bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of fluid-phase behavior using transition-matrix Monte Carlo: Binary Lennard-Jones mixtures
Authors: Shen Vincent K. , Errington Jeffrey R.
Published: February 2005

We present a novel computational methodology for determining fluid-phase equilibria in binary mixtures. The method is based on a combination of highly efficient transition-matrix Monte Carlo and histogram reweighting. In particular, a directed grand-canonical transition-matrix Monte Carlo scheme is used to calculate the particle-number probability distribution, after which histogram reweighting is used as a postprocessing procedure to determine the conditions of phase equilibria. To validate the methodology, we have applied it to a number of model binary Lennard-Jones systems known to exhibit nontrivial fluid-phase behavior. Although we have focused on monatomic fluids in this work, the method presented here is general and can be easily extended to more complex molecular fluids. Finally, an important feature of this method is the capability to predict the entire fluid-phase diagram of a binary mixture at fixed temperature in a single simulation.
Ice nucleation on BaF2(111)
Authors: Conrad Peter , Ewing George E. , Karlinsey Robert L. , Sadtchenko Vlad
Published: February 2005

The mechanism of heterogeneous ice nucleation on inorganic substrates is not well understood despite work on AgI and other materials over the past 50years. We have selected BaF2 as a model substrate for study since its (111) surface makes a near perfect match with the lattice of the basal face of Ih ice and would appear to be an ideal nucleating agent. Two series of experiments were undertaken. In one, nucleation of thin film water formed from deposition of vapor on BaF2(111) faces was explored with the finding that supercooling to −30°C was required before freezing occurred. In the other series, nucleation of liquid water on submerged BaF2 crystals was studied. Here supercooling to −15°C was needed before ice formed. The reason why BaF2 is such a poor nucleating agent contains clues to realistic mechanisms of heterogeneous nucleation. Our explanation of these results follows the model of Fletcher [J. Chem. Phys. 29, 572 (1958)] who showed that heterogeneous ice nucleating ability depends on how well ice wets a substrate. In this view, a smooth BaF2(111) face is poor at nucleation because ice only partially wets its surface. In an extension of Fletcher’s model, our calculations, consistent with the experimental results demonstrate that the pitting of a submerged BaF2 crystal dramatically improves its ice nucleating ability.
Analytical time-dependent density functional derivative methods within the RI-J approximation, an approach to excited states of large molecules
Authors: Rappoport Dmitrij , Furche Filipp
Published: February 2005

Time-dependent density functional theory (TDDFT) is now well established as an efficient method for molecular excited state treatments. In this work, we introduce the resolution of the identity approximation for the Coulomb energy (RI-J) to excited state gradient calculations. In combination with nonhybrid functionals, the RI-J approximation leads to speed ups in total timings of an order of magnitude compared to the conventional method; this is demonstrated for oligothiophenes with up to 40 monomeric units and adamantane clusters. We assess the accuracy of the computed adiabatic excitation energies, excited state structures, and vibrational frequencies on a set of 36 excited states. The error introduced by the RI-J approximation is found to be negligible compared to deficiencies of standard basis sets and functionals. Auxiliary basis sets optimized for ground states are suitable for excited state calculations with small modifications. In conclusion, the RI-J approximation significantly extends the scope of applications of analytical TDDFT derivative methods in photophysics and photochemistry.
Improved modification for the density-functional theory calculation of thermodynamic properties for C–H–O composite compounds
Authors: Liu Min Hsien , Chen Cheng , Hong Yaw Shun
Published: February 2005

A three-parametric modification equation and the least-squares approach are adopted to calibrating hybrid density-functional theory energies of C1–C10 straight-chain aldehydes, alcohols, and alkoxides to accurate enthalpies of formation ΔHf and Gibbs free energies of formation ΔGf, respectively. All calculated energies of the C–H–O composite compounds were obtained based on B3LYP∕6-311++G(3df,2pd) single-point energies and the related thermal corrections of B3LYP∕6-31G(d,p) optimized geometries. This investigation revealed that all compounds had 0.05% average absolute relative error (ARE) for the atomization energies, with mean value of absolute error (MAE) of just 2.1 kJ/mol (0.5 kcal/mol) for the ΔHf and 2.4 kJ/mol (0.6 kcal/mol) for the ΔGf of formation.
Effects of lateral diffusion on morphology and dynamics of a microscopic lattice-gas model of pulsed electrodeposition
Authors: Frank Stefan , Roberts Daniel E. , Rikvold Per Arne
Published: February 2005

The influence of nearest-neighbor diffusion on the decay of a metastable low-coverage phase (monolayer adsorption) in a square lattice-gas model of electrochemical metal deposition is investigated by kinetic Monte Carlo simulations. The phase-transformation dynamics are compared to the well-established Kolmogorov–Johnson–Mehl–Avrami theory. The phase transformation is accelerated by diffusion, but remains in accord with the theory for continuous nucleation up to moderate diffusion rates. At very high diffusion rates the phase-transformation kinetic shows a crossover to instantaneous nucleation. Then, the probability of medium-sized clusters is reduced in favor of large clusters. Upon reversal of the supersaturation, the adsorbate desorbs, but large clusters still tend to grow during the initial stages of desorption. Calculation of the free energy of subcritical clusters by enumeration of lattice animals yields a quasiequilibrium distribution which is in reasonable agreement with the simulation results. This is an improvement relative to classical droplet theory, which fails to describe the distributions, since the macroscopic surface tension is a bad approximation for small clusters.
Hydrated copper and gold monovalent cations: Ab initio study
Authors: Lee Han Myoung , Min Seung Kyu , Lee Eun Cheol , Min Jung-Hyun , Odde Srinivas , Kim Kwang S.
Published: February 2005

