bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of ferromagnetic transition under pressure by a commercial Hall device
Authors: Markovich V. , Gorodetsky G. , Shtrikman S.
Published: July 2002

It is demonstrated that Hall devices can be successfully utilized for measurements of ferromagnetic transition temperatures under hydrostatic pressure. Results observed for ferromagnetic single crystal La0.94Mn0.98O3 and ceramic manganite La0.84Zn0.07Mn0.95O3 at pressures up to 0.94 GPa are reported.
Experimental and theoretical investigation of the rotational structure of the Al–H2/D2 complex
Authors: Tan Xiaofeng , Dagdigian Paul J. , Williams Jason , Alexander Millard H.
Published: July 2002

The rotational structure of the Al–H2/D2 complex is investigated in a collaborative experimental and theoretical study. The isotopomeric complexes were prepared in a pulsed supersonic beam, and their 3d←3p and 4p←3d electronic transitions recorded through laser fluorescence excitation spectroscopy. Transitions to quasibound excited vibronic levels were observed by monitoring emission from lower excited Al atomic levels, formed by nonradiative decay of the excited complex. In some bands, the Lorentzian width was sufficiently narrow that resolved rotational lines were observed. Rotational analysis of several bands which typify the two different patterns of observed rotational structure is presented. The derived rotational constants and parity splitting parameters for the ground Al(3p)–oH2/pD2 bend-stretch levels were compared with constants computed from fits to J- and parity-dependent energies calculated with new Al(3p)–H2 potential energy surfaces (PESs), which extend those recently reported by Williams and Alexander [J. Chem. Phys. 112, 5722 (2000)] by inclusion of the dependence on the H2 bond distance. The experimental and computed rotational constants were found to be in very good agreement. This provides strong support for the reliability of the calculated PESs.
Systematic treatment of relativistic effects accurate through arbitrarily high order in α2
Authors: Barysz Maria
Published: July 2002

A systematic method for the generation of two-component relativistic Hamiltonians for use in relativistic quantum chemistry is presented and discussed. The free particle Foldy–Wouthuysen transformation of the Dirac Hamiltonian is performed prior to the determination of the block-diagonalizing unitary transformation. The latter can be determined iteratively through arbitrarily high leading order with respect to α (fine structure constant). Certain freedom in the initialization of the iterative solution leads to the whole class of two-component Hamiltonians h2k which are exact through the order of α2k and differ in contributions of all higher orders in α2. The efficiency of different iterative schemes is analyzed. Also the relation between the present method and the Douglas–Kroll approximation is investigated. The performance of two-component Hamiltonians for k=2, 3, and 4 is studied numerically in calculations of energies of the 1s1/2 level in heavy hydrogen-like ions. Their performance in calculations of the valence-determined atomic and molecular properties is investigated by computing the ionization potential of Au and spectroscopic constants of the AuH molecule. The total energy of these systems strongly depends on the level of exactness with respect to α2. However, for moderately relativistic systems the α4-class Hamiltonian derived in this paper is found to be sufficient for accurate calculations of valence-determined properties.
Two-dimensional spatial distribution of electric-discharge plasma around a frictional interface between dielectric surfaces
Authors: Miura Takashi , Nakayama Keiji
Published: July 2002

Electric-discharge plasma and thermal radiation were directly observed in measuring two-dimensional spatial distributions of photon emission during friction between a diamond spherical surface and crystalline Al2O3 surface in ambient air. By using spectral filters, a clear difference was observed between the spatial distribution of the radiation from the plasma about the frictional point and that of the thermal radiation. This report elucidates the spatial distribution of the electric-discharge plasma on the interface between dielectric surfaces during friction, which then allows one to estimate the potential difference between the two dielectric surfaces at the discharge sites.
Biexciton emission from ZnO/Zn0.74Mg0.26O multiquantum wells
Authors: Sun H. D. , Makino T. , Segawa Y. , Kawasaki M. , Ohtomo A. , Tamura K. , Koinuma H.
Published: July 2002

Luminescence due to the radiative recombination of localized biexcitons has been observed at low temperature (5 K) in ZnO/Zn0.74Mg0.26O multiquantum wells grown by laser-molecular-beam epilaxy on a lattice-matched ScAlMgO4 substrate (0001). The emission components due to the recombination of localized excitons and biexcitons and due to the exciton–exciton scattering were verified by examining their relative energy positions and intensity dependence on excitation power density. The excitation threshold for biexciton emission was significantly lower than that for exciton–exciton scattering. The binding energy of biexcitons in multi-quantum wells is largely enhanced by quantum confinement effect.
Strings in AdS3 and the SL(2,R) WZW model. II: Euclidean black hole
Authors: Maldacena Juan , Ooguri Hirosi , Son John
Published: July 2002

