bluuebunny/crossref_metadata_2025 huggingface dataset:
Brownian simulations of a network of reptating primitive chains
Authors: Masubuchi Yuichi , Takimoto Jun-Ichi , Koyama Kiyohito , Ianniruberto Giovanni , Marrucci Giuseppe , Greco Francesco
Published: July 2002

A new model for Brownian dynamics simulations of entangled polymeric liquids is proposed here. Chains are coarse grained at the level of segments between consecutive entanglements; hence, the system is in fact a network of primitive chains. The model incorporates not only the “individual” mechanisms of reptation and tube length fluctuation, but also collective contributions arising from the 3D network structure of the entangled system, such as constraint release. Chain coupling is achieved by fulfilling force balance on the entanglement nodes. The Langevin equation for the nodes contains both the tension in the chain segments emanating from the node and an osmotic force arising from density fluctuations. Entanglements are modeled as slip links, each connecting two chain strands. The motion of monomers through slip links, which ultimately generates reptation as well as tube length fluctuations, is also described by a suitable Langevin equation. Creation and release of entanglements is controlled by the number of monomers at the chain ends. In a creation event, the partner chain segment is chosen randomly among those spatially close to the advancing chain end. To validate the model, equilibrium dynamics simulations were run for monodisperse linear chains containing up to Z=40 entanglements. The results show, in agreement with experiments, (i) a Z3.5±0.1 dependence of the longest relaxation time, (ii) a Z−2.4±0.2 dependence of the self-diffusion coefficient, and (iii) a relaxation modulus proportional to the square of the end-to-end vector correlation function, consistently with the dynamic tube dilation concept.
Measuring the ion current in high-density plasmas using radio-frequency current and voltage measurements
Authors: Sobolewski Mark A.
Published: July 2002

The total current or flux of ions striking the substrate is an important parameter that must be tightly controlled during plasma processing. Several methods have recently been proposed for monitoring the ion current in situ. These methods rely on passive, noninvasive measurements of the radio frequency (rf) current and voltage signals that are generated by plasma-processing equipment. The rf measurements are then interpreted by electrical models of the plasma discharge. Here, a rigorous and comprehensive test of such methods was performed for high-density discharges in argon at 1.33 Pa (10 mTorr) in an inductively coupled plasma reactor, at inductive source powers of 60–350 W, rf bias powers up to 150 W, and rf bias frequencies of 0.1–10 MHz. Model-based methods were tested by comparison to direct, independent measurements of the ion current at the substrate electrode made using lower frequency (10 kHz) rf bias and modulated rf bias. Errors in two model-based methods are identified and explained by effects that are present in the high-density plasmas but are not included in the models. A third method, based on a new, more accurate numerical sheath model, gives values of the ion current in agreement with the independent measurements.
On the two-dimensional character of the superconducting transition in underdoped high-temperature superconductors
Authors: Sergeeva G. G.
Published: July 2002

The 3D→2D dimensional crossover in the superconducting state of quasi-two-dimensional high-Tc superconductors is investigated. The consistent pattern of the superconducting state of 2D and 3D systems is used to find a universal temperature dependence of the ratio of the squares of the penetration depths of a magnetic field directed along the ĉ axis, i.e., λ2(0)/λ2(T/Tc), from which one can obtain a relation between the superconducting transition temperature Tc and λ−2(0). This yields the temperature Tcr of the dimensional crossover as the boundary of the region of “two-dimensionality” of the superconducting state, where for T>Tcr the results of measurements of the penetration depth begin to deviate from the universal dependence found. It is shown that the size of the region of three-dimensional superconducting fluctuations can be determined from measurements of λ(T/Tc) and the resistance along the ĉ axis and turns out to be finite, attesting to the 2D character of the superconducting transition in quasi-2D HTSCs.
Nonrigidity, delocalization, spatial confinement and electronic-vibrational spectroscopy of anthracene–helium clusters
Authors: Heidenreich Andreas , Even Uzi , Jortner Joshua
Published: July 2002

In this paper we present quantum mechanical calculations for the energetics, nuclear dynamics, spectral shifts, and vibrational level structure of anthracene⋅Hen (n=1,2) clusters in the ground (S0) and in the first spin-allowed excited (S1) electronic states. The anthracene–He potential in the S0 state was described in terms of a sum of Lennard-Jones atom–atom potentials, while the potential in the S1 state also included changes in dispersive energy and in repulsive interactions. Variational calculations were conducted for anthracene⋅He1. For anthracene⋅He2 we carried out configuration interaction calculations with the wave functions consisting of Hartree products, accounting for boson permutation symmetry. Extensive, anisotropic, one-dimensional spatial delocalization of the He atoms on the anthracene microsurface, which originates from large-scale confinement by the aromatic molecule, is exhibited, being further enhanced by repulsive interactions in the S1 state and by the He–He repulsion. The anomalous size-dependence of the (red) spectral shifts for the S0→S1 electronic origin arises from mutually canceling dispersive and repulsive contributions which, together with the electronic-vibrational level structure, manifest quantum effects of anisotropic spatial delocalization, confinement and He–He interaction in nonrigid clusters.
Chemical and thermal reduction of thin films of copper (II) oxide and copper (I) oxide
Authors: Kirsch P. D. , Ekerdt J. G.
Published: July 2002

