bluuebunny/crossref_metadata_2025 huggingface dataset:
Transient development of instabilities in a uniformly driven layer
Authors: Deng Rensheng , Wang Chi-Hwa
Published: November 2006

We present a nonlinear stability analysis on a uniformly driven granular layer to investigate the transient development of the instabilities induced by small perturbations. A continuum model based on the grain kinetic theory was adopted to trace the pattern evolution from the original unstable base state to a new base state and was numerically solved using a finite-element method. In the stability diagram characterized by two operating parameters, dimensionless mass holdup (Mt), and energy input (Qt), one point (Mt=4.75, Qt=58.31) in the stationary mode is selected to examine the fate of the two-dimensional density wave found from the linear stability analysis. Upon perturbing the base state, the bed height, solid fraction, particle velocities, and granular temperature profiles move away from their original to new, steady values after undergoing some oscillations. The stripes of solid fraction and granular temperature profiles finally change into a layer-like pattern, while the top surface is not uniform anymore but appears as periodic peaks and valleys due to the existence of uneven vertical velocities during the evolution. It is found that the finite amplitude of the initial perturbations is important for the pattern transition, and the evolution time increases with the increasing energy input and the increasing collision restitution coefficient. With the top surface of the granular layer being fixed at its original position, the oscillatory feature in the evolution curves of mass and temperature profiles disappears, and a lower solid fraction is obtained as compared to that observed in the presence of a moving upper boundary.
Modeling of smoothening effect on morphologies of annealed submicron nickel particles used for electrically conductive adhesives
Authors: Goh C. F. , Gan Z. H. , Mhaisalkar S. G. , Boey F. Y. C. , Gusak A. M. , Teo P. S.
Published: November 2006

Microelectronic packaging has been accelerating towards adoption of solutions that offer lower cost, higher electrical performance, and better reliability. Flip chip technology lends itself excellently to these goals. The development of anisotropic and isotropic conductive adhesives (ACA and ICA, respectively) as an alternative to solder bumps has received extensive attention in flip chip packaging as it offers an array of advantages such as finer pitch interconnects, green processes, low cost, and low temperature processing. Nickel, with its lower cost than that of silver and better thermal stability than that of copper, appears to be a viable candidate material for ICA and ACA applications. In the present study, a spiky surface morphology of nickel particles synthesized by means of a hydrothermal reduction method was clearly observed. The spiky morphology may have a detrimental effect on the conductivity of the ICA and ACA flip chip interconnect due to its smaller contact surface and propensity towards oxidation. The morphology of the nickel surface can be changed by annealing the particles between 270 and 360°C for 4–12h. A smoothening effect leading to more or less smooth spherical particles along with an increase in conductivity was observed after annealing at 360°C for up to 12h. An analytical model considering surface diffusion is established here to describe the temperature activated and energy minimization driven smoothening of the Ni particles.
Magnetic reconfiguration of MnAs∕GaAs(001) observed by magnetic force microscopy and resonant soft x-ray scattering
Authors: Coelho L. N. , Neves B. R. A. , Magalhães-Paniago R. , Vicentin F. C. , Westfahl H. , Fernandes R. M. , Iikawa F. , Däweritz L. , Spezzani C. , Sacchi M.
Published: November 2006

We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on GaAs(001) during the coexistence of hexagonal/orthorhombic phases using polarized resonant (magnetic) soft x-ray scattering and magnetic force microscopy. The results of the diffuse satellite x-ray peaks were compared to those obtained by magnetic force microscopy and suggest a reorientation of ferromagnetic terraces as temperature rises. By measuring hysteresis loops at these peaks we show that this reorientation is common to all ferromagnetic terraces. The reorientation is explained by a simple model based on the shape anisotropy energy. Demagnetizing factors were calculated for different configurations suggested by the magnetic images. We noted that the magnetic moments flip from an in-plane monodomain orientation at lower temperatures to a three-domain out-of-plane configuration at higher temperatures. The transition was observed when the ferromagnetic stripe width L is equal to 2.9 times the film thickness d. This is in good agreement with the expected theoretical value of L=2.6d.
Spectroscopy and optically stimulated luminescence of Al2O3:C using time-resolved measurements
Authors: Yukihara E. G. , McKeever S. W. S.
Published: November 2006

This paper reports the observation of ultraviolet (UV) emission at 335nm in the optically stimulated luminescence (OSL) of carbon-doped aluminum oxide (Al2O3:C) and presents results on the investigation of the OSL properties of this band, including its dose response, time dependence after irradiation, and dependence of the OSL signal on the type of radiation. Time-resolved OSL measurements were used to separate the UV emission band from the dominant OSL emission band of Al2O3:C, namely, the F-center luminescence at 420nm. A comparison of the OSL properties of the UV and F-center emission bands is important for various dosimetric applications because the relative contribution of the UV and F-center emissions to the OSL signal varies with readout technique and optical filters used in the readout equipment. The UV emission band is found to show an ionization density dependence that is different from the dependence observed for the F-center emission, and an increase in intensity with time elapsed after beta irradiation. These results are relevant for OSL dosimetry of radiation fields containing heavy charged particles, such as the space radiation field and the secondary fields created by interactions of matter with energetic neutrons, as well as for understanding of the basic OSL mechanism in Al2O3:C.
Photogating carbon nanotube transistors
Authors: Marcus Matthew S. , Simmons J. M. , Castellini O. M. , Hamers R. J. , Eriksson M. A.
Published: November 2006

