bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced flux pinning in Ag-sheathed Bi(Pb)–Sr–Ca–Cu–O superconductors tapes with addition of magnetic nanorod γ-Fe2O3
Authors: Lau K. T. , Yahya S. Y. , Abd-Shukor R.
Published: August 2006

The pinning strength of the Ag-sheathed Bi(Pb)–Sr–Ca–Cu–O tapes with the addition of magnetic nanorod γ-Fe2O3 is studied using the exponent n values obtained from power law current-electric field (I-E) characteristics. The starting composition of the γ-Fe2O3 added samples is Bi1.6Pb0.4Sr2Ca2Cu3O10-(γ-Fe2O3)0.01. The rate of decrease of n values in applied fields for the tapes with nano-γ-Fe2O3 is found lower than the tapes without nano-γ-Fe2O3. This indicates that the pinning strength for the tapes with nano γ-Fe2O3 is relatively higher than the tapes prepared without nano γ-Fe2O3. The results are supported by two conventional methods which are normally used to compare the pinning strength of superconductor tapes. Magnetic impurities generally suppress superconductivity. However, our study shows that magnetic nanorod γ-Fe2O3 enhanced the pinning strength of Ag-sheathed Bi–Sr–Ca–Cu–O superconductor tapes. This is consistent with previous calculations on frozen flux superconductor with magnetic nanorod as pinning centers.
High quality factor resonance at room temperature with nanostrings under high tensile stress
Authors: Verbridge Scott S. , Parpia Jeevak M. , Reichenbach Robert B. , Bellan Leon M. , Craighead H. G.
Published: August 2006

Quality factors as high as 207 000 are demonstrated at room temperature for radio-frequency silicon nitride string resonators with cross sectional dimensions on the scale of 100nm, made with a nonlithographic technique. A product of quality factor and surface to volume ratio greater than 6000nm−1 is presented, the highest yet reported. Doubly clamped nanostring resonators are fabricated in high tensile-stress silicon nitride using a nonlithographic electrospinning process. We fabricate devices with an electron beam process, and demonstrate frequency and quality factor results identical to those obtained with the nonlithographic technique. We also compare high tensile-stress doubly clamped beams with doubly clamped and cantilever resonators made of a lower stress material, as well as cantilever beams made of the high stress material. In all cases, the doubly clamped high stress beams have the highest quality factors. We therefore attribute the high quality factors to high tensile stress. Potential dominant loss mechanisms are discussed, including surface and clamping losses, and thermoelastic dissipation. Some practical advantages offered by these nanostrings for mass sensing are discussed.
Role of defects in transport through a quantum dot single electron transistor
Authors: Singh K. P. , Lamba Subhalakshmi , Joshi S. K. , Lamba Sushil
Published: August 2006

The effect of a single dotlike defect on the transport through a quantum dot single electron transistor weakly coupled to external leads is studied. It is found that the conductance profile is changed significantly by the quantum mechanical tunneling between the dot and the defect and the interactions between them, both of which are dependent on the distance between the dot and the defect, as also by the morphology of the defect. In particular, we find that even a very small strength of interdot interaction has a major influence on the transport and must be taken into account in device fabrication.
Photothermal excitation of microcantilevers in liquids
Authors: Ramos D. , Tamayo J. , Mertens J. , Calleja M.
Published: August 2006

We report the selective excitation of the flexural modes of microcantilevers in aqueous solutions, by applying the photothermal excitation technique. The experiments show that a particular vibration mode can be efficiently excited by focusing the intensity-modulated laser beam on regions of high curvature of the vibration shape. In addition, the resulting resonant peaks in liquid appear distorted by an amplitude component that decreases with the frequency. This distortion produces a shift of the resonance to lower frequencies. A theoretical model based on the transformation of optical energy into mechanical energy via an intermediate thermal stage is proposed to interpret the experimental results. The theory shows that the driven oscillation of the cantilever depends on the curvature of the eigenmode at the excitation position and the heating induced by the excitation laser, which decreases with the frequency. The results reported here set the basis for efficient excitation of high vibration modes in liquids and for optimized design of optically driven microresonators.
Nonlinear electric field effects in the magnetoresistance of n-type GaSb
Authors: Ghezzi C. , Magnanini R. , Parisini A. , Longo M.
Published: August 2006

Non-Ohmic effects in the magnetoresistance of n-type GaSb epitaxial layers are investigated as the magnetic field is varied through a critical Bc value (Bc=9.1T) where the electron system undergoes a metal-insulator transition. The low temperature Ohmic conductivity is mainly due to either free or localized electrons, depending on B, so that different pictures are proposed to explain the observed nonlinear behaviors when the electric field is increased. The experiment is made possible by the availability of samples with electron densities slightly lower than 1016cm−3, grown by molecular beam epitaxy, in order that the electron gas can be driven to the metal-insulator transition at an experimentally accessible value of B. Both the longitudinal (B‖I) and transverse (B⊥I) configurations are investigated.
Liquid crystal pretilt angle control using nanotextured surfaces
Authors: Yeung Fion Sze-Yan , Xie Feng-Chao , Wan Jones Tsz-Kai , Lee Fuk Kay , Tsui Ophelia K. C. , Sheng Ping , Kwok Hoi-Sing
Published: August 2006

