bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of the average molecular weight and the concentration of plasticizer on the orientational dynamics of chromophores in guest-host polymers
Authors: Ribierre J. C. , Mager L. , Gillot F. , Fort A. , Méry S.
Published: September 2006

We report experimental results obtained from dielectric spectroscopy, electro-optic, and second harmonic generation dynamics measurements on different polymers doped with nonlinear optical chromophores. The polymers considered in this work are polyvinylcarbazole plasticized by ethylcarbazole and polystyrenes with different average molecular weights. Altogether the influence of temperature, average molecular weight of the polymer host, and concentration of plasticizer on the orientational processes of chromophores is investigated and described by polymer rheology laws. Finally, these results, which identify the major role played by the average molecular weight of polymers on the orientational dynamics of chromophores, suggest another possible way of optimization for low glass transition temperature photorefractive polymers, where fast orientational response times are required.
Electromagnetic phenomena in heterogeneous media: Effective properties and local behavior
Authors: Bottauscio Oriano , Manzin Alessandra , Chiadó Piat Valeria , Codegone Marco , Chiampi Mario
Published: September 2006

The purpose of this paper is the use of a mathematical homogenization approach based on the multiple scale expansion theory for modeling the electromagnetic phenomena arising in heterogeneous media under an imposed magnetic flux. The attention is focused on the analysis and discussion of the merits and limits of this theoretical approach in reproducing not only the effective macroscopic properties but also the local behavior, under a wide frequency range and considering different constitutive and geometrical parameters. The results show that the proposed method is able to predict local and integral physical quantities, ranging from a substantially global behavior in the whole media to significantly localized effects determined by the microscopic structure.
Combined optical and electrical studies of the effects of annealing on the intrinsic states and deep levels in a self-assembled InAs quantum-dot structure
Authors: Lin S. W. , Song A. M. , Rigopolis N. , Hamilton B. , Peaker A. R. , Missous M.
Published: September 2006

The effects of postgrowth rapid thermal annealing on the electronic states in a relatively long wavelength (∼1.3μm), self-assembled InAs∕GaAs quantum-dot structure are investigated. We combine optical and electrical experiments, i.e., photoluminescence (PL) and deep-level transient spectroscopy (DLTS) measurements, to identify the underlying physical processes responsible for the changes in the PL spectra at different annealing temperatures. Physical parameters of the intrinsic and deep-level states are quantitatively determined in the DLTS experiments. These include the thermal excitation energies, densities, and their changes with the annealing temperature. We observe that the densities of the deep levels that coexist in the quantum-dot layer decrease and a new deep level, about 0.62eV below the GaAs conduction band edge, is formed at elevated temperatures. Both effects explain the variations in the PL spectra. Moreover, beyond what can be revealed in the PL experiments, the DLTS spectra show a more complex electronic structure of both optically active and inactive states.
Dielectric properties of (110) oriented PbZrO3 and La-modified PbZrO3 thin films grown by sol-gel process on Pt(111)∕Ti∕SiO2∕Si substrate
Authors: Parui Jayanta , Krupanidhi S. B.
Published: September 2006

Highly (110) preferred orientated antiferroelectric PbZrO3 (PZ) and La-modified PZ thin films have been fabricated on Pt∕Ti∕SiO2∕Si substrates using sol-gel process. Dielectric properties, electric field induced ferroelectric polarization, and the temperature dependence of the dielectric response have been explored as a function of composition. The Tc has been observed to decrease by ∼ 17°C per 1mol% of La doping. Double hysteresis loops were seen with zero remnant polarization and with coercive fields in between 176 and 193kV∕cm at 80°C for antiferroelectric to ferroelectric phase transformation. These slim loops have been explained by the high orientation of the films along the polar direction of the antiparallel dipoles of a tetragonal primitive cell and by the strong electrostatic interaction between La ions and oxygen ions in an ABO3 perovskite unit cell. High quality films exhibited very low loss factor less than 0.015 at room temperature and pure PZ; 1 and 2mol% La doped PZs have shown the room temperature dielectric constant of 135, 219, and 142 at the frequency of 10kHz. The passive layer effects in these films have been explained by Curie constants and Curie temperatures. The ac conductivity and the corresponding Arrhenius plots have been shown and explained in terms of doping effect and electrode resistance.
A medium energy ion scattering and x-ray photoelectron spectroscopy study of physical vapor deposited thin cerium oxide films on Si(100)
Authors: Barnes R. , Starodub D. , Gustafsson T. , Garfunkel E.
Published: September 2006

40 Å thick cerium oxide films have been grown on Si(100) substrates via physical vapor deposition of cerium metal in an oxygen background. The films have been characterized for their composition and thermal properties upon deposition and under different annealing conditions via x-ray photoelectron spectroscopy and medium energy ion scattering (MEIS) and their morphology using atomic force microscopy. By reoxidizing the films in O218 gas and using MEIS, we investigated the processes responsible for film formation. We found that annealing the as-deposited samples to 750°C produced a cerium silicate film with a sharp silicate:silicon interface. Our results show that the oxygen transport in both the oxide and silicate films occurs via an exchange mechanism.
Physical and electrical characterizations of ultrathin Si-rich Hf-silicate film and Hf-silicate/SiO2 bilayer deposited by atomic layer chemical vapor deposition
Authors: Kim Jaehyun , Yong Kijung
Published: September 2006

