bluuebunny/crossref_metadata_2025 huggingface dataset:
Understanding the inelastic electron-tunneling spectra of alkanedithiols on gold
Authors: Solomon Gemma C. , Gagliardi Alessio , Pecchia Alessandro , Frauenheim Thomas , Di Carlo Aldo , Reimers Jeffrey R. , Hush Noel S.
Published: March 2006

We present results for a simulated inelastic electron-tunneling spectra (IETS) from calculations using the “gDFTB” code. The geometric and electronic structure is obtained from calculations using a local-basis density-functional scheme, and a nonequilibrium Green’s function formalism is employed to deal with the transport aspects of the problem. The calculated spectrum of octanedithiol on gold(111) shows good agreement with experimental results and suggests further details in the assignment of such spectra. We show that some low-energy peaks, unassigned in the experimental spectrum, occur in a region where a number of molecular modes are predicted to be active, suggesting that these modes are the cause of the peaks rather than a matrix signal, as previously postulated. The simulations also reveal the qualitative nature of the processes dominating IETS. It is highly sensitive only to the vibrational motions that occur in the regions of the molecule where there is electron density in the low-voltage conduction channel. This result is illustrated with an examination of the predicted variation of IETS with binding site and alkane chain length.
Vortex cores, strain cells, and filaments in quasigeostrophic turbulence
Authors: Petersen Mark R. , Julien Keith , Weiss Jeffrey B.
Published: March 2006

We present numerical simulations of decaying two-dimensional (2D) and three-dimensional quasigeostrophic (3D QG) turbulence. The resulting vorticity fields are decomposed into three components: the vortex cores, the strain cells, and the background. In 2D, the vortex cores induce five times the energy as the background, while in 3D QG the background plays a more dominant role and induces the same amount of energy as the vortex cores, quantifying previous observations that 3D QG has a more active filamentary background. The probability density function of the total velocity field is nearly Gaussian in 3D QG but significantly less so in 2D. In both 2D and 3D QG, the velocities induced by the vortex cores and the strain cells are non-Gaussian. In both 2D and 3D QG turbulence, the enstrophy spectrum of the background is close to k−1 predicted by inverse cascade theories.
Mechanisms for deposition and resuspension of heavy particles in turbulent flow over wavy interfaces
Authors: Marchioli Cristian , Armenio Vincenzo , Salvetti Maria Vittoria , Soldati Alfredo
Published: March 2006

It has been long recognized that turbulent flow over steep waves can produce coherent flow structures of different temporal and spatial scales. In particular, quasistreamwise vortices grow up on the upslope side of the wave and interact with geometry-dependent vortical structures, aligned spanwise and located within the recirculation bubble in the wave trough, thus creating the conditions for the development of a three-dimensional highly turbulent flow field. In this work, we have analyzed the trajectories of O(105) small dense particles (either in solid form or in the form of liquid droplets) released into a turbulent air flow over waves precisely to clarify the role of coherent vortical structures in controlling particle deposition and resuspension. The three-dimensional time-dependent flow field at Reτ=170 is calculated using large-eddy simulation, and the dynamics of individual different-sized particles is described using a Lagrangian approach. Drag, gravity, and lift are used in the equation of motion for particles that have no influence on the flow field. Particle-wall interaction is fully elastic. Our findings show that different-sized particles interact selectively with vortical flow structures, producing different distribution patterns and dispersion rates qualitatively depending on the particle-to-fluid time-scale ratio. Specifically, we find that quasistreamwise vortices on the upslope side of the wave control particle dispersion and eventual segregation in the flow separation region downstream the wave crest as well as in the shear layer forming behind the wave, just above the separation region. These vortices generate momentum mixing events that also entrain and move particles towards and away from the wavy wall. This process is similar to the sweep/ejection cycle occurring in turbulent flow over a flat boundary layer.
An efficient wall model for large-eddy simulation based on optimal control theory
Authors: Templeton Jeremy A. , Wang Meng , Moin Parviz
Published: March 2006

Large-eddy simulation is currently very expensive for high Reynolds number attached flows due to the need to resolve the wall layer. In order to reduce this expense, wall modeling has been proposed to provide approximate boundary conditions to the LES that allow the wall layer to be unresolved. Unfortunately, subgrid scale modeling errors and numerical errors are important in this region when a coarse grid is used, necessitating a wall model that can compensate for their effects. Optimal control theory has been used to provide such models in the past, but is impractical for complex flows due to the need to provide a target mean velocity profile and the computational expense involved with solving gradient-based optimization problems. In this paper we address the latter issue by reformulation of the optimization problem to include only data near the wall. Further approximations have been made to the Navier-Stokes and adjoint equations used in the optimization process that significantly reduce the computational cost. Results will be presented comparing this method with other control-based and standard wall models.
Transitions to nematic states in homogeneous suspensions of high aspect ratio magnetic rods
Authors: Gopinath A. , Mahadevan L. , Armstrong R. C.
Published: March 2006

Isotropic-nematic and nematic-nematic transitions from a homogeneous suspension of high aspect ratio magnetic rods are studied for both Maier-Saupe and Onsager excluded volume potentials. Asymptotic analysis in the vicinity of critical points yields insight into the stability and type of polarized nematic states emanating from nonpolarized equilibrium states. This, in conjunction with recently published global numerical results, yields a unified picture of the bifurcation diagram and provides a convenient base state to study effects of external orienting fields.
Stretching of polymers around the Kolmogorov scale in a turbulent shear flow
Authors: Davoudi Jahanshah , Schumacher Jörg
Published: March 2006