We consider the one-loop partition function for Euclidean BTZ black hole back-grounds or equivalently thermal AdS3 backgrounds which are quotients of H3 (Euclidean AdS3). The one-loop partition function is modular invariant and we can read off the spectrum which is consistent to that found in hep-th/0001053. We see long strings and discrete states in agreement with the expectations.
Abnormal ferroelectric properties of compositionally graded Pb(Zr,Ti)O3 thin films with LaNiO3 bottom electrodes
Authors: Bao Dinghua , Wakiya Naoki , Shinozaki Kazuo , Mizutani Nobuyasu , Yao Xi
Published: July 2002

We report abnormal ferroelectric properties of compositionally graded Pb(Zr,Ti)O3 thin films on LaNiO3-coated SiO2/Si substrates, where Pb(Zr,Ti)O3 and LaNiO3 films were prepared by a metalorganic decomposition technique and a sol–gel technique, respectively. It was found that the hysteresis loops of the Pb(Zr,Ti)O3 graded films measured by the conventional Sawyer–Tower method shifted along the polarization axis, i.e., they showed polarization offsets when applied by an alternating electric field. The polarization offsets were 82.5 μC/cm2 at 270 kV/cm and 62.5 μC/cm2 at 185 kV/cm for the up-graded film and the down-graded film, respectively. The absolute magnitude of the polarization offsets was closely related to the magnitude of the driving electric field, and the direction of the polarization offsets depended on the direction of the composition gradient with respect to the substrate. Analysis indicated that the polarization offsets did not originate from asymmetric contact effects, oxygen vacancies, alignment of defect dipoles, and/or electron injection. These results showed that polarization offsets in hysteresis loops were the intrinsic characteristic of the compositionally graded Pb(Zr,Ti)O3 thin films.
An analysis and implementation of a general coupled cluster approach to excitation energies with application to the B2 molecule
Authors: Hald Kasper , Jørgensen Poul , Olsen Jeppe , Jaszuński Michał
Published: July 2002

A general scheme is presented for the calculation of excitation energies using the standard coupled cluster hierarchy and a simple implementation is described for the higher standard models. An error analysis is performed to find to what order excitation energies in different coupled cluster models are correct. The analysis includes both the standard coupled cluster hierarchy as well as the approximate models and considers excitations to states that are dominated by one, two, and three electron replacements compared to the reference state. Calculations are presented up to the quadruple excitation level for the open shell B2 molecule using an excited closed shell state as reference state to emphasize the usefulness of the order analysis. The coupled cluster excitation energies are compared to full configuration interaction results.
Measurement of the transfer function of the steering filter of the Virgo super attenuator suspension
Authors: Ballardin G. , Braccini S. , Bradaschia C. , Casciano C. , Cavalieri R. , Cecchi R. , Chickarmane V. , Dattilo V. , Di Virgilio A. , Ferrante I. , Fidecaro F. , Frasconi F. , Gaddi A. , Gennai A. , Giazotto A. , Holloway L. , Lomtazde T. , Paoletti F. , Pasqualetti A. , Passaquieti R. , Passuello D. , Poggiani R. , Taddei R. , Viceré A. , Zhang Z. , Cuoco E. , Losurdo G. , Ni Wei-Tou , Wu Jeah-Sheng , Chang Chun-Hsiung
Published: July 2002

The optical elements of the Virgo antenna are supported and isolated from seismic noise disturbances by super attenuator (SA) suspensions. The steering filter (SF) is a component of the SA, designed for the mirror control. The dynamical properties of the SF are described by transfer functions, which have been measured in order to define the control strategy; the results have made it possible to set and tune the parameters of a simulation of the SA. The measuring devices were linear voltage differential transducers: they were found to be quite effective and flexible in usage. An auto-regressive model was used to fit the experimental data, implementing the linear relation between the input forces and the resulting motion. The ability of the model to reproduce the experimental behavior was a clear indication of the good data quality, showing that the contaminating noise was under control. The simulation was able to reproduce the qualitative behavior, and the simulation parameters were estimated, with 10% and 20% accuracy. The estimations were later found to be consistent with the measurements taken on a full SA equipped with the same steering filter. Unknown spectral structures were found only above 20 Hz.
Green’s function calculation of electron spin polarization. IV. Radical pairs in micelles
Authors: Neufeld A. A. , Doktorov A. B. , Pedersen J. Boiden
Published: July 2002