A scalable and economical method to control the pretilt alignment of nematic liquid crystal (LC) to ∼45° has long been sought as it constitutes the foundation of some key technologies of LC displays. We demonstrate that nanotextured surfaces fabricated by mixing horizontal and vertical polyimides allow complete LC pretilt control from ∼0° to 90°. Devices made with these surfaces show response times four times superior to the state-of-the-art panel.
Fibrous nanocomposites with interface stress: Hill’s and Levin’s connections for effective moduli
Authors: Chen Tungyang , Dvorak George J.
Published: August 2006

We study the macroscopic behavior of solids containing circular cylindrical nanoinclusions of the same size with surface effects prevailing at interfaces. The overall thermomechanical properties of these solids are shown to comply with two sets of exact connections. The first set, similar to Hill’s universal connections, provides two constraints among the three axisymmetric overall elastic moduli. The second set relates the effective coefficients of thermal expansion to the effective moduli, in analogy with Levin’s formula. In contrast to the classical results, the presence of surface effects makes both sets of connections dependent on the absolute size of the nanoinclusions.
Transparent conductive Al-doped ZnO films for liquid crystal displays
Authors: Oh Byeong-Yun , Jeong Min-Chang , Moon Tae-Hyoung , Lee Woong , Myoung Jae-Min , Hwang Jeoung-Yeon , Seo Dae-Shik
Published: August 2006

Al-doped zinc oxide (ZnO:Al) films were applied to liquid crystal displays (LCDs) as transparent electrodes substituting indium tin oxide (ITO). While the ZnO:Al-based twisted nematic LCD cell showed similar operational behavior to ITO-based counterpart, its electro-optical (EO) and residual dc (r-dc) characteristics were somewhat improved. Capacitance-voltage relations suggested that these improved EO and r-dc characteristics of the ZnO:Al-based LCD cell are due to the substantially lower density of charge carrier trapping centers in the polyimide layer∕electrode interface region, demonstrating high application potential of ZnO:Al films as transparent electrodes of LCDs.
Magnetic susceptibility reduction method for magnetically labeled immunoassay
Authors: Hong Chin-Yih , Wu C. C. , Chiu Y. C. , Yang S. Y. , Horng H. E. , Yang H. C.
Published: August 2006

In addition to synthesizing biofunctionalized magnetic nanopaticles for the purpose of magnetically labeling biomolecules, a system to measure the ac magnetic susceptibility of the labeled sample was developed. When a targeted biomolecule was mixed with magnetic fluid possessing biofunctionalized magnetic nanoparticles, portions of magnetic nanoparticles agglomerated to form clusters due to the association with the targeted biomolecule. Due to the formation of magnetic clusters, the measured ac magnetic susceptibility reduced. The relationship between the mixed-frequency ac magnetic susceptibility reduction and the amount of the detected biomolecule was established.
Characterization of CaCu3Ti4O12 varistor-capacitor ceramics by impedance spectroscopy
Authors: Guo Mingsen , Wu Tianshu , Liu Tao , Wang Sheng-Xiang , Zhao Xing-Zhong
Published: August 2006

The varistor-capacitor property of CaCu3Ti4O12 ceramics has been characterized by a combination of ac impedance spectroscopy and dc measurement. The results demonstrate that the non-Ohmic property and ultrahigh dielectric constant of CaCu3Ti4O12 ceramics are associated with the grain-boundary behavior. The I-V and C-V characteristics of grain-boundary regions in CaCu3Ti4O12 ceramics are explained on the basis of a back-to-back potential barrier model. We suggest that the origin of the potential barrier is related to Cu ions segregated to grain boundaries.
Coherent diffraction and random scattering in thiol-ene–based holographic polymer-dispersed liquid crystal reflection gratings
Authors: Sutherland R. L. , Tondiglia V. P. , Natarajan L. V. , Lloyd P. F. , Bunning T. J.
Published: August 2006

Bragg diffraction and random scattering in reflective holographic polymer-dispersed liquid crystal gratings are modeled using a matrix approach for a stack of low-high index layer pairs and an effective medium theory. Scattering is due to both random roughness of layer interfaces and random index variations within the layers. These are related to random liquid crystal droplet size and location as well as random orientation of the symmetry axes of bipolar droplets. Characteristic parameters governing coherent diffraction efficiency and random scattering are obtained partly from experiments, where possible, and partly from calculations based on a model of an effective medium applied to the grating. Calculations of grating transmittance are then compared to experimental transmittance spectra. Effects of scattering, primarily a decrease in baseline transmittance with wavelength and a small reduction in diffraction efficiency at the Bragg wavelength, are found to be due primarily to index inhomogeneities within the liquid-crystal-rich layers of the grating.
Ion-induced electron emission from different crystalline phases of ZrO2
Authors: Matulevich Y. T. , Lee Min-Suk , Kim Jae-Hyuk , Choi Jong-Seo , Kim Suk-Ki , Suh Soon-Sung , Zang Dong-Sik , Aarik Jaan , Aidla Aleks , Aints Märt , Raud Jüri , Kirm Marco
Published: August 2006

A detailed study of ZrO2 as a candidate material for a protective layer of plasma display panels has been performed. The ion-induced electron emission coefficient γ and the firing voltage were measured for cubic, tetragonal, and monoclinic ZrO2 films. Different crystalline phases of ZrO2 showed a very small difference in the coefficient γ and the firing voltage. Compared to MgO, the coefficient γ was smaller and, as a result, the firing voltage was higher for ZrO2.
Near-ultraviolet optical absorption behavior of TiO2–Al2O3 multilayer films
Authors: Omari M. A. , Sorbello R. S. , Aita C. R.
Published: August 2006