Chemical and thermal reduction of copper oxide thin films (∼20 Å) has been studied with x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) for application in microelectronic device interconnects. XPS showed that copper (I) oxide (Cu2O) and copper (II) oxide (CuO) were reduced to copper metal at 400 K within a 30 min exposure to deuterium atoms (D*) and molecules at 1×10−4 Torr. Similarly, XPS showed that Cu2O was reduced to copper metal at 400 K within a 30 min exposure to methyl radicals (CH3*) and acetone molecules at 1×10−4 Torr. After D* exposure, TPD showed O leaves the Cu2O surface as D2O from 400 K to 700 K with a peak desorption temperature of 510 K. After CH3* exposure, TPD showed O leaves the Cu2O surface as CO2 over a range from 400 to 700 K with a peak temperature at 590 K. With carbon impurity below the XPS detection limit, thermal reduction of CuO to Cu2O was verified by XPS near 890 K. TPD experiments showed that O leaves the CuO surface as O2 at 890 K. Without surface C, thermal reduction of Cu2O was not observed up to 1073 K. Reduction of Cu2O without reactive radical species (D* or CH3*) was negligible. These results suggest that thin films of copper oxide can be reduced at 400 K with D* and CH3*.
On-board projection of a microwave plasma upstream of a Mach 6 bow shock
Authors: Exton R. J. , Balla R. J. , Shirinzadeh B. , Brauckmann G. J. , Herring G. C. , Kelliher W. C. , Fugitt J. , Lazard C. J. , Khodataev K. V.
Published: July 2002

High power, pulsed microwaves are beamed forward of a model in a Mach 6 flow using an on-board, Ku-band horn. A thin “precursor” plasma is generated just ahead of the bow shock in the low density free stream. Temporal and spatial analysis of the microwave E-field and plasma conductivity in the supersonic flow field indicate that the plasma reflects a large proportion of the microwave power in an individual pulse as soon as the plasma frequency exceeds the wave frequency. Possible methods for more efficient deposition of power in the free stream are discussed which could lead to increased interaction with the shock.
Hybrid exchange-correlation functional determined from thermochemical data and ab initio potentials
Authors: Wilson Philip J. , Bradley Thomas J. , Tozer David J.
Published: July 2002

Multiplicative potentials, appropriate for adding to the non-multiplicative fractional orbital exchange term in the Kohn–Sham equations, are determined from correlated ab initio electron densities. The potentials are examined graphically and are used in conjunction with conventional thermochemical data to determine a new hybrid exchange-correlation functional, denoted B97-2. Calculations using B97-2 are compared with those from (a) the B97-1 functional [J. Chem. Phys. 109, 6264 (1998)], which has the same functional form and fraction of orbital exchange, but was fitted to just thermochemical data; and (b) the widely used B3LYP functional [J. Chem. Phys. 98, 5648 (1993)]. B97-2 atomization energies are close to those from B97-1; total electronic energies and ionization potentials are less accurate, but remain an improvement over B3LYP. Molecular structures from all three functionals are comparable. Static isotropic polarizabilities improve from B3LYP to B97-1 to B97-2; the B97-2 functional underestimates experimental values, which is consistent with the neglect of zero-point vibrational corrections. NMR shielding constants—determined as the conventional second derivative of the electronic energy—improve from B3LYP to B97-1 to B97-2. Shieldings determined directly from these DFT electron densities using the recently proposed MKS approach [Chem. Phys. Lett. 337, 341 (2001)] are two to three times more accurate than the conventional shieldings, and exhibit an analogous improvement across the three functionals. Classical reaction barriers for sixteen chemical reactions improve significantly from B3LYP to B97-1 to B97-2. The introduction of multiplicative potentials into semi-empirical hybrid functional development therefore appears beneficial.
Ab initio molecular dynamics simulation of liquid water and water–vapor interface
Authors: Vassilev Peter , Hartnig Christoph , Koper Marc T. M. , Frechard Frédéric , van Santen Rutger A.
Published: July 2002

The results of ab initio molecular dynamics simulations of liquid water and liquid water–vapor interface using the Perdew-Wang 91 (PW91) exchange-correlation functional are presented. The structural and transport properties of liquid water are comparable to the previous results using Becke-Lee-Yang-Parr (BLYP) functional and experimental data. The shape and the position of the first peak in the oxygen–oxygen radial distribution function is in good agreement with the most recent neutron diffraction data. The ab initio molecular dynamics simulation of liquid water–vapor interface, which is the first of its kind, suggests a preferred orientation of the surface water dipole towards the bulk region.
High-yield synthesis of single-walled carbon nanotubes by gravity-free arc discharge
Authors: Kanai M. , Koshio A. , Shinohara H. , Mieno T. , Kasuya A. , Ando Y. , Zhao X.
Published: July 2002

Single-walled carbon nanotubes (SWNTs) have been produced in high yield by the dc arc discharge technique under gravity-free conditions. Gravity-free conditions can reduce the convection flow of the buffer gas during the arc discharge, which results in the increase of high thermostatic volume around the arc flame. The yields of both the total soot and SWNTs in soot are significantly increased. Raman spectroscopy and transmission electron microscopy analyses reveal that the diameter distribution of SWNTs has shifted to a larger diameter region under the gravity-free condition. The annealing process in high-thermostatic atmosphere is a crucial experimental factor to selectively synthesize a certain diameter SWNT in high yield.
Eley–Rideal and hot atom reactions between hydrogen atoms on Ni(100): Electronic structure and quasiclassical studies
Authors: Guvenc Ziya B. , Sha Xianwei , Jackson Bret
Published: July 2002

