bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic-scale analysis of deposition and characterization of a-Si:H thin films grown from SiH radical precursor
Authors: Sriraman Saravanapriyan , Aydil Eray S. , Maroudas Dimitrios
Published: July 2002

Growth of hydrogenated amorphous silicon films (a-Si:H) on an initial H-terminated Si(001)(2×1) substrate at T=500 K was studied through molecular-dynamics (MD) simulations of repeated impingement of SiH radicals to elucidate the effects of reactive minority species on the structural quality of the deposited films. The important reactions contributing to film growth were identified through detailed visualization of radical–surface interaction trajectories. These reactions include (i) insertion of SiH into Si–Si bonds, (ii) adsorption onto surface dangling bonds, (iii) surface H abstraction by impinging SiH radicals through an Eley–Rideal mechanism, (iv) surface adsorption by penetration into subsurface layers or dissociation leading to interstitial atomic hydrogen, (v) desorption of interstitial hydrogen into the gas phase, (vi) formation of higher surface hydrides through the exchange of hydrogen, and (vii) dangling-bond-mediated dissociation of surface hydrides into monohydrides. The MD simulations of a-Si:H film growth predict an overall surface reaction probability of 95% for the SiH radical that is in good agreement with experimental measurements. Structural and chemical characterization of the deposited films was based on the detailed analysis of evolution of the films’ structure, surface morphology and roughness, surface reactivity, and surface composition. The analysis revealed that the deposited films exhibit high dangling bond densities and rough surface morphologies. In addition, the films are abundant in voids and columnar structures that are detrimental to producing device-quality a-Si:H thin films.
Dissociative ionization of ICl studied by ion imaging spectroscopy
Authors: Yamada Hidetaka , Taniguchi Nori , Kawasaki Masahiro , Matsumi Yutaka , Gordon Robert J.
Published: July 2002

The speed and angular distributions of I+ ions, produced when ICl molecules were exposed to both ultraviolet and visible radiation at 304+608 nm, 355+608 nm, and 304+532 nm, were measured by velocity map imaging. An intense central feature in the I+ images was observed to be very sensitive to the polarization of the ultraviolet light and is attributed to a dissociative ionization mechanism involving three-body fragmentation: ICl+hv (visible)+3hv (ultraviolet)→I++Cl+e−. The effect of varying the delay between the visible and ultraviolet radiation on the I+ images suggests that an intermediate gateway state of ICl reached by absorption of one photon of visible light mediates the transition to the superexcited dissociative ionization state.
Self-consistent model for quantum well infrared photodetectors with thermionic injection under dark conditions
Authors: Ryzhii V. , Ryzhii M. , Liu H. C.
Published: July 2002

We present a self-consistent analytical model describing transport processes in quantum well infrared photodetectors (QWIPs) under dark conditions. The model takes into account electron thermionic emission from the quantum wells, thermionic injection from the emitter contact, and features of transport and capture in the self-consistent electric field in the QWIP active region. Using an assumption that the rates of the electron escape from and capture into a QW are functions of the electric fields only in the barriers sandwiching this QW, we calculate the electric field and charge distributions as well as dark current–voltage characteristics. We clarify the origin of steep dark current characteristics. It is confirmed that the effect of the emitter contact substantially weakens with increasing number of QWs in the QWIP structure.
Dependence of exchange bias energy on spin projections at (La,Ca)MnO3 ferromagnetic/antiferromagnetic interfaces
Authors: Christides C. , Moutis N. , Komninou Ph. , Kehagias Th. , Nouet G.
Published: July 2002

Strained epitaxial bilayers and multilayers consisting of La1−xCaxMnO3 ferromagnetic (FM) layers (x=0.33, 0.4) and La0.33Ca0.67MnO3 antiferromagnetic (AF) layers were grown on (001)LaAlO3 to study the evolution of exchange coupling interactions. The epitaxy was revealed by conventional and high resolution electron microscopy. An out-of-plane lattice expansion is observed mainly on the FM layers that induces a spontaneous magnetization component normal to the film plane. Field-cooling experiments with the applied field parallel and perpendicular to the film plane exhibit loop-shifts (exchange biasing) and enhanced coercivities that depend on the spin projections at the AF/FM interfaces.
Structure–property relationships in pure and acceptor-doped Ba1−xSrxTiO3 thin films for tunable microwave device applications
Authors: Cole M. W. , Hubbard C. , Ngo E. , Ervin M. , Wood M. , Geyer R. G.
Published: July 2002

The influence of low concentration (1 mol %) Mg doping on the structural, microstructural, surface morphological, and dielectric properties of Ba1−xSrxTiO3 (BST) thin films has been measured and analyzed. The films were fabricated on MgO and Pt–Si substrates via the metalorganic solution deposition technique using carboxylate–alkoxide precursors and postdeposition annealed at 800 °C (film/MgO substrates) and 750 °C (film/Pt–Si substrates). The structure, microstructure, surface morphology, and film/substrate compositional quality were analyzed by glancing angle x-ray diffraction, field emission scanning microscopy, atomic force microscopy, and Auger electron spectroscopy studies. Dielectric properties of unpatterned films were measured at 10 GHz using a coupled, tuned split dielectric resonator system, and at 100 kHz using metal–insulator–metal capacitors. The Mg-doped BST films exhibited improved dielectric and insulating properties compared to the undoped Ba0.6Sr0.4TiO3 thin films. The improved dielectric properties, low leakage current, and good dielectric tunability of the low level Mg-doped BST thin films merits strong potential for utilization in tunable microwave devices.
Adsorption of water on the BaF2(111) surface
Authors: Nutt David R. , Stone Anthony J.
Published: July 2002

