bluuebunny/crossref_metadata_2025 huggingface dataset:
Reaction of C2H2+ (n·ν2, m·ν5) with NO2: Reaction on the singlet and triplet surfaces
Authors: Boyle Jason M. , Bell David M. , Anderson Scott L.
Published: January 2011

Integral cross sections and product recoil velocity distributions were measured for reaction of C2H2+ with NO2, in which the C2H2+ reactant was prepared in its ground state, and with mode-selective excitation in the cis-bend (2ν5) and CC stretch (n·ν2, n = 1, 2). Because both reactants have one unpaired electron, collisions can occur with either singlet or triplet coupling of these unpaired electrons, and the contributions are separated based on distinct recoil dynamics. For singlet coupling, reaction efficiency is near unity, with significant branching to charge transfer (NO2+), O− transfer (NO+), and O transfer (C2H2O+) products. For triplet coupling, reaction efficiency varies between 13% and 19%, depending on collision energy. The only significant triplet channel is NO+ + triplet ketene, generated predominantly by O− transfer, with a possible contribution from dissociative charge transfer at high collision energies. NO2+ formation (charge transfer) can only occur on the singlet surface, and appears to be mediated by a weakly bound complex at low energies. O transfer (C2H2O+) also appears to be dominated by reaction on the singlet surface, but is quite inefficient, suggesting a bottleneck limiting coupling to this product from the singlet reaction coordinate. The dominant channel is O− transfer, producing NO+, with roughly equal contributions from reaction on singlet and triplet surfaces. The effects of C2H2+ vibration are modest, but mode specific. For all three product channels (i.e., charge, O−, and O transfer), excitation of the CC stretch fundamental (ν2) has little effect, 2·ν2 excitation results in ∼50% reduction in reactivity, and excitation of the cis-bend overtone (2·ν5) results in ∼50% enhancement. The fact that all channels have similar mode dependence suggests that the rate-limiting step, where vibrational excitation has its effect, is early on the reaction coordinate, and branching to the individual product channels occurs later.
Evaluation of a digital data acquisition system and optimization of n-γ discrimination for a compact neutron spectrometer
Authors: Giacomelli L. , Zimbal A. , Reginatto M. , Tittelmeier K.
Published: January 2011

A compact NE213 liquid scintillation neutron spectrometer with a new digital data acquisition (DAQ) system is now in operation at the Physikalisch-Technische Bundesanstalt (PTB). With the DAQ system, developed by ENEA Frascati, neutron spectrometry with high count rates in the order of 5×105 s−1 is possible, roughly an order of magnitude higher than with an analog acquisition system. To validate the DAQ system, a new data analysis code was developed and tests were done using measurements with 14-MeV neutrons made at the PTB accelerator. Additional analysis was carried out to optimize the two-gate method used for neutron and gamma (n-γ) discrimination. The best results were obtained with gates of 35 ns and 80 ns. This indicates that the fast and medium decay time components of the NE213 light emission are the ones that are relevant for n-γ discrimination with the digital acquisition system. This differs from what is normally implemented in the analog pulse shape discrimination modules, namely, the fast and long decay emissions of the scintillating light.
Quasiclassical trajectory calculations for Li(22PJ) + H2 → LiH(X1Σ+) + H: Influence by vibrational excitation and translational energy
Authors: Hsiao Ming-Kai , Lin King-Chuen , Hung Yu-Ming
Published: January 2011

Ab initio potential energy surfaces and the corresponding analytical energy functions of the ground 1A′ and excited 2A′ states for the Li(22P) plus H2 reaction are constructed. Quasiclassical trajectory calculations on the fitted energy functions are performed to characterize the reactions of Li(22P) with H2(v = 0, j = 1) and H2(v = 1, j = 1) as well as the reaction when the vibrational energy is replaced by collision energy. For simplicity, the transition probability is assumed to be unity when the trajectories go through the crossing seam region and change to the lower surface. The calculated rotational distributions of LiH(v = 0) for both H2(v = 0, j = 1) and H2(v = 1, j = 1) reactions are single-peaked with the maximum population at j′ = 7, consistent with the previous observation. The vibrational excitation of H2(v = 1) may enhance the reaction cross section of LiH(v′ = 0) by about 200 times, as compared to a result of 93–107 reported in the experimental measurements. In contrast, the enhancement is 3.1, if the same amount of energy is deposited in the translational states. This endothermic reaction can be considered as an analog of late barrier. According to the trajectory analysis, the vibrational excitation enlarges the H–H distance in the entrance channel to facilitate the reaction, but the excess energy may not open up additional reaction configuration.
A low-wavenumber-extended confocal Raman microscope with very high laser excitation line discrimination
Authors: Lebedkin Sergei , Blum Carolin , Stürzl Ninette , Hennrich Frank , Kappes Manfred M.
Published: January 2011

