bluuebunny/crossref_metadata_2025 huggingface dataset:
On the paradox of thermocapillary flow about a stationary bubble
Authors: Yariv Ehud , Shusser Michael
Published: August 2006

When a stationary bubble is exposed to an external temperature gradient, Marangoni stresses at the bubble surface result in fluid motion. A straightforward attempt to calculate the influence of this thermocapillary flow upon the temperature distribution fails to provide a well-behaved solution [Balasubramaniam and Subramanian, Phys. Fluids 16, 3131 (2004)]. This problem is revisited here using a regularization procedure which exploits the qualitative disparity in the long-range flow fields generated by a stationary bubble and a moving one. The regularization parameter is an (exponentially small) artificial bubble velocity, which reflects the inability of any asymptotic expansion to satisfy the condition of exact bubble equilibrium. The solution is obtained using asymptotic matching of two separate Reynolds-number expansions: an inner expansion, valid at the bubble neighborhood, and a remote outer expansion, valid far beyond the familiar Oseen region. This procedure provides a well-behaved solution, which is subsequently used to evaluate the convection-induced correction to the hydrodynamic force exerted on the bubble. The independence of that correction upon the artificial velocity confirms the adequacy of the regularization procedure to describe the stationary-bubble case. The ratio of the calculated force to that pertaining to the classical pure-conduction limit [Young, Goldstein, and Block, J. Fluid Mech. 6, 350 (1959)] is given by 1−Ma∕8+o(Ma), where Ma is a radius-based Marangoni number.
Band gap engineering and spatial confinement of optical phonon in ZnO quantum dots
Authors: Lin Kuo-Feng , Cheng Hsin-Ming , Hsu Hsu-Cheng , Hsieh Wen-Feng
Published: August 2006

Both band gap engineering and spatial confinement of optical phonon were observed depending upon the size of ZnO quantum dots at room temperature. Size-dependent blueshifts of photoluminescence and absorption spectra reveal the quantum confinement effect. The measured Raman spectral shift and asymmetry for the E2(high) mode caused by localization of optical phonons agree well with that calculated by using the modified spatial correlation model.
Control of subband energy levels of quantum dots using InGaAs gradient composition strain-reducing layer
Authors: Amano T. , Yamauchi S. , Sugaya T. , Komori K.
Published: August 2006

We propose a method to control the subband energy levels of quantum dots (QDs) using an InGaAs gradient composition strain-reducing layer (GC-SRL). A large band shift of 70meV was realized using a GC-SRL at the fourth-order energy level in both the samples. In addition, the QDs with and without a GC-SRL exhibited an exponential and constant increase in the subband space, respectively. These results indicate square-well-shaped and crucible-shaped potential band structures. The GC-SRL enabled the control of not only the subband energy but also the confinement energy of these potential structures.
Dipole active vibrations and dipole moments of N2 and O2 physisorbed on a metal surface
Authors: Gustafsson K. , Andersson S.
Published: August 2006

We have, in infrared reflection absorption measurements, observed narrow dipole active absorption lines associated with the fundamental internal vibrational transitions of N2 and O2 physisorbed at 30K on the chemically inert Pt(111)(1×1)H surface. Such transitions are forbidden for free homonuclear molecules and become dipole active at a metal surface due to polarization induced surface dipole moments. The measurements show that the internal stretch vibration frequencies are lowered by 7–8cm−1 relative to the gas phase values. The measured static and dynamic dipole moments are in the ranges of 0.06–0.07 and 0.001–0.002D, respectively. We find that good estimates of the induced dynamic as well as the static dipole moments can in general be obtained from a van der Waals model but that the ratios of the measured static and dynamic moments indicates a need for a refinement of the dipole moment function.
Study of the validity of the phase-integral connection formula for potential barriers of arbitrary thickness
Authors: Thylwe Karl-Erik , Yngve Staffan , Fröman Per Olof
Published: August 2006

The validity of the phase-integral connection matrix for potential barriers is discussed in a concrete way for a particular parabolic barrier, for the symmetric Eckart-Epstein barrier, and for the inverted Morse potential, when the first-order phase-integral approximation is used.
Electromagnetism theory of waveguide array electro-optical scanner: Model and characteristics of optical field distribution in the waveguide array
Authors: Li Jiali , Shi Shunxiang , Liu Jifang , Sun Yanling , Dong Hongzhou , Liang Huawei
Published: August 2006

Waveguide array electro-optical scanner (WAEOS) technology is presented. Based on the theory of the dielectric slab optical waveguide and the coupled wave equations of infinite waveguide array given by Yariv and Yeh [Optical Waves in Crystals (Wiley, New York, 1983)], the coupled wave equations are modified for adapting the finite waveguide array of the WAEOS. A model of the optical field distribution in the waveguide array is derived, which determines the output field of the WAEOS. Some characteristics of the optical field distribution with uniform incidence and single waveguide incidence are given.
A two-dimensional hybrid model of the Hall thruster discharge
Authors: Parra F. I. , Ahedo E. , Fife J. M. , Martínez-Sánchez M.
Published: August 2006