Physisorption of water on the (111) surface of barium fluoride is investigated using accurate intermolecular potentials. A revised version of the successful ASP–W2 water potential is developed together with a new potential describing the interaction between water and the barium fluoride surface. Isolated water molecules are found to have a binding energy of −39.8 kJ mol−1. Monolayer and multilayer coverages are also investigated and compared with previous experimental and theoretical work. We find no evidence to support epitaxial growth of ferroelectric ice on this surface and suggest reasons for this.
Theoretical calculation of the electro-absorption spectrum of the α-sexithiophene single crystal
Authors: Andrzejak Marcin , Petelenz Piotr , Slawik Michał , Munn R. W.
Published: July 2002

An extended two-dimensional analogue of the Merrifield model of the mixing between Frenkel and charge-transfer excitons is used to calculate the electro-absorption spectrum of the α-sexithiophene single crystal. The model reflects the symmetry of the crystal and takes into account all the major interactions between the molecules. The input parameters are estimated from independent quantum-chemical and micro-electrostatic calculations. The simulated spectrum is in very good agreement with experiment, both in shape and in absolute amplitude. The results demonstrate that the eigenstates of the crystal between 2.55 and 2.85 eV are primarily of charge-transfer parentage, so that charge-transfer contributions dominate the electro-absorption spectrum in that region. This first successful reproduction of the electro-absorption spectrum of a single crystal is a stringent test of the theoretical description that confirms its validity.
Photoionization dynamics of the B 1E″ state of ammonia
Authors: Pratt S. T.
Published: July 2002

Two-photon excitation was used to excite a number of vibronic bands within the B 1E″ state of ammonia, including a number of combination bands involving the umbrella mode, ν2, and the asymmetric bend, ν4. Photoelectron spectra following single-photon ionization of these levels by the same laser provide insight into the character of the intermediate levels, and resolve questions raised by previous photoelectron studies via the B 1E″ state. Two-color double-resonance spectra were also recorded via selected rotational levels of the B 1E″ 2341 state. These spectra show complex series of resonances that appear to converge to the X 2A2″ 2341 state of the ion. Photoelectron spectra for a large number of these resonances show that Δv2=−1 and Δv4=−1 processes dominate, but that the branching between the two processes depends strongly on the specific resonance.
On the spectroscopy of AgCl: A complete active space self-consistent field+averaged coupled pair functional study
Authors: Ramı́rez-Solı́s A.
Published: July 2002

The two lowest-lying (X and 2)1Σ+ states, the first Σ+3, Π,3 Π,1 Δ,3 and Δ1 excited states of the AgCl molecule have been studied through extensive complete active space SCF+averaged coupled pair functional calculations, using a 19-active-electron relativistic effective core potential (RECP) for Ag, a 7-active electron RECP for chlorine and large optimized Gaussian basis sets for both atoms. The 2 1Σ+ and Σ+3 excited states present shallow relative minima very near the equilibrium geometry of the ground state, while the lowest Π3,1 states were found to be totally repulsive in the internuclear range studied. The Δ3,1 states present very shallow minima around 5.2 a.u. The calculated spectroscopic constants for the ground- and excited states are compared with the available experimental data and have been found in good agreement. Even though the Π3 state is repulsive, it lies very close in energy to the 2 1Σ+ one near the equilibrium geometry of the ground state; thus, a strong Π–2 3Σ+1 mixture through the spin–orbit interaction is predicted to occur that will lead to the fine-structure B state responsible for the recently revised B←X transitions in AgCl. The C–X transition observed at 43 500 cm−1, appears now to arise from a higher-lying root of the Σ+1 or Π manifolds, perhaps the third Σ+1 root, while the D–X system seems to arise from the Δ←3Σ+1 transition around 49 000 cm−1.
Nonlinear δf simulation studies of intense charged particle beams with large temperature anisotropy
Authors: Startsev Edward A. , Davidson Ronald C. , Qin Hong
Published: July 2002

In this paper, a 3D nonlinear perturbative particle simulation code (BEST) [H. Qin, R. C. Davidson, and W. W. Lee, Phys. Rev. ST Accel. Beams 3, 084401 (2000)] is used to systematically study the stability properties of intense non-neutral charged particle beams with large temperature anisotropy (T⊥b≫T∥b). The most unstable modes are identified, and their eigenfrequencies, radial mode structure, and nonlinear dynamics are determined for axisymmetric perturbations with ∂/∂θ=0.
Epitaxial La-doped SrTiO3 on silicon: A conductive template for epitaxial ferroelectrics on silicon
Authors: Liu B. T. , Maki K. , So Y. , Nagarajan V. , Ramesh R. , Lettieri J. , Haeni J. H. , Schlom D. G. , Tian W. , Pan X. Q. , Walker F. J. , McKee R. A.
Published: July 2002