The reactions of gas-phase H (or D) atoms with D (or H) atoms adsorbed onto a Ni(100) surface are studied. Electronic structure calculations based on density functional theory are used to examine the interaction of H atoms with the Ni(100) surface, as well as the interactions between two H atoms near the metal surface. A model potential-energy surface based on ideas from effective medium theory is fit to the results of these electronic structure calculations. Quasiclassical trajectory methods are used to simulate the interaction of low energy H and D atom beams with H and D-covered Ni(100) surfaces. It is found that hot-atom processes dominate the formation of molecular hydrogen. The distribution of energy in the product molecules is examined with regard to the various pathways available for reaction. The initial adsorbate coverage is varied and is shown to control the relative amounts of reflection, reaction, sticking, and subsurface penetration. Our results are compared with those from similar studies on Cu(111) and available experimental data for Ni(100).
Observation of 4H–SiC to 3C–SiC polytypic transformation during oxidation
Authors: Okojie Robert S. , Xhang Ming , Pirouz Pirouz , Tumakha Sergey , Jessen Gregg , Brillson Leonard J.
Published: July 2002

We have observed the formation of single and multiple stacking faults that sometimes give rise to 3C–SiC bands in a highly doped n-type 4H–SiC epilayer following dry thermal oxidation. Transmission electron microscopy following oxidation revealed single stacking faults and bands of 3C–SiC in a 4H–SiC matrix within the 4H–SiC epilayer. These bands, parallel to the (0001) basal plane, were not detected in unoxidized control samples. In addition to the 3.22 eV peak of 4H–SiC, Cathodoluminescence spectroscopy at 300 K after oxidation revealed a spectral peak at 2.5 eV photon energy that was not present in the sample prior to oxidation. The polytypic transformation is tentatively attributed to the motion of Shockley partial dislocations on parallel (0001) slip planes. The generation and motion of these partials may have been induced by stresses caused either by the heavy doping of the epilayer or nucleation from defect.
Morphological stability analysis of directional solidification into an oscillatory fluid layer
Authors: Volfson Dmitri , Viñals Jorge
Published: July 2002

We study the stability of a planar solid-melt boundary during directional solidification of a binary alloy when the solid is being periodically vibrated in the direction parallel to the boundary (or equivalently, under a far field uniform and oscillatory flow parallel to the planar boundary). The analysis is motivated by directional solidification experiments under the low level residual acceleration field characteristic of a microgravity environment, and possible effects on crystal growth in space. It is known that periodic modulation of the solid-melt interface under the conditions stated induces second order stationary streaming flows within a boundary layer adjacent to the interface, the thickness of which is the same as the wavelength of the modulation. We derive an effective solute transport equation by averaging over the fast time scale of the oscillatory flow, and obtain the resulting dispersion relation for a small disturbance of a planar interface. We find both regions of stationary and oscillatory instability. For small ratios of the viscous to solutal layer thicknesses, s, the flow generally destabilizes the planar interface. For s≃1, the flow stabilizes the stationary branch, but it can also excite an oscillatory instability. For large s, the effect of the flow is small.
Size-, shape-, and position-controlled GaAs nano-whiskers
Authors: Ohlsson B. J. , Björk M. T. , Magnusson M. H. , Deppert K. , Samuelson L. , Wallenberg L. R.
Published: July 2002

We have developed a technique for the synthesis of size-selected, GaAs, epitaxial nano-whiskers, grown on a crystalline substrate. As catalysts, we used size-selected gold aerosol particles, which enabled us to fully vary the surface coverage independently of the whisker diameter. The whiskers were rod shaped, with a uniform diameter between 10 and 50 nm, correlated to the size of the catalytic seed. Furthermore, by the use of nano-manipulation of the aerosol particles by means of atomic force microscopy, we can nucleate individual nano-whiskers in a controlled manner at specific positions on a substrate with accuracy on the nm level.
Preparation and structure analysis of titanium oxide nanotubes
Authors: Du G. H. , Chen Q. , Che R. C. , Yuan Z. Y. , Peng L.-M.
Published: July 2002

Well crystallized nanoscale tubular materials have been synthesized via the reaction of TiO2 crystals of either anatase or rutile phase and NaOH aqueous solution. The atomic structure of the synthesized tubular material is imaged by high-resolution transmission electron microscopy (HRTEM), and the composition of individual tubular structures is determined using selected area energy dispersive X-ray spectroscopy (EDX). Our results show that the tubular materials are well crystallized tubes with an average diameter of about 9 nm and little dispersion, and are composed of mainly titanium and oxygen. The atomic ratio of O/Ti is found, however, to vary from tube to tube. Detailed electron and x-ray diffraction studies show that the structure of our titanium oxide nanotubes do not agree with those made of TiO2 crystals with either anatase or rutile phase. HRTEM observations revealed that the titanium oxide nanotubes usually have multiple shells, in analogy with multiwalled carbon nanotubes, but the shell spacing is about 0.75 nm which is much larger than that of the carbon nanotube, and the atomic structures of different shells are well correlated.
Investigation of oxidation mechanism for ohmic formation in Ni/Au contacts to p-type GaN layers
Authors: Lee Chi-Sen , Lin Yow-Jon , Lee Ching-Ting
Published: July 2002

