bluuebunny/crossref_metadata_2025 huggingface dataset:
Description of fluid dynamics and coupled transports in models of a laminar flow diffusion chamber
Authors: Trávníčková Tereza , Havlica Jaromír , Ždímal Vladimír
Published: August 2013

The aim of this study is to assess how much the results of nucleation experiments in a laminar flow diffusion chamber (LFDC) are influenced by the complexity of the model of the transport properties. The effects of the type of fluid dynamic model (the steady state compressible Navier-Stokes system for an ideal gas/parabolic profile approximation) and the contributions of the coupled terms describing the Dufour effects and thermodiffusion on the predicted magnitude of the nucleation maxima and its location were investigated. This study was performed on the model of the homogeneous nucleation of an n-butanol-He vapor mixture in a LFDC. The isothermal dependencies of the nucleation rate on supersaturation were determined at three nucleation temperatures: 265 K, 270 K, and 280 K. For this purpose, the experimental LFDC data measured by A. P. Hyvärinen et al. [J. Chem. Phys. 124, 224304 (2006)10.1063/1.2200341] were reevaluated using transport models at different levels of complexity. Our results indicate that the type of fluid dynamical model affects both the position of the nucleation maxima in the LFDC and the maximum value of the nucleation rate. On the other hand, the Dufour effects and thermodiffusion perceptibly influence only the value of the maximal nucleation rate. Its position changes only marginally. The dependence of the maximum experimental nucleation rate on the saturation ratio and nucleation temperature was acquired for each case. Based on this dependence, we presented a method for the comparison and evaluation of the uncertainties of simpler models' solutions for the results, where we assumed that the model with Navier-Stokes equations and both coupled effects taken into account was the basis. From this comparison, it follows that an inappropriate choice of mathematical models could lead to relative errors of the order of several hundred percent in the maximum experimental nucleation rate. In the conclusion of this study, we also provide some general recommendations concerning the proper choice and setting of the mathematical model of transport processes in the LFDC.
A Chebyshev method for state-to-state reactive scattering using reactant-product decoupling: OH + H2 → H2O + H
Authors: Cvitaš Marko T. , Althorpe Stuart C.
Published: August 2013

We extend a recently developed wave packet method for computing the state-to-state quantum dynamics of AB + CD → ABC + D reactions [M. T. Cvitaš and S. C. Althorpe, J. Phys. Chem. A 113, 4557 (2009)]10.1021/jp8111974 to include the Chebyshev propagator. The method uses the further partitioned approach to reactant-product decoupling, which uses artificial decoupling potentials to partition the coordinate space of the reaction into separate reactant, product, and transition-state regions. Separate coordinates and basis sets can then be used that are best adapted to each region. We derive improved Chebyshev partitioning formulas which include Mandelshtam-and-Taylor-type decoupling potentials, and which are essential for the non-unitary discrete variable representations that must be used in 4-atom reactive scattering calculations. Numerical tests on the fully dimensional OH + H2 → H2O + H reaction for J = 0 show that the new version of the method is as efficient as the previously developed split-operator version. The advantages of the Chebyshev propagator (most notably the ease of parallelization for J > 0) can now be fully exploited in state-to-state reactive scattering calculations on 4-atom reactions.
Linear-scaling time-dependent density-functional theory in the linear response formalism
Authors: Zuehlsdorff T. J. , Hine N. D. M. , Spencer J. S. , Harrison N. M. , Riley D. J. , Haynes P. D.
Published: August 2013

We present an implementation of time-dependent density-functional theory (TDDFT) in the linear response formalism enabling the calculation of low energy optical absorption spectra for large molecules and nanostructures. The method avoids any explicit reference to canonical representations of either occupied or virtual Kohn-Sham states and thus achieves linear-scaling computational effort with system size. In contrast to conventional localised orbital formulations, where a single set of localised functions is used to span the occupied and unoccupied state manifold, we make use of two sets of in situ optimised localised orbitals, one for the occupied and one for the unoccupied space. This double representation approach avoids known problems of spanning the space of unoccupied Kohn-Sham states with a minimal set of localised orbitals optimised for the occupied space, while the in situ optimisation procedure allows for efficient calculations with a minimal number of functions. The method is applied to a number of medium sized organic molecules and a good agreement with traditional TDDFT methods is observed. Furthermore, linear scaling of computational cost with system size is demonstrated on (10,0) carbon nanotubes of different lengths.
Morphology of growth and first to fourth moments in a Monte Carlo particle-deposition model with a novel adherence-potential barrier
Authors: Galindo J. L. , Huertas R.
Published: August 2013

