bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastic properties of Pd40Cu30Ni10P20 bulk glass in supercooled liquid region
Authors: Nishiyama N. , Inoue A. , Jiang J. Z.
Published: July 2002

In situ ultrasonic measurements for the Pd40Cu30Ni10P20 bulk glass in three states: Glassy solid, supercooled liquid, and crystalline, have been performed. It is found that velocities of both longitudinal and transverse waves and elastic moduli (shear modulus, bulk modulus, Young’s modulus, and Lamé parameter), together with Debye temperature, gradually decrease with increasing temperature through the glass transition temperature as the Poisson’s ratio increases. The behavior of the velocity of transverse wave vs. temperature in the supercooled liquid region could be explained by viscosity flow, rather than the two different crystallization processes in the region, suggested in the literature. No decomposition was detected at a temperature only 5 K below the crystallization temperature.
Photodesorption of disilane physisorbed on hydrogen terminated Si(100) and the dramatic consequences of weak molecular chemisorption
Authors: Wright Steven , Hasselbrink Eckart
Published: July 2002

The adsorption of disilane (Si2H6) on D terminated Si(100) has been investigated using photodesorption to probe the coverage over a range of surface temperatures. Incident Si2H6 molecules can become molecularly (nondissociatively) chemisorbed after being trapped in a physisorbed precursor state. At temperatures higher than approximately 116 K, nearly all the physisorbed molecules desorb rather than chemisorb. Photofragmentation products are readily observed when chemisorbed Si2H6 is irradiated with 6.4 eV photons but surprisingly, none are observed in the case of physisorbed Si2H6. Consequently, Si can be deposited by reactions of the photofragmentation products with the surface, but only at temperatures lower than 116 K. The photodesorption cross section is also extremely dependent on the chemical nature of the Si(100) surface. It is high when the surface is terminated with a monodeuteride phase but much smaller when dideuteride species are present, suggesting that the presence of surface gap states is of importance. We consider how desorption might be initiated by electron attachment or by electron-hole recombination at the surface.
Characterization of photonic dots in Si/SiO2 thin-film structures
Authors: Porjo N. , Kuusela T. , Heikkilä L.
Published: July 2002

We have studied bright electroluminescent dots on a single SiO2 layer and on Si/SiO2 layer pairs prepared by chemical-vapor deposition onto the silicon wafer substrate. The size, density, and brightness distribution of the dots and the fractal dimension of the light maps have been characterized using charge-coupled-device (CCD) camera techniques, which allow reliable quantitative analysis. We have found that the fractal dimension of the light maps depends on the structure of the first silicon dioxide layer. CCD images and atomic-force microscope and scanning tunneling microscopy analysis have been compared in order to find a spatial correlation between the dots and the surface morphology or electrical properties of the thin films, but no correspondence was found, indicating that the origin of the dots must be in the interface between the silicon substrate and the first oxide layer. We have also shown that the dots are not related to heating of the sample.
Characteristics of asymmetric superconducting quantum interference devices
Authors: Testa G. , Sarnelli E. , Pagano S. , Calidonna C. R. , Furnari M. Mango
Published: July 2002

Direct current-superconducting quantum interference devices (SQUIDs) characterized by asymmetric Josephson junctions have never been used for applications. In order to demonstrate their potential advantages, a throughout numerical analysis of different asymmetric configurations has been carried out. A damping resistance has been included in the SQUID circuit and the thermal noise associated with junction and damping resistances has been considered in the numerical model. Magnetic modulations and flux noise spectral densities have been computed as a function of many parameters (bias current, asymmetry, SQUID inductance, and damping resistance) and the performance of symmetric and asymmetric devices have been compared. The results show that, by properly optimizing the SQUID design, asymmetric SQUIDs are characterized by higher magnetic flux to voltage transfer coefficient and lower flux noise. As a result, asymmetric configurations can be very useful in all the applications where high sensitivity is required.
Optical properties of atomic boron in rare gas matrices: An ultraviolet-absorption/laser induced fluorescence study with ab initio and diatomics-in-molecules molecular dynamics analysis
Authors: Kiljunen Toni , Eloranta Jussi , Ahokas Jussi , Kunttu Henrik
Published: July 2002

Boron atoms have been produced by laser ablation and trapped in solid Ar, Kr, and Xe matrices. In addition to stable doublet absorptions at 210–230 nm, broad temperature dependent bands less in blue have been recorded. With the aid of laser induced fluorescence measurements, ab initio pair potential calculations, and diatomics-in-molecules simulations these novel broad lines at 241 nm in Ar, 247 nm in Kr, and 254 nm in Xe were assigned to boron 3s(2S)←2p(2P) transitions. The thermal behavior of the broad absorption bands is similar to what was reported for boron atoms detected by electron paramagnetic resonance [J. Chem. Phys. 114, 7144 (2001)]. The revised interpretation based on the new spectroscopic observations is in good agreement with theoretical predictions.
Thermochemical Properties of Si2F6 and SiF4 in Gas and Condensed Phases
Authors: Lyman John L. , Noda Tetsuji
Published: July 2002