The fundamental optical absorption edge of sputter-deposited titania-alumina (TiO2–Al2O3) multilayers on fused SiO2 substrates is studied by near ultraviolet-visible spectrophotometry. We examine a family of films with bilayer architecture Λ=9–72nm TiO2∕7nm Al2O3 (TiO2 volume fraction from 0.56 to 0.91). Neither the TiO2 or Al2O3 layers have long-range crystallographic order. The absorption coefficient α in the region of optical density from 2 to 7 is determined as a function of energy E. The first significant finding is that the onset of absorption for these multilayers is determined by their TiO2 component, independent of TiO2 layer thickness. The nondirect optical gap Eo is determined by extrapolating linear α1∕2 vs E curves to α=0. Eo=2.95 (±0.02)eV for all multilayers, identical to Eo for a single layer TiO2 film with rutile short-range atomic order. The second significant finding is that the strength of α in the multilayers is diminished compared to a Vegard’s rule analysis based on the weighted sum of α(TiO2) and α(Al2O3). A modified Vegard’s rule model is proposed to account for the possible electronic effect of Al2O3 on TiO2 at an interface.
Reliability of thermally oxidized SiO2∕4H-SiC by conductive atomic force microscopy
Authors: Fiorenza Patrick , Raineri Vito
Published: August 2006

The dielectric breakdown (BD) kinetics of silicon dioxide (SiO2) thin films thermally grown on 4H-SiC was determined by comparison between I-V measurements on large area (up to 1.96×10−5cm2) metal-oxide-semiconductor structures and conductive atomic force microscopy (C-AFM). C-AFM clearly images the weak breakdown single spots under constant voltage stresses. The stress time on the single C-AFM tip dot was varied from 2.5×10−3to1×10−1s. The density of BD spots, upon increasing the stress time, exhibits an exponential trend. The Weibull slope and the characteristic time of the dielectric BD events have been determined by direct measurements at nanometer scale allowing to demonstrate that the percolative model is valid for thermal oxide on 4H-SiC.
Effect of Ba doping on magnetic, ferroelectric, and magnetoelectric properties in mutiferroic BiFeO3 at room temperature
Authors: Wang D. H. , Goh W. C. , Ning M. , Ong C. K.
Published: August 2006

Ba doped BiFeO3 compounds were prepared by a solid-state reaction. X-ray diffraction showed that Bi1−xBaxFeO3 was single phase up to x=0.25. These samples exhibited magnetism and ferroelectricity simultaneously at room temperature. The magnetoelectric coupling was evidenced by the increase of the dielectric constant with the increase of the applied magnetic field. For Bi0.75Ba0.25FeO3 with ΔH=8kOe, the values of [εr(H)−εr(0)]∕εr(0) are 1.7% and 1% for 80 and 300K, respectively.
Electrical conductivity enhanced dielectric and ferroelectric properties of interface-coupled ferroelectric superlattices
Authors: Zhou Y. , Shin F. G.
Published: August 2006

We have developed a model to theoretically study the interplay of the effects of interfacial coupling and electrical conductivity on the ferroelectric and dielectric properties of superlattices consisting of alternating ferroelectric and paraelectric layers based on the Landau-Ginzburg theory. The qualitative predictions of the model are compared with recent experimental results for “symmetric” and “asymmetric” superlattices. It is shown that the consideration of time-dependent space-charge-limited conductivity and “interface structure” can satisfactorily account for the enhancement of the overall ferroelectric and dielectric properties of ferroelectric-paraelectric superlattices.
A comparison of the predictions of a simple kinetic theory with experimental and numerical results for a vibrated granular bed consisting of nearly elastic particles of two sizes
Authors: Wildman R. D. , Jenkins J. T. , Krouskop P. E. , Talbot J.
Published: August 2006

A comparison of the predictions of a simple kinetic theory with experimental and numerical results for a vibrated granular bed consisting of nearly elastic particles of two sizes has been performed. The results show good agreement between the data sets for a range of numbers of each size of particle, and are particularly good for particle beds containing similar proportions of each species. The agreement suggests that such a model may be a good starting point for describing polydisperse systems of granular flows.
Thermal influence on passing of polarized light through the SnO2:In2O3 layers
Authors: Davidenko N. A. , Pavlov V. A. , Chuprina N. G. , Davidenko I. I. , Baath L.
Published: August 2006

Thermal dependent changes of light depolarization degree were observed in the SnO2:In2O3 (ITO) layers deposited onto the flat glass substrates which are used usually as electrodes for optoelectronic devices. The observed effect is reversible. It can be attributed to the changes of nanostructure geometry in the bulk of the ITO layer as well as on its surface. Such geometric changes involve dispersion of polarized light. The investigated effect should be taken into consideration when developing optoelectronic devices because it can provoke distortion of the optical information field.
Capacitance of spherical dielectric layers
Authors: Topasna G. A.
Published: August 2006

