bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermoelectric properties of Sn-filled skutterudites
Authors: Nolas G. S. , Takizawa H. , Endo T. , Sellinschegg H. , Johnson D. C.
Published: July 2002

Thermal conductivity, resistivity, Seebeck coefficient, and structure measurements of CoSb3 with tin interstitially placed in the voids are reported. These tin-filled skutterudites were synthesized under high pressure and temperature conditions; they cannot be synthesized under “normal” synthesis approaches. The tin atoms exhibit very large atomic displacement parameters indicating a large “rattling” motion inside their atomic “cages.” The disorder induced by the Sn atoms is a very good phonon scattering mechanism. The thermal conductivity of these compounds is very low with a temperature dependence that is atypical of simple solids. The tin-filled compounds exhibit n-type semiconducting behavior with relatively high Seebeck coefficients for compounds whose electronic properties have not been optimized. The potential for thermoelectric applications is discussed.
Electron transport mobility in a δ-doped double quantum well structure
Authors: Sahu T. , Patnaik J.
Published: July 2002

The electron transport mobility μn in a GaAs/AlxGa1−xAs coupled double quantum well structure has been studied. The central barrier is delta-doped with Si so that we have two sheets of two dimensional electron gas separated by a positively charged donor layer. The subband electron wave functions and the energy levels are numerically obtained as a function of barrier height, barrier width, well width, and doping concentration. The screened ionized impurity potential is obtained in terms of the static dielectric response function within the random phase approximation. The effect of tunneling of electrons through the barrier, screening of ionized impurities, and intersubband scattering on μn of the double channel system is investigated. It would be interesting to compare our results for μn with experiment, when available.
Polarization reversal kinetics of a lead zirconate titanate thin-film capacitor for nonvolatile memory
Authors: Seike Aya , Amanuma Kazushi , Kobayashi Sota , Tatsumi Toru , Koike Hiroki , Hada Hiromitsu
Published: July 2002

The switching kinetics of polarization reversal using lead zirconate titanate (PZT) thin-film capacitors for nonvolatile memories has been analyzed in terms of the Ishibashi model. A practical 16 kbit PZT memory was used in this work. The polarization reversal ratio agrees well with the theoretical model of the first order dimensional domain growth. The domain growth proceeds outwardly with one-dimensional movement, and domain reversal occurs with existing nuclei and is completed before any other significant nucleation takes place. The characteristic domain growth time t0 is 3 ns. This reveals that the domain switching is substantially fast, and therefore it does not affect the read-out operation of the ferroelectric memories.
Deformation and failure modes of adhesively bonded elastic layers
Authors: Crosby Alfred J. , Shull Kenneth R. , Lakrout Hamed , Creton Costantino
Published: July 2002

Adhesively bonded elastic layers with thicknesses that are small relative to their lateral dimensions are used in a wide variety of applications. The mechanical response of the compliant layer when a normal stress is imposed across its thickness is determined by the effects of lateral constraints, which are characterized by the ratio of the lateral dimensions of the layer to its thickness. From this degree of confinement and from the material properties of the compliant layer, we predict three distinct deformation modes: (1) edge crack propagation, (2) internal crack propagation, and (3) cavitation. The conditions conductive for each mode are presented in the form of a deformation map developed from fracture mechanics and bulk instability criteria. We use experimental data from elastic and viscoelastic materials to illustrate the predictions of this deformation map. We also discuss the evolution of the deformation to large strains, where nonlinear effects such as fibrillation and yielding dominate the failure process.
Pyramid-shaped pixels for full-color organic emissive displays
Authors: Yang Yang , Chang Shun-Chi
Published: July 2002

Organic electroluminescent emissive displays are composed by pixels consisting red-green-blue (RGB) light-emitting diodes (LEDs) in a planar arrangement. When operated, these RGB LEDs are biased independently to produce the required color. In this manuscript, we describe a promising pixel structure, the pyramid-shaped pixel (PSP) for the integration of organic light-emitting diodes (OLEDs) in full-color organic emissive displays. The RGB light-emitting diodes are constructed on the walls of the pyramid structure. When operated, the RGB LEDs emit photons through the base of the pyramid structure, hence these RGB LEDs share the same emissive area to produce the required color. The PSP structure offers the advantage of being a full color emissive pixel comprising of individual RGB OLEDs with very high resolution. In addition, pyramid pixel does not require shadow mask to pattern the organic materials during the vacuum deposition process.
Preparation and transport properties of polycrystalline Bi and Bi–SiO2 nanocomposites
Authors: Brochin F. , Lenoir B. , Devaux X. , Martin-Lopez R. , Scherrer H.
Published: July 2002