Two models of particle depositions with different adhesion dynamics have been developed and studied using Monte Carlo simulations. They have been compared for the complete range of Peclet numbers and they differ in the particle-adhesion dynamics, with an adherence-potential barrier in one of them. In general, the time course of the two models is different in ballistic cases, but similar in diffusive cases. Thus, they complement each other, enabling a description of the morphology for most cases. Depending on their deposits, these systems have been characterized morphologically, as compact, homogeneous, or open. They have also been described by the following parameters, which have been analyzed: from the first to the fourth moments, porosity, and the fractal dimension. Also, the time course of the parameters has been studied and described, especially the Anomalous and Family-Vicsek scaling. The third and fourth moments and the fractal dimension have been characterized, defining different stages in their time course, and studying their behavior for the complete range of Peclet numbers. Moreover, the inclusion of an adherence-potential barrier enables the description of new universality class intervals needed to explain recent experimental systems, e.g., at the edges of evaporated drops of colloidal suspensions, electro-convection on nematic liquid crystals, and polynuclear growth model. Finally, the characterization of the systems required not only a dynamic exponent (β) by universality classes but also a third (skewness) and fourth (kurtosis) moments for a complete description.
A comparative study of electronic and structural properties of polycrystalline and epitaxial magnetron-sputtered ZnO:Al and Zn1-xMgxO:Al Films—Origin of the grain barrier traps
Authors: Bikowski André , Ellmer Klaus
Published: August 2013

Homoepitaxial and heteroepitaxial ZnO, ZnO:Al, and Zn1-xMgxO:Al films have been grown by magnetron sputtering from ceramic targets at substrate temperatures between 200 °C and 500 °C. We studied the relation between the electronic transport and structural properties for the epitaxially grown films and compared it to the properties of polycrystalline films by means of X-ray diffraction, transmission electron microscopy and optical reflectance and transmittance measurements. The results show that the epitaxial growth of ZnO:Al and Zn1-xMgxO:Al thin films, which has been observed for nearly all films prepared on single crystalline substrates, will not significantly improve the electronic transport properties in comparison to polycrystalline films unless the grain boundaries are eliminated completely. The grain boundary defect densities of about 3 × 1013 cm−2 are nearly independent on the structural quality of the different polycrystalline, hetero- and homoepitaxial films. This clearly proves that the grain boundary defects are not caused by crystallographic defects, but, most probably, by the dopant aluminium.
Quantum mechanics/molecular mechanics dual Hamiltonian free energy perturbation
Authors: Polyak Iakov , Benighaus Tobias , Boulanger Eliot , Thiel Walter
Published: August 2013

The dual Hamiltonian free energy perturbation (DH-FEP) method is designed for accurate and efficient evaluation of the free energy profile of chemical reactions in quantum mechanical/molecular mechanical (QM/MM) calculations. In contrast to existing QM/MM FEP variants, the QM region is not kept frozen during sampling, but all degrees of freedom except for the reaction coordinate are sampled. In the DH-FEP scheme, the sampling is done by semiempirical QM/MM molecular dynamics (MD), while the perturbation energy differences are evaluated from high-level QM/MM single-point calculations at regular intervals, skipping a pre-defined number of MD sampling steps. After validating our method using an analytic model potential with an exactly known solution, we report a QM/MM DH-FEP study of the enzymatic reaction catalyzed by chorismate mutase. We suggest guidelines for QM/MM DH-FEP calculations and default values for the required computational parameters. In the case of chorismate mutase, we apply the DH-FEP approach in combination with a single one-dimensional reaction coordinate and with a two-dimensional collective coordinate (two individual distances), with superior results for the latter choice.
Probing local structure of pyrochlore lead zinc niobate with synchrotron x-ray absorption spectroscopy technique
Authors: Kanchiang Kanokwan , Pramchu Sittichain , Yimnirun Rattikorn , Pakawanit Phakkhananan , Ananta Supon , Laosiritaworn Yongyut
Published: August 2013

Local structure of lead zinc niobate (PZN) ceramic, synthesized via B-site oxide precursor route in atmospheric pressure, was investigated using synchrotron x-ray absorption spectroscopy (XAS) technique. The x-ray absorption near-edge structure (XANES) simulation was first carried out. The XANES simulation results indicate that the PZN ceramic is in pyrochlore phase having Zn2+ substituted on Nb5+ site. Afterwards, the extended x-ray absorption fine structure (EXAFS) analysis was performed to extract the bond length information between Zn2+ and its neighboring atoms. From the EXAFS fitting, the bond length between Zn2+ and Pb2+ in the pyrochlore phase was found to be longer than the previously reported bond length in the perovskite phase. Further, with the radial distribution information of Zn2+'s neighboring atoms, the formation energies along the precursor-to-pyrochlore and precursor-to-perovskite reaction paths were calculated using the density functional theory (DFT). The calculated results show that the formation energy of the perovskite phase is noticeably higher than that of the pyrochlore phase, which is influenced by the presence of energetic Pb2+ lone pair, as the perovskite phase has shorter Zn2+ to Pb2+ bonding. This therefore suggests the steric hindrance of Pb2+ lone pair and the mutual interactions between Pb2+ lone pair and Zn2+ are main causes of the instability of lead zinc niobate in the perovskite structure and confirm the efficacy of XAS and DFT analysis in revealing local structural details of complex pyrochlore materials.
Enhancing the incorporation compatibility of molybdenum oxides in organic light emitting diodes with gap state formations
Authors: Wang Po-Sheng , Lo Yuan-Yen , Tseng Wei-Hsuan , Chen Mei-Hsin , Wu Chih-I
Published: August 2013