Optoelectronic measurements of carbon nanotube transistors have shown a wide variety of sensitivites to the incident light. Direct photocurrent processes compete with a number of extrinsic mechanisms. Here we show that visible light absorption in the silicon substrate generates a photovoltage that can electrically gate the nanotube device. The photocurrent induced by the changing gate voltage can be significantly larger than that due to direct electron-hole pair generation in the nanotube. The dominance of photogating in these devices is confirmed by the power and position dependence of the resulting photocurrent. The power dependence is strongly nonlinear and photocurrents are measured through the device even when the laser illuminates up to 1mm from the nanotube.
Size-dependent energy band gap and dielectric constant within the generalized Penn model applied to a semiconductor nanocrystallite
Authors: Sharma A. C.
Published: November 2006

We calculated size-dependent single particle energy eigenvalues, wave functions, energy band gap, and dielectric constant for a semiconductor nanocrystallite, within the framework of the two band Penn model. The finite size effects are presented as size-dependent correction terms to the energy eigenvalues and the energy band gap of the bulk semiconductor. Ours is a self-consistent calculation within the Penn model [Phys. Rev. 128, 2093 (1962)], providing a much deeper understanding of the size dependences of energy band gap and the dielectric constant, as compared to those performed in the past using the so called generalized Penn model. Computed results on energy band gap and dielectric constant from our investigations exhibit very good agreement with those obtained from experimental measurements and the detailed numerical calculations performed using techniques such as density functional approach. Unlike our calculation, prior reported calculations of dielectric constant with the use of the generalized Penn model completely ignore or do not properly take into account the size dependences of oscillator strength and the energy band gap. It is shown that the size dependences of oscillator strength and the energy band gap play an important role and they lead to a fractional exponent (between 1 and 2) of size (radius) that appears in the formula for the dielectric constant of a nanocrystallite.
Influence of trace precursors on mass transport and growth rate during sublimation deposition of AlN crystal
Authors: Li Yanxin , Brenner Donald W.
Published: November 2006

A parameter-free model using ab initio chemical potentials is developed to analyze transport and growth rate in sublimation deposition of AlN. The model predicts that spontaneous mass transport is limited by a chemical potential barrier, which in turn determines the range of effective source-crystal temperature drops. The contributions to the barrier from Al and N2, which are the dominant species in the gas phase, almost cancel so that the barrier is sensitive to trace species such as Al3N. This result explains the experimentally observed decrease in growth rate with increasing inlet pressure as being due to a decreasing Al3N concentration. Using first principles chemical potentials may present potential opportunities to study other practical systems in which trace species plays an important role but where transport coefficients have to be determined empirically. Examples include atmospheric transport of contaminants and photosynthetic assimilation of CO2.
Effects of N2+ ion implantation on phase transition in Ge2Sb2Te5 films
Authors: Kim YoungKuk , Baeck J. H. , Cho M.-H. , Jeong E. J. , Ko D.-H.
Published: November 2006

The phase transitions of Ge2Sb2Te5 (GST) films after bombardment with 40keV N2+ ions were investigated. Comparing the nitrogen incorporated GST films with a pure GST film, the suppression of a crystalline grain growth was more effective in the N2+ implanted GST film than in a nitrogen codeposited GST film, i.e., x-ray diffraction data showed that the intensities of the crystalline diffraction peaks were decreased and the full widths at half maximum were broader than that of a pure GST film. This suppression of crystallization owing to the incorporation of nitrogen drastically reduced the roughness of surface morphology and decreased the electrical conductivity of the crystalline film. A near edge x-ray absorption fine structure experiment and x-ray photoemission spectroscopy data demonstrated that the suppression of crystalline grain growth is due to the formation of Ge3N4 and interstitial N2 molecules. In N2+ implanted GST films, in particular, interstitial N2 molecules played a major role in the suppression of crystallization.
Lifetime study of polymer light-emitting electrochemical cells
Authors: Zhang Yanguang , Gao Jun
Published: November 2006