We present numerical studies of stretching of Hookean dumbbells in a turbulent Navier-Stokes flow with a linear mean profile, ⟨ux⟩=Sy. In addition to the turbulence features beyond the viscous Kolmogorov scale η, the dynamics at the equilibrium extension of the dumbbells significantly below η is well resolved. The variation of the constant shear rate S causes a change of the turbulent velocity fluctuations on all scales and thus of the intensity of local stretching rate of the advecting flow. The latter is measured by the maximum Lyapunov exponent λ1 which is found to increase as λ1∼S3∕2, in agreement with a dimensional argument. The ensemble of up to 2×106 passively advected dumbbells is advanced by Brownian dynamics simulations in combination with a pseudospectral integration for the turbulent shear flow. Anisotropy of stretching is quantified by the statistics of the azimuthal angle ϕ which measures the alignment with the mean flow axis in the x-y shear plane, and the polar angle θ which determines the orientation with respect to the shear plane. The asymmetry of the probability density function (PDF) of ϕ increases with growing shear rate S. Further, the PDF becomes increasingly peaked around mean flow direction (ϕ=0). In contrast, the PDF of the polar angle θ is symmetric and less sensitive to changes of S.
Hydrodynamic model for a vacuum arc operated with background gas: Theory and experimental validation
Authors: Grondona D. , Kelly H. , Minotti F. O.
Published: March 2006

A stationary, one-dimensional fluid model is presented to describe the interelectrode region of a nonfiltered vacuum arc operated with a background gas. The model includes the electron energy equation and the main elastic and inelastic atomic processes among metallic ions, electrons, and gas particles. To validate the model predictions an experimental study of the plasma-neutral gas structure, using a titanium (Ti) cathode and argon (Ar) as the background gas, is presented. The measured electron temperature and the experimental dependence on the pressure of neutral Ti and Ar spectroscopic emission lines are well reproduced, using a simple atomic model to interpret the plasma radiation emission.
Nonlinear instabilities in a vertical pipe flow discharging from a cylindrical container
Authors: Sanmiguel-Rojas E. , Fernandez-Feria R.
Published: March 2006

We report results from three-dimensional numerical simulations of the incompressible flow in a vertical pipe of circular cross-section discharging from a cylindrical container. Natural Coriolis forces due to Earth rotation trigger the instability of the axisymmetric flow, and nonlinear spiral waves with azimuthal wave number ∣n∣=3 are formed above a critical Reynolds number based on the pipe flow rate (ReQ). We characterize this critical Reynolds number as a function of the Coriolis parameter (F), that is proportional to the square of the radius of the container. As a difference with previous numerical works on nonlinear instabilities and transition in a pipe flow, here the nonlinear disturbances needed to trigger the instabilities are not artificially introduced inside the pipe flow, but naturally produced by Coriolis forces, the amplitude of these disturbances being characterized by a nondimensional Coriolis parameter. We find that the pipe flow can be unstable for ReQ as low as 300 for the largest value of F considered. We also discuss the relevance of the residual swirl introduced by natural Coriolis forces in triggering the nonlinear traveling waves.
Direct spectroscopic evidence of field-induced solid-state chain conformation transformation in a ferroelectric relaxor polymer
Authors: Zhang Shihai , Chu Baojin , Neese Bret , Ren Kailiang , Zhou Xin , Zhang Q. M.
Published: March 2006

Ferroelectric relaxor P(VDF-TrFE-CFE) terpolymer can generate a hysteresis-free polarization of 47mC∕m2 at an electric field of 100MV∕m and is attractive for many potential actuating and sensing applications. Fourier transform infrared spectra under electric field reveal that this large polarization originates from the field-induced crystalline phase transformation. The nonpolar TGTG′ chain conformation in P(VDF-TrFE-CFE) can be converted into polar trans (Tm>4 and T3G) conformation under high electric field and the latter can be effectively aligned to the applied field direction. Furthermore, this conformation transformation is completely reversible and no hysteresis can be observed during the switching of applied electric field. In contrast, normal ferroelectric polymer P(VDF-TrFE) exhibits square polarization loop with very small reversible polarization.
Chemical etching study of probe-grown ultrathin nano-oxides by atomic force microscopy
Authors: Chung H. J. , Xie X. N. , Sow C. H. , Wee A. T. S.
Published: March 2006

We report the study of the etching characteristics of atomic force microscopy (AFM) probe-grown ultrathin oxides (AFM oxides, up to 5nm thick). In our method, an AFM localized depth analysis technique was employed to monitor the atomic layer-by-layer etching of AFM oxides. Insights into the growth mode and etching mechanism of AFM oxides were acquired on the basis of the etching results: it was found that AFM oxide growth is related to Si out-diffusion. For the formation of ultrathin oxides in ambient conditions, it is evident that oxidation-enhanced Si diffusion facilitates the layer-by-layer oxide growth in AFM anodic oxidation. The etching rate of ultrathin AFM oxides is dependent on the SiOH silanol reactive sites. Thermal annealing could reduce the content of silanol groups and enhance the chemical stability of AFM oxides against etching.
Free-surface flows with large slopes: Beyond lubrication theory
Authors: Snoeijer Jacco H.
Published: March 2006