A capacitor device is modeled consisting of a close-packed arrangement of spheres as the dielectric layer. The top electrode is deposited directly on the upper half of the top layer of spheres and bottom electrode is a flat conductor. Expressions are derived for the capacitance for both single and multiple layers of spheres. It is found that a single layer of spheres has the largest increase in capacitance over that of a perfect parallel plate capacitor that has a dielectric thickness equal to the diameter of the spheres. The model predicts a decrease in capacitance as the number of sphere layers increases. The relative uncertainty in capacitance equals the relative uncertainty in the diameter of the spheres for both single and multiple layers.
Epitaxial growth and optical properties of semiconductor quantum wires
Authors: Wang Xue-Lun , Voliotis Valia
Published: August 2006

In this paper we present a review on major advances achieved over the past ten years in the field of fabrication of semiconductor quantum wires (QWRs) using epitaxial growth techniques and investigation of their optical properties. We begin the review with a brief summary on typical epitaxial QWRs developed so far. We next describe the state-of-the-art structural qualities of epitaxial QWRs in terms of (i) size uniformity between wires, (ii) heterointerface uniformity, (iii) crystal purity, and (iv) strength of lateral quantum confinement. Several prominent breakthroughs have been accomplished concerning the improvements of wire qualities, including (i) realization of V-shaped GaAs∕AlGaAs QWRs in the “real one-dimensional” (1D) regime in which exciton states can extend coherently over distances exceeding 1μm, (ii) reduction of residual impurity concentrations in V-shaped GaAs∕AlGaAs QWRs to a level comparable to that in an equivalent quantum well (QWL), which resulted in the semiconductor QWR with room-temperature photoluminescence efficiency exceeding that of a QWL, and (iii) reduction of the multimonolayer (ML) interface fluctuations on the second-grown arm QWL surface, in old-generation T-shaped GaAs∕AlGaAs QWRs, to the single-ML level. The second part of this article is devoted to the discussion of optical properties of epitaxial QWRs, such as exciton dynamics, fine structure of exciton levels, and nonlinear effects, studied by means of high-spatial resolution spectroscopy, i.e., microphotoluminescence experiments. We will concentrate our discussions on V-shaped GaAs∕AlGaAs QWRs and put an emphasis on demonstrating how the interface quality influences wire’s optical properties. The properties of QWRs in the “zero-dimensional quantum box regime” and QWRs in the real 1D regime will be presented in separate sections. We will show that the realization of QWRs in the real 1D regime makes possible the investigation of intrinsic 1D effects by focusing on a single perfect 1D wire region using microscopic techniques. This has led to important results, for instance, (i) the demonstration of the square-root dependence of 1D exciton radiative recombination lifetimes down to a temperature as low as 10K (limited by the experimental setup) and (ii) the clear demonstration of the existence of Mott transition in a 1D exciton system which is a fundamental problem under long debate.
Electrostatic field induced patterning: A method for controlling the anisotropy of thin films of polar organic molecules
Authors: Verpoort S. , Draude A. , Meinhardt R. , Franke H. , Lessard R. A.
Published: August 2006

The molecular orientation of anisotropic organic molecules also depends on the presence of electrostatic charges on the surface of low conductive substrates. Unknown charges may be the origin of poor reproducibility, while controlled charge patterns may be used as a tool for the preparation of partly oriented films. Different techniques for the creation of surface charge patterns have been applied: friction with a pen or brush, corona discharge, sliding electrical contact wire, exposure with ultraviolet light (UV exposure), and electron-beam (e-beam) recording. The orientational order was checked by measuring the angular dependence of the absorption constant.
Direct simulation Monte Carlo convergence behavior of the hard-sphere-gas thermal conductivity for Fourier heat flow
Authors: Rader D. J. , Gallis M. A. , Torczynski J. R. , Wagner W.
Published: August 2006

The convergence behavior of the direct simulation Monte Carlo (DSMC) method is systematically investigated for near-continuum, one-dimensional Fourier flow. An argon-like, hard-sphere gas is confined between two parallel, fully accommodating, motionless walls of unequal temperature. The simulations are performed using four variations based on Bird’s DSMC algorithm that differ in the ordering of the move, collide, and sample operations. The primary convergence metric studied is the ratio of the DSMC-calculated bulk thermal conductivity to the infinite-approximation Chapman-Enskog (CE) theoretical value, although temperature and heat flux are also considered. Ensemble, temporal, and spatial averaging are used to reduce statistical errors to levels that are small compared to the discretization errors from the time step (Δt), the cell size (Δx), and the number of computational particles per cell (Nc). The errors from these three parameters are determined using over 700 individual cases selected from the ranges 0.05<Δt∕to<1 (to is the molecular mean collision time), 0.05<Δx∕λo<1 (λo is the molecular mean free path), and 7≤Nc≤480. The infinite-particle-number (Nc→∞) convergence behavior for the thermal-conductivity ratio is found to be second-order in both time step and cell size, in good agreement with previous theoretical predictions based on Green-Kubo theory. For vanishing time step and cell size, the finite-particle-number convergence behavior is found to be O(1∕Nc) if ∼30 or more particles per cell are used. The observed convergence behavior is found to be more complicated when all three discretization parameters are finite. As discretization errors are systematically reduced, the DSMC-calculated conductivity is shown to approach the infinite-approximation CE theoretical value to within 1 part in 104.
Hydrogen absorption kinetics and optical properties of Pd-doped Mg thin films
Authors: Pasturel M. , Slaman M. , Schreuders H. , Rector J. H. , Borsa D. M. , Dam B. , Griessen R.
Published: August 2006