We describe the simple modification of a confocal Raman imaging microscope to incorporate two ultra-narrow holographic notch filters. The modified microscope rejects the laser excitation line (Rayleigh peak) by a discrimination factor of ∼1011 and allows simultaneous measurements of Stokes/anti-Stokes Raman shifts as close as ∼10/20 cm−1 to the Rayleigh line. The extremely high rejection ratio of the Rayleigh peak results in its intensity becoming comparable to typical Raman scattering signals. This is essential for micro-Raman spectroscopy and imaging in the low-wavenumber region. We illustrate the resulting performance with measurements on silicon/silica, sapphire, sulfur, L-cystine, as well as on single-walled carbon nanotubes (SWNTs). We find that both aggregated (bulk) and individual (deposited on substrate) SWNTs demonstrate strong and broad characteristic Raman features below ∼100 cm−1—in a region which has remained essentially unexplored in measurements of bulk SWNT samples and which has so far been inaccessible for Raman spectroscopy of individual SWNTs.
Design, construction, and performance of a real-time holographic refractometry prototype for liquid analysis
Authors: Barbosa E. A. , Silva D. M. , Preto A. O. , Verzini R.
Published: January 2011

The development and the performance of a portable holographic refractometer prototype for liquid measurement employing multimode diode lasers with emission centered at 662 nm as light sources is reported. Due to the multiwavelength character of the holographic recording, a synthetic wavelength was generated, and the diffracted wave intensity was thus modulated as a function of the optical path difference between the reference and the object beams. The transparent test cell containing the liquid was placed at the reference-beam arm of the optical setup, while the contour interferogram generated on the holographic image of a flat object was used for fringe counting. A change ΔL on the liquid column length is proportional to the Δp running fringes on the object image, and from this relation the refractive index of the test liquid was obtained. The holograms were recorded on a photorefractive Bi12TiO20 crystal whether using a single multimode diode laser or by combining two diode lasers. In the latter configuration the synthetic wavelength can be varied in order to enhance the measurement sensitivity and/or to allow the analysis of turbid liquids. The size of the whole prototype is 54 × 22 × 14 cm3. The refractive indexes of ethanol/water mixtures with different concentrations were measured, as well as the NaCl concentrations in aqueous solutions were determined upon comparison with an empirical curve. In both cases the results were compared with the ones obtained through an Abbe refractometer.
A new Lanczos-based algorithm for simulating high-frequency two-dimensional electron spin resonance spectra
Authors: Chiang Yun-Wei , Freed Jack H.
Published: January 2011

The Lanczos algorithm (LA) is a useful iterative method for the reduction of a large matrix to tridiagonal form. It is a storage efficient procedure requiring only the preceding two Lanczos vectors to compute the next. The quasi-minimal residual (QMR) method is a powerful method for the solution of linear equation systems, Ax = b. In this report we provide another application of the QMR method: we incorporate QMR into the LA to monitor the convergence of the Lanczos projections in the reduction of large sparse matrices. We demonstrate that the combined approach of the LA and QMR can be utilized efficiently for the orthogonal transformation of large, but sparse, complex, symmetric matrices, such as are encountered in the simulation of slow-motional 1D- and 2D-electron spin resonance (ESR) spectra. Especially in the 2D-ESR simulations, it is essential that we store all of the Lanczos vectors obtained in the course of the LA recursions and maintain their orthogonality. In the LA-QMR application, the QMR weight matrix mitigates the problem that the Lanczos vectors lose orthogonality after many LA projections. This enables substantially more Lanczos projections, as required to achieve convergence for the more challenging ESR simulations. It, therefore, provides better accuracy for the eigenvectors and the eigenvalues of the large sparse matrices originating in 2D-ESR simulations than does the previously employed method, which is a combined approach of the LA and the conjugate-gradient (CG) methods, as evidenced by the quality and convergence of the 2D-ESR simulations. Our results show that very slow-motional 2D-ESR spectra at W-band (95 GHz) can be reliably simulated using the LA-QMR method, whereas the LA-CG consistently fails. The improvements due to the LA-QMR are of critical importance in enabling the simulation of high-frequency 2D-ESR spectra, which are characterized by their very high resolution to molecular orientation.
Investigation of electrical shading effects in back-contacted back-junction silicon solar cells using the two-dimensional charge collection probability and the reciprocity theorem
Authors: Reichel C. , Granek F. , Hermle M. , Glunz S. W.
Published: January 2011

In this study, short-circuit current losses in high-efficiency n-type back-contacted back-junction silicon solar cells caused by the electrical shading effect have been investigated by two-dimensional simulations of the charge carrier collection probability. Based on the reciprocity theorem, the homogenous partial differential equation describing the probability of charge carriers being collected by the p-n junction on the rear side of the solar cell has been solved numerically using the finite element method implemented in the partial differential equation solver COMSOL Multiphysics. The method has been applied to study the impact of geometrical parameters of the solar cell, such as the pitch distance, as well as the emitter and back surface field width, on the local and global internal quantum efficiency and on the short-circuit current density. The influence of the rear surface recombination velocity of an undiffused gap and the effective rear surface recombination velocity of the back surface field region on the short-circuit current density is also presented. It has been found that the width and the surface recombination velocity of the undiffused gap on the rear side of the solar cell have a strong impact on the charge collection probability in the base. Thus, the surface recombination velocity of the undiffused gap has to be minimized or the undiffused gap has to be reduced or omitted completely in order to increase the short-circuit current density significantly. Furthermore, it has been found that low base doping concentrations are essential for minimizing the effective rear surface recombination velocity of the back surface field. It has also been shown that in the case of low base doping concentrations, the short-circuit current density reaches its maximum value and that in this case it is nearly independent of the back surface field doping concentration.
Sedimentation of aggregating colloids
Authors: Whitmer Jonathan K. , Luijten Erik
Published: January 2011