Use of an epitaxial conducting template has enabled the integration of epitaxial ferroelectric perovskites on silicon. The conducting template layer, LaxSr1−xTiO3 (LSTO), deposited onto (001) silicon wafers by molecular-beam epitaxy is then used to seed {001}-oriented epitaxial perovskite layers. We illustrate the viability of this approach using PbZr0.4Ti0.6O3 (PZT) as the ferroelectric layer contacted with conducting perovskite La0.5Sr0.5CoO3 (LSCO) electrodes. An important innovation that further facilitates this approach is the use of a low-temperature (450 °C) sol–gel process to crystallize the entire ferroelectric stack. Both transmission electron microscopy and x-ray diffraction analysis indicate the LSCO/PZT/LSCO/LSTO/Si heterostructures are epitaxial. The electrical response of ferroelectric capacitors (for pulse widths down to 1 μs) measured via the underlying silicon substrate is identical to measurements made using conventional capacitive coupling method, indicating the viability of this approach.
Direct measurement of contact temperature using Seebeck potential
Authors: Liao C. N. , Tu K. N.
Published: July 2002

As the feature size of integrated circuit technology is shrinking continuously into submicron region, contacts in the circuit have become a major concern in terms of resistance and Joule heating. A high contact resistance not only increases the RC delay but also causes extra Joule heating. However, it is extremely difficult to measure the temperature at a tiny contact using conventional methods. This paper presents a method of direct measurement of contact temperature using four-terminal Kelvin test structures of metal in contact with Si. The temperature at the contact can be determined from the builtup Seebeck potential in the Kelvin circuit during electrical stressing once the relative Seebeck coefficient of the metallization with respect to Si is known. The Seebeck potential is extracted from the voltage measurement across the contacts. In the study, the Ti/Si contacts on both p+-Si and n+-Si with contact area ranging from 20×20 μm2 to 6×6 μm2 were examined using this method. When applying a current density of 2.8×104 A/cm2, the contact temperature is found to increase as much as by 48 °C.
Combination of magnetic force microscopy with in situ magnetoresistance measurements
Authors: Buntinx D. , Volodin A. , Haesendonck C. Van
Published: July 2002

We have combined magnetic force microscopy in the presence of an external magnetic field with in situ magnetoresistance measurements. This provides the unique opportunity of studying the interplay between the domain structure and the magnetoresistance of mesoscopic ferromagnetic thin film structures. We examined 2 μm wide Co zigzag wires because of their pronounced shape induced anisotropy effects. By combining zigzag wires in a Wheatstone bridge geometry, the hysteresis behavior for zigzag wires parallel and perpendicular to the magnetic field can be compared directly. The magnetoresistance measurements provide quantitative information about the difference in resistivity between the two configurations due to the anisotropic magnetoresistance effect and the different domain structure.
Photoluminescence study of self-assembled InAs/GaAs quantum dots covered by an InAlAs and InGaAs combination layer
Authors: Zhang Z. Y. , Xu B. , Jin P. , Meng X. Q. , Li Ch. M. , Ye X. L. , Wang Z. G.
Published: July 2002

We have fabricated a quantum dot (QD) structure for long-wavelength temperature-insensitive semiconductor laser by introducing a combined InAlAs and InGaAs overgrowth layer on InAs/GaAs QDs. We found that QDs formed on GaAs (100) substrate by InAs deposition followed by the InAlAs and InGaAs combination layer demonstrate two effects: one is the photoluminescence peak redshift towards 1.35 μm at room temperature, the other is that the energy separation between the ground and first excited states can be up to 103 meV. These results are attributed to the fact that InAs/GaAs intermixing caused by In segregation at substrate temperature of 520 °C can be considerably suppressed by the thin InAlAs layer and the strain in the quantum dots can be reduced by the combined InAlAs and InGaAs layer.
Ab Initio and density functional theory study of the interaction in formamide and thioformamide dimers and trimers
Authors: Cabaleiro-Lago Enrique M. , Otero Jesús Rodrı́guez
Published: July 2002

In this work, clusters consisting of two and three formamide or thioformamide molecules were subjected to ab initio and density functional theory calculations using the aug-cc-pvdz/cc-pvdz basis set. Formamide and thioformamide dimers were both found to exhibit five different minima on their potential surfaces involving hydrogen bonds of the N–H⋯X=C or C–H⋯X=C (X=O, S) type. The most stable structure in both cases is a cyclic configuration of C2h symmetry involving two identical N–H⋯X=C bonds. The interaction energy for such a structure is −60 and −48 kJ/mol for formamide and thioformamide, respectively. Based on the calculations, each N–H⋯X=C bond contributes −30 kJ/mol to it in formamide and −24 kJ/mol in thioformamide. On the other hand, each N–H⋯X=C bond contributes −9.7 kJ/mol in formamide and −11.7 kJ/mol in thioformamide. The interaction causes appreciable distortion in the molecules, particularly in the N–H groups involved in a hydrogen bond, which are lengthened by up to 0.019 and 0.013 Å in formamide and thioformamide, respectively. The trimer structures identified on the potential surfaces of formamide and thioformamide are cyclic configurations capable of establishing 3 or 4 hydrogen bonds. While formamide tends to adopt planar configurations (the most stable of which possesses an interaction energy of −105 kJ/mol), thioformamide forms preferentially nonplanar structures (the most stable being a nonplanar cyclic configuration with an interaction energy of −88 kJ/mol). The contribution of nonadditive pairwise terms is not particularly significant in either compound, which suggests the absence of substantial cooperative phenomena in the trimers. However, this contribution is crucial with a view to determining the stability sequence for the trimers, where the most stable structures result from the contribution of nonadditive pairwise terms (up to 15% of the overall interaction energy for the most stable thioformamide trimer). The interaction shifts the frequencies of modes closely involved in it. Thus, the N–H symmetric stretching frequency is redshifted by more than 300 cm−1 and the NH2 wagging frequency is blueshifted to a similar extent. As a rule, frequency shifts are less marked in the thioformamide clusters; both substances, however, exhibit identical trends.
Effects of morphology on the electronic and transport properties of Sn-based clathrates
Authors: Møllnitz Lone , Blake Nick P. , Metiu Horia
Published: July 2002