Hf-silicate films and Hf-silicate/SiO2 bilayers were fabricated on Si(100) to study SiO2 buffer layer effects. Hf-silicate layers were grown by atomic layer chemical vapor deposition using alternate supply of tetrakis-diethylamido-hafnium (Hf[N(C2H5)2]4) and tetra-n-butyl-orthosilicate [Si(OnBu)4] precursors. Ultrathin SiO2 buffer layers effectively suppressed Hf-rich phases and dislocations found at Hf-silicate/Si interfaces in Hf-silicate samples. These effects resulted in the significantly improved electrical properties of bilayers, compared to Hf-silicate films, such as low leakage current density (Jg), low flatband voltage (Vfb) shift, and high breakdown voltage (VBD).
Capillary pressure of a liquid in a layer of close-packed uniform spheres
Authors: Slobozhanin Lev A. , Alexander J. Iwan D. , Collicott Steven H. , Gonzalez S. Roberto
Published: September 2006

The capillary pressure in liquid partially filling the pore space in a layer of equidimensional close-packed spheres has been calculated numerically and studied experimentally. The case of square packing when the centers of the spheres are in the same plane and lie at the corners of a square receives primary consideration for zero gravity. In the absence of gravity, the menisci shapes of a liquid that occupies some fraction of the pore space are constructed using the Surface Evolver code. The mean curvature (and, hence, the capillary pressure) of the liquid surface is calculated. The dependence of capillary pressure on the liquid volume is obtained for selected contact angles in the range 0⩽θ⩽π. The evolution of the shape of the liquid’s free surface and the capillary pressure under quasistatic infiltration and drainage can be deduced from these results. The maximum pressure difference between liquid and gas required for a meniscus passing through a pore is calculated and compared with that for hexagonal packing and with an approximate solution given by Mason and Morrow. The lower and upper critical liquid volumes that determine the stability limits for the equilibrium of a capillary liquid in contact with spheres in a square packed array are tabulated for a set of contact angles. To assess the applicability of the obtained results to multiple layers, the possibility that the constructed menisci intersect spheres from adjacent layers has been analyzed. The effect of gravity has also been examined. For square packing the dependence of capillary pressure on liquid volume is constructed for selected contact angles and Bond numbers, Bo, and compared with the case of zero gravity (Bo=0). Maximum capillary pressures are calculated for a set of contact angles and Bo=5 in the limiting cases of square packing and hexagonal packing. Experiments were performed for a layer of square-packed spheres to compare with numerical predictions.
Two-dimensional model problem to explain counter-rotating vortex pair formation in a transverse jet
Authors: Muppidi Suman , Mahesh Krishnan
Published: September 2006

A two-dimensional model problem is used to study the evolution of the cross section of a transverse jet and the counter-rotating vortex pair (CVP). The solution to the model problem shows deformation of the jet similar to that observed in a transverse jet, and also yields a CVP. These phenomena are explained in terms of the acceleration the jet experiences in the direction of the cross-flow, and the pressure field around the jet. The initial stages of the jet’s evolution are at constant acceleration while the later stages are at constant velocity. The effects of Reynolds number and velocity ratio on the evolution of the jet are used to explain the dependence of CVP formation on velocity ratio, as observed by Smith and Mungal [J. Fluid Mech. 357, 83 (1998)].
Pertubations on a liquid curtain near break-up: Wakes and free edges
Authors: Roche J. S. , Le Grand N. , Brunet P. , Lebon L. , Limat L.
Published: September 2006

We report experiments on liquid curtains falling between two vertical wires. The flow is mainly driven by gravity, so that the Weber number (We) (ratio of momentum flux to twice the surface tension) is close to zero at the top of the curtain and increases downstream, with the possible existence of a location where We equals 1 (which turns out to be a singular point in the sheet, in terms of waves propagation). In the present paper, we focus on the curtain response to localized perturbations, i.e., formation of either surface waves or free edges behind a thin needle touching the curtain, with a special emphasis to what happens near the break-up limit. We extract and compare the shapes of two kind of “wakes” left behind the obstacle: classical triangular wake of standing sinuous waves and stationary hole involving two free edges pinned on the needle. It is found that these two wakes are very similar for high enough We, but behave very differently when We reaches 1 from above; the sinuous wake disappears, while the “hole wake” still exists, and its shape becomes rounded. Below We=1, the hole can either stay stable, oscillate or expand and break the curtain. We provide exact analytical expressions for stationary free-edges that compare very well with experiments.
Scaling of dark count rate with active area in 1.06μm photon-counting InGaAsP∕InP avalanche photodiodes
Authors: Dauler E. A. , Hopman P. I. , McIntosh K. A. , Donnelly J. P. , Duerr E. K. , Magliocco R. J. , Mahoney L. J. , Molvar K. M. , Napoleone A. , Oakley D. C. , O’Donnell F. J.
Published: September 2006

