bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of applied mechanical strain on the ferroelectric and dielectric properties of Pb(Zr0.35Ti0.65)O3 thin films
Authors: Kelman Maxim B. , McIntyre Paul C. , Hendrix Bryan C. , Bilodeau Steven M. , Roeder Jeffrey F.
Published: May 2003

A wafer bending method has been devised to impose biaxial strains on Pb(Zr0.35Ti0.65)O3 (PZT) thin films ranging in thickness from 700 to 4000 Å grown by metal-organic chemical vapor deposition. The ferroelectric and dielectric properties of PZT capacitors were investigated while the film was placed under biaxial tension. It was observed that biaxial strains as small as 0.08% can reversibly reduce the remanent polarization of PZT films by 12 to 14% for all film thicknesses. The small-signal capacitance measured at voltages significantly larger than the switching voltage increased with increasing biaxial tension. These observations present clear evidence of room temperature strain accommodation in PZT thin films by reversible 90° domain wall motion that changes the volume fraction of the film that switches during electrical testing.
Raman scattering of germanium nanocrystals embedded in glass matrix under hydrostatic pressure
Authors: Liu L. , Shen Z. X. , Teo K. L. , Kolobov A. V. , Maeda Y.
Published: May 2003

We have studied the hydrostatic pressure dependence of Ge nanocrystals embedded into SiO2 matrix on quartz-glass substrate by Raman scattering at room temperature. The pressure coefficient (dω/dP=0.77 cm−1 kbar−1) obtained for the Ge–Ge mode in the nanocrystals is found to be almost twice as large compared with its corresponding bulk value (dω/dP=0.39 cm−1 kbar−1). We explained our results using a simple elastic model, which describes the effective pressure transmitted from the matrix to the nanocrystals.
Implant isolation of Zn-doped GaAs epilayers: Effects of ion species, doping concentration, and implantation temperature
Authors: Deenapanray Prakash N. K. , Gao Q. , Jagadish C.
Published: May 2003

The electrical isolation of Zn-doped GaAs layers grown by metalorganic chemical vapor deposition was studied using H, Li, C, and O ion implantation. The ion mass did not play a significant role in the stability of isolation, and a similar activation energy of ∼(0.63±0.03 eV) was obtained for isolation using either H or O ions. Furthermore, the isolation was stable against isochronal annealing up to 550 °C as long as the ion dose was 2–3.5 times the threshold dose for complete isolation, Dth, for the respective ion species. By studying the thermal stability and the temperature dependence of isolation, we have demonstrated the various stages leading to the production of stable isolation with the increasing dose of 2 MeV C ions. For ion doses less than 0.5Dth, point defects which are stable below 250 °C are responsible for the degradation of hole mobility and hole trapping. The stability of isolation is increased to ∼400 °C for a dose Dth due to the creation of defect pairs. Furthermore, the hopping conduction mechanism is already present in the damaged epilayer implanted to Dth. Higher order defect clusters or complexes, such as the arsenic antisite, AsGa, are responsible for the thermal stability of implantation isolation at 550 °C. The substrate temperature (−196–200 °C) does not have an effect on the isolation process further revealing that the stability of isolation is related to defect clusters and not point-like defects. An average number of eight carbon ions with energy of 2 MeV are required to compensate 100 holes, which provides a general guideline for choosing the ion dose required for the isolation of a GaAs layer doped with a known Zn concentration. A discussion of the results on the implantation isolation of p-GaAs previously reported in the literature is also included.
Physics of perpendicular magnetic recording: Playback
Authors: Litvinov Dmitri , Kryder Mark H. , Khizroev Sakhrat
Published: May 2003

The physics of the playback process in perpendicular recording is explored. It is shown that due to the existence of effectively two layers of the “magnetic charges” in a perpendicular medium, the stray field sensed by a reader rolls off with the areal density essentially differently than it does in longitudinal recording. Comparing three recording modes, longitudinal recording and perpendicular recording without and with a soft underlayer (SUL), at equivalent conditions, with a 10 nm thick recording layer at a bit aspect ratio of 1:1, the density roll-off is the fastest for the longitudinal mode, while it is the slowest for the perpendicular mode without a SUL. It is shown that although at relatively low linear and track densities the use of a SUL increases the playback signal, the signal does not depend on the use of a SUL at high densities. It is shown that for both perpendicular modes, although at sufficiently low track densities (below ∼50 ktpi) the signal disappears at relatively low linear densities, there is a significant nonzero signal even at zero linear density if the track density is sufficiently high (above ∼300 ktpi). Using the magnetic image model, it is shown that, fundamentally, the use of a SUL could not improve the resolution of a recording system. As an example, the Reciprocity Principle is utilized to analyze and compare four different reader designs.
Extremely sensitive light-induced reorientation in nondoped nematic liquid crystal cells due to photoelectric activation of the interface
Authors: Pagliusi P. , Cipparrone G.
Published: May 2003