Density-functional calculations are used to study the electronic structure and transport properties of the type-I clathrates, K8Sn46, and K8Sn44□2, (□ is a missing Sn atom), and the type-III clathrate K8Sn25. We show K8Sn44□2 to be more stable than the defect-free K8Sn46, with K8Sn46 being metallic and K8Sn44□2; semimetallic. K8Sn25 is a zintl-phase semiconductor with a band gap of 0.5 eV. It has flatter bands than the type-I clathrates and can be expected to exhibit a smaller electrical conductivity but much larger Seebeck coefficient than the almost zintl K8Sn44□2. The figure of merit of the type-III clathrate is an order of magnitude larger than its type-I counterparts. This would make a suitably doped K8Sn25 at least as good a thermoelectric as the best Ge-based type-I clathrates, and potentially better, depending on the momentum relaxation time for carriers in this material.
Lattice dynamics and methyl rotational excitations of 2-butyne
Authors: Kirstein O. , Prager M. , Johnson M. R. , Parker S. F.
Published: July 2002

On the basis of the recently determined low temperature crystal structure the lattice dynamics of 2-butyne and a single particle methyl rotational potential are calculated using pair potential parameters given by Williams in 1974 within the model of semirigid molecules. In the regime of lattice modes the existence of four methyl librational bands with significant dispersion can explain the measured density of states. The single particle librational energy obtained for the rotational potential from the Schrödinger equation of the methyl rotor coincides well with the mean librational band energy. The calculated single particle tunneling frequency is only 17% lower than observed in experiment. Similarly well the activation energy is reproduced. Although the pair potential parameters developed for aromatic compounds need some scaling they are useful for materials with triple bond carbons.
Theoretical and experimental study of the intermediate Sm CFI 2* and the Sm CFI 1* phases in antiferroelectric liquid crystals
Authors: Čepič M. , Gorecka E. , Pociecha D. , Žekš B. , Nguyen H. T.
Published: July 2002

Within the new phenomenological model of antiferroelectric liquid crystals, structures of the two phases, the Sm CFI2* and the Sm CFI1* that appear in the temperature range between the Sm C* and the Sm CA* phase, are found. The structure of the Sm CFI2* and the Sm CFI1* phases is consistent with extensive optical, electro-optic, and dielectric measurements performed on the liquid crystal R-10OTBBB1M7, for which resonant x-ray scattering studies have previously proven that these two phases have the three-layer (Sm CFI1*) and the four-layer (Sm CFI2*) periodic structure, respectively. The biaxiality of the four-layer structure of the Sm CFI2* phase is directly observed experimentally for the first time at the temperature where the optical pitch is unwound. The theoretical model allows for experimentally consistent phase sequence, the order of transitions, and the temperature dependence of optical properties including the optical helicoidal modulation inversion within the temperature range of the Sm CFI2* phase.
Electron correlation corrected static polarizabilities of polymers with linear and cyclic conjugated elementary cells
Authors: Otto P. , Martinez A. , Czaja A. , Ladik J.
Published: July 2002

We have calculated the static polarizabilities of polymers with small linear and medium-size cyclic elementary cells and their constituent molecules. The basis set dependence has been investigated for the average and the longitudinal polarizabilities. The influence on the results of the selected method for the calculation (finite field or coupled perturbed Hartree–Fock) was analyzed. The effect of the electron correlation for this property in these systems has been calculated for the first time in the frames of the crystal orbital approach. The results show that this correction amounts up to 35% of the ab initio Hartree–Fock values.
Molecular integrals evaluated over contracted Gaussian functions by using auxiliary contracted hyper-Gaussian functions
Authors: Honda Hiroaki , Yamaki Takayoshi , Obara Shigeru
Published: July 2002

General recurrence formulas for evaluating molecular integrals over contracted Cartesian Gaussian functions are derived by introducing auxiliary contracted hyper-Gaussian (ACH) functions. By using a contracted Gaussian function, this ACH represents an extension of the Gaussian function named derivative of Fourier-kernel multiplied Gaussian [J. Chem. Phys. 94, 3790 (1991)]. The ACH is reducible to contracted Cartesian Gaussian functions, contracted modified Hermite Gaussian functions, and to contracted Gaussian functions multiplied by phase factors, or the so-called GIAO, and is also reducible to various spatial operators necessary for ab initio molecular orbital calculations. In our formulation, all molecular integrals are expressed in terms of ACH. Therefore, the formulations have wide applicability for calculating various kinds of molecular integrals in ab initio calculations. Recursive calculations based on our formulation do not depend on the number of contraction terms, because the contraction step is completed at the evaluation of the initial integrals. Therefore, we expect that more efficient recursive calculations will be accomplished by using our formulas for evaluating molecular integrals over contracted Gaussian functions.
Dielectric and mechanical relaxation of cresolphthalein–dimethylether
Authors: Paluch M. , Roland C. M. , Best A.
Published: July 2002