The available data for determining the thermochemical parameters for Si2F6 and SiF4 has been assembled. With this information, the thermochemical functions for Si2F6 and SiF4 have been calculated as ideal gases in the harmonic oscillator approximation. These results are presented in the format of the NIST–JANAF Thermochemical Tables, as well as in the format of the CHEMKIN Thermochemical Database. Using the vapor pressure data, the thermochemical parameters for Si2F6 in the solid phase and SiF4 in both the solid and liquid phases have been calculated. These results are compared with available experimental and calculated data. The earlier results are in good agreement with our results. However, the Si2F6 results were not previously available in the JANAF format, and the results for both species are on a much firmer footing with the new experimental data we used. The anharmonicity constants measured earlier for SiF4 permitted a determination of the effect of anharmonicity on the thermochemical properties for this species. For this species at least, the harmonic oscillator approximation is very good. Differences between the harmonic and anharmonic calculations were only significant at temperatures above 1000 K.
Photon energy dependence of the sign of the current-induced absorption polarization sensitivity in degenerate semiconductors
Authors: Ryvkin B. S. , Avrutin E. A. , Walker A. C.
Published: July 2002

We theoretically analyze the effect of current-induced polarization dependence of absorption (fundamental and intersubband) and gain in degenerate semiconductors. The sign of the currentinduced polarization selectivity of absorption is shown to depend on the energy of the initial (or final) state of the transition with respect to the Fermi level and, therefore, on the photon energy. The effect is predicted to be important for understanding and, potentially, engineering polarization properties of devices such as vertical-cavity surface-emitting lasers.
Flow fields with constant rate of strain along streamlines
Authors: Sciffer Scott
Published: July 2002

In order to study the effect of strain rate on entrained bubbles it would be useful to produce a flow in which the bubbles are subjected to uniform rates of strain. However it is shown that the only steady two-dimensional or axisymmetric flows with constant components of the rate of strain tensor in streamline coordinates are the trivial cases of rigid body motion.
On the breaking of standing waves by falling jets
Authors: Longuet-Higgins Michael S. , Dommermuth Douglas G.
Published: July 2002

By direct numerical time stepping, using two different methods of calculation, we follow the development of a standing gravity wave on the surface of deep water when it is given an initial energy exceeding that of the most energetic periodic wave of the same wavelength. Two aspects of the motion are studied in detail: the sequence of wave profiles close to the instant when the maximum surface slope angle is close to 45° and, second, the conditions under which a sharp cusp is formed at the free surface. When a cusp is formed, it can fall vertically into the wave trough, enclosing “bubbles of air.”
Theoretical performance and structure optimization of 3.5–4.5 μm InGaSb/InGaAlSb multiple-quantum-well lasers
Authors: Andreev A. D. , O’Reilly E. P. , Adams A. R. , Ashley T.
Published: July 2002

We present a comprehensive theoretical investigation to optimize 3.5–4.5 μm InGaSb/InGaAlSb quantum-well (QW) lasers grown on ternary InGaSb substrates. We use an eight-band k⋅P Hamiltonian to calculate the Auger recombination and optical absorption coefficients in the active region, as well as the gain and threshold characteristics. The dominant Auger process involves hole excitation from the QW to unbound valence states. For structure optimization we varied the Ga content in the substrate and QW barrier layers. The optimized structure was obtained by maximizing the strain in the QWs, despite the Auger coefficient also increasing with strain. It is, therefore, demonstrated that the main aim for midinfrared laser optimization can be minimization of the threshold carrier density rather than reduction of the Auger coefficient.
Thermocapillary migration of long bubbles in polygonal tubes. I. Theory
Authors: Mazouchi Ali , Homsy G. M.
Published: July 2002

Thermocapillary migration of long gas bubbles in cylinders of regular-polygonal or rectangular cross sections is studied. An imposed axial temperature gradient produces a gradient of surface tension leading to a steady migration of the bubble towards the hotter region. A leading order approximation for the bubble migration speed is found by computing the volume flux of liquid through the corner regions of the cylinder, which provide a parallel channel for the flow of fluid driven by the Marangoni stress. A global mass balance is used to relate the dimensionless bubble speed to the modified capillary number Δσ*=γTβa/σ, where β is the temperature gradient, a the tube length scale, σ the mean surface tension, and γT the temperature coefficient of surface tension. The dimensionless bubble speed is found to be linear in this parameter at leading order. The approximation is improved by accounting for the deposition of a thin film on the cylinder walls at small capillary number. A modified Landau–Levich equation governs the thin film profile, the solution of which allows the calculation of the additional flux so produced. It is found that corrections in the bubble velocity are nonlinear in Δσ*. For regular-polygonal tubes with small numbers of sides or low aspect ratio rectangular cross sections, most of the flux passes through the corner regions, while for larger numbers of sides or large aspect ratios, flow in the thin film regions dominates and the results tend toward those for cylindrical tubes.
Trial wave functions for the calculation of vibrational states of molecules using quantum Monte Carlo
Authors: da Silva Washington B. , Acioli Paulo H.
Published: July 2002

In this article we test three kinds of trial wave functions for the calculation of vibrational excited states of molecules using quantum Monte Carlo. We begin our study with the basis set originally used by Bernu and co-workers and further modified by Acioli and Soares Neto. The second set tested was the simplified Morse oscillator-like with harmonic coupling (SMOL-HC) proposed by Brown et al. to study the vibrational spectra of C3. Finally we proposed a third basis set, based on the previous two. This basis set keeps the anharmonicity of the SMOL-HC basis but with well conditioned Hamiltonian and overlap matrices. The calculations were performed in the H2, H3+, and H2O molecules. The results indicate that the basis sets proposed in this work yield more accurate results with a smaller number of basis functions.
CaBi
2
Ta
2
O
9
ferroelectric thin films prepared by pulsed laser deposition
Authors: Das Rasmi R. , Rodriguéz R. J. , Katiyar Ram S. , Krupanidhi S. B.
Published: July 2002