We report an investigation of the extremely sensitive molecular reorientation in pure nematic liquid crystal film induced by the combined application of low dc electric field (less than 0.1 V/μm) and very low intensity optical irradiation (few mW/cm2). The effect is observed in planar cells of well-known commercial nematic mixture (E7) aligned with rubbed polyvinyl alcohol layers, which exhibit photorefractive-like effect. We analyze the dependence of the photoinduced changes in birefringence upon the applied dc voltage and the light intensity. According to our results we believe that the effect is due to photoinduced recombination of the opposite charged carriers accumulated near the interface. In the low dc voltage regime (a few volts) the voltage mainly drops on the electric double layers at the interfaces as a consequence of dc field collected charge carriers from liquid crystalline and polymeric films to the border surfaces. Irradiation with appropriate wavelength reduces the interfacial charges density, because of photoinduced carrier injection and recombination processes, and consequently, induces a relocation of the electric field from the surface to the liquid crystal bulk. The light-induced additional electric field component in the nematic film results in a lowering of the Fréedericksz threshold or an enhanced molecular reorientation.
Behavior of oxidation-induced stacking faults in annealed Czochralski silicon doped by nitrogen
Authors: Yang Deren , Chu Jia , Xu Jin , Que Duanlin
Published: May 2003

After oxidation at 1150 °C, oxidation-induced stacking faults (OSFs) in nitrogen-doped Czochralski crystal silicon (NCZ–Si) preannealed at 750 °C for 16 h followed by annealing at 1100 °C were investigated. It was observed that the size of OSFs in NCZ–Si samples was larger than one in the conventional Czochralski silicon (CZ–Si) samples. With the increase of the oxidation duration, the size of the OSFs in NCZ–Si decreased, while that of the OSFs in CZ–Si increased. It was also found that platelet oxygen precipitates and dislocations generated in NCZ–Si, while octahedral oxygen precipitates and stacking faults formed in CZ–Si. It is suggested that the induced dislocations in NCZ–Si could absorb self-interstitial atoms so that the OSFs shrank and dissolved with the oxidation time.
Transmission characterization of drilled alternating-layer three-dimensional photonic crystals
Authors: Kuramochi Eiichi , Notomi Masaya , Yokohama Itaru , Takahashi Jun-ichi , Takahashi Chiharu , Kawashima Takayuki , Kawakami Shojiro
Published: May 2003

To investigate the photonic band structure of a drilled alternating-layer photonic crystal (DALPC), light transmittances were measured. Electron beam lithography was used to prepare Si/SiO2/air DALPCs that had four periods in the z direction with air holes penetrating the whole structure. The transmittances of TE/TM polarized lights were measured for three directions (Γ–X, Γ–Y, Γ–Z). There was a common transmittance attenuated band at a wavelength of around 1.4 μm in the transmission spectra for all measured directions and polarizations. This agreed well with the photonic band gap (PBG) predicted by calculation. The results supported the good potential of Si-based DALPCs as full PBG materials.
Assisted-pinched transport of heavy-ion beams in a fusion chamber
Authors: Welch D. R. , Genoni T. C. , Rose D. V. , Oliver B. V. , Clark R. E. , Olson C. L. , Yu S. S.
Published: May 2003

In heavy-ion inertial confinement fusion, ion beams are transported several meters through the reactor chamber to the target. This standoff distance mitigates damage to the final focus magnets and chamber walls from the target explosion. A promising transport scheme makes use of a preformed discharge channel to confine and guide the beams. In this assisted-pinched transport scheme, many individual beams are merged into two high-current beams for two-sided illumination of the fusion target. The beams are combined and focused outside the chamber before propagating at small radius in the discharge channel to the target. A large beam divergence can be contained by the strong magnetic field resulting from the roughly 50-kA discharge current. Using a hybrid particle-in-cell simulation code, we examine the dynamics of heavy-ion inertial confinement fusion driver-scale beams in this transport mode. Results from detailed two-dimensional simulations of assisted-pinched transport in roughly 1-Torr Xe suggest that the Xe plasma becomes sufficiently conductive to limit self-field effects and achieve good transport efficiency. Coupling to a published target design is calculated. In addition, results from a semianalytic theory for resistive hose growth are presented that explain three-dimensional simulation results.
Effects of seed dot layer and thin GaAs spacer layer on the structure and optical properties of upper In(Ga)As quantum dots
Authors: He J. , Zhang Y. C. , Xu B. , Wang Z. G.
Published: May 2003

The structure and optical properties of In(Ga)As with the introduction of InGaAlAs or InAlAs seed dot layers are investigated. The area density and size homogeneity of the upper InGaAs dots are efficiently improved by the introduction of a buried layer of high-density dots. Our explanation for the realization of high density and size homogeneity dots is presented. When the GaAs spacer layer is too thin to cover the seed dots, the upper dots exhibit some optical properties like those of a quantum well. By analyzing the growth dynamics, we refer to this kind of dot as an empty-core dot.
Bias dependence of the tunneling magnetoresistance in double spin-filter junctions
Authors: Xie Zheng-Wei , Li Bo-Zang
Published: May 2003