To investigate the function and mechanisms of oxidation, we present the ohmic performances for Ni/Au contacts to p-type GaN treated with various conditions. When the p-type GaN sample was preoxidized at 750 °C for 30 min in air ambient and then treated with (NH4)2Sx solution, we routinely obtained a specific contact resistance of 4.5×10−6 Ω cm2 for the Ni/Au contacts to samples alloyed at 500 °C for 10 min in air ambient. The fact that, in this configuration, ohmic performance improved one order of magnitude [compared with (NH4)2Sx surface treatment], is attributable to the strengthened formation of GaOx (aided by the preoxidation process), as well as the fact that more holes were induced on the oxidation-free p-type GaN surface.
Nucleation in binary polymer blends: A self-consistent field study
Authors: Wood Stephen M. , Wang Zhen-Gang
Published: July 2002

We study the structure and thermodynamics of the critical nuclei in metastable binary polymer blends using the self-consistent field method. At the mean-field level, our results are valid throughout the entire metastable region and provide a smooth crossover from the classical capillary-theory predictions near the coexistence curve to the density functional predictions of Cahn and Hilliard (properly transcribed into expressions involving the parameters of the binary polymer blends) near the spinodal. An estimate of the free energy barrier provides a quantitative criterion (the Ginzburg criterion) for the validity of the (mean-field) self-consistent approach. The region where mean-field theory is valid and where there can be a measurable nucleation rate is shown to be poorly described by the existing limiting theories; our predictions are therefore most relevant in this region. We discuss our results in connection with recent experimental observations by Balsara and co-workers.
Mesoscopic quantum oscillations of the resistance in the intermediate state of type-I superconductors
Authors: Chiang Yu. N. , Shevchenko O. G.
Published: July 2002

Quantum interference effects in the conductance of SNS systems formed in type-I superconductors in the intermediate state are investigated. The investigation is extended to the SNS system of the intermediate state of indium in a macroscopic hybrid sample with single-crystal copper, for a sample configuration corresponding to a doubly connected “Andreev interferometer.” Quantum oscillations of the resistance are detected in ultrapure samples of Pb, Sn, and In of various geometries and are investigated as a function of temperature, or of critical magnetic field, in the normal regions of domains of the intermediate state. It is shown that the nature of the oscillations observed is analogous to that of the Aharonov–Bohm dissipative oscillations for the ballistic trajectories of coherent excitations near an NS boundary.
On the magnetoresistance of the organic complexes (BEDT–TTF)2MHg(SCN)4
Authors: Peschansky V. G. , Atalla R.
Published: July 2002

According to band calculations of the electron energy spectrum of the organic complexes (BEDT–TTF)2MHg(SCN)4, where M is one of the metals of the group K, Rb, Tl, there should be, in addition to the main group of charge carriers with a quasi-two-dimensional spectrum, a group of charge carriers with a quasi-one-dimensional spectrum [R. Rossenau et al., J. Phys. (Paris) 6, 1527 (1996)]. We consider galvanomagnetic phenomena in quasi-two-dimensional conductors with a multisheet Fermi surface, and show that it is possible to determine the degree of quasi-one-dimensionality of an additional sheet of the Fermi surface having the form of corrugated planes.
n
+
-
GaN
formed by Si implantation into p-GaN
Authors: Sheu J. K. , Tun C. J. , Tsai M. S. , Lee C. C. , Chi G. C. , Chang S. J. , Su Y. K.
Published: July 2002

Si + 28 implantation into Mg-doped GaN, followed by thermal annealing in N2 was performed to achieve n+-GaN layers. Multiple implantation was used to form a uniform Si implanted region. It was found that the carrier concentration of the films changed from 3×1017 cm−3 (p-type) to 5×1019 cm−3 (n-type) when the samples were annealed in N2 ambient at 1000 °C. The activation efficiency of Si in Mg-doped GaN was as high as 27%. In addition, planar GaN n+–p junctions formed by Si-implanted GaN:Mg were also achieved.
Analysis of the magnetic coupling in binuclear complexes. II. Derivation of valence effective Hamiltonians from ab initio CI and DFT calculations
Authors: Calzado Carmen J. , Cabrero Jesús , Malrieu Jean Paul , Caballol Rosa
Published: July 2002

Most interpretations of the magnetic coupling J between two unpaired electrons rest upon simple valence models that involve essentially the ferromagnetic direct exchange contribution, Kab, and the antiferromagnetic effect of the delocalization resulting from the interaction between neutral and ionic determinants, tab, whose energy difference is U. Ab initio valence-only calculations give very poor estimates of J, whatever the definition of the magnetic orbitals, and large CI expansions are required to evaluate it properly. It is, however, possible to define valence effective Hamiltonians from the knowledge of the eigenenergies and the eigenvectors of these accurate CI calculations. When applied to four different complexes, this strategy shows that spin polarization may change the sign of the direct exchange interaction, Kab, and that dynamical correlation results in a dramatic reduction of the effective repulsion U. The present article also shows how Kab, tab, and U effective parameters can be extracted from density functional theory (DFT) calculations and that the typical overestimation of J in DFT can be attributed to an excessive lowering of the effective on-site repulsion.
Influence of Fe, Zr, and Cu on the microstructure and crystallographic texture of melt-spun 2:17 Sm–Co ribbons
Authors: Yan A. , Bollero A. , Müller K. H. , Gutfleisch O.
Published: July 2002