We investigate the combined effects of gravity, attractive interactions, and Brownian motion in suspensions of colloidal particles and nonadsorbing polymer. Depending on the effective strength of gravitational forces, resulting from a density mismatch between the colloids and the solvent, and the magnitude and range of the depletion interactions induced by the polymer, sedimentation in these suspensions can result in an equilibrium structure or a kinetically arrested state. We employ large-scale molecular dynamics simulations to systematically classify the different regimes that arise as a function of attraction strength and gravitational stress. Whereas strong attractions lead to cluster aggregation and low-density arrested states, moderate attractions can enhance crystallization of the colloidal particles in the sediment. We make direct comparisons to experimental results to infer general conclusions about the mechanisms leading to mechanically stable sediments.
Semiclassical dynamics simulations of charge transport in stacked π-systems
Authors: Plasser Felix , Lischka Hans
Published: January 2011

Charge transfer processes within stacked π-systems were examined for the stacked ethylene dimer radical cation with inclusion of a bridge containing up to three formaldehyde molecules. The electronic structure was treated at the complete active space self-consistent field and multireference configuration interaction levels. Nonadiabatic interactions between electronic and nuclear degrees of freedom were included through semiclassical surface hopping dynamics. The processes were analyzed according to fragment charge differences. Static calculations explored the dependence of the electronic coupling and on-site energies on varying geometric parameters and on the inclusion of a bridge. The dynamics simulations gave the possibility for directly observing complex charge transfer and diabatic trapping events.
Direct observation of the substitution effects on the hydrogen bridge dynamics in selected Schiff bases—A comparative molecular dynamics study
Authors: Jezierska-Mazzarello Aneta , Panek Jarosław J. , Vuilleumier Rodolphe , Koll Aleksander , Ciccotti Giovanni
Published: January 2011

We have studied substituent effects on the properties of the intramolecular hydrogen bond of some ortho-hydroxy Schiff bases using density functional theory (DFT) based first-principle molecular dynamics (FPMD) and path integral molecular dynamics. The studied compounds possess a strong intramolecular hydrogen bond (r(O⋅⋅⋅N) ≤ 2.6 Å), which can be tuned by substitution to either (i) enhance the basicity of the acceptor moiety by induction effects or (ii) decrease the hydrogen bond length through steric repulsion. DFT calculations and FPMD were employed to investigate structural and dynamical properties of the selected molecules, while quantum effects on the structural properties were assessed using path integral FPMD. The simulations were performed in vacuo and in the solid state to study the influence of the environment on the hydrogen bond and spectroscopic properties. We give computational support to the suggestion that induction effects are less effective to tune the intramolecular hydrogen bond properties of the discussed ortho-hydroxy Schiff bases than the steric or the environmental effects.
An apparatus for simultaneous measurement of electrical conductivity and thermopower of thin films in the temperature range of 300–750 K
Authors: Ravichandran J. , Kardel J. T. , Scullin M. L. , Bahk J.-H. , Heijmerikx H. , Bowers J. E. , Majumdar A.
Published: January 2011

An automated apparatus capable of measuring the electrical conductivity and thermopower of thin films over a temperature range of 300–750 K is reported. A standard dc resistance measurement in van der Pauw geometry was used to evaluate the electrical conductivity, and the thermopower was measured using the differential method. The design of the instrument, the methods used for calibration, and the measurement procedure are described in detail. Given the lack of a standard National Institute of Standards and Technology (Gaithersburg, Md.) sample for high temperature thermopower calibration, the disclosed calibration procedure shall be useful for calibration of new instruments.
Effects of osmolytes on the helical conformation of model peptide: Molecular dynamics simulation
Authors: Mehrnejad Faramarz , Ghahremanpour Mohammad Mehdi , Khadem-Maaref Mahmoud , Doustdar Farahnoosh
Published: January 2011

Co-solvents such as glycerol and sorbitol are small organic molecules solvated in the cellular solutions that can have profound effects on the protein structures. Here, the molecular dynamics simulations and comparative structural analysis of magainin, as a peptide model, in pure water, 2,2,2-trifluoroethanol/water, glycerol/water, and sorbitol/water are reported. Our results show that the peptide NMR structure is largely maintained its native structure in osmolytes–water mixtures. The simulation data indicates that the stabilizing effect of glycerol and sorbitol is induced by preferential accumulation of glycerol and sorbitol molecules around the nonpolar and aromatic residues. Thus, the presence of glycerol and sorbitol molecules decreases the interactions of water molecules with the hydrophobic residues of the peptide, and the alpha helical structure is stabilized.
Thermal exchange bias field drift in field cooled Mn83Ir17/Co70Fe30 thin films after 10 keV He ion bombardment
Authors: Ehresmann Arno , Schmidt Christoph , Weis Tanja , Engel Dieter
Published: January 2011

