correlation functions of the interface shape. It follows from numerical estimates that elastic resonant scattering in quantum wells should be observed at an rms roughness height of the order of the atomic monolayer thickness.     

Highly efficient catalytic system for electrophilic amination of arylzinc reagents

The effect of catalyst on the yield of amine in the amination of three classes of arylzinc reagents with acetone oxime O‐tosylate was investigated. Since they allowed the preparation of arylamines in excellent yields in the presence of a minimum amount of copper (I) or copper (II) compounds, the catalytic systems using copper (I) or copper (II) combined with a P‐, N‐ or S‐donor ligand were revealed to be the best catalysts for the electrophilic amination of arylzinc reagents with acetone oxime O‐tosylate in the presence of DMPU. Copyright © 2009 John Wiley & Sons, Ltd.     

Design assessment of a 150 kWt CFBC Test Unit

For clean and efficient energy generation from coal, the most suitable technology known to date is ‘Fluidized Bed Combustion’ technology. Applications of circulating fluidized bed (CFB) combustion technology have been steadily increasing in both capacity and number over the past decade. Designs of these units have been based on the combustion tests carried out in pilot scale facilities to determine the combustion and desulfurization characteristics of coal and limestone reserves in CFB conditions. Similarly, utilization of Turkish lignites in CFB boilers necessitates adaptation of CFB combustion technology to these resources. However, the design of these test units are not based on firing coals with high ash, volatile matter and sulfur contents like Turkish lignites. For this purpose, a 150 kWt CFB combustor test unit is designed and constructed in Chemical Engineering Department of Middle East Technical University, based on the extensive experience acquired at the existing 0.3 MWt Bubbling Atmospheric Fluidized Bed Combustor (AFBC) Test Rig. Following the commissioning tests, a combustion test is carried out for investigation of combustion characteristics of Çan lignite in CFB conditions and for assessment of the design of test unit. Comparison of the design outputs with experimental results reveals that most of the predictions and assumptions have acceptable agreement with the operating conditions. In conclusion, the performance of 150 kWt CFBC Test Unit is found to be satisfactory to be utilized for the long term research studies on combustion and desulfurization characteristics of indigenous lignite reserves in circulating fluidized bed combustors.     

Interference of additional modes in spectra of opal-like photonic crystals in the multiple diffraction regime

The mechanisms of formation of Bragg reflectance and transmittance spectra have been investigated for films of three-dimensional opal-like photonic crystals. Attention has been drawn to the fact that the spectra exhibit an additional short-period interference structure of the novel type due to the multiple Bragg diffraction of light. The spectra have been calculated in the framework of the dynamical three-wave diffraction model taking into account strong spatial modulation of the dielectric function. It has been found that short-period oscillations appear in the spectra due to the spatial quantization of additional modes with a low group velocity.     

Amplitude-phase reflectance spectra of amorphous silicon-based Bragg structures

Amplitude-phase spectra of light reflection from distributed Bragg reflectors and Fabry-Pérot microcavities based on a-Si: H/a-SiO_x: H thin films have been studied. The frequency dependence of the phase difference between the amplitude p-and s-light reflection coefficients within the photonic band gap is measured. The phase spectrum exhibits predominantly a monotonic, close-to-linear frequency behavior, except for spectral regions near the stop band edges and near the singularities related to the microcavity eigenmodes. The experimental spectra are compared with theoretical calculations based on the transfer matrix method and approximate analytical relations. A method based on analyzing amplitude-phase reflectance spectra is proposed for structural characterization of multilayer microcavity systems.     