A comprehensive study on melt-spun 2:17 Sm–Co ribbons prepared at low wheel speed was undertaken in order to understand the effects of composition on the microstructure and formation of crystallographic texture. A high degree of texture was observed in melt-spun SmCo8.5 and Sm(Co0.9Fe0.1)8.5 ribbons prepared at 5 m/s with easy magnetization axis being parallel to the longitudinal direction of the ribbons. However, Zr was found to reduce the crystallographic texture and the Sm(Co0.74Fe0.1Zr0.04Cu0.12)z ribbon is isotropic. Scanning electron microscopy (SEM) observation showed that the formation of texture is attributed to a regular dendritic structure, with their long axis parallel to the longitudinal direction of the ribbons. The size and alignment of dendritic grains are significantly modified by Zr and Cu substitution for Co and the Sm(Co0.74Fe0.1Zr0.04Cu0.12)z ribbons are composed of many small areas with different alignments of the dendrites. X-ray diffraction patterns of crushed isotropic (d⩽20 μm) ribbons of Sm(Co0.74Fe0.1Zr0.04Cu0.12)8.5 show very high degree of texture and thus these ribbons have potential applications in preparing anisotropic bonded magnets. After an appropriate annealing, a coercivity of over 2.5 T has been obtained in Sm(Co0.74Fe0.1Cu0.12Zr0.04)z ribbons with z=7.5 and 8.0, and the coercivity mechanism is typical of the domain-wall pinning. This is consistent with transmission electron microscopy observation, which reveals a cellular and lamellar structure in the as-annealed ribbons.
Collision-induced dissociation of formaldehyde cations: The effects of vibrational mode, collision energy, and impact parameter
Authors: Liu Jianbo , Van Devener Brian , Anderson Scott L.
Published: July 2002

We report a study of collision-induced dissociation (CID) of H2CO+, including measurement of the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the CID integral and differential cross sections. CID was studied for collision with both Xe and Ne, and the Ne results provide a very detailed probe of energy transfer collisions leading to CID. The CID appearance threshold is found to depend only on total energy, but for all energies above threshold, vibrational energy is far more effective at driving CID than Ecol, with some mode-specificity. Results are fit with an impact parameter-based mechanism, and considerable insight is obtained into the origins of the Ecol and vibrational effects. A series of ab initio and RRKM calculations were also performed to help interpret the results.
Coherent state semiclassical initial value representation for the Boltzmann operator in thermal correlation functions
Authors: Makri Nancy , Miller William H.
Published: July 2002

A semiclassical methodology for evaluating the Boltzmann operator entering semiclassical approximations for finite temperature correlation functions is described. Specifically, Miller’s imaginary time semiclassical approach is applied to the Herman–Kluk coherent state initial value representation (IVR) for the time evolution operator in order to obtain a coherent state IVR for the Boltzmann operator. The phase-space representation gives rise to exponentially decaying factors for the coordinates and momenta of the real time trajectories employed in the dynamical part of the calculation. A Monte Carlo procedure is developed for evaluating dynamical observables, in which the absolute value of the entire exponential part of the integrand serves as the sampling function. Numerical tests presented show that the methodology is accurate as well as stable over the temperature range relevant to chemical applications.
Features of the x-ray bremsstrahlung in the scattering of intermediate-energy electrons on atoms of inert elements
Authors: Verkhovtseva É. T. , Gnatchenko E. V.
Published: July 2002

The results of experimental and theoretical studies of the differential x-ray bremsstrahlung spectra emitted in the scattering of intermediate-energy electrons on atoms of inert elements are presented in generalized form. The experimental studies were done using the low-temperature gas-jet method of generation of electromagnetic radiation. The features of the bremsstrahlung and the relationships governing the novel form of bremsstrahlung due to polarization effects are established. It is shown that the Born approximation is inapplicable for considering the bremsstrahlung cross sections in the scattering of electrons of intermediate energies on atoms of inert elements.
IR spectra of cryocondensates of an isotopic water mixture on thermocycling
Authors: Aldijarov A. , Drobyshev A. S. , Sarsembinov S. S.
Published: July 2002

The IR spectra are measured for thin films of vacuum condensates of an isotopic water mixture in the proportions H2O (10%), HDO (50%), and D2O (40%). The condensation was done on an Invar metal mirror in the temperature interval 30–130 K at a pressure of the gas phase during cryocondensation of 6.7×10−4 Pa. Warming of these cryocondensates is accompanied by structural transformations which lead to a change of the energy of the hydrogen bonds between molecules. Calculations for the various types of stretching vibrations of the mixture show that this corresponds to a decrease of the enthalpy of the hydrogen bond with increasing temperature. Increasing the temperature of the cryocondensates formed at 30 K leads to a significant decrease of the frequencies of the absorption bands corresponding to deformation vibrations of the D2O and HDO molecules. In the frequency range of the librational vibrations a substantial change of the spectrum of the cryocondensate of the isotopic water mixture is observed with increasing temperature. The cause of this is apparently thermally stimulated proton–proton and deuteron–deuteron exchange between the D2O and HDO molecules.
Characteristics of the electric field accompanying a longitudinal acoustic wave in a metal. Anomaly in the superconducting phase
Authors: Avramenko Yu. A. , Bezuglyi E. V. , Burma N. G. , Kolobov I. G. , Fil’ V. D. , Shevchenko O. A. , Gokhfeld V. M.
Published: July 2002

