Impurities in Magnetic Insulators Studied by Low-Temperature Nuclear Orientation

Although the effects of impurities in metallic magnets have been extensively studied there have been far fewer experiments on impurities in insulating magnets, and, in an effort to understand this problem, we have investigated the effect of impurities in various systems. In MnCl₂⋅4H₂O samples doped with 1.6, 4.0 and 6.2% Co, the ⁵⁴Mn NMRON line is significantly broadened compared to the line in the pure crystal and shows structure on the low frequency side. The nuclear spin-lattice relaxation time is also lower by an order of magnitude. ⁵⁴Mn has also been doped into FeCl₂⋅4H₂O for which a 4-sublattice structure has been proposed and some details of this structure have been obtained. This revised version was published online in September 2006 with corrections to the Cover Date.     

Coherent transmission and angular structure of light scattering by monolayer films of polymer dispersed liquid crystals with inhomogeneous boundary conditions

We consider monolayer polymer films with oriented droplets of a nematic liquid crystal (LC). Relations for the coherent transmission coefficients of a layer of oriented ellipsoidal droplets and for the intensity of light scattered by monolayers of spherical and spheroidal droplets have been obtained. The amplitude-phase screen model and the interference approximation of the theory of multiple wave scattering have been used. To describe light scattering by an individual ellipsoidal droplet with inhomogeneous surface binding, we have developed an anomalous diffraction approximation. For monolayers of spherical LC droplets, the coherent scattering coefficients and the angular scattering structure have been analyzed. The internal structure of nematic droplets have been calculated by the relaxation method based on the solution of the minimization problem of the free energy volume density. We have studied basic regular features of light scattering by a monolayer with homogeneous and inhomogeneous boundary conditions at the LC-polymer interface. We show that, for films that contain droplets with inhomogeneous boundary conditions of the tangentially normal type, the angular structure of the scattered light is asymmetric with respect to the polar scattering angle.     

A LEED study of the Si{100}(1 × 1)H surface structure

Extensive LEED intensity-energy data have been collected for a Si{100}(1 × 1)H surface and dynamical theory LEED calculations have been performed using an ideal unreconstructed Si(100) surface to model this structure. Agreement between experiment and theory is good indicating that the probable structure for this surface does involve (weakly scattering) H atoms on the “dangling bonds” of an unreconstructed Si(100) surface, and that difficulties in achieving good agreement between experiment and theory for the clean Si{100}(2 × 1) surface is more probably due to deficiencies in the model structure than to deficiencies in the non-structural aspects of the LEED theory.     

Micromagnetic structure of the domain wall with Bloch lines in an electric field

The micromagnetic structure of the domain wall (DW) with periodically distributed horizontal Bloch lines in a ferromagnetic film in an external electric field has been studied. The effect of the electric field on the internal DW micromagnetic structure is caused by inhomogeneous magnetoelectric coupling. Possible scenarios of the DW internal structure transformations implemented with varying the electric fields strength have been analyzed in detail. For each scenario, static characteristics of the system, such as the energy, DW profile, DW effective thickness, and electric polarization have been calculated.     

Simulation design for the structures of Bragg fibers

The propagation characteristics which are influenced by the structures of Bragg fibers have been researched in this paper. Simulation software has been set up to analyze periodic structure, pseudo-periodic structure and non-axial symmetric structure for Bragg fibers. Three dimensional field distributions and the axial propagation flux characteristics which are influenced by the number of radial cladding for high and low refraction index media have been researched by computer simulation. Optimum structure simulation design has an important meaning for the manufacture of Bragg fibers.     