The enhancement of injection current and luminance in organic light emitting diodes is achieved by annealing molybdenum oxide (MoO3) hole injecting layers prior to the deposition of hole transport layers. While there is no benefit by the incorporation of non-annealed MoO3 in devices using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as the hole transport layers, the annealed MoO3 layers exhibit a significant improvement in hole injection from indium tin oxide anodes to TAPC. X-ray photoemission spectroscopy reveals the change of oxidation states of Mo atoms in MoO3 films due to the annealing process. The gap state formation is verified by ultra-violet photoemission spectroscopy. A more energetically favorable band alignment is obtained at the interface between the annealed MoO3 and TAPC, resulting in improved hole injection efficiency. The overall performance of OLEDs can be enhanced by adopting annealed MoO3 in most of the hole transport layers.
Surface photoconductivity of organosilicate glass dielectrics induced by vacuum-ultraviolet radiation
Authors: Zheng H. , Nichols M. T. , Pei D. , Nishi Y. , Shohet J. L.
Published: August 2013

The temporary increase in the electrical surface conductivity of low-k organosilicate glass (SiCOH) during exposure to vacuum-ultraviolet radiation (VUV) is investigated. To measure the photoconductivity, patterned “comb structures” are deposited on dielectric films and exposed to synchrotron radiation in the range of   8–25  eV, which is in the energy range of most plasma vacuum-ultraviolet radiation. The change in photo surface conductivity induced by VUV radiation may be beneficial in limiting charging damage of dielectrics by depleting the plasma-deposited charge.
Refined algebraic quantisation in a system with nonconstant gauge invariant structure functions
Authors: Martínez-Pascual Eric
Published: August 2013

In a previous work [J. Louko and E. Martínez-Pascual, “Constraint rescaling in refined algebraic quantisation: Momentum constraint,” J. Math. Phys. 52, 123504 (2011)]10.1063/1.3664336, refined algebraic quantisation (RAQ) within a family of classically equivalent constrained Hamiltonian systems that are related to each other by rescaling one momentum-type constraint was investigated. In the present work, the first steps to generalise this analysis to cases where more constraints occur are developed. The system under consideration contains two momentum-type constraints, originally abelian, where rescalings of these constraints by a non-vanishing function of the coordinates are allowed. These rescalings induce structure functions at the level of the gauge algebra. Providing a specific parametrised family of real-valued scaling functions, the implementation of the corresponding rescaled quantum momentum-type constraints is performed using RAQ when the gauge algebra: (i) remains abelian and (ii) undergoes into an algebra of a nonunimodular group with nonconstant gauge invariant structure functions. Case (ii) becomes the first example known to the author where an open algebra is handled in refined algebraic quantisation. Challenging issues that arise in the presence of non-gauge invariant structure functions are also addressed.
Self-organized one-atom thick fractal nanoclusters via field-induced atomic transport
Authors: Batabyal R. , Mahato J. C. , Das Debolina , Roy Anupam , Dev B. N.
Published: August 2013

We report on the growth of a monolayer thick fractal nanostructures of Ag on flat-top Ag islands, grown on Si(111). Upon application of a voltage pulse at an edge of the flat-top Ag island from a scanning tunneling microscope tip, Ag atoms climb from the edge onto the top of the island. These atoms aggregate to form precisely one-atom thick nanostructures of fractal nature. The fractal (Hausdorff) dimension, DH = 1.75 ± 0.05, of this nanostructure has been determined by analyzing the morphology of the growing nanocluster, imaged by scanning tunneling microscopy, following the application of the voltage pulse. This value of the fractal dimension is consistent with the diffusion limited aggregation (DLA) model. We also determined two other fractal dimensions based on perimeter-radius-of-gyration (DP) and perimeter-area (D′P) relationship. Simulations of the DLA process, with varying sticking probability, lead to different cluster morphologies [P. Meakin, Phys. Rev. A 27, 1495 (1983)]; however, the value of DH is insensitive to this difference in morphology. We suggest that the morphology can be characterized by additional fractal dimension(s) DP and/or D′P, besides DH. We also show that within the DLA process DP = DH [C. Amitrano et al., Phys. Rev. A 40, 1713 (1989)] is only a special case; in general, DP and DH can be unequal. Characterization of fractal morphology is important for fractals in nanoelectronics, as fractal morphology would determine the electron transport behavior.
A simple sub-nanosecond ultraviolet light pulse generator with high repetition rate and peak power
Authors: Binh P. H. , Trong V. D. , Renucci P. , Marie X.
Published: August 2013

We present a simple ultraviolet sub-nanosecond pulse generator using commercial ultraviolet light-emitting diodes with peak emission wavelengths of 290 nm, 318 nm, 338 nm, and 405 nm. The generator is based on step recovery diode, short-circuited transmission line, and current-shaping circuit. The narrowest pulses achieved have 630 ps full width at half maximum at repetition rate of 80 MHz. Optical pulse power in the range of several hundreds of microwatts depends on the applied bias voltage. The bias voltage dependences of the output optical pulse width and peak power are analysed and discussed. Compared to commercial UV sub-nanosecond generators, the proposed generator can produce much higher pulse repetition rate and peak power.
Bayesian soft X-ray tomography using non-stationary Gaussian Processes
Authors: Li Dong , Svensson J. , Thomsen H. , Medina F. , Werner A. , Wolf R.
Published: August 2013