To understand the hydration phenomena of noble transition metals, we investigated the structures, hydration energies, electronic properties, and spectra of the Cu+(H3O)1–6 and Au+(H2O)1–6 clusters using ab initio calculations. The coordination numbers of these clusters are found to be only two, which is highly contrasted to those of Ag+(H2O)n (which have the coordination numbers of 3–4) as well as the hydrated alkali metal ions (which have the coordination numbers of ∼6). For the possible identification of their interesting hydration structures, we predict their IR spectra for the OH stretch modes.
Quasi-unary homogeneous nucleation of H2SO4-H2O
Authors: Yu Fangqun
Published: February 2005

We show that the binary homogeneous nucleation (BHN) of H2SO4-H2O can be treated as quasi-unary nucleation of H2SO4 in equilibrium with H2O vapor. A scheme to calculate the evaporation coefficient of H2SO4 molecules from H2SO4-H2O clusters is presented and a kinetic model to simulate the quasi-unary nucleation of H2SO4-H2O is developed. In the kinetic model, the growth and evaporation of sulfuric acid clusters of various sizes are explicitly simulated. The kinetic quasi-unary nucleation model does not have two well-recognized problems associated with the classical BHN theory (violation of the mass action law and mismatch of the cluster distribution for monomers) and is appropriate for the situations where the assumption of equilibrium cluster distribution is invalid. The nucleation rates predicted with our quasi-unary kinetic model are consistent with recent experimental nucleation experiments in all the cases studied, while the most recent version of the classical BHN model systematically overpredicts the nucleation rates. The hydration of sulfuric acid clusters, which is not considered in the classical model but is accounted for implicitly in our kinetic quasi-unary model, is likely to be one of physical mechanisms that lead to lower nucleation rates. Further investigation is needed to understand exactly what cause the difference between the kinetic quasi-unary model and the classical BHN model.
Photoelectron spectroscopy of chromium-doped silicon cluster anions
Authors: Zheng Weijun , Nilles J. Michael , Radisic Dunja , Bowen Kit H.
Published: February 2005

The photoelectron spectra of chromium-doped silicon cluster anions, CrSin−, were measured over the size range, n=8–12. Their vertical detachment energies were measured to be 2.71, 2.88, 2.87, 2.95, and 3.18eV, respectively. Our results support theoretical calculations by Khanna, Rao, and Jena [Phys. Rev. Lett. 89, 016803 (2002)] which found CrSi12 to be an enhanced stability (magic) cluster with its chromium atom encapsulated inside a silicon cage and with its magnetic moment completely quenched by the effects of the surrounding cage.
Characteristic modes and evolution processes of shear-layer vortices in an elevated transverse jet
Authors: Huang Rong F. , Lan Jen
Published: February 2005

Characteristics and evolution processes of the traveling coherent flow structure in the shear layer of an elevated round jet in crossflow are studied experimentally in an open-loop wind tunnel. Streak pictures of the smoke flow patterns illuminated by the laser-light sheet in the median and horizontal planes are recorded with a high speed digital camera. Time histories of the instantaneous velocity of the vortical flows in the shear layer are digitized by a hot-wire anemometer through a high-speed data acquisition system. By analyzing the streak pictures of the smoke flow visualization, five characteristic flow structures, mixing-layer type vortices, backward-rolling vortices, forward-rolling vortices, swing-induced mushroom vortices, and jet-type vortices, are identified in the shear layer evolving from the up-wind edge of the jet exit. The behaviors and mechanisms of the vortical flow structure in the bent shear layer are prominently distinct in different flow regimes. The frequency characteristics, Strouhal number, power-spectrum density functions, autocorrelation coefficient, as well as the time and length scales of the coherent structure and the Lagrangian integral scales are obtained by processing the measured instantaneous velocity data. The Strouhal number is found to decay exponentially with the increase of the jet-to-crossflow momentum flux ratio. The autocorrelation coefficients provide the information for calculating the statistical time scales of the coherent structure and the integral time scales of turbulence fluctuations. The corresponding length scales of the vortical structure and the integral length scales of turbulence in the shear layer are therefore obtained and discussed.
Precharging strategy to accelerate spin-transfer switching below the nanosecond
Authors: Devolder T. , Chappert C. , Crozat P. , Tulapurkar A. , Suzuki Y. , Miltat J. , Yagami K.
Published: February 2005

We compared different ways of inducing magnetization switching by spin momentum transfer in pillar shaped CoFe∕Cu∕CoFe trilayers using sub-ns-current pulses. In comparison with switching induced by a single sub-ns pulse, precharging the device with a bias current prior to the application of the pulse proved to lower the required peak current. Precharging is efficient for pulses ranging from 2ns down to at least 200ps. Simulations indicate that the bias current prepares the magnetization in a precession state that provides an enhanced susceptibility to the spin torque of the pulsed current. The precession settling time is typically 2ns, hence the precharging strategy loses its efficiency for longer pulses, in agreement with experiments.
Using fluorescence resonance energy transfer to measure distances along individual DNA molecules: Corrections due to nonideal transfer
Authors: Sabanayagam Chandran R. , Eid John S. , Meller Amit
Published: February 2005