Structural transformation of vapor grown carbon nanofibers studied by HRTEM

Vapor grown carbon nanofibers have been extensively manufactured and investigated in recent years. In this study commercially available vapor grown carbon nanofibers subjected to different processing and post processing conditions were studied employing high resolution TEM images. The analysis showed that the fibers consist primarily of conical nanofibers, but can contain a significant amount of bamboo nanofibers. Most conical nanofibers were found to consist of an ordered inner layer and a disordered outer layer, with the cone angle distribution of the inner layers indicating that these cannot have a stacked cone structure but are compatible with a cone-helix structure. Fibers that have been heat treated to temperatures above 1,500 °C undergo a structural transformation with the ordered inner layers changing from a cone-helix structure to a highly ordered multiwall stacked cone structure. The bamboo nanofibers were found to have a tapered multiwall nanotube structure for the wall and a multishell fullerene structure for the cap of each segment, surrounded by a disordered outer layer. When these fibers are heat treated the disordered outer layers transform to an ordered multiwall nanotube structure and merge with the wall of each segment. The end caps of each segment transform from a smooth multiwall fullerene structure to one consisting of disjointed graphene planes. A reaction-diffusion mechanism is proposed to explain the growth and structure of the bamboo nanofibers.     

Optical properties of aperiodic thin-layer structures: Effective refractive index

Simple analytical expressions for the effective refractive index of an aperiodic thin-layer structure have been obtained and analyzed. The expressions correctly describe the optical properties of the structure in a wide range of wavelengths. The optical properties that are general for the structures of this class have been established. The validity of the results is shown by a numerical experiment.     

The input impedance of the dielectric resonator antenna

Dielectric cylinders of very high permittivity have been used in the past as resonant cavities, but since the structure is not enclosed by metallic walls, electromagnetic fields do exist beyond the geometrical boundaries of the structure and part of the power is radiated. Through the proper choice of geometry and permittivity this radiation can become the dominant feature of the structure and become an efficient antenna for use at millimeter wave frequencies. Both experimental and theoretical investigations of a variety of these dielectric resonator antennas have been undertaken. In particular, the input impedance of a probe-fed cylindrical structure was examined in detail and a comparison of theoretical and experimental results was made.     

Phase transformations studies on a disordered structure of lead iodide

Phase transformation studies on disordered crystals of lead iodide grown by the “gel” technique have been performed. It has been shown that the degree of disorder is reduced as a result of annealing, and that an ordered structure may appear. In some cases the disorder is completely eliminated and only the ordered structure is the product of transformation. The results have been explained in terms of the annihilation of randomly distributed stacking faults and the nucleation of new ordered stacking faults, or the migration of extended defects to a more stable configuration. Electron microscope evidence is presented in support of the above explanation.     

Degradation of GaAsP photocathodes under bombardment by ions of residual gases

Processes occurring in the near-surface region of semiconductor photocathodes based on an epitaxial multilayer GaAsP structure activated by a CsO monolayer have been investigated via secondary ion mass spectroscopy, X-ray luminescence spectroscopy, and electron microscopy. The main mechanisms of photoemitter degradation have been revealed. The main cause of degradation is low-energy heavy ion implantation into the near-surface region of the cathode, which changes the electron structure of the surface.     

Scanning tunnelling microscopy of charge-density waves in transition metal chalcogenides

We have used scanning tunnelling microscopes (STMs) operating at liquid helium and liquid nitrogen temperatures to image the charge-density waves (CDWs) in transition metal chalcogenides. The layer structure dichalcogenides TaSe₂, TaS₂, NbSe₂, VSe₂, TiSe₂ and TiS₂ have been studied including representative polytype phases such as 1T, 2H and 4Hb. Experimental results are presented for the complete range of CDW amplitudes and structures observed in these materials. In most cases both the CDW and the surface atomic structure have been simultaneously imaged. Results on the trichalcogenide NbSe₃ are also included.

The formation of the CDW along with the associated periodic lattice distortion gaps the Fermi surface (FS) and modifies the local density-of-states (LDOS) detected by the tunnelling process. The tunnelling microscopes have been operated mostly in the constant current mode which maps the LDOS at the position of the tunnelling tip. The relative amplitudes and profiles of the CDW superlattice and the atomic lattice have been measured and confirm on an atomic scale the CDW structures predicted by X-ray, electron and neutron diffraction. The absolute STM deflections are larger than expected for the CDW induced modifications of the LDOS above the surface and possible enhancement mechanisms are reviewed.