An analytic expression is derived for the electron spin polarization (CIDEP) of neutral radicals pairs in micellar solutions at high magnetic fields. It is valid for arbitrary reactivities of the singlet and triplet states of the radical pair complex, additional dephasing processes at the contact distance, and arbitrary values of the singlet-triplet mixing and the micellar parameters. The expression is obtained from our previously derived general formula by explicitly evaluating all the fundamental quantities, that are expressed in terms of the Green’s function of the diffusive motion in a micelle and which apparently has not been given before. The exchange interaction is assumed to decrease exponentially with the separation between the radicals and to be characterized by a decay length lx and a maximum value J0. The micelle is modeled by a potential well. The only condition for the applicability of the expression is that the exchange interaction is locally weak, i.e., (J0lx2/Dm)2≪1, but it may be integrally strong; Dm is the relative diffusion constant for the radicals in the micelle. A comparison with numerical results showed that the expression is very accurate, no deviations were found.
Accurate ab initio study on the spectroscopy of Ag and Ag+ including spin-orbit couplings aimed at molecular calculations
Authors: Ramírez-Solís A. , Vallet V. , Teichteil Ch. , Leininger T. , Daudey J. P.
Published: July 2002

Very accurate ab initio electronic + spin-orbit calculations of the lowest-lying states of the Ag atom and Ag+ cation have been performed through the CASSCF + ACPF + EPCISO method, using the Stuttgart small-core (19 active electrons) relativistic effective core potential (RECP) as well as its associated 2D spin-orbit effective potential. An ad hoc spin-orbit P-symmetry pseudopotential for the 2P state adapted to this 19-e RECP and basis set was extracted. The Stuttgart basis set was augmented to a large valence Gaussian basis set (8s8p7d3f3g/6s6p4d3f3g) in order to reproduce at best the experimental 2S-2D and 2S-2P transition energies as well as the ionization potential (IP) of Ag, which play a crucial role for the accurate description of the spectroscopy in silver-containing molecular systems. A detailed discussion on the multiple schemes used to deal with the differential d10 vs d9 electronic correlation for these two excited states is given. The role of the 4s and 4p (core) shells on the 2S-2D and 2S-2P transition energies and the IP is carefully studied and discussed. The core–core correlation is found to play a minor role while an insufficient treatment of the core-valence electronic correlation is responsible for the main differential d10 vs d9 correlation energy error between the 2S-2D and 2S-2P transition energies. For the neutral atom, the 2D5/2-2D3/2 and 2P3/2-2P1/2 splittings are in excellent agreement with the experimental ones. However, the relative calculated energetic ordering for the 2D5/2,2D3/2,2P3/2, and 2P1/2 fine structure components is critically dependent on the J-averaged purely electronic ACPF 2P and 2D energies of the parent states. The 3D fine-structure splitting for the ion is also found in good agreement with the experiment.
A multireference valence bond approach to electronic excited states
Authors: Martinazzo Rocco , Famulari Antonino , Raimondi Mario , Bodo Enrico , Gianturco Franco A.
Published: July 2002

A new spin coupled multireference balence bond scheme for the treatment of electronic excited states is presented. The method involves the specific optimization of excited spin coupled wave functions which act as reference functions and are used to introduce correlation corrections directly in the excited state of interest. The variational optimization of such excited wave functions is achieved by using simple orthogonality constraints on the spin coupled orbitals and it preserves all the appealing features of the valence bond method. The applications to the lower lying states of [LiH2]+ and BeH show the advantages and some of the limitations of this new approach.
An insoluble surfactant model for a vertical draining free film with variable surface viscosity
Authors: Naire S. , Braun R. J. , Snow S. A.
Published: July 2002

A mathematical model is constructed to study the evolution of a vertically oriented, thin, free liquid film draining under gravity when there is an insoluble surfactant, with finite variable surface viscosity, on its free surface. Lubrication theory for this free film results in three coupled nonlinear partial differential equations describing the free surface shape, the surface velocity and the surfactant transport, at leading order. In the limit of large surface viscosity and the Marangoni effect, the evolution of the free surface is that of a rigid film. For mobile films with small surface viscosity, transition from a mobile to an essentially immobile film is observed for large Marangoni effects. It is also verified that stable aqueous films can be formed in the regime of high surfactant concentrations. The theoretical results are compared with experiment; the purpose of both is to act as a model problem to evaluate the effectiveness of surfactants for potential use in foam-fabrication processes.
Charge carrier transport and recombination at the interface between disordered organic dielectrics
Authors: Arkhipov V. I. , Emelianova E. V. , Bässler H.
Published: July 2002