Thin films of CaBi2Ta2O9 (CBT) were deposited on Pt/TiO2/SiO2/Si substrates using the pulsed laser deposition technique at temperatures ranging from 500 to 700 °C. The presence of (115) and (0010¯) orientations confirm the phase formation at the lower temperature (500 °C). Microstructure evolution of CBT films with oxygen pressure of 100–200 mTorr at a substrate temperature of 650 °C shows that the films deposited at lower pressure have a relatively smaller grain size and less surface roughness. The films grown at 650 °C exhibited a maximum polarization of (2Pm) 17 μC/cm2, remanent polarization of (2Pr) 8 μC/cm2 and coercive field of (Ec) 128 kV/cm, with fatigue endurance up to 1010 switching cycles. The higher dielectric constant (∼115 at 100 kHz) with a relatively lower dissipation factor (0.02) at higher growth temperature (700 °C) was explained by the increased grain size. The higher leakage current density (∼10−7 A/cm2) at higher deposition temperature is attributed to the interfacial diffusion of the film and the substrate.
Utility function and cooperativity in binding systems
Authors: Ben-Naim A.
Published: July 2002

The utility function for hemoglobin is defined as the amount of oxygen transported by a single hemoglobin molecule per roundtrip from the lungs (the loading terminal) to the various tissues (the unloading terminal). Traditionally, the efficiency of transporting any ligand by a binding system has been inferred from the cooperativity of the binding process. The larger the (positive) cooperativity, the steeper the binding curve (the so-called binding isotherm), and hence the larger efficiency. This monotonic relationship between cooperativity and efficiency is indeed valid for some simple systems, such as a two-site system. It is also valid for more complicated multisubunit systems for which all cooperativities are pairwise additive. We found that when the binding process is nonadditive, efficiency does not follow straightforwardly from cooperativity. Hemoglobin is a typical, highly nonadditive binding system, for which efficiency should be studied in its own right and independently of cooperativity.
Molecular dynamics studies of supercooled ethanol
Authors: Sesé G. , Palomar R.
Published: July 2002

Molecular dynamics simulations have been performed to get some insight into the dynamical properties of supercooled ethanol. The temperature dependence of translational and reorientational dynamics of the system has been analyzed. For all the correlators under study, a two-step relaxation behavior has been encountered. For short times, there is an initial Gaussian decay. For very long times, the relaxation shows a nonexponential dependence. For intermediate times and for temperatures close to a critical temperature, a nondiffusive regime appears (β relaxation). The predictions of the ideal version of the mode-coupling theory have been tested and a reasonable agreement has been found for the dynamics of the molecular centers-of-mass.
Defects in 30 keV Er+-implanted SiO2/Si studied by positron annihilation and cathodoluminescence
Authors: Hirata K. , Arai H. , Kawasuso A. , Sekiguchi T. , Kobayashi Y. , Okada S.
Published: July 2002

Defects in SiO2 (48 nm)/Si induced by 30 keV Er ion implantation were studied by positron annihilation. Depth-selective information on defects for samples implanted with doses of 3.0×1014 and 1.5×1015 Er/cm2 was obtained by a variable-energy positron beam by measuring Doppler broadening of positron annihilation γ rays as a function of incident positron energy. Comparison of the results by Doppler broadening with those by electron spin resonance after annealing indicates that the types of defects (which predominantly exist in the SiO2 layer) depend on implantation dose. The annealing temperature dependence of positron data is compared with that of the cathodoluminescence intensity at 1.54 μm, and the possible effect of defects on luminescence intensity is discussed.
On the crossing of electronic energy levels of diatomic molecules at the large-D limit
Authors: Shi Qicun , Kais Sabre , Remacle Françoise , Levine R. D.
Published: July 2002

Analytical and numerical results are presented for the intersection of electronic energies of the same space symmetry for electrons in the field of two Coulomb centers in D-dimensions. We discuss why such crossings are allowed and may be less “exceptional” than one could think because even for a diatomic molecule there is more than one parameter in the electronic Hamiltonian. For a one electron diatomic molecule at the large-D limit, the electronic energies are shown analytically to diverge quadratically from the point of their intersection. The one electron two Coulomb centers problem allows a separation of variables even when the charges on the two centers are not equal. The case of two electrons, where their Coulombic repulsion precludes an exact symmetry, is therefore treated in the large-D limit. It is then found that, in addition to the quadratic intersection, there is also a curve crossing where the energies diverge linearly.
Towards a molecular logic machine
Authors: Remacle F. , Levine R. D.
Published: July 2002