Particle-in-cell methods are used for ions and neutrals. Probabilistic methods are implemented for ionization, charge-exchange collisions, gas injection, and particle-wall interaction. A diffusive macroscopic model is proposed for the strongly magnetized electron population. Cross-field electron transport includes wall collisionality and Bohm-type diffusion, the last one dominating in most of the discharge. Plasma quasineutrality applies except for space-charge sheaths, which are modeled taking into consideration secondary-electron-emission and space-charge saturation. Specific weighting algorithms are developed in order to fulfil the Bohm condition on the ion flow at the boundaries of the quasineutral domain. The consequence is the full development of the radial plasma structure and correct values for ion losses at lateral walls. The model gains in insight and physical consistency over a previous version, but thrust efficiency is lower than in experiments, indicating that further model refinement of some phenomena is necessary.
The influence of different species’ granular temperatures on segregation in a binary mixture of dissipative grains
Authors: Yoon David K. , Jenkins James T.
Published: August 2006

We employ a kinetic theory for binary mixtures of slightly inelastic, frictionless particles to study segregation in a uniformly agitated system under gravity, where large particles are dilute in a dense gas of small particles. We take the initial motion of large particles as an indicator of segregation. By incorporating the effects of two different granular temperatures, we show that, although the temperature differences can be significant with increasing differences in the mass or size of the particles, the segregation is not greatly affected. However, we find that there is a small range in size and material density ratios where the initial direction of segregation from a well-mixed state is reversed due to nonequipartition.
The electro-optical and electrochromic properties of electrolyte-liquid crystal dispersions
Authors: Cupelli Daniela , De Filpo Giovanni , Chidichimo Giuseppe , Nicoletta Fiore Pasquale
Published: August 2006

Liquid crystals are known to exhibit a reversible color change by applying a direct current electric field, if a small amount of quaternary ammonium salts is dissolved into them. Applications of such an electrochromic liquid crystal cell have been proposed as interesting laser-addressed writing and image storage devices. Liquid crystal dispersions are composite materials formed by liquid crystal droplets embedded in either a polymer or a monomer matrix. Thin films of liquid crystal dispersions can be turned from an opaque to a transparent state by application of a suitable alternating current electric field. Herein, we report our investigations on electrolyte-liquid crystal dispersions, which show independent electro-optical and electrochromic properties characterized by fast bleaching times. This cell involves the reorientation of liquid crystal molecules, trapped in droplets, for the electro-optical changes from the opaque to transparent state and the formation of complexes at the cathode, between the positive ions of electrolyte and liquid crystal dispersed in the matrix, for the electrochromic changes from the bleached to colored state. The device is able to change its electro-optical transmittance within few milliseconds and its color within few seconds.
Cooling field dependence of exchange bias in phase-separated La0.88Sr0.12CoO3
Authors: Tang Yan-kun , Sun Young , Cheng Zhao-hua
Published: August 2006

We report the observation of exchange bias phenomena in the hole-doped perovskite cobaltite La0.88Sr0.12CoO3 in which a spontaneous phase separation occurs. When the sample is cooled in a static magnetic field through a freezing temperature, the magnetization hysteresis loops shift to the negative field. Moreover, the exchange bias strongly depends on the cooling field. These results highlight the important role of a glassy interface between the intrinsic inhomogeneous phases in a phase-separated system.
Rossiter’s formula: A simple spectral model for a complex amplitude modulation process?
Authors: Delprat Nathalie
Published: August 2006

Spectral analysis of pressure signals recorded for high subsonic compressible flows past open cavities generally reveals frequency components of strong magnitudes (Rossiter modes). These components are not multiples of each other in the case of shallow cavities. This nonharmonic property is examined through a nonlinear modeling of the spectral distributions. For that purpose, a signal processing interpretation of the Rossiter formula is developed, which combines nonlinear distortion and amplitude modulation. The consequences of this new interpretation on the prediction of frequencies, on mode identification, and on mode interactions are discussed as well as the underlying physical mechanisms.
Effect of threading dislocation density on Ni∕n-GaN Schottky diode I-V characteristics
Authors: Arehart A. R. , Moran B. , Speck J. S. , Mishra U. K. , DenBaars S. P. , Ringel S. A.
Published: August 2006

The impact of threading dislocation density on Ni∕n-GaN Schottky barrier diode characteristics is investigated using forward biased current-voltage-temperature (I-V-T) and internal photoemission (IPE) measurements. Nominally, identical metal-organic chemical vapor deposition grown GaN layers were grown on two types of GaN templates on sapphire substrates to controllably vary threading dislocation density (TDD) from 3×107to7×108cm−2. I-V-T measurements revealed thermionic emission to be the dominant transport mechanism with ideality factors near 1.01 at room temperature for both sample types. The Schottky barrier heights showed a similar invariance with TDD, with measured values of 1.12–1.13eV obtained from fitting the I-V-T results to a thermionic emission-diffusion model. The I-V-T results were verified by IPE measurements made on the same diodes, confirming that the Ni∕n-GaN barrier heights do not show a measurable TDD dependence for the TDD range measured here. In apparent contrast to this result is that the measured forward bias I-V characteristics indicate a shift in the observed forward bias turn-on voltage such that at the higher TDD value investigated here, a larger turn-on voltage (lower current) is observed. This difference is attributed to localized current blocking by high potential barrier regions surrounding threading dislocations that intersect the Ni∕GaN interface. A simple model is presented that reconciles both the observed voltage shift and variations in the extracted Richardson constant as a function of threading dislocation density. With this model, an average local barrier surrounding dislocation of ∼0.2V is obtained, which diverts current flow across the forward biased Schottky interface to nondislocated regions.
Multibit memories using a structure of SiO2/partially oxidized amorphous Si∕HfO2
Authors: Park Sangjin , Cha Young-Kwan , Cha Daigil , Park Youngsoo , Yoo In-Kyeong , Lee Jung-Hyun , Seol Kwang Soo , Choi Suk-Ho
Published: August 2006