In the 2H trigonal prismatic coordination phases the CDWs involve a relatively small charge transfer and the atomic structure dominates the STM images. In the 1T octahedral coordination phases the charge transfer is large and the CDW structure dominates the STM image with an anomalously large enhancement of the STM profile. Systematic comparison of the STM profiles with band structure and FS information is included.

In the case of the 4Hb mixed coordination phases at the lowest temperatures two nearly independent CDWs form in alternate sandwiches. STM studies on 4Hb crystals with both octahedral and trigonal prismatic surface sandwiches have been carried out. The STM scans detect the relative strengths of the two CDWs as well as the interactions between the two types of CDW structure.

The STM scans are also able to detect defects and domain structure in the CDW image. Several examples will be given demonstrating the potential of the STM to detect these local variations in LDOS on an atomic scale. In contrast to the layer structure crystals the linear chain compound NbSe₃ shows a complex surface atomic structure as well as the formation of two CDWs. The surface atomic structure is resolved in the STM scans and profiles have detected the presence of the CDW modulation at 77K and 4.2K. These results demonstrate the feasibility of detecting CDW structure in the presence of complex atomic structure and using materials where dynamical CDW effects can also be studied by STM.

The range of STM results presented here show that the STM scans are extremely sensitive to the detail of the CDW structure and its effect on the LDOS. Although much of this structure has been deduced from diffraction studies, the ability to examine the CDW structure on an atomic scale with the STM is new. The sensitivity of the STM method suggests potential applications to a wide range of electronic structures in materials.     

Magnetic properties of the CuCoAlBO<Subscript>5</Subscript> single crystal

High-quality CuCoAlBO₅ single crystals have been grown, and their crystal structure, magnetic susceptibility, and magnetization have been studied. It has been established that the CuCoAlBO₅ compound is an uncompensated antiferromagnet or ferrimagnet with a small magnetic moment and the magnetic ordering temperature T _N = 28 K. A model has been proposed for the magnetic structure. A strong anisotropy of the magnetic properties has been revealed.     

Feature Analysis and Modeling of the Network Community Structure

Community structure has an important influence on the structural and dynamic characteristics of the complex systems.So it has attracted a large number of researchers.However,due to its complexity,the mechanism of action of the community structure is still not clear to this day.In this paper,some features of the community structure have been discussed.And a constraint model of the community has been deduced.This model is effective to identify the communities.And especially,it is effective to identify the overlapping nodes between the communities.Then a community detection algorithm,which has linear time complexity,is proposed based on this constraint model,a proposed node similarity model and the Modularity Q.Through some experiments on a series of real-world and synthetic networks,the high performances of the algorithm and the constraint model have been illustrated.     

A study of mechanical twinning inα-solid solutions of the system Cu-Ge

Electron and optical microscopy have been used to study the structure and distribution of mechanical twins in polycrystalline Cu-Ge alloys containing up to 9.5 at.% Ge which had been deformed at room temperature. The probability of twinning occurring increases continuously with rise in concentration of the alloy up to the solubility limit. A reduction in the grain size of a polycrystalline aggregate impedes the development of twinning. It is shown that the twins have an imperfect structure even in the initial stage of growth.     

Microstructure and Mechanical Properties of Multilayer α-AlN/α-BCN Coating as Functions of the Current Density during Sputtering of a B<Subscript>4</Subscript>C Target

Multilayer AlN/BCN coating of nanometer scale have been prepared by magnetron sputtering of Al and B₄C targets in an argon–nitrogen atmosphere during deposition on a Si substrate. These coating have an X-ray amorphous structure and the maximum Knoop hardness of 27 GPa (at the current density 100 mA). The first-principle molecular dynamics calculations show that the B4–BN layer is dynamically unstable; thus, it will not be epitaxial and will be amorphous or have a structure different from the B4–BN structure. The thermal vacuum annealing from 600 to 800°C of samples with multilayer nanosized coating leads to the decrease in the Knoop hardness to 18 GPa; however, the coating structure is retained X-ray amorphous.     