In this study, a Bayesian based non-stationary Gaussian Process (GP) method for the inference of soft X-ray emissivity distribution along with its associated uncertainties has been developed. For the investigation of equilibrium condition and fast magnetohydrodynamic behaviors in nuclear fusion plasmas, it is of importance to infer, especially in the plasma center, spatially resolved soft X-ray profiles from a limited number of noisy line integral measurements. For this ill-posed inversion problem, Bayesian probability theory can provide a posterior probability distribution over all possible solutions under given model assumptions. Specifically, the use of a non-stationary GP to model the emission allows the model to adapt to the varying length scales of the underlying diffusion process. In contrast to other conventional methods, the prior regularization is realized in a probability form which enhances the capability of uncertainty analysis, in consequence, scientists who concern the reliability of their results will benefit from it. Under the assumption of normally distributed noise, the posterior distribution evaluated at a discrete number of points becomes a multivariate normal distribution whose mean and covariance are analytically available, making inversions and calculation of uncertainty fast. Additionally, the hyper-parameters embedded in the model assumption can be optimized through a Bayesian Occam's Razor formalism and thereby automatically adjust the model complexity. This method is shown to produce convincing reconstructions and good agreements with independently calculated results from the Maximum Entropy and Equilibrium-Based Iterative Tomography Algorithm methods.
Capacitive detection of buried interfaces by a dynamic surface force apparatus
Authors: Villey Richard , Piednoir Agnès , Sharma Preeti , Cottin-Bizonne Cécile , Cross Benjamin , Phaner-Goutorbe Magali , Charlaix Élisabeth
Published: August 2013

We present here a new type of distance sensor mounted on a Surface Force Apparatus (SFA), able to detect the position of a buried interface and therefore the thickness of a thin solid or soft matter film coating the SFA surface(s). This sensor relies on the capacitance created by the two metallized surfaces of the SFA. An harmonic oscillation of these polarized surfaces creates a pico- to femto-amps current indicating their relative position. One of the specificities of this sensor is the relatively weak polarization used for the measurements, minimizing the electrical forces and their impact on other interactions, hydrodynamical and mechanical forces measured by the SFA. This original and simple design is of high interest for studying the viscoelastic properties of thin films, to detect adsorbed liquid layers or slippage at liquid-solid interfaces, or even to study complex fluids such as ionic liquids under polarization. We demonstrate the use of this sensor to study the flow boundary condition of silicon oil on a metal surface, and the elastic modulus of a thin elastomer layer.
Photoluminescence decay rate engineering of CdSe quantum dots in ensemble arrays embedded with gold nano-antennae
Authors: Haridas M. , Basu J. K. , Tiwari A. K. , Venkatapathi M.
Published: August 2013

We discuss experimental results on the ability to significantly tune the photoluminescence decay rates of CdSe quantum dots embedded in an ordered template, using lightly doped small gold nanoparticles (nano-antennae), of relatively low optical efficiency. We observe both enhancement and quenching of photoluminescence intensity of the quantum dots varying monotonically with increasing volume fraction of added gold nanoparticles, with respect to undoped quantum dot arrays. However, the corresponding variation in lifetime of photoluminescence spectra decay shows a hitherto unobserved, non-monotonic variation with gold nanoparticle doping. We also demonstrate that Purcell effect is quite effective for the larger (5 nm) gold nano-antenna leading to more than four times enhanced radiative rate at spectral resonance, for largest doping and about 1.75 times enhancement for off-resonance. Significantly for spectral off-resonance samples, we could simultaneously engineer reduction of non-radiative decay rate along with increase of radiative decay rate. Non-radiative decay dominates the system for the smaller (2 nm) gold nano-antenna setting the limit on how small these plasmonic nano-antennae could be to be effective in engineering significant enhancement in radiative decay rate and, hence, the overall quantum efficiency of quantum dot based hybrid photonic assemblies.
Transport phenomena in intrinsic semiconductors and insulators at high current densities: Suppression of the broken neutrality drift
Authors: Mnatsakanov T. T. , Levinshtein M. E. , Tandoev A. G. , Yurkov S. N.
Published: August 2013

It is shown that, in addition to the diffusion and broken neutrality drift (BND) modes well-known for insulators and very lightly doped semiconductors, the quasineutral drift (QND) mode is possible. The transition from the BND to QND mode is accompanied by the appearance of a portion with a very sharp current rise in the current-voltage characteristic. This effect is observed in a new type of semiconductor detectors (CIDs, Current Injected Detectors) of high-intensity neutron and proton radiation, suggested, in particular, for Large Hadron Collider. The effect is unambiguously attributed now to the presence of radiation-induced deep centers in a semiconductor. It is shown, however, in this paper that the effect of a very sharp rise in current upon a slight increase in voltage is even possible when there are no deep centers. An equation adequately describing the possible transport modes in intrinsic semiconductors and insulators is derived. The results of an analytical study are confirmed by an adequate simulation.
Nonlinear oscillations following the Rayleigh collapse of a gas bubble in a linear viscoelastic (tissue-like) medium
Authors: Hua Chengyun , Johnsen Eric
Published: August 2013