With the intention of providing reference materials for research, manufacture and application of magnetoresistance devices, we calculate the dependences of tunneling magnetoresistance (TMR) in a NM/FI/FI/NM double spin-filter junction (DSFJ) on the bias (voltage) and, secondarily, on the thickness, barrier height and molecular field of FIs [here the NM and FI represent the nonmagnetic electrode and the ferromagnetic insulator (semiconductor) spacer, respectively]. Our results show that for the TMR of the DSFJ besides its very high value it does not decrease monotonously and rapidly with a rise of bias, but increases slowly at first and then decreases after a maximum value is reached. This feature is in distinct contrast to the ordinary magnetic tunnel junction FM/NI/FM and thus facilitates the application of the DSFJ as a magnetoresistance device [here FM and NI denote the ferromagnetic electrode and the nonmagnetic insulator (semiconductor) spacer, respectively]. The influence of the thickness, barrier height and, especially, the molecular field of FIs on TMR and its bias dependence is also not negligible.
Prevention of degradation of (Ba,Sr)TiO3 during forming gas anneal by a SiO2 capped (Ba,Sr)RuO3 electrode
Authors: Kim Young-Bae , Park Jeong-Hee , Hong Duck-Hwa , Choi Duck-Kyun , Yoo Cha-Young , Horii Hideki
Published: May 2003

Degradation of (Ba,Sr)TiO3 during a forming gas anneal was examined and the effect of a SiO2 capped (Ba,Sr)RuO3 electrode was studied. All the samples were prepared by a rf magnetron sputtering technique and the forming gas (10% H2+90% N2) anneal was carried out at 400 °C for 30 min. The (Ba,Sr)RuO3 film directly exposed to H2 ambient was damaged severely, which resulted in the reduction and phase separation into BaO and Ru. On the other hand, the SiO2 capped (Ba,Sr)RuO3 film was not damaged during H2 annealing. By adopting the SiO2 capped (Ba,Sr)RuO3 as an electrode of (Ba,Sr)TiO3, the degradation of (Ba,Sr)TiO3 could be inhibited. SiO2 capping also decreased the extent of the reduction of a Pt/(Ba,Sr)TiO3/Pt structured capacitor, which is known to degrade dramatically. It is believed that the major cause of the tolerance to the forming gas anneal is the blocking capability of SiO2 against the diffusion of reaction products such as H2O.
Superconductivity near critical temperature
Authors: Pan Xing-Bin
Published: May 2003

In this paper we study the superconductivity for a sample subjected to an applied magnetic field and slightly below the critical temperature Tc. We use the Ginzburg–Landau functional to estimate the value of the critical field Hc, and examine the superconductivity when temperature is close to Tc and the applied field is below Hc.
Structure and magnetic properties of PrCo6.7−xCuxTi0.3 compounds
Authors: Zhang Jian , Wang Fang , Rong Chuan-bing , Zhang Hong-wei , Zhang Shao-ying , Shen Bao-gen
Published: May 2003

The phase structure and magnetic properties of PrCo6.7−xCuxTi0.3 (x=0–1) compounds have been studied. The compounds were prepared by arc melting and subsequent annealing. With an increase of Cu content, the 2:17 phase disappears and the single 1:7 phase with TbCu7-type structure is formed for samples x>0.4. Tc2:17 and Tc1:7 in samples x=0–0.4 decrease with an increase of Cu content. Tc1:7 in samples x>0.4, however, is almost the same. For samples x>0.4, the study of their anisotropy indicates that these samples have uniaxial anisotropy at 293 K. The addition of Cu weakens the anisotropy of the Co sublattice and enhances the anisotropy of the Pr sublattice, which causes the spin reorientation temperature to increase.
Bias and self-bias of magnetic macroparticle filters for cathodic arc plasmas
Authors: Byon Eungsun , Anders André
Published: May 2003

Curved magnetic filters are often used for the removal of macroparticles from cathodic arc plasmas. This study addresses the need to further reduce losses and improving plasma throughput. The central figure of merit is the system coefficient κ defined as a filtered ion current normalized by the plasma-producing arc current. The coefficient κ is investigated as a function of continuous and pulsed magnetic field operation, magnetic field strength, external electric bias, and arc amplitude. It increases with positive filter bias but saturates at about 15 V for relatively low magnetic field (∼10 mT), whereas stronger magnetic fields lead to higher κ with saturation at about 25 V. Further increase of positive bias reduces κ. These findings are true for both pulsed and continuous filters. Bias of pulsed filters has been realized using the voltage drop across a self-bias resistor, eliminating the need for a separate bias circuit. Almost 100 A of filtered copper ions have been obtained in pulsed mode, corresponding to κ≈0.04. The results are interpreted by a simplified potential trough model.
Charge trapping related threshold voltage instabilities in high permittivity gate dielectric stacks
Authors: Zafar Sufi , Callegari Alessandro , Gusev Evgeni , Fischetti Massimo V.
Published: May 2003

An experimental and modeling study of charge trapping related threshold voltage shifts in Al2O3 and HfO2 n-type field effect transistors (nFET) is reported. The dependence of threshold voltage, subthreshold slope, and gate leakage currents on stressing time and injected charge carrier density are investigated as a function positive bias stress voltage and temperature. Based on experimental data, a model for trapping of charges in the existing traps is developed. The model is similar to SiO2 charge trapping models with one exception. Unlike SiO2 models, the model assumes a continuous distribution in trapping capture cross sections. The model predicts that threshold voltage would increase with a power law dependence on stressing time and injected charge carrier density (Ninj) in the initial stages of stressing. The model calculates threshold voltage shifts as a function of stress time and Ninj, thereby provides estimates of threshold voltage shifts after 10 years lifetime. It also provides insights into the nature of traps by estimating trapping capture cross sections. The calculated results are shown to be consistent with both Al2O3 and HfO2 data over several decades of stressing time and Ninj. Using the model, a comparison between Al2O3 and HfO2 is made. In addition, the model is compatible with charge trapping data reported by other research groups.
Triboemission behavior of photons at dielectric/dielectric sliding: Time dependence nature at 10−4–104 s
Authors: Nevshupa Roman A. , Nakayama Keiji
Published: May 2003