Finite state logic machines can be realized by pump–probe spectroscopic experiments on an isolated molecule. The most elaborate setup, a Turing machine, can be programmed to carry out a specific computation. We argue that a molecule can be similarly programmed, and provide examples using two photon spectroscopies. The states of the molecule serve as the possible states of the head of the Turing machine and the physics of the problem determines the possible instructions of the program. The tape is written in an alphabet that allows the listing of the different pump and probe signals that are applied in a given experiment. Different experiments using the same set of molecular levels correspond to different tapes that can be read and processed by the same head and program. The analogy to a Turing machine is not a mechanical one and is not completely molecular because the tape is not part of the molecular machine. We therefore also discuss molecular finite state machines, such as sequential devices, for which the tape is not part of the machine. Nonmolecular tapes allow for quite long input sequences with a rich alphabet (at the level of 7 bits) and laser pulse shaping experiments provide concrete examples. Single molecule spectroscopies show that a single molecule can be repeatedly cycled through a logical operation.
Free-standing SiGe-based nanopipelines on Si (001) substrates
Authors: Schmidt O. G. , Jin-Phillipp N. Y.
Published: July 2002

Thin solid films form nanopipelines if the films are released from a substrate and put back onto their own surface. We give a detailed description of free-standing SiGe-based nanopipelines created on Si (001) substrates. The initial layer sequence is grown by molecular beam epitaxy and comprises SiGe-based epitaxial layers grown on a Ge sacrificial layer. After selectively etching away the Ge sacrificial layer, SiGe nanopipelines have formed on the surface. Nanopipelines as long as 20 μm with diameters ranging from 50 to 530 nm are fabricated. We show that SiGe nanopipelines perform multiple revolutions if selective etching is carried out long enough. Adding carbon to Si epitaxial layers is proposed to extend the design freedom of Si-based nanopipelines and nanotubes.
Direct measurement of water self-diffusion in hardening blast furnace slag cement pastes by means of nuclear magnetic resonance techniques
Authors: Nestle N. , Galvosas P. , Kärger J.
Published: July 2002

Recently, we have reported the unusual nuclear spin relaxation behavior of water in hardening blast furnace slag mixtures and suggested an explanation for the observed changes in the spin relaxation times based on diffusion effects in inner magnetic field gradients in the paste. Meanwhile, we have succeeded in measuring the self-diffusion coefficient of water in the hardening pastes directly by means of a special pulsed field gradient technique in which the effects of internal magnetic field gradients in a sample can be compensated for. Compared to water self-diffusion in ordinary cement materials, we find a much stronger decrease of the self-diffusion coefficient in the slag based cements.
Dependence of the giant magnetoresistance on the concentration of magnetic particles in granular composites
Authors: Xu C. , Hui P. M. , Li Z. Y.
Published: July 2002

We study the dependence of giant magnetoresistance (GMR) on the volume concentration of magnetic particles in a magnetic granular composite via a Monte Carlo method and by modeling the composite as a random resistor network. We assume the nanosized magnetic particles are spherical in shape and are randomly distributed in a square or cubic lattice. The uniaxial anisotropy of the particles and the classical dipolar interaction among the particles are taken into account. By considering the difference in electron scatterings for spin-up and spin-down conduction electrons at the magnetic and nonmagnetic interface, and the scatterings within the magnetic regions and the nonmagnetic host medium in the composite, the value of GMR is found to depend sensitively on the spatial distribution of the particles, the magnetic states of the particles, and the densities of the spin polarized conduction electrons. There is an optimum concentration (about 25% in two-dimensional and 30% in three-dimensional cases) of magnetic particles at which the magnetoresistance shows a maximum. This phenomenon was also observed in experiments.
Interaction of copper metallization with rare-earth metals and silicides
Authors: Molnár G. L. , Petö G. , Zsoldos E. , Horváth Z. E.
Published: July 2002

Solid-phase reactions of copper films with underlying gadolinium, erbium, and erbium–silicide layers on Si(100) substrates were investigated. For the phase analysis, x-ray diffraction and cross-sectional transmission electron microscopy were used. In the case of Cu/Gd/Si(100), an orthorhombic GdSi2 formed, and, at higher temperatures, copper aggregated into islands. Annealed Cu/Er/Si(100) samples resulted in a hexagonal Er5Si3 phase. In the Cu/ErSi2−x/Si system, the copper catalyzes the transformation of the highly oriented hexagonal ErSi2−x phase into hexagonal Er5Si3. Diverse phase developments of the samples with Gd and Er are based on reactivity differences of the two rare-earth metals.
Magnetic anisotropy in a permalloy microgrid fabricated by near-field optical lithography
Authors: Li S. P. , Lebib A. , Peyrade D. , Natali M. , Chen Y. , Lew W. S. , Bland J. A. C.
Published: July 2002

We report the fabrication and magnetic properties of permalloy microgrids prepared by near-field optical lithography and characterized using high-sensitivity magneto-optical Kerr effect techniques. A fourfold magnetic anisotropy induced by the grid architecture is identified.
Characterization of electron-irradiated n-GaN
Authors: Goodman S. A. , Auret F. D. , Legodi M. J. , Beaumont B. , Gibart P.
Published: July 2002

Using deep level transient spectroscopy, we have investigated the electron trap defects introduced in n-GaN grown using the epitaxial lateral overgrowth technique during high energy electron irradiation from a Sr90 radionuclide source. The results indicate that the major electron-irradiation-induced defect labeled ER3 is not a single defect level but is made up of at least three defect levels (ER3b–ER3d). One of these defects, ER3d, has an activation energy and introduction rate of 0.22 eV and 0.43 cm−1, respectively. The total introduction rate of the three defects (ER3b–ER3d) is approximately 1.0 cm−1.
Vacuum electron emission with low turn-on electric field from hydrogenated amorphous carbon thin films
Authors: Xu Jun , Huang Xiaohui , Li Wei , Wang Li , Huang Xinfan , Chen Kunji , Xu Jianbin , Wilson I. H.
Published: July 2002