Bismuth–silica nanocomposites and polycrystalline bismuth were prepared via powder metallurgy in order to study the influence of silica inclusions on the thermoelectric properties of bismuth. Bi–SiO2 powders containing from 0.5 to 15 vol. % of silica and pure Bi powders were produced by an arc-plasma processing. Transmission electron microscopy investigations revealed the presence of a nanometric silica shell around the Bi grains. The powders were cold pressed and sintered close to the melting temperature of bismuth. The bulk microstructures are very different for the bismuth and the Bi–SiO2 nanocomposites because silica, which is primarily dispersed at grain boundaries, inhibits the grain growth during sintering. The electrical resistivity was measured from 5 to 300 K, while the thermoelectric power and the thermal conductivity were measured from 65 to 300 K on both polycrystalline bismuth and Bi–SiO2 samples containing 0.5, 4, and 15 vol. % of silica, respectively. The transport properties are mainly discussed with regard to the microstructures. In spite of a strong reduction of the thermal conductivity for the nanocomposites, the thermoelectric figure of merit is not improved compared to bismuth due to a dominating concurrent increase of the electrical resistivity resulting from a finite-size effect.
Refractive index and degree of inhomogeneity of nanocrystalline TiO2 thin films: Effects of substrate and annealing temperature
Authors: Mosaddeq-ur-Rahman Md. , Yu Guolin , Soga Tetsuo , Jimbo Takashi , Ebisu Hiroshi , Umeno Masayoshi
Published: July 2002

Nanocrystalline TiO2 thin films, deposited on single crystal Si (100) substrates under different temperature conditions by the sol–gel dip coating method, have been investigated for their optical properties using ultraviolet-visible spectroscopic ellipsometry. A gradual increase in refractive index, n, with increasing annealing temperature up to 600 °C, and thereafter a sharp increase in n at 800 °C of annealing temperature have been observed. For the heat-treated and low temperature (400 °C) annealed films, n is found to be higher at the film–substrate interface than at the film surface and the refractive index gradient slightly increases for the annealed sample. However, for the 600 °C temperature annealed film, the refractive index gradient significantly decreases and the film appears to be almost homogeneous. These results are in sharp contrast with those for the films deposited on a vitreous silica substrate where n was found to be higher at the film surface than at the film–substrate interface and the refractive index gradient increased with increasing annealing temperature. For the high temperature (800 °C) annealed sample on the Si substrate, formation of a thick SiO2 interfacial layer has been observed and the degree of homogeneity deteriorates severely.
Photoluminescence of disordered ABO3 perovskites
Authors: Pizani P. S. , Leite E. R. , Pontes F. M. , Paris E. C. , Rangel J. H. , Lee E. J. H. , Longo E. , Delega P. , Varela J. A.
Published: July 2002

Intense photoluminescence in highly disordered (amorphous) BaTiO3, PbTiO3, and SrTiO3 prepared by the polymeric precursor method was observed at room temperature. The emission band maxima from the three materials are in the visible region and depend on the exciting wavelength. The origin of the photoluminescence was not exactly identified. However, the line shape indicates that confinement effects are not probable. The experimental results indicate that it could be related to the disordered perovskite structure.
Element specific x-ray magnetic circular dichroism magnetization curves using total electron yield
Authors: Goering Eberhard , Fuss Alex , Weber Wibke , Will Judith , Schütz Gisela
Published: July 2002

We used x-ray magnetic circular dichroism (XMCD) to obtain element specific magnetization curves. The most common way measuring XMCD in the soft x-ray region is total electron yield (TEY), because of the easy experimental setup and high signal-to-noise ratio compared to fluorescence yield. Surprisingly TEY was used only marginally for magnetization curves in the past. We show that the major problem using field dependent TEY is the complex current behavior as a function of applied magnetic fields, which directly results in corresponding normalization problems. We developed a simple but effective normalization method in combination with symmetry arguments to obtain magnetization curves using TEY as a probe for XMCD. Quick (2–6 min) and high quality data could be extracted in a wide range of high magnetic fields. In comparison to fluorescence yield hysteresis loops, those measurements could be easily done at synchrotron bending magnet beamlines and no high brilliant undulator sources and/or long measurement times are necessary. As an example we compare hysteresis loops of the colossal magnetoresistance compound La0.7Ca0.3MnO3 measured in situ by XMCD at the Mn L3 edge and conventional ex situ measurements.
Small polaron and bipolaron transport in antimony oxide doped barium hexaferrites
Authors: Brahma P. , Banerjee S. , Chakraborty S. , Chakravorty D.
Published: July 2002

Direct current electrical resistivity measurements on specimens with composition BaO (6−x) Fe2O3, x Sb2O3 with x having values of 0.0, 0.025, 0.05, 0.1, and 0.3, respectively, were carried out in the temperature range of 120–300 K. Chemical analyses showed the presence of Sb3+ and Sb5+ ions as well as Fe2+ and Fe3+ ions in the doped specimens. The doped specimens were found to have reduced resistivities. The analysis of the data shows that the electrical conductivity in this system arises due to a combined contribution of both small polaron hopping between Fe2+ and Fe3+ sites and bipolaronic transport between Sb3+ and Sb5+ sites.
Rheology of lubricant basestocks: A molecular dynamics study of C30 isomers
Authors: Moore J. D. , Cui S. T. , Cochran H. D. , Cummings P. T.
Published: July 2002

