X-ray Diffraction Study of the Structure and Defect System of Yb-Activated (Na0.5Gd0.5)WO4 Crystals

Nominally pure and Yb-activated Czochralski-grown (Na_0.5Gd_0.5)WO₄ crystals (scheelite type) were studied by x-ray diffraction (phase analysis, unit-cell parameters, and structure refinement by the Rietveld method and single-crystal techniques). A pseudotetragonal (orthorhombic) superstructure was revealed, which was shown to have doubled unit-cell parameters compared to the scheelite cell. Some of the crystals were found to contain tungsten and oxygen vacancies, which give rise to additional optical absorption. The effects of the growth and heat-treatment conditions and Yb activation on the structure, defect system, and additional absorption spectrum of (Na_0.5Gd_0.5)WO₄ are analyzed.     

A Comparison of Extinction Efficiencies in the Eikonal and the Anomalous Diffraction Approximations for Spheres

Abstract

Extinction curves in the eikonal approximation and the anomalous diffraction approximation have been compared with exact extinction curves for homogeneous spheres of various refractive indices. Our results show the anomalous diffraction approximation to be more suitable for extinction calculations.     

Solid State Properties and Photographic Activity of Crystalline Ag2S- and (Ag,AU)2S-Specks at the Surface of Silver Halide Crystals

In the present paper a model is proposed according to which the photographic activity of silver sulphide specks is explained on behalfof the solid state properties of crystalline cubic silver sulphide, Epitaxial deposit ion of the laller is cia imed to occur at least partly at the (III) faces of AgBr during chemical sensitization, The disorder of the cation sublallice in cubic silver sulphide is con idered to be the physical base of highly efficient electron trapping, whereas the high electronic conductivity is rcsponsible for its catalytic activity towards developmcnt (fog). Excess silvcr in non stoichiometric cubic silver sulphide plays a very important role in its photographic behaviour inclUding development kinetics, The model provides a coherent explanation of a great number of experimental facts including the effects introduced by gold ions during sulphur sensitization.     

Breakdown of Zero-Energy Quantum Hall State in Graphene in the Light of Current Fluctuations and Shot Noise

We have investigated the cross-over from Zener tunneling of single charge carriers to avalanche type of bunched electron transport in a suspended graphene Corbino disk in the zeroth Landau level. At low bias, we find a tunneling current that follows the gyrotropic Zener tunneling behavior. At larger bias, we find an avalanche type of transport that sets in at a smaller current the larger the magnetic field is. The low-frequency noise indicates strong bunching of the electrons in the avalanches. On the basis of the measured low-frequency switching noise power, we deduce the characteristic switching rates of the avalanche sequence. The simultaneous microwave shot noise measurement also reveals intrinsic correlations within the avalanche pulses and indicate a decrease in correlations with increasing bias.     

Vapor Agglomerates and Dry Spots as Precursors of the Subcooled Liquid Boiling Crisis in a Channel

High Temperature - The mechanism of the formation of large vapor agglomerates was experimentally studied in a subcooled water flow boiling in a channel at a heat flux that approaches the critical...     

PAN基原丝预氧丝炭纤维微结构的几种新的X射线衍射散射定量表征技术

介绍几种新近开发的针对PAN基原丝、预氧丝和炭丝微结构和结构缺陷的X射线衍射与散射定量表征技术,它们是:纤维材料中柱状(椭球状)纳米取向微孔体积分数X射线小角散射定量表征方法;纤维材料中柱状(椭球状)纳米取向微孔取向角的小角散射定量表征方法;预氧丝中链状相与环化相体积分数的小角衍射定量表征方法以及炭纤维中乱层石墨相与单层石墨相的X射线广角衍射定量表征方法等。     

Asymmetry of the Forward and Backward Raman Gain Coefficient at 1.54 mum in Methane

The threshold pump energies at 1.06 mum and Raman gain coefficients of stimulated Raman scattering (SRS) at 1.54 mum have been measured in both single-pass and half-resonator configurations at various methane pressures. In the single-pass configuration, the Raman gain coefficients of the backward and forward SRS are 0.32 and 0.23 cm/GW at 95 atm, respectively. The backward Raman gain coefficient is higher than the forward Raman gain coefficient. This gain reduction is caused by the depletion of local pump intensity, the phase-matching conditions, and the transient effect of the high amplification rate in the forward SRS process. In the half-resonator configuration, the Raman gain coefficient was 0.46 cm/GW at 75 atm of methane.     