An analytic model of charge carrier hopping at the interface separating two disordered organic materials in organic light-emitting diodes (OLEDs) is formulated. It is shown that the rate of charge carrier injection across such an interface is much lower than the rate of first interfacial jumps, most of which are followed by return carrier jumps back into initially occupied sites. In OLEDs, the rate of injection across the interface determines the leakage current while the rate of radiative recombination is proportional to the first-jump rate. Therefore, the marked difference between the injection rate and the rate of first carrier jumps across the interface implies a much higher efficiency of carrier recombination at the interface than predicted by the conventional Langevin theory of recombination.
r-axis sound speed and elastic properties of sapphire single crystals
Authors: Winey J. M. , Gupta Y. M. , Hare D. E.
Published: July 2002

The sound speed was measured for plane wave propagation along the normal to the (11̄02) plane of sapphire single crystals. The measured r-axis sound speed is significantly different (∼7%) from measured c- and a-axis sound speeds. These results contradict the previous assumption that the mechanical response of sapphire is nearly isotropic. Also, the r axis is not a pure mode direction for longitudinal acoustic wave propagation. The measured r-axis sound speed was not consistent with calculations using the elastic constants reported for sapphire. This discrepancy arises because the elastic constants determined previously used crystallographic conventions different from those in current use. By rotating the coordinate system used for describing the material properties of sapphire in earlier work, consistency is achieved with the current crystallographic conventions. The effect of this coordinate transformation on the second-order elastic constants, the third-order elastic constants, and the photoelastic constants of sapphire is discussed.
Combination of a Besocke-type scanning tunneling microscope with a scanning electron microscope
Authors: Emundts A. , Coenen P. , Pirug G. , Voigtländer B. , Bonzel H. P. , Wynblatt P.
Published: July 2002

The article describes the combination of a Besocke-type scanning tunneling microscope (STM) with a scanning electron microscope (SEM) in an ultrahigh vacuum (UHV) environment. The open design of the Besocke STM allows the SEM to be implemented as an add-on of a high resolution electron column and a secondary electron detector. The combined instrument is capable of atomic resolution imaging by STM and real time SEM imaging. SEM resolution down to about 80 nm was achieved. Simultaneous operation of STM and SEM is possible. The operation and performance of the combined instrument is illustrated by a variety of examples. Although the instrument is suitable for a wide range of applications where a combination of atomic resolution with lower magnification imaging is required, its operation in an UHV environment makes it particularly appropriate for the study of reactive metal surfaces.
Advantage of anti-Stokes Raman scattering for high-temperature measurements
Authors: Fujimori Hirotaka , Kakihana Masato , Ioku Koji , Goto Seishi , Yoshimura Masahiro
Published: July 2002

We present the results of experiments that assess the viability of anti-Stokes scattering to investigate in situ materials at high temperatures. Both anti-Stokes and Stokes Raman measurements have been performed at various high temperatures using hafnia as a test material. As compared with Stokes Raman spectra, anti-Stokes spectra were observed with lower thermal emission backgrounds in accordance with Planck’s equation. The intensity ratio of anti-Stokes to Stokes scattering approaches 1 as the temperature increases at high temperatures satisfying the Boltzmann distribution law. These results clearly demonstrate the advantage and feasibility of anti-Stokes Raman scattering for the elimination of the thermal emission in comparison with Stokes scattering.
Superconducting Nb-film LC resonator
Authors: Finne A. , Grönberg L. , Blaauwgeers R. , Eltsov V. B. , Eska G. , Krusius M. , Ruohio J. J. , Schanen R. , Suni I.
Published: July 2002

Sputtered Nb thin-film LC resonators for low frequencies at 0.5 MHz have been fabricated and tested in the temperature range 0.05–1 K in magnetic fields up to 30 mT. Their Q value increases towards decreasing temperature as Q∝T−0.5 and reaches ∼103 at 0.05 K. As a function of magnetic field Q is unstable and displays variations up to 50%, which are reproducible from one field sweep to the next. These instabilities are attributed to dielectric losses in the plasma deposited SiO2 insulation layer, since the thin-film coil alone reaches a Q≳105 at 0.05 K.
Estimating bias error distributions
Authors: Liu Tianshu , Finley Tom D.
Published: July 2002