From pressure–volume–temperature measurements, in combination with published dielectric spectroscopy results, we assess the volume dependence of structural relaxation in cresolphthalein–dimethylether (KDE). Structural relaxation in KDE cannot be described as either a volume activated or thermally activated process, and unlike results for some other molecular glass formers, temperature is not the dominant control variable. The inflection point of the PVT data yields a glass temperature, Tg, that corresponds to a dielectric relaxation time equal to 10 s. There is a marked variation with pressure, dTg/dP=307 K/GPa at low pressure. We also carried out dynamic mechanical measurements on KDE near Tg. The shape of the mechanical and dielectric relaxation functions, as well as both the magnitude and temperature dependence of the respective relaxation times, are the same. Such equivalence is not generally expected, and arises herein due to the rigid structure of the KDE molecule.
Stabilization of sawteeth in tokamaks with toroidal flows
Authors: Kleva Robert G. , Guzdar Parvez N.
Published: July 2002

Sheared toroidal flows with a magnitude of the order of the sound speed are shown to stabilize sawteeth in tokamaks. In the absence of flows, tokamak equilibria in which the central safety factor q is less than unity are unstable to resistive tearing modes (resistive internal kink modes) with toroidal mode number n=1. As the ratio β of the plasma pressure to the magnetic field pressure increases, the growth rate of the n=1 mode rises because of the increasing pressure gradient. However, the addition of a toroidal flow to the equilibrium has a stabilizing effect. As the magnitude of the toroidal flow approaches the sound speed, the n=1 resistive tearing mode can be completely stabilized by the flow, eliminating sawteeth.
Effects of inelastic scattering on direct tunneling gate leakage current in deep submicron metal–oxide–semiconductor transistors
Authors: Alam K. , Zaman S. , Chowdhury M. M. , Khan M. R. , Haque A.
Published: July 2002

Direct tunneling gate leakage current in metal–oxide–semiconductor (MOS) structures with ultrathin gate oxides is studied. The effects of inelastic scattering of inversion carriers in the gate-oxide region is taken into account in the current calculation. Open boundary conditions, incorporating the effects of wave function penetration into the gate oxide, are used to solve Schrödinger’s equation. The proposed technique, based on the Green’s function formalism, is numerically efficient and does not require determination of complex eigenenergies of a non-Hermitian matrix. Self-consistent calculations for n-type MOS devices are compared with experimental results. Excellent agreement between simulated and measured data is obtained when appropriate spatial and gate bias dependence of the inelastic scattering rate is taken into account. It is shown that due to inelastic scattering, at low gate voltages, the gate current increases significantly in devices with oxide thickness equal to 2 nm or higher. However, when the oxide thickness is reduced below 2 nm, inelastic scattering has no significant effect on gate current. The existing mismatch at lower gate voltages between experimental and modeled direct tunneling currents in devices with gate-oxide width equal to or greater than 2 nm is explained in terms of inelastic scattering effects.
Impact of Sr content on dielectric and electrical properties of pulsed laser ablated SrBi2Ta2O9 thin films
Authors: Bhattacharyya S. , Laha Apurba , Krupanidhi S. B.
Published: July 2002

The ferroelectric properties of SrBi2Ta2O9 thin films grown by laser ablation were investigated as a function of the Sr+2 content in the films. Different target compositions were used to obtain films with different Sr+2/Ta+5 ratios. The initial composition was according to the stoichiometric composition (1/2), and the Sr+2/Ta+5 ratio was varied to 0.7/2.0. It was seen that the remanent polarization showed a consistent increase, as the film became more deficient of “Sr+2” up to a certain extent. Similarly, a decrease in the dielectric constant and the leakage current with the decrease of Sr+2 in the film was observed. The dielectric transition temperature showed an increase with the reduction of Sr+2 content and was seen to approach the bulk value.
Eutectic growth: A closed problem for the solution of the steady-state growth of lamellar eutectics
Authors: Anestiev L. , Froyen L.
Published: July 2002

The Jackson and Hunt theory was modified in order to get a better understanding of the driving forces of the eutectic’s growth kinetics. A solution of the diffusion problem, based on more rigorous boundary conditions, was obtained and kinetic members were added in order to account for the growth kinetics of the specific phases composing the eutectic. It was unambiguously shown that the eutectic’s growth problem is a closed problem, e.g., no additional assumptions are needed in order to obtain the eutectic growth velocity as a function of the phase’s composition and the lamellae’s width. The model proposed in the present article was further used to model the growth kinetics of different alloys, which exhibit eutectic structure and to derive some important kinetic parameters from the existing experimental data.
Bulk photothermal model for laser ablation of polymers by nanosecond and subpicosecond pulses
Authors: Bityurin N. , Malyshev A.
Published: July 2002