Memory capacitors with a structure of SiO2/partially oxidized amorphous Si (a-Si)/HfO2 have been prepared by sequential processes: atomic layer deposition (ALD) of 6nm a-Si on 3.5nm SiO2, thermal oxidation at 900°C, and another ALD of 12nm HfO2. The memory devices offer hybrid type of charge memory: the interface states of partially oxidized a-Si∕SiO2 tend to act as hole traps, resulting in a negative shift of flatband voltage in capacitance-voltage (C-V) curve, and the partially oxidized a-Si∕HfO2 interface has dominantly electron-trap centers, leading to a positive voltage shift. By this hybrid effect, the memory window in C-V curve is observed to be enlarged enough to realize four-level (2bit) memories, which is demonstrated through measurements of program/erase speeds and charge-loss rates.
Mechanisms for the activation of ion-implanted Fe in InP
Authors: Cesca T. , Verna A. , Mattei G. , Gasparotto A. , Fraboni B. , Impellizzeri G. , Priolo F.
Published: August 2006

In this paper we present structural and electrical investigations on high temperature Fe-implanted InP. The aim of the work is to relate the lattice position of the implanted atoms after annealing treatments (from 300to600°C) with their electrical activation as compensating deep traps and to draw a comprehensive picture of the activation mechanisms. The overall results demonstrate that the electrical behavior and the Fe2+ deep trap activation properties are strictly connected to the annealing evolution of the implant-induced damage and to the escape process of the Fe atoms from substitutional sites, which in turn is controlled by the background doping density in the substrates.
Role of hole playing in improving performance of organic light-emitting devices with an Al2O3 layer inserted at the cathode-organic interface
Authors: Zhang S. T. , Zhou Y. C. , Zhao J. M. , Zhan Y. Q. , Wang Z. J. , Wu Y. , Ding X. M. , Hou X. Y.
Published: August 2006

The role of hole playing in improving electron injection in the presence of an Al2O3 layer at the organic-cathode interface is discussed. It is deduced that, according to the model of tunneling barrier reduction and the carrier transporting mechanism in organic light-emitting devices, electron injection will be enhanced only if holes are injected and accumulated at the organic-buffer interface. The validity of this analysis is well confirmed by experimental results. The observed abnormal characteristic of operating voltage varying with the Al2O3 layer thickness and the efficiency improvement are also well explained by the model.
Composition and orientation dependence of dielectric and piezoelectric properties in poled Pb(Mg1∕3Nb2∕3)O3–PbTiO3 crystals
Authors: Feng Zuyong , Zhao Xiangyong , Luo Haosu
Published: August 2006

Temperature and orientation dependence of dielectric constant and discharging current was investigated in the poled single crystals of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 with x varying from 0.24 to 0.38, furthermore, composition and orientation dependence of the room temperature piezoelectric constant d33 was also summarized. Temperature, frequency, orientation, and composition dependence of the dielectric constant in the poled crystals shows that domain configurations and phase transitions in the poled crystals strongly depend on both composition and orientation. Compared to the monodomain crystals, the poled polydomain crystals, especially near the morphotropic phase boundary, presented large piezoelectric and dielectric susceptibilities. High piezoelectric response of these ⟨001⟩ poled crystals with x between 0.28 and 0.30 in a broad temperature scope makes them the most promising candidate for potential practical applications.
Doping induced surface chemistry dictating the characteristics of Schottky contacts to III-V nitride semiconductors
Authors: Alam J. , Noor Mohammad S.
Published: August 2006

While lightly doped semiconductors are preferred for Schottky contacts, heavily doped semiconductors are preferred for Ohmic contacts. The upper limit of doping for Schottky contacts and the lower limit of doping for Ohmic contacts have not, however, been quantified. To address this problem, the influence of doping induced surface chemistry on the electrical characteristics of Schottky diodes has been studied. Hall measurement, current-voltage measurements, and transmission electron microscopy have been performed. The most remarkable observation of the study is a relationship between the surface dislocation density and the ideality factor of the Schottky diodes. The observation leads to a conclusion that the upper limit of the semiconductor doping for Schottky diodes should be such that it leads to negligible barrier height reduction without the creation of excessive surface dislocation density.
Using a family of dividing surfaces normal to the minimum energy path for quantum instanton rate constants
Authors: Li Yimin , Miller William H.
Published: August 2006

One of the outstanding issues in the quantum instanton (QI) theory (or any transition-state-type theory) for thermal rate constants of chemical reactions is the choice of an appropriate “dividing surface” (DS) that separates reactants and products. (In the general version of the QI theory, there are actually two dividing surfaces involved.) This paper shows one simple and general way for choosing DSs for use in QI theory, namely, using the family of (hyper) planes normal to the minimum energy path on the potential energy surface at various distances s along it. Here the reaction coordinate is not one of the dynamical coordinates of the system (which will in general be the Cartesian coordinates of the atoms), but rather simply a parameter which specifies the DS. It is also shown how this idea can be implemented for an N atom system in three-dimensional space in a way that preserves overall translational and rotational invariance. Numerical application to a simple system (the collinear H+H2 reaction) is presented to illustrate the procedure.
Multiferroicity in polarized single-phase Bi0.875Sm0.125FeO3 ceramics
Authors: Yuan G. L. , Or Siu Wing
Published: August 2006