Single molecule fluorescence resonance energy transfer has been extensively used to measure distance changes and kinetics in various biomolecular systems. However, due to complications involving multiple de-excitation pathways of the dyes, the absolute inter-dye distance information has seldom been recovered. To circumvent this we directly probe the relative variations in the quantum yield of individual fluorophores. B-DNA was used as a scaffold to position the donor (Cy3 or TMR) at precise distances from the acceptor (Cy5) within the Förster radius. We found that the variation in the Cy3 quantum yield is ∼5 times larger than that of TMR. By taking into account the molecule-to-molecule variability in the acceptor/donor quantum yield ratio, the apparent fluorescence resonance energy transfer efficiencies were scaled to yield the theoretical values. We obtained very good agreement with a physical model that predicts distances along B-DNA.
Preparation of As-doped p-type ZnO films using a Zn3As2∕ZnO target with pulsed laser deposition
Authors: Vaithianathan Veeramuthu , Lee Byung-Teak , Kim Sang Sub
Published: February 2005

We report the preparation of arsenic doped p-type ZnO films using a Zn3As2∕ZnO target by pulsed laser deposition. Zn3As2 was used as a p-type dopant source material for arsenic doping in ZnO. The existence of As in the As-doped ZnO films was confirmed by the x-ray photoelectron spectroscopy study. The p-type behavior of the As-doped ZnO films was determined by the Hall and photoluminescence measurements. Room temperature Hall measurements revealed that the As-doped ZnO films exhibited p-type conductivity after being annealed at 200°C in N2 ambient for 2min with the hole concentrations varied between 2.48×1017 and 1.18×1018cm−3. The resistivity and carrier mobility of the As-doped p-type ZnO films were in the range of 2.2–6.7Ωcm and 0.83–11.4cm2∕Vs, respectively. The low temperature photoluminescence measurements confirmed the peak associated with the neutral-acceptor bound exciton (A0X) emission in the As-doped p-type ZnO films.
Quantitative high-pressure pair distribution function analysis of nanocrystalline gold
Authors: Martin C. David , Antao Sytle M. , Chupas Peter J. , Lee Peter L. , Shastri Sarvjit D. , Parise John B.
Published: February 2005

Using a diamond anvil cell with high-energy monochromatic x rays, we have studied the total scattering of nanocrystalline gold to 20Å−1 at pressures up to 10GPa in a hydrostatic alcohol pressure-medium. Through direct Fourier transformation of the structure function [S(Q)], pair distribution functions (PDFs) [G(r)] are calculated without Kaplow-type iterative corrections. Quantitative high-pressure PDF (QHP-PDF) analysis is performed via full-profile least-squares modeling and confirmed through comparison of Rietveld analysis of Bragg diffraction. The quality of the high pressure PDFs obtained demonstrates the integrity of our technique and suggests the feasibility of future QHP-PDF studies of liquids, disordered solids, and materials at phase transition under pressure.
Easy axis alignment of chemically partially ordered FePt nanoparticles
Authors: Kang Shishou , Jia Zhiyong , Shi Shifan , Nikles David E. , Harrell J. W.
Published: February 2005

Partially ordered Fe53Pt47 nanoparticles with size around 8nm were prepared by the simultaneous decomposition of iron pentacarbonyl and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. X-ray diffraction measurements reveal that as-made FePt particles were partially transformed into the ordered L10 phase with some weak superlattice peaks. The room-temperature hysteresis loop and remanence curve suggest a broad distribution of anisotropies in the partially ordered particles. By coating the partially ordered FePt nanoparticles with a polyvinylchloride polymer binder, the particles could be re-dispersed in cyclohexanone. Furthermore, the easy axis of the particles coated with the polyvinylchloride polymer binder could be aligned under an external field. Easy axis alignment was confirmed from both alternating gradient magnetometer and x-ray diffraction measurements.
Upconversion photoluminescence in poly(ladder-type-pentaphenylene) doped with metal (II)-octaethyl porphyrins
Authors: Baluschev S. , Keivanidis P. E. , Wegner G. , Jacob J. , Grimsdale A. C. , Müllen K. , Miteva T. , Yasuda A. , Nelles G.
Published: February 2005

We report on the optimization of the upconversion photoluminescence in films of conjugated polymers doped with platinum porphyrin. The upconversion emission was observed to take place at pump intensities as low as 0.5kW∕cm2. Comparison between the photoluminescence integral intensity of polyfluorene and ladder-type pentaphenylene polymer shows a five-fold increase of the efficiency of the upconversion process for the latter. The higher upconversion efficiency can be partially attributed to the reduced reabsorption of the photoluminescence in the case of the ladder-type pentaphenylene matrix.
Dispersion of thermo-optic coefficient in porous silicon layers of different porosities
Authors: Moretti Luigi , De Stefano Luca , Mario Rossi Andrea , Rendina Ivo
Published: February 2005

The thermo-optic coefficient (dn∕dT) of porous silicon has been measured. The measurements are based on a simple technique. The experimental data, in the wavelength range between 500 and 1100nm and at different porosities, have been compared with the Bruggeman and Landau–Lifshitz models. The results show good agreement in case of the Bruggeman model for porosities lower than 0.7, while for higher porosities the Landau model is in better agreement. An estimation of dn∕dT as a function of the porosity at the fiber optic communication wavelength of 1.55μm has also been reported.
In situ and real-time characterization of metal-organic chemical vapor deposition growth by high resolution x-ray diffraction
Authors: Kharchenko A. , Lischka K. , Schmidegg K. , Sitter H. , Bethke J. , Woitok J.
Published: February 2005