We have performed extensive equilibrium and nonequilibrium molecular dynamics (EMD and NEMD) simulations of three isomers of C30H62 at temperatures of 311 and 372 K employing a united atom model. Using the rotational relaxation time calculated from the EMD simulation, the Rouse model predicts a zero-shear viscosity for n-triacontane within 16% of the value determined by NEMD. Compared to experiment, NEMD and the united atom model underpredict the kinematic viscosities of n-triacontane and 9-n-octyldocosane but accurately predict the values for squalane (within 15%). In addition, the predicted values of the kinematic viscosity index for both 9-n-octyldocosane and squalane are in quantitative agreement with experiment and represent the first such predictions by molecular simulation. This same general potential model and computational approach can be used to predict this important lubricant property for potential lubricants prior to their synthesis, offering the possibility of simulation-guided lubricant design. © 2000 American Institute of Physics.
Multipartite generalization of the Schmidt decomposition
Authors: Carteret H. A. , Higuchi A. , Sudbery A.
Published: July 2002

We find a canonical form for pure states of a general multipartite system, in which the constraints on the coordinates (with respect to a factorizable orthonormal basis) are simply that certain ones vanish and certain others are real. For identical particles they are invariant under permutations of the particles. As an application, we find the dimension of the generic local equivalence class.
Gaussian-3X (G3X) theory: Use of improved geometries, zero-point energies, and Hartree–Fock basis sets
Authors: Curtiss Larry A. , Redfern Paul C. , Raghavachari Krishnan , Pople John A.
Published: July 2002

A modification of G3 theory incorporating three changes is presented. The three new features include: (1) B3LYP/6-31G(2df,p) geometry; (2) B3LYP/6-31G(2df,p) zero-point energy; and (3) addition of a g polarization function to the G3Large basis set for second-row atoms at the Hartree–Fock level. Extension of G3 theory in this manner, referred to as G3X, is found to give significantly better agreement with experiment for the G3/99 test set of 376 reaction energies. Overall the mean absolute deviation from experiment decreases from 1.07 kcal/mol (G3) to 0.95 kcal/mol (G3X). The largest improvement occurs for nonhydrogens. In this subset of energies the mean absolute deviation from experiment decreases from 2.11 to 1.49 kcal/mol. The increased accuracy is due to both the use of new geometries and the larger Hartree–Fock basis set. In addition, five other G3 methods are modified to incorporate these new features. Two of these are based on reduced orders of perturbation theory, G3X(MP3) and G3X(MP2), and have mean absolute deviations for the G3/99 test set of 1.13 and 1.19 kcal/mol, respectively. The other three methods are based on scaling the energy terms, G3SX, G3SX(MP3), and G3SX(MP2). They have mean absolute deviations of 0.95, 1.04, and 1.34 kcal/mol, respectively.
Domain wall motion and its contribution to the dielectric and piezoelectric properties of lead zirconate titanate films
Authors: Xu F. , Trolier-McKinstry S. , Ren W. , Xu Baomin , Xie Z.-L. , Hemker K. J.
Published: July 2002

In this article, domain wall motion and the extrinsic contributions to the dielectric and piezoelectric responses in sol–gel derived lead zirconate titanate (PZT) films with compositions near the morphotropic phase boundary were investigated. It was found that although the films had different thicknesses, grain sizes, and preferred orientations, similar intrinsic dielectric constants were obtained for all films between 0.5 and 3.4 μm thick. It was estimated that about 25%–50% of the dielectric response at room temperature was from extrinsic sources. The extrinsic contribution to the dielectric constant of PZT films was mainly attributed to 180° domain wall motion, which increased with both film thickness and grain size. In studies on the direct and converse longitudinal piezoelectric coefficients of PZT films as a function of either stress or electric driving field, it was found that the ferroelastic non-180° domain wall motion was limited. Thus extrinsic contributions to the piezoelectric response were small in fine grain PZT films (especially those under 1.5 μm in thickness). However, as the films became thicker (>5μm), nonlinear behavior between the converse piezoelectric coefficient and the electric driving field was observed. This indicated that there was significant ferroelectric non-180° domain wall motion under high external excitation in thicker films. The activity of the non-180° domain walls was studied through non-180° domain switching. For fine grain films with film thicknesses less than 2 μm, non-180° switching was negligible. Transmission electron microscopy plan-view micrographs evidenced non-180° domain fringes in these films, where the vast majority of grains were 50–100 nm in diameter and showed a single set of domain fringes. Taken together, these measurements suggest that the pinning of non-180° domain walls is very strong in films with thickness less than 2 μm. In thicker films, non-180° domain switching was evidenced when the poling field exceeded a threshold field. The threshold field decreased with an increase in film thickness, suggesting more non-180° domain wall mobility in thicker films. Non-180° domain switching in large grained PZT films was found to be much easier and more significant than in the fine grained PZT films.
Enhanced ultraviolet photoluminescence from SiO2/Ge:SiO2/SiO2 sandwiched structure
Authors: Shen J. K. , Wu X. L. , Yuan R. K. , Tang N. , Zou J. P. , Mei Y. F. , Tan C. , Bao X. M. , Siu G. G.
Published: July 2002

SiO 2 / Ge:SiO 2 / SiO 2 sandwiched structure was fabricated for exploring efficient light emission. After annealed in N2 (O2<1%), this structure shows three photoluminescence (PL) bands at 293, 395, and 780 nm. The intensity of the 395 nm band is largely enhanced in comparison with that from the monolayered Ge:SiO2 film. Spectral analyses suggest that the three PL bands originate from S1→S0, T∑(T∏)→S0, and T∏′→S0 optical transitions in GeO color centers, respectively. The improvement of the GeO density resulting from the confinement on Ge diffusion is responsible for the enhanced ultraviolet PL. This structure is expected to have important applications in optoelectronics.
Supernarrowing mirrorless laser emission in dendrimer-doped polymer waveguides
Authors: Otomo Akira , Yokoyama Shiyoshi , Nakahama Tatsuo , Mashiko Shinro
Published: July 2002

