Structural transitions in model beta-sheet tapes

We present a molecular-scale simulation study of the structural transitions between helicoidal, helical, and tubular geometries in supramolecular beta-sheet tapes. Such geometries have been observed in different self-assembled amyloid systems (based on either natural or synthetic peptides) for which the beta-sheet tapes represent the simplest fibrillar aggregates. A coarse-grained model for the beta-sheet tapes is proposed, with chiral degrees of freedom and asymmetrical chemical properties, which provides a quantitative characterization of the structural transitions. A quantitative connection is established between the molecular properties and the elastic parameters of the supramolecular tapes.     

Test Elements and Test Rank of a Free Metabelian Group

Siberian Mathematical Journal -     

Base fields of csp-rings

We study into the question of which fields may serve as base fields for csp-rings. It is proved that every algebraic extension of a field Q is the base field of some csp-ring. Also it shown that in studying base fields, we may confine ourselves to treating only csp-rings of idempotent cocharacteristic, or only regular csp-rings.     

Photoluminescence properties of silicon nanocrystals grown by nanosecond laser ablation of solid-state targets in an inert gas atmosphere

It was found that the photoluminescence intensity decay kinetics of nanocrystalline silicon layers formed by nanosecond laser ablation of crystalline silicon targets in a helium atmosphere exhibit a power-law behavior with an exponent from 0.9 to 1.5, depending on the temperature and luminescence photon energy in the range of 1.4–1.8 eV, which indicates photoexcited carrier recombination controlled by dissipative tunneling processes in silicon nanocrystal ensembles in a suboxide matrix.     

Analysis of dynamic light scattering of poly(N-isopropylacrylamide) across the collapse transition

Summary We analyse data from the dynamic light scattering of poly(N-isopropylacrilamide) in water solution as we cross the collapse transition. Experimental data are interpreted by the Gaussian self-consistent Zimm model that takes into account two- and three-body excluded-volume effects, and Oseen hydrodynamic interactions, as well as by the standard cumulant and Contin analyses. By fitting the dynamic structure factor we extract the temperature dependence of the diffusion coefficientD and the first relaxation time τ₁ across the collapse transition for a range of scattering angles. The relaxation time τ₁ possesses a characteristic peak at about 32.4 °C due to slowing of the internal motions of the polymer at the theta-point, and a minimum at 33.4 °C. We interpret this as a combination of collapse closely followed by the growth of critical correlations. At large scattering angles we reach the universalk ³ regime, and observe that this behaviour vanishes at the onset of collapse transition. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

The role of phase-matching and nanocrystal-size effects in three-wave mixing and CARS processes in porous gallium phosphide

Nonlinear-optical interactions, such as second-harmonic and sum-frequency generation and coherent anti-Stokes Raman spectroscopy (CARS), are investigated in porous GaP for the first time by means of a novel laser source based on mode-locked picosecond Nd³⁺:YVO₄ laser and subsequent continuum generation in an optical fiber. The efficiency of the former two nonlinear optical processes is shown to be strongly dependent on the wavelengths of the interacting waves and tends to increase with the decrease of the excitation wavelength. The power of the generated second-harmonic and sum-frequency increases by a factor of 2 and 30, respectively, compared to the crystalline GaP. In contrast, the CARS signal in porous GaP is found to be less efficient than one in crystalline GaP. The observed results are explained in terms of competition of the phase-matching effects in GaP nanocrystals and the enhanced photon lifetime in scattering porous GaP layers. PACS 42.25.Dd; 42.65.Ky; 42.70.Nq; 81.07.Bc     

Adsorption activity and selectivity of the surface of InP-CdS system semiconductors with respect to toxic microimpurities

The mechanism of and rules governing the interaction of carbon oxides (CO and CO₂) and ammonia with the surface of binary (InP and CdS) and quaternary ((InP) _x (CdS)_{1 ? x} ) InP-CdS system components were studied by piezoelectric quartz microweighing and IR and Raman spectroscopy. The results obtained, acid-base and other physicochemical adsorbent properties, and the electronic structure of adsorbate molecules led us to conclude that the adsorption of the gases at temperatures above 297 K was predominantly chemical in nature and largely occurred according to the donor-acceptor mechanism with the participation of coordination unsaturated “metallic” atoms. The adsorbabilities of the gases on the same adsorbent under equal conditions were different and increased in the series a(CO₂) → a(NH₃) → a(CO). The acid-base, adsorption, and optical properties of adsorbents changed in parallel, which was indicative of the same origin of active adsorption centers and surface biographical states. This also allowed the adsorption activity of a surface to be predicted from its acid-base properties and the volumetric properties of the initial binary compounds and elementary components. The behaviors of (InP) _x (CdS)_{1 ? x} solid solutions and binary system components (InP and CdS) had common and special features. Surface characteristic-composition diagrams were used to find the system component most active with respect to CO, which was the (InP)_0.95(CdS)_0.05 solid solution. It was used to create a high-sensitivity and selectivity sensor.     

Enhanced Raman scattering in multilayer structures of porous silicon

Multiple enhancement of the Raman scattering efficiency is observed in porous‐silicon‐based one‐dimensional photonic bandgap (PBG) structures with tunable reflection and dispersion under excitation at 1.06 µm. The experimental results are explained as being due to the resonant increase in the effective Raman susceptibility at light wavelengths close to the PBG edges. This effect is discussed in view of possible applications in the Raman spectroscopy of molecules embedded in porous media as well as in the Raman laser based on silicon. Copyright © 2011 John Wiley & Sons, Ltd.