The thermal exchange bias field drift of sputter deposited Mn83Ir17(15 nm)/Co70Fe30(10 nm)/Ta thin films at room temperature after 10 keV He+ ion bombardment in an externally applied in-plane magnetic field for different ion fluences was studied. Although field cooling of the layer system resulted in a temporally stable exchange bias field at room temperature the exchange bias field starts to drift after ion bombardment like in non-annealed samples. Between 1 and 648 h after ion bombardment a logarithmic increase in the absolute magnitude of the exchange bias field is observed. A tentative model is presented for its description based on noninteracting domains in the antiferromagnet. A comparison between experimental data and the model reveals the delicate interplay between the ion bombardment modified average antiferromagnetic anisotropy constants, exchange coupling constants, and relaxation time distributions in the polycrystalline layer system influencing the thermal drift velocities.
Microwave N2–Ar plasma torch. II. Experiment and comparison with theory
Authors: Henriques J. , Tatarova E. , Dias F. M. , Ferreira C. M.
Published: January 2011

Spatially resolved emission spectroscopy techniques have been used to determine the gas temperature, the electron, and N2+ ion densities and the relative emission intensities of radiative species in a microwave (2.45 GHz) plasma torch driven by a surface wave. The experimental results have been analyzed in terms of a two-dimensional theoretical model based on a self-consistent treatment of particles kinetics, gas dynamics, and wave electrodynamics. The measured spatial variations in the various quantities agree well with the model predictions. The radially averaged gas temperature is around 3000 K and varies only slowly along the discharge zone of the source but it drops sharply down to about 400 K in the postdischarge. The experimental wave dispersion characteristics nearly follow the theoretical ones, thus confirming that this plasma source is driven by a surface wave.
Effect of the concentration on sol–gel transition of telechelic polyelectrolytes
Authors: Zhang Ran , Shi Tongfei , Li Hongfei , An Lijia
Published: January 2011

Telechelic polyelectrolytes, bearing short hydrophobic blocks at both ends, will ionize into polyions and their counterions when dissolved in water. With the variation of concentration, the interplay between short range attraction and the long range electrostatic interaction as well as the counterion distribution exerts a major influence on the chain conformations (two basic conformations: loop and nonloop, the latter can be subdivided into three association types: free, dangling, and bridge), the cluster structure and the forming of a physical gel. For weak hydrophobic interaction, the relative strong electrostatic interaction dominates the gelation progress; sol–gel transition occurs at higher concentrations due to electrostatic screening and mainly involves the forming of stretched nonloop conformations such as dangling and bridge. While for strong hydrophobic interaction, the hydrophobic interaction dominates and the electrostatic interaction provides a contribution to the formation of gels by maintaining a spatial swelling structure, resulting in a much lower concentration region of sol–gel transition; besides, the sol–gel transition is characterized by the competition of the forming of loop and bridge chains.
Bäcklund transformation, Lax pair, and solutions for the Caudrey–Dodd–Gibbon equation
Authors: Qu Qi-Xing , Tian Bo , Sun Kun , Jiang Yan
Published: January 2011

By using Bell polynomials and symbolic computation, we investigate the Caudrey–Dodd–Gibbon equation analytically. Through a generalization of Bells polynomials, its bilinear form is derived, based on which, the periodic wave solution and soliton solutions are presented. And the soliton solutions with graphic analysis are also given. Furthermore, Bäcklund transformation and Lax pair are derived via the Bells exponential polynomials. Finally, the Ablowitz-Kaup-Newell-Segur system is constructed.
Invited Article: CO2 laser production of fused silica fibers for use in interferometric gravitational wave detector mirror suspensions
Authors: Heptonstall A. , Barton M. A. , Bell A. , Cagnoli G. , Cantley C. A. , Crooks D. R. M. , Cumming A. , Grant A. , Hammond G. D. , Harry G. M. , Hough J. , Jones R. , Kelley D. , Kumar R. , Martin I. W. , Robertson N. A. , Rowan S. , Strain K. A. , Tokmakov K. , van Veggel M.
Published: January 2011

In 2000 the first mirror suspensions to use a quasi-monolithic final stage were installed at the GEO600 detector site outside Hannover, pioneering the use of fused silica suspension fibers in long baseline interferometric detectors to reduce suspension thermal noise. Since that time, development of the production methods of fused silica fibers has continued. We present here a review of a novel CO\documentclass[12pt]{minimal}\begin{document}$_2$\end{document}2 laser-based fiber pulling machine developed for the production of fused silica suspensions for the next generation of interferometric gravitational wave detectors and for use in experiments requiring low thermal noise suspensions. We discuss tolerances, strengths, and thermal noise performance requirements for the next generation of gravitational wave detectors. Measurements made on fibers produced using this machine show a 0.8% variation in vertical stiffness and 0.05% tolerance on length, with average strengths exceeding 4 GPa, and mechanical dissipation which meets the requirements for Advanced LIGO thermal noise performance.
Strong correlation treated via effective hamiltonians and perturbation theory
Authors: Li Manni Giovanni , Aquilante Francesco , Gagliardi Laura
Published: January 2011