Single-phase Bi0.875Sm0.125FeO3 ceramics were prepared, and their crystal structure and multiferroic properties were studied. The triclinic structure with P1 space group was confirmed in the ceramics by refining the x-ray diffraction data. Large piezoelectric d33 coefficient of 29pC∕N, together with high remnant polarization of 15.09μC∕cm2, was measured at room temperature, suggesting the existence of long-range ferroelectric order on a macroscopic scale. The observed small remnant magnetization of 0.071emu∕g at room temperature as a result of the collapse of the space-modulated spin structure indicated the presence of long-range canted antiferromagnetic order on a macroscopic scale. The coexistence of the long-range ferroelectric and canted antiferromagnetic orders allowed the magnetoelectric effect below the antiferromagnetic Néel temperature of 265°C near which magnetoelectric coupling was obvious.
Multicomponent diffusion revisited
Authors: Lam S. H.
Published: August 2006

The derivation of the multicomponent diffusion law is revisited. Following Furry [Am. J. Phys. 16, 63 (1948)], Williams [Am. J. Phys. 26, 467 (1958); Combustion Theory, 2nd ed. (Benjamin/Cummings , Menlo Park, CA,1985)] heuristically rederived the classical kinetic theory results using macroscopic equations, and pointed out that the dynamics of the mixture fluid had been assumed inviscid. This paper generalizes the derivation, shows that the inviscid assumption can easily be relaxed to add a new term to the classical diffusion law, and the thermal diffusion term can also be easily recovered. The nonuniqueness of the multicomponent diffusion coefficient matrix is emphasized and discussed.
Transitional and weakly turbulent flow in a rotating magnetic field
Authors: Stiller J. , Fraňa K. , Cramer A.
Published: August 2006

The early stage of turbulent flow driven by a rotating magnetic field is studied via direct numerical simulations and electric potential measurements for the case of a cylindrical geometry. The numerical results show that the undisturbed flow remains stable up to the linear stability limit (Tac), whereas small perturbations may initiate a nonlinear transition at subcritical Taylor numbers. The observed instabilities occur randomly as isolated pairs of Taylor-Görtler vortices, which grow from spots to long tubes until they are dissipated in the lid boundary layers. At 7.5Tac, the flow is governed by large-scale three-dimensional fluctuations and may be characterized as weakly turbulent. Taylor-Görtler vortices provide the major turbulence mechanism, apart from oscillations of the rotation axis. As the vortices tend to align with the azimuthal direction, they result in a locally two-dimensional turbulence pattern.
Exact solutions for transient mixing of two gases of different densities
Authors: Sánchez Antonio L. , Vera Marcos , Liñán Amable
Published: August 2006

This Brief Communication presents a number of exact solutions describing the transient mixing of two gases of different molecular weights. Descriptions are given for both the concentration field and the associated induced motion in one-dimensional spherical, cylindrical, and planar configurations, including mixing layers, pockets, coflow jets, and concentrated mass sources.
Mixing characteristics and structure of a turbulent jet diffusion flame stabilized on a bluff-body
Authors: Kim Seung Hyun , Pitsch Heinz
Published: August 2006

Flow dynamics, scalar mixing, and pollutant formation in a turbulent jet diffusion flame stabilized on a bluff-body are investigated using large-eddy simulation. The density weighted filtered equations for the flow and mixing fields are solved using dynamic models for the subfilter quantities. Subfilter combustion processes are modeled by the conditional filtering method. An integrated formulation that considers only axial variation of conditionally filtered quantities is presented. Results show that vortex shedding from the coflow stream and its interaction with the high speed main jet play an important role in the generation of high dissipation layers in the intense mixing region. The mechanisms that generate the high dissipation layers in the intense mixing region are identified. The relatively uniform composition of combustion products in the recirculation zone helps the flame stabilization by maintaining low scalar dissipation rate and high temperature in the vicinity of the stoichiometric surfaces. The present integrated formulation is shown to reproduce these characteristics of the mixing field and to predict the flame structure and NO formation well. The weighted integral formulation of the conditional velocity allows the entrainment of the combustion products in the intense mixing region into the recirculation zone. The proper prediction of low conditional scalar dissipation in the recirculation zone is shown to be crucial for accurately describing the stabilization process. The decrease of NO at the end of the recirculation zone is reproduced due to the well-predicted mixing characteristics.
Yang–Mills fields on CR manifolds
Authors: Barletta Elisabetta , Dragomir Sorin , Urakawa Hajime
Published: August 2006