Tunneling in rf-SQUIDs and Tests of Quantum Mechanics at the Macroscopic Level

Superconducting devices such as rf-SQUIDs have been proposed to test the validity of quantum mechanics by means of Bell-like inequalities involving different-time correlation probabilities for measurements of magnetic flux. We calculate the quantum mechanical violations to such temporal Bell inequalities taking into account the effect of the measurement process on the probability distribution for the outcomes. We define a general criterion quantifying the observability of the violations and show that it is not fulfilled for the various experimental configurations proposed so far.     

Comparison of Electronic Speckle Laser Interferometry Hole-Drilling and X-ray Diffraction Techniques for Determination of Residual Stresses in the Heat Treated Steels

Carburizing is widely used to improve wear resistance and fatigue life of high duty machine parts. Fatigue performance of the carburized components is greatly dependent on the residual stress state in the surface layer. The aim of this paper is to measure the depth profiles of residual stresses in the carburized steels by electronic speckle laser interferometry (ESPI) assisted hole-drilling, and to compare the results with those measured by X-ray diffraction technique. To comprehend the differences in the residual stress state, the low-C steel components were carburized, and then, tempered in the range of 180–600 \(^{\circ }\)C. Microstructural investigations and hardness measurements were also conducted. The results obtained from both techniques gave identical results, and showed that the beneficial compressive residual stresses exist at the surface after carburizing, and their magnitudes decrease with increasing tempering temperature. It was concluded that ESPI assisted hole-drilling, with optimized drilling and stress calculation parameters, is suitable for determining the residual stress state of the carburized and tempered steels.     

The Effect of the Polarization Characteristics of Probe Light on the Signal of Optically Detected Magnetic Resonance in Magnetometric and Gyroscopic Quantum Sensors

Technical Physics Letters - We consider the effect of the polarization characteristics of probe light on the signal of optically detected magnetic resonance in quantum sensors, including quantum...     

On the Phase Distributions in Images and Fraunhofer Diffraction Patterns Produced by a Thin,Non-aberrant Lens Illuminated by a Monochromatic Point Source of Light

The paraxial approximation is used to show that the phase distribution in the image plane for an object which itself introduces no phase distortions is that which would be obtained from a monochromatic point source placed in the Fraunhofer diffraction plane and vice versa. The idea of image formation as amplitude modulation of a carrier is suggested.     

Thermalization of X-rays in evaporated tin and bismuth films used as the absorbing materials in X-ray calorimeters

We have investigated the use of evaporated tin and bismuth films as the absorbing materials in X-ray calorimeters. When the films were deposited directly on monolithic silicon calorimeters, the output signal from both Sn and Bi devices was strongly dependent on the location of the absorption event relative to the ion-implanted thermistors, presumably indicating thermistor sensitivity to a non-thermal spectrum of phonons. With Sn films we also observed that a component of the thermalization proceeded slowly, relative to a complete thermalization reference. The thermalization function could be modified by trapping magnetic flux within the film. In order to distinguish thermalization effects in the films from the thermistor sensitivity to energetic phonons, we deposited Sn and Bi films on thin Si substrates which we then affixed to calorimeters using epoxy. With glued Sn films, we were able to attain as good as 13.6 eV resolution of 6 keV X-rays with no excess broadening of the line beyond the width of the baseline, while similarly made Bi devices showed excess broadening.     