The description of free-surface flows can often be simplified to thin-film (or lubrication) equations, when the slopes of the liquid-gas interface are small. Here, we present a long-wavelength theory that remains fully quantitative for steep interface slopes, by expanding about Stokes flow in a wedge. For small capillary numbers, the variations of the interface slope are slow and can be treated perturbatively. This geometry occurs naturally for flows with contact lines: we quantify the difference with ordinary lubrication theory through a numerical example and analytically recover the full Cox-Voinov asymptotic solution.
On the generation of a reverse von Kármán street for the controlled cylinder wake in the laminar regime
Authors: Bergmann Michel , Cordier Laurent , Brancher Jean-Pierre
Published: March 2006

In this Brief Communication we are interested in the maximum mean drag reduction that can be achieved under rotary sinusoidal control for the circular cylinder wake in the laminar regime. For a Reynolds number equal to 200, we give numerical evidence that partial control restricted to an upstream part of the cylinder surface may considerably increase the effectiveness of the control. Indeed, a maximum value of relative mean drag reduction equal to 30% is obtained when applying a specific sinusoidal control to the whole cylinder, where up to 75% of reduction can be obtained when the same control law is applied only to a well-selected upstream part of the cylinder. This result suggests that a mean flow correction field with negative drag is observable for this controlled flow configuration. The significant thrust force that is locally generated in the near wake corresponds to a reverse von Kármán vortex street as commonly observed in fish-like locomotion or flapping wing flight. Finally, the energetic efficiency of the control is quantified by examining the power saving ratio: it is shown that our approach is energetically inefficient. However, it is also demonstrated that for this control scheme the improvement of the effectiveness generally occurs along with an improvement of the efficiency.
Drag reduction in turbulent channel flow with periodically arrayed heating and cooling strips
Authors: Yoon Hyun Sik , El-Samni Osama A. , Chun Ho Hwan
Published: March 2006

A new technique giving significant drag reduction in turbulent shear flows has been proposed by using the buoyancy effect to generate periodic spanwise motion. Such spanwise motion can be obtained by arranging heating and cooling strips periodically aligned in the spanwise direction of a vertical channel, where the streamwise mean flow is perpendicular to the gravity vector. The strip size has been changed in order to obtain the optimum size corresponding to the maximum drag reduction. Series of direct numerical simulation have been performed where the bulk Reynolds number, Rem=Umδ∕ν is fixed at 2270 whereas the Grashof number is changed to between 106 and 107. At the lowest Grashof number, the buoyancy forces are not strong enough to disturb the flow and no apparent variation of drag compared to the plane channel is observed. However, as the Grashof number increases, considerable drag reduction can be obtained. At the highest Grashof number, an optimum strip size of about 250 wall units gives drag reduction of about 35%. The greater the Grashof number, the smaller the strip size which attains the maximum drag reduction. The similarity between the induced lateral motions by the buoyancy forces and those induced by the spanwise wall oscillations is highlighted.
Theoretical analysis of the effect of insoluble surfactant on the dip coating of chemically micropatterned surfaces
Authors: Tiwari Naveen , Davis Jeffrey M.
Published: March 2006

Microfluidic flow on chemically heterogeneous surfaces is a useful technique with applications ranging from selective material deposition to the self-assembly of nanostructures. The recent theoretical analysis by Davis [Phys. Fluids 17, 038101 (2005)] of the dip coating of a pure fluid onto vertical, wetting stripes surrounded by nonwetting regions quantified the experimentally observed deviations from the classical Landau-Levich result due to lateral confinement of the fluid by chemical surface patterning. In this present work, the analysis of dip coating of these heterogeneous surfaces is extended to a liquid containing an insoluble surfactant. Using matched asymptotic expansions based on lubrication theory in the limit of a small capillary number, the thickness of the deposited liquid film and the surfactant concentration in the deposited monolayer are predicted for a wide range of fluid properties and process parameters. The increase in the deposited film thickness is shown analytically to be limited by a multiplicative factor of 41∕3 times the result for a pure liquid. Numerical results demonstrate that the thickening due to Marangoni effects is nonmonotonic in the capillary number because of the competition between viscous stresses, Marangoni stresses, and surface diffusion.
Transient electron excitation in laser-induced plasma-assisted ablation of transparent materials
Authors: Hanada Yasutaka , Sugioka Koji , Obata Kotaro , Garnov Serge V. , Miyamoto Iwao , Midorikawa Katsumi
Published: March 2006

We investigate the mechanism of laser-induced plasma-assisted ablation (LIPAA), by which high-quality and high-efficiency ablation of transparent materials, such as glass, can be performed with a single conventional pulsed laser. The laser-induced plasma induces transient absorption of the laser beam (532nm) by the glass substrate. The origin of the transient absorption is electron excitation by ions with kinetic energy more than approximately 10eV in the plasma, which is observed by measuring transient polarization change in the glass substrate applied with a high external pulsed electric field during the plasma-assisted electron excitation (plasma-conductivity measurement). A possible mechanism of LIPAA is proposed based on the results obtained.
A thermal model for static current characteristics of AlGaN∕GaN high electron mobility transistors including self-heating effect
Authors: Chang Yuancheng , Zhang Yimen , Zhang Yuming , Tong K. Y.
Published: March 2006