The lifetime and degradation characteristics of frozen-junction polymer light-emitting electrochemical cells have been studied. Two seemingly minor factors, the end group of the same luminescent polymer and the type of substrate on which the device is constructed, have shown profound effects on the stress behaviors of these devices operated at a nominal operating temperature of 200K. In devices made on glass substrates, an anomalous large increase in light intensity and driving voltage has been observed under constant current stress measurement. This has been attributed to the relaxation of the electrochemical doping, which leads to reduced photoluminescence quenching and increased resistance in the polymer film. The doping relaxation is accelerated by the poor thermal conductivity of glass substrate, which causes significant temperature rise in the device due to self-heating. Evidence also suggests that doping relaxation can even occur at below the glass transition temperature of the polymer electrolyte. All devices are shown to be short lived when operated at or above 250K due to fast doping relaxation. However, devices made on sapphire substrates based on 3,5-dimethyl phenyl-terminated poly[5-(2′- ethylhexyloxy)-2-methoxy-1, 4-phenylene vinylene] are remarkably stable when operated at 200K, with an estimated half-life of 700h even when stressed at a very high current density of 1A∕cm2.
Optimal estimation for large-eddy simulation of turbulence and application to the analysis of subgrid models
Authors: Moreau A. , Teytaud O. , Bertoglio J. P.
Published: November 2006

The tools of optimal estimation are applied to the study of subgrid models for large-eddy simulation of turbulence. The concept of optimal estimator is introduced and its properties are analyzed in the context of applications to a priori tests of subgrid models. Attention is focused on the Cook and Riley model in the case of a scalar field in isotropic turbulence. Using DNS data, the relevance of the β assumption is estimated by computing (i) generalized optimal estimators and (ii) the error brought by this assumption alone. Optimal estimators are computed for the subgrid variance using various sets of variables and various techniques (histograms and neural networks). It is shown that optimal estimators allow a thorough exploration of models. Neural networks are proved to be relevant and very efficient in this framework, and further usages are suggested.
Overlap fluctuations from the Boltzmann random overlap structure
Authors: Barra Adriano , De Sanctis Luca
Published: November 2006

We investigate overlap fluctuations of the Sherrington-Kirkpatrick mean field spin glass model in the framework of the Random Overlap Structure (ROSt). The concept of ROSt has been introduced recently by Aizenman and co-workers, who developed a variational approach to the Sherrington-Kirkpatrick model. Here we propose an iterative procedure to show that, in the so-called Boltzmann ROSt, Aizenman-Contucci polynomials naturally arise for almost all values of the inverse temperature (not in average over some interval only). These polynomials impose restrictions on the overlap fluctuations in agreement with Parisi theory.
Step edge growth of Co nanoislands on Cu(111) surface
Authors: Chang H. W. , Yuan F. T. , Yao Y. D. , Cheng W. Y. , Su W. B. , Chang C. S. , Lee C. W. , Cheng W. C.
Published: November 2006

Step edge growth of Co nanoislands on Cu(111) surface have been investigated by scanning tunneling microscopy (STM). The cobalt atoms cluster at the upper step edges and form bilayer islands of 2nm in diameter (about nine Co atoms in width) initially during the initial stage of Co deposition. This result is in accordance with the total energy calculations within density functional theory. Besides, the size and amount of nanoislands increase with increasing coverage. The average number of Co atoms contained in one island increases with a rate of 375 atoms per monolayer (ML). The statistics data on the STM images indicate that the cobalt nanoislands preferentially grow at the upper step edge during the first stage of Co deposition, then toward terrace, and finally, the growth rate of islands in edge is almost the same as that in terrace for Co thickness above 0.78–1.42 ML.
On kernel formulas and dispersionless Hirota equations of the extended dispersionless BKP hierarchy
Authors: Chen Yu-Tung , Tu Ming-Hsien
Published: November 2006

We derive dispersionless Hirota equations of the extended dispersionless BKP(EdBKP) hierarchy proposed by Takasaki from the method of kernel formula provided by Carroll and Kodama. Moreover, we verify associativity equations (WDVV equations) in the EdBKP hierarchy from dispersionless Hirota equations and give a realization of associative algebra with structure constants expressed in terms of the residue formula.
Size effect in barium titanate powders synthesized by different hydrothermal methods
Authors: Sun Weian
Published: November 2006

The size effect in barium titanate (BaTiO3) was investigated both experimentally and theoretically. Tetragonal BaTiO3 powders with average sizes from 80to420nm were directly prepared by different hydrothermal methods. The tetragonality of the hydrothermal BaTiO3 decreased with decreasing particle size, which exhibited a dependence on the synthesis method. A phenomenological model for the size effect was proposed to interpret the experimental observations. The influence of the defects, mainly the lattice hydroxyl, on the size effect was investigated to understand the correlation between the size effect and synthesis condition. The permittivities of BaTiO3 powder at different particle sizes were calculated, which predicted a maximum permittivity of over 16 000 around the room-temperature critical size of ∼70nm. The prediction was in good accordance with the experimental data reported recently.
Impedance of space-charge-limited currents in organic light-emitting diodes with double injection and strong recombination
Authors: Pitarch Ángeles , Garcia-Belmonte Germà , Bisquert Juan , Bolink Henk J.
Published: November 2006