We study pseudo Yang–Mills fields on a compact strictly pseudoconvex CR manifold M, i.e., the critical points of the functional PYM(D)=12∫M∥πHRD∥2θ∧(dθ)n, where D is a connection in a Hermitian CR holomorphic vector bundle (E,h)→M. Let Ω={φ<0}⊂Cn be a smoothly bounded strictly pseuodoconvex domain and g the Bergman metric on Ω. We show that boundary values Db of Yang–Mills fields D on (Ω,g) are pseudo Yang–Mills fields on ∂Ω, provided that iTRDb=0 and iNRD=0 on H(∂Ω). If S1→C(M)→πM is the canonical circle bundle and π*D is a Yang–Mills field with respect to the Fefferman metric Fθ of (M,θ) then D is a pseudo Yang–Mills field on M. The Yang–Mills equations δπ*DRπ*D=0 project on the Euler–Lagrange equations δbDRD=0 of the variational principle δPYM(D)=0, provided that iTRD=0. When M has vanishing pseudohermitian Ricci curvature the pullback π*D of the (CR invariant) Tanaka connection D of (E,h) is a Yang–Mills field on C(M). We derive the second variation formula {d2PYM(Dt)∕dt2}t=0=∫M⟨SbD(φ),φ⟩θ∧(dθ)n, Dt=D+At [provided that D is a pseudo Yang–Mills field and φ≡{dAt∕dt}t=0∊Ker(δD)], and show that SbD(φ)≡ΔbDφ+RbD(φ), φ∊Ω0,1[Ad(E)], is a subelliptic operator.
SiGe amorphization during Ge condensation in silicon germanium on insulator
Authors: Balakumar S. , Lo G. Q. , Tung C. H. , Kumar R. , Balasubramanian N. , Kwong D. L. , Ong C. S. , Li M. F.
Published: August 2006

High temperature oxidation and annealing are found to be desirable to achieve a uniform germanium (Ge) profile in silicon germanium on insulator (SGOI) layers formed using germanium condensation technique. This work investigates the amorphization mechanism, which was considered as an undesirable consequence of lowering the temperature during Ge condensation when oxidations are conducted at 900 or 950°C. A dual layer SGOI stack, composed of an amorphous layer above a crystalline layer, was obtained at low temperature condensation as characterized using transmission electron microscope and Auger analysis. A possible mechanism is presented. Si0.4Ge0.6OI and Si0.25Ge0.75OI layers can be achieved without amorphization through oxidation/annealing at 1050°C and two oxidations at 1050 and 1000°C, respectively.
The role of streamwise perturbations in pipe flow transition
Authors: Mellibovsky Fernando , Meseguer Alvaro
Published: August 2006

The phenomenon of subcritical transition in Hagen-Poiseuille or pipe flow is explored for a wide range of Reynolds numbers within the interval Re∊[2.5×103,1.26×104] by means of a computational method that numerically resolves the transitional dynamics with nearly 3.5×104 degrees of freedom on a medium aspect-ratio domain of length 32π∕5. The aim of this exploration is to provide a theoretical characterization of the basin of attraction of the basic regime by measuring the minimal amplitude of an initial global perturbation leading to transition. The analysis is based on a particular theoretical scenario that considers streamwise-independent finite amplitude initial vortical perturbations that trigger global transition via optimal inflectional instabilities of streamwise-dependent modes with selected axial wave numbers. Disturbances consisting of 1, 2, and 3 pairs of vortices are investigated. Special attention is given to relaminarization phenomena that is frequently observed for low Reynolds numbers. Long lasting turbulent regimes and relaminarized flows are distinguished by means of time integrations of suitable length between Tmin=600 and Tmax=1000 advective time units. Some transitional runs are specifically analyzed to exemplify the transition scenario under investigation and its independence of pipe length is verified with a few computations on a longer pipe of length 32π (1.4×105 degrees of freedom). For large values of the Reynolds number, a theoretical scaling law for the threshold amplitude of a perturbation required to trigger transition is provided. Different types of perturbations seem to respond to different scaling laws.
Hydrodynamic and hydromagnetic energy spectra from large eddy simulations
Authors: Haugen Nils Erland L. , Brandenburg Axel
Published: August 2006

Direct and large eddy simulations of hydrodynamic and hydromagnetic turbulence have been performed in an attempt to isolate artifacts from real and possibly asymptotic features in the energy spectra. It is shown that in a hydrodynamic turbulence simulation with a Smagorinsky subgrid scale model using 5123 mesh points, two important features of the 40963 simulation on the Earth simulator [Y. Kaneda et al., Phys. Fluids 15, L21 (2003)] are reproduced: a k−0.1 correction to the inertial range with a k−5∕3 Kolmogorov slope and the form of the bottleneck just before the dissipative subrange. Furthermore, it is shown that, while a Smagorinsky-type model for the induction equation causes an artificial and unacceptable reduction in the dynamo efficiency, hyper-resistivity yields good agreement with direct simulations. In the large-scale part of the inertial range, an excess of the spectral magnetic energy over the spectral kinetic energy is confirmed. However, a trend toward spectral equipartition at smaller scales in the inertial range can be identified. With magnetic fields, no explicit bottleneck effect is seen.
Mg
B
2
tunnel junctions with native or thermal oxide barriers
Authors: Singh R. K. , Gandikota R. , Kim J. , Newman N. , Rowell J. M.
Published: August 2006

Mg B 2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers therefore survive the deposition of the second electrode at 300°C, even over junction areas of ∼1mm2. Studies of such junctions and those of the type MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers grown on MgB2 exhibit a wide range of barrier heights and widths.
Partition selection in multiscale turbulence modeling
Authors: Ramakrishnan Srinivas , Collis S. Scott
Published: August 2006