In a variety of biomedical engineering applications, cavitation occurs in soft tissue, a viscoelastic medium. The present objective is to understand the basic physics of bubble dynamics in soft tissue. To gain insights into this problem, theoretical and numerical models are developed to study the Rayleigh collapse and subsequent oscillations of a gas bubble in a viscoelastic material. To account for liquid compressibility and thus accurately model large-amplitude oscillations, the Keller-Miksis equation for spherical bubble dynamics is used. The most basic linear viscoelastic model that includes stress relaxation, viscosity, and elasticity (Zener, or standard linear solid) is considered for soft tissue, thereby adding two ordinary differential equations for the stresses. The present study seeks to advance past studies on cavitation in tissue by determining the basic effects of relaxation and elasticity on the bubble dynamics for situations in which compressibility is important. Numerical solutions show a clear dependence of the oscillations on the viscoelastic properties and compressibility. The perturbation analysis (method of multiple scales) accurately predicts the bubble response given the relevant constraints and can thus be used to investigate the underlying physics. A third-order expansion of the radius is necessary to accurately represent the dynamics. Key quantities of interest such as the oscillation frequency and damping, minimum radius, and collapse time can be predicted theoretically. The damping does not always monotonically decrease with decreasing elasticity: there exists a finite non-zero elasticity for which the damping is minimum; this value falls within the range of reported tissue elasticities. Also, the oscillation period generally changes with time over the first few cycles due to the nonlinearity of the system, before reaching an equilibrium value. The analytical expressions for the key bubble dynamics quantities and insights gained from the analysis may prove valuable in the development and optimization of certain biomedical applications.
On the multistream approach of relativistic Weibel instability. I. Linear analysis and specific illustrations
Authors: Ghizzo A. , Bertrand P.
Published: August 2013

A one-dimensional multistream formalism is extended for the study of temperature anisotropy driven Weibel-type instabilities in collisionless and relativistic plasma. The formulation is based on a Hamiltonian reduction technique using the invariance of generalized canonical momentum in transverse direction. The Vlasov-Maxwell model is expressed in terms of an ensemble of one-dimensional Vlasov-type equations, coupled together with the Maxwell equations in a self-consistent way. Although the model is fundamentally nonlinear, this first of three companion papers focuses on the linear aspect. Dispersion relations of the Weibel instability are derived in the linear regime for different kinds of polarization of the electromagnetic potential vector. The model allows new unexpected insights on the instability: enhanced growth rates for the Weibel instability are predicted when a dissymmetric distribution is considered in p⊥. In the case of a circular polarization, a simplification of the linear analysis can be obtained by the introduction of the “multiring” approach allowing to extend the analytical model of Yoon and Davidson [Phys. Rev. A 35, 2718 (1987)]. Applications of this model are left to the other two papers of the series where specific problems are addressed pertaining to the nonlinear and relativistic dynamics of magnetically trapped particles met in the saturation regime of the Weibel instability.
On the multistream approach of relativistic Weibel instability. II. Bernstein-Greene-Kruskal-type waves in magnetic trapping
Authors: Ghizzo A.
Published: August 2013

The stationary state with magnetically trapped particles is investigated at the saturation of the relativistic Weibel instability, within the “multiring” model in a Hamiltonian framework. The multistream model and its multiring extension have been developed in Paper I, under the assumption that the generalized canonical momentum is conserved in the perpendicular direction. One dimensional relativistic Bernstein-Greene-Kruskal waves with deeply trapped particles are addressed using similar mathematical formalism developed by Lontano et al. [Phys. Plasmas 9, 2562 (2002); Phys. Plasmas 10, 639 (2003)] using several streams and in the presence of both electrostatic and magnetic trapping mechanisms.
On the multistream approach of relativistic Weibel instability. III. Comparison with full-kinetic Vlasov simulations
Authors: Ghizzo A.
Published: August 2013

The saturation of the Weibel instability in the relativistic regime is investigated within the Hamiltonian reduction technique based on the multistream approach developed in paper I in the linear case and in paper II for the nonlinear saturation. In this work, the study is compared with results obtained by full kinetic 1D2V Vlasov-Maxwell simulations based on a semi-Lagrangian technique. For a temperature anisotropy, qualitatively different regimes are realized depending on the excitation of the longitudinal (plasma) electric field, in contrast with the existing theories of the Weibel instability based on their purely transverse characters. The emphasis here is on gaining a better understanding of the nonlinear aspects of the Weibel instability. The multistream model offers an alternate way to make calculations or numerical experiments more tractable, when only a few moments of the velocity distribution of the plasma are considered.
Two-band finite difference method for the bandstructure calculation with nonparabolicity effects in quantum cascade lasers
Authors: Ma Xunpeng , Li Kangwen , Zhang Zuyin , Hu Haifeng , Wang Qing , Wei Xin , Song Guofeng
Published: August 2013

We present a two-band finite difference method for the bandstructure calculation of quantum cascade lasers (QCLs) based on the equivalent two-band model of the nonparabolic Schrödinger equation. Particular backward and forward difference forms are employed in the discretization procedure instead of the common central difference form. In comparison with the linearization approach of the nonparabolic Schrödinger equation, the method is as accurate and reliable as the linearization approach, while the velocity of the method is faster and the matrix elements are more concise, therefore making the method more practical for QCLs simulations.
Optimization of silver-dielectric-silver nanoshell for sensing applications
Authors: Shirzaditabar Farzad , Saliminasab Maryam
Published: August 2013