In order to develop optical fiber hydrogen sensors, thin film materials with a high optical contrast between the metallic and hydrided states are needed. Magnesium exhibits such a contrast but cannot be easily hydrogenated at room temperature. However, thin films of Pd-doped Mg (MgPdy with 0.023⩽y⩽0.125) prepared by magnetron dc sputtering can easily be hydrided at room temperature and 0.5bar H2 within a few minutes. The rate of first hydrogenation increases linearly with increasing Pd concentration. Hydrogenation induces high variations of transmission (ΔT up to 20%) and reflection (ΔR up to 70%) of light (0.5eV⩽ℏω⩽6.0eV corresponding to 2500nm⩾λ⩾210nm). The optical properties can be understood by considering Pd as a deep donor in semiconducting MgH2.
Cathodoluminescence microscopy and spectroscopy of GaN epilayers microstructured using surface charge lithography
Authors: Díaz-Guerra C. , Piqueras J. , Volciuc O. , Popa V. , Tiginyanu I. M.
Published: August 2006

Cathodoluminescence (CL) microscopy and spectroscopy have been used to investigate the optical properties of GaN microstructures patterned by Ar+ ion irradiation and subsequent photoelectrochemical (PEC) etching. Monochromatic CL images and CL spectra reveal an enhancement of several defect-related emission bands in a 10μm wide area around each microstructure. In addition, columnar nanostructures and nanoetch pits were found in the PEC etched areas. CL emission of the nanocolumns is dominated by free electron to acceptor transitions, while excitonic luminescence prevails in the rest of the etched GaN layers. Investigation of the sidewalls of the microstructures reveals that a CL emission band centered at about 3.41eV, attributed to excitons bound to structural defects, is effectively suppressed after PEC etching only in the observed nanocolumns.
Dynamic coupling of piezoelectric effects, spontaneous polarization, and strain in lattice-mismatched semiconductor quantum-well heterostructures
Authors: Willatzen M. , Lassen B. , Lew Yan Voon L. C. , Melnik R. V. N.
Published: August 2006

A static and dynamic analysis of the combined and self-consistent influence of spontaneous polarization, piezoelectric effects, lattice mismatch, and strain effects is presented for a three-layer one-dimensional AlN∕GaN wurtzite quantum-well structure (with GaN as the central quantum-well layer). It is shown that, contrary to the assumption of Fonoberov and Balandin [J. Appl. Phys. 94, 7178 (2003); J. Vac. Sci. Technol. B 22, 2190 (2004)], even in cases with no current transport through the structure, the strain distributions are not well captured by minimization of the strain energy only and not, as is in principle required, the total free energy including electric and piezoelectric coupling and spontaneous polarization contributions. Furthermore, we have found that, when an ac signal is imposed through the structure, resonance frequencies exist where strain distributions are even more strongly affected by piezoelectric-coupling contributions depending on the amount of mechanical and electrical losses in the full material system.
Raman characteristics and nonlinear optical properties of tellurite and phosphotellurite glasses containing heavy metal oxides with ultrabroad Raman bands
Authors: Murugan G. Senthil , Suzuki Takenobu , Ohishi Yasutake
Published: August 2006

A family of glasses suitable for ultrabroadband Raman amplifications was designed and fabricated. In addition to the bandwidth the gain characteristics of these glasses are expected to be much higher than those of the silica based materials. The thermal stabilities of these glasses in view of fiber fabrication were discussed. The various structural units which contribute to the overall Raman band were unraveled. Accordingly the various bands were tailored in such a way to get a wide continuous Raman band. The nonlinear optical properties which are vital for optical switching and other nonlinear processes were studied via third-harmonic generation measurements.
Three-dimensional morphology evolution of SiO2 patterned films under MeV ion irradiation
Authors: Otani Kan , Chen Xi , Hutchinson John W. , Chervinsky John F. , Aziz Michael J.
Published: August 2006

We have measured the evolving three-dimensional (3D) morphology of patterned SiO2 stripes on Si substrates induced by 3MeV O++ ion irradiation. We develop a 3D constitutive relation to describe anisotropic deformation, densification, and flow. We use this constitutive relation in a finite element model that simulates the experimental morphology evolution, and we find excellent agreement between simulated and measured profiles. The model should be useful in predicting morphology evolution in complex three-dimensional structures under MeV ion irradiation.
Calorimetric characterization of photoinduced relaxation in GeSe9 glass
Authors: Lucas Pierre , King Ellyn A.
Published: August 2006

Enthalpy relaxation is measured during sub-band-gap irradiation of a GeSe9 sample. The glass undergoes rapid photorelaxation and reaches a saturated state that appears to correspond to a dynamic equilibrium between photoexcitation and thermodynamically driven relaxation. The activation energy for structural relaxation is measured and found to be small enough that the glass can fully relax in a few years at room temperature. A comparison of isochronal optical and thermal treatments shows that photorelaxation is not induced by laser heating but is rather due to an optically induced softening of the glass network. The photorelaxation process is interpreted as optically lifting the kinetic impediment to relaxation at room temperature. The kinetics of optically and thermally activated relaxation both show a nonexponential response, which can be fitted with a Kohlrausch function. It is shown that the relaxation kinetics increase with higher irradiation intensity.
Stacking faults and twin boundaries in fcc crystals determined by x-ray diffraction profile analysis
Authors: Balogh Levente , Ribárik Gábor , Ungár Tamás
Published: August 2006