Hydrogenated amorphous carbon films were deposited with and without hydrogen gas dilution in a small dc bias-assisted plasma chemical vapor deposition system. The field emission characteristics were investigated and compared. It was found that the vacuum electron emission with a very low turn-on electric field (∼0.5 V/μm) could be achieved, which is comparable with the value obtained from nitrogen-doped diamond materials. The improvement of field emission property in the present carbon films was tentatively attributed to the field enhancement effect due to the electronic structure inhomogeneity of the carbon films and the reduction of the surface emission barrier due to the hydrogen termination.
Selective adsorption and patterning of Si nanoparticles fabricated by laser ablation on functionalized self-assembled monolayer
Authors: Hata K. , Fujita M. , Yoshida S. , Yasuda S. , Makimura T. , Murakami K. , Shigekawa H. , Mizutani W. , Tokumoto H.
Published: July 2002

We demonstrate an in situ selective adsorption of Si nanoparticles fabricated by laser ablation on a functionalized self-assembled monolayer (SAM). Si nanoparticles adsorbed on –CH3 terminated a SAM while Si particles did not adsorb on –NH2, –F, –OH, and –COOH, terminated SAMs. The end group of a SAM solely determines the selectivity against Si nanoparticle adsorption. We utilized the screening ability of functionalized SAMs to pattern Si nanoparticles onto desired locations on a Si substrate.
Double textured cylindrical block copolymer domains via directional solidification on a topographically patterned substrate
Authors: Park Cheolmin , Cheng Joy Y. , Fasolka Michael J. , Mayes Anne M. , Ross C. A. , Thomas Edwin L. , De Rosa Claudio
Published: July 2002

Directional solidification of cylinder forming block copolymer films confined between a directionally crystallizing solvent (benzoic acid) and a topographically patterned silicon substrate imparts a particular orientation to the block copolymer microdomains that is dependent of the solidification direction and the local film thickness. The substrate features (30 nm high, 2μm wide square mesas on a 4μm sq lattice) shape the film morphology by periodically modulating the local film thickness. Thicker regions between substrate features (plateaus) exhibit in-plane cylinders aligned in the crystallization direction and thinner regions over the substrate features (mesas) display vertically aligned cylindrical domains. This approach is a simple and general technique for engineering an intended domain orientation in specific areas of a block copolymer film. Development of this method for nanolithographic applications is demonstrated through oxygen plasma reactive ion etching of the patterned cylindrical domains.
Internal self-ordering in In(Sb,As), (In,Ga)Sb, and (Cd,Zn,Mn)Se nano-agglomerates/quantum dots
Authors: Möck P. , Topuria T. , Browning N. D. , Booker G. R. , Mason N. J. , Nicholas R. J. , Dobrowolska M. , Lee S. , Furdyna J. K.
Published: July 2002

Nano-agglomerates of In(Sb,As) in InAs, (In,Ga)Sb in GaSb, and (Cd,Zn,Mn)Se in (Zn,Mn)Se are classified by transmission electron microscopy. In scanning transmission electron microscopy, atomic resolution Z-contrast images reveal different modes of internal compositional modulation on the atomic length scale, resulting for all three material systems in nano-agglomerates of an appropriate size that may constitute a new type of quantum dot. For other nano-agglomerates of In(Sb,As) in InAs and (In,Ga)Sb in GaSb, we observed a second type of nanoscale ordering that results in nano-agglomerates with an internal compositional modulation on a length scale of a few nm. Both types of compositional modulation are discussed as having arisen from a rather long-term structural response to a combination of internal and external strains.
Visualization of mode transformation in a planar waveguide splitter by near-field optical phase imaging
Authors: Balistreri M. L. M. , Korterik J. P. , Kuipers L. , van Hulst N. F.
Published: July 2002

The gradual transformation of a guided TM00 mode into an “intermediate” double mode by a splitting junction has been investigated with a phase-sensitive photon scanning tunneling microscope. Field profiles and wave vectors of the modes have been directly determined from the phase information. Via a Fourier analysis of the measured phase and amplitude maps the decay of the TM00 mode and buildup of the intermediate mode have been directly visualized. Phase singularities and phase jumps in the transition region underline the mode transformation process. Finally, a partial polarization conversion of the TM modes to TE-polarized modes has been observed.
Harmonic collective modes in atomic liquids
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: July 2002

In an earlier paper we developed a collective motion description of atomic liquids, by expressing the quantum Hamiltonian in terms of rotated components of the density Fourier transform, which allowed the treatment of potential interaction terms without approximations. In the present paper, we further explore the physical content of our formalism and we show how it can be used for calculations in a real system.
Room-temperature lasing of InGaN multiquantum-well hexagonal microfacet lasers by current injection
Authors: Akasaka Tetsuya , Ando Seigo , Nishida Toshio , Saito Hisao , Kobayashi Naoki
Published: July 2002