A thermal model of AlGaN∕GaN high electron mobility transistors (HEMTs) has been developed based on a quasi-two-dimensional numerical solution of Schrödinger’s equation coupled with Poisson’s equation. The static current characteristics of HEMT devices have been obtained with the consideration of the self-heating effect on related parameters including polarization, electron mobility, saturation velocity, thermal conductivity, drain and source resistance, and conduction-band discontinuity at the interface between AlGaN and GaN. The simulation results agree well with our experimental data. It has also been demonstrated that the reduction of the saturation drain current at high power dissipation is primarily due to the decrease of electron mobility in the channel. The proposed model is valuable for predicting and evaluating the performance of different device structures and packages for various applications.
Recombination property of nitrogen-acceptor-bound states in ZnO
Authors: Yang X. D. , Xu Z. Y. , Sun Z. , Sun B. Q. , Ding L. , Wang F. Z. , Ye Z. Z.
Published: March 2006

The recombination property of nitrogen (N)-related acceptor-bound states in ZnO has been investigated by photoluminescence (PL), time-resolved PL, and selective PL. Several possible recombination processes were discussed by analyzing the relaxation and recombination properties under large Coulomb interaction. It is strongly suggested that bound exciton emission dominates the recombination process related to the N acceptor. The recombination lifetime is 750ps and the binding energy is 67meV for N-acceptor-bound exciton at low temperature.
Oxidation of Si nanocrystals fabricated by ultralow-energy ion implantation in thin SiO2 layers
Authors: Coffin H. , Bonafos C. , Schamm S. , Cherkashin N. , Assayag G. Ben , Claverie A. , Respaud M. , Dimitrakis P. , Normand P.
Published: March 2006

The effect of thermal treatments in nitrogen-diluted oxygen on the structural characteristics of two-dimensional arrays of Si nanocrystals (NCs) fabricated by ultralow-energy ion implantation (1 keV) in thin silicon dioxide layers is reported. The NC characteristics (size, density, and coverage) have been measured by spatially resolved electron-energy-loss spectroscopy by using the spectrum-imaging mode of a scanning transmission electron microscope. Their evolution has been studied as a function of thermal treatment duration at a temperature (900 °C) below the SiO2 viscoelastic point. An extended spherical Deal-Grove [J. Appl. Phys. 36, 3770 (1965)] model for self-limiting oxidation of embedded silicon NCs has been carried out. It proposes that the stress effects, due to oxide deformation, slow down the NC oxidation rate and lead to a self-limiting oxide growth. The model predictions show a good agreement with the experimental results. Soft oxidation appears to be a powerful way for manipulating the NC size distribution and surface density.
Probing the intermixing in In(Ga)As∕GaAs self-assembled quantum dots by Raman scattering
Authors: Ibáñez J. , Cuscó R. , Hernández S. , Artús L. , Henini M. , Patanè A. , Eaves L. , Roy Mervyn , Maksym P. A.
Published: March 2006

We show that Raman scattering is a sensitive technique for probing the degree of Ga intermixing in In(Ga)As∕GaAs self-assembled quantum dots (QDs). The shifts of the QD phonon frequency that we observe are explained by the modification of the strain due to Ga incorporation into the QDs from the GaAs matrix during growth. Using an elastic continuum model, we estimate the average In content of the dots from the QD phonon frequency. The varying amount of intermixing in QDs grown with different In compositions, QD layer thicknesses, growth temperatures, and stacking spacer layer thicknesses are investigated. The Raman data indicate that Ga intermixing is larger for QD samples with low In(Ga)As coverage thickness and∕or high growth temperature and, in multilayered systems, for samples with small GaAs spacer layers.
Left-handed material anisotropy effect on guided wave electromagnetic fields
Authors: Krowne Clifford M.
Published: March 2006

Effect of anisotropy on the physical tensor description of the negative index of refraction material acting as a substrate is found on the electromagnetic field distributions. This is done for the case of a microstrip structure, whose configuration is commonly used in microwave and millimeter wave integrated circuits. These ab initio studies have been done self-consistently with a computer code using a full-wave integral equation numerical method based on generalized Green’s function utilizing appropriate boundary conditions. Field distributions are provided over two decades of frequency in the cross section of the uniform guided wave structure, from 0.2to20GHz. It is found that modifying the tensor can allow control because the wave changes volumetrically, or switches from volumetric to surface, in its distribution of fields.
In situ spectroscopic ellipsometry in plasma-assisted molecular beam epitaxy of InN under different surface stoichiometries
Authors: Yoshitani Masayoshi , Akasaka Koichiro , Wang Xinqiang , Che Song-Bek , Ishitani Yoshihiro , Yoshikawa Akihiko
Published: March 2006

InN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy under different surface stoichiometries were characterized by in situ spectroscopic ellipsometry in the range from 0.731eV (1697nm) to 4.95eV (250nm). Nitrogen polarity InN epilayers were grown at 600°C on GaN-underlayer/sapphire substrate. The surface stoichiometry during growth was changed by varying the indium-beam flux under the same nitrogen-beam flux. It was found that the pseudodielectric functions were drastically affected by the surface stoichiometry. The dielectric functions of InN grown under different stoichiometries were obtained. Both real and imaginary parts of the dielectric functions tended to be larger with increasing In-beam flux.
Molecular modeling of metastable FeB49 phase evolution in laser surface engineered coating
Authors: Balani K. , Agarwal A. , Dahotre Narendra B.
Published: March 2006