We present an x-ray diffractometer for the analysis of epitaxial layers during (in situ) metal-organic chemical vapor deposition (MOCVD). Our diffractometer has a conventional x-ray source, does not need a goniometer stage, and is not sensitive to precise adjustment of the samples before measurement. It allows us to perform measurements within a few seconds even from rotating and wobbling samples. The first results of laboratory tests performed with our x-ray diffraction system show that it is well suited for in situ and real-time monitoring of the MOCVD growth process. We were able to measure the growth rate of a cubic GaN layer and the intensity and peak position of Bragg reflections of the growing layer in less than 20 s only.
Magnetic field stabilization by temperature control of an azimuthally varying field cyclotron magnet
Authors: Okumura S. , Arakawa K. , Fukuda M. , Nakamura Y. , Yokota W. , Ishimoto T. , Kurashima S. , Ishibori I. , Nara T. , Agematsu T. , Sano M. , Tachikawa T.
Published: February 2005

A magnetic field drift, gradual decrease of the order of 10−4 in several tens of hours, was observed with the beam intensity decrease in an operation of an azimuthally varying field (AVF) cyclotron. From our experimental results, we show that the temperature increase of the magnet iron by the heat transfer from the excitation coils can induce such change of the magnetic field as to deteriorate the beam quality. The temperature control of the magnet iron was realized by thermal isolation between the main coil and the yoke and by precise control of the cooling water temperature of the trim coils attached to the pole surfaces in order to prevent temperature change of the magnet iron. The magnetic field stability of ±5×10−6 and the beam intensity stability of ±2% have been achieved by this temperature control.
Spin-echo small angle neutron scattering in Delft
Authors: Rekveldt M. Theo , Plomp Jeroen , Bouwman Wim G. , Kraan Wicher H. , Grigoriev Serguei , Blaauw Menno
Published: February 2005

We describe two spin-echo instruments for neutron small angle scattering, which have been installed at the reactor institute in Delft. The first setup is using a monochromatic beam and magnetized foils as spin flippers, while the second uses resonant spin flippers in a pulsed neutron beam. The components that play an essential role for operation are described in some detail. Each setup has specific advantages in its range of spin-echo lengths that covers the range of correlation lengths that could be measured. This is demonstrated in a comparative measurement, the setup with magnetized foils measuring at spin-echo-lengths up to 20 μm and the setup with resonant flippers measuring in the range up to 0.5 μm.
Emergence of a new feature in the high pressure–high temperature relaxation spectrum of tri-propylene glycol
Authors: Prevosto D. , Capaccioli S. , Lucchesi M. , Rolla P. A. , Paluch M. , Pawlus S. , Zioło J.
Published: February 2005

We investigated dielectric relaxation of a tri-propylene glycol system under high compression. By increasing temperature and pressure we observed that a new relaxation process emerges from the low frequency tail of the structural peak. This new peak starts to be visible at about 0.5 GPa and becomes clearly evident at 1.7 GPa. However, this additional peak merges again with the structural one as the glass transition is approached, since it has a weaker temperature dependence. This finding enriches the relaxation scenario of molecular glass formers confirming that the application of very high hydrostatic pressure can favor the detection of new relaxation or otherwise unresolved processes in supercooled liquid systems.
Time resolved resonance Raman observation of the extreme protonation forms of a radical zwitterion in water
Authors: Tripathi G. N. R.
Published: February 2005

The reactions of the aqueous proton with the zwitterionic p-aminophenoxyl radical in strongly basic to extremely acidic aqueous solutions have been investigated using time-resolved resonance Raman spectroscopy. The dynamic stability of the different protonation forms of the radical, observed on the microsecond time scale in this work, has been achieved by controlling the proton exchange rate in water. In strongly acidic solutions we observe a rare ring-H+ bonded dication species, a key intermediate in the amine hydrolysis. The neutral p-aminophenoxyl radical undergoes NH2-deprotonation in strongly basic aqueous solutions, which has no analogues in closed-shell amines.
Characterization of heteroepitaxial Na0.5K0.5NbO3/La0.5Sr0.5CoO3 electro-optical cell
Authors: Khartsev S. I. , Grishin M. A. , Grishin A. M.
Published: February 2005

Heteroepitaxial Na0.5K0.5NbO3(1.5μm)/La0.5Sr0.5CoO3(0.5μm) (NKN/LSCO) films were grown on an Al2O3(011̱2) single crystal (r-cut sapphire) by rf-magnetron sputtering (NKN) and pulsed laser deposition (LSCO) techniques. Prism coupling waveguide refractometry has been employed to characterize vertical capacitive electro-optical cells with 2×8mm2 semitransparent Au top electrodes. Fitting reflectivity spectra to Fresnel formulas yields extraordinary and ordinary refractive indices ne=2.232 and no=2.234 as well as electro-optic coefficient r13=17.4pm∕V. Dispersion of the refraction index follows the Sellmeier formula n2=1+3.46∕[1−(244nm∕λ)2] in the range from 400 nm to 850 nm.
GaN metal-oxide-semiconductor high-electron-mobility-transistor with atomic layer deposited Al2O3 as gate dielectric
Authors: Ye P. D. , Yang B. , Ng K. K. , Bude J. , Wilk G. D. , Halder S. , Hwang J. C. M.
Published: February 2005