A supernarrowing laser spectrum is observed for mirrorless polymer waveguide lasers containing rhodamine B cored dendrimers. The moderate scattering from the aggregated dendrimers in the host polymer matrix is responsible for the mirrorless lasing. However, the long amplification path through the waveguide produces a directional beam and supernarrowing a spectrum in scattering lasers.
A very low temperature scanning tunneling microscope for the local spectroscopy of mesoscopic structures
Authors: Moussy N. , Courtois H. , Pannetier B.
Published: July 2002

We present the design and operation of a very low temperature scanning tunneling microscope (STM) working at 60 mK in a dilution refrigerator. The STM features both atomic resolution and micron-sized scanning range at low temperature. We achieved an efficient thermalization of the sample while maintaining a clean surface for STM imaging. Our spectroscopic data show unprecedented energy resolution. We present current–voltage characteristics and the deduced local density of states of hybrid superconductor–normal metal systems. This work is the first experimental realization of a local spectroscopy of mesoscopic structures at very low temperature.
Atomic force microscopy study of plastic deformation and interfacial sliding in Al thin film: Si substrate systems due to thermal cycling
Authors: Chen M. W. , Dutta I.
Published: July 2002

A method is proposed to measure the plastic deformation of thin metallic films on Si substrates induced by thermal cycling. The cross-sectional profiles of pattern-grown square Al films with a thickness of ∼250 nm and a size of ∼6 μm×6 μm were measured before and after thermal cycling by employing an atomic force microscope. With the assistance of statistical analysis, the change in the size and shape of the thin films were determined. Based on theoretical considerations, the thermal cycling deformation of thin films is attributed to creep and plasticity effects, accommodated by diffusion-controlled interfacial sliding.
Multiple sensor stabilization system for local probe microscopes
Authors: Altmann Stephan M. , Lenne Pierre-François , Heinrich Hörber J.-K.
Published: July 2002

We introduce the concept of a multiple sensor stabilization system (MSS) applicable to all local probe microscopes. With this it is possible to separate distance control and sensor-sample-interaction signals with unprecedented stability. The MSS uses two or more sensors of an array of local probe sensors, which are coupled via rigid low-mass short-distance connections. At least one of these sensors is employed to always provide a continuous and independent feedback signal. Using this, the distance between any other sensor of the array and the sample surface can be controlled, under ambient and in situ conditions, with the resolution and range of the designated sensors on time scales of up to hours. The concept of MSS is applicable to the whole range and any conceivable combination of local probe techniques, especially all other scanning near field probes. MSS offers particularly large advantages for spectroscopic applications. We demonstrate its utility by the example of an atomic force microscope using a commercially available array of cantilevers. By using two cantilever sensors for position control it is possible to eliminate all drift between the sample and the tip’s position. The high potential of the MSS is illustrated by two applications: a thermal noise-reduction based approach with minimal contact forces and the first pN-“force clamp” for single molecule force spectroscopy.
Laser drilling of silicon nitride and alumina ceramics: A numerical and experimental study
Authors: Atanasov P. A. , Eugenieva E. D. , Nedialkov N. N.
Published: July 2002

Laser drilling of silicon nitride (Si3N4) and alumina (Al2O3) ceramics targets was studied theoretically and experimentally. A one-dimensional model based on the heat-transfer equation was developed in order to describe the process. It includes a change of the type of the equivalent heat source during the laser pulse. The drilling of Si3N4 and Al2O3 ceramic slabs was performed by using a TEM00 Q-switched 10 ns pulse Nd:YAG laser. When the absorption of the plasma formed during the drilling process was taken into account, the theoretical results obtained agreed well with the experimental ones.
Single-crystal magnetotunnel junctions
Authors: Wulfhekel W. , Klaua M. , Ullmann D. , Zavaliche F. , Kirschner J. , Urban R. , Monchesky T. , Heinrich B.
Published: July 2002

We have grown epitaxial single-crystal magnetotunnel junctions using Fe(001) substrates, MgO(001) spacers and Fe top electrodes. We have used scanning tunneling microscopy and atomic force microscopy to measure the tunneling characteristics as a function of position and demonstrated that local tunneling can be obtained such that the buried MgO can be characterized with nm resolution. Local I(V) curves revealed that most of the area had intrinsic tunneling properties corresponding to the proper MgO tunneling barrier. A small fraction of the scanned areas showed localized spikes in the tunneling current which are most likely caused by defects in the MgO.
Raman studies of the temperature dependence of the internal vibrations of the ammonium ions in the mixed crystals K1−xAxDP and R1−xAxDP
Authors: Van’kevich A. V. , Popkov Yu. A. , Taranova I. A.
Published: July 2002