In this article we formulate a general methodology for estimating the bias error distribution of a device in a measurement domain from less accurate measurements when a minimal number of standard values (typically no more than two values) are available. A new perspective is that the bias error distribution can be found as a solution of an intrinsic functional equation in a domain. Based on this theory, the scaling- and translation-based methods for determining the bias error distribution are developed. These methods are applicable to virtually any device as long as the bias error distribution of the device can be sufficiently described by a power series (a polynomial) or a Fourier series in a domain. These methods were validated through computational simulations and laboratory calibration experiments for a number of different devices.
Photodetachment of F−(H2O)n (n=1–4): Observation of charge-transfer states [F−(H2O)n+] and the transition state of F+H2O hydrogen abstraction reaction
Authors: Yang Xin , Wang Xue-Bin , Wang Lai-Sheng
Published: July 2002

We report a photoelectron spectroscopic study of F−(H2O)n (n=1–4) at 193 and 157 nm. In addition to a detachment feature from F−, we observed a higher binding energy feature in all spectra due to ionization of the solvent molecules leading to F−(H2O)n+ charge-transfer states. Detachment of an electron from F− in F−(H2O) reaches the transition state region of the reaction, F+H2O→HF+OH. The bound to non-bound transition resulted in a long tail at the low binding energy side in the photoelectron spectrum of F−(H2O). We estimated that the neutral F⋯H2O complex at the anion geometry, which is near the transition state region of the H-abstraction reaction, is 6±1 kcal/mol above the F+H2O asymptote, consistent with a previous estimation of a 4 kcal/mol entrance barrier height for this reaction.
Multidimensional motion measurement of a bimorph-type piezoelectric actuator using a diffraction grating target
Authors: Kim Jong-Ahn , Bae Eui Won , Kim Soo Hyun , Kwak Yoon Keun
Published: July 2002

Precision actuators, such as pick-up actuators for HDDs or CD-ROMs, mostly show multidimensional motion. So, to evaluate them more completely, multidimensional measurement is required. Through structural variation and optimization of the design index, the performance of a measurement system can be improved to satisfy the requirement of this application, and so the resolution of each axis is higher than 0.1 μm for translation and 0.5 arcsec for rotation. Using this measurement system, the multidimensional motion and frequency transfer functions of a bimorph-type piezoelectric actuator are obtained.
Deviations from ideal behavior in isotopic mixtures of ammonia
Authors: Lopes J. N. Canongia , Rebelo L. P. N. , Calado J. C. G.
Published: July 2002

The vapor pressure isotope effect between samples of isotopically substituted ammonia, NH3, ND3 and NH315, and their mixtures was measured as a function of temperature (225<T/K<259) and mixture composition. The p(x,T) results were used to calculate the excess molar Gibbs energy function of the mixtures. The GE(x,T) data were fitted to a temperature dependent Redlich–Kister equation and compared with theoretical calculations based on a modified version of the theory of isotope effects in mixtures. The theoretical estimates are in good agreement with the experimental data, confirming the mechanisms responsible for the deviations from ideality in such “almost ideal” systems.
Equilibrium thermodynamic theory for the evaluation of temperature distributions in overdriven steady-plane wave fronts
Authors: Sano Tomokazu , Sano Yukio
Published: July 2002

Temperature distributions in overdriven steady wave fronts in solid 2024 Al shocked up to 80 GPa, in which no melting occurred in the wave fronts, were evaluated using the equilibrium thermodynamic theory. The effective strain increments were sufficiently small and the effective temperature rise times were sufficiently long with respect to electron–phonon relaxation times to justify using equilibrium thermodynamics. In addition, the sufficiently large viscous-stress components supported the efficacy of the thermodynamic theory. The same is true of shocks up to 250 GPa in solid Pt and 230 GPa in solid Fe. Furthermore, the influence of viscous stress was examined by evaluating the temperature distributions for inviscid 2024 Al, Pt, and Fe solids using the equations for temperature derived from the Mie–Grüneisen equation. Finally, we demonstrate that there might be a solid–liquid–solid Hugoniot between the solid and liquid–solid Hugoniots for Fe and estimate the solid–liquid–solid and liquid–solid Hugoniots.
Energetics and cooperativity in three-center hydrogen bonding interactions. II. Intramolecular hydrogen bonding systems
Authors: Parra Rubén D. , Gong Bing , Zeng X. C.
Published: July 2002