InGaN multiquantum-well (MQW) hexagonal microfacet (HMF) lasers have been fabricated by electron cyclotron resonance dry etching of as-grown planar InGaN MQW laser structures followed by metalorganic vapor phase epitaxy regrowth of p-GaN to form mirror facets of the lasers. The mirror facets are smooth and vertical {112̄0} facets of the regrown p-GaN. The HMF lasers lase at 401 nm under pulsed operation at room temperature. There is no significant difference in threshold current density for different cavity lengths of 480 and 770 μm, indicating that the mirror facets act as total reflectors.
Void formation by thermal stress concentration at twin interfaces in Cu thin films
Authors: Sekiguchi A. , Koike J. , Kamiya S. , Saka M. , Maruyama K.
Published: July 2002

A void formation mechanism was investigated in an electroplated copper thin film on Ta/SiO2/Si. Microstructural observation after thermal cycling indicated that void formation occurred at intersecting points or terminating corners of annealing twins. The calculated stress distribution was compared with experimental results of the void formation tendency. An excellent correlation was found between void formation sites and stress concentration sites. Electron diffraction analysis revealed that most twin interfaces in Cu thin films are incoherent {322} planes. The stress concentration drives diffusion along incoherent twin interfaces of {322} and leads to void formation at twin interfaces and corners.
Beneficial effect of Gd substitution on magnetic properties of magnetically anisotropic SmCo5 ribbons
Authors: Zhang Wen-yong , Shen Bao-gen , Cheng Zhao-hua , Li Long , Zhou Yu-qing , Li Jian-qi
Published: July 2002

Magnetically anisotropic Sm1−xGdxCo5 (x=0, 0.2, 0.5) ribbons are produced by conventional melt spinning. The coercivity Hc increases from 5 kOe for x=0 to 11 kOe for x=0.5. Microstructure analyses show that this abnormal change of the coercivity is ascribed to the decrease of the quantity of the 2:7 phase, which serves as a center for a reverse domain. The remanence ratio also increases from 0.80 for x=0 to 0.95 for x=0.5. An energy product of 16 MGOe at room temperature for the Sm0.8Gd0.2Co5 ribbons has been achieved without the need of a complex heat treatment. The temperature characteristic of remanence is substantially improved due to the change of the microstructure arising from Gd substitution.
Amplification of transverse acoustic phonons in quantum well heterostructures with piezoelectric interaction
Authors: Komirenko S. M. , Kim K. W. , Demidenko A. A. , Kochelap V. A. , Stroscio M. A.
Published: July 2002

We have analyzed amplification of transverse phonons confined in quantum well (QW) heterostructures through piezoelectric electron–phonon interaction with drifting electrons. It was found that this mechanism of interaction couples the low-dimensional electrons and the shear-horizontal (SH) confined phonons. We have studied the electrostatic potential accompanying the SH waves and found that efficient interaction can be achieved for the lowest antisymmetric SH phonon branch in a narrow band of phonon frequencies. For AlGaAs QWs the amplification coefficient was calculated to be on the order of 100 cm−1 in the sub-THz phonon frequency range. These results suggest an electrical method for coherent phonon generation in the technologically well-developed AlGaAs QW heterostructures.
On the gravitomagnetic effects in cylindrically symmetric space–times
Authors: Herrera L. , Santos N. O.
Published: July 2002

Using gyroscopes we generalize results, obtained for the gravitomagnetic clock effect in the particular case when the exterior space–time is produced by a rotating dust cylinder, to the case when the vacuum space–time is described by the general cylindrically symmetric Lewis space–time. Results are contrasted with those obtained for the Kerr space–time.
Nanotweezers consisting of carbon nanotubes operating in an atomic force microscope
Authors: Akita Seiji , Nakayama Yoshikazu , Mizooka Syotaro , Takano Yuichi , Okawa Takashi , Miyatake Yu , Yamanaka Sigenori , Tsuji Masashi , Nosaka Toshikazu
Published: July 2002

We have developed nanotweezers consisting of carbon nanotubes that will operate in an atomic force microscope. The two nanotubes were attached on the metal electrodes patterned on a conventional Si tip and their fixations were made by carbon deposition. These processes were made under the view of a scanning electron microscope. The application of a dc voltage to the two nanotube arms induces their movement to approach each other. The numerical simulation by taking into account the balance between the electrostatic attraction and the bending moment of the nanotubes well explains the motion of the nanotube arms.
Small doped He3 clusters: A systematic quantum chemistry approach to fermionic nuclear wave functions and energies
Authors: Jungwirth Pavel , Krylov Anna I.
Published: July 2002

A novel approach for calculating nuclear wave functions and energies of He3 clusters doped with an atomic or molecular impurity is developed. It adopts the systematic and well developed methodology of quantum chemistry employing an analogy between electrons bound by Coulomb forces to the nuclei and fermionic He3 atoms clustered around a dopant species. The differences primarily concern the different shapes of the helium–helium and helium–impurity potentials and the larger mass of the He3 atom, as compared to electronic structure problems. A new integral evaluation procedure is outlined, as well as the necessary modifications to electronic structure codes. Tests against numerically exact calculations for Imp–3He (Imp=Ne, Ar, Kr, Xe, and SF6) complexes show that a modest set of 15 basis functions provides accurate and converged results. Calculations for the lowest triplet state of the SF6(3He)2 cluster, where fermionic statistics comes into play in the orbital part of the helium nuclear wave function, are presented. The triplet state is bound by 22 μhartree with respect to dissociation into He3+SF6–3He. The applicability of the new method to larger systems is discussed.
Analytical simulation of microwave reflectometry of a plasma cylinder
Authors: Bruskin L. G. , Mase A.
Published: July 2002