The Raman spectra in the frequency region corresponding to the internal vibrations of the ammonium ions are investigated in the temperature range 4.2–300 K for the mixed crystals K1−x(NH4)xH2PO4 with different ammonium concentrations (x=0.22, 0.32, 0.53, 0.74, and 0.82). Analysis of the temperature dependence of the frequencies and half-widths of the internal vibration lines of the ammonium ions confirms that in the paraelectric phase the position of the ammonium ions in the crystal lattice is not fixed: they execute hops with a change in the position of the center of mass. The activation energies of these reorientation processes are determined. It is found that at temperatures of the order of 110–120 K for the K1−x(NH4)xH2PO4 crystals of all concentrations a correlated fixing of the ammonium ion in the lattice occurs, leading to the formation of an antiferroelectric cluster structure. For the crystals with concentrations x=0.22, 0.32, and 0.53 this structure is preserved in the structural glass phase down to liquid helium temperatures, while for the crystals with x=0.74 and 0.82 it is preserved to the temperatures of the antiferroelectric phase transition TN (68.5 and 80.5 K, respectively). Analogous processes are also observed in the isostructural crystal Rb0.20(NH4)0.80H2PO4.
Phenomenologically derived electric field-temperature phase diagrams and piezoelectric coefficients for single crystal barium titanate under fields along different axes
Authors: Bell A. J.
Published: July 2002

The possible domain states of perovskite ferroelectrics under applied fields are reviewed and, as an illustration, a phenomenological study of barium titanate is carried out. Electric field-temperature phase diagrams, the polarization, and the lattice strain of barium titanate single crystals are calculated from the Landau–Ginzburg–Devonshire theory of ferroelectrics for applied fields up to 20 MV m−1 and for temperatures from 1 to 450 K. The calculations are carried out for fields applied along the pseudocubic [001], [101], and [111] axes, revealing the temperature and field dependence of all the ferroelectric phase transitions. Large piezoelectric coefficients can be identified close to field-induced transitions. Good agreement is seen with experimental data for the piezoelectric coefficient parallel to [001] over a wide range of temperature. The series of transitions predicted for increasing field parallel to [111] at room temperature is qualitatively similar to that observed experimentally but with somewhat larger critical fields and lower piezoelectric coefficients. Similarities are noted between the phase diagrams for fields along [001] and [111] and those describing PbBa2+ and ZrTi4+ substitutions.
IUPAC-NIST Solubility Data Series. 73. Metal and Ammonium Formate Systems
Authors: Balarew Christo , Dirkse Thedford P. , Golubchikov Oleg A. , Salomon Mark , Balarew Christo , Golubchikov Oleg A. , Dirkse Thedford P. , Trendafilova Stoyanka , Tepavitcharova Stefka , Ageyeva Tatiana , Baldini Primo , D’Andrea Guilio
Published: July 2002

This volume reviews the metal and ammonium formate solubility data published up to 1995. So far as the editors are aware, all the solubility data published during this period have been reviewed. Preference has been given to data published in numerical form. Data that appeared only in graphical form may not appear in this volume. In each section the metal atoms are arranged in the order (group) in which they appear in the Periodic Table. Metal formates are crystalline solids having some interesting chemical and physical properties. Several of these salts are important because they have nonlinear optical properties. Specific examples are: LiCHO2⋅H2O (3), NaCHO2 (4), Sr(CHO2)2, and Sr(CHO2)2⋅2H2O, Ba(CHO2)2, formates of Sc, Y and the rare earth elements having the general formula Me(CHO2)3⋅nH2O (where Me=Sc, Y, La, Ce, Pr, Nd, Sm…Lu) and some double salts and mixed salts such as NaCd(CHO2)3, BaCd(CHO2)4⋅2H2O and Li0.9Na0.1CHO2. Some metal formates have useful electric or magnetic characteristics. Thus, Cu(CHO2)2⋅4H2O has ferroelectric properties, Cu(CHO2)2 is ferromagnetic, Mn(CHO2)2⋅2H2O is antiferromagnetic, CuBa2(CHO2)6⋅4H2O is paramagnetic, and the formates of Ca, Cd, and Sr have elastic and thermoplastic properties. Bivalent metal formates could be used as precursors for the production of catalysts because they show excellent miscibility in the solid state, i.e., they form mixed crystals that dissociate at relatively low temperatures (about 300 °C) to form the respective oxides and mixed oxides. There are also additional smaller-scale uses of metal formates. The wide interest in the applications and uses of metal formates will lead to an interest in seeking methods for the preparation of these materials. Solubility data for the metal formates will be helpful in devising the methods of preparation. Therefore, this volume has been prepared to present and evaluate solubility data for the binary, ternary and multicomponent systems containing metal formates in aqueous and in nonaqueous solutions.
Thermal stability of laser-produced iron nitrides
Authors: Han M. , Carpene E. , Landry F. , Lieb K.-P. , Schaaf P.
Published: July 2002