Interstitial iron-boride phases have been a subject of research interest for a long time owing to their useful properties. Metastable FeB49 phase evolved during nonequilibrium laser surface engineering was investigated along with FeB, Fe2B, and Fe3B phases. Theoretical x-ray diffraction spectrum derived from numerically constructed FeB49 crystal matched with the experimental diffraction pattern from laser surface engineered boride coating on the steel substrate. Furthermore, employment of ab initio SIESTA 1.3 molecular modeling for computation of total crystal energy elucidated instability of the FeB49 phase. The generation of thermodynamically nonequilibrium phase FeB49 along with Fe3B phase was further confirmed by selected area diffraction and high-resolution transmission electron microscopy analyses.
Structure, Curie temperature, and magnetostriction of SmZn1−xMnx alloys
Authors: Wang B. W. , Sun Y. , Cao S. Y. , Huang W. M. , Weng L.
Published: March 2006

The structure, Curie temperature, and magnetostriction of SmZn1−xMnx polycrystalline crystals were investigated by x-ray diffraction, vibrating-sample magnetometer, and standard strain gauge techniques. It is found that SmZn1−xMnx alloys are nearly a single Sm(Zn,Mn) phase with the CsCl-type cubic structure up to x=0.2. The Curie temperature of SmZn1−xMnx alloys quickly increases with increasing Mn when x⩽0.05 and gradually does in the range of 0.05<x⩽0.2. The substitution of Mn for Zn in the SmZn compound has a marked effect on improving the Curie temperature and makes it reach 255K when x=0.2. The magnetization of SmZn1−xMnx alloys increases with increasing Mn content in the range of 0<x⩽0.2 and the magnetostriction increases in the range of 0<x⩽0.15.
Electric fatigue in antiferroelectric ceramics induced by bipolar electric cycling
Authors: Zhou Longjie , Zuo R. Z. , Rixecker G. , Zimmermann A. , Utschig T. , Aldinger F.
Published: March 2006

The fatigue behavior of antiferroelectric lanthanum-doped lead zirconate stannate titanate bulk material induced by bipolar cycling was investigated. Strain and polarization hysteresis loops, acoustic emissions (AEs), and biaxial strength were monitored. The material showed a high resistance to electric fatigue, concerning the losses in maximum strain, switchable polarization, and biaxial strength as well as modifications of AE patterns and microstructure. Fatigue microcracking is weak in the material during cycling. The pinning of antiferroelectric and ferroelectric domains by point defect agglomeration is discussed as the main fatigue mechanism. A diffuse antiferroelectric-ferroelectric phase transition and an asymmetry of the strain hysteresis loop due to the electric cycling are described and explained as a result of the pinning process and offset polarization.
Switching properties of nonlinear electron-wave directional couplers
Authors: Paspalakis Emmanuel , Terzis Andreas F.
Published: March 2006

We study two types of nonlinear electron-wave directional couplers, one with constant coupling coefficient and another with a variable coupling coefficient, and present analytical and numerical results for their switching properties.
Room temperature rectifying characteristics of epitaxial Y1Ba2Cu3−xZnxO7−δ (x=0.0,0.2) and Nb:SrTiO3 (Nb: 0.05%, 0.1%, 0.5%) heterojunctions
Authors: Ramadan W. , Ogale S. B. , Dhar S. , Fu L. F. , Browning N. D. , Venkatesan T.
Published: March 2006

We report on the fabrication and electrical characterization of epitaxial metal-semiconductor junctions between Y1Ba2Cu3O7−δ (YBCO) (optimally doped and Zn doped) and (001) Nb:SrTiO3 with different Nb concentrations (0.05%, 0.1%, and 0.5%). The current-voltage characteristics of such epitaxial junctions are nonlinear and rectifying, and these are dramatically enhanced with decreasing Nb concentration and Zn doping. Indeed, for the case of 0.05% Nb:STO, reverse breakdown voltage as high as −18V(−28V) is realized for optimally doped (Zn doped) YBCO. These data are analyzed within the framework of thermionic emission∕diffusion models for Schottky and metal-insulator-semiconductor-type junctions.
Multilevel based analysis of the thermoluminescence of CaSO4:RE (RE=Tm, Dy, Tb, and Sm)
Authors: Shinsho Kiyomitsu , Suzuki Yuta , Harada Kazumasa , Yamamoto Yusuke , Urushiyama Akio
Published: March 2006

Multitrapping levels feature of thermoluminescent (TL) phosphors of CaSO4:RE (RE=Dy, Tb, and Sm) were analyzed using a variable heating rate method (including extremely slow heating rates of 0.001°Cs−1), the postannealing method, and the analysis of TL decay curves using x-ray induced luminescence (XIL) spectroscopy after x-ray excitation was cut off at temperatures between 100 and 240 °C. All the data confirmed the multilevel feature of the TL levels. The ranges of activation energies E of these components in the main glow were determined to be (a) 1.64-2.03 eV for CaSO4:Dy, (b) 1.66-2.01 eV for CaSO4:Tb, and (c) 1.66-2.00 eV for CaSO4:Sm. The frequency factor s was determined to be 1019s−1 for all components of the phosphors. The previously reported TL properties of the CaSO4:Tm phosphor were reinvestigated using the XIL decay analysis. Assuming that the most populated level is located at the center of the main glow, the four phosphors have similar E values of ∼1.8eV. The present results imply that the multicomponent TL properties of rare-earth-doped CaSO4 phosphors are characterized by a lattice of CaSO4 involving defects introduced around RE3+ ions, and the glow curve properties are essentially not dependent on the rare-earth species introduced.
The effective magnetoelectroelastic moduli of matrix-based multiferroic composites
Authors: Srinivas Shashidhar , Li J. Y. , Zhou Y. C. , Soh A. K.
Published: March 2006