We report on a GaN metal-oxide-semiconductor high-electron-mobility-transistor (MOS-HEMT) using atomic-layer-deposited (ALD) Al2O3 as the gate dielectric. Compared to a conventional GaN high-electron-mobility-transistor (HEMT) of similar design, the MOS-HEMT exhibits several orders of magnitude lower gate leakage and several times higher breakdown voltage and channel current. This implies that the ALD Al2O3∕AlGaN interface is of high quality and the ALD Al2O3∕AlGaN∕GaN MOS-HEMT is of high potential for high-power rf applications. In addition, the high-quality ALD Al2O3 gate dielectric allows the effective two-dimensional (2D) electron mobility at the AlGaN∕GaN heterojunction to be measured under a high transverse field. The resulting effective 2D electron mobility is much higher than that typical of Si, GaAs or InGaAs metal-oxide-semiconductor field-effect-transistors (MOSFETs).
Device annealing effect in organic solar cells with blends of regioregular poly(3-hexylthiophene) and soluble fullerene
Authors: Kim Youngkyoo , Choulis Stelios A. , Nelson Jenny , Bradley Donal D. C. , Cook Steffan , Durrant James R.
Published: February 2005

Here we report enhanced efficiency bulk heterojunction organic solar cells using blend films of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) that are subjected to a thermal annealing process. Blend films (P3HT:PCBM=1:1 by weight) were prepared using chlorobenzene and 1,2-dichlorobenzene in order to investigate the role of the solvent. Irrespective of the chosen solvent, the optimal device annealing temperature was found to be 140 °C. The highest power conversion efficiency, 3% under air mass 1.5 simulated solar illumination (100mW∕cm2), was achieved by device annealing at 140 °C for 15 min using blend films prepared from chlorobenzene (2.3% for 1,2-dichlorobenzene).
Electronic structure of Li-inserted V6O13 battery cathodes: Rigid band behavior and effects of hybridization
Authors: Schmitt T. , Augustsson A. , Nordgren J. , Duda L.-C. , Höwing J. , Gustafsson T. , Schwingenschlögl U. , Eyert V.
Published: February 2005

Resonant soft x-ray emission (SXE) spectroscopy was used to study the electronic structure of LixV6O13 battery cathodes. We observe that the V 3d-bands of V6O13 exhibit a rather rigid behavior. Upon lithiation, electrons enter the top of the valence band and add intensity to the corresponding part of the V L-emission spectrum without significantly distorting the lower lying bands. We perform ab initio calculations which are in good agreement with the experimental results. Moreover, we find that lithiation leads to an overall decrease of the V 3d–O 2p hybridization. In contrast to x-ray diffraction, it is possible to study charge transfer effects in Li-batteries with SXE spectroscopy over the entire lithiation range.
Dual-gate pentacene organic field-effect transistors based on a nanoassembled SiO2 nanoparticle thin film as the gate dielectric layer
Authors: Cui Tianhong , Liang Guirong
Published: February 2005

In this paper, we report the fabrication of dual-gate organic field-effect transistors (OFETs) using self-assembled SiO2 and thermal oxide as gate dielectric materials and pentacene as a semiconductor. The top dielectric layer was formed by the low-cost and low-temperature self-assembly with SiO2 nanoparticles 45nm in diameter. The fabricated dual-gate pentacene field-effect transistor (FET) has a threshold voltage of −2.2V, a field-effect mobility of 0.1cm2∕Vs, an Ion∕off ratio of 3.8×103, and a slope of 1.3V∕decade. Compared to a single gate OFET, dual-gate FET has better performance with higher drain output current at the relatively low operating voltage, larger field-effect mobility, and better channel controllability by separately adjusting two gate biases.
Anisotropic screening effects on electron-ion bremsstrahlung process in an anisotropic plasma
Authors: Jung Young-Dae
Published: February 2005

The plasma screening and electric field effects on the low-energy electron-ion Coulomb bremsstrahlung process are investigated for both soft and hard photon radiation in an anisotropic plasma in the presence of an external electric field. The impact-parameter trajectory analysis is applied to the motion of the projectile electron in order to investigate the variation of the bremsstrahlung cross section as a function of the drift velocity, impact parameter, Debye length, and radiation photon energy. The results show that the bremsstrahlung radiation cross section decreases with increasing bremsstrahlung photon energy. The screening effect is more significant for the soft photon case. It is also found that the electric field effect enhances the bremsstrahlung cross section for soft photon radiation and large impact parameters and increases with increasing photon energy.
Resonant Raman scattering in strained and relaxed InGaN∕GaN multi-quantum wells
Authors: Lazić S. , Moreno M. , Calleja J. M. , Trampert A. , Ploog K. H. , Naranjo F. B. , Fernandez S. , Calleja E.
Published: February 2005

The effects of the composition and strain in InGaN∕GaN multi-quantum wells on their phonon frequencies have been determined using resonant Raman scattering in a wide energy range. In pseudomorphic quantum wells a strong compensation of both effects occurs, resulting in the InGaN A1LO phonon frequency being almost independent on In concentration. In relaxed quantum wells the A1LO frequency is clearly below the GaN value and depends on the excitation energy, as reported in thick films. This variation, together with the resonance profile, gives a direct estimate of the In concentration and its fluctuations.
Ion-beam-induced nanosmoothening and conductivity enhancement in ultrathin metal films
Authors: Mitra Partha , Hebard Arthur F.
Published: February 2005