Near-Infrared Spectral Emissivity of Cu, Ag, and Au in the Liquid and Solid States at Their Melting Points

Normal spectral emissivities of liquid and solid Cu, Ag, and Au have been determined at their melting points over a wavelength range 1000 to 2500 nm using an apparatus that consists of a cold crucible and a diffraction grating spectrometer. For the noble metals, the emissivities of liquid phases are systematically larger than those of solid phases over the measured wavelength range, and the wavelength dependence of the liquid is similar to that of the solid. The measured emissivities for the liquid metals are compared with those deduced from the optical constants measured by Miller and Krishnan et al. The present results for liquid Cu and Au are in good agreement with the data of Krishnan et al., but not with those of Miller for Cu, which suggests that the optical constants measured by Krishnan et al. for liquid Cu are more accurate than those of Miller. The present data for liquid Ag and Au are in excellent agreement with all previously reported data. For the solid metals at their melting point, a semi-empirical estimation of the emissivity was carried out based upon the Drude model incorporating the effects of interband absorption and a frequency-dependent scattering rate, ^–1()= ^–1 ₀+b ². The values of ^–1 ₀ and b at the melting point are obtained by fitting the modified Drude model to the measurement results for the solid noble metals.     

The nature of the lamellar overgrowth in polyethylene shish-kebab fibres as revealed by small-angle X-ray scattering and electron microscopy

The melting, dissolution and crystallization behaviour of the lamellar overgrowth in polyethylene shish-kebab fibres have been studied by small-angle X-ray scattering and electron microscopy. SAXS experiments in which fibres were heatedin situ demonstrated the irreversible reorganization of the lamellar overgrowth already at 60° C. Reorganization continued in an inhomogeneous manner until the fibre lost its porosity around 140° C. The morphology that developed upon cooling depended on the crystallization temperature as well as on the maximum temperature attained before crystallization. Transmission electron microscopy observations showed the influence of molecular weight on aggregation. The SAXS patterns obtained duringin situ dissolution experiments disclosed that the lamellae dissolved in dodecane above 115° C. Recrystallization of the lamellae on to the backbones was almost complete within 10 min at 110° C. These high temperatures, as compared with single-crystal behaviour, indicate that the cilia nucleated with exceptionally high stem lengths on to the backbones. SAXS of fibres elongated to a maximum ratio of 1.5 at 90° C demonstrated the role of the lamellar overgrowth as a matrix between the elementary fibrils. The shish-kebab morphology could be restored after elongation by selective dissolution of the lamellae and recrystallization.     

Effects of the Non Equilibrium Phonons and the Band Non Parabolicity on the Small Signal High Frequency ac Mobility in Narrow Gap Semiconductors in the Extreme Quantum Limit at Low Temperatures

The small signal high frequency ac mobility of the hot electrons in n-HgCdTe and n-InSb has been calculated in the extreme quantum limit at low temperatures considering the non equilibrium phonon distribution as well as the thermal phonon distribution. The energy and the momentum losses of the carriers have been considered due to acoustic phonon scattering via deformation potential and piezoelectric coupling. The ac mobility is found to remain constant up to about 110 GHz for n-HgCdTe and up to about 100 GHz for n-InSb and then it decreases at higher frequencies. The ac mobility for the non equilibrium phonon distribution at lower frequency is found to be higher compared to the thermal phonon distribution and the variation at higher frequency is faster for the thermal phonons. The inclusion of the non equilibrium phonons increases the cut off frequency i.e. the cut off frequency is higher for the non equilibrium phonon distribution. The phase lag of drift velocity is found to increase with the frequency both for the nonequilibrium and the thermal phonon distribution respectively. The influence of the band non parabolicity based on simplified Kane’s model for the extreme quantum limit has also been investigated on the ac mobility of hot electrons and the phase angle. It is observed that at lower frequencies the normalized ac mobility is same both for the parabolic and the non parabolic band but at higher frequencies it is higher for the non parabolic than that of the parabolic band structure. The phase angle increases with frequency and is found to be higher for the parabolic band compared to the non parabolic band. These results can be explained by the Drude theory for ac conductivity.      

X-ray Diffraction Study of the Structure and Defect System of Nominally Pure and Er<Superscript>3+</Superscript>- and Ce<Superscript>3+</Superscript>-Activated (Na<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>)MoO<Subscript>4</Subscript> Crystals

Nominally pure and activated (Er³⁺ and Er³⁺ + Ce³⁺) sodium lanthanum molybdate single crystals grown by the Czochralski technique from the (Na_0.5La_0.5)MoO₄ melt in different environments and then oxidized in air at 100°C are characterized in detail by x-ray diffraction (lattice parameters and structure refinement by the Rietveld method and single-crystal techniques). The results confirm that (Na_0.5La_0.5)MoO₄ crystallizes in a tetragonal scheelite structure (sp. gr. I₁/a). The crystals (especially unannealed crystals grown in neutral atmosphere) are shown to contain oxygen vacancies. In addition, some of the samples contain Mo vacancies. The Er³⁺ distribution over some of the activated crystals is highly inhomogeneous. As a result, the crystals contain Er-enriched zones with a distorted scheelite structure.     