A systematic procedure is developed to evaluate the density of planar defects together with dislocations and crystallite or subgrain size by x-ray line profile analysis in fcc crystals. Powder diffraction patterns are numerically calculated by using the DIFFAX software for intrinsic and extrinsic stacking faults, and twin boundaries for the first 15 Bragg reflections up to 20% fault density. It is found that the Bragg reflections consist of five subreflection types categorized by specific selection rules for the hkl indices in accordance with the theory of Warren [Prog. Met. Phys. 8, 147 (1959)]. It is shown that the profiles of the subreflections are Lorentzian-type functions. About 15 000 subreflections are evaluated for their full widths of half maxima and their positions relative to the exact Bragg angle. These values are parametrized as a function of the density and type of planar faults. A whole profile fitting procedure, previously worked out for determining the dislocation structure and crystallite size distributions, is extended for planar fault by including these data into the software. The method is applied to evaluate twin densities in nanocrystalline and submicron grain-size copper specimens. It is found that twinning becomes substantial under a critical crystallite or subgrain size of about 40nm, in accordance with other observations.
Nonuniform current flow in superconductor-fullerene-superconductor junctions
Authors: Shaternik V. E. , Ivanjuta A. N. , Shaternik A. V.
Published: August 2006

Superconducting structures of the superconductor-fullerene-superconductor type are prepared using thin films of a molybdenum-rhenium alloy. The current-voltage characteristics obtained experimentally are compared with the theoretical characteristics calculated for models of multiple Andreev reflections of quasiparticles. The nonuniform current flow through the fullerene thin film is discussed.
Vibrational spectrum of disordered linear chain with light isotopic impurities. Region of the initial band
Authors: Ivanov M. A. , Molodid V. S. , Skripnik Yu. V.
Published: August 2006

The vibrational spectrum of disordered linear chains with light impurities of substitution is calculated within the region of the unperturbed band. The overall structure of the spectrum and the spectral shift function are determined. The results of a numerical calculation are compared with the known analytical expressions. The character of the statistical distribution of vibrational frequencies over different parts of the spectrum is investigated, and it is shown that at a sufficient impurity concentration in not-too-wide spectral intervals a completely random distribution of eigenfrequencies is realized. It is found that, with increasing concentration, sequences of minima and maxima arise with increasing clarity in the high-frequency half of the band, their positions being independent of the concentration and clumped towards the outer edge of the band. An expression giving the values of these special frequencies is found.
Influence of the negative oxygen ions on the structure evolution of transition metal oxide thin films
Authors: Mráz Stanislav , Schneider Jochen M.
Published: August 2006

The energy distributions of O− ions of magnetron sputtered Nb, Ta, Zr, and Hf in an Ar∕O2 atmosphere were measured as a function of the oxygen partial pressure. Three ion populations were detected in the plasma: high, medium, and low energy ions, with energies corresponding to the target potential, half of the target potential, and <150eV, respectively. The ion energy distribution functions were compared to distributions obtained based on Sigmund’s linear collision cascade sputtering theory. If the surface binding energy is assumed to be equal to the heat of formation, good agreement between the experiment and theory was achieved. From correlating the measured ion energy distributions with previously published phase stability data [Ngaruiya et al., Appl. Phys. Lett. 85, 748 (2004)], it can be deduced that large fluxes of medium and high energy O− ions comparable to the fluxes of the low energy O− ions enable formation of crystalline transition metal oxide thin films during low temperature growth. The presented data here may be of general relevance for understanding the structure evolution of thin oxide films.
Plasma modeling for ultrashort pulse laser ablation of dielectrics
Authors: Jiang L. , Tsai H. L.
Published: August 2006

In ultrashort pulse (<10ps) laser ablation of dielectrics, affected materials are first transformed into absorbing plasma with metallic properties and, then, the subsequent laser-plasma interaction causes material removals. For ultrashort-pulse laser ablation of dielectrics, this study proposes a model using the Fokker-Planck equation for electron density distribution, a plasma model for the optical properties of ionized dielectrics, and quantum treatments for electron heating and relaxation time. The free electron density distribution of the plasma within the pulse duration is then used to determine the ablation crater shape. The predicted threshold fluences and ablation depths for barium aluminum borosilicate and fused silica are in agreement with published experimental data. It is found that the significantly varying optical properties in time and space are the key factors determining the ablation crater shape. The effects of fluence and pulse duration are also studied.
Structural implications of ring shape, dimension, and metal atom insertion in nanosized cyclic oligothiophenes: Joint Raman and density functional theory study
Authors: Casado Juan , Zgierski Marek Z. , Fuhrmann Gerda , Bäuerle Peter , López Navarrete Juan T.
Published: August 2006

This paper reports on a combined spectroscopic and density functional theory study on a series of cyclo(diacetylenes-oligothiophenes) with different shapes and dimensions. The case of a macrocycle fragmented by the insertion of Pt metallic atoms is illustrated as well. The analysis covers the neutral and oxidized samples. A full assignment of the spectra has been proposed, supported by BP86/cc-pVDZ theoretical calculations. Ring shape and molecular dimension have been related with the CC∕C–C structural parameters and Raman wave numbers. Platinum atom insertion within the acetylene spacer seems to soften ring strain owing to the rupture of the strained circular shape. Oxidation causes an overall downshift of the Raman wave numbers likely due to the skeletal relaxation of CC conjugated path, while the ring shape of each macrocycle is conserved upon oxidation.
Effects of annealing on the electrical properties of TiO2 films deposited on Ge-rich SiGe substrates
Authors: Chakraborty S. , Bera M. K. , Maiti C. K. , Bose P. K.
Published: August 2006