The impedance model for a one-carrier space-charge-limited (SCL) current has been applied to explain some experimental features of double carrier organic light-emitting diodes. We report the analytical model of impedance of bipolar drift transport in SCL regime in the limit of infinite recombination. In this limit the ac impedance function is identical to that of a single carrier device, with a transit time modified by the sum of mobilities for electrons and holes, μn+μp. The static capacitance C(ω→0) is a factor of 34 lower than the geometric capacitance, as observed for single carrier devices, but it is shifted to higher frequencies. It follows that impedance measurements in the dual-carrier organic diodes with strong recombination provide the combination of μn+μp. For the mobilities of the different carriers to be determined separately, additional information is required.
Characterization of neutral species densities in dual frequency capacitively coupled photoresist ash plasmas by optical emission actinometry
Authors: Worsley M. A. , Bent S. F. , Fuller N. C. M. , Dalton T.
Published: November 2006

Reactive neutral species densities for various conditions in dual frequency capacitively coupled discharges of Ar∕O2, Ar∕N2, and Ar∕H2 were determined using optical emission spectroscopy, Kr actinometry, and modeling. The reactive neutral species probed in this work include O, O2, N, N2, H, and H2. Densities are reported as a function of pressure (5–60mTorr), percent Ar in the feed gas (1%–86%), source power (50–800W), and bias power (0W, 200W). It was found that increasing the pressure from 5to60mTorr resulted in order of magnitude increases in atomic species densities for all ash chemistries. At 30mTorr, percent dissociation is relatively low (⩽15%) for all species. Also, at 30mTorr, the addition of Ar resulted in a small decrease in N and H densities, but an order of magnitude increase in O density. Based on modeling, it is proposed that the increase in O density is due to an increasing contribution of Penning dissociation with increasing Ar density. Only the source power contributed significantly to O and N radical densities, but 200W bias power generated a significant H radical density above that generated via the source power. Details of these results are discussed in comparison with theory and literature.
Microscopic theory for interface fluctuations in binary liquid mixtures
Authors: Hiester Thorsten , Dietrich S. , Mecke Klaus
Published: November 2006

Thermally excited capillary waves at fluid interfaces in binary liquid mixtures exhibit simultaneously both density and composition fluctuations. Based on a density functional theory for inhomogeneous binary liquid mixtures we derive an effective wavelength dependent Hamiltonian for fluid interfaces in these systems beyond the standard capillary-wave model. Explicit expressions are obtained for the surface tension, the bending rigidities, and the coupling constants of compositional capillary waves in terms of the profiles of the two number densities characterizing the mixture. These results lead to predictions for grazing-incidence x-ray scattering experiments at such interfaces.
Theoretical and experimental analysis of current spreading in AlGaInP light emitting diodes
Authors: Perks R. M. , Porch A. , Morgan D. V. , Kettle J.
Published: November 2006

The profile of light emission from aluminum gallium indium phosphide (AlGaInP) light emitting diodes has been studied and compared with the theoretical modeling of current spreading using a lossy transmission line model of current injected into the active region. Discrepancies between the experimentally determined emission profile and the theoretically predicted current injected into the active region have been analyzed and explained using a Monte Carlo ray tracing simulation which considers the trajectories of the photons after leaving the active region and the particular geometry of the device current, die size, and current spreading layer resistance upon the emitted emission profile. The combination of the current spreading and ray tracing models give very good agreement with the experimental emission profiles for both thin and thick current spreading layers.
Quantum calculations of correlated electron-ion collisions in a strong laser field
Authors: Rascol G. , Bachau H. , Tikhonchuk V. T. , Kull H.-J. , Ristow T.
Published: November 2006

The energy spectrum and angular distribution of electrons scattered by an ion in a strong laser field are investigated as a function of the incident electron velocity for small impact parameters. The energy distribution has been calculated quantum-mechanically by a method of wave-packet scattering from a three-dimensional hydrogen-like Coulomb potential. It is compared with the energy distribution from the classical instantaneous collision model, and the quantum limitations are evaluated. The backscattered particles can have enhanced scattering rates and a very large energy gain due to the effect of correlated collisions. Their spectrum displays a ring structure similar to the rescattering plateau in the above-threshold ionization of neutral atoms. The effect of these large-angle scattering effects on the electron acceleration and heating is also discussed.
Three-dimensional wave packets in a compressible boundary layer
Authors: Forgoston Eric , Tumin Anatoli
Published: November 2006

A three-dimensional wave packet generated by a local disturbance in a two-dimensional hypersonic boundary layer flow is studied with the aid of the previously solved initial-value problem. The solution to this problem can be expanded in a biorthogonal eigenfunction system as a sum of modes consisting of continuous and discrete spectra of temporal stability theory. A specific disturbance consisting of an initial temperature spot is considered, and the receptivity to this initial temperature spot is computed for both the two-dimensional and three-dimensional cases. Using previous analysis of the discrete and continuous spectrum, the inverse Fourier transform is computed numerically. The two-dimensional inverse Fourier transform is calculated for two discrete modes: Mode F and Mode S. The Mode S result is compared with an asymptotic approximation of the Fourier integral, which is obtained using the Gaussian model as well as the method of steepest descent. It is shown that the method of steepest descent provides an excellent approximation to the more computationally intensive numerical evaluation of the inverse Fourier transform. Additionally, the three-dimensional inverse Fourier transform is found using an asymptotic approximation of the Fourier integral. A main feature of the resulting three-dimensional wave packet is its two-dimensional nature, which arises from an association of Mode S with Mack’s second mode.
Electromigration enhanced intermetallic growth and void formation in Pb-free solder joints
Authors: Chao Brook , Chae Seung-Hyun , Zhang Xuefeng , Lu Kuan-Hsun , Ding Min , Im Jay , Ho Paul S.
Published: November 2006