Parametric methods for frequency-selective MR spectroscopy-a review

Accurate quantitation of the spectral components in a pre-selected frequency band for magnetic resonance spectroscopy (MRS) signals is a frequently addressed problem in the MR community. One obvious application for such a frequency-selective technique is to lower the computational burden in situations when the measured data sequence contains too many samples to be processed using a standard full-spectrum method. Among the frequency-selective methods previously proposed in the literature, only a few possess the two features of primary concern: high robustness against interferences from out-of-band components and low computational complexity. In this survey paper we consider five spectral analysis methods which can be used for MRS signal parameter estimation in a selected frequency band. We re-derive the filter diagonalization method (FDM) in a new way that allows an easy comparison to the other methods presented. Then we introduce a frequency-selective version of the method of direction estimation (MODE) which has not been applied to MR-spectroscopy before. In addition, we present a filtering and decimation technique using a maximum phase bandpass FIR-filter and relate it to a similar ARMA-modeling approach known as SB-HOYWSVD (sub-band high-order Yule-Walker singular value decomposition). Finally, we study the numerical performances of these four methods and compare them to that of the recently introduced SELF-SVD (Singular Value Decomposition-based method usable in a SELected Frequency band) in several examples using simulated MR data, and discuss the benefits and disadvantages of each technique.     

Thermodynamics and general relativity could determine the geometry of the universe

We introduce a suggestive model where certain quantities in Friedmann models are treated like their thermodynamic counterparts; temperature entropy, Gibbs energy, and so on. Within this model, changes in the symmetry of the universe are interpreted as first- or second-order phase transitions. The thermodynamics we introduce give us a new way of determining the geometry of the universe. By choosing a specific local equation of state (P=), we show that with respect to the thermodynamics we have introduced, it is always more advantageous for the universe to be in a Bianchi V (open) symmetric state.This essay received an honorable mention from the Gravity Research Foundation for the year 1986 — Ed.     

Application of two kinetic methods to evaluate the trap energy of lithium fluoride + sodium fluoride thermoluminescent phosphor

A mixture of equal quantities of powdered LiF and NaF was doped with 0.2% each of Ti, Mg and Mn and heated to obtain a LiF+NaF phosphor. After grinding and sieving, the thermoluminescent phosphor was used to obtain the thermoluminescence characteristics such as the glow curve; the dose-response curve, sensitivity, reproducibility and linearity were all established. The main goal was to introduce two new methods for the calculation of trap energy: the single-peak-differential and single-peak-integration methods, both modifications of the basic method of Randall-Wilkins. The plots used to calculate the trap energy were In[(d/dT)1/(1-)]vs. 1/T and ln[ln(1-)/(T _o-T)]vs. 1/T, respectively. These linear plots with slopes of (E/k) gave trap energies ofE=0.547 eV andE=0.551 eV, respectively.     

Sensitive and Accurate Determination of Cobalt at Trace Levels by Slotted Quartz Tube-Flame Atomic Absorption Spectrometry Following Preconcentration with Dispersive Liquid–Liquid Microextraction

Slotted quartz tube-flame atomic absorption spectrometry (SQT-FAAS) was used as a sensitive technique for the determination of cobalt, an element has toxic effects on living organisms at high doses. For the preliminary preconcentration of cobalt prior to the analysis, dispersive liquid–liquid microextraction (DLLME) was used. The instrument is equipped with a SQT to further increase the sensitivity by increasing the residence time of cobalt atoms in the light path emitted by a hollow cathode lamp. In the complex formation step, pH, the volume of buffer solution, the concentration of 1,5-diphenylcarbazone, and the volume of ligand were optimized. In addition, all of the system parameters, including the type and volume of the extraction solvent, dispersant type and volume of solvent, type of salt and the volume for the dispersion liquid–liquid microextraction, were optimized to obtain the lowest detection limit. Under the optimum conditions, the detection power of FAAS was improved by a factor of 86.56 fold using DLLME-SQT-FAAS. The limit of detection for the DLLME-SQT-FAAS system was 0.97?µg L^?1. The applicability of the developed method was verified in tap and waste water samples by spiking measurements. The percentage recovery values for these were determined to be 91.7% and 111.0%, respectively.