Reducing the active area of InGaAsP∕InP avalanche photodiodes operated in Geiger mode is investigated for reducing the dark count rate. The dark count rate in Geiger mode is found to scale linearly with the detector’s active area for mesa diameters of 10, 15, and 20μm. Scaling the mesa size from 20to10μm results in a reduction of the room-temperature dark count rate from 290to44kHz at ∼4.5V of overbias, while the photon detection efficiency is ∼45% for all device diameters. The trade-offs associated with shrinking the active area, including reduced optical coupling and higher on-state resistance, are discussed briefly.
Enhanced room temperature ferromagnetism in Co- and Mn-ion-implanted silicon
Authors: Bandaru P. R. , Park J. , Lee J. S. , Tang Y. J. , Chen L.-H. , Jin S. , Song S. A. , O’Brien J. R.
Published: September 2006

The authors report on ferromagnetism at room temperature in cluster-free, cobalt- and manganese-ion-implanted crystalline silicon. Through magnetic and structural analysis it is shown that the ion-implanted Si consists of two layers of Co- and Mn-containing silicon: (1) an amorphous Si layer on the surface and (2) single crystalline Si beneath. The amorphous layer shows very little magnetism by itself but seems to be responsible for partially canceling out or masking the ferromagnetism in the crystalline Si. The authors also observe that etching of the amorphous Si layer dramatically enhances the measured magnetism by as much as 400%.
Micromagnetic simulation of angular dependence of coercivity in Sm(Co,Fe,Cu,Zr)z magnets
Authors: Chen Ren-Jie , Zhang Hong-Wei , Rong Chuan-Bing , Sun Ji-Rong , Shen Bao-Gen
Published: September 2006

The angular dependence of coercivity in Sm(Co,Fe,Cu,Zr)z magnets is investigated with the micromagnetic finite element method. The results show that the mechanism of the magnetization reversal changes from the domain wall pinning to magnetization uniform reversal with the increase of the angle between the easy axes and the applied field. According to the results of calculation, the microstructural parameter αφ is determined.
Long-range surface plasmon-polariton mode cutoff and radiation in embedded strip waveguides
Authors: Breukelaar Ian , Charbonneau Robert , Berini Pierre
Published: September 2006

Using a fully vectorial formulation, the normal mode analysis method is used to model the radiative spreading of surface plasmon-polariton mode input into regions where the bound surface mode is cut off or radiative. The analysis was performed at λ0=1.55μm for embedded finite width strip metal waveguides, where the metal was Au and the surrounding dielectrics were SiO2 and LiNbO3. Mode cutoff is induced by an asymmetry between the index of refraction of the top clad and the bottom clad. Comparisons between predictions and experimental results show very good agreement. The analysis was performed for a range of waveguide dimensions, and results show that a change in insertion loss of better than 20dB is possible for an appropriate waveguide and dielectric asymmetry. Practical applications to make use of this cutoff effect are outlined.
Determination of the aspect ratio statistical distribution of gold nanorods in solution from a theoretical fit of the observed inhomogeneously broadened longitudinal plasmon resonance absorption spectrum
Authors: Eustis Susie , El-Sayed Mostafa A.
Published: September 2006

The determination of the statistical distribution of aspect ratios of a nanorod solution is desirable for experimentally synthesized solutions. The traditional method of using transmission electron microscopy (TEM) images for size determination gives statistically incorrect values due to distortions introduced by TEM sample preparation and by difficulties in counting a sufficiently large number of rods. In the present work, we propose a method to obtain the aspect ratio distribution using the observed longitudinal surface plasmon resonance absorption spectrum of gold nanorods in solution. The observed inhomogeneously broadened spectrum is fitted with a collection of homogeneously broadened spectra of nanorods each with a specific aspect ratio and population contribution using Gans extension [Ann. Phys. 47, 270 (1915)] of Mie theory. The fit generates an aspect ratio distribution for the rods in solution from which the median value and the statistical distribution are determined. This method is statistically more accurate, more convenient, and less expensive than the traditional method of TEM analysis on a solid substrate.
A pump-and-probe method for the characterization of nonlinear material parameters within Fabry-Pérot microcavities
Authors: Gulde Stephan , Jochim Selim , Moll Nikolaj , Mahrt Rainer F.
Published: September 2006

We present a pump-and-probe method based on optical Fabry-Pérot resonators that allows the characterization of third-order nonlinear materials relevant for all-optical technologies. The experiments show particular nonlinear optical resonator dynamics, which we analyze by numerical simulations. We highlight the pros and cons of our method compared with existing nonlinear material-characterization methods.
Quantum transport in two- and three-dimensional nanoscale transistors: Coupled mode effects in the nonequilibrium Green’s function formalism
Authors: Luisier Mathieu , Schenk Andreas , Fichtner Wolfgang
Published: September 2006

In this article, we study the coupled mode space approach to nonequilibrium Green’s function (NEGF) simulation. When the lateral confinement of nanoscale devices changes abruptly and the correlation functions arising from coupled mode effects are improperly evaluated in the current and charge density calculations, it becomes difficult to solve nonequilibrium Green’s function equations self-consistently with Poisson’s equation because discrepancies appear in the charge distribution. To avoid this complication, two- and three-dimensional structures with a constant or a slightly varying confinement are often considered in the NEGF coupled mode space approach. We present a rigorous derivation of the method starting from the definition of the Green’s function and its expansion in a coupled mode space, where current and charge density equations fully account for the coupling effects. Excellent agreement with real space calculation demonstrates the strength of the method and its applicability to the simulation of two- and three-dimensional nanoscale metal-oxide-semiconductor field-effect transistors with abruptly flared-out source/drain contacts in the ballistic limit.
As-doped p-type ZnO films by sputtering and thermal diffusion process
Authors: Wang Peng , Chen Nuofu , Yin Zhigang , Yang Fei , Peng Changtao , Dai Ruixuan , Bai Yiming
Published: September 2006