The variational multiscale (VMS) method for large-eddy simulation (LES) is a promising new approach that employs variational projection to achieve a priori scale separation in lieu of traditional spatial filtering. However, depending on the numerical method used, VMS may not be convenient in all spatial directions. We apply the VMS methodology to a numerical method that does not support explicit scale separation in the wall-normal direction for turbulent channel flow. Similar to the common LES practice of filtering only in the planes, variational projection is performed only in the planes and this strategy is found to be as successful as the full VMS method. However, in all VMS approaches, the partition between the large and small scales and the overall resolution are crucial parameters for obtaining quality solutions. By applying scale separation in just one of the coordinate directions, we have developed a consistent method for partition and resolution selection in channel flow that is related to the physical structures in the near-wall region. The idea behind this approach can, potentially, be extended for informed VMS parameter selection in other flows.
A minimal multiscale Lagrangian map approach to synthesize non-Gaussian turbulent vector fields
Authors: Rosales Carlos , Meneveau Charles
Published: August 2006

A simple method is proposed to generate synthetic vector fields as surrogates for turbulent velocity fields. The method is based on the minimal Lagrangian map, by which an initial Gaussian field generated using random-phase Fourier modes is deformed. The deformation is achieved by moving fluid particles of a sequence of low-pass filtered fields at their fixed velocity for some scale-dependent time-interval, interpolating onto a regular grid, and imposing the divergence-free condition. Statistical analysis shows that the resultant non-Gaussian field displays many properties commonly observed in turbulence, ranging from skewed and intermittent velocity gradient and increment probability distributions, preferential alignment of vorticity with intermediate strain rate, and nontrivial vortex stretching statistics. Differences begin to appear only when interrogating the data with measures associated with intense vortex tubes that are conspicuously absent in the synthetic field. To explore the dynamical implications of these observations, the synthetic non-Gaussian fields are used as initial conditions in DNS and LES of decaying isotropic turbulence, and results are compared with initializations using Gaussian fields. The non-Gaussian synthetic fields yield more realistic results with significantly shortened initial adjustment periods.
Translation and oscillation of a bubble under axisymmetric deformation
Authors: Shaw Stephen J.
Published: August 2006

In this work the nonlinear interactions between the axisymmetric shape distortions, the axial translational motion, and the volume oscillations of a gas bubble in an inviscid, incompressible liquid are considered. Representing the surface deformation by a complete set of Legendre polynomials and assuming that both this deformation and the translational motion are small, a Lagrangian energy formulation is used to derive a system of equations valid to third order in these interaction terms. The effects of surface tension and pressure are also accounted for to this order. No restriction is placed on the size of the volume oscillations. Examination of the translational motion equation indicates that while at second order the interaction of two neighboring shape modes can cause the bubble to move, at third order any combination of three odd shape modes or one odd and two even shape modes can result in the bubble moving. Third order interactions between any three even shape modes or one even and two odd modes will contribute to the volume oscillations of the bubble. Restricting attention to the first fifteen shape modes, a truncated subset of equations is obtained. For this system, the bubble is given an initial static deformation in shape and the process by which the other modes become excited is considered in detail for some chosen examples. The role of second and third order interactions is identified and general relations which determine which terms can interact to excite a given shape mode are given.
Rotational dynamics of a diatomic solute in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate
Authors: Shim Youngseon , Jeong Daun , Choi M. Y. , Kim Hyung J.
Published: August 2006

Reorientational time correlation functions Cl(t)(≡⟨Pl[cosθ(t)]⟩) for a diatomic solute in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF6−) are analyzed via molecular dynamics computer simulations, where ⟨…⟩ denotes an equilibrium ensemble average, Pl the lth order Legendre polynomial and θ(t) the angle between the solute orientation at time t and its initial direction. Overall results are indicative of heterogeneous dynamics in EMI+PF6−. For a small nondipolar solute, Cl(t) are well-described as stretched exponential functions in wide time ranges. One striking feature is that after rapid initial relaxation, C2(t) decays more slowly than C1(t). As a result, the correlation time associated with the former is considerably longer than that with the latter. This is ascribed to solvent structural fluctuations, which allow large-amplitude solute rotations. As the solute size grows, relaxation of Cl(t) approaches exponential decay.
Gallium/aluminum nanocomposite material for nonlinear optics and nonlinear plasmonics
Authors: Krasavin A. V. , MacDonald K. F. , Schwanecke A. S. , Zheludev N. I.
Published: August 2006

We report on a new type of composite metallic structure for nonlinear optics and nonlinear plasmonics, created by grain boundary penetration of gallium into an aluminum film. These composite films form mirrorlike interfaces with silica and show an exceptionally broadband phase-transition-based nonlinear response to optical excitation.
Optimized fabrication and electrical analysis of silver nanowires templated on DNA molecules
Authors: Park Sung Ha , Prior Matthew W. , LaBean Thomas H. , Finkelstein Gleb
Published: August 2006

We report on the electrical conductivity measurement of silver nanowires templated on native λ-bacteriophage and synthetic double-stranded DNA molecules. After an electroless chemical deposition, the metallized DNA wires have a diameter down to 15nm and are among the thinnest metallic nanowires available to date. Two-terminal I-V measurements demonstrating various conduction behaviors are presented. DNA templated functional nanowires may, in the near future, be targeted to connect at specific locations on larger-scale circuits and represent a potential breakthrough in the self-assembly of nanometer-scale structures for electronics layout.
Crystallization of helical oligomers with chirality selection. I. A molecular dynamics simulation for bare helix
Authors: Yamamoto Takashi
Published: August 2006