The main feature of polymeric materials is the hierarchy of bonds between molecular groups. This feature is explicitly taken into account in a model of photothermal laser ablation of polymers derived in the present work. According to this model the reason for laser ablation is a photothermal bond-breaking reaction within the bulk of material originated from laser heating. In order to address the movement of the interface between gaseous and condensed phases, we change the Stefan-like boundary condition considered previously for the Frenkel–Wilson one. In the latter case the activation energy for elimination of a short enough polymer chain from the surface is proportional to the sum of the energies of weak bonds connecting this chain with the surface. We compare predictions of this model with the previously derived Stefan-like bulk model and with the surface photothermal evaporation model with respect to kinetics and dynamics of laser ablation by both nanosecond and subpicosecond pulses, including ablation by two subpicosecond pulses. This consideration suggests experimental evidence that allows one to distinguish between surface and bulk photothermal ablation mechanisms. The parameters used in numerical calculation correspond to the KrF excimer laser ablation of polyimide. The results of the theoretical modeling are compared with the existing data on kinetics of nanosecond laser ablation as well as with findings of time-resolved UV subpicosecond laser ablation experiments.
Vacuum ultraviolet emission dynamics of a coplanar electrode microdischarge: dependence on voltage and Xe concentration
Authors: Lee W.-G. , Shao M. , Gottschalk J. R. , Brown M. , Compaan A. D.
Published: July 2002

We report nanosecond time-resolved measurements of the vacuum ultraviolet (VUV) emission and current waveforms for a planar, surface-discharge plasma display electrode in Xe/Ne mixtures. The 173 nm excimer emission is separately resolved from the overall VUV emission by a removable fused silica filter. The time difference between the maximum of excimer emission and that of resonant atomic emission is measured at high Xe concentration. The decay time of the overall VUV emission decreases strongly as the Xe concentration increases due to three-body collisions leading to Xe2 formation. The decay time constants versus Xe concentration and pressure are measured and compared with published rate constants. The efficiency of VUV emission increases rapidly from 4% to 10% Xe, but only marginally with further increase to 30% Xe. For fixed Xe ratio, the VUV emission efficiency drops with increasing driving voltage.
Ions in holes: An experimental study of ion distributions inside surface features on radio-frequency-biased wafers in plasma etching discharges
Authors: Woodworth J. R. , Miller P. A. , Shul R. J. , Abraham I. C. , Aragon B. P. , Hamilton T. W. , Willison C. G.
Published: July 2002

We present an experimental study of ion fluxes, energy distributions, and angular distributions inside surface features on radio frequency-biased wafers in high-density, inductively driven discharges in argon. Specifically, we present data on ion distributions at the bottom of 100-μm-square, 400-μm-deep “holes” in the wafer. Transmission of ions to the bottom of the holes increases with increasing ion energy and decreases as the sheath size becomes comparable to the hole size. Ion energy distributions at the bottom of the holes are narrower than distributions on the flat wafer surface. The flux of ions remains normal to the wafer surface over most of the hole area but the flux of ions within 6 μm of the wall is angled towards the wall. The observed trends are consistent with effects expected due to bowing of the plasma sheath around the surface features on the wafer. Scattering of ions off sidewalls contributes at most, only a small part of the ion flux reaching the bottom of the hole.
Preparation of La0.9Sr0.1Ga0.85Mg0.15O2.875 thin films by pulsed-laser deposition and conductivity studies
Authors: Joseph M. , Manoravi P. , Tabata H. , Kawai T.
Published: July 2002

Thin films La0.9Sr0.1Ga0.85Mg0.15O2.875 (LSGMO), an oxide ion conducting electrolyte material, which has potential application in oxygen sensor development and in solid oxide fuel cells, have been prepared by pulsed-laser deposition. Preparation of thin film under high-vacuum condition gives a stoichiometric LSGMO, whereas film prepared under oxygen atmosphere is not stoichiometric. Highly oriented films of LSGMO are obtained on SrTiO3 {STO(001)}, whereas polycrystalline films are obtained on sapphire(0001), quartz, and calcia-stabilized zirconia substrates. Film conductivity is measured by ac impedance analysis. Film on sapphire shows higher conductivity compared to that of the film on STO by a factor of 10 at 730 °C. The activation energies for films on sapphire and STO are 0.7 and 0.82 eV, respectively, in the temperature region between 630 and 930 °C.
An analytical approach to the determination of surface temperature history from subsurface data
Authors: Fivez J. , Thoen J.
Published: July 2002

In this article we address the problem of the reconstruction of surface temperature history of a semi-infinite solid from subsurface temperature wave data at one instant. This problem proves particularly interesting in relation to the geothermal history and the detection of climatic changes using borehole data. In particular we present exact analytical results valuable for this important inverse problem. We show how this provides us with a straightforward method which eliminates the need of foreknowledge typical of the current numerical approaches.
Relationship between magnetic shielding and magnetizability
Authors: Janowski Tomasz , Woliński Krzysztof
Published: July 2002

A simple relation between magnetic shielding and magnetizability has been found. Its validity has been first shown by numerical calculations and than proven analytically. This fundamental relation shows that the magnetic shielding tensor integrated over a whole space is simply proportional to the magnetizability tensor with a constant factor equal to two-thirds of the vacuum permeability with a negative sign.
Effect of vacancies on the charge-transfer energy in the anthracene crystal
Authors: Tsiaousis D. , Munn R. W.
Published: July 2002

