A dual-catalysis approach to the asymmetric Steglich rearrangement and catalytic enantioselective addition of O-acylated azlactones to isoquinolines

A dual-catalysis approach, namely the combination of an achiral nucleophilic catalyst and a chiral anion-binding catalyst, was applied to the Steglich rearrangement to provide α,α-disubstituted amino acid derivatives in a highly enantioselective fashion. Replacement of the nucleophilic co-catalyst for isoquinoline resulted in a divergent reaction pathway and an unprecedented transformation of O-acylated azlactones. This strategy provided highly substituted α,β-diamino acid derivatives with excellent levels of stereocontrol.     

Physical properties,crystal and magnetic structure of layered Fe<Subscript>1.11</Subscript>Te<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> superconductors

The physical and structural properties of Fe_1.11Te and Fe_1.11Te_0.5Se_0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe_1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe_1.11Te_0.5Se_0.5 compound, the superconducting transition with T _c = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H _c2 is obtained by measuring the resistivity under different magnetic fields. The Kim’s critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe_1.11Te_0.5Se_0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe_1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.     

Lattice dynamics of strontium tungstate

We report here measurements of the phonon density of states and the lattice dynamics calculations of strontium tungstate (SrWO₄). At ambient conditions this compound crystallizes to a body-centred tetragonal unit cell (space group I4₁/a) called scheelite structure. We have developed transferable interatomic potentials to study the lattice dynamics of this class of compounds. The model parameters have been fitted with respect to the experimentally available Raman and infra-red frequencies and the equilibrium unit cell parameters. Inelastic neutron scattering measurements have been carried out in the triple-axis spectrometer at Dhruva reactor. The measured phonon density of states is in good agreement with the theoretical calculations, thus validating the interatomic potential developed.      

Design and Optimization of Nonlinear Tapers using Particle Swarm Optimization

In this paper the design and optimization of a nonlinear diameter taper, connecting the output section of a gyrotron cavity to the uniform output waveguide section, is presented. The design of a nonlinear taper of a 42 GHz, 200 kW CW gyrotron operating in the TE_0,3 cavity mode with axial output collection has been taken as a case study. The taper synthesis has been carried out considering a raised cosine type of nonlinear taper and the analysis is done using a dedicated scattering matrix code. In addition, an improved particle swarm optimization - an evolutionary optimization - algorithm is used for the design optimization of this nonlinear taper. The optimum design of the taper shows the effectiveness of the presented method.     

Analytical modeling and ATLAS simulation of N+-InP/n0-In0.53Ga0.47As/p+-In0.53Ga0.47As p-i-n photodetector for optical fiber communication

In this paper we report an analytical modeling of N⁺-InP/n⁰-In_0.53Ga_0.47As/p⁺-In_0.53Ga_0.47As p-i-n photodetector for optical fiber communication. The results obtained on the basis of our model have been compared and contrasted with the simulated results using ATLAS? and experimental results reported by others. The photodetector has been studied in respect of energy band diagram, electric field profile, doping profile, dark current, resistance area-product, quantum efficiency, spectral response, responsivity and detectivity by analytical method using closed form equations and also been simulated by using device simulation software ATLAS? from SILVACO® international. The photodetector exhibits a high quantum efficiency ～90%, responsivity ～1.152–1.2 A/W in the same order as reported experimentally by others, specific detectivity ～5 × 10⁹ cm Hz^1/2 W^?1at wavelength 1.55–1.65 μm, dark current of the order of 10^?11 A at reverse bias of 1.5 V and 10^?13–10^?12 A near zero bias. These values are comparable to those obtained for practical p-i-n detectors. The estimated noise equivalent power (NEP) is of the order of 2.5 × 10^?14 W.     

Band head spin assignment of superdeformed bands in Hg isotopes through power index formula

The power index formula has been used to obtain the band head spin(I_0) of all the superdeformed(SD) bands in Hg isotopes. A least squares fitting approach is used. The root mean square deviations between the determined and the observed transition energies are calculated by extracting the model parameters using the power index formula. Whenever definite spins are available, the determined and the observed transition energies are in accordance with each other. The computed values of dynamic moment of inertia J~((2)) obtained by using the power index formula and its deviation with the rotational frequency is also studied. Excellent agreement is shown between the calculated and the experimental results for J~((2)) versus the rotational frequency. Hence, the power index formula works very well for all the SD bands in Hg isotopes expect for ~(195)Hg(2, 3, 4).     

Systematic study of rigid triaxiality in Ba–Pt nuclei and role of $$$$ subshell effect

A systematic variation of shape variables \(\beta \) and \(\gamma \) with N and \(N_\mathrm {p}N_\mathrm {n}\) is studied in the framework of an asymmetric rotor model of Davydov and Filippov for the \(Z=\) 50–82, \(N=\) 82–126 major shell space. The role of the \(Z=64\) subshell in producing smooth systematics has been discussed. The quadrupole moments are extracted after considering both axially symmetric and axially asymmetric nuclei. The correlation of \(\beta \) with \(\gamma \) together with the measured quadrupole moments indicates that \(\gamma \)-rigidity is better observed in nuclei with modest deformation.     

Control of vortex shedding behind circular cylinder for flows at low Reynolds numbers

It has been observed by researchers in the past that vortex shedding behind circular cylinders can be altered, and in some cases suppressed, over a limited range of Reynolds numbers by proper placement of a second, much smaller, ‘control’ cylinder in the near wake of the main cylinder. Results are presented for numerical computations of some such situations. A stabilized finite element method is employed to solve the incompressible Navier–Stokes equations in the primitive variables formulation. At low Reynolds numbers, for certain relative positions of the main and control cylinder, the vortex shedding from the main cylinder is completely suppressed. Excellent agreement is observed between the present computations and experimental findings of other researchers. In an effort to explain the mechanism of control of vortex shedding, the streamwise variation of the pressure coefficient close to the shear layer of the main cylinder is compared for various cases, with and without the control cylinder. In the cases where the vortex shedding is suppressed, it is observed that the control cylinder provides a local favorable pressure gradient in the wake region, thereby stabilizing the shear layer locally. Copyright © 2001 John Wiley & Sons, Ltd.