An analytical solution of a time-dependent two-dimensional (2D) full-wave equation is obtained for the case of microwave propagation in a plasma with axial symmetry. The 2D structure of the electromagnetic wave in a nonmagnetized plasma (or an ordinary wave in a magnetized plasma) is studied for the general case of arbitrary time dependence of the incident wave emitted by the horn. The solutions cover the cases of conventional microwave reflectometry as well as the ultrashort pulse reflectometry of fusion and processing plasma. Analytical expressions can be further applied to study plasma density fluctuations as well as effects of plasma curvature and multidimensionality. The 2D plasma cylinder model is deemed by the authors to be more realistic as compared to the one-dimensional plasma slab model previously employed in all the analytical and most numerical treatments since the plasma in the fusion toroidal devices, mirror machines, and plasma processing chambers can be considered axially symmetric on the scale relevant to microwave reflectometry.
A subgrid model for nonlinear functions of a scalar
Authors: Pantano C. , Sarkar S.
Published: July 2002

In applications of large eddy simulation of turbulent flows, subgrid models are often required for closure of strongly nonlinear functions of a scalar. The Arrhenius dependence of the reaction rate on temperature, T, the T4 dependence of radiation heat transfer, as well as the species mass fractions and temperature dependence on the mixture fraction in solutions of the strained laminar flamelet model are among some of the problems of interest. A moment-based reconstruction methodology is proposed here in which the scalar field is estimated by an approximate deconvolution operation but, unlike the usual deconvolution operation with given coefficients, the coefficients in the expansion are obtained by requiring that the statistical filtered moments of the scalar field up to a certain order are matched. The estimated scalar field is then used as a surrogate for the exact scalar field to directly calculate the subgrid contribution. Tests of the proposed approach are performed by using our direct numerical simulation database of scalar transport in a turbulent shear layer using two filter sizes: 12 points and 6 points per vorticity thickness. It is found that a simple moment-based model with one coefficient performs well for polynomial nonlinearities. The performance of the model in the case of an exponential Arrhenius-type nonlinearity is generally good and can be very good depending on the stoichiometric mixture fraction and the filter size.
Magnetic entropy change in Ni50.1Mn20.7Ga29.6 single crystal
Authors: Hu Feng-xia , Sun Ji-rong , Wu Guang-heng , Shen Bao-gen
Published: July 2002

Magnetic entropy change (ΔS) of a single crystal Ni50.1Mn20.7Ga29.6 alloy under different applied fields has been investigated near the martensitic–austenitic structural transition temperature of 219 K. With increasing applied field from 0.2 to 5 T, the sign of ΔS changes from positive to negative. At 0.8 T the positive ΔS reaches a maximum value of about 6.0 J/kg  K, and above 0.8 T the increase of applied field results in a negative ΔS. At 5 T ΔS reaches −6.0 J/kg K. The interesting behavior of ΔS is attributed to the first-order martensitic–austenitic structural transition at 219 K on heating. The phenomenon of the considerable magnetic entropy change under the relatively low applied field of 0.8 T and the easy adjustment of the martensitic–austenitic transition temperature indicate that Ni–Mn–Ga single crystal materials may have a possibility for practical applications as magnetic refrigerants.
Fast signal processing of a yttrium–aluminum–perovskite:Ce detector for synchrotron x-ray experiments
Authors: Harada Masaaki , Sakurai Kenji , Saitoh Kazuhiro , Kishimoto Shunji
Published: July 2002

An amplifier has been developed to form narrow pulses of less than 100 ns for a YAP:Ce scintillator, which appears promising as a detector for high-counting rate x-ray measurements. The performance of the detector system has been evaluated with monochromatic 8, 16.5, and 25 keV synchrotron x-ray photons at the Photon Factory. The whole deadtime obtained was 84 ns, which is around 3.5 times the decay time of the scintillation (25 ns), indicating that the present system is almost optimum. It has been found that the counting loss for 1 M counts/s is only 8%–9%, and that the detector can count extremely strong photons up to 5 M counts/s.
Electronics and signal processing for the multichord far-infrared polarimeter of the RFX experiment
Authors: Zilli E. , Milani F. , O’Gorman M. , Giudicotti L. , Prunty S. L.
Published: July 2002