In order to explore whether vacancies could trap charge-transfer (CT) states, the polarization and charge-quadrupole energies of CT configurations near vacancies in anthracene are calculated. Polarization and charge–quadrupole energies of single charges and of CT configurations in perfect crystals and of single charges near vacancies are recalculated treating anthracene as 14 submolecules at the heavy atoms, rather than three at the centers of the rings as in previous work. This improves agreement with experiment. A vacancy adjacent to either of the two lowest-energy CT configurations reduces the dielectric screening of the Coulomb stabilization, thereby stabilizing the configuration by typically 20–50 meV. However, for some higher-energy CT configurations a vacancy on or near the CT axis can actually increase the screening and destabilize the configuration by up to 40 meV. A vacancy also changes the charge–quadrupole energy by as much as ±250 meV, so that this effect dominates (as for single charges), leading to traps as deep as 300 meV for the two lowest-energy CT configurations. Such traps could reduce the efficiency of charge-carrier photogeneration by enhancing geminate recombination of CT configurations.
Ignition phenomena in combustion synthesis: An experimental methodology
Authors: Monagheddu M. , Bertolino N. , Giuliani P. , Zanotti C. , Tamburini U. Anselmi
Published: July 2002

An experimental methodology is presented to investigate the ignition in combustion synthesis. The aim of this work is to study the ignition phenomena, separating the initiation from the propagation process. The reactions were activated by a CO2 laser with a power range from 20 to 60 W. The experimental configuration was set to avoid the sample heat losses. All the experiments were performed in thermal explosion mode. The studied system was Ni–Al at three compositions (Ni:Al=3:1, Ni:Al=1:1 and Ni:Al=1:3). The results indicate that the ignition temperature is not affected by the laser power applied, but it is characteristic for each composition. A theoretical approach, aimed to the ignition energy calculation, supports the experiments. The calculated ignition energy, for the investigated powder mixtures, is equal to 17±0.5 kJ/molat, independent of stoichiometric ratio and of used heating ramps. A mechanistic interpretation of the ignition is attempted to compare experimental results and numerical simulations.
Numerical simulation of metal-halide lamp using a time-dependent two-dimensional model
Authors: Hashiguchi S. , Hatase K. , Mori S. , Tachibana K.
Published: July 2002

Numerical simulation of a vertically held metal-halide lamp (Hg/Na/Sc system) is presented. The lamp was driven by an ac current of 60 Hz. The model took convection and diffusion of various species into account. A method for rapid calculation of optically thick radiation power was developed. Density of scandium ions and sodium ions has a minimum on the central axis due to radial ambipolar diffusion. Density of metallic atoms and ions and metal iodine molecules decreases with increasing axial distance from the bottom of the tube. This tendency was explained by convection and diffusion. The density of scandium ions depends on the direction of the current; the density was greater at upward current than at downward current.
Investigation of the piezoelectric polarization in (In,Ga)N/GaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy
Authors: Waltereit P. , Craven M. D. , DenBaars S. P. , Speck J. S.
Published: July 2002

We have studied the structural and optical properties of a series of (In,Ga)/GaN multiple quantum wells with identical thicknesses but varied In content grown by plasma-assisted molecular beam epitaxy. Careful choice of the growth parameters returns samples with smooth and abrupt interfaces. The shift of the photoluminescence transition energy with externally applied biaxial tension was investigated. We observed a redshift for small In contents while a blueshift was detected for higher In contents. This result is in qualitative agreement with self-consistent band profile calculations taking into account both band gap deformation potentials and piezoelectric polarization charges in these structures. However, the reduction of the polarization induced quantum-confined Stark effect is well in excess of that conventionally calculated for this material system. We attribute this observation to a substantial deviation of the piezoelectric polarization constants of strained layers from those calculated for unstrained material. This finding is shown to be in agreement with recent calculations of the piezoelectric polarization charges for biaxially strained (Al,Ga,In)N layers.
Using Λ-doublet ratios to understand collision geometry in direct bimolecular reactions
Authors: Wojcik Michael D. , Fletcher T. Rick
Published: July 2002

A model is developed which can be used to understand how collision geometry can dictate product state lambda doublet ratios in direct bimolecular reactions that produce diatomic products in a Π molecular energy state. The predicted lambda doublet ratio is A′/A″=2 for unconstrained collision geometry, while the two limiting case geometries of exclusive end-on collisions and exclusive side-on collisions yield lambda doublet ratios of A′/A″=∞ and A′/A″=1, respectively. The theory is derived and used to interpret the experimental lambda doublet ratio of OH (v=0) from the H+N2O→OH+N2 reaction.
Calculation of the transport properties of carbon dioxide. I. Shear viscosity, viscomagnetic effects, and self-diffusion
Authors: Bock Steffen , Bich Eckard , Vogel Eckhard , Dickinson Alan S. , Vesovic Velisa
Published: July 2002