Titanium oxide (TiO2) high-k dielectric layers were deposited on the Si0.3Ge0.7 substrates by plasma enhanced chemical vapor deposition (PECVD) at low temperature (150°C). To study the effects of annealing on the electrical properties, the TiO2 films were annealed in pure nitrogen ambient in the temperature range of 400–600°C. Good electrical performance for the gate dielectrics was observed for the dielectric films annealed up to 500°C, in terms of interface state density, leakage current, and charge trapping properties. Annealing at 600°C is found to degrade the electrical properties due to Ge segregation and subsequent diffusion into the TiO2 layer. Schottky emission was found to be the dominant leakage current conduction mechanism in PECVD TiO2 films.
Phonon confinement and self-limiting oxidation effect of silicon nanowires synthesized by laser ablation
Authors: Fukata N. , Oshima T. , Okada N. , Murakami K. , Kizuka T. , Tsurui T. , Ito S.
Published: August 2006

The phonon confinement and self-limiting oxidation effects of silicon nanowires (SiNWs) synthesized by laser ablation were investigated. The size of SiNWs was controlled by the synthesis parameters during laser ablation and the subsequent thermal oxidation. Thermal oxidation increases the thickness of the SiNWs’ surface oxide layer, resulting in a decrease in their crystalline Si core diameter. This effect causes a downshift and asymmetric broadening of the Si optical phonon peak due to phonon confinement, while excess oxidation causes an upshift due to compressive stress. The compressive stress retarded the oxidation of the SiNWs by self-limiting oxidation effect. This result shows that the Si core diameter can be controlled by compressive stress.
Room temperature ferroelectric properties of Mn-substituted BiFeO3 thin films deposited on Pt electrodes using chemical solution deposition
Authors: Singh S. K. , Ishiwara H. , Maruyama K.
Published: August 2006

Mn-substituted BiFeO3 (BFO) thin films were fabricated by chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) structures. X-ray diffraction analysis revealed that the BFO lattice was somewhat distorted when 5% of Fe atoms were substituted with Mn atoms, but no secondary phase appeared by Mn substitution up to 20%. The leakage current density at higher electric field than 0.6MV∕cm decreased by Mn substitution of 3%–5%, compared with a pure BFO film. Because of the low leakage current density in the high electric field region, well saturated polarization hysteresis loops with remanent polarization of 100μC∕cm2 were observed in the 5% Mn-substituted BFO films at a measurement frequency of 1kHz.
Interplay between stimulated emission and singlet-singlet annihilation in oligothiophene dioxide thin films
Authors: Lattante S. , De Giorgi M. , Barbarella G. , Favaretto L. , Gigli G. , Cingolani R. , Anni M.
Published: August 2006

We have studied the optical properties of different thienyl-S,S-dioxide oligothiophenes under strong excitation. No stimulated emission is observed in neat films due to singlet-singlet bimolecular annihilation. The bimolecular recombination constant is quantified from a rate equation model in the limit of constant annihilation rate. We demonstrate that tunable stimulated emission in the range 490–660nm, due to optical gain with an estimated cross section of the order of σg≈10−17cm2, can be obtained by blending the active molecule with small quantities of inert polycarbonate poly(bisphenol-A-carbonate) (PC). The presence of amplified spontaneous emission (ASE) for a PC:active molecule relative concentration as small as 1:500 suggests that the polycarbonate role is to reduce the intermolecular diffusion rate rather than to isolate the active molecules. Moreover, for higher PC content, a continuous decrease of the bimolecular quenching role is observed. These results demonstrate that the absence of stimulated emission in neat films is not necessarily due to intrinsic molecular properties, as strong ASE can be obtained by slightly modifying the molecule interaction during the deposition process. This approach could allow a considerable extension of the number of molecules showing stimulated emission for organic laser applications.
Field emission current saturation of aligned carbon nanotube—Effect of density and aspect ratio
Authors: Patra S. K. , Mohan Rao G.
Published: August 2006

The investigation of field emission (FE) properties of carbon nanotube film reveals a strong dependency on the density and its aspect ratio. Nonlinearity in the Fowler-Nordheim (F–N) plot has been explained in terms of change in work function of carbon due to heating during FE. A characteristic knee in the F-N plot, separates the linear region from the nonlinear region. Iknee value turns out to be a basic parameter to define the FE process. It has been shown that an optimum density and length of the nanotubes have high efficiency of field emission. An explanation has been given for the phenomena responsible for efficient field emission in short nanotubes.
Spin configuration and magnetostrictive properties of Laves compounds TbxDy0.7−xPr0.3(Fe0.9B0.1)1.93(0.10⩽x⩽0.28)
Authors: Liu J. J. , Ren W. J. , Zhang Z. D. , Li D. , Li J. , Zhao X. G. , Liu W. , Or S. W.
Published: August 2006