In this paper, resonance light scattering (RLS) properties of a silver-dielectric-silver nanoshell, based on quasi-static approach and plasmon hybridization theory, are investigated. Scattering spectrum of silver-dielectric-silver nanoshell has two intense and clearly separated RLS peaks and provides a potential for biosensing based on surface plasmon resonance and surface-enhanced Raman scattering. The two RLS peaks in silver-dielectric-silver nanoshell are optimized by tuning the geometrical dimensions. In addition, the optimal geometry is discussed to obtain the high sensitivity of silver-dielectric-silver nanoshell. As the silver core radius increases, the sensitivity of silver-dielectric-silver nanoshell decreases whereas increasing the middle dielectric thickness increases the sensitivity of silver-dielectric-silver nanoshell.
Influence of film thickness in THz active metamaterial devices: A comparison between superconductor and metal split-ring resonators
Authors: Singh Ranjan , Roy Chowdhury Dibakar , Xiong Jie , Yang Hao , Azad Abul K. , Taylor Antoinette J. , Jia Q. X. , Chen Hou-Tong
Published: August 2013

We experimentally demonstrate thickness-dependent resonance tuning in planar terahertz superconducting metamaterials. Inductive-capacitive resonance of arrays of split-ring resonators fabricated from 50, 100, and 200 nm thick YBa2Cu3O7−δ (YBCO) and gold films were characterized and compared as a function of temperature. In the YBCO metamaterials the resonance frequency strongly depends on the thickness, and they show high thermal tunability in both resonance strength and frequency below the superconducting transition temperature, where the imaginary conductivity varies by three orders of magnitude. In contrast, the resonance in the gold metamaterials exhibits little thickness-dependence and very small tunability.
Diffuse phase transition and electrical properties of lead-free piezoelectric (LixNa1-x)NbO3 (0.04 ≤ x ≤ 0.20) ceramics near morphotropic phase boundary
Authors: Mitra S. , Kulkarni A. R. , Prakash Om
Published: August 2013

Temperature-dependent dielectric permittivity of lead-free (LixNa1-x)NbO3 for nominal x = 0.04–0.20, prepared by solid state reaction followed by sintering, was studied to resolve often debated issue pertaining to exactness of morphotropic phase boundary (MPB) location besides structural aspects and phase stability in the system near MPB. Interestingly, a diffuse phase transition has been observed in the dielectric permittivity peak arising from the disorder induced in A-site and structural frustration in the perovskite cell due to Li substitution. A partial phase diagram has been proposed based on temperature-dependent dielectric permittivity studies. The room temperature piezoelectric and ferroelectric properties were investigated and the ceramics with x = 0.12 showed relatively good electrical properties (d33 = 28 pC/N, kp = 13.8%, Qm = 440, Pr = 12.5 μC/cm2, Ec = 43.2 kV/cm, and Tm = 340 °C). These parameter values make this material suitable for piezoelectric resonator and filter applications. Moreover, a high dielectric permittivity (ε′r = 2703) with broad diffuse peak near transition temperature, and low dielectric loss (<4%) over a wide temperature range (50–250 °C) found in this material may also have a potential application in high-temperature multilayer capacitors in automotive and aerospace related industries.
Turbulent energy exchange: Calculation and relevance for profile prediction
Authors: Candy J.
Published: August 2013

The anomalous heat production due to turbulence is neither routinely calculated in nonlinear gyrokinetic simulations nor routinely retained in profile prediction studies. In this work, we develop a symmetrized method to compute the exchange which dramatically reduces the intermittency in the time-dependent moment, thereby improving the accuracy of the time-average. We also examine the practical impact on transport-timescale simulations, and show that the exchange has only a minor impact on profile evolution for a well-studied DIII-D discharge.
Transport of electron beams with initial transverse-longitudinal correlation
Authors: Harris J. R. , Lewellen J. W. , Poole B. R.
Published: August 2013

When an electron beam whose current varies in time is extracted from a DC gun, the competition between the time-dependent space charge force and the time-independent focusing force will cause a correlation between radius, divergence, current, and position along the beam. This correlation will determine the beam's configuration in trace space, and together with the design of the downstream transport system, will determine the quality of the transport solutions that can be obtained, including the amplitude of the mismatch oscillations occurring in each slice of the beam. Recent simulations of a simplified diode with Pierce-type focusing operating at nonrelativistic voltages indicated that the radius and divergence of beams extracted from such guns can be approximated to high accuracy as linear functions of current. Here, we consider the impact of this dependence on the beam configuration in trace space and investigate the implications for matching and transport of such correlated beams in uniform linear focusing channels.
Conductance fluctuation and shot noise in disordered graphene systems, a perturbation expansion approach
Authors: Zhuang Jia Ning , Wang Jian
Published: August 2013