Paracrystallinity-modulus relationships in kevlar aramid fibers

Meridional x-ray reflections of Kevlar (TM) aramid fibers have been analyzed to provide direct information about fine structure in the direction of the fiber axis. In particular, these reflections have been studied to provide evidence for a one-dimensional paracrystalline structure in Kevlar and to seek quantitative structure-property relationships based on a paracrystalline structure model. In a series of Kevlar fibers an excellent correlation is found between paracrystalline distortion factor g_II and tensile modulus, with g_II decreasing from 2.33% to 1.69% as modulus increases from 75 GPa to 125 GPa. Surprisingly, however, there is no systematic increase in axial crystallite length within this series in spite of heat treatment.     

Inverted yablonovite fabricated by the direct laser writing method and its photonic structure

Three-dimensional photonic crystals with an inverted yablonovite structure have been fabricated by the direct laser writing method based on the two-photon polymerization of a photosensitive material. The correspondence of the structure of the samples to the inverted yablonovite lattice has been confirmed by scanning electron microscopy. The photonic band structure of inverted yablonovite, as well as a number of related photonic materials with an fcc lattice, has been calculated. It has been found that the photonic properties of opal and yablonovite are opposite: the complete photonic band gap appears in inverted opal and direct yablonovite and is absent in direct opal and inverted yablonovite. A method for the fabrication of ideal three-dimensional photonic structures having the complete photonic band gap in the infrared and visible spectral ranges has been discussed.     

Study of the electronic structure of single-walled carbon nanotubes filled with cobalt bromide

Single-walled carbon nanotubes have been filled with cobalt bromide. The microstructure, optical properties, and effect of the introduced CoBr₂ compound on the electronic structure of the nanotubes have been studied. It has been shown that the electron density in the resulting nanocomposites is transferred from the walls of the nanotubes to the nanocrystals of cobalt bromide, which is an electron acceptor.     

Electrical properties of the metal-insulator nanoparticles-metal structure

The electrical properties of the metal-insulator-metal (MIM) structure with a monolayer of Al₂O₃ or ZrO₂ spherical nanoparticles as an insulator have been investigated. It has been experimentally found that the MIM structure can be in a weakly conducting or highly conducting state, depending on the applied voltage. A phenomenon has been revealed that the differential resistance of the MIM structure is negative and approaches zero (reversibly) with an increase in the electric current. After treatment of the MIM structure in the medium of atomic oxygen or atomic hydrogen, the electrical conductivity of the structure increases and depends on the polarity of the applied voltage. It has been established that the differential resistance of the structure in the form of a monolayer of nickel and copper spherical nanoparticles located between the planar metal electrodes is positive and decreases with an increase in the applied voltage.     

Optimization of the CIGS based thin film solar cells: Numerical simulation and analysis

Chalcopyrite Cu(In,Ga)Se (CIGS) is a very promising material for thin film photovoltaics and offers a number of interesting advantages compared to the bulk silicon devices. CIGS absorbers today have a typical thickness of about 1–2 μm. However, on the way toward mass production, it will be necessary to reduce the thickness even further. This paper indicates a numerical study to optimization of CIGS based thin film solar cells. An optimum value of the thickness of this structure has been calculated and it is shown that by optimizing the thickness of the cell efficiency has been increases and cost of production can be reduces. Numerical optimizations have been done by adjusting parameters such as the combination of band gap and mismatch as well as the specific structure of the cell. It is shown that by optimization of the considered structure, open circuit voltage increases and an improvement of conversion efficiency has been observed in comparison to the conventional CIGS system. Capacitance–voltage characteristics and depletion region width versus applied voltage for optimized cell and typical cell has been calculated which simulation results predict that by reducing cell layers in the optimized cell structure, there is no drastically changes in depletion layer profile versus applied voltage. From the simulation results it was found that by optimization of the considered structure, optimized value of CIGS and transparent conductive oxide thickness are 0.3 μm and 20 nm and also an improvement of conversion efficiency has been observed in comparison to the conventional CIGS which cell efficiency increases from 17.65 % to 20.34%, respectively.