We present a systematic in situ study of the effect of postdeposition low-energy (200eV) ion bombardment on resistance and surface topography of ultrathin iron (<50Å) and copper (<130Å) films. The ion-beam-induced nanosmoothening occurs while material is being removed and gives rise to an initial decrease in resistance followed by a steady increase as the film is subsequently uniformly eroded. The shunt resistance associated with the resistance decrease is found to be independent of the thickness of the underlying film, thus indicating that the conductivity enhancement is due primarily to surface modification.
Tunable distributed feedback lasing with narrowed emission using holographic dynamic gratings in a polymeric waveguide
Authors: Tsutsumi Naoto , Fujihara Arata
Published: February 2005

The letter presents reliable tunable distributed feedback (DFB) lasing achieved by holographic dynamic gratings in a polymeric waveguide including conjugated organic compound. Tunable lasing from 484 to 508nm was simply achieved by rotating a Lloyd mirror configuration. Lasing spectra indicate that the present DFB lasing has the contribution of periodic spatial variation of the refractive index in the gain medium in addition to that of periodic spatial variation of the gain itself.
Magnetization dynamics and thermal stability in patterned media
Authors: Moritz J. , Dieny B. , Nozières J. P. , van de Veerdonk R. J. M. , Crawford T. M. , Weller D. , Landis S.
Published: February 2005

The dynamic coercivity of patterned media with perpendicular magnetic anisotropy is measured over ten orders of magnitude using magnetic field pulses from magnetic recording heads with pulse widths in the range 150ps to 1s. The local field generated by write heads allows the study of the switching dynamics of individual bits. Existing models have been applied for a quantitative analysis of the data. As a result, we found the written bits are stable for at least 10years. Individual dot magnetization switching has been achieved with subnanosecond pulses showing that very-high-data-rate recording is possible in patterned media.
Electrical properties of InAlP native oxides for metal–oxide–semiconductor device applications
Authors: Cao Y. , Zhang J. , Li X. , Kosel T. H. , Fay P. , Hall D. C. , Zhang X. B. , Dupuis R. D. , Jasinski J. B. , Liliental-Weber Z.
Published: February 2005

Data are presented on the insulating properties and capacitance–voltage characteristics of metal–oxide–semiconductor (MOS) device-thickness (below ∼100nm) native oxides formed by wet thermal oxidation of thin InAlP epilayers lattice matched to GaAs. Low leakage current densities of J=1.4×10−9A∕cm2 and J=8.7×10−11A∕cm2 are observed at an applied field of 1MV∕cm for MOS capacitors fabricated with 17 and 48nm oxides, respectively. Transmission electron microscopy images show that the In-rich interfacial particles which exist in 110nm oxides are absent in 17nm oxide films. Quasistatic capacitance–voltage measurements of MOS capacitors fabricated on both n-type and p-type GaAs show that the InAlP oxide–GaAs interface is sufficiently free of traps to support inversion, indicating an unpinned Fermi level. These data suggest that InAlP native oxides may be a viable insulator for GaAs MOS device applications.
Dielectric properties of WS2-coated multiwalled carbon nanotubes studied by energy-loss spectroscopic profiling
Authors: Stolojan Vlad , Silva S. R. P. , Goringe Michael J. , Whitby R. L. D. , Hsu Wang K. , Walton D. R. M. , Kroto Harold W.
Published: February 2005

We investigate experimentally the electronic properties of the coating for multiwalled carbon nanotubes covered in tungsten disulfide (WS2) of various thicknesses. Coatings of thicknesses between 2 and 8 monolayers (ML) are analyzed using energy-loss spectroscopic profiling (ELSP), by studying the variations in the plasmon excitations across the coated nanotube, as a function of the coating thickness. We find a change in the ELSP for coatings above 5 ML thickness, which we interpret in terms of a change in its dielectric properties.
Efficient second-harmonic generation of tunable femtosecond pulses into the blue in periodically poled KTP
Authors: Fayaz G. R. , Ghotbi M. , Ebrahim-Zadeh M.
Published: February 2005

We report efficient single-pass second-harmonic generation of tunable femtosecond pulses into the blue in periodically poled KTP (PPKTP). Using a 2 mm crystal with 4 μm grating period and femtosecond fundamental pulses at 76 MHz, we show that average powers of up to 320 mW in the blue at conversation efficiencies as high as 20% are attainable despite the limitations of group velocity mismatch and spectral acceptance in PPKTP. Accurate measurements of phase-matching acceptance in PPKTP have been performed and excellent agreement between experimental data and calculations is confirmed. Using temperature phase matching, a significant tuning range of 422–427 nm in the blues is also demonstrated. Cross-correlation measurements of the fundamental and second-harmonic pulses in a 500 μm thick β-BaB2O4 result in blue pulse durations of 700–800 fs for 130 fs fundamental pulses.
Capacitance-voltage spectroscopy of silicon nanodots
Authors: Su Alex Yu-Kwen , Hwang H. L. , Pilkuhn M. H. , Pei Z.
Published: February 2005

Frequency-dependent capacitance-voltage spectroscopy was applied to investigate the carrier transport dynamics in a silicon nanodots resonant tunneling device structure. Two negative differential resistance (NDR) regions in the current-voltage characteristics were found in this investigated structure. Two anomalous regions were also found in the capacitance-voltage spectroscopy, which coincide with the NDR regions in the current-voltage characteristics. The origin of the anomalous phenomenon was attributed to the mesoscopic quantum capacitance due to the holes transport through the energy states associated with the Si nanodots. An equivalent circuit model was proposed to quantitatively evaluate the frequency dependence of the capacitance-voltage spectroscopy.
Evolution of leakage paths in HfO2∕SiO2 stacked gate dielectrics: A stable direct observation by ultrahigh vacuum conducting atomic force microscopy
Authors: Kyuno K. , Kita K. , Toriumi A.
Published: February 2005