Photon triboemission behavior was studied experimentally during sliding of a diamond pin on a soda-lime glass in Ar gas atmosphere under reduced gas pressures of 40–90 Pa in time scales of 10−4–104 s. The patterns of the photon emission behavior were classified into two main types. On the time scale of 10−3–100 s, the pattern consisted of pretty equally spaced peaks. The mean period between the peaks was about 30 ms. The peaks occurred in groups, which lasted several hundreds of a millisecond. The groups of peaks also had a periodicity, which coincided with one of the disk rotation. In a time scale of 101–104 s, the mean photon emission intensity decreased with an asymptotic low value. This behavior corresponds to the transition from wear-in to subsequent steady wear periods. The behavior of photon emission was interpreted in terms of the tribomicroplasma concept due to electric discharge of the surrounding gas. The interplay between the charge accumulation and the charge dissipation is responsible for the formation of various triboemission patterns.
Extended methods of molecular dynamic simulations under hydrostatic pressure and/or isostress
Authors: Aoki Keiko M. , Yoneya Makoto , Yokoyama Hiroshi
Published: May 2003

By introducing new cell dynamics to the Parrinello–Rahman method, we propose extended methods for molecular dynamics simulations under hydrostatic pressure and/or isostress. The new method leads straightforwardly to the virial theorem. The obtained equations of motions are invariant under transformations between spatially periodic simulation cells. Molecular dynamics simulations of the crystalline phases of spheres and elongated molecules are conducted.
Long-wavelength optical phonons and mode behavior of Pb1−xSrxSe thin films
Authors: Chen J. , Shen W. Z.
Published: May 2003

Far-infrared (FIR) reflection measurements have been employed to study the long-wavelength optical phonons and their mode behavior in PbSrSe thin films grown by molecular beam epitaxy on BaF2 substrates with different Sr concentrations. The optical phonon frequencies are obtained by calculating FIR reflection spectra using a multioscillator model and Kramers–Kronig dispersion analysis. Both the PbSe-like and SrSe-like optical phonon reflection bands have been identified in ternary PbSrSe thin films. The modified random-element-isodisplacement model has been used to analyze the long-wavelength optical mode behavior of the PbSrSe alloy. A mixed mode of the Pb1−xSrxSe alloy has been revealed with critical point of x=0.71, which can be further confirmed by a simple atomic mass criterion.
Growth and optical properties of SrBi2Nb2O9 ferroelectric thin films using pulsed laser deposition
Authors: Yang Pingxiong , Carroll David L. , Ballato John , Schwartz Robert W.
Published: May 2003

High quality SrBi2Nb2O9 ferroelectric thin films were fabricated on platinized silicon using pulsed laser deposition assisted with dc glow discharge plasma. Microstructure and ferroelectric properties of the films were characterized. Optical properties of the thin films were studied by spectroscopic ellipsometry and photoluminescence from the ultraviolet to the infrared region. Optical constants, n∼0.56 in the infrared region and n∼2.24 in the visible spectral region, were determined through multilayer analyses on their respective pseudodielectric functions. The band-gap energy is estimated to be 3.60 eV. A photoluminescence peak at 0.78 μm, whose intensity decreases with decreasing temperature, was observed when excited with subband-gap energy (2.41 eV). This emission process may involve intermediate defect states at the crystallite boundaries. A possible mechanism for the observed photoluminescence, a Nb4+–O− exciton in the NbO6 octahedron, is discussed.
Detailed interpretation of electron transport in n-GaN
Authors: Mavroidis C. , Harris J. J. , Kappers M. J. , Humphreys C. J. , Bougrioua Z.
Published: May 2003

We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on two silicon doped GaN epitaxial layers grown by two different metalorganic chemical vapor deposition sources on sapphire substrates. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness, for 1.62 and 3.92 μm thick films. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that in the undoped layer, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Using a model based upon a doped layer with one type of shallow donor, plus compensating acceptors, in parallel with an interface layer which shows impurity-band conduction, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors in the doped region were also found. An important result is that for the carrier concentration fitting, when the temperature dependence of the activation energy is taken into account, the fitted energies are closer to those predicted by free and bound carrier screening. Finally the Fermi level was found to be in the range ∼56–84 meV below the conduction band minimum at room temperature and moved closer to the donor levels at low temperatures.
Controllability properties for finite dimensional quantum Markovian master equations
Authors: Altafini Claudio
Published: May 2003

Various notions from geometric control theory are used to characterize the behavior of the Markovian master equation for N-level quantum mechanical systems driven by unitary control and to describe the structure of the sets of reachable states. It is shown that the system can be accessible but neither small-time controllable nor controllable in finite time. In particular, if the generators of quantum dynamical semigroups are unital, then the reachable sets admit easy characterizations as they monotonically grow in time. The two level case is treated in detail.
Low-lying electronic states of difluorodioxirane
Authors: Song Jie , Khait Yuriy G. , Wang Hefeng , Hoffmann Mark R.
Published: May 2003