This article describes the realization and testing of the electronic system which forms part of the multichannel far-infrared (FIR) polarimeter for the RFX machine, a plasma confinement experiment with Reversed Field Pinch (RFP) configuration. The electronic system, which comprises the detectors, the signal-processing electronics, and the motion electronics for the half-wave plate movement, is described. Emphasis is placed in the analysis of the polarimeter signals, which permits an in-depth understanding of the performance of the data processing electronics and the role of the various sources of noise in the system. After a brief outline of the basic principle of the measurement, the choice of detectors and their characteristics are described in order to achieve the best performances at the FIR wavelength (λ=118.8 μm) of interest. Various tests, which are described, confirmed the need for a specifically designed pyroelectric detector capable of operating in the hostile magnetic environment near the machine. The processing of the raw polarimeter signals to produce the required sum and difference signals and to convert them into dc signals with 3 ms time constant is presented. These signals are synchronous with a chopper signal on the FIR beam and are subsequently fed to a lock-in amplifier. An accurate analysis of the data processing procedure is described, which helps to clarify the understanding of the output signals that are eventually recorded in the data acquisition system. In particular, various sources of noise, such as thermal noise of the detectors, laser fluctuations, spurious signals at harmonics of the chopper frequency, and phase jitter of the chopper, are evaluated, discussed, and compared with the observed signals. Finally, the control circuitry for the movement of the half-wave plates, both for manual control and for the programmed sequences of zero-search and calibration performed by a PLC control system, is described. Calibration curves obtained during experiments are also reported.
Nonintegrable reductions of the self-dual Yang–Mills equations in a metric of plane wave type
Authors: Kapadia Devendra A.
Published: July 2002

Symmetry reductions of the self-dual Yang–Mills equations for SL(2,C) bundles with the background metric ds2=2 du dv−dx2+f2(u)dy2 are considered. One of the field components in the reduced equations can be cast into Jordan normal form after gauge transformations. The reduced equations for the two possible normal forms are equivalent, respectively, to certain generalizations of the Korteweg–de Vries (KdV) equation and the nonlinear Schrödinger (NLS) equation. It is shown that the generalized KdV and NLS equations fail the Painlevé test except when the metric is flat. The generalized KdV equation is transformed to a simple form in the case when f(u)=ua and it is shown that one may obtain either the KdV equation or the cylindrical KdV equation by this method only when the metric is flat.
Right-angle wiggling electron paramagnetic resonance spectroscopy
Authors: Eichel Rüdiger-A. , Schweiger Arthur
Published: July 2002

Right-angle wiggling (RAW) electron paramagnetic resonance (EPR) spectroscopy is introduced. In this two-dimensional pulse EPR technique the orientation of the external magnetic field B0 is varied during the pulse sequence. For this purpose an additional sinusoidal field of variable amplitude ΔB0⊥ is applied perpendicular to the orientation of B0. RAW-EPR is an alternative and experimentally much less demanding technique to right-angle spinning for performing anisotropy-resolved EPR experiments [Sierra and Schweiger, Mol. Phys. 95, 973 (1998)]. It can be applied to improve the resolution of EPR spectra of orientationally disordered systems or to separate overlapping single-crystal EPR spectra, and to facilitate an unambiguous interpretation of these spectra. A detailed theoretical description of RAW-EPR is given. Model calculations for systems with axial and orthorhombic symmetry, as well as a number of RAW-EPR experiments on paramagnetic systems with anisotropic interactions are presented.
Distance and entropy for density matrices
Authors: Harriman John E.
Published: July 2002

Density matrices for systems with a finite number of states are considered as elements in a vector space. A density matrix that is a convex combination of density matrices that are unitary transformations of some initial density matrix will be closer to, or possibly at the same distance from, the density matrix for the ensemble in which all states are equally occupied, compared with the initial density matrix. This distance is correlated, for few-state systems, with a function simply related to the entropy. This function increases (entropy decreases) with distance from the equal-occupancy density matrix. Other properties of the function are also established.
Random growth statistics of long-chain single molecule poly-(p-phenylene vinylene)
Authors: Claudio Gil C. , Bittner Eric R.
Published: July 2002

Structures of poly-(p-phenylene vinylene) (PPV) were generated using a random growth algorithm. The algorithm assumes that the configuration of a part of the polymer (a few monomers length) can be sampled from the configuration of a PPV trimer. The probabilities of the configurations of the trimer are taken as the Boltzmann weight of the energies. We constructed several types of polymers with different numbers of cis-defects which were added to the polymer either uniformly or randomly distributed within the entire polymer. Polymer characteristics, such as conjugation length, end-to-end distance, and radius of gyration, were also calculated. The trends of these characteristics were found to be inversely proportional with the number of cis-defects in the polymer. Although average conjugation lengths are generally independent of the distribution of cis-defects, the morphology of the polymer is dependent on cis-defect distribution. This suggests that conformational disorder rather than cis-defect density is the determining factor in exciton localization and diffusion in these systems. Finally, we derive a simple model similar to the Ising model that relates the energy needed to break conjugation to the average conjugation length.
Concurrent x-ray diffractometer for high throughput structural diagnosis of epitaxial thin films
Authors: Ohtani M. , Fukumura T. , Kawasaki M. , Omote K. , Kikuchi T. , Harada J. , Ohtomo A. , Lippmaa M. , Ohnishi T. , Komiyama D. , Takahashi R. , Matsumoto Y. , Koinuma H.
Published: July 2002

We have developed a concurrent x-ray diffractometer that concurrently measures spatially resolved x-ray diffraction (XRD) spectra of epitaxial thin films integrated on a substrate. A convergent x-ray is focused into stripe on a substrate and the diffracted beam is detected with a two-dimensional x-ray detector. The obtained snapshot image represents a mapping of XRD intensity with the axes of the diffraction angle and the position in the sample. In addition to the parallel XRD measurements of thin films with various compositions and structures, two-dimensional spatial mapping of XRD peak with a resolution of ∼100 μm is demonstrated. This technique will provide us a high throughput characterization method of various devices composed of epitaxial films.