A conducting atomic force microscopy (C-AFM) in ultrahigh vacuum (UHV) is used to directly observe the evolution of leakage path in HfO2∕SiO2 stacked gate dielectrics. Thanks to the UHV environment, reproducible results for both positive and negative tip biases are obtained without material formation on the surface, which has been a problem for atmospheric C-AFM. It is found that the density of leakage spots increases exponentially as a function of tip bias and that it is a large factor for leakage current increase.
Observation of magnetization reversal in epitaxial Gd0.67Ca0.33MnO3 thin films
Authors: Ma Yanwei , Guilloux-Viry M. , Barahona P. , Peña O. , Moure C.
Published: February 2005

High-quality epitaxial thin films (∼200nm thick) of Gd0.67Ca0.33MnO3 have been deposited onto (100) SrTiO3 substrates by pulsed-laser deposition. Enhanced properties in comparison with bulk samples were observed. The magnetic transition temperature (Tc) of the as-grown films is much higher than the corresponding bulk values. Most interestingly, magnetization measurements performed under small applied fields, exhibit magnetization reversals below Tc, no matter whether the film is field cooled (FC) or zero-field cooled (ZFC). A rapid magnetization reversal occurs at 7K when field cooled, while as for the ZFC process the magnetization decreases gradually with increasing temperatures, taking negative values above 7K and changing to positive values again, above 83K. In higher magnetic fields the magnetization does not change sign. The reversal mechanism is discussed in terms of a negative exchange f-d interaction and magnetic anisotropy, this later enhanced by strain effects induced by the lattice mismatch between the film and the substrate.
Polymer microcantilevers fabricated via multiphoton absorption polymerization
Authors: Bayindir Z. , Sun Y. , Naughton M. J. , LaFratta C. N. , Baldacchini T. , Fourkas J. T. , Stewart J. , Saleh B. E. A. , Teich M. C.
Published: February 2005

We have used multiphoton absorption polymerization to fabricate a series of microscale polymer cantilevers. Atomic force microscopy has been used to characterize the mechanical properties of microcantilevers with spring constants that were found to span more than four decades. From these data, we extracted a Young’s modulus of E=0.44GPa for these microscale cantilevers. The wide stiffness range and relatively low elastic modulus of the microstructures make them attractive candidates for a range of microcantilever applications, including measurements on soft matter.
Fourier transform analysis method for modeling the positions and properties of cavity defects in Fabry–Pérot laser diodes
Authors: Bream P. J. , Bull S. , Harrison I. , Sujecki S. , Larkins E. C.
Published: February 2005

A simple and fast model is presented, which allows the determination of defect positions within a high-power laser emitter cavity and an estimation of their transmission properties. The model is based upon the calculation of the cavity transmission spectrum below threshold and the analysis of its fast Fourier transform. Modeled and experimental results are compared, showing good correlation. The speed and simplicity of the model means that it is applicable as a screening process for the detection and characterization of defects in manufactured lasers.
Ohmic and degradation mechanisms of Ag contacts on p-type GaN
Authors: Song June-O , Kwak Joon Seop , Park Yongjo , Seong Tae-Yeon
Published: February 2005

The electrical and optical properties of the Ag contacts have been investigated as a function of the annealing temperature. The as-deposited contact becomes good Ohmic with contact resistivity of 2.47×10−4Ωcm2 and produces reflectance of ∼84% when annealed at 330°C for 1mm in air ambient. However, annealing at 530°C results in nonlinear current–voltage behavior and degraded reflectance. The light output of InGaN∕GaN blue light emitting diodes (LEDs) fabricated with the as-deposited Ag contact shows somewhat better output performance than those of LEDs with the annealed Ag contacts. Based on the transmission electron microscopy-energy dispersive x-ray analysis, synchrotron x-ray diffraction, and x-ray photoemission spectroscopy results, possible mechanisms for the ohmic formation and degradation of the Ag contacts are described.
Conserved patterns of protein interaction in multiple species
Authors: Sharan Roded , Suthram Silpa , Kelley Ryan M. , Kuhn Tanja , McCuine Scott , Uetz Peter , Sittler Taylor , Karp Richard M. , Ideker Trey
Published: February 2005

To elucidate cellular machinery on a global scale, we performed a multiple comparison of the recently available protein–protein interaction networks ofCaenorhabditis elegans,Drosophila melanogaster, andSaccharomyces cerevisiae. This comparison integrated protein interaction and sequence information to reveal 71 network regions that were conserved across all three species and many exclusive to the metazoans. We used this conservation, and found statistically significant support for 4,645 previously undescribed protein functions and 2,609 previously undescribed protein interactions. We tested 60 interaction predictions for yeast by two-hybrid analysis, confirming approximately half of these. Significantly, many of the predicted functions and interactions would not have been identified from sequence similarity alone, demonstrating that network comparisons provide essential biological information beyond what is gleaned from the genome.
Dietary Administration of Colesevelam Hydrochloride Does Not Affect Fertility or Reproductive Performance in Rats
Authors: Marquis Judith K. , Dagher Rafif , Jones Michael R.
Published: February 2005