Helical polymers often exhibit pronounced chirality recognition during crystallization. By molecular dynamics simulation, we have already shown that the helical polymers crystallize with or without marked chirality selection depending on structural details of the polymer molecules. We have there classified the helical polymers into two categories: the bare helices made of only backbone atoms which show rather tolerant chirality selection, and the general helices with large side groups showing strict chirality recognition. Polymer crystallization is in general largely hampered and retarded by slow dynamics of the entangled chains, and therefore short helical oligomers are very suitable models for studying the chiral crystallization. We here report on molecular simulations of crystallization in the bare helical oligomer molecules by the use of Monte Carlo and molecular dynamics simulations. First we confirm the low temperature chiral crystal phase and the reversible order-disorder transition. We also observe frequent inversions of the helical sense, and the helix reversal defects propagating along the chains. Then we investigate crystallization from the melt into the chiral crystal phase. We find that the crystallization rate depends very sensitively on the degree of undercooling. The crystallization is found to be the first order transition that conforms well to the traditional picture of crystal growth in small molecules. Even when the crystallization directly into the chiral crystal phase is conducted, marked chirality selections are not observed at the early stage of crystallization; the chains adhere to the crystal surfaces selecting their helical senses rather at random resulting in racemic crystallites. The isothermal crystallization for a sufficiently long time, however, yields lamellar crystals composed of well-developed chiral domains, the growth of which seems to be accomplished through the transition back into the ordered chiral crystal phase.
Control of the spin Hall current in two dimensional electronic gas
Authors: Cheche T. O. , Barna E.
Published: August 2006

The intrinsic spin Hall conductivity is obtained for a two-dimensional electronic gas (2DEG) in the presence of strain, Rashba coupling, and an external in-plane applied magnetic field. The conduction electrons of [001] oriented quantum well are used to model the 2DEG. The spin current value is dependent on the stress applied in direction [111].
Suppression of thermocapillary instability in a falling film
Authors: Frank A. M.
Published: August 2006

The ability of feedback control to suppress a thermocapillary instability in a film flow is investigated numerically. The film is falling down a plate with the built-in heater that provides significant streamwise temperature gradient. This is known to induce spanwise instability leading to rivulet flow that inhibits heat transfer and causes film rupture. To prevent this instability, a feedback loop controlling the temperature of the heater is introduced. Either film surface elevation or surface temperature is utilized as an input feedback signal. It is shown that control can considerably flatten the film surface and provide a twofold increase of the threshold temperature for the film rupture.
How kinetics drives the two- to three-dimensional transition in semiconductor strained heterostructures: The case of InAs∕GaAs(001)
Authors: Arciprete F. , Placidi E. , Sessi V. , Fanfoni M. , Patella F. , Balzarotti A.
Published: August 2006

The two- to three-dimensional growth mode transition in the InAs∕GaAs(001) heterostructure has been investigated by means of atomic force microscopy. The kinetics of the density of three-dimensional islands indicates two transition onsets at 1.45 and 1.59 ML of InAs coverage, corresponding to two separate families, small and large dots. According to the scaling analysis and volume measurements, the transition between the two families of quantum dots and the explosive nucleation of the large ones is triggered by the erosion of the step edges.
Near infrared absorption and room temperature photovoltaic response in AlN∕GaN superlattices grown by metal-organic vapor-phase epitaxy
Authors: Baumann E. , Giorgetta F. R. , Hofstetter D. , Golka S. , Schrenk W. , Strasser G. , Kirste L. , Nicolay S. , Feltin E. , Carlin J. F. , Grandjean N.
Published: August 2006

We report on intersubband absorption of near infrared radiation in AlN∕GaN superlattice structures grown by metal-organic vapor-phase epitaxy. A good correlation between well thickness and absorption peak energy was obtained. One sample shows a photovoltaic signal which overlaps well with the corresponding absorption curve at around 1.5μm (830meV), a common wavelength in optical fiber telecommunication systems. This photovoltaic signal is strongest at temperatures around 75K and persists up to room temperature. The frequency response of this sample was measured with a modulated 1.5μm laser diode. The amplitude of the response was highest for a frequency of 36kHz.
Nondipolar liquid migration on carbon nanotube films at low field
Authors: Kuo H.-F. , Lien D.-H. , Hsu W.-K.
Published: August 2006

Thin films made by NH4OH treated single-walled carbon nanotubes exhibit a hydrophilic property and are capable of transporting nondipolar droplet at low field. The underlying mechanism is discussed and verified.
Evidence of electron and hole inversion in GaAs metal-oxide-semiconductor capacitors with HfO2 gate dielectrics and α-Si∕SiO2 interlayers
Authors: Koester S. J. , Kiewra E. W. , Sun Yanning , Neumayer D. A. , Ott J. A. , Copel M. , Sadana D. K. , Webb D. J. , Fompeyrine J. , Locquet J.-P. , Marchiori C. , Sousa M. , Germann R.
Published: August 2006