High level ab initio calculations have been carried out on a number of intramolecular two- and three-center H-bonding structures containing the amide motif, NHC=O. These structures serve as model systems for probing the stability of intramolecular three-center H bonds of the type A1HA2. Particular attention is given to cooperative effects in the three-center H-bonding structures. A systematic assessment of cooperativity for intramolecular H-bonding systems is presented.
Third-order nonlinear optical properties of fractal- and nonfractal-structured oligomers modeled after dendron parts in Cayley-tree-type dendrimers
Authors: Nakano Masayoshi , Fujita Harunori , Takahata Masahiro , Yamaguchi Kizashi
Published: July 2002

As an extension of our previous study on polarizabilities [J. Chem. Phys. 115, 1052 (2001)], we investigate the molecular third-order nonlinear optical properties, i.e., the second hyperpolarizabilities (γ), of three types of model oligomers (made of phenylene vinylenes), i.e., para-oligomers and meta-oligomers with fractal and nonfractal structures, which are considered to be models of dendron parts involved in Cayley-tree-type dendrimers. Similarly to the polarizability case, the chain-length dependence of γ of para-oligomers is found to be much larger than that of nonfractal meta-oligomers, while fractal meta-oligomers exhibit an intermediate chain-length dependency of γ between them. The spatial contributions of π-electrons to γ of these systems are elucidated using hyperpolarizability density analysis. It is found that the spatial contributions of π-electrons to γ of nonfractal meta-oligomers are much smaller than those of para-oligomers and are well decoupled at the meta-substituted benzene rings, while the fractal meta-oligomers show significantly large spatial contributions of π-electrons to γ in their linear-leg (para-substituted phenylene vinylene) regions similarly to the case of para-oligomers although the distributions are also well decoupled at the meta-substituted benzene rings. This feature supports the distinct features of chain-length dependency of γ for these oligomers.
Quantitative diode laser absorption spectroscopy near 2 μm with high precision measurements of CO2 concentration
Authors: Gagliardi G. , Restieri R. , De Biasio G. , De Natale P. , Cotrufo F. , Gianfrani L.
Published: July 2002

A diode laser sensor was developed to measure CO2 concentration using absorption spectroscopy at 2 μm. The simplicity of the optical setup and detection electronics makes this spectrometer very compact and well suited for field studies, while the use of optical fibers enables one to probe remote environments. This sensor is capable of providing absolute measurements, with the concentration being inferred from the directly measured absorption spectra, without using calibration cells. This ensures versatility as well as widespread applicability, and a linear response over a large dynamic range. Its analytical capabilities were carefully tested by a large variety of measurements, also with use of a certified CO2/air mixture. The precision of mixing ratio retrievals was about 0.2% while the accuracy was 0.9% for measurements of ambient CO2 concentration. The spectrometer is also capable of providing high precision measurements of the water vapor concentration in ambient air. Specifically, simultaneous detection of both molecular species is demonstrated. Although the performance of our spectrometer can be improved with some effort, it is already competitive with commercially available infrared gas analyzers for atmospheric monitoring applications. In addition, it is an attractive tool for continuous measurements of volcanic gas concentrations.
InAs/GaSb-based lateral current injection laser
Authors: Laikhtman B. , Luryi S. , Belenky G.
Published: July 2002

We propose a contact structure for InAs/GaSb multilayers, such that electrons and holes are selectively injected in alternating layers. This enables the implementation of a far-infrared lateral current injection laser based on the InAs/GaSb superlattice. Preliminary calculation of the gain shows that both surface- and edge-emitting designs are possible.
Grain boundary ridge on sintered bonds between ice crystals
Authors: Adams E. E. , Miller D. A. , Brown R. L.
Published: July 2002

Well-sintered snow examined in a scanning electron microscope revealed a newly observed morphological structure that protrudes into the pore space along ice grain boundaries. We have termed this a “grain boundary ridge.” Grain boundary diffusion is a sintering process that occurs at the interface of two crystals, whereby mass migrates from the center of the contact to the surface of the bond. Since mass tends to sublimate from sharp features toward smaller curvature surfaces through vapor diffusion, a ridge developed by grain boundary diffusion will readily sacrifice mass to the surrounding ice surfaces. A mass balance between vapor and grain boundary diffusion based on the observed geometry is considered. This analysis indicates grain boundary diffusion may play a far more significant role than generally acknowledged. While this study was restricted to ice, it may have implications for other crystalline materials.
Numerical modeling of initially turbulent wakes with net momentum
Authors: Gourlay Michael J. , Arendt S. C. , Fritts D. C. , Werne J.
Published: July 2002

