Combination transitions due to stretching and librations of OH⁻ ions in LiNbO₃

A new absorption band has been detected at 4009 cm?1 in stoichiometric LiNbO? single crystals. Based on present and earlier experiments and recent calculations, this band has been assigned as a combination transition involving the OH stretching and an OH librational mode. The librational mode participating in this combination is of lower fundamental frequency than that contributing to the combination band at 4415 cm1 observed by Gr?ne and Kapphan [J. Phys. Condens. Matter 7, 3051 (1995)].     

Persistence of zero velocity fronts in reaction diffusion systems

Steady, nonpropagating, fronts in reaction diffusion systems usually exist only for special sets of control parameters. When varying one control parameter, the front velocity may become zero only at isolated values (where the Maxwell condition is satisfied, for potential systems). The experimental observation of fronts with a zero velocity over a finite interval of parameters, e.g., in catalytic experiments [Barelko et al., Chem. Eng. Sci., 33, 805 (1978)], therefore, seems paradoxical. We show that the velocity dependence on the control parameter may be such that velocity is very small over a finite interval, and much larger outside. This happens in a class of reaction diffusion systems with two components, with the extra assumptions that (i) the two diffusion coefficients are very different, and that (ii) the slowly diffusing variables has two stable states over a control parameter range. The ratio of the two velocity scales vanishes when the smallest diffusion coefficient goes to zero. A complete study of the effect is carried out in a model of catalytic reaction. (c) 2000 American Institute of Physics.     

Optoelectronic,Aggregation, and Redox Properties of Double-Rotor Boron Difluoride Hydrazone Dyes

We develop the chemistry of boron difluoride hydrazone dyes (BODIHYs) bearing two aryl substituents and explore their properties. The low-energy absorption bands (λ_max=427–464 nm) of these dyes depend on the nature of the N-aryl groups appended to the BODIHY framework. Electron-donating and extended π-conjugated groups cause a redshift, whereas electron-withdrawing groups result in a blueshift. The title compounds were weakly photoluminescent in solution and strongly photoluminescent as thin films (λ_PL=525–578 nm) with quantum yields of up to 18 % and lifetimes of 1.1–1.7 ns, consistent with the dominant radiative decay through fluorescence. Addition of water to THF solutions of the BODIHYs studied causes molecular aggregation which restricts intramolecular motion and thereby enhances photoluminescence. The observed photoluminescence of BODIHY thin films is likely facilitated by a similar molecular packing effect. Finally, cyclic voltammetry studies confirmed that BODIHY derivatives bearing para-substituted N-aryl groups could be reversibly oxidized (E_ox1=0.62–1.02 V vs. Fc/Fc⁺) to their radical cation forms. Chemical oxidation studies confirmed that para-substituents at the N-aryl groups are required to circumvent radical decomposition pathways. Our findings provide new opportunities and guiding principles for the design of sought-after multifunctional boron difluoride complexes that are photoluminescent in the solid state.     

Möbius characterization of hemispheres

In this paper we generalize the Möbius characterization of metric spheres as obtained in Foertsch and Schroeder [4] to a corresponding Möbius characterization of metric hemispheres.     

New Raman Spectroscopic Data of the Almahata Sitta Meteorite

The archaeological site of Pintia (Padilla de Duero/Peñafiel, Valladolid), considered one of the first cities on the Iberian Peninsula, has yielded very interesting archaeological records regarding the different functional areas that involve the oppidum. Pottery, metal, and bone/skeletal remains are very well preserved, being optimal for analysis and study of the ancient food contained, such as milk-based products, animal fats, beer, and wine. The combination of Raman Microscopy (RM) with other analytical techniques such as Light Microscopy (LM) and Environmental Scanning Electron Microscopy equipped with Energy Dispersive X-ray Spectroscopy (ESEM-EDX) becomes a powerful tool to obtain information on the possible content in the ceramic receptacles from historical-archaeological heritage. It can lead to an interpretation of their specific use or functionality. In this work, a unique sample preparation was used for all techniques, which lead to a very simple procedure and documentation. The degradation and aging of natural fibers and the presence of organic matter with low crystallinity and calcium oxalate–related residuals can be studied by these combined techniques. The archaeological residuals can be separated from modern contamination coming from agriculture processes, surrounding land minerals, or fresh vegetal matter.     