We propose a new approach to determine a suitable zeroth-order wavefunction for multiconfigurational perturbation theory. The same ansatz as in complete active space (CAS) wavefunction optimization is used but it is split in two parts, a principal space (A) and a much larger extended space (B). Löwdin's partitioning technique is employed to map the initial eigenvalue problem to a dimensionality equal to that of (A) only. Combined with a simplified expression for the (B) portion of the wavefunction, we are able to drastically reduce the storage and computational demands of the wavefunction optimization. This scheme is used to produce reference wavefunctions and energies for subsequent second-order perturbation theory (PT2) corrections. Releasing the constraint of computing the exact CAS energy and wavefunction prior to the PT2 treatment introduces a nonstandard paradigm for multiconfigurational methods. Based on the results of test calculations, we argue that principal parts with only few percents of the total number of CAS configurations could provide final multiconfigurational PT2 energies of the same accuracy as in the standard paradigm. In the future, algorithmic improvements for this scheme will bring into reach active spaces much beyond the present limit of CAS-based methods, therefore allowing for accurate studies of systems featuring strong correlation.
Thermal frequency noise in dynamic scanning force microscopy
Authors: Colchero J. , Cuenca M. , Martínez J. F. González , Abad J. , García B. Pérez , Palacios-Lidón E. , Abellán J.
Published: January 2011

Thermal fluctuation of the cantilever position sets a fundamental limit for the precision of any scanning force microscope. In the present work we analyze how these fluctuations limit the determination of the resonance frequency of the tip-sample system. The basic principles of frequency detection in dynamic scanning force microscopy are revised and the precise response of a typical frequency detection unit to thermal fluctuation of the cantilever is analyzed in detail. A general relation for thermal frequency noise is found as a function of measurement bandwidth and cantilever oscillation. For large oscillation amplitude and low bandwidth, this relation converges to the result known from the literature, while for low oscillation amplitude and large bandwidth, we find that the thermal frequency noise is equal to the width of the resonance curve and, therefore, stays finite, contrary to what is predicted by the relation known so far. The results presented in this work fundamentally determine the ultimate limits of dynamic scanning force microscopy.
Atomic effect algebras with compression bases
Authors: Caragheorgheopol Dan , Tkadlec Josef
Published: January 2011

Compression base effect algebras were recently introduced by Gudder [Demonstr. Math. 39, 43 (2006)]. They generalize sequential effect algebras [Rep. Math. Phys. 49, 87 (2002)] and compressible effect algebras [Rep. Math. Phys. 54, 93 (2004)]. The present paper focuses on atomic compression base effect algebras and the consequences of atoms being foci (so-called projections) of the compressions in the compression base. Part of our work generalizes results obtained in atomic sequential effect algebras by Tkadlec [Int. J. Theor. Phys. 47, 185 (2008)]. The notion of projection-atomicity is introduced and studied, and several conditions that force a compression base effect algebra or the set of its projections to be Boolean are found. Finally, we apply some of these results to sequential effect algebras and strengthen a previously established result concerning a sufficient condition for them to be Boolean.
Structural and electronic properties of hydrogenated polymorphous silicon films deposited at high rate
Authors: Soro M. Y. , Gueunier-Farret M. E. , Kleider J. P.
Published: January 2011

Structural and electronic defect-related properties of hydrogenated polymorphous silicon (pm-Si:H) films deposited at high rates of up to 9 Å/s have been investigated. Raman spectroscopy reveals the usual peaks characteristic of the amorphous nature of the material, as well as intermediate peaks associated with the effect of nanocrystallites in the amorphous matrix. These results have been correlated with infrared measurements. Transport and defect-related properties deduced from a set of complementary techniques show that hydrogenated polymorphous silicon presents better properties compared to the standard amorphous silicon (a-Si:H). In particular, capacitance measurements on Schottky diodes reveal a very low density of states at the Fermi level. Based on experimental results, band gap states modeling has been achieved for pm-Si:H and a-Si:H by means of a numerical calculation software. Some parameters that are not directly measured with experiments, such as capture cross sections, could be deduced from simulations and differences between pm-Si:H and a-Si:H could be further revealed.
Double step structure and meandering due to the many body interaction at GaN(0001) surface in N-rich conditions
Authors: Załuska-Kotur Magdalena A. , Krzyżewski Filip , Krukowski Stanisław
Published: January 2011

Growth of gallium nitride on GaN(0001) surface is modeled by Monte Carlo method. Simulated growth is conducted in N-rich conditions, hence it is controlled by Ga atoms surface diffusion. It is shown that dominating four-body interactions of Ga atoms can cause step flow anisotropy. Kinetic Monte Carlo simulations show that parallel steps with periodic boundary conditions form double terrace structures, whereas initially V-shaped parallel step train initially bends and then every second step moves forward, building regular, stationary ordering as observed during metal organic vapor phase epitaxy or hydride vapor phase epitaxy growth of GaN layers. These two phenomena recover surface meandered pair step pattern observed, since 1953, on many semiconductor surfaces, such as SiC, Si, or GaN. Change in terrace width or step orientation particle diffusion jump barriers leads either to step meandering or surface roughening. Additionally it is shown that step behavior changes with the Schwoebel barrier height. Furthermore, simulations under conditions corresponding to very high external particle flux result in triangular islands grown at the terraces. All structures, emerging in the simulations, have their corresponding cases in the experimental results.
Vacuum fluorescent displays utilizing ZnO nanoparticles
Authors: Aaronson Charles H. , Amekura Hiro , Sato Yoshitaka , Kishimoto Naoki
Published: January 2011