In this paper, we develop a mean field Mori-Tanaka model [T. Mori and K. Tanaka, Acta. Metall. 21, 571 (1973)] to calculate the effective magnetoelectroelastic moduli of matrix-based multiferroic composites, emphasizing the effects of shape and orientation distribution of second phase particles that have not been investigated before. Through a systematic study, it is observed that laminated composites are optimal for magnetoelectric coefficient a11, while fibrous composites are optimal for a33. In addition, these coupling coefficients are maximum when the second phase particles are aligned. It is also postulated that the large discrepancy between theoretical predictions and experimental measurements for magnetoelectric coefficients of multiferroic composites previously reported is partly due to the orientation distribution of second phase particles, which has not been considered before in theoretical modeling. When our calculations take the orientation distribution of second phase particles into account with appropriate texture coefficient, good agreement with experimental data is observed.
Raman scattering from epitaxial HfN layers grown on MgO(001)
Authors: Stoehr M. , Seo H.-S. , Petrov I. , Greene J. E.
Published: March 2006

Stoichiometric single-crystal HfN layers grown on MgO(001) are analyzed by Raman spectroscopy. Second-order Raman scattering predominates, but first-order modes in the acoustic and optical ranges are also visible. The latter indicates that the Oh symmetry of NaCl-structure HfN is broken. The large mass difference between Hf and N leads to a correspondingly large separation, 250cm−1, between the first-order acoustic and optical bands. Within this gap, four Raman lines are clearly observed. The first three are the second-order transverse acoustic mode (240cm−1), the sum of the first-order transverse and longitudinal acoustic modes (280cm−1), and the second-order longitudinal acoustic mode (325cm−1). The fourth line at 380cm−1 is identified as the difference between the first-order optical and acoustic modes. The observed first-order Raman scattering, as well as the width of the gap between the first-order acoustic and optical modes, is in good agreement with previously calculated HfN phonon density of states.
Electronic excitation and optical responses of metal-nanoparticle composites under heavy-ion implantation
Authors: Plaksin Oleg , Takeda Yoshihiko , Amekura Hiroshi , Kishimoto Naoki
Published: March 2006

Measurements of optical transmission and ion-induced photon emission (IIPE) during implantation of copper ions into silica glass (a-SiO2) have been combined in order to study formation of Cu nanoparticles. It has been shown that in situ measurements are more advantageous than conventional experimental approaches of examining the properties of ion-implanted nanocomposites solely after implantation. Series of experiments have been done to prove that the band of IIPE at 545–550 nm originates from Cu+ solutes in a-SiO2. The combination of in situ optical techniques provides means of monitoring Cu nanoparticles and Cu solutes via the optical absorption in the range of the surface-plasmon resonance (SPR) of nanoparticles and the IIPE of Cu+ solutes. It has been shown that a simple linear approximation can be used to separate optical bands of defects, Cu solutes, and nanoparticles. Ion-induced transient optical absorption has been found in the ranges of SPR and defect bands. The transient optical response in the range of SPR has been tentatively attributed to a thermomodulation effect. We analyze relationships between electronic excitation, radiation-induced optical responses, and kinetics of nanoparticle formation. Several stages of nanoparticle formation have been distinguished: accumulation of Cu solutes, precipitation of Cu atoms, coalescence, growth of nanoparticles, and saturation of nanocomposites.
Electronic structure of Nb2N and NbN thin films
Authors: Sanjinés R. , Benkahoul M. , Papagno M. , Lévy F. , Music D.
Published: March 2006

The electronic structure of the hexagonal β-Nb2N, hexagonal δ′-NbN, and cubic δ-NbN thin films have been investigated by x-ray photoemission spectroscopy. Calculated density of states for these phases are also presented and used in the interpretation of the experimental valence band spectra. The experimental energy distribution curves are consistent with the calculated densities of states. Comparing hexagonal to cubic phases, both β-Nb2N and δ′-NbN are more covalent than the cubic phase NbN. Prominent covalent bonding in hexagonal β-Nb2N and δ′-NbN nitrides can be related with the higher hardness values as compared to that of the cubic phase. This study discloses relevant observations important for both the technological applications and fundamental properties.
Influence of metal trapping on the shape of cavities induced by high energy He+ implantation
Authors: El Bouayadi R. , Regula G. , Lancin M. , Pichaud B. , Desvignes M.
Published: March 2006

In He implantation induced cavities highly contaminated with metals (Au, Ni, Pt) we found that, when no three-dimensional structure is observed, the shape of the cavities can be strongly modified depending on the nature of the metal and on its trapped quantity. The equilibrium shape of cavities is the Wulff shape associated with the minimum surface energy which can be determined using the code WULFFMAN. On the basis of these computations the effect of a metal chemisorption may be accounted for. At very low coverage (far below 1%) there is no effect to be expected. At coverages between 1% and 10%, independent of the nature of the metal, a reduction of the specific surface energy of the vicinal surfaces may produce spherical cavities. Eventually for coverages close to one monolayer, the specific surface energy of the concerned metal will drive the cavities toward spherical or highly facetted shapes depending on whether the specific energy of the metal is smaller or higher than the vicinal one of silicon.
Roles of microcrystalline silicon p layer as seed, window, and doping layers for microcrystalline silicon p-i-n solar cells
Authors: Fujibayashi Takashi , Kondo Michio
Published: March 2006