The temperature dependence of the amplitude and phase of the electric potential arising at a plane boundary of a conductor when a longitudinal acoustic wave is incident normally on it is investigated theoretically and experimentally. The surface potential is formed by two contributions, one of which is spatially periodic inside the sample, with the period of the acoustic field; the second is aperiodic and arises as a result of an additional nonuniformity of the electron distribution in a surface layer of the metal. In the nonlocal of parameters region the second contribution is dominant. The phases of these contributions are shifted by approximately π/2. For metals found in the normal state the experiment is in qualitative agreement with the theory. The superconducting transition is accompanied by catastrophically rapid vanishing of the electric potential, in sharp contrast to the theoretical estimates, which predict behavior similar to the BCS dependence of the attenuation coefficient for longitudinal sound.
Room-temperature homogeneous nucleation synthesis and thermal stability of nanometer single crystal CeO2
Authors: Zhou X.-D. , Huebner W. , Anderson H. U.
Published: July 2002

Nanometer (about 4∼5 nm) CeO2 single crystals were first synthesized by room-temperature homogeneous nucleation; the size was determined by electron microscopy and specific surfaced area of the particles. Modeling revealed that the surface energy of as-synthesized nanometer single crystals was in the range of 2.8–3.7 J/m2. Crystal growth mechanisms change over the temperature regimes, from boundary diffusion over low-temperature regime (Ea=0.16 eV) to bulk diffusion (Ea=0.50 eV) over high-temperature region.
Imaging electron density in a two-dimensional electron gas
Authors: LeRoy B. J. , Topinka M. A. , Westervelt R. M. , Maranowski K. D. , Gossard A. C.
Published: July 2002

Spatial profiles of the electron density in a two-dimensional electron gas were obtained from the spacing of interference fringes in coherent electron flow. Images of electron flow from a quantum point contact formed in a GaAs/AlGaAs heterostructure were recorded with a liquid He cooled scanned probe microscope. The images are decorated with interference fringes spaced by half the Fermi wavelength; the fringe spacing measures the electron density below the scanned probe microscope tip. As the density is decreased with a back gate, the fringe spacing increases in agreement with a planar capacitor model.
Fractal analysis of size effects and surface morphology effects in catalysis and electrocatalysis
Authors: Avnir D. , Carberry J. J. , Citri O. , Farin D. , Grätzel M. , McEvoy A. J.
Published: July 2002

Fractal geometry tools are used in order to analyze several related problems in surface science, catalysis, and electrocatalysis. The effects of complex morphologies of adsorbents, catalysts, and electrodes on various molecular processes with these materials are determined both theoretically and experimentally. It is shown that fractal geometry provides a convenient and natural tool for the elucidation of geometry-performance relations in heterogeneous chemistry. Issues covered are particle size effects in physisorption and chemisorption; morphology effects on a variety of catalytic processes with unsupported catalysts (including coal liquefaction, alkene polymerizations, oxidations, dehydrogenations, and esterifications); surface accessibility effects on molecular interactions in an Eley–Rideal mechanism; surface patterning effects on concentration profiles near the surface; and electrode-morphology effects on a variety of electrochemical and electrocatalytic processes. The domains of applicability of the fractal approach to these problems is discussed.
Estimates of stability time for nearly integrable systems with a quasiconvex Hamiltonian
Authors: Lochak P. , Neishtadt A. I.
Published: July 2002

A Hamiltonian system differing from an integrable system by a small perturbation ≂ε is analyzed. According to the Nekhoroshev theorem, the changes in the perturbed motion of the ‘‘action’’ variables of the unperturbed system are small over a time interval which increases exponentially in length as ε decreases linearly. If the unperturbed Hamiltonian is a quasiconvex function of these ‘‘actions,’’ the changes in them remain small (≂ε1/2n) over a time interval on the order of exp(const/ε1/2n), where n is the number of degrees of freedom of the system.
The dynamical structure of tremor in tardive dyskinesia
Authors: Newell K. M. , Gao F. , Sprague R. L.
Published: July 2002

The movement disorder syndrome of tardive dyskinesia arises as a consequence of prolonged regimens of neuroleptic medication, and is characterized, although not exclusively, by jerky and sometimes rhythmical stereotypical motions in a wide range of muscle systems. It is well established that the degree and variability of tremor in tardive dyskinesia is greater than that in normal age-matched subjects. The findings from the current experiment show that the dimension of the tardive dyskinetic finger tremor time series is systematically lower than that evident in normal finger tremor. Furthermore, the variability of finger motion in both groups is inversely related to the dimension of the respective attractor dynamic. The neuroleptic medication appears to constrain the degrees of freedom regulated in organization of the motor system.
Parallel signal processing workstation environment
Authors: Mähönen Petri
Published: July 2002