The electronic structures of the A11 ground state and several low-lying excited states of difluorodioxirane (CF2O2) were investigated using the recently revised, multireference, second-order Generalized Van Vleck Perturbation Theory (GVVPT2) method for the description of electron correlation. Multiconfigurational self-consistent field calculations that included 19 molecular orbitals demonstrated that the electronic structures of all considered states are substantially multiconfigurational, with maximum configuration amplitudes not larger than 0.9. Equilibrium geometries of ground and first excited A11 states and the energetically lowest lying B1 and B2 triplet and singlet states were optimized at the GVVPT2 level. Additionally, the nature of and vertical excitation energy to the 1 1A2 state are evaluated. Most of the considered states are predicted to be thermodynamically stable relative to their adiabatic dissociation limits and, at least some of them, should be observable in absorption spectra. The O–O bond lengths in the excited states are predicted to be significantly larger (1.94–2.34 Å) than in the ground state (1.578 Å), which contains the longest known bond of this type.
Hermite Gaussian auxiliary functions for the variational fitting of the Coulomb potential in density functional methods
Authors: Köster Andreas M.
Published: May 2003

The use of primitive Hermite Gaussian auxiliary functions for the variational fitting of the Coulomb potential is discussed. Integral recurrence relations for three-center electron repulsion integrals and integral sums with these auxiliary functions are derived. An asymptotic expansion of three-center electron repulsion integrals with primitive Hermite Gaussian auxiliary functions is presented. The recurrence relations for the asymptotic expanded integrals are given. Integral pathway diagrams and density matrix assignment diagrams are presented for all target integrals. The calculations of the three-center electron repulsion integrals of alkenes indicate that the new algorithms represent a substantial performance improvement.
Acidic protons before take-off: A comparative jet Fourier transform infrared study of small HCl– and HBr–solvent complexes
Authors: Fárnı́k Michal , Weimann Marcus , Suhm Martin A.
Published: May 2003

A comprehensive study of the proton vibrational dynamics in isolated (HX)m⋅(M)n (X=Cl, Br, and M=H2O, D2O, CH3OH) clusters is presented. Mid-IR (1900–3900 cm−1) spectra are measured in pulsed, seeded slit nozzle expansions. A close similarity among the HCl and HBr systems is revealed and exploited for cluster assignment. Distinct absorptions by small HX and mixed HX–M clusters are superimposed on broadbands, partly due to electrolytically dissociated HX in larger mixed clusters. Assignments of bands corresponding to mixed dimers and trimers based on their X–H and O–H stretching fundamentals are provided. The broad Cl–H stretching band profile of the 1:2 complex is indicative of sub-picosecond intracluster vibrational energy redistribution. The assignments are compared to matrix isolation experiments, when available. Systematic quantum chemistry calculations are performed for HCl–water complexes with up to three molecules and their isotopomers. Hydrogen bond induced frequency shifts of Cl–H stretching vibrations are best described at the coupled-cluster [CCSD(T)] level of theory. Lower level correlated quantum-chemical calculations clearly overestimate Cl–H frequency shifts. The low abundance of mixed clusters in the size range close to electrolytic dissociation (n=3–5) is discussed.
Response to “Comment on ‘Direct determination of the fragility indices of glassforming liquids by differential scanning calorimetry: Kinetic versus thermodynamic fragilities’ ” [J. Chem. Phys. 118, 10351 (2003)]
Authors: Wang Li-Min , Angell C. Austen
Published: May 2003

We give a brief review of how activation energies from scan-rate-dependent Tg studies have been correlated with those for viscosity and other relaxation processes, in order to place the present study in context with previous work, including that referred to in the comment. Then we examine the ensuing question of the extent to which kinetic fragility can be predicted from thermodynamic data. We present a new correlation involving only the entropy of fusion and the jump in heat capacity at Tg, that is followed by most nonchain liquids. Polymer liquids, as the comment authors have shown, have different behavior, consistent with earlier failures to identify Kauzmann and Vogel temperatures in these materials.
Investigating a link between all-atom model simulation and the Ising-based theory on the helix–coil transition. II. Nonstationary properties
Authors: Takano Mitsunori , Nakamura Hironori K. , Nagayama Kuniaki , Suyama Akira
Published: May 2003

The all-atom and the Ising-based models have both played their own roles to help our understanding of helix–coil transition. In this study, we address to what degree these two theoretical models can be consistent with each other in the nonstationary regime, complementing the preceding equilibrium study. We conducted molecular dynamics simulations of an all-atom model polyalanine chain and Monte Carlo simulations of a corresponding kinetic Ising chain. Nonstationary properties of each model were characterized through power spectrum, Allan variance, and autocorrelation analyses regarding the time course of a system order parameter. A clear difference was indicated between the two models: the Ising-based model showed a Lorentzian spectrum in the frequency domain and a single exponential form in the time domain, whereas the all-atom model showed a 1/f spectrum and a stretched exponential form. The observed stretched exponential form is in agreement with a very recent T-jump experiment. The effect of viscous damping on helix–coil dynamics was also studied. A possible source of the observed difference between the two models is discussed by considering the potential energy landscape, and the idea of dynamical disorder was introduced into the original Glauber model in the hope of bridging the gap between the two models. Other possible sources, e.g., the limitations of the Ising framework and the validity of the Markovian dynamics assumption, are also discussed.
Accelerated Stokesian dynamics: Brownian motion
Authors: Banchio Adolfo J. , Brady John F.
Published: May 2003

