Optical Properties of Three-Dimensional InGaP(As) Islands Formed by Substitution of Fifth-Group Elements

Optics and Spectroscopy - The optical properties of three-dimensional quantum-size InGaPAs islands, which are formed by substitution of phosphorous by arsenic in an InGaPAs layer deposited on GaAs...     

Optical Gain of 1550-nm Range Multiple-Quantum-Well Heterostructures and Limiting Modulation Frequencies of Vertical-Cavity Surface-Emitting Lasers Based on Them

Results of investigation of 1550 nm range stripe semiconductor lasers fabricated from heterostructures with different designs of the gain medium are presented. It is shown that the proposed designs of the gain medium allow obtaining the effective lasing at high level of total optical losses, comparable with the typical optical losses in the vertical-cavity surface-emitting lasers. The evaluation of modal gain in different types of the gain mediummade it possible to estimate the possible frequencies of the small-signal modulation of vertically emitting lasers and proposed the ways to increase them up to 20 GHz or more.     

Specific features of praseodymium-doping induced changes in the critical temperature and energy spectrum parameters of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> in the presence of calcium ions in the lattice

The results of the experimental study of the specific features of changes in the temperature and concentration dependences of the thermopower coefficient and the critical temperature in the Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y system as the praseodymium content increases are presented. The results obtained have been analyzed based on the narrow band model, the energy spectrum and charge carrier system parameters have been determined, and their behavior with an increase in the doping level has been analyzed. It has been found that both superconducting properties and parameters of the normal state of Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y vary differently in various doping ranges. Based on a comparison of the results obtained with the available data for the case of single doping of YBa₂Cu₃O _y with praseodymium, conclusions have been drawn regarding the mechanism of the energy spectrum modification in the studied compound. The Fermi-level pinning effect has been revealed in the region of a local peak of the density-of-states function, and the energy position of this peak has been determined. It has been shown that the consideration of the Fermi level dynamics caused by the specific features of the structure and transformation of the Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y energy spectrum upon doping makes it possible to explain the observed dependence of the critical temperature on the praseodymium content.     

Computer simulation of a twisted nanotube buckling

We develop procedures for solving the problems of dynamic nanostructure deformation and buckling numerically. The procedures are based on discretization with respect to time of the nonlinear equations of molecular mechanics whose matrices and vectors are determined using the Morse potential for the central forces of interaction between atoms and fictitious truss elements accounting for the variations of the angle between atomic bonds. To determine the critical values of deformation parameters and the shapes of buckling nanostructures we use a stability loss criterion for solutions to nonlinear ordinary differential equations on a finite time interval. We implemented our procedures in the PIONER code, using which we solve the problem of a twisted nanotube buckling in the conditions of a quasistatic deformation. To determine the postcritical equilibrium modes we solve the same problem in a dynamic formulation. We show that the modes of equilibrium configurations of the nanotube in the initial postcritical deformation correspond to a buckling mode obtained both at the bifurcation point of quasistatic solutions and at the quasibifurcation point of dynamic solutions.     

Comparative analysis of the electronic structure of positional isomers: Indole-isoindole

On the basis of an analysis of the canonical and localized molecular orbitals of indole and isoindole, calculated in the SCF and CNDO/2 approximations, as well as an x-ray crystallographic investigation of 2-methyl-isoindole, a comparison of the electronic structure of the positional isomers was made. The 10-electronic system of isoindole is more integral than for indole: isoindole is a single 10-electronic system with an appreciable localization of the bonds in the carbocyclic portion of the bicycle; the electronic structure of indole can be represented in a first approximation as the aggregate of three weakly interacting -subsystems: the benzene ring, the double bond between the - and -carbon atoms, and the free electron pair of the nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1497–1503, November, 1984.     

Isoelectronic analogs of indolizine

A new method is offered for synthesizing hitherto unknown compounds of the pyrrolo[1, 2-a]benzimidazole series, involving heating quaternary salts prepared from 1, 2-dialkylbenzimidazoles and -halogenoketones with sodium carbonate solution. The method is used to prepare 6 bases iso-electronic with the corresponding indolizine derivatives. The salts of the pyrrolo[1, 2-a]benzimidazoles give 2 series of cyanine dyes, those with a chain at the 1,1 positions, and those with one at the 3, 3 positions. The anomalous value of the vinylene shift of the absorption maximum observed on passing from monomethine- to trimethinecyanines, is due to steric hindrance in molecules of monomethinecyanines.For previous parts see [1–4].     

Synthesis of 2-cyanomethylazaheterocycles

2-(α-Aryloxyacetyl)cyanomethyl-, 2-(α-1-adamantylacetyl)cyanomethyl-, 2-(α-adamantoyl)cyanometyl-, and 2-(α-1-naphthylacetyl)cyanomethylazaheterocycles have been synthesized by treating 2-cyanomethylazaheterocycles with carboxylic acid chlorides. The 2-cyanomethylbenzimidazoles exist in the enaminoketone form. The biological activity of the synthesized compounds was studied and substances possessing fungicidal activity discovered. T. G. Shevchenko University, Kiev 252033. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 520–522, April, 1997.