Evidence of inversion in GaAs metal-oxide-semiconductor capacitors with HfO2 gate dielectrics and α-Si∕SiO2 interlayers is reported. Capacitors formed on n-GaAs with atomic layer-deposited HfO2 displayed C-V characteristics with minimum Dit of 7×1011cm−2∕eV, while capacitors with molecular beam epitaxy-deposited HfO2 on p-GaAs had Dit=3×1012cm−2∕eV. Lateral charge transport was confirmed using illuminated C-V measurements on capacitors fabricated with thick Al electrodes. Under these conditions, capacitors on n-GaAs (p-GaAs) showed “low-frequency” C-V behavior, indicated by a sharp capacitance increase and saturation at negative (positive) gate bias, confirming the presence of mobile charge at the semiconductor/dielectric interface.
Clarification of Mn–Zn interaction for InMnP:Zn epilayer by photoluminescence and x-ray photoelectron spectroscopy
Authors: Shon Yoon , Lee Sejoon , Jeon H. C. , Lee S.-W. , Kim D. Y. , Kang T. W. , Kim Eun Kyu , Yoon Chong S. , Kim C. K. , Park Y. J. , Lee Jeoung Ju
Published: August 2006

Transition related to the Mn–Zn interaction was observed in photoluminescence (PL) study of the InMnP:Zn epilayer and the peak position blueshifted with increasing Mn concentration. X-ray photoelectron spectroscopy was used to clarify the blueshift of the PL peak. The binding energy shifts of Mn 2p and Zn 2p core levels indicative of the interaction between Mn and Zn were observed. This mutual interaction between Mn 2p and Zn 2p agrees with the result that the Mn-related transition in InMnP:Zn codoped with Zn is shifted to the higher energy region in comparison with InMnP without additional doping of Zn.
Quasipassive positioning platform for nanoscale management of in-plane motion
Authors: Li Biao , Zhu Yu , Sharon Andre
Published: August 2006

As leading edge technology pursues a common trend of working on smaller and smaller scales, there is increasing demand on the motion management at the nanometer range. In this letter, we report a two-axis quasipassive positioning device capable of four degrees of freedom in-plane motion. The concept comprises of a platform suspended by tensile stressed flexure elements on either side. By selectively trimming the stress elements, the equilibrium position can be biased to one side or another, enabling nanoscale movement between the suspended platform and the base. Focused ion beam experiment demonstrates that such platform enables positioning accuracy on the order of tens of nanometers.
On the role of frustration on the glass transition and polyamorphism of mesoscopically heterophase liquids
Authors: Bakai A. S.
Published: August 2006

The model of heterophase fluctuations is developed accounting frustration of the mesoscopic solidlike fluctuons. Within the framework of this model, the glass transition and polyamorphous transformations are considered. It is shown that the frustration increases the temperature range in which the heterophase liquid state exists. the upper and lower boundaries of this temperature range are determined. These boundaries separate different phase states—amorphous solid, heterophase liquid, and fluid phases. Polyamorphous liquid-liquid transitions in the liquid are investigated. Frustration can call forth continuous fluid-solid phase transformation avoiding the first- or second-order phase transition. Conditions under which the first-order phase transition fraction takes place are formulated. Two scenarios of the first-order liquid-liquid polyamorphous transformation are described. As an example the glacial phase formation and the first-order liquid-liquid phase transition in triphenyl phosphate are considered and discussed. Impact of frustration on the liquid crystallization and crystallinity of the glassy state is studied.
Photoluminescence evaluation of defects generated during SiGe-on-insulator virtual substrate fabrication: Temperature ramping process
Authors: Wang Dong , Ii Seiichiro , Nakashima Hiroshi , Ikeda Ken-ichi , Nakashima Hideharu , Matsumoto Koji , Nakamae Masahiko
Published: August 2006

Crystal qualities of Si∕SiGe∕Si-on-insulator structures with different SiGe thicknesses were evaluated by photoluminescence (PL). The wafers were annealed at different temperatures with a ramping rate of 5°C∕min. Free exciton PL peaks were clearly observed for the as-grown wafers and decreased with an increase in the annealing temperature. For the annealed wafers, defect-related PL signals were observed at around 0.82, 0.88, 0.95, and 1.0eV, which varied according to the annealing temperature and the SiGe thickness. They were also correlated to dislocation-related defects by transmission electron microscopy.
Measuring plasmon-resonance enhanced third-harmonic χ(3) of Ag nanoparticles
Authors: Liu Tzu-Ming , Tai Shih-Peng , Yu Che-Hang , Wen Yu-Chieh , Chu Shi-Wei , Chen Lung-Jin , Prasad Muppa Ramakrishna , Lin Kuan-Jiuh , Sun Chi-Kuang
Published: August 2006

By coinciding the plasmon frequency with the third-harmonic frequency of the excitation light, the authors determined the plasmon-resonance enhanced optical third-harmonic-generation (THG) susceptibility of a polyvinylpyrrolidone-coated Ag nanoparticle with a 5–7nm diameter. With dispersed Ag nanoparticles on a quartz surface and through measuring the frequency dependent THG intensities, interface THG showed evident enhancement when the third harmonic of excitation matched the Ag-nanoparticle’s plasmon-resonant frequency. According to the effective medium theory and by analyzing the interface THG under focused Gaussian beams, the ensemble-averaged χ(3)(3ω:ω,ω,ω) of a Ag nanoparticle can be estimated to be on the order of 2×10−11esu.