Quantum dots confined in an organic drop as luminescent probes for detection of selenium by microfluorospectrometry after hydridation: study of the quenching mechanism and analytical performance

Following a preliminary work (Costas-Mora, I.; Romero, V.; Pena-Pereira, F.; Lavilla, I.; Bendicho, C. Anal. Chem.2011, 83, 2388-2393), a quenching mechanism has been established for the selective detection of Se (as selenium hydride) by microfluorospectrometry using CdSe quantum dots (QDs) as luminescent probes stabilized with hexadecylamine and confined in an organic droplet. For this purpose, luminescence, luminescence lifetime, UV-vis absorption, total reflection X-ray fluorescence, transmission electron microscopy, and atomic force microscopy measurements were performed. The presence of stabilizing agents of QDs in the droplet was found to cause a critical effect on both extraction efficiency of selenium hydride in the drop and luminescence quenching. A self-quenching mechanism due to the aggregation of QDs is suggested. Aggregation is thought to occur as a result of the binding between selenide trapped into the organic drop as selenium hydride and Cd(2+) present in the surface of QDs, which in turn, may cause the loss of stabilizing hexadecylamine groups. After full optimization of main variables influencing the luminescent response, the analytical performance was established. A detection limit as low as 0.08 μg L(-1) Se(IV) and a repeatability expressed as relative standard deviation of 4.6% were obtained. The method was validated against CRM NWTM-27.2 lake water, and a recovery study was performed with synthetic seawater. The use of CdSe as luminescent probes in an organic drop may constitute an extremely selective, sensitive, and miniaturized assay for in situ detection of Se(IV) in water.     

Preparation of an Alkali-Element or Alkali-Earth-Element-Doped CeO2–Sm2O3 System and Its Operation Properties as the Electrolyte in Planar Solid Oxide Fuel Cells

Ceria–samaria (CeO₂–Sm₂O₃) is one of the most interesting fluorite oxides since its ionic conductivity is higher than that of yttria-stabilized zirconia in air. However, these CeO₂ -based oxides are partially reduced and develop electronic conductivity under fuel cell operating conditions. In their application to the SOFC system, their current densities and power densities are not at a satisfactory level. For the development of high-performance CeO₂ electrolytes, it is important that the fluorite lattice of CeO₂-based oxide be improved from the viewpoint of crystallography. In this study, it is assumed that the reduction of Ce⁴⁺ in the fluorite lattice was inhibited by expansion of the CeO₂ lattice. In order to investigate the contribution of the expanded CeO₂ lattice to reduction resistance, CeO₂–Sm₂O₃ solid solution, calcia-doped CeO₂–Sm₂O₃ solid solution, and a small amount of alkali element-doped CeO₂–Sm₂O₃ -based oxide were prepared for comparison. It was found that the calcia or a small amount of alkali element-doped CeO₂ solid solution enhanced the oxide ionic conductivity. The power density of the latter showed a high value at 800°C. It is concluded that the improved fuel cell performance can be attributed to the good reduction resistance in the fuel cell atmosphere.     

Charge transfer and partial pinning at the contacts as the origin of a double dip in the transfer characteristics of graphene-based field-effect transistors

We discuss the origin of an additional dip other than the charge neutrality point observed in the transfer characteristics of graphene-based field-effect transistors with a Si/SiO2 substrate used as the back-gate. The double dip is proved to arise from charge transfer between the graphene and the metal electrodes, while charge storage at the graphene/SiO2 interface can make it more evident. Considering a different Fermi energy from the neutrality point along the channel and partial charge pinning at the contacts, we propose a model which explains all the features observed in the gate voltage loops. We finally show that the double dip enhanced hysteresis in the transfer characteristics can be exploited to realize graphene-based memory devices.