This paper presents results from the first fully three-dimensional direct numerical simulations of initially turbulent wakes with net momentum in unstratified and density stratified fluids. The initial conditions contain a super-position of an initially axisymmetric mean streamwise velocity profile plus a spectrally specified fluctuation velocity field with initially incoherent phases to model initial turbulence. To provide evidence in favor of their validity, we compare results from these simulations with previous measurements behind towed bodies in wind tunnels and towing tanks, and also compare with theories of turbulent wakes. Comparisons with laboratory flow experiments provide agreement, both with statistical quantities and vortex structures and evolution. We subsequently investigate open questions by analysis of the fully three-dimensional flow. Coherent vortices in stratified wakes have their origins in the vortex geometry of the mean wake flow, and do not require stratification or coherent seeding in the initial velocity fluctuations. We conclude that the simulations provide a trustworthy and valuable complement to wake research, and that the vortex structures result from a combination of the necessity that vortices form loops and diffusion of vorticity to smooth the loops into rings.
Room-temperature 1.54 μm electroluminescence from Er-doped silicon-rich silicon oxide films deposited on n+-Si substrates by magnetron sputtering
Authors: Ran G. Z. , Chen Y. , Qin W. C. , Fu J. S. , Ma Z. C. , Zong W. H. , Lu H. , Qin J. , Qin G. G.
Published: July 2002

Room-temperature 1.54 μm electroluminescence (EL) was compared for Au/SiO2:Er/n+-Si and Au/SiOx:Si:Er/n+-Si diodes under reverse bias. The 18 nm thick SiO2:Er and SiOx:Si:Er films were deposited by the magnetron sputtering technique. The maximum of the EL intensity was reached after annealing at 900 °C (SiO2:Er) and 800 °C (SiOx:Si:Er). The threshold potential of the EL was about 4 and 6 V for the Au/SiOx:Si:Er/n+-Si and Au/SiO2:Er/n+-Si diodes, respectively. The power efficiency was larger for the Au/SiOx:Si:Er/n+-Si diode than that for the Au/SiO2:Er/n+-Si diode by six times. Our experimental results demonstrate that the existence of Si nanoclusters reduces the threshold potentials of the EL and strongly enhances the power efficiency for Er3+ EL.
Effect of frequency in the deposition of microcrystalline silicon from silane discharges
Authors: Amanatides E. , Mataras D. , Rapakoulias D. E.
Published: July 2002

The influence of frequency in the range from 13.56 to 50 MHz, on the properties of 2% silane in hydrogen 0.5 Torr discharges used for the deposition of microcrystalline silicon thin films, has been investigated. The experiments were carried out under constant power conditions as determined through Fourier transform voltage and current measurements. The increase of frequency leads to a decrease of the rf field, an extension of the bulk, and a marked increase of the electron density and the amount of power consumed by electrons. These changes induce a decrease of the rate of high-energy electron–molecule collision processes (>10.5 eV) at higher frequencies and an enhancement of lower energy processes. Thus, there is a significant increase in the hydrogen flux toward surfaces, which can explain the beneficial effect of frequency to the crystallinity of μc-Si:H thin films. At the same time, SiH4 electron impact dissociation is enhanced mainly due to the increase of electron density. On the contrary, ionization is not favored by the increase of frequency and the calculated ion flux toward the film surface indicates that the role of ions in a possible enhancement of the surface mobility of the film precursors is minor. The observed increase of the deposition rate is further discussed in terms of the nature of the film precursors and the spatial distribution of their production.
Growth and electrical transport of germanium nanowires
Authors: Gu G. , Burghard M. , Kim G. T. , Düsberg G. S. , Chiu P. W. , Krstic V. , Roth S. , Han W. Q.
Published: July 2002

Single crystalline germanium nanowires have been synthesized from gold nanoparticles based on a vapor–liquid–solid growth mechanism. Germanium powder was evaporated at 950 °C, and deposited onto gold nanoparticles at 500 °C using argon as a carrier gas. The diameter of the germanium nanowires ranged from 20 to 180 nm when gold thin films were utilized as the substrate, while the nanowires grown from 10 nm Au particles showed a narrower diameter distribution centered at 28 nm. The growth direction of germanium nanowires is along the [111] direction, determined by high resolution transmission electron microscopy. Transport measurements on individual Ge nanowires indicate that the wires are heavily doped during growth and that transport data can be explained by the thermal fluctuation tunneling conduction model.
Diffusion coefficient of the order parameter in the early stages of the time evolution of InGaN phase separation
Authors: Okumura Toshiyuki , Akagi Yoshiro
Published: July 2002