Colesevelam hydrochloride (HCl) (WelChol; Sankyo Pharma) is a novel, highly potent, bile acid-binding polymer used for the treatment of hypercholesterolemia. The primary aim of this study was to determine the effects of dietarily administered colesevelam HCl on fertility and reproductive performance parameters. To assess these effects, sexually mature Sprague-Dawley rats were randomized to one of five treatment groups: feed alone, feed plus control article (SigmaCell), or feed plus colesevelam HCl 200, 1000, or 2000 mg/kg/day. Male and female rats were administered the appropriate group agent for 28 and 15 days, respectively, and were subsequently paired together for cohabitation and mating. Females continued to receive the test agent in their dietary formulation through presumed gestation day (GD) 7. Presumed pregnant females underwent cesarean section on GD 20. Food consumption rate, body weight, gross necropsy, and standard preclinical tests for reproduction and fertility were performed for each test animal. No statistically significant differences were found between control and drug-treated groups for any tested endpoints of reproduction. All animals placed in cohabitation successfully mated. Uterine and litter end points were unaffected by dosages of colesevelam HCl as high as 2000 mg/kg/day. There were no significant differences between treatment group litter averages in the number of corpora lutea, implantation sites, litter size, live fetuses, body weights, early/late resorptions, and the number of dams with viable fetuses. In addition, no external alterations of fetal morphology were attributable to treatment with colesevelam HCl when administered up to the embryo implantation stage. In male animals, no significant differences were found between the colesevelam HCl and control study groups in the average caudal epididymal sperm count or sperm concentration, total number of motile and nonmotile sperm, and the total percentage of motile sperm. Based on these data, colesevelam HCl does not have any significant adverse reproductive or fertility effects in rats, even when administered at doses approximately 30 times greater than the approved clinical dose.
The molecular mechanisms underlying BiP-mediated gating of the Sec61 translocon of the endoplasmic reticulum
Authors: Alder Nathan N. , Shen Ying , Brodsky Jeffrey L. , Hendershot Linda M. , Johnson Arthur E.
Published: January 2005

The Sec61 translocon of the endoplasmic reticulum membrane forms an aqueous pore that is gated by the lumenal Hsp70 chaperone BiP. We have explored the molecular mechanisms governing BiP-mediated gating activity, including the coupling between gating and the BiP ATPase cycle, and the involvement of the substrate-binding and J domain–binding regions of BiP. Translocon gating was assayed by measuring the collisional quenching of fluorescent probes incorporated into nascent chains of translocation intermediates engaged with microsomes containing various BiP mutants and BiP substrate. Our results indicate that BiP must assume the ADP-bound conformation to seal the translocon, and that the reopening of the pore requires an ATP binding–induced conformational change. Further, pore closure requires functional interactions between both the substrate-binding region and the J domain–binding region of BiP and membrane proteins. The mechanism by which BiP mediates translocon pore closure and opening is therefore similar to that in which Hsp70 chaperones associate with and dissociate from substrates.
Human foetal osteoblastic cell response to polymer-demixed nanotopographic interfaces
Authors: Lim Jung Yul , Hansen Joshua C , Siedlecki Christopher A , Runt James , Donahue Henry J
Published: February 2005

Nanoscale cell–substratum interactions are of significant interest in various biomedical applications. We investigated human foetal osteoblastic cell response to randomly distributed nanoisland topography with varying heights (11, 38 and 85 nm) produced by a polystyrene (PS)/polybromostyrene polymer-demixing technique. Cells displayed island-conforming lamellipodia spreading, and filopodia projections appeared to play a role in sensing the nanotopography. Cells cultured on 11 nm high islands displayed significantly enhanced cell spreading and larger cell dimensions than cells on larger nanoislands or flat PS control, on which cells often displayed a stellate shape. Development of signal transmitting structures such as focal adhesive vinculin protein and cytoskeletal actin stress fibres was more pronounced, as was their colocalization, in cells cultured on smaller nanoisland surfaces. Cell adhesion and proliferation were greater with decreasing island height. Alkaline phosphatase (AP) activity, an early stage marker of bone cell differentiation, also exhibited nanotopography dependence, i.e. higher AP activity on 11 nm islands compared with that on larger islands or flat PS. Therefore, randomly distributed island topography with varying nanoscale heights not only affect adhesion-related cell behaviour but also bone cell phenotype. Our results suggest that modulation of nanoscale topography may be exploited to control cell function at cell–biomaterial interfaces.
Native-state dynamics of the ubiquitin family: implications for function and evolution
Authors: Marianayagam Neelan J , Jackson Sophie E
Published: February 2005

Protein dynamics are integral to protein function. In recent years, the use of computer simulation to understand the molecular motions of proteins has become widespread. However, there are few such studies which compare the dynamics of proteins that are structurally and functionally related. In this study, we present native-state molecular dynamic simulations of four proteins which possess a ubiquitin-like fold. Three of these proteins are thought to have evolved from a common ancestral ubiquitin-like protein and have similarities in their function. A fourth protein, which is structurally homologous but which appears to have a different function, is also studied. Local fluctuations in the native state simulations are analysed, and conserved motions of the C-α backbone atoms are identified in residues which are important for function. In addition, the global dynamics of the proteins are analysed using the essential-dynamics method. This analysis reveals a slightly higher degree of conservation in dynamics for the three proteins which are functionally related. Both the global and local analyses illustrate how nature has optimized and conserved protein motions for specific biological activity within the ubiquitin family.