Zinc-oxide (ZnO) nanoparticles (NPs) fabricated by ion implantation combined with thermal oxidation were installed into vacuum fluorescent displays. The cathode-ray induced luminescence observed was strongly dependent on sample oxidation temperature. Raising the temperature form 650 to 800 °C, the relative intensity of the defect band around 480 nm to that of the free-exciton recombination at 380 nm increased. With increasing accelerating voltage, the free-exciton peak of ZnO NPs shifted to a longer wavelength. This was ascribed to a temperature increase due to cathode ray irradiation because the shift was only observed during continuous wave mode operation and not during pulsed mode operation. As the oxidation temperature was further raised to 900 °C strong green band emission around 520 nm became the only luminescence band. This was due to the formation of the Zn2SiO4 phase from the reaction between ZnO NPs and the SiO2 substrate, which was confirmed by x-ray diffraction, x-ray photoelectron spectroscopy, and optical absorption spectroscopy.
Numerical simulation of field-cooled and zero field-cooled processes for assembly of superparamagnetic nanoparticles with uniaxial anisotropy
Authors: Usov N. A.
Published: January 2011

The results of the numerical simulation of field-cooled and zero field-cooled (ZFC) experiments in a dilute assembly of superparamagnetic nanoparticles with uniaxial anisotropy are presented. The numerical simulation uses a solution of the kinetic rate equations for population numbers of the potential wells. The particle relaxation times are rigorously obtained from the corresponding Fokker–Planck equation. For an assembly of particles with a single diameter a monotonic decrease in a blocking temperature as a function of the applied magnetic field is found, the blocking temperatures of aligned and randomly oriented assemblies being close. For an assembly with lognormal volume distribution the location of the maximum at the ZFC assembly magnetization differs considerably from the blocking temperature of particles with the average diameter. Equating of both quantities may lead to a considerable overestimation of the particle effective anisotropy constant. The effective blocking temperature of the assembly with lognormal volume distribution may experience nonmonotonic magnetic field dependence, as the particles of large diameters begin to contribute to the assembly magnetization with the increase in the applied field. A prominent dependence of the ZFC assembly magnetization maximum on the width of the lognormal distribution is also revealed.
Development of a compact, lightweight, mobile permanent magnet system based on high Tc Gd-123 superconductors
Authors: Tomita Masaru , Fukumoto Yusuke , Suzuki Kenji , Ishihara Atsushi , Muralidhar Miryala
Published: January 2011

We have designed and fabricated a compact, lightweight, mobile permanent high-Tc superconducting magnet system using melt-processed GdBa2Cu3Oy rings. Magnetic field distribution inside the annular magnet reached 0.75 T to 2.65 T, for one to ten rings, respectively. The increasing number of rings enhanced magnetic field in the center position and caused the field to be flatter over the entire surface. Using this new permanent magnet, we successfully magnetized several melt-processed bulk samples at 77 K. Use of simple superconducting permanent magnet for magnetizing bulk melt-processed materials can open new technological windows in various industrial areas.
X-ray characterization of Ge epitaxially grown on nanostructured Si(001) wafers
Authors: Zaumseil P. , Yamamoto Y. , Bauer A. , Schubert M. A. , Schroeder T.
Published: January 2011

Selective growth of Ge on nanostructured Si(001) wafers is studied to evaluate the applicability of nanoheteroepitaxy approaches on the Ge–Si system for photonics applications under particular consideration of possible growth mask materials. A gate spacer technology established in advanced silicon microelectronics is used to generate a periodic array of nanoscaled Si pillars. The spacing of these Si pillars is 360 nm; diameter and height are about 100 nm, which is still above the expected geometrical parameters to obtain a compliant behavior of the Si lattice in the pillars. Ge dots are deposited by reduced pressure chemical-vapor deposition on top of the Si pillars. The characterization is done by different x-ray diffraction methods and transmission electron microscopy. It is found that even 30 nm thick Ge dots are fully relaxed and they exhibit a clear network of misfit dislocations. Significantly thicker Ge dots generate additional structural defects, mainly microtwins. A strain partitioning between Si pillar and Ge dot was not observed. The main reason for this is not the geometry of pillars but the compressive in-plane strain inside the uncovered Si pillar caused by the surrounding SiO2 layer required as masking material to guaranty the selective growth of Ge on top of the pillars only. High temperature diffraction experiments show that a partial annealing of this strain is possible.
Fabrication of iodine-doped pentacene thin films for organic thermoelectric devices
Authors: Hayashi K. , Shinano T. , Miyazaki Y. , Kajitani T.
Published: January 2011

We investigate the relation between the morphology and thermoelectric properties of iodine-doped pentacene thin films grown on the glass substrate. The electrical conductivity and thermoelectric power factor are enhanced with decreasing the film thickness and growth temperature due to the volumetric increase in the thin film phase relative to the bulk phase in the film. The 115 and 190 nm thin films grown at room temperature, whose majority phase is the thin film phase, exhibit high power factor of 1.3×10−5 W m−1 K−2. Also, we demonstrate an improved stability of thermoelectric properties by covering the iodine-doped film with a polyimide tape.
Growth and magnetism of low-temperature deposited Fe/Si(111) films as an intermediate layer for suppression of silicide formation
Authors: Tu Wen-Ting , Wang Chih-Hsiung , Huang Ya-Yun , Lin Wen-Chin
Published: January 2011