A kinetic analysis was formulated for electromigration enhanced intermetallic evolution of a Cu–Sn diffusion couple in the Sn-based Pb-free solder joints with Cu under bump metallurgy. The simulated diffusion couple comprised the two terminal phases, Cu and Sn, as well as the two intermetallic phases, Cu3Sn and Cu6Sn5, formed between them. The diffusion and electromigration parameters were obtained by solving the inverse problem of the electromigration enhanced intermetallic growth, and they were compatible with the literature values. Finite difference method was applied to the whole simulated domain to solve for the mass transport kinetics within the intermetallic phases and across each interface of interest. Simulation showed that, when electromigration effect was absent (zero current), intermetallic growth followed a parabolic law, suggesting a diffusion controlled mechanism for thermal aging. However, under significant current stressing (4×104A∕cm2), the growth of the dominant intermetallic Cu6Sn5 clearly followed a linear law, suggesting a reaction controlled mechanism for electromigration. Simulation results were consistent with the experimental observations. The analysis of vacancy transport was also incorporated with the model, and the results showed substantial increase in vacancy concentration at the Cu6Sn5 phase near the Cu3Sn∕Cu6Sn5 interface. The peaking of the vacancy concentration explains the substantial Kirkendall void formation under electromigration at this region.
Electric-field-induced orthorhombic to monoclinic MB phase transition in [111] electric field cooled Pb(Mg1∕3Nb2∕3O3)–30%PbTiO3 crystals
Authors: Cao Hu , Li Jie-fang , Viehland D.
Published: November 2006

Structural phase transformations of [111] electric field cooled (FC) Pb(Mg1∕3Nb2∕3)O3–0.30PbTiO3 crystals have been performed by x-ray diffraction. A phase sequence of cubic (C)→tetragonal (T)→orthorhombic (O)→monoclinic (M) was found in the FC condition. The finding of a=b>c demonstrates that the M phase is of the MB type.
Observation of unusual magnetic behavior: Spin reorientation transition in thick Co–Fe–Ta–B glassy films
Authors: Sharma Parmanand , Kimura Hisamichi , Inoue Akihisa
Published: November 2006

Atomically smooth Co–Fe–Ta–B glassy films were deposited on variety of substrates (Si, SiO2, and keptone). An extensive magnetic characterization in the temperature range from 5to330K is reported for the films of thickness up to ∼5.5μm. A reversible spin reorientation transition (SRT) from in-plane single domainlike state to out-of-plane multidomain state with increase in measuring temperature from 5to330K was observed in the films of thickness up to ∼2.5μm, in contrast to previously reported ultrathin ferromagnetic films of transition metals consisting of about half a dozen of monolayers. The SRT temperature (TSRT) is dependent on the film thickness and the applied magnetic field and is not governed by the temperature dependent magnetocrystalline anisotropy or the anisotropy at the film-substrate interface, which are the most common cause for the SRT in magnetic materials. Atomic relaxation has significant influence on SRT. The relaxed state results in a shift in TSRT to higher temperature or disappearance of SRT. We have compared our results with the reported data on ultrathin ferromagnetic films and shown that the atomic randomness and the strains/stress are responsible for the SRT in present case.
A comparative study of simulated and experimentally obtained nonuniformity in thermal and electrical properties of conducting films
Authors: Mishra Shramana , Dizon Jonathan R. , Aga Roberto S. , Wu Judy Z.
Published: November 2006

The effect of localized microwave absorption in conducting thin films had been simulated using a heat diffusion model to quantitatively evaluate the electrical voltage maps obtained recently using a near-field scanning microwave microprobe (NSMM) in combination with the I-V characterization. In the experiment, the focused microwave emitted from the NSMM at a sample spot of dimension comparable to the NSMM tip dimension changes the local resistivity of the sample via microwave heating. By measuring the voltage response while scanning the NSMM, nonuniformity in the electrical current distribution in the sample can be resolved. To quantitatively assess the effect of various parameters, a theoretical simulation of the temperature profile has been made and related to the voltage response profile obtained experimentally. The variation of the induced voltage due to microwave absorption with time, microwave input power, and the thickness of the film has also been studied and the simulation compares well with experiment. Discussions on discrepancies between simulation and experiment will also be presented.
Surface switching of rotating fluid in a cylinder
Authors: Suzuki Toshiyuki , Iima Makoto , Hayase Yumino
Published: November 2006