A new Stokesian dynamics (SD) algorithm for Brownian suspensions is presented. The implementation is based on the recently developed accelerated Stokesian dynamics (ASD) simulation method [Sierou and Brady, J. Fluid Mech. 448, 115 (2001)] for non-Brownian particles. As in ASD, the many-body long-range hydrodynamic interactions are computed using fast Fourier transforms, and the resistance matrix is inverted iteratively, in order to keep the computational cost O(N log N). A fast method for computing the Brownian forces acting on the particles is applied by splitting them into near- and far-field contributions to avoid the O(N3) computation of the square root of the full resistance matrix. For the near-field part, representing the forces as a sum of pairwise contributions reduces the cost to O(N); and for the far-field part, a Chebyshev polynomial approximation for the inverse of the square root of the mobility matrix results in an O(N1.25 log N) computational cost. The overall scaling of the method is thus roughly of O(N1.25 log N) and makes possible the simulation of large systems, which are necessary for studying long-time dynamical properties and/or polydispersity effects in colloidal dispersions. In this work the method is applied to study the rheology of concentrated colloidal suspensions, and results are compared with conventional SD. Also, a faster approximate method is presented and its accuracy discussed.
Three-dimensional Raman free electron laser in a waveguide with annular electron beam
Authors: Farokhi B. , Family Z. , Maraghechi B.
Published: May 2003

A linear theory for a free electron laser with a three-dimensional helical wiggler and axial magnetic field in the collective regime is presented. The configuration consists of an annular electron beam propagating inside a cylindrical waveguide. Parametric decay of the wiggler pump wave, in the beam frame, into a space-charge wave and an electric-magnetic waveguide mode is analyzed in three dimensions. A nonlinear wave equation for the three-wave interaction is derived and employed to obtain a formula for the spatial growth rate of the excited eigenmodes. Numerical analysis is conducted to study the growth rate, radiation wavelength, and required relativistic factor as functions of axial magnetic field B0. Variation of growth rate with the beam thickness and the radius of the equilibrium trajectory was also studied.
Electron trajectories in the magnetic field of capillary discharge: Application to laser wakefield accelerators in plasma channel
Authors: Hafizi B. , Gordon D. F. , Zigler A. , Ting A.
Published: May 2003

Capillary discharge plasma channels have demonstrated guiding of laser beams and may be employed to extend the length of laser-driven accelerators. However, the magnetic field associated with the discharge current may affect the injection of electrons into the capillary. A model for magnetic field generation is presented and analyzed. For a wide range of parameters of interest the magnetic field outside the channel is well described by a diffusion equation. Analytical solutions of the diffusion equation are obtained and compared with numerical solutions. Trajectories, for test electrons starting from outside the capillary, are described and discussed. For typical parameters the deflection of electrons by the magnetic field is small compared with the acceptance of the wakefield.
Growth and characterization of Fe-doped Pb(Zn1/3Nb2/3)O3−PbTiO3 single crystals
Authors: Zhang Shujun , Lebrun Laurent , Jeong D.-Y. , Randall Clive A. , Zhang Qiming , Shrout Thomas R.
Published: May 2003

Fe-doped 0.955Pb(Zn1/3Nb2/3)O3−0.045PbTiO3 (PZNT4.5) single crystals were grown using a high temperature flux technique and the dielectric, piezoelectric, electromechanical, electrostrictive, and electro-optic properties were characterized. The room temperature dielectric permittivity along the <001> direction after poling was about ∼6000 (5000−7000), increased compared to that of pure PZNT4.5 crystals ∼3000−5000. The Curie temperature (TC) and the rhombohedral to tetragonal phase transition temperature (Tr−t) of doped crystals were about 143 and 92 °C, respectively, more than 20 °C lower than that of the pure crystals. The remnant polarization (Pr) and coercive field (Ec) of <001> oriented doped crystals measured at 1 Hz and 20 kV/cm field were ∼27.3 μC/cm2 and 4.7 kV/cm, respectively, the latter significantly greater than the Ec of the pure crystals ∼3 kV/cm. Piezoelectric coefficients (d33) of <001> oriented doped PZNT4.5 crystals were ∼2500−3000 pC/N, being much higher than that of the pure (1400–2000 pC/N). The electric field induced strain was up to 0.5% with minimal hysteresis at low electric field (E∼15 kV/cm). The longitudinal electromechanical coupling factor k33 was 92%–93%. The shear mode and lateral mode properties were also studied with piezoelectric coefficient d15 of ∼3500 pC/N, d31 of ∼−1800 pC/N, and electromechanical coupling of k15∼93%, k31∼87%, respectively. The electrostrictive coefficients of Fe-doped PZNT4.5 crystals, Q11∼0.099 m4/C2, Q12∼−0.048 m4/C2, Q44∼0.012 m4/C2, and Qh∼0.0033 m4/C2 were calculated and compared to those of PZNT4.5 crystals. The apparent electro-optic coefficients of doped crystals were measured to be r33*∼118 pm/V and r13*∼29 pm/V, respectively.
Nonequilibrated situations of pulse modulated Ar–H2 and Ar–N2 thermal plasmas at atmospheric pressure
Authors: Paul K. C. , Ishigaki T. , Mostaghimi J. , Sakuta T.
Published: May 2003