As-doped p-type ZnO films were grown on GaAs by sputtering and thermal diffusion process. Hall effect measurements showed that the as-grown films were of n-type conductivity and they were converted to p-type behavior after thermal annealing. Moreover, the hole concentration of As-doped p-type ZnO was very impressible to the oxygen ambient applied during the annealing process. In addition, the bonding state of As in the films was investigated by x-ray photoelectron spectroscopy. This study not only demonstrated an effective method for reliable and reproducible p-type ZnO fabrication but also helped to understand the doping mechanism of As-doped ZnO.
Magnetoelectric effect of mildly conducting magnetostrictive/piezoelectric particulate composites
Authors: Zhou Yan , Shin F. G.
Published: September 2006

A relatively simple model has been developed to study the magnetoelectric effect of magnetostrictive/piezoelectric dilute particulate composites. For illustrative purpose, we calculate the magnetoelectric voltage coefficients of particulate composites of NiFe2O4 and lead zirconate titanate. By treating the inclusion/matrix as mildly conducting materials, we have reproduced all the key experimental features of magnetoelectric behavior of the particulate composites. In addition, the qualitative dependence of the calculated magnetoelectric voltage coefficients on the constituent’s electrical conductivity is also in good agreement with recent experimental observations.
Structural defects in multiferroic BiMnO3 studied by transmission electron microscopy and electron energy-loss spectroscopy
Authors: Yang H. , Chi Z. H. , Yao L. D. , Zhang W. , Li F. Y. , Jin C. Q. , Yu R. C.
Published: September 2006

The multiferroic material BiMnO3 synthesized under high pressure has been systematically studied by transmission electron microscopy and electron energy-loss spectroscopy, and some important structural defects are revealed in this multiferroic material. The frequently observed defects are characterized to be Σ3(11¯1) twin boundaries, Ruddlesden-Popper [Acta Crystallogr. 11, 54 (1958)] antiphase boundaries, and ap⟨001⟩p superdislocations connected with a small segment of Ruddlesden-Popper defect. These defects are present initially in the as-synthesized sample. In addition, we find that ordered voids (oxygen vacancies) are easily introduced into the multiferroic BiMnO3 by electron-beam irradiation.
Dislocation dynamics in strain relaxation in GaAsSb∕GaAs heteroepitaxy
Authors: Pérez Rodríguez B. , Mirecki Millunchick J.
Published: September 2006

The real-time stress evolution has been investigated during molecular-beam epitaxial growth of GaAs1−xSbx∕GaAs metamorphic buffer. These real-time data were obtained using an in situ multibeam optical sensor measurement and has been combined with detailed analysis of data obtained from x-ray diffraction, transmission electron microscopy, and atomic force microscopy. We compare the strain relaxation of two different compositions of GaAs1−xSbx, and correlated the development of dislocation structure and morphology. Several distinct stages of the strain relaxation were observed during growth, which can be separated in three main regimes: pseudomorphic growth, fast strain relaxation, and saturation. Transmission electron microscopy data show that GaAs0.5Sb0.5 buffer layers have a larger fraction of pure-edge dislocations that arise during the earliest stages of growth. This could have a significant influence in the fabrication of buffer layers, since pure edges are favored over the threading dislocations. The strain relaxation profile for each film was modeled using a modified model of Dodson and Tsao [Phys. Rev. B 38, 12383 (1988)] that takes into account the elastic interactions of misfit dislocations. The model results agree with the experimental data and show that interaction of misfit dislocations is responsible for the large residual stress. In addition, following the description developed by Dodson and Tsao [Phys. Rev. B 38, 12383 (1988)] for the rate of dislocation multiplication, we were able to determine the line density of threading dislocations from the experimental data. This has a potential application in the design of metamorphic buffer layers because our observations are made in real time on individual growth, without the need of external characterization to measure the dislocation density.
Variable energy positron beam analysis of vacancy defects in laser ablated SrTiO3 thin films on SrTiO3
Authors: McGuire S. , Keeble D. J. , Mason R. E. , Coleman P. G. , Koutsonas Y. , Jackson T. J.
Published: September 2006

Vacancies are known often to be the dominant type of point defects in perovskite oxides, but their presence can rarely be directly determined. Positron annihilation methods have unique sensitivity to vacancy-related defects and the variable energy positron beam (VEPB) techniques allow near-surface depth profiling. The characterization of strontium titanate (SrTiO3) thin films, deposited on SrTiO3 substrates by pulsed laser deposition, using Doppler broadening of the annihilation radiation detected VEPB measurements, is reported. The laser repetition rate and the postgrowth anneal conditions for the samples were varied. VEPB measurements clearly detected the deposited SrTiO3 layer (the increase in the Doppler broadened line shape parameter S compared to the substrate was ∼3.5%) and showed the presence of a high concentration of open volume defects. The magnitude of the change in S and the lack of dependence on anneal oxygen partial pressure provided evidence that strontium vacancies and VSr-oxygen vacancy defects were responsible for positron trapping. Three of the films studied have also been carefully studied by transmission electron microscopy (TEM). Fitting of the VEPB profiles using thicknesses obtained from TEM provided evidence that the homogeneous growth region, observed by TEM, has a lower vacancy defect content compared to the surface columnar region.
Nonlinear dynamics and breakup of compound jets
Authors: Suryo Ronald , Doshi Pankaj , Basaran Osman A.
Published: September 2006