We study the surface shape of water in an open cylinder driven by constant rotation of the bottom. Around the critical Reynolds number for the laminar-turbulent transition, the surface deformation, which is of the order of the container size, shows an aperiodic switching phenomenon between an axisymmetric shape and a nonaxisymmetric shape. The axisymmetric shape is observed as a steady state when the Reynolds number is smaller than that in the switching region, while the nonaxisymmetric shape is observed as a (quasi-) periodic state in which the surface rotates at almost constant angular velocity when the Reynolds number is larger than that in the switching region. A detailed analysis for the surface shape suggests that the flow with the nonaxisymmetric shape is turbulent.
Integrable and superintegrable systems with spin
Authors: Winternitz Pavel , Yurduşen İsmet
Published: November 2006

A system of two particles with spin s=0 and s=12, respectively, moving in a plane is considered. It is shown that such a system with a nontrivial spin-orbit interaction can allow an eight dimensional Lie algebra of first-order integrals of motion. The Pauli equation is solved in this superintegrable case and reduced to a system of ordinary differential equations when only one first-order integral exists.
Crooks equation for steered molecular dynamics using a Nosé-Hoover thermostat
Authors: Procacci Piero , Marsili Simone , Barducci Alessandro , Signorini Giorgio F. , Chelli Riccardo
Published: November 2006

The Crooks equation [Eq. (10) in J. Stat. Phys. 90, 1481 (1998)], originally derived for microscopically reversible Markovian systems, relates the work done on a system during an irreversible transformation to the free energy difference between the final and the initial state of the transformation. In the present work we provide a theoretical proof of the Crooks equation in the context of constant volume, constant temperature steered molecular dynamics simulations of systems thermostated by means of the Nosé-Hoover method (and its variant using a chain of thermostats). As a numerical test we use the folding and unfolding processes of decaalanine in vacuo at finite temperature. We show that the distribution of the irreversible work for the folding process is markedly non-Gaussian thereby implying, according to Crooks equation, that also the work distribution of the unfolding process must be inherently non-Gaussian. The clearly asymmetric behavior of the forward and backward irreversible work distributions is a signature of a non-Markovian regime for the folding/unfolding of decaalanine.
A high-temperature Raman spectroscopic investigation of the potassium tetrasilicate in glassy, supercooled, and liquid states
Authors: Kalampounias A. G. , Yannopoulos S. N. , Papatheodorou G. N.
Published: November 2006

Raman spectra of K2Si4O9 were measured over a broad temperature range including the glassy, supercooled, and molten states in an effort to follow the structural changes caused by temperature variation. Potassium tetrasilicate glass has been prepared using a containerless method and a CO2 laser for heating and melting the samples and thus avoiding contamination induced by the walls of the crucibles. Systematic Raman intensity measurements caused by temperature variation have been performed in order to elucidate the induced structural changes in the high-frequency stretching and in the three- and four-membered ring breathing vibration regions. The high-frequency symmetric stretching vibrations of the nonbridging Si–O bond are associated to the presence of two distinct types of tetrahedral units with terminal oxygen atoms. The low-frequency Raman spectra reveal the, well resolved, presence of the boson peak at temperatures above the melting point. The temperature dependence of the boson peak energy has also been determined and compared with that of the sound velocities of potassium tetrasilicate. The results are discussed in the context of recent experimental and theoretical works.
Chemistry of Cu deposition by Cu(hfac)(tmvs) monitored by Li+ ion attachment mass spectrometry
Authors: Fujii Toshihiro , Arulmozhiraja Sundaram , Nakamura Megumi , Shiokawa Yoshiro
Published: November 2006

Ion attachment mass spectrometry was used for continuous in situ analysis of coordinated products formed during copper chemical vapor deposition (Cu CVD) by Cu(1,1,1,5,5,5-hexafluoroacetylacetonate)(vinyltrimethylsilane) [Cu(hfac)(tmvs)] in a simple tubular reactor. This study of the thermally labile Cu(hfac)(tmvs) demonstrated the utility of this method for detecting molecular ions of labile compounds. The results demonstrate the feasibility of monitoring the deposition chemistry of Cu(hfac)(tmvs) by generating Li+ adduct molecular ions by means of the Li+ ion attachment technique. The reaction pathways for Cu CVD by Cu(hfac)(tmvs) were studied by analysis of the reaction products. H(hfac) and tmvs were identified as the main products when Cu(hfac)tmvs was heated at temperatures ranging from room temperature to 160°C in the reactor. The rate constant for Cu deposition, k (1∕s), was determined to be 1.6×106exp(−10.2kcal∕molRt).
Experimental and numerical investigation of the competition between axisymmetric time-periodic modes in an enclosed swirling flow
Authors: Lopez J. M. , Cui Y. D. , Lim T. T.
Published: November 2006