Low temperature (LT: 100 K) deposition of Fe on Si(111)7×7 surface effectively reduces Fe-silicide formation at the Fe/Si interface, as compared with conventional room temperature (RT) growth. The interface condition of 5–15 monolayers (ML) LT-Fe/Si(111) remains stable at least up to 350 K. Si segregation was observed after annealing at 400 K. LT-grown Fe films also reveal a relatively flat surface morphology with a roughness of 0.4–0.6 nm. Thus, LT-Fe films were suggested as an intermediate layer for the subsequent RT-growth of Fe. We use a single domain model of magnetic anisotropy to fit the magnetic coercivity evolution of n ML RT-Fe on 5 ML LT-Fe/Si(111). Accordingly, we deduce the surface and volume-contributed magnetic anisotropy for discussion.
An electrode-free method of characterizing the microwave dielectric properties of high-permittivity thin films
Authors: Bovtun V. , Pashkov V. , Kempa M. , Kamba S. , Eremenko A. , Molchanov V. , Poplavko Y. , Yakymenko Y. , Lee J. H. , Schlom D. G.
Published: January 2011

A thin dielectric resonator consisting of a dielectric substrate and the thin film deposited upon it is shown to suffice for microwave characterization and dielectric parameter measurement of high-permittivity thin films without electrodes. The TE01δ resonance mode was excited and measured in thin (down to 0.1 mm) rectangular- or disk-shaped low-loss dielectric substrates (D∼10 mm) with permittivity ε′≥10 inserted into a cylindrical shielding cavity or rectangular waveguide. The in-plane dielectric permittivity and losses of alumina, DyScO3, SmScO3, and (LaAlO3)0.29(SrAl1/2Ta1/2O3)0.71 (LSAT) substrates were measured from 10 to 18 GHz. The substrate thickness optimal for characterization of the overlying thin film was determined as a function of the substrate permittivity. The high sensitivity and efficiency of the method, i.e., of a thin dielectric resonator to the dielectric parameters of an overlying film, was demonstrated by characterizing ultrathin strained EuTiO3 films. A 22 nm thick EuTiO3 film grown on a (100) LSAT substrate and strained in biaxial compression by 0.9% exhibited an increase in microwave permittivity at low temperatures consistent with it being an incipient ferroelectric; no strain-induced ferroelectric phase transition was seen. In contrast, a 100 nm thick EuTiO3 film grown on a (110) DyScO3 substrate and strained in biaxial tension by 1% showed two peaks as a function of temperature in microwave permittivity and loss. These peaks correspond to a strain-induced ferroelectric phase transition near 250 K and to domain wall motion.
Influence of film thickness on the physical properties of manganite heterojunctions
Authors: Gao Weiwei , Sun Xuan , Wang Jing , Shang Dashan , Shen Baogen , Sun Jirong
Published: January 2011

Rectifying and photoelectronic properties of the La0.67Ba0.33MnO3/SrTiO3:Nb junctions with the film thickness from d=0.5 to 30 nm have been systematically studied. It is found that the electronic transport of the junction is dominated by quantum tunneling or thermoionic emission when film thickness is below or above 1 nm. The rectifying ratio and ideality factor, correspondingly, experience a sudden change as film thickness grows from 0.5 to 1 nm and a smooth variation with film thickness above 1 nm. The threshold film thickness for the establishment of a mature depletion layer is therefore 1 nm. The photoemission properties of the junctions also exhibit a strong dependence on film thickness. As experimentally shown, the photocurrent vanishes in the zero thickness limit, and grows rapidly with the increase in film thickness until d=6 nm, where a maximal photocurrent of ∼770 nA/mm2 under the irradiance of the laser of 5 mW and 532 nm is obtained. After this maximum, an increase-to-decrease turning appears with further increasing film thickness. Taking into account the finite diffusion distance of the photocarriers and the strain-enhanced charge trapping in ultrathin film junctions, a theoretical description that well reproduces the experiment results can be obtained, which reveals the severe depression of finite diffusion distance of the extra carriers on photocurrent. The maximal diffusion distance thus obtained is ∼3.5 nm. Similar analyses have been performed for the La0.67Ca0.33MnO3/SrTiO3:Nb junctions, and the corresponding diffusion distance there is ∼1.5 nm.
Formation and annealing of dislocation loops induced by nitrogen implantation of ZnO
Authors: Perillat-Merceroz Guillaume , Gergaud Patrice , Marotel Pascal , Brochen Stephane , Jouneau Pierre-Henri , Feuillet Guy
Published: January 2011

Although zinc oxide is a promising material for the fabrication of short wavelength optoelectronic devices, p-type doping is a step that remains challenging for the realization of diodes. Out of equilibrium methods such as ion implantation are expected to dope ZnO successfully provided that the non-radiative defects introduced by implantation can be annealed out. In this study, ZnO substrates are implanted with nitrogen ions, and the extended defects induced by implantation are studied by transmission electron microscopy and x-ray diffraction (XRD) before and after annealing at 900 °C. Before annealing, these defects are identified to be dislocation loops lying either in basal planes in high N concentration regions, or in prismatic planes in low N concentration regions, together with linear dislocations. An uniaxial deformation of 0.4% along the c axis, caused by the predominant basal loops, is measured by XRD in the implanted layer. After annealing, prismatic loops disappear while the density of basal loops decreases and their diameter increases. Moreover, dislocation loops disappear completely from the subsurface region. XRD measurements show a residual deformation of only 0.05% in the implanted and annealed layer. The fact that basal loops are favored against prismatic ones at high N concentration or high temperature is attributed to a lower stacking fault energy in these conditions. The coalescence of loops and their disappearance in the subsurface region are ascribed to point defect diffusion. Finally, the electrical and optical properties of nitrogen-implanted ZnO are correlated with the observed structural features.
Low dark current metal-semiconductor-metal ultraviolet photodetectors based on sol-gel-derived TiO2 films
Authors: Xie Yannan , Huang Huolin , Yang Weifeng , Wu Zhengyun
Published: January 2011