The roles of the hydrogenated microcrystalline silicon (μc-Si:H) p layer in the μc-Si:H p-i-n solar cell fabricated by plasma-enhanced vapor deposition are determined through evaluation of the photovoltaic characteristics of solar cells fabricated by varying the deposition time of p layer. Mechanisms of p-layer growth are analyzed with in situ Auger electron spectroscopy and ex situ Raman scattering spectroscopy. Each successive regime of film growth including an amorphous silicon layer, an incubation layer containing crystalline silicon nuclei, and a layer filled with conical crystalline silicon grains that evolves in the p-layer process leads to diverse changes in the crystalline development of the subsequent μc-Si:H i layer and in the characteristics of the solar cell.
In-rich InGaN∕GaN quantum wells grown by metal-organic chemical vapor deposition
Authors: Kwon Soon-Yong , Kim Hyun Jin , Na Hyunseok , Kim Young-Woon , Seo Hui-Chan , Kim Hee Jin , Shin Yoori , Yoon Euijoon , Park Yoon-Soo
Published: March 2006

Growth mechanism of In-rich InGaN∕GaN quantum wells (QWs) was investigated. First, we examined the initial stage of InN growth on GaN template considering strain-relieving mechanisms such as defect generation, islanding, and alloy formation at 730 °C. It was found that, instead of formation of InN layer, defective In-rich InGaN layer with thickness fluctuations was formed to relieve large lattice mismatch over 10% between InN and GaN. By introducing growth interruption (GI) before GaN capping at the same temperature, however, atomically flat InGaN∕GaN interfaces were observed, and the quality of In-rich InGaN layer was greatly improved. We found that decomposition and mass transport processes during GI in InGaN layer are responsible for this phenomenon. There exists severe decomposition in InGaN layer during GI, and a 1-nm-thick InGaN layer remained after GI due to stronger bond strength near the InGaN∕GaN interface. It was observed that the mass transport processes actively occurred during GI in InGaN layer above 730 °C so that defect annihilation in InGaN layer was greatly enhanced. Finally, based on these experimental results, we propose the growth mechanism of In-rich InGaN∕GaN QWs using GI.
Rotational and vibrational state distributions of H2 activated on a heated tungsten filament
Authors: Umemoto Hironobu , Ansari S. G. , Matsumura Hideki
Published: March 2006

The rotational and vibrational state distributions of H2 activated on a heated tungsten filament were determined by employing a coherent anti-Stokes Raman scattering technique to examine the contribution to the catalytic chemical vapor deposition process. The rotational excitation could be confirmed and the distribution was Boltzmann-like. When the filament temperature was 2700 K and the H2 pressure was over 1.3 kPa, the rotational temperature monitored 5 cm under the filament was around 1200 K; it showed a sharp decrease below 670 Pa and it was 700 K at 67 Pa. This decrease in the rotational temperature suggests the importance of relaxation processes on the chamber walls. The first vibrationally excited H2 molecules could also be identified at pressures over 670 Pa and the vibrational temperature was not much different from the rotational one. This vibrational temperature is much lower than those in typical H2 plasma, showing that the direct vibrational excitation of H2 on hot filaments is inefficient compared to its dissociation to two H atoms.
Structure and magnetic properties of iron nanoparticles stabilized in carbon
Authors: Enz Thorsten , Winterer Markus , Stahl Branko , Bhattacharya Sarbari , Miehe Gerhard , Foster Keir , Fasel Claudia , Hahn Horst
Published: March 2006

Nanoparticles composed of iron and carbon have been produced by chemical vapor synthesis. A detailed structural, electronic, and magnetic characterization has been performed by several methods. The atomic arrangement in the as-prepared particles is strongly affected and stabilized by excess carbon. Small clusters of different ferrous phases are the building blocks of the particles. Due to the in situ formation of a carbonaceous shell the particles are stable against oxidation at ambient conditions. The magnetic properties are influenced by the exceptionally small particle size. The particles exhibit superparamagnetic behavior with a blocking temperature of 30K and the temperature dependence of the magnetization is governed by the finite size of the system.
Room-temperature 1.6μm light emission from InAs∕GaAs quantum dots with a thin GaAsSb cap layer
Authors: Liu H. Y. , Steer M. J. , Badcock T. J. , Mowbray D. J. , Skolnick M. S. , Suarez F. , Ng J. S. , Hopkinson M. , David J. P. R.
Published: March 2006

It is demonstrated that the emission of InAs quantum dots (QDs) capped with GaAsSb can be extended from 1.28to1.6μm by increasing the Sb composition of the capping layer from 14% to 26%. Photoluminescence excitation spectroscopy is applied to investigate the nature of this large redshift. The dominant mechanism is shown to be the formation of a type-II transition between an electron state in the InAs QDs and a hole state in the GaAsSb capping layer. The prospects for using these structures to fabricate 1.55μm injection lasers are discussed.
Theoretical calculation of the excited states of the KCs molecule including the spin-orbit interaction
Authors: Korek M. , Moghrabi Y. A. , Allouche A. R.
Published: March 2006