Time-periodic vortex flows in an enclosed circular cylinder driven by the rotation of one endwall are investigated experimentally and numerically. This work is motivated partly by the linear stability analysis of Gelfgat et al. [J. Fluid Mech. 438, 363 (2001)], which showed the existence of an axisymmetric double Hopf bifurcation, and the purpose of the experiment is to see if the nonlinear dynamics associated with this double Hopf bifurcation can be captured under laboratory conditions. A glycerin/water mixture was used in a cylinder with variable height-to-radius ratios between Γ=1.67 and 1.81, and Reynolds numbers between Re=2600 and 2800 (i.e., in the neighborhood of the double Hopf). Hot-film measurements provide, for the first time, experimental evidence of the existence of an axisymmetric double Hopf bifurcation, involving the competition between two stable coexisting axisymmetric limit cycles with periods (nondimensionalized by the rotation rate of the endwall) of approximately 31 and 22. The dynamics is also captured in our nonlinear computations, which clearly identify the double Hopf bifurcation as “type I simple,” with the characteristic signatures that the two Hopf bifurcations are supercritical and that there is a wedge-shaped region in (Γ,Re) parameter space where both limit cycles are stable, delimited by Neimark-Sacker bifurcation curves.
Excitonic absorption in ZnI2 films
Authors: Vedeshwar Agnikumar G. , Tyagi Pankaj
Published: November 2006

Optical absorption measurements were carried out at room temperature on stoichiometric and well characterized vacuum evaporated ZnI2 films as function of film thickness and heat-treatment temperatures. The films were transparent and tetragonally structured. X-ray diffraction analysis reveals residual strain on the crystal planes parallel to film thickness. Optical absorption shows three distinct peaklike structures at 3, 3.5 (attributed to be excitonic), and 4.1eV. The residual strains as well as all the absorption structures vary with film thickness and treatment temperatures. Combining all the data from the two experiments, we find that the two excitonic peaks blueshift with residual strain of (104). The strain-free values for all the above quantities have been determined by the extrapolation of their linear dependence on residual strain. The excitonic absorption observed at room temperature shows a large excitonic binding energy of 150meV.
Hybrid quantum/classical path integral approach for simulation of hydrogen transfer reactions in enzymes
Authors: Wang Qian , Hammes-Schiffer Sharon
Published: November 2006

A hybrid quantum/classical path integral Monte Carlo (QC-PIMC) method for calculating the quantum free energy barrier for hydrogen transfer reactions in condensed phases is presented. In this approach, the classical potential of mean force along a collective reaction coordinate is calculated using umbrella sampling techniques in conjunction with molecular dynamics trajectories propagated according to a mapping potential. The quantum contribution is determined for each configuration along the classical trajectory with path integral Monte Carlo calculations in which the beads move according to an effective mapping potential. This type of path integral calculation does not utilize the centroid constraint and can lead to more efficient sampling of the relevant region of conformational space than free-particle path integral sampling. The QC-PIMC method is computationally practical for large systems because the path integral sampling for the quantum nuclei is performed separately from the classical molecular dynamics sampling of the entire system. The utility of the QC-PIMC method is illustrated by an application to hydride transfer in the enzyme dihydrofolate reductase. A comparison of this method to the quantized classical path and grid-based methods for this system is presented.
Self-similarity of asymmetric sand-ripple profiles formed under nonlinear shoaling waves
Authors: Testik F. Y. , Voropayev S. I. , Balasubramanian S. , Fernando H. J. S.
Published: November 2006

We report the results of laboratory experiments conducted to study equilibrium profiles of asymmetric sand ripples that form under nonlinear shoaling waves. Waves were generated in a large wave tank with a sandy slope that mimics the oceanic coastal zone. The measurements revealed that, to the first approximation, the equilibrium asymmetric sand ripples profiles can be considered as self-similar. In the experimental parameter range considered (wave asymmetry A=1.3–5; mobility parameter Ψ=8–70; shoaling coefficient S=1.03–1.6), the ripple profiles can be described by a simple similarity profile of “sawtooth” shape with “universal” onshore θ1 (≈34°) and offshore θ2 (≈18°) ripple slope angles. The results may be useful in parameterization of coastal bed roughness, in underwater acoustics and sediment transport applications.
New regimes of stochastic wave growth: Theory, simulation, and comparison with data
Authors: Robinson P. A. , Li B. , Cairns I. H.
Published: November 2006

Stochastic growth theory (SGT) of bursty waves is generalized and it is shown that the theory of “elementary bursts,” previously used to describe bursty emission in certain solar plasmas, is a limiting case of the generalized theory. New regimes of strong and weak stochastic growth are found, the boundaries separating the regimes are elucidated, and a reduced-parameter quasilinear model is used to constrain growth dynamics. The analytic results are then compared with simulations using the reduced-parameter model. Upon re-analysis of data from situations previously studied using SGT or other theories, including spacecraft data and results of particle-in-cell and quasilinear simulations, good agreement is found with the predictions of the generalized theory. In particular, data collapse of stochastic wave statistics is accomplished onto a universal curve with no free parameters.
Vibrationally mediated photodissociation of ammonia: The influence of N–H stretching vibrations on passage through conical intersections
Authors: Hause Michael L. , Yoon Y. Heidi , Crim F. Fleming
Published: November 2006