The titanium dioxide (TiO2) films prepared by sol-gel processing were used to fabricate metal-semiconductor-metal ultraviolet photodetectors. A very low dark current of 5.38 pA (current density of 3.84 nA/cm2) at 5 V bias is obtained, which is ascribed to the high effective Schottky barrier between Au and TiO2 films. The x-ray photoelectron spectroscopy analysis demonstrates that the concentration of oxygen vacancies is very low in the surface of the TiO2 films, which is responsible for the high effective Schottky barrier. The devices exhibit a cutoff wavelength at about 380 nm and a large UV-to-visible rejection ratio (340 versus 400 nm) of three orders of magnitude. The peak responsivity of the devices is 17.5 A/W at 5 V bias, indicating the presence of internal photoconductive gain induced by desorption of oxygen on the TiO2 surface.
Comparison between rough and smooth plates within the same Rayleigh–Bénard cell
Authors: Tisserand J.-C. , Creyssels M. , Gasteuil Y. , Pabiou H. , Gibert M. , Castaing B. , Chillà F.
Published: January 2011

In a Rayleigh–Bénard cell at high Rayleigh number, the bulk temperature is nearly uniform. The mean temperature gradient differs from zero only in the thin boundary layers close to the plates. Measuring this bulk temperature allows to separately determine the thermal impedance of each plate. In this work, the bottom plate is rough and the top plate is smooth; both interact with the same bulk flow. We compare them and address in particular the question whether the influence of roughness goes through a modification of the bulk flow.
Density-dependent analysis of nonequilibrium paths improves free energy estimates II. A Feynman–Kac formalism
Authors: Minh David D. L. , Vaikuntanathan Suriyanarayanan
Published: January 2011

The nonequilibrium fluctuation theorems have paved the way for estimating equilibrium thermodynamic properties, such as free energy differences, using trajectories from driven nonequilibrium processes. While many statistical estimators may be derived from these identities, some are more efficient than others. It has recently been suggested that trajectories sampled using a particular time-dependent protocol for perturbing the Hamiltonian may be analyzed with another one. Choosing an analysis protocol based on the nonequilibrium density was empirically demonstrated to reduce the variance and bias of free energy estimates. Here, we present an alternate mathematical formalism for protocol postprocessing based on the Feynmac–Kac theorem. The estimator that results from this formalism is demonstrated on a few low-dimensional model systems. It is found to have reduced bias compared to both the standard form of Jarzynski's equality and the previous protocol postprocessing formalism.
Adhesion of tungsten carbide reinforced amorphous hydrocarbon thin films (WC/a-C:H) to steel substrates for tribological applications
Authors: Evans R. D. , Shiller P. J. , Howe J. Y.
Published: January 2011

We have explored the adhesive interlayer structure for a tungsten carbide reinforced amorphous hydrocarbon thin film coating (WC/a-C:H) that demonstrated excellent coating adhesion under highly stressed tribological contact. Transmission electron microscopy (TEM) analysis including cross-sectional imaging, electron diffraction, and energy dispersive spectroscopy was performed on abrupt and gradient interfaces within the multilayer film architecture. Interpretation of these results is aided by quantum mechanical calculations that were performed to investigate bonding interactions of the Cr adhesive interlayer to the Fe substrate surface within a ∼3 nm thick interfacial region. Low levels of oxygen present in the coating deposition chamber during deposition were found at the Fe–Cr interface using high-resolution TEM. Molecular orbital calculations for a linear three-atom molecular model Fe–O–Cr demonstrate the role of O in strengthening Fe to Cr bonding within that interfacial region.
Electrical transport properties of the Si-doped cubic boron nitride thin films prepared by in situ cosputtering
Authors: Ying J. , Zhang X. W. , Yin Z. G. , Tan H. R. , Zhang S. G. , Fan Y. M.
Published: January 2011

Si-doped cubic boron nitride (c-BN) films with various Si concentrations were achieved by in situ cosputtering during ion beam assisted deposition. Effects of the Si concentration and rapid thermal annealing (RTA) conditions on the electrical transport properties of Si-doped c-BN thin films were investigated systematically. The results suggest that the optimum RTA condition is at the temperature of 1000 °C for 3 min. The resistance of Si-doped c-BN films gradually decreases as the Si concentration increases, indicating an electrical doping effect of the Si impurity. The temperature dependent electrical conductivity of the Si-doped c-BN films suggests that different conduction mechanisms are dominant over the different temperature ranges. Based on the Davis–Mott model, we propose that the extended-state conduction, band tail-state conduction and short-range hopping conduction are responsible for the respective temperature ranges. In addition, the reduction in activation energy of Si impurities is observed as the Si concentration increases.