Finite-amplitude deformation and breakup of a compound jet, whose core and shell are both incompressible Newtonian fluids, that is surrounded by a passive gas are analyzed computationally by a temporal analysis. The means is a method of lines algorithm in which the Galerkin/finite element method with elliptic mesh generation is used for spatial discretization and an adaptive finite difference method is employed for time integration. The dynamics are initiated by subjecting the inner and the outer interfaces of a quiescent compound jet to axially periodic perturbations that are either in phase (ω=0) or π radians out phase (ω=π), where ω is the phase shift between the disturbances imposed on the two interfaces. The initial growth rates of disturbances obtained from computations are compared and demonstrated to be in excellent agreement with predictions of linear theory [Chauhan et al., J. Fluid Mech. 420, 1 (2000)]. Computations reveal that recirculating flows occur commonly during the deformation and pinch-off of compound jets, and hence render inapplicable the use of slender-jet type approximations for analyzing the dynamics in such cases. Moreover, as the deformations of one or both of the interfaces of the compound jet grow, the resulting shapes at the incipience of pinch-off are asymmetric and lead to the formation of satellite drops. Calculations are carried out over a wide range of Reynolds numbers of the core fluid, ratios of the viscosity and density of the shell fluid to those of the core fluid, ratio of the surface tension of the outer interface to the interfacial tension of the inner interface, the ratio of the unperturbed radius of the outer cylindrical interface to that of the inner cylindrical interface, wavenumber, and perturbation amplitudes to determine their effects on breakup time and whether both interfaces pinch at the same instant in time to result in the formation of compound drops. Conditions are also identified for which the dynamical response of compound jets subjected to initial perturbations with ω=0 differ drastically from those subjected to ones with ω=π.
The empirical dependence of radiation-induced charge neutralization on negative bias in dosimeters based on the metal-oxide-semiconductor field-effect transistor
Authors: Benson Chris , Albadri Abdulrahman , Joyce Malcolm J. , Price Robert A.
Published: September 2006

The dependence of radiation-induced charge neutralization (RICN) has been studied in metal-oxide-semiconductor field-effect transistor (MOSFET) dosimeters. These devices were first exposed to x rays under positive bias and then to further dose increments at a selection of reverse bias levels. A nonlinear empirical trend has been established that is consistent with that identified in the data obtained in this work. Estimates for the reverse bias level corresponding to the maximum rate of RICN have been extracted from the data. These optimum bias levels appear to be independent of the level of initial absorbed dose under positive bias. The established models for threshold voltage change have been considered and indicate a related nonlinear trend for neutralization cross section σN as a function of oxide field. These data are discussed in the context of dose measurement with MOSFETs and within the framework of statistical mechanics associated with neutral traps and their field dependence.
Extended Hückel theory for band structure, chemistry, and transport. I. Carbon nanotubes
Authors: Kienle D. , Cerda J. I. , Ghosh A. W.
Published: September 2006

We describe a semiempirical atomic basis extended Hückel theoretical (EHT) technique that can be used to calculate bulk band structure, surface density of states, electronic transmission, and interfacial chemistry of various materials within the same computational platform. We apply this method to study multiple technologically important systems, starting with carbon nanotubes and their interfaces and silicon-based heterostructures in our follow-up paper [D. Kienle et al., J. Appl. Phys. 100, 043715 (2006), following paper]. We find that when it comes to quantum transport through interesting, complex heterostructures including gas molecules adsorbed on nanotubes, the Hückel band structure offers a fair and practical compromise between orthogonal tight-binding theories with limited transferability between environments under large distortion and density functional theories that are computationally quite expensive for the same purpose.
Lattice vibration and thermal diffusion of Yb doped KLu(WO4)2 single crystal
Authors: Ling Z. C. , Xia H. R. , Sun S. Q. , Ran D. G. , Liu F. Q. , Zhao P. , Gao C. Y. , Zhang J. X. , Wang J. Y.
Published: September 2006

Yb doped KLu(WO4)2 (Yb:KLuW) single crystal was grown by the method of top-seeded-solution growth slow-cooling method. Its lattice vibrational modes were calculated by using space group theory, and polarized Raman spectra were measured at room temperature with different scattering geometry projections. The powder transmission of Fourier-transform infrared spectra was also performed as a supplement for Raman spectra to study the lattice vibration of the laser crystal. The main Raman peaks and infrared transmission bands were attributed to the internal mode WO6 octahedrons in the crystals. Two kinds of link ways of WO6 octahedrons implying large distortion had led the Raman peaks to shift to different positions. Large distortions of the WO6 octahedrons in the crystal had led to the splitting of some Raman peaks and even the activation of ν6 mode, which is originally a silent mode by the selection rule. As a self-Raman crystal, the linewidth of the Raman peaks ν1 and ν2 reached 12.5 and 16.6cm−1, with relaxation times of 849.4 and 639.6fs, respectively, indicating that it is a promising stimulated Raman scattering crystal. Thermal diffusion measurement of Yb:KLuW showed large anisotropy which could be explained by the different link ways of the WO6 octahedrons.
Nonlinear dynamics in a backward-facing step flow
Authors: Rani H. P. , Sheu Tony W. H.
Published: September 2006