We report the investigation of conductance fluctuation and shot noise in disordered graphene systems with two kinds of disorder, Anderson type impurities and random dopants. To avoid the traditional exact but time-consuming approach, known as brute-force calculation, which is somehow impractical at low doping concentration, we develop an expansion method based on the coherent potential approximation (CPA) to calculate the average of four Green's functions, and the results are obtained by truncating the expansion up to 6th order in terms of “single-site-T-matrix.” Since our expansion is with respect to “single-site-T-matrix” instead of disorder strength W, good result can be obtained at 6th order for finite W. We benchmark our results against brute-force method on disordered graphene systems as well as the two dimensional square lattice model systems for both Anderson disorder and the random doping. The results show that in the regime where the disorder strength W is small or the doping concentration is low, our results agree well with the results obtained from the brute-force method. Specifically, for the graphene system with Anderson impurities, our results for conductance fluctuation show good agreement for W up to 0.4t, where t is the hopping energy. While for average shot noise, the results are good for W up to 0.2t. When the graphene system is doped with low concentration 1%, the conductance fluctuation and shot noise agrees with brute-force results for large W which is comparable to the hopping energy t. At large doping concentration 10%, good agreement can be reached for conductance fluctuation and shot noise for W up to 0.4t. We have also tested our formalism on square lattice with similar results. Our formalism can be easily combined with linear muffin-tin orbital first-principles transport calculations for light doping nano-scaled systems, making prediction on variability of nano-devices.
Influence of thermal growth parameters on the SiO2/4H-SiC interfacial region
Authors: Pitthan E. , Lopes L. D. , Palmieri R. , Corrêa S. A. , Soares G. V. , Boudinov H. I. , Stedile F. C.
Published: August 2013

In order to elucidate the origin of SiC electrical degradation from thermal oxidation, 4H-SiC substrates were thermally oxidized under different conditions of time and pressure. Results from nuclear reaction analyses were correlated to those from electrical measurements. Although the increase in the flatband voltage shift and in the film thickness were related to the oxidation parameters, the results exclude the thickness of the SiO2/4H-SiC interfacial region and the amount of residual oxygen compounds present on the SiC surface as the main cause of the electrical degradation from the SiC oxidation.
Paramagnetic FexTa1-x alloys for engineering of perpendicularly magnetized tunnel junctions
Authors: Gottwald Matthias , Kan Jimmy J. , Lee Kangho , Kang Seung H. , Fullerton Eric E.
Published: August 2013

Exchange coupling between two magnetic layers through an interlayer is of broad interest for numerous recent applications of nano-magnetic systems. In this letter, we study ferromagnetic exchange coupling through amorphous paramagnetic Fe-Ta alloys. We show that the exchange coupling depends exponentially on spacer thickness and scales with the Fe-Ta susceptibility, which can be tuned via the alloy composition and/or temperature. Such materials are of high interest for the engineering of perpendicularly magnetized CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of magnetic layers with differing crystalline lattices, suppresses dead layers, and can act as an inter-diffusion barrier during annealing.
Thermal conductivity of argon at high pressure from first principles calculations
Authors: Chernatynskiy Aleksandr , Phillpot Simon R.
Published: August 2013

We present calculations of the thermal conductivity of fcc Argon at high pressures (pressure range is 10–150 GPa, temperatures range is 400–1200 K) from first principles in the framework of density functional theory and solution of the Boltzmann Transport Equation. Local density approximation (LDA) and generalized gradient approximation (GGA) produce similar thermal conductivities, with differences accounted by the known overbinding and underbinding of the LDA and GGA, correspondingly. Thermal conductivities at all considered pressures and temperatures are found to be consistent with the results of previous molecular dynamics simulations based on classical 2-body potentials. However, they are not consistent with recent experimental findings. Possible reasons for this disagreement are discussed. In addition, in light of our calculations, we critically examine analytically tractable approximations for thermal conductivity as applied to solid argon.
First-principles study of negative thermal expansion in zinc oxide
Authors: Wang Zhanyu , Wang Fei , Wang Lei , Jia Yu , Sun Qiang
Published: August 2013

We present the first-principles calculations of vibrational and thermal properties for wurtzite and zinc-blende zinc oxide (ZnO) within DFT and quasi-harmonic approximation, especially for their negative thermal expansion (NTE) behavior. For the wurtzite and zinc-blende phases, negative thermal expansions are obtained at T < 95 K and T < 84 K, respectively. For the wurtzite structure, calculated phonon frequencies and mode Grüneisen parameters of low-energy modes are in good agreement with that determined experimentally. And the thermal expansion coefficient is found to be in good agreement with the experimental results. Like many other NTE semiconductors, detailed study of both phases shows that maximum contribution to NTE comes from low-frequency transverse acoustic modes, while for the wurtzite structure the contribution of longitudinal acoustic and lowest-energy optical modes is not ignorable. From the specific analysis of the vibration modes, we found that the negative thermal expansion in ZnO is dominated by the tension effect.
Trapping behavior of Shockley-Read-Hall recombination centers in silicon solar cells
Authors: Gogolin R. , Harder N. P.
Published: August 2013