Velocity map ion imaging of the H atoms formed in the photodissociation of vibrationally excited ammonia molecules measures the extent of adiabatic and nonadiabatic dissociation for different vibrations in the electronically excited state. Decomposition of molecules with an excited symmetric N–H stretch produces primarily ground state NH2 along with a H atom. The kinetic energy release distribution is qualitatively similar to the ones from dissociation of ammonia excited to the electronic origin or to several different levels of the bending vibration and umbrella vibration. The situation is very different for electronically excited molecules containing a quantum of antisymmetric N–H stretch. Decomposition from that state produces almost solely electronically excited NH2*, avoiding the conical intersection between the excited state and ground state surfaces. These rotationally resolved measurements agree with our previous inferences from lower resolution Doppler profile measurements. The production of NH2* suggests that the antisymmetric stretching excitation in the electronically excited molecule carries it away from the conical intersection that other vibrational states access.
Type I vacuum solutions with aligned Papapetrou fields: An intrinsic characterization
Authors: Ferrando Joan Josep , Sáez Juan Antonio
Published: November 2006

We show that Petrov type I vacuum solutions admitting a Killing vector whose Papapetrou field is aligned with a principal bivector of the Weyl tensor are the Kasner and Taub metrics, their counterpart with timelike orbits and their associated windmill-like solutions, as well as the Petrov homogeneous vacuum solution. We recover all these metrics by using an integration method based on an invariant classification which allows us to characterize every solution. In this way we obtain an intrinsic and explicit algorithm to identify them.
Effect of solvation and confinement on the trans-gauche isomerization reaction in n-butane
Authors: Travis Karl P. , Searles Debra J.
Published: November 2006

The effect of solvation and confinement on the conformational equilibria and kinetics of n-butane is examined using molecular dynamics simulations of the bulk and confined fluids and compared to appropriately chosen reference states. Clear evidence for a solvent shift of the preferred conformation in bulk n-butane is found. At a temperature of 292K and a density of 6.05nm−3 a small solvent shift in favor of gauche is observed (similar to previously reported values), and the shift increases substantially with an increase in density to 8.28nm−3. The rate of torsional interconversion from the trans to the gauche state, calculated using the relaxation function method, was found to increase with increasing temperature and density. The rate constants kTG and kGT have an Arrhenius temperature dependence yielding activation energies significantly lower than the trans-gauche and gauche-trans barrier heights in the torsional potential for a free molecule, depending on the density. In the confined phase, we considered the same densities as simulated in the bulk phase, and for four different values of the physical pore width (∼1.5–4.0nm). At the high density, we find that the position of the trans-gauche equilibrium is displaced towards excess trans compared with the bulk phase, reflecting the confinement and interactions of the molecules with the pore wall. The isomerization rate is found to decrease with decreasing pore width. Again, we find that the kinetics obeys an Arrhenius rate law and the activation energy for the trans-gauche and gauche-trans interconversions is slightly smaller than that of the bulk fluid at the same density.
The interrelation between adhesion, contact creep, and roughness on the life of gold contacts in radio-frequency microswitches
Authors: Gregori Giuliano , Clarke David R.
Published: November 2006

With repeated actuation, an increasing adhesive force develops between the gold contacts of radio-frequency microswitches until failure eventually occurs by stiction. Detailed characterization of the contact forces and the contact surfaces as a function of actuation cycles for “cold-switched” devices indicates that the increase in adhesive forces in air is attributed to mechanical creep of the polycrystalline gold contacts. The increase in adhesion is observed to be associated with an increase in contact area and depth of contact impression as well as asperity flattening. These morphological observations are related to the propensity for stiction using two nondimensional numbers, the plasticity index ψ and the adhesion parameter θ. Trajectories of the evolution of contact roughness in terms of these two numbers provide insight into the design of contacts to resist stiction.
Stress gradients in CrN coatings
Authors: Janssen G. C. A. M. , Tichelaar F. D. , Visser C. C. G.
Published: November 2006

Stress in hard films is the net sum of tensile stress generated at the grain boundaries, compressive stress due to ion peening, and thermal stress due to the difference in thermal expansion of the coating and substrate. The tensile part due to grain boundaries is thickness dependent. The other two contributions are not thickness dependent. Summation of the three components leads to a stress gradient in the coating. In the present paper it is demonstrated that adding the three contributions mentioned above yields a good description of the observed dependence of stress on thickness in CrN coatings.
The symmetry of single-molecule conduction
Authors: Solomon Gemma C. , Gagliardi Alessio , Pecchia Alessandro , Frauenheim Thomas , Di Carlo Aldo , Reimers Jeffrey R. , Hush Noel S.
Published: November 2006

We introduce the conductance point group which defines the symmetry of single-molecule conduction within the nonequilibrium Green’s function formalism. It is shown, either rigorously or to within a very good approximation, to correspond to a molecular-conductance point group defined purely in terms of the properties of the conducting molecule. This enables single-molecule conductivity to be described in terms of key qualitative chemical descriptors that are independent of the nature of the molecule-conductor interfaces. We apply this to demonstrate how symmetry controls the conduction through 1,4-benzenedithiol chemisorbed to gold electrodes as an example system, listing also the molecular-conductance point groups for a range of molecules commonly used in molecular electronics research.