A numerical investigation has been conducted to explore the complex nonlinear nature of flow in a backward-facing step channel by the simulated results, which include the bifurcation diagram, limit cycle oscillations, power spectrums, and phase portraits. For small values of Reynolds number, the flow was steady and laminar. When the Reynolds number was amplified, the flow becomes unsteady with the initiation of a supercritical Hopf bifurcation. The flow path is trapped by the stable limit cycles, and this system is made to proceed with a sustained oscillation. As the Reynolds number was amplified further, the stability of the investigated system keeps decreasing through a sequence of frequency-doubling bifurcations. The fundamental frequency of high amplitude was identical to the most amplified mode of Kelvin-Helmholtz instability oscillations in the shear layer. Frequencies with a small amplitude result in a slower development of the Kelvin-Helmholtz instability and are responsible for the roll-up of the shear layer. The phase portrait shows the evolution of a chaotic attractor from a simple periodic attractor. Prior to the onset of the chaotic motion, pitchfork bifurcation showed its existence.
Long plasma channels in segmented capillary discharges
Authors: Levin M. , Pukhov A. , Zigler A. , Sugiyama K. , Nakajima K. , Hubbard R. F. , Ting A. , Gordon D. F. , Sprangle P. , Kaganovich D.
Published: September 2006

Guided propagation of ultrashort (100fs) high intensity (1016Wcm−2) laser pulses over distances up to 12.6cm using optimized segmented capillary is reported. A new diagnostic technique is presented in which the transport of a guided laser pulse at different delay times from the initiation of the discharge is sampled on a single discharge shot. The current waveform was optimized to obtain a long lasting, deep radial profile. Radial profiles with the maximal electron density from 4×1017 to 2×1018cm−3 and up to 25% deep were obtained thereby, whereas longitudinal profiles were found to be remarkably uniform. The potential application of these long channels to the laser wake field accelerator is discussed.
An analytical solution for rapidly distorted turbulent shear flow in a rotating frame
Authors: Akylas E. , Kassinos S. C. , Langer C. A.
Published: September 2006

In this study we apply rapid distortion theory to the case of nonstratified homogeneous turbulence that is sheared in a frame that counter-rotates at a rate that matches in magnitude the rotation associated with the mean shear. In the inviscid case, analytical solutions are worked out for the evolution of the components of the Reynolds stresses and the structure dimensionality tensor, and these are shown to equal each other. The results are compared to direct numerical simulations data with which they proved to be in good agreement, especially in terms of the Reynolds shear stress and of the dimensionless tensor components. Finally, the development of the structure of a passive scalar field with a constant mean gradient is investigated, and remarkable analogies are shown to exist between this case and the case of shear in a fixed frame.
Low-temperature M=3 flow deceleration by Lorentz force
Authors: Nishihara Munetake , Rich J. William , Lempert Walter R. , Adamovich Igor V. , Gogineni Sivaram
Published: September 2006

This paper presents results of cold magnetohydrodynamic (MHD) flow deceleration experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M=3 nitrogen and air flows in the presence of transverse direct current electric field and transverse magnetic field. MHD effect on the flow is detected from the flow static pressure measurements. Retarding Lorentz force applied to the flow produces a static pressure increase of up to 17%–20%, while accelerating force of the same magnitude results in static pressure increase of up to 5%–7%. The measured static pressure changes are compared with modeling calculations using quasi-one-dimensional MHD flow equations. Comparison of the experimental results with the modeling calculations shows that the retarding Lorentz force increases the static pressure rise produced by Joule heating of the flow, while the accelerating Lorentz force reduces the pressure rise. The effect is produced for two possible combinations of the magnetic field and transverse current directions producing the same Lorentz force direction (both for accelerating and retarding force). This demonstrates that the observed static pressure change is indeed due to the MHD interaction, and not due to Joule heating of the flow in the crossed discharge. No discharge polarity effect on the static pressure was detected in the absence of the magnetic field. The fraction of the discharge input power going into Joule heat in nitrogen and dry air, inferred from the present experiments, is low, α=0.1, primarily because energy remains frozen in the vibrational energy mode of nitrogen. This result provides first direct evidence of cold supersonic flow deceleration by Lorentz force.
Anisotropic developments for homogeneous shear flows
Authors: Cambon Claude , Rubinstein Robert
Published: September 2006