Atmospheric pressure inductively coupled thermal plasmas with pulsing mode are generated for 104 slpm standard liters per minute total flow rate of Ar−H2 and Ar−N2 mixtures (Ar 94% molar concentration) inside a torch of 55 mm inner diameter. The inverter-type power source of 22 kW maximum output supplied coil current of 1 MHz fundamental frequency at 17.3 kW level, and the coil current is modulated by an external pulsed signal to produce the desired pulse modulated plasmas. Spectroscopic measurements are carried out for the temporal radiation intensity and coil current intensity. In addition, a time-dependent, two-dimensional model, which works with coil current input, is solved by using the same coil current profile measured in the experiment. Experimental and theoretical results for 67% duty factor with 10 ms on-time are compared. The theoretical predictions are found comparable with the experimental findings although the theoretical time-dependent response is seen slower, especially over the time just after on-pulsation. An estimation of electron temperature helps to describe this discrepancy fairly adequately. Experimentally the shimmer current level is found to vary with gas composition as well as with on- and off-time of pulsing signal. Both the experimental and theoretical findings suggest that plasma responds slowly at any axial positions below midcoil compared to that at midcoil level.
Dynamics of OH formation in photodissociation of pyruvic acid at 193 nm
Authors: Dhanya Suresh , Maity Dilip K. , Upadhyaya Hari P. , Kumar Awadhesh , Naik Prakash D. , Saini Rameshwar D.
Published: May 2003

The dynamics of the formation of OH radical upon 193 nm excitation of pyruvic acid has been investigated by the laser-photolysis laser-induced-fluorescence technique. OH radicals were generated in the ground electronic state, with no vibrational excitation. The estimated rotational temperature is 720±90 K, and the translational energy is 18.7±6.5 kcal mol−1. Ab initio calculations on excited electronic states were performed at the configuration interaction with single electronic excitation level with 6-31+G(d,p) basis function. All low-lying electronic excited states (S1–S3 and T1–T6) were characterized and the transitions were identified. A transition state for the C–OH dissociation channel has been obtained from the T1 state with a late exit barrier. A mechanism for the formation of OH radicals involving internal conversion and intersystem crossing from the initially populated S3 state to T1 state and the dissociation from the T1 potential energy surface with the calculated barrier is proposed, which reproduces the observed partitioning of available energy in the fragments according to the hybrid model.
Mass analyzed threshold ionization spectroscopy of indoline cation: Cyclization effect and large amplitude vibrations
Authors: Lin Jung Lee , Lin Jieli , Wu Rong Hu , Tzeng Wen Bih
Published: May 2003

We have applied mass analyzed threshold ionization (MATI) spectroscopy to record the vibrational spectra of the indoline cation by ionizing through several vibrational levels in the S1 state. The adiabatic ionization energy is determined to be 59 269±5 cm−1. The present experimental results show that only one stable configuration is involved in the photo-excitation processes, as predicted by ab initio calculations. Comparing the electronic transition and ionization energies of indoline with those of aniline and N-ethylaniline suggests that the cyclization of the five-membered ring leads to a redshift in both quantities. When the intermediate states involve large amplitude vibrations, the intense bands that appeared in the MATI spectra correspond to similar vibrational patterns, indicating that the structure of indoline in the D0 state resembles that in the S1 state. The puckering of the five-membered ring, the ring flopping (butterfly), the out-of-plane ring twisting, and the N-inversion of indoline in the D0 state are observed with frequencies of 92, 187, 235, and 706 cm−1, respectively. These spectral bands may be regarded as the characteristic features of the indoline cation for molecular identification.
Theoretical investigation of discharge parameters in magnetized radio frequency excited CO2 lasers
Authors: Tavassoli H. , Sohbatzadeh F. , Latifi H.
Published: May 2003

In the present paper the magnetic field effects on discharge parameters in rf excited CO2 lasers are calculated. A rf generated plasma imbedded in an external, constant, and homogeneous magnetic field is considered. The continuity equation is used to derive the electron density. Quasineutrality condition and ambipolar diffusion are used. Electron attachment coefficient is neglected. Local electric field, local electron density, and thickness of charge layers are derived as a function of distance from the electrodes and magnetic field. The thickness of charge layers in the presence of magnetic field is always smaller than one without the magnetic field. When the magnetic field increases, the electron density increases in all regions of discharge, and the electric field reduces in the charge layers but increases in the middle part of discharge.
Ultrasonic measurement of the porosity and tortuosity of air-saturated random packings of beads
Authors: Fellah Z. E. A. , Berger S. , Lauriks W. , Depollier C. , Trompette P. , Chapelon J. Y.
Published: May 2003