We have carried out a study involving a model for the diffusion mechanism of group III atoms in an InGaN alloy semiconductor that uses a cell dynamical system (CDS) approach. In this CDS analysis, we focused on gathering information concerning the In-rich region with a dot-like structure that is formed at an early stage in the time evolution of phase separation of this alloy. A similar dot-like structure is known to appear in the active layer of InGaN-based multiquantum well laser diodes. Thus, we can determine the diffusion coefficient of group III atoms in InGaN by comparing the dot-like structures observed by cross-sectional transmission electron microscopy with those obtained by the CDS numerical approach. In order to perform this comparison quantitatively such that we could use it to calculate the diffusion coefficient, we derived a relationship between the rule of mapping used in CDS, and the discretization of the Cahn–Hilliard–Cook equation, which describes the dynamics of phase separation with a conserved order parameter. The value obtained for the diffusion coefficient is 4×10−16 cm2/s at a growth temperature of 800 °C. From this result, we can conclude that the diffusion of group III atoms arises from atomistic migration on the surface of an InGaN active layer during growth, instead of diffusion within the solid phase.
Size-dependent hot-electron dynamics in small Pdn−-clusters
Authors: Pontius N. , Lüttgens G. , Bechthold P. S. , Neeb M. , Eberhardt W.
Published: July 2002

Using time-resolved photoelectron spectroscopy we show that electron relaxation processes via inelastic electron–electron scattering are efficient energy dissipation channels not only in bulk metals but also in extremely small transition metal clusters. The photoelectron spectra of optically excited Pd3−, Pd4−, and Pd7− reveal effective electron relaxation times of less than 100 fs. Moreover the relaxation times vary with cluster size. In comparison to simple metal clusters the bulklike inelastic scattering rates in open d-shell transition metal clusters are attributed to the larger valence electron level density. An energy transfer to the vibrational degrees of freedom occurs within 10 ps.
Crystal orbital calculation of coupled Hartree–Fock dynamic (hyper)polarizabilities for prototype π-conjugated polymers
Authors: Gu Feng Long , Bishop David M. , Kirtman Bernard
Published: July 2002

Ab initio static and dynamic (hyper)polarizability calculations at the time-dependent coupled-perturbed Hartree–Fock level have been carried out on polyyne, all-trans polyacetylene (PA), and trans-transoïd polymethineimine by a noniterative method. The dependence on basis set and on the number of neighboring unit cells exactly included has been investigated. The 6-31G basis set with nine neighboring unit cells, and long-range Coulomb interactions taken into account, was utilized in the final calculations. Although in many instances our results are the first that have been obtained we compare with oligomer calculations wherever possible. For PA the theoretical result is also compared with experiment and the main reasons for the existing discrepancy are addressed.
The production and decay kinetics of ClOO in water and freon-11: A time-resolved resonance raman study
Authors: Hayes Sophia C. , Thomsen Carsten L. , Reid Philip J.
Published: July 2002

The production of ClOO following OClO photolysis in water and fluorotrichloromethane (freon-11) is investigated using time-resolved resonance Raman (TRRR) spectroscopy. Stokes spectra are obtained as a function of time following OClO photoexcitation using pump and probe wavelengths of 390 and 260 nm, respectively. Scattering assignable to ClOO is observed, and appears with a time constant of 27.9±4.5 ps in water and 172±30 ps in freon-11. The ClOO intensity decays with a time constant of ∼398±50 ps in water and 864±200 ps in freon-11. Although the production and decay kinetics are solvent dependent, the quantum yield for ClOO production is similar between water and freon-11. Femtosecond pump–probe studies designed to monitor the evolution in optical density at 390 and 260 nm following OClO photoexcitation are also presented. These studies demonstrate that geminate recombination of the primary photoproducts is less efficient in freon-11 relative to water. This result taken in combination with the solvent invariance of the ClOO-production quantum yield indicates that ClOO is not formed via geminate recombination. Instead, the results presented here suggest that OClO photoisomerization results in the production of ClOO. Finally, the vibrational energy content of ClOO upon internal conversion to the ground state is studied through comparison of the ClOO Raman and absorption cross sections to those predicted using computational methods. These studies suggest that ground-state ClOO is produced with minimal excess vibrational energy. The