The general decomposition of the spectral correlation tensor Rij(k) by Cambon et al. [J. Fluid Mech. 202, 295 (1989); 337, 303 (1997)] into directional and polarization components is applied to the representation of Rij(k) by spherically averaged quantities. The decomposition splits the deviatoric part Hij(k) of the spherical average of Rij(k) into directional and polarization components Hij(e)(k) and Hij(z)(k). A self-consistent representation of the spectral tensor is constructed in terms of these spherically averaged quantities. The directional and polarization components must be treated independently: representation of the spectral tensor using the spherical average Hij(k) alone proves to be inconsistent with Navier-Stokes dynamics. In particular, a spectral tensor consistent with a prescribed Reynolds stress is not unique. Since spherical averaging entails a loss of information, the description of an anisotropic correlation tensor by spherical averages is limited to weak departures from isotropy. The degree of anisotropy permitted is restricted by realizability requirements. More general descriptions can be given using a higher-order expansion of the spectral tensor. Directionality is described by a conventional expansion in spherical harmonics, but polarization requires an expansion in special tensorial quantities generated by irreducible representations of the rotation group SO3. These expansions are considered in more detail in the special case of axial symmetry.
Selective adsorption of benzoic acid species on patterned OH∕Si(100) surface
Authors: Ihm Kyuwook , Han Jin Hee , Kim Bongsoo , Chung Sukmin , Hwang Chan-Cuk , Kang Tai-Hee , Kim Ki-Jeong , Jung Yu Jin , An Ki-Seok
Published: September 2006

It has recently been observed that benzoic acid strongly reacts with OH group on the silicon surface. Here, by defining the area in which OH group is adsorbed on the Si surface, the selective adsorption of benzoic acid species was attempted. The patterned OH∕Si surface was prepared by irradiating the zeroth order beam from the bending magnet of the synchrotron facility through the gold mesh placed in front of the OH∕Si sample. For discerning the selectively adsorbed molecule by x-ray photoelectron emission microscopy (X-PEEM) at N k edge, 4-nitrobenzoic acid was utilized instead of benzoic acid. Near edge x-ray absorption fine structure spectra at carbon and oxygen k edges were in good accord with the previous results obtained from the benzoic acid system. The X-PEEM images around N k edge clearly showed that the molecules adsorb only on the area in which OH groups remain.
Coexistence of low-field positive and negative magnetic entropy change in SmMn2Ge2
Authors: Han Zhida , Wu Hongliang , Wang Dunhui , Hua Zhenghe , Zhang Chengliang , Gu Benxi , Du Youwei
Published: September 2006

Polycrystalline SmMn2Ge2 has been prepared by arc melting method. Large magnetic entropy changes of −3.7 and 3.3J∕kgK are found under a low magnetic field change of 5kOe, near two first-order transition temperatures, T1(112K) and T2(152K), respectively. The coexistence of the large low-field positive and negative magnetic entropy change suggests a potential of SmMn2Ge2 as a magnetic refrigerant in the corresponding temperature range.
Analysis of second harmonic generation in photonic-crystal-assisted waveguides
Authors: D’Orazio Antonella , de Ceglia Domenico , De Sario Marco , Prudenzano Francesco , Bloemer Mark J. , Scalora Michael
Published: September 2006

We study second harmonic generation in a planar dielectric waveguide having a low-index, polymer core layer, bounded by two multilayer stacks. This geometry allows exceptionally strong confinement of the light at the fundamental wavelength inside the core region with virtually zero net propagation losses for distances that exceed several centimeters, provided that material and scattering losses are neglected. A phase-matched configuration of the waveguide is reported in which the pump signal is the lowest-order mode of the waveguide, and the generated second harmonic signal corresponds to the third propagation mode of the waveguide. Using a polymer waveguide core, having χ(2)∼100pm∕V, we predict a conversion efficiency of approximately 90% after a propagation distance of 2mm, using peak pump intensities inside the core of the waveguide of 1.35GW∕cm2. If the waveguide core contains polymer layers with different glass transition temperatures, the layers can be poled independently to maximize the overlap integral, and similar pump depletions may be achieved over a distance of approximately 500μm.
Electronic resonant images of an ion implanted Si(111) substrate observed by wavelength tunable optical second harmonic microscopy
Authors: Sano Haruyuki , Saito Jun , Ikeda Junichi , Mizutani Goro
Published: September 2006

This paper demonstrates that the spatial distribution of electronic states of an arsenic ion implanted Si(111) substrate can be observed by using a wavelength tunable second harmonic (SH) microscope in a wide photon energy range from 2ℏω=1.96to5.19eV. The contrast in the SH intensity images between the As-doped area and the nondoped area depends greatly on the SH photon energy. For 2ℏω>3eV, optical second harmonic generation (SHG) from the nondoped area was stronger than from the doped area, and the contrast was reversed for 2ℏω⩽2.33eV. The contrast in the SH intensity images was considerably different from that in the linear optical reflection images, indicating that spectroscopic SH microscopy can provide different informations on electronic levels from that associated with the linear optical response. It is suggested that the larger SH intensity from the nondoped area for 2ℏω>3eV results from the resonant SHG enhancement effect associated with the bulk Si E1 (3.4eV) and E2 (4.3eV) gaps. In the case of the doped area, a small resonant enhancement of the SH intensity was observed around 2ℏω=2.33eV. This resonance may result from an energy level created by the ion implantation.
Photovoltaic characterization of concentrator solar cells by localized irradiation
Authors: Katz Eugene A. , Gordon Jeffrey M. , Tassew Wondesen , Feuermann Daniel
Published: September 2006