The spin configuration and magnetostrictive properties of TbxDy0.7−xPr0.3(Fe0.9B0.1)1.93 (0.10⩽x⩽0.28) Laves compounds are investigated. The spin phase diagram for the compounds is constructed to illustrate both the composition dependence of the Curie point TC and spin reorientation temperature TSR and the arrangement for the easy magnetization direction (EMD). It is found that when x is increased from 0.16 to 0.28, TC increases from 617to623K, while TSR decreases from 375to160K at which the EMD reorients from ⟨100⟩ to ⟨111⟩ axis. The magnetization and magnetostriction of the compounds are measured as a function of applied field. The magnetocrystalline anisotropy constant K1 and the ratio λa∕K1 reach a minimum and a maximum value at x=0.22, respectively. The single phase Tb0.22Dy0.48Pr0.3(Fe0.9B0.1)1.93 compound has the lowest magnetocrystalline anisotropy, the highest magnetostriction to anisotropy ratio, and a large spontaneous magnetostriction λ111 of ∼1500ppm at room temperature. In view of these properties, the compound is a promising magnetostrictive material.
Ordering in linear multipolar colloids driven by an external field
Authors: Alarcón-Waess O.
Published: August 2006

An approach to describe a linear multipolar colloid driven by an external field is developed by considering a colloid which in absence of the field is low structured and its coupling potential is axially symmetric. The equilibrium correlation of one component of the orientation tensor, self and collective, is computed up to linear order in density, which can be measured in an appropriate light scattering experiment. The self-correlation is written in terms of the second and fourth order parameters. All the equilibrium quantities are computed up to two-body level. This is done by assuming that the two-body equilibrium density function is given by the Boltzmann distribution, whereas the one-body density function is computed as solution of the equilibrium N-body Smoluchowski equation in the absence of hydrodynamic interactions. These observables, self and collective, as well as the second and fourth order parameters are able to describe when the colloid would evolve to an orientationally ordered phase. Explicit results for the dipole and quadrupole moments are reported. These results predict a different alignment with the external field for each moment. A relationship is provided between second and fourth order parameters, predicting the critical value of the external field in which the colloid goes into an axially symmetric phase.
Experimental studies of the converse flexoelectric effect induced by inhomogeneous electric field in a barium strontium titanate composition
Authors: Fu John Y. , Zhu Wenyi , Li Nan , Cross L. Eric
Published: August 2006

Recent experimental studies have shown that some inhomogeneously strained bulk solid dielectrics have large flexoelectric coefficients. Here we report the experimental observation of the converse flexoelectric effect, i.e., the elastic stress induced in a Ba0.67Sr0.33TiO3 (BST) composition trapezoid block under the inhomogeneous electric fields generated by its chosen boundary shape. This BST composition has a very high electric susceptibility value χ and a ferroelectric Curie point that is close to room temperature. The quantitative measurement of the flexoelectric coefficient μ11 at a temperature just above the Curie point indicates that an enhanced converse flexoelectric effect exists in the BST sample in good agreement with the earlier measured direct effect. This observed phenomenon agrees with the theoretical predictions of Tagantsev’s phenomenological model. Furthermore, our experimental results suggest that we may be able to develop a method to design some unusual but useful piezocomposites with microscale artificial geometry shapes, which are completely fabricated from nonpiezoelectric materials.
Nonlinear optical transitions of GaAs∕AlGaAs asymmetric double-well structures
Authors: Kim E. H. , Shin Y. H. , Kim Yongmin , Noh S. J. , Perry C. H. , Simmons J. A. , Crooker S. A. , Takamasu T.
Published: August 2006

A thin AlGaAs barrier (2.5nm) inserted into a GaAs∕AlGaAs single heterojunction formed a square and a wedge-shaped triangular quantum well in the conduction band. In such a structure, the valence band does not have tunnel-coupled energy levels. Hence, the photogenerated valence holes tend to move to the GaAs flatband region. This asymmetric quantum structure showed intense nonlinear photoluminescence emission behavior with external excitation power and magnetic field. Increasing the external laser power simply caused the number of photogenerated holes to increase near the interface close to the quantum well, which then recombined with the conduction band electrons. External magnetic fields resulted in an increased Coulomb attraction which generated dynamic movement of valence holes. The migration of unbound holes to the interface region from the GaAs flatband area produced highly nonlinear optical transitions in magnetic fields.
Visible and near-infrared photoluminescences of europium-doped titania film
Authors: Jia C. W. , Xie E. Q. , Zhao J. G. , Sun Z. W. , Peng A. H.
Published: August 2006

Eu 3 + -doped TiO2 films were prepared on silicon substrates by sol-gel method. Anatase and rutile phases appear when the samples were heat treated in oxygen atmosphere at 500 and 900°C, respectively. Photoluminescence (PL) properties were investigated under the excitation of 325nm He–Cd laser. Visible PL peaking at 543, 598, 620, 665, and 694nm are founded which are ascribed to D15→F17, D05→Fj7(j=1,2,3,4) transitions of Eu3+ ions and the PL intensities reach maximum when the sample was heat treated at 700°C. Compared with the PL spectra of pure TiO2, energy transfer from self-trapped exciton state to rare-earth ions is considered to exist in Eu3+-doped TiO2 system. After the sample was heat treated at higher temperatures than 700°C, the PL intensity of Eu3+ ions decreases obviously, while near-infrared PL (815nm) was detected which is due to the defect states associated with Ti3+ ions. Through the analysis of excitation and emission mechanism, we conclude that visible and near-infrared PLs are two competitive processes and energy back transfer from Eu3+ ions to TiO2 host is responsible for the decrease of visible PL intensity and the increase of near-infrared PL intensity.