In the pore space of packed grain material, transport properties are characterized by macroscopic parameters. Some of them, porosity and tortuosity, are measured for random packing of glass beads and compared with evaluations made during previous studies. A simple method for measuring porosity and tortuosity for air-saturated granular media is given. This method is based on a temporal model of the direct and inverse scattering problem for the propagation of transient ultrasonic waves in a homogeneous isotropic slab of porous material with a rigid frame. Viscous and thermal losses of the medium are described by temporal operators describing memory effects in a porous medium. Because of the high attenuation of acoustic waves in air-saturated packings of beads, the wave reflected by this medium is equivalent to the wave reflected by the first interface. Using this approximation, a simple relation between porosity, tortuosity, the angle of incidence, and the reflected wave is obtained. Porosity and tortuosity are thus measured via reflected waves in the porous medium at oblique incidence. Experimental and numerical validation results for this method are given for a sample of random packings of glass beads.
Origin and effect of impurity incorporation in plasma-enhanced ZrO2 deposition
Authors: Cho Byeong-Ok , Lao Sandy X. , Chang Jane P.
Published: May 2003

The origin of impurity incorporation in the ZrO2 films deposition by plasma-enhanced chemical vapor deposition with Ar-carried zirconium t-butoxide (ZTB) and O2 was delineated by optical emission spectroscopy, quadrupole mass spectrometry, transmission Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy. Hydrocarbons were the predominant contaminants in films obtained with only ZTB in the plasma, while their fractions decreased rapidly with the increasing O2 to ZTB-carrying Ar flow rate ratio (O2/Ar), and eventually became negligible in the oxygen-rich condition at O2/Ar⩾2. However, the increasing amount of oxygen led to the formation of hydrogen carbonate and formate from CO chemisorption at O2/Ar=0.5 to mostly bidentate carbonate from CO2 chemisorption at O2/Ar=2. The films from all conditions contained a significant amount of hydrogen-bonded hydroxyl groups. At higher O2/Ar ratios, the decrease in hydrocarbon concentration and the increase in carbonate fractions resulted in the increase in the dielectric constant and the negative oxide trapped charges.
Photoelectron energy shift induced by microfocused x rays in micrometer-thick insulating layers
Authors: Shin H. J. , Song H. J. , Lee M. K. , Kim G. B. , Hong C. K.
Published: May 2003

The temporal changes of the kinetic energy spectra of photoelectrons emitted from micrometer-thick insulating layers, SiO2, and photoresist layers, were investigated with microfocused soft x rays in soft x-ray spectromicroscopy. The energy spectra of the insulators shifted up to several tens of electronvolts toward lower energies within seconds of the initial exposure. The amount of the energy shift depended on the thickness of the insulators. For the photoresist insulator, which was susceptible to radiation damage, the energy shift then decreased as the exposure time increased. The main cause of this decrease is attributed to the increase of conductivity by the x-ray-induced chemical state change of the insulator along the x-ray path. It was also demonstrated that by choosing appropriate time and energy in detecting photoelectrons the spectromicroscopy could be used as a depth probe of the conducting microstructures covered by insulating layers.
Angular dependence of the coercivity and remanence of ferromagnetic nanowire arrays
Authors: Han G. C. , Zong B. Y. , Luo P. , Wu Y. H.
Published: May 2003

Magnetization properties of magnetic nanowire arrays are studied on various ferromagnetic materials grown in anodic alumina (alumite) and track etched polycarbonate (PCTE) membranes by pulsed electrodeposition. Magnetization curves were measured as functions of wire material, field orientation, and wire length. The coercivity (Hc) and remanent squareness (S) of the various wire arrays were derived from hysteresis loops as a function of angle (θ) between the field and wire axis. For PCTE membranes, Hc(θ) curves for CoNiFe, NiFe, and Co nanowire arrays all show an otherwise-bell-type variation, while they change shapes from the otherwise bell to bell type for Ni nanowire arrays as the wire diameter decreases to 30 nm. These characteristics can be understood based on different magnetization reversal mechanisms of small wires. The effect of magnetostatic interaction among wires on the magnetic properties was examined by changing the wire lengths in alumite membranes. It is found that the interaction reduces Hc and S values significantly and may cause the overall easy axis change from parallel to perpendicular to the wire axis. However, the interaction is much weaker than expected from an independent precession theory. The strong coupling among the wire may also induce a change of magnetization reversal mechanism.
Density functional calculations of the structures and binding energies of Ni2Cn clusters (n=7–11)
Authors: Longo R. C. , Gallego L. J.
Published: May 2003

We report the structures and binding energies of Ni2Cn clusters (n=7–11) as computed using the density functional ab initio method SIESTA. We found no three-dimensional ground state structures in any case, in contrast with the behavior of M2Cn clusters with early 3d transition metals M.
Lattice-model study of the thermodynamic interplay of polymer crystallization and liquid–liquid demixing
Authors: Hu Wenbing , Frenkel Daan , Mathot Vincent B. F.
Published: May 2003

We report Monte Carlo simulations of a lattice-polymer model that can account for both polymer crystallization and liquid–liquid demixing in solutions of semiflexible homopolymers. In our model, neighboring polymer segments can have isotropic interactions that affect demixing, and anisotropic interactions that are responsible for freezing. However, our simulations show that the isotropic interactions also have a noticeable effect on the freezing curve, as do the anisotropic interactions on demixing. As the relative strength of the isotropic interactions is reduced, the liquid–liquid demixing transition disappears below the freezing curve. A simple extended Flory–Huggins theory accounts quite well for the phase behavior observed in the simulations.