Influence of Ga/(Ga + In) grading on deep‐defect states of Cu(In,Ga)Se2 solar cells

The benefits of gallium (Ga) grading on Cu(In,Ga)Se₂ (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive‐level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free‐carrier densities in the CIGS absorber, and their impact on the cell's open‐circuit voltage V_oc. The parameter most constraining the cell's V_oc is found to be the deep‐defect density close to the space charge region (SCR). In ungraded devices, high deep‐defect concentrations (4.2 × 10¹⁶cm^–3) were observed near the SCR, offering a source for Shockley–Read–Hall recombination, reducing the cell's V_oc. In graded devices, the deep‐defect densities near the SCR decreased by one order of magnitude (2.5 × 10¹⁵ cm^–3) for back surface graded devices, and almost two orders of magnitude (8.6 × 10¹⁴ cm^–3) for double surface graded devices, enhancing the cell's V_oc. In compositionally graded devices, the free‐carrier density in the absorber's bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Fast Laplace Transforms for the Exponential Radon Transform

The Fourier slice theorem for the standard Radon transform generalizes to a Laplace counterpart when considering the exponential Radon transform. We show how to use this fact in combination with algorithms for the unequally spaced fast Laplace transform to construct fast and accurate methods for computing both the forward exponential Radon transform and the corresponding back-projection operator.     

Feature of the magnetotransport of a quasi-one-dimensional electrons on the superfluid helium

The magnetotransport in a nondegenerate quasi-one-dimensional (Q1D) electron system over superfluid helium has been investigated experimentally. The measurements are performed in the presence of a perpendicular magnetic field B up to 2.6 T in the temperature range T=0.48–2.05 K in the system of conducting channels of 100–400 nm width. It is shown that the value of longitudinal magnetoresistance ρ_xx increases with B. In the electron-gas scattering region (T>0.9 ), the behaviour of ρ_xx agrees with classical Drude law. In the quantum transport regime, the self-consistent Born approximation (SCBA) theory for a 2D electron system over liquid helium describes the experimental data qualitatively. The deviation due to the difference of the experimentally studied Q1D system of the electrons in a parabolic potential well differs from theoretically analysed one. The experimental data agree with the theoretical calculation for the Q1D electron system at the weak magnetic field and the low temperature.

The negative magnetoresistance of the conducting channels has been observed in both the gas- and the ripplon-scattering region. These effects have been explained by weak carrier localization on the gas atoms at high temperature and by display of the quantum magnetotransport features in a mesoscopic system at low temperature.     

Globally optimal superconducting magnets part II: symmetric MSE coil arrangement

A globally optimal superconducting magnet coil design procedure based on the Minimum Stored Energy (MSE) current density map is outlined. The method has the ability to arrange coils in a manner that generates a strong and homogeneous axial magnetic field over a predefined region, and ensures the stray field external to the assembly and peak magnetic field at the wires are in acceptable ranges. The outlined strategy of allocating coils within a given domain suggests that coils should be placed around the perimeter of the domain with adjacent coils possessing alternating winding directions for optimum performance. The underlying current density maps from which the coils themselves are derived are unique, and optimized to possess minimal stored energy. Therefore, the method produces magnet designs with the lowest possible overall stored energy. Optimal coil layouts are provided for unshielded and shielded short bore symmetric superconducting magnets.     

Operation of pulsed dye laser with an intracavity phase step

Effects of an intracavity phase step on the spectral and spatial characteristics of pulsed dye laser radiation are studied. In the broadband operation regime of the dye laser, the lasing spectrum was strongly, periodically modulated. In the narrowband regime, periodic alteration between TEM₀₀ and TEM₀₁-like beam profiles is observed on tuning the laser wavelength. Two different spectral periods of these effects are recognized. The shorter period corresponds to double passage and the longer one to apparent single passage of intracavity light through the phase step on a cavity round trip. Depending on the position of the phase step along the laser cavity, single-passage period, double-passage period, or a superposition of both periods appeared in the broadband emission spectrum and in the beam profile alteration sequence. These observations demonstrate several possibilities for the control of spectral, spatial, and phase characteristics of laser beams by simple intracavity phase elements.