Laser nitriding is a very efficient method to improve the mechanical properties, surface hardness, corrosion, and wear resistance of iron and steel, with the advantages of a high nitrogen concentration, fast treatment, and accurate position control, and without any undesired heating effect on the substrate. However, the stability of laser-produced iron nitrides is still under investigation. This article reports investigations of the thermal stability of these iron nitrides upon annealing treatments, which were conducted both in vacuum and air. The phase and elemental composition of the nitride layers were deduced from conversion electron Mössbauer spectroscopy, resonant nuclear reaction analysis, and grazing incidence x-ray diffraction. The surface hardness was measured by the nanoindentation method. In laser-nitrided iron, two critical temperatures are found: at 523 K the predominant iron-nitride phase changes from the γ/ε to the γ′ phase. When the temperature exceeds 773 K, all of the nitrogen has escaped from the surface layer. For annealing in air the nitrogen escapes completely already at 673 K, where a thick oxide layer has formed. Stainless steel proved to be more stable than iron, and even up to 973 K no new phases or oxides were produced, here, also, only at 973 K the nitrogen content decreased significantly. Therefore, laser-nitrided stainless steel is well suited for applications.
Sputtering and in-plane texture control during the deposition of MgO
Authors: Dong Liang , Zepeda-Ruiz Luis A. , Srolovitz David J.
Published: July 2002

Molecular dynamics simulations are performed to study the fundamental role of the ion beam in determining the in-plane texture of 〈100〉 oriented (out-of-plane) MgO films during ion beam assisted deposition (IBAD). Sputter yields are determined as a function of in-plane orientation for Ar ion beams. The minimum sputter yield exists at an ion beam orientation corresponding to the MgO 〈110〉 direction. The finite width of the sputter yield minimum is attributable to two main factors: (i) only a fraction of the incident ions are oriented to travel directly down the center of the channel and (ii) ions that are not exactly parallel to the channeling direction may channel. While the simulations imply that it is possible to in-plane orient {001} MgO films using IBAD, there are fundamental limitations on the degree of ordering that can be achieved.
Nanocrystalline iron at high pressure
Authors: Chen B. , Penwell D. , Kruger M. B. , Yue A. F. , Fultz B.
Published: July 2002

X-ray diffraction measurements were performed on nanocrystalline iron up to 46 GPa. For nanocrystalline ε-Fe, analysis of lattice parameter data provides a bulk modulus, K, of 179±8 GPa and a pressure derivative of the bulk modulus, K′, of 3.6±0.7, similar to the large-grained control sample. The extrapolated zero-pressure unit cell volume of nanocrystalline ε-Fe is 22.9±0.2 Å3, compared to 22.3±0.2 Å3 for large-grained ε-Fe. No significant grain growth was observed to occur under pressure.
Drops pushing by Marangoni forces
Authors: Savino R. , Monti R. , Alterio G.
Published: July 2002

Numerical simulations and laboratory experiments show that Marangoni effects are responsible for a pushing force on a droplet interacting with a solid wall in the presence of a temperature gradient. The results clarify questions of interest in the field of material science, in particular, they may provide some explanation for the minority phase separation observed during solidification processes under microgravity conditions.
Output-coupling semiconductor saturable absorber mirror
Authors: Spühler G. J. , Reffert S. , Haiml M. , Moser M. , Keller U.
Published: July 2002

We present a semiconductor saturable absorber mirror (SESAM), which also acts as an output coupler at the same time. The influence of the output coupler transmission onto the absorber parameters is investigated theoretically, as well as experimentally. A passively Q-switched Nd:YVO4 microchip laser is built using such a nonlinear output coupler, yielding clean pulses of 143 ps duration, 48 nJ energy, and 572 W peak power. This result is compared with the traditional approach, where the SESAM is not used as an output coupler.
Melting and crystallization in Ni nanoclusters: The mesoscale regime
Authors: Qi Yue , Çağin Tahir , Johnson William L. , Goddard William A.
Published: July 2002

We studied melting and freezing of Ni nanoclusters with up to 8007 atoms (5.7 nm) using molecular dynamics with the quantum-Sutten–Chen many-body force field. We find a transition from cluster or molecular behavior below ∼500 atoms to a mesoscale nanocrystal regime (well-defined bulk and surface properties) above ∼750 atoms (2.7 nm). We find that the mesoscale nanocrystals melt via surface processes, leading to Tm,N=Tm,bulk−αN−1/3, dropping from Tm,bulk=1760 K to Tm,336=980 K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For N>400 the supercooled clusters transform to FCC grains, but smaller values of N lead to a glassy structure with substantial icosahedral character.
A Lax pair for the two dimensional Euler equation
Authors: Li Yanguang (Charles)
Published: July 2002

A Lax pair for the 2D Euler equation is found.
Competition between agostic WCH2+ and HWCH+: A joint experimental and theoretical study
Authors: Simon Aude , Lemaire Joël , Boissel Pierre , Maı̂tre Philippe
Published: July 2002

We present both theoretical and experimental photodissociation results on the products of the methane dehydrogenation by W+ in the gas phase. We show that the reaction may lead to two isomers: whereas only the methylidenetungsten WCH2+ had been proposed, we show that the hydridomethylidynetungsten HWCH+ can also be formed. Both density functional and highly correlated ab initio quantum chemical calculations have been performed using a relativistic core potential for W+. Spin–orbit couplings have been evaluated semiempirically. We found the HWCH+ and the WCH2+ isomers to be nearly degenerate, the latter structure exhibiting a strong agostic distortion. Photodissociation of the mass selected [W,C,2H]+ product of the reaction has been carried out in a Fourier transform ion cyclotron resonance mass spectrometer. The H+WCH+ channel has been observed as the major photofragmentation channel and a photodissociation threshold of 2.5±0.1 eV has been derived. This low-energy value is in good agreement with the thermodynamic threshold determined theoretically. These results suggest a very large bond energy associated with the triple WC bond in WCH+ (about 158 kcal/mol).
Terahertz pulse propagation through small apertures
Authors: Mitrofanov Oleg , Lee M. , Hsu J. W. P. , Pfeiffer L. N. , West K. W. , Wynn J. D. , Federici J. F.
Published: July 2002