For the molecule KCs the potential energy has been calculated for the 72 lowest molecular states Ω. Using an ab initio method the calculation is based on nonempirical pseudopotentials within the range of 5.0a0–34.0a0 of the internuclear distance R. Gaussian basis sets have been used for both atoms and spin-orbit effects have been taken into account through a semiempirical spin-orbit pseudopotential added to the electrostatic Hamiltonian. The spectroscopic constants for 60 states have been calculated by fitting the calculated energy values to a polynomial in terms of the internuclear distance R. The components of the spin-orbit splitting for (1,2,5,6)Π3 and (1)Δ3 have been identified. The comparison of the present results with those available in the literature shows a very good agreement, while the other results, to the best of our knowledge, are given here for the first time.
β -plane turbulence in a basin with no-slip boundaries
Authors: Kramer W. , van Buren M. G. , Clercx H. J. H. , van Heijst G. J. F.
Published: March 2006

On a domain enclosed by no-slip boundaries, two-dimensional, geostrophic flows have been studied by numerical simulations of the Navier-Stokes equation with the β-plane approximation at intermediate Reynolds numbers and a range of values for β. The β effect causes a refinement of the flow structures, and the presence of basin modes has been revealed by means of frequency spectra. The presence and apparent stability of basin modes on a domain enclosed by no-slip boundaries is a rather surprising observation, because these modes are solutions of the inviscid flow equations on a bounded domain (with free-slip boundaries). To understand the persistence of these basin modes, the viscous boundary layers near the no-slip walls have been investigated. The mean flow in forced simulations shows a zonal band structure, much unlike the regular Fofonoff-like solution observed when free-slip boundary conditions are used.
Imaging of ferroelectric films under an applied electric field by scanning electron microscopy
Authors: Lev U. , Zolotoyabko E.
Published: March 2006

We report on scanning electron microscopy (SEM) measurements under external electric fields as a tool for studying electric-field-induced processes in ferroelectric thin films. Epitaxial films of BaTiO3, grown by metal-organic chemical-vapor deposition on MgO substrates, served as samples in this research. Electric fields of up to 6MV∕m were applied to interdigital electrodes having 5μm blank intervals between metallic strips deposited on the top of the films. In situ SEM measurements under applied electric fields revealed time-dependent variations in contrast, attributed to domain rearrangements. The results obtained are analyzed in terms of nucleation and growth of energetically favorable domains. The critical electric field of about 10MV∕m for the 90°-domain flips deduced from these measurements is in agreement with previously published data.
Effect of pressure on magnetocrystalline anisotropy and magnetic phase diagram of ThFe11Cx (x=1.5 and 1.8) compounds
Authors: Isnard O. , Arnold Z. , Kamarad J. , Buschow K. H. J.
Published: March 2006

The magnetic properties of the ThFe11Cx (x=1.5 and 1.8) compounds have been investigated at ambient and high pressures. The thermal evolution of the saturation magnetization has been determined up to 550K. These compounds are found to have a reorientation of their easy magnetization axis, which is aligned along the tetragonal c axis at room temperature but lies within the basal plane at 4K. The reorientation of magnetization results from competing magnetic anisotropy of inequivalent iron sites. The spin reorientation temperatures are 257 and 202K for ThFe11C1.5 and ThFe11C2, respectively, showing a significant effect of the carbon concentration. The anisotropy parameters have been obtained on both sides of the spin reorientation transition in the 4–300K temperature range. The effect of pressure on the spin reorientation temperature has been investigated up to 10kbars. This spin reorientation transition is found to decrease significantly upon increasing pressure with rates of −9.4 and −8K∕kbar for ThFe11C1.5 and ThFe11C1.8, respectively. A magnetic phase diagram is proposed for both ThFe11Cx compounds.
Magneto-optical properties of La0.7Sr0.3MnO3 thin films with perpendicular magnetic anisotropy
Authors: Liu H. L. , Lu K. S. , Kuo M. X. , Uba L. , Uba S. , Wang L. M. , Jeng H.-T.
Published: March 2006

We report the magneto-optical (MO) properties of compressively strained La0.7Sr0.3MnO3 (LSMO) thin films epitaxially grown on a LaAlO3 substrate. The magnetic force microscope images show the stripe magnetic domains, characteristic of films with the perpendicular magnetic anisotropy (PMA). The optical reflectance and transmittance of the samples were measured over a broad energy range from the far infrared through the ultraviolet. To extract the optical constants of the films, we analyzed all of the layers of this thin-film structure using a Drude-Lorentz model. From the parameters obtained, we compute the optical constants, such as frequency-dependent optical conductivity and the diagonal components of the dielectric tensor. Moreover, the MO polar Kerr spectra of the samples were measured in an applied magnetic field of 1.5T between 0.74 and 5.8eV. The off-diagonal components of the dielectric tensor were then calculated by analyzing Kerr rotation, ellipticity, and the determined diagonal elements of the dielectric tensor. These functions yield information about the spin-dependent electronic structures of the LSMO thin films. We observed several MO-active transitions above 2eV. These bands arise from the Mn-d(t2g) to Mn-d(eg) in the majority-spin channel and the onset of O-2p to Mn-d(eg) and O-2p to Mn-d(t2g) transitions in the minority-spin channel, in accord with the band-structure calculations. Most interestingly, their peak position and intensity are dependent on the film thickness, suggesting possible applications for tailoring MO responses in these PMA thin films.