The modified k·p method to investigate polarization effects in nitride quantum-well devices

The modified k·p method (which includes both stress and polarization effects) has been used to investigate recombination phenomena in nitride quantum-well (QW) devices. Within their volumes, both spontaneous and piezoelectric polarization have been found to have an essential influence on carriers behaviour. In particular, as a result of the quantum-confined Stark effect, energy of radiation emitted within the AlGaN/GaN/AlGaN QW has been found to decrease rapidly with an increase in the AlN mole fraction of the barrier material, which means—with an increase in mechanical stresses at the GaN/AlGaN heterojunctions. It should also be stressed that screening of polarization effects induced by free carriers at least partly reduced the above stress influence. So both effects, polarization and its screening, are equally important in exact modelling of an operation of nitride QW devices.     

Stereoselective alkylation of thiacalix[4]arenes

A direct tetraalkylation of thiacalix[4]arenes using procedures well established in ‘classical’ calixarene chemistry usually gives the 1,3-alternate conformers as the main product (n-PrI/K₂CO₃/acetone) while the cone conformers are obtained only in very poor yields (n-PrI/NaH/DMF). Surprisingly, the so far almost inaccessible cone conformers can be prepared in high yields using the two-step procedure: dialkylation-dialkylation, opening the way for their further utilisation in supramolecular chemistry.     

Who's afraid of reduced-rank parameterizations of multivariate models? Theory and example

Reduced-rank restrictions can add useful parsimony to coefficient matrices of multivariate models, but their use is limited by the daunting complexity of the methods and their theory. The present work takes the easy road, focusing on unifying themes and simplified methods. For Gaussian and non-Gaussian (GLM, GAM, mixed normal, etc.) multivariate models, the present work gives a unified, explicit theory for the general asymptotic (normal) distribution of maximum likelihood estimators (MLE). MLE can be complex and computationally hard, but we show a strong asymptotic equivalence between MLE and a relatively simple minimum (Mahalanobis) distance estimator. The latter method yields particularly simple tests of rank, and we describe its asymptotic behavior in detail. We also examine the method's performance in simulation and via analytical and empirical examples.     

New chiral catalysts: synthesis and fragmentation pattern of C9-o-silanized cinchonidines

Summary We synthesized C9-O-silyl ethers of cinchonidine and dihydrocinchonidine (1-8), seven of which have not been known before. The structures of the compounds were verified by ESI-ion-trap MS and ¹H-NMR. Fragmentation of silyl ethers containing Si-phenyl bonds was studied for the first time. The compounds obtained will be utilized as chiral catalysts of certain asymmetric syntheses.</o:p>     

On the possibility of detecting gluon condensation from the spectra of heavy quarkonia

We compute the effect of the gluon condensate on the spectra of heavy quarkonia, taking into account the condensate correlation length. We compare the results obtained for the hydrogen-like model with a more realistic model. The effects are shown to be very small both in bottomonium and toponium.     

Domain structure fluctuations kinetics of amorphous alloys at low-temperature annealing

Previously reported spin orientation fluctuations of the amorphous Fe₈₁B₁₉ alloy at temperatures well below the onset of crystallization were directly shown by the time scanning experiment at 250 °C. The connection of this effect with the magnetostriction field and α-Fe nuclei at the contact surface was proved by the CEMS spectra and by vanishing the effect at both the etched-off Fe₈₁B₁₉ and a zero magnetostriction samples.     

Thermal,spectral and biological properties of Zn(II) complex compounds with phenazone

The thermal decomposition of the complexes Zn(form)₂⋅2phen (I), Zn(ac)₂⋅2phen (II), Zn(prop)₂⋅2phen (III), Zn(but)₂⋅2phen (IV), where phen=phenazone, form=formiate, ac=acetate, prop=propionate, but=butyrate has been studied in air by TG/DTG and DTA methods. The possible mechanism of the thermal decomposition was proposed. The final product of thermal decomposition was ZnO. IR data show unidentate coordination of carboxylate group to Zn(II) ion. The complexes were tested against various strains of microorganisms and their efficiency decrease in the sequence yeasts >bacteria>filamentous fungi.     

Analysis of correlation between percolation concentration and elongation at break in filled electroconductive epoxy-based adhesives

Electrical conductivity and elongation at break of epoxy filled with electroconductive carbon black, graphite or with silver-coated basalt particles or fibres were investigated in this paper. Percolation concentrations were determined to be 14 vol% for epoxy/carbon black composites, 22 vol% for epoxy/graphite composites, 28-29 vol% for both epoxy/silver-coated basalt particles and fibres. The steepest increase in electrical conductivity and the most pronounced decrease in elongation at break occurs at similar filler concentration range for all investigated systems. A good correlation between phenomenological model, introduced in [J. Mater. Sci. Lett. 18 (1998) 1457] and experimental data for all investigated systems was observed.     

Planar waveguide lasers and structures created by laser ablation — an overview

The parameters of planar and channel waveguide lasers fabricated by different techniques are reviewed. Materials and properties of thin films used for creation of planar waveguide lasers are summarized. The parameters of active and passive planar waveguides created by the method of pulsed laser deposition and problems of laser deposition are described. The aspects of laser generation of planar waveguide lasers, resonators and channel waveguides are presented. The results of our experiments of laser deposition of thin Ti:sapphire layers and thin layers of Nd:YAG and Nd:YAP are briefly discussed. This work was supported by the grant of Czech Grant Agency No. 102/96/0429.     

Mathematical modelling of rock bolt systems I

The main goal of the paper is to give a variational formulation of the behaviour of bolt systems in rock mass. The problem arises in geomechanics where bolt systems are applied to reinforce underground openings by inserting steel bars or cables. After giving a variational formulation, we prove the existence and uniqueness and some other properties.