Transport properties of pure carbon dioxide have been calculated from the intermolecular potential using the classical trajectory approach. Results are reported for shear viscosity, viscomagnetic coefficients, and self-diffusion in the dilute-gas limit and in the temperature range of 200–1500 K for the three recently proposed carbon dioxide potential energy hypersurfaces. Agreement with the measurements is, in general, within the experimental error. The calculations indicate that the corrections in the second-order approximation and those due to the angular-momentum polarization for the viscosity are small, <1% in the temperature range considered. The very good agreement of the calculated values for the Bukowski et al. potential energy hypersurface (1999) with the experimental viscosity data is consistent with the rigid-rotor assumption made in the calculations being reasonable for the three properties considered.
Infrared spectra of C2H4–HCl complex
Authors: Çarçabal Pierre , Seurre Nathalie , Chevalier Michèle , Broquier Michel , Brenner Valérie
Published: July 2002

We report the first rotationally resolved observation of the infrared spectrum of the molecular complex C2H4–HCl. The complex was produced by a supersonic expansion through a pulsed slit jet. By means of a high-resolution tunable diode laser spectrometer, we have recorded the spectrum of the HCl stretching vibration for the isotopes C2H4–H35Cl and C2H4–H37Cl. From the analysis of the spectra, we determined the rotational constants and vibrational frequencies of both isotopes. These experimental results have been interpreted in view of obtaining information on the intermolecular interaction. The experimental data have been complemented by standard coupled cluster singles and doubles model including connected triple excitations with a correlation consistent polarized valence triple zeta basis set ab initio followed by grid calculations, in order to study the anharmonicity, the coupling between intramolecular and intermolecular motions, and the basis set superposition error effects. The results obtained in this study are compared to those of our previous work on the C2H2–HCl [J. Chem. Phys. 113, 4876 (2000)].
Quantum hydrodynamic model for the enhanced moments of inertia of molecules in helium nanodroplets: Application to SF6
Authors: Lehmann Kevin K. , Callegari Carlo
Published: July 2002

The increase in moment of inertia, ΔI, of SF6 in helium nanodroplets is calculated using the quantum hydrodynamic approach [Callegari et al., Phys. Rev. Lett. 83, 5058 (1999); 84, 1848 (2000)], which we extend here to an explicit three-dimensional treatment. Three plausible helium densities are reconstructed by interpolation of previously published “density cuts” in terms of an expansion into cubic harmonics (several interpolation strategies are presented). This allows us to predict a value of ΔI that ranges from as low as 30 u⋅Å2 to as high as 318 u⋅Å2. The lower limit reproduces the prediction of Kwon et al. [J. Chem. Phys. 113, 6469 (2000)], who use the same hydrodynamic model and an unpublished density based upon a Path Integral Monte Carlo calculation. These values can be compared with the experimentally measured ΔI (310±10 u⋅Å2) for large (N⩾103 He atoms), and with Fixed Node, Diffusion Monte Carlo calculations by Lee, Farrelly, and Whaley [Phys. Rev. Lett. 83, 3812 (1999)], which found ΔI=290–305 u⋅Å2 for N=8–20 helium atoms. The present results show that the value of ΔI obtained from the hydrodynamic model is quite sensitive to physically reasonable variations in the helium density; therefore one has to be careful as to which density to use. Because the model is based upon the assumption that the helium is in the ground “quasienergy” state of the helium-molecule time-dependent potential, we propose that calculations should be done using densities calculated at 0 K rather than at finite temperature. We have extended our original algorithm to also handle irregular boundaries. We find that in the present case the calculated value of ΔI only changes by a few percent.
Depolarization as a probe for ultrafast reorientation of diatomics in condensed phase: ClF versus I2 in rare gas solids
Authors: Bargheer M. , Gühr M. , Schwentner N.
Published: July 2002

Polarization dependent femtosecond pump-probe spectra display characteristic vibrational wave packet dynamics of ClF in Ar (isotropic cage and small fragment size) and I2 in Kr (cylindrical cage and large fragments). The intensity ratio of the signals for pumping with parallel versus crossed polarization with respect to the probe pulse is close to the value 1/3, as expected for full photoselection immediately after excitation. For ClF this ratio depolarizes to unity within τr=1.2 ps, showing the ultrafast randomization of the orientation of the molecular bond due to fragment scattering off the matrix cage. The direction of the I2 bond is geometrically fixed by the Kr matrix and the ratio remains constant.
Calculation of binary hard-sphere mixture radial distribution functions at contact from an equation of state
Authors: Viduna D. , Smith W. R.
Published: July 2002

A new method for calculation of the radial distribution functions (RDFs) at contact from a specified hard-sphere mixture equation of state (EOS) is proposed. The method is applied to two available accurate EOS expressions and new analytical formulas for the RDFs are obtained for binary additive hard-sphere mixtures. The results of the new formulas are compared with available computer simulation data and with those of other RDF expressions from the literature. The evidence to date suggests that the new formula is more accurate than alternative formulas currently available.
Effective attractions between like-charged colloidal particles
Authors: Carbajal-Tinoco M. D. , González-Mozuelos P.
Published: July 2002

We apply the Ornstein–Zernike equations together with the extended Zerah–Hansen approach, described in a previous paper [J. Chem. Phys. 109, 11074 (1998)], to determine the microstructure of a colloidal suspension at finite concentrations. Using these results as an input, the effective pair potential between two colloidal particles is also calculated following the general approach described in that paper. It is found that, for sufficiently large concentrations, this effective potential develops an attractive well with a minimum located at a distance of almost four macroparticle diameters. It is also shown that this attraction is generated by the inversion of the sign of the effective charged distribution surrounding each macroparticle involved in the electrostatic component of the effective pair potential.