In this work, a parallel signal processing environment will be introduced. An initial testing system has been implemented and a specific parallel hardware and software are under consideration. An underwater acoustic lattice code is used as a case study in the prototype environment. The experiments prove that the parallel signal processing environment suggested will be a powerful tool for both physicists and engineers. The major improvement of the approach is the combination of a parallelism and a neural network pattern recognition. Another innovation is use of a dynamic lattice code instead of the fixed-size lattice code.
Simulating solar cells with a microcomputer
Authors: Morales-Acevedo Arturo , Arizpe Humberto
Published: July 2002

Designing modern silicon solar cells in order to get the maximum efficiency requires the inclusion of effects such as the variation of diffusion length, and electric field as a function of impurity concentration in nonhomogeneous silicon. In this paper, it is shown that it is possible to develop an efficient computer program to simulate the I–V curves of solar cells, which takes into account such effects, when low-injection conditions apply to the semiconductor. The program can be run in any PC microcomputer, without any coprocessor, giving results similar to larger programs, in a fraction of the time usually required for this purpose. The program allows the inclusion of internal resistances (series and shunt) in order to compare the ideal and real behavior of a given solar cell. Conversion efficiency, open circuit voltage, short circuit current, and filling factor are obtained for a set of fabrication parameters, such as substrate and emitter dopings, thicknesses, and surface recombination velocities. The results are in agreement with recent experimental results.
Monte Carlo simulation of Comptonization in inhomogeneous media
Authors: Hua Xin-Min
Published: July 2002

Comptonization is the process in which the photon spectrum changes due to multiple Compton scatterings in the electronic plasma. It plays an important role in the spectral formation of astrophysical x-ray and gamma-ray sources. There are several intrinsic limitations to the analytical method in dealing with the Comptonization problem and Monte Carlo simulation provides one of the few alternatives. We describe an efficient Monte Carlo method that can solve the Comptonization problem in a fully relativistic way. We expanded the method so that it is capable of simulating Comptonization in the media where electron density and temperature vary discontinuously from one region to the other and in the isothermal media where density varies continuously along photon paths. The algorithms are presented in detail to facilitate computer code implementation. We also present a few examples of its application to astrophysical research. © 1997 American Institute of Physics.
The spin-wave spectrum of layered magnetic thin films
Authors: van Stapele R. P. , Greidanus F. J. A. M. , Smits J. W.
Published: July 2002

The ferromagnetic resonance spectrum of a layered magnetic thin film is expected to show a number of standing spin-wave resonances with a wavelength that matches the thickness of the film. For the case of perpendicular resonance such spectra were calculated for some typical films in which magnetic layers are alternated with weaker magnetic layers. Some useful approximations are discussed. The results of the calculations are compared with experimental perpendicular spectra measured on films in which fifty Permalloy layers alternate with Ni layers.
Characterization of high-purity Si-doped molecular beam epitaxial GaAs
Authors: Skromme B. J. , Bose S. S. , Lee B. , Low T. S. , Lepkowski T. R. , DeJule R. Y. , Stillman G. E. , Hwang J. C. M.
Published: July 2002

High-purity, lightly Si-doped (μ77∼70 000–126 000 cm2/V s and n77∼2–8×1014 cm−3) molecular beam epitaxy (MBE) GaAs layers have been characterized using variable-temperature Hall effect and C-V measurements, photothermal ionization spectroscopy, low-temperature photoluminescence (PL), and deep level transient spectroscopy (DLTS). The spectroscopic measurements of the residual donors and acceptors indicate that the pronounced increase in carrier concentration which is observed with increasing As flux (for a constant Ga flux) results from incorporation of additional residual S donors from the As source material, and not from reductions in the Si acceptor concentration or residual C acceptor concentration. The increase in carrier concentration with As flux is considerably more pronounced when using an alternative source of As, which introduces both S and 3 additional donor species. The C acceptor concentration increases with As flux using either As source, although the increase is much stronger with the alternative source. The dependence of C concentration on the As source implies that the As source itself contributes at least part of the C background. The Si acceptor concentration is negligible for the range of growth conditions that were used. Close compensation between the residual S donors and C acceptors may account for the high resistivity previously observed in undoped samples grown in this system using the purer As source. The PL data exhibit very weak ‘‘defect’’-related emissions in the 1.504–1.512 and 1.466–1.482 eV ranges; evidence is presented supporting the existence of a correlation between these two sets of peaks, in agreement with the work of Briones and Collins.

Temperature and excitation intensity-dependent PL measurements are used to demonstrate conclusively that the peaks in the 1.466–1.482 eV range are donor-to-acceptor and band-to-acceptor in nature, involving normal shallow donors and at least four different acceptor levels whose exact origin is unknown. The ‘‘defect’’ peak intensity is larger in the less pure material which contains more C, implying that the ‘‘defects’’ may be C related. Several electron traps including M1, M3, and M4 are observed in the DLTS spectra, and the C-V measurements give a total trap concentration of ∼3×1013 cm−3.
Two-beam photoacoustic phase measurement of the thermal diffusivity of solids
Authors: Pessoa O. , Cesar C. L. , Patel N. A. , Vargas H. , Ghizoni C. C. , Miranda L. C. M.
Published: July 2002

A simple method is demonstrated for obtaining the thermal diffusivity of solids, by measuring the phase lag between a front and rear illumination, at a single chopping frequency. The method is tested using some semiconductor and glass samples.
An analytical study of the p/n junction space-charge region under high forward voltage
Authors: Park Jong-Sik , Lindholm Fred A. , Neugroschel Arnost
Published: July 2002