Transmission of single-cycle terahertz pulses through subwavelength apertures is experimentally studied in the near-field zone. Measurements of throughput for aperture sizes d as small as λ/300 show that the theoretical d3 law requires a correction term which takes into account the physical thickness of the aperture screen. Frequency dependent transmission of the broad band pulses explains changes in their spectral and temporal characteristics. Practical application of small apertures in near-field microscope probes allows achievement of spatial resolution of 7 μm for pulses with a broad spectral content of λ=120–1500 μm.
Characterizing the function of unstructured proteins: Simulations of charged polymers under confinement
Authors: Bright Joanne N. , Stevens Mark. J. , Hoh Jan , Woolf Thomas B.
Published: July 2002

Experimental findings that some polypeptides may be unstructured and behave as entropically driven polymeric spacers in biological systems motivates a study of confined polymers. Here we examine the confinement of neutral, polyampholyte, and polyelectrolyte polymers between two parallel surfaces using course grained models and molecular dynamics. Forces between the confining surfaces are determined for different polymer classes and as a function of chain length, charge sequence (pattern) and degree of confinement. Changes in chain properties are also evaluated under these conditions. The results reinforce the significance of length and net charge for predicting chain properties. In addition the clustering of charge along the chain appears to be critical, and changes in cluster size and distribution produce dramatic changes in chain behavior.
Spin-valve transistor with an Fe/Au/Fe(001) base
Authors: Sato R. , Mizushima K.
Published: July 2002

A spin-valve transistor with an epitaxial Fe/Au/Fe(001) base was formed on n-GaAs, the characteristics of which were examined under a magnetic field for emitter voltages up to 3 V. The transfer ratio of the transistor, i.e., the ratio of collector-to-emitter current exceeded 10−3 at 3 V, preserving the magnet current ratio, i.e., the ratio of collector current in the parallel-to-antiparallel magnetic configuration well above 100%. It was suggested that the transfer ratio would be further enhanced by improving the flatness of the tunnel junction for injecting electrons from the emitter into the base, as well as by increasing the electron transmittance at the base/collector interface.
ArI
2
(X)→Ar+I
2
(B)
photodissociation: Comparison between linear and T-shaped isomers dynamics
Authors: Roncero Octavio , Lepetit Bruno , Beswick J. Alberto , Halberstadt Nadine , Buchachenko Alexei A.
Published: July 2002

Quantum dynamical calculations on ArI2 photodissociation have been performed using ab initio and semi-empirical potential energy surfaces, which support both linear and T-shaped isomers in the ground electronic state. Whereas the photon absorption spectra for the T-shaped isomer consist of narrow and intense bands, those for the linear isomer result from the superposition of a continuous background and peaks due to linear quasi-bound states. Vibrational distributions for the linear isomer are broader than those originating from the T-shaped one. Rotational distributions for the linear isomer are smooth and characteristic of a fast dissociation dynamics, whereas those for the T-shaped isomer are highly oscillatory. Implications of these results on the interpretation of experimental data are discussed.
Inverse bremsstrahlung in a plasma with electron temperature anisotropy
Authors: Ferrante G. , Zarcone M. , Uryupin S. A.
Published: July 2002

Inverse bremsstrahlung absorption of electromagnetic radiation in plasma with anisotropic two-temperature bi-Maxwellian electron distribution function over velocities is investigated. In the case of a weak field, absorption is more effective if the radiation field is polarized in the plane in which the plasma electrons have the smaller of the two temperatures. In the case when the distribution function is highly anisotropic, absorption changes strongly when the field polarization changes its direction with respect to the temperature anisotropy axis. In the intermediate domain, when the field is strong in directions not very close to that of the larger temperature, both absorption efficiency and degree of its anisotropy decrease. The conditions when the absorbed power practically does not depend on the field are established. Finally, in the case of a strong field, absorption decreases further while the degree of anisotropy is a weakly changing logarithmic function of effective electron temperatures.
Residual strain effects on the two-dimensional electron gas concentration of AlGaN/GaN heterostructures
Authors: Martı́nez-Criado G. , Cros A. , Cantarero A. , Ambacher O. , Miskys C. R. , Dimitrov R. , Stutzmann M. , Smart J. , Shealy J. R.
Published: July 2002

Ga-face AlGaN/GaN heterostructures with different sheet carrier concentrations have been studied by photoluminescence and Raman spectroscopy. Compared to bulk GaN, an energy shift of the excitonic emission lines towards higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. This strain was confirmed by the shift of the E2 Raman line, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced. The spontaneous and piezoelectric polarizations for each layer of the heterostructures have been also calculated. The analysis of these quantities clarified the influence of the residual stress on the sheet electron concentration (ns). Possible causes for the discrepancies between the calculated and experimentally determined sheet carrier densities are briefly discussed.
Improved diffusion barrier by stuffing the grain boundaries of TiN with a thin Al interlayer for Cu metallization
Authors: Nam Ki Tae , Datta Arindom , Kim Soo-Hyun , Kim Ki-Bum
Published: July 2002