We investigate the correlation between increased apparent carrier lifetime in photoconductance-based lifetime measurements and actually reduced recombination lifetime as measured by photoluminescence measurements. These findings are further reconfirmed by I-V curve measurements of solar cells. In particular, we show experimental results for lifetime samples and solar cells with and without hydrogen passivation. In the samples and solar cells without hydrogen passivation, we find both a stronger trapping behavior and a lower recombination lifetime. Our model provides a consistent description of the observation of both, the increased apparent lifetime from carrier trapping and the decreasing recombination lifetime. In our model, both are caused by a single physical mechanism; i.e., by Recombination-Active-Trap (RAT) states. Upon fitting the experimental lifetime data, we find that the RAT-defect parameters for the hydrogen-passivated and non-hydrogen-passivated lifetime samples and solar cells are identical except for the defect concentration: hydrogen-passivation reduced the defect density by 50% in both, the lifetime samples and solar cells. We conclude that trapping should be considered as an indication for hidden, yet potentially strongly increased, low injection recombination activity.
Degradation mechanism in simplified phosphorescent organic light-emitting devices utilizing one material for hole transport and emitter host
Authors: Zhang Yingjie , Abdelmalek Mina M. A. , Wang Qi , Aziz Hany
Published: August 2013

Simplified phosphorescent organic light-emitting devices (PHOLEDs) have better efficiency performance but their lifetime is shorter compared to that of standard PHOLEDs. In this work, we find that their shorter lifetime is due to exciton-induced degradation of the indium tin oxide (ITO)/organic interface. The excitons are created by the recombination of electrons leaked from the emission layer with holes injected from the ITO. Reducing electrons at the interface reduces this degradation process, hence increases device lifetime. Introducing a layer of 2,6-bis[3-(carbazol-9-yl)phenyl] pyridine (26DCzPPy) at the ITO interface results in an increase in device lifetime by one order of magnitude while maintaining the same efficiency.
Extraordinary acoustic transmission through ultrathin acoustic metamaterials by coiling up space
Authors: Li Yong , Liang Bin , Zou Xin-ye , Cheng Jian-chun
Published: August 2013

We show that by coiling up space through curled channels, a two-dimensional ultrathin acoustic metamaterial can be constructed to support the extraordinary acoustic transmission. The exotic phenomenon intrinsically stems from Fabry-Pérot resonances while the thickness of the structure is kept deep subwavelength. Broadband unity transmission is observed at a specific incident angle. It is also demonstrated that the coiling factor plays a dominant role in determining the property of transmission peak. Our results should contribute to the understanding of the underlying physics of structures by coiling up space and may potentially benefit their practical application.
Electron spin-interaction effects on the ponderomotive magnetization and radiation in degenerate quantum plasmas
Authors: Jung Young-Dae , Hong Woo-Pyo
Published: August 2013

The spin interaction effects on the Karpman-Washimi ponderomotive magnetization are investigated in quantum plasmas. The induced Karpman-Washimi magnetization and cyclotron frequency due to the ponderomotive interaction are obtained as functions of the electron quantum wave length, Fermi wave number, wave frequency, and wave number. It is found that the spin interaction correction significantly enhances the Karpman-Washimi magnetization. It is also found the Karpman-Washimi magnetization increases with increasing Fermi wave number. In addition, it is found that the spin correction effect strongly enhances the radiation spectrum. It would be expected that the Karpman-Washimi interaction can be a possible terahertz radiation mechanism in a quantum plasma.
Non-equilibrium atmospheric pressure microplasma jet: An approach to endoscopic therapies
Authors: Zuo Xiao , Wei Yu , Wei Chen Long , Dong Meng Yue ,   Plasma Medicine Team
Published: August 2013

Atmospheric pressure microplasma jet generated in a long hollow core optical fiber is studied to verify the potential feasibility of endoscopic therapies. Thermal damage and electric shock to the human body were suppressed by two technical methods, i.e., the high-voltage resistant flexible tube wrapped on the optical fiber and a power resistor of 100 kΩ connected between the power supply and the copper foil electrode. Optical emission spectra analysis indicated that many kinds of active radicals like excited atomic O and OH, were generated in the microplasma jet. In addition, the applications of the microplasma jet on sterilization and lung cancer cell apoptosis were presented. After 5 min of exposures to the microplasma jet, the cell viability and the bacillus subtilis replication decreased to about 3% and zero, respectively. More investigations are needed to improve the plasma-aided endoscopic therapies.
Circular polarization of obliquely propagating whistler wave magnetic field
Authors: Bellan P. M.
Published: August 2013

The circular polarization of the magnetic field of obliquely propagating whistler waves is derived using a basis set associated with the wave partial differential equation. The wave energy is mainly magnetic and the wave propagation consists of this magnetic energy sloshing back and forth between two orthogonal components of magnetic field in quadrature. The wave electric field energy is small compared to the magnetic field energy.
Nucleation of reversed domain and pinning effect on domain wall motion in nanocomposite magnets
Authors: Li Z. B. , Shen B. G. , Niu E. , Sun J. R.
Published: August 2013

The magnetization behaviors show a strong pinning effect on domain wall motion in optimally melt-spun Pr8Fe87B5 ribbons at room temperature. According to analysis, the coercivity is determined by the nucleation field of reversed domain, and the pinning effect, which results from the weak exchange coupling at interface, makes domain nucleation processes independent and leads to non-uniform magnetization reversals. At a temperature of 60 K, owing to the weak exchange coupling between soft-hard grains, magnetization reversal undergoes processes of spring domain nucleation in soft grains and irreversible domain nucleation in hard grains, and the pinning effect remains strong among hard grains.