The behavior of the p/n junction space-charge region under high forward voltage is studied quantitatively and qualitatively by linearizing the Poisson equation. As examples, the symmetric step and linearly graded junction as special cases of a symmetric exponential doping profile are investigated. From the analytical solutions and their interpretation we obtain a detailed physical insight about the behavior of the space-charge region capacitance and positions of the space-charge region edges.
Effects of densification on fluorescence spectra and glass structure of Eu3+-doped borate glasses
Authors: Soga N. , Hirao K. , Yoshimoto M. , Yamamoto H.
Published: July 2002

Densified glass specimens of 90 B2O3⋅10 Na2O⋅1 Eu2O3 were obtained by applying hydrostatic pressure up to 6 GPa at various temperatures from 250 to 900 °C, and their densities and inhomogeneous bandwidths of Eu3+ fluorescence spectra were determined in order to follow a structural change taking place during densification. The results indicate that the role of hydrostatic pressure is first to eliminate the atomic scale voids usually appearing when quenched from high temperatures and then to increase the fluctuation of local fields around Eu3+ probably due to the distortion of glass network accompanied with a wide variation of bond length. The molecular dynamics simulation of the densified state was also carried out to support the above conclusion.
TiAl3 formation by furnace annealing of Ti/Al bilayers and the effect of impurities
Authors: Zhao X.-A. , So F. C. T. , Nicolet M.-A.
Published: July 2002

Reactions of Ti/Al couples induced by furnace annealing were investigated (at elevated temperature) using large-grained Al substrates and vacuum-evaporated bilayers of both sequences. 4 He MeV backscattering spectrometry was principally used to monitor the reactions. Profiles of oxygen impurity were obtained by elastic 16 O(α,α)16 O resonant scattering. In the range of 460–515 °C, TiAl3 forms as a laterally uniform layer at the Ti/Al interface. The thickness of this compound layer increases as (annealing time)1/2. The activation energy is 1.9–2.0±0.1 eV. For evaporated bilayers on an oxidized Si substrate, the sequence of the bilayers does not effect the growth mechanism of TiAl3 , but the growth rate of samples with the Ti on top is lower than that of samples with Al on top, that is, oxygen in Ti/Al samples can reduce the reaction rate by decreasing the pre-exponential factor. Oxygen already contained in the Ti film and oxygen from the annealing ambient are incorporated in the growing TiAl3 compound during thermal annealing. In addition, a TiAl3 layer also forms at the free Ti surface during vacuum annealing when the oxygen-containing contaminants in the ambient are minimized. So far, we succeeded in accomplishing this only for large-grained Al substrates. We conclude that the formation of the TiAl3 compound at the surface is controlled by nucleation and depends sensitively on the condition of the surface layer of the Ti film.
Laser action of optically pumped atomic titanium vapor
Authors: Ninomiya H. , Hirata K.
Published: July 2002

Laser action has been observed on the titanium 551.4 nm (3D01-3F2) transition. A nitrogen laser is used to produce the titanium vapor by irradiating a metal plate, and the titanium atoms are optically pumped by another nitrogen laser.
Deep donor levels (D
X centers) in III-V semiconductors
Authors: Mooney P. M.
Published: July 2002

D X centers, deep levels associated with donors in III-V semiconductors, have been extensively studied, not only because of their peculiar and interesting properties, but also because an understanding of the physics of these deep levels is necessary in order to determine the usefulness of III-V semiconductors for heterojunction device structures. Much progress has been made in our understanding of the electrical and optical characteristics of DX centers as well as their effects on the behavior of various device structures through systematic studies in alloys of various composition and with applied hydrostatic pressure. It is now generally believed that the DX level is a state of the isolated substitutional donor atom. The variation of the transport properties and capture and emission kinetics of the DX level with the conduction-band structure is now well understood. It has been found that the properties of the deep level when it is resonant with the conduction band, and is thus a metastable state, are similar to its characteristics when it is the stable state of the donor. And it has been consistently found that there is a large energy difference between the optical and thermal ionization energies, implying that this deep state is strongly coupled to the crystal lattice. The shifts in the emission kinetics due to the variation in the local environment of the donor atom suggest that the lattice relaxation involves the motion of an atom (the donor or a neighboring atom) from the group-III lattice site toward the interstitial site. Total energy calculations show that such a configuration is stable provided that the donor traps two electrons, i.e., has negative U. Verification of the charge state of the occupied DX level is needed as well as direct evidence for its microscopic structure.
Effect of frequency on local thermodynamic equilibrium conditions in an inductively coupled argon plasma at atmospheric pressure
Authors: Mostaghimi Javad , Boulos Maher I.
Published: July 2002

A theoretical investigation of the effect of induction frequency shows that deviations from local thermodynamic equilibrium (LTE) are strongly related to this parameter. Computations are carried out for an argon plasma at atmospheric pressure over the frequency range 3–40 MHz. Higher frequencies result in lower-temperature levels in the torch, and also the difference between the electron and the atom/ion temperatures is increased. This is in agreement with the observations of other investigators. The results of the proposed model also show good agreement with the measured temperature profiles. Similar calculations, which are based on the LTE assumption, are reported and compared with the present non-LTE model.