The ability to determine the macroscopic parameters that characterize photovoltaic performance, including their spatial dependence, especially at high flux, is demonstrated with extensive solar measurements on high-efficiency concentrator solar cells. Two case studies explore (a) the impact of inhomogeneous flux distribution on photovoltaic behavior, (b) establishing how solar cell parameters vary across the cell surface (of particular interest for deployment in high-concentration optical systems), and (c), the sensitivity of photovoltaic parameters to the spatial variation of series resistance that stems from nonuniform cell metallization. In the process, we elucidate current-voltage trends unique to strongly inhomogeneous illumination and to series resistance losses at high flux.
Thermal transport in nanocrystalline materials
Authors: Zhong Zhanrong , Wang Xinwei
Published: September 2006

In this work, thermal transport in nanocrystalline materials is studied using large-scale equilibrium molecular dynamics simulation. Nanocrystalline materials with different grain sizes are studied to explore how and to what extent the size of nanograins affects the thermal conductivity and specific heat. Substantial thermal conductivity reduction is observed and the reduction is stronger for nanocrystalline materials with smaller grains. On the other hand, the specific heat of nanocrystalline materials shows little change with the grain size. Based on the calculated thermal conductivity, the thermal resistance at grain boundaries is calculated and found to be in the order of 10−9m2K∕W. The simulation results are compared with the thermal transport in freestanding nanograins based on molecular dynamics simulation. Further discussions are provided to explain the fundamental physics behind the observed thermal phenomena in this work.
Confined magnetic stray field from a narrow domain wall
Authors: You Chun-Yeol
Published: September 2006

It is found that a strongly confined magnetic stray field can be generated from a narrow domain wall. The field strength is stronger than from an ordinary domain wall because the wall is localized within a few tens of nanometers in its lateral dimensions. Such a narrow domain wall can be formed in an interlayer exchange coupled system for which only a part of one of the ferromagnetic layers is coupled with the other ferromagnetic layer. Since the coupled and free regions have different local switching fields, a domain wall will form at the interface between the two regions. The interlayer coupling and Zeeman energies play an important role in the domain wall energy equations.
Epitaxial growth of non-c-axis-oriented ferroelectric rare-earth element-substituted bismuth titanate thin films on Si(100)
Authors: Lee Sung Kyun , Hesse Dietrich , Gösele Ulrich
Published: September 2006

Non-c-axis-oriented ferroelectric La-substituted Bi4Ti3O12 (BLT) and Nd-substituted Bi4Ti3O12 (BNT) epitaxial thin films of (104) orientation were grown on buffered Si(100) substrates by pulsed laser deposition. Electrically conducting (111)-oriented SrRrO3 layers on Pt(111)∕YSZ(100)∕Si(100) were used as an effective epitaxial template for the subsequent growth of the ferroelectric thin films. X-ray diffraction and transmission electron microscopy characterizations revealed that the SrRuO3 layers and the overlying ferroelectric thin films were multiply twinned due to the fourfold symmetry of YSZ(100) and the threefold symmetry of the Pt(111) plane. The BNT films showed ∼1.5 times higher remanent polarization (2Pr) of 38.7μC∕cm2 than the BLT films (2Pr=26.0μC∕cm2). The polarization enhancement for BNT films stems from an enhanced structural distortion of the TiO6 octahedra in the pseudoperovskite block by the Nd3+ ions (with a smaller ion radius than La3+) substituting for Bi3+. These (104)-oriented BLT and BNT films on buffered Si(100) exhibited a good fatigue endurance. No significant change in the shape of the hysteresis loops was observed even after 1×1011 switching cycles.
Tunable giant magnetoresistance with magnetic barriers
Authors: Papp G. , Peeters F. M.
Published: September 2006

The temperature dependent giant magnetoresistance effect is investigated theoretically in a magnetically modulated two-dimensional electron gas, which can be realized by depositing two parallel ferromagnets on top and bottom of a heterostructure. Assuming a single particle effective mass Hamiltonian and ballistic transport, the giant magnetoresistance ratio (MRR) is calculated. It is shown that MRR can be tuned by the magnetic field and with the width of the bottom ferromagnet. The MRR shows strong dependence on temperature, but studies indicate that the system even at 4K has 4×104% MRR and it can be used as a device for realistic electron density.
Influence of target properties and laser fluence on energetic protons accelerated by a laser-produced plasma
Authors: Nayuki Takuya , Oishi Yuji , Fujii Takashi , Takano Kei , Wang Xiaofang , Andreev Alexander Alekseevitch , Nemoto Koshichi , Ueda Ken-ichi
Published: September 2006

The dependences of the observed maximum proton energy on laser fluence, and the type and thickness of the target material were studied. The maximum proton energy depended on the laser fluence rather than on the laser intensity for laser pulses with energies between 40 and 430mJ and pulse durations between 55 and 400fs. The maximum proton energy increased as the product of target thickness and target mass density decreased. When a polyimide tape target of 7.5μm thickness was irradiated, the observed maximum proton energy and flux, counted as the number of etched tracks with a solid angle of 0.25sr, were 3.3±0.1MeV and 4×106∕MeV∕shot, respectively. For efficient proton acceleration when a wide ion emission angle is acceptable, polyimide was suitable for the tape target because it contains much hydrogen, and furthermore, it has a low mass density and a high tensile strength.