A laterally segregated diffusion barrier was investigated for Cu metallization. In this scheme, the intended final structure is composed of two different barrier materials; one is the parent barrier layer (TiN, in our case) and the other (Al2O3, in this case) is segregated laterally along the grain boundaries of the parent barrier layer. As a result, the fast diffusion paths, the so-called grain boundaries of the parent diffusion barrier, are effectively passivated. To realize this type of barrier experimentally, the TiN(5 nm)/Al(2 nm)/TiN(5 nm) structure was fabricated by sequential sputtering and compared with TiN(10 nm) as a diffusion barrier against Cu. The etch pit test results indicated that the barrier with the Al interlayer prevented Cu diffusion into the Si up to 650 °C, which is 250 °C higher than achieved by a TiN(10 nm) barrier.
Energy broadening due to photoion space charge in a high resolution laser photoelectron source
Authors: Bömmels J. , Leber E. , Gopalan A. , Weber J. M. , Barsotti S. , Ruf M.-W. , Hotop H.
Published: July 2002

The results of experimental and theoretical studies, aiming at a quantitative characterization of photoion-induced energy broadening effects in a laser photoelectron source, are reported. The electron source is based on two-step cw laser photoionization of potassium atoms in a collimated beam. In the experimental studies, the attachment spectra for the formation of (N2O)9O− cluster ions through a narrow vibrational Feshbach resonance (full width at half maximum 2.3 meV) were measured as a function of the photocurrent. The theoretical studies involved Monte Carlo simulations of the broadening effects and were based on potential distributions caused by realistic spatial distributions of the photoions. Using the corresponding electric field distribution, trajectories were calculated for a representative ensemble of electrons, and effective electron energy distributions were obtained from averages over the electron trajectories in the volume relevant for electron attachment. Furthermore, the effects of additional weak electric fields, applied along the atomic beam direction, have been simulated. For our geometry (ionization volume about 2 mm3) the effective space charge related energy width is found to be about 16 μeV/pA.
Computing phase equilibria by parallel excluded volume tempering
Authors: Vlugt Thijs J. H. , Dünweg Burkhard
Published: July 2002

We present a Monte Carlo scheme for the computation of phase equilibria at high densities. At these high densities, all conventional simulation techniques that rely on insertions and deletions of particles, e.g., the Gibbs ensemble technique, will have problems because the acceptance probability for these moves is very low. Furthermore, the efficiency of these methods strongly depends on the complexity of the system, e.g., degree of polymerization and branching of the components. Our new method is based upon simulating a path of independent systems in the grand-canonical ensemble. Each system has a slightly different interaction potential, ranging from a full excluded volume potential to an ideal gas, as well as different imposed chemical potentials of each component. This path is constructed in such a way that the average number of molecules of a specific component per system is constant along the path. To sample all systems of the path efficiently, we apply a parallel tempering procedure to exchange configurations of two adjacent systems. The advantage of these exchanges is that, for the full excluded volume system, one does not have to rely on particle insertions and deletions in this system to sample the full phase space, but rather on particle insertions and deletions in systems with soft interactions. Without excluded volume interactions, the acceptance of insertions is independent of molecular size and shape; hence our method does not suffer from the problems of the conventional methods. We have tested our method for very simple systems (Lennard-Jones particles) and found exact agreement with Gibbs ensemble simulations. For these simple systems the conventional techniques to compute phase equilibria are much more efficient. However, we expect that for long chain molecules this situation will be reversed.
Coupled-cluster theory for excited electronic states: The full equation-of-motion coupled-cluster single, double, and triple excitation method
Authors: Kucharski Stanisław A. , Włoch Marta , Musiał Monika , Bartlett Rodney J.
Published: July 2002

The equation-of-motion coupled-cluster method with the full inclusion of the single, double, and triple excitations (EOM-CCSDT) has been formulated and implemented. The proper factorization procedure ensures that the method scales as n8, i.e., in the same manner as the standard CCSDT method for ground states. The method has been tested on the vertical excitation energies of the N2 and CO molecules for several basis sets up to 92 basis functions. The full inclusion of the triple excitations improves the EOM-CCSD results by up to 0.2 eV for considered systems.
Influence of the crystal structure and chemical order on the magnetic and magneto-optical properties of equiatomic CoPt alloy
Authors: Uba L. , Uba S. , Horpynyuk O. , Antonov V. N. , Yaresko A. N.
Published: July 2002

The influence of the structure-dependent electronic effects on the magnetic, magneto-optical (MO), and anisotropic properties of equiatomic CoPt alloy was modeled with the use of fully relativistic linear-muffin-tin-orbital calculations. We found large differences between the MO spectra of the alloy of the AuCu (L11) and CuPt (L10) superlattice type structures, as well as a strong sensitivity of the spectra on the degree of chemical order. Very good agreement between the experimental and theoretical spectra for both a partially ordered and a fully disordered alloy, modeled by a Monte Carlo simulation, is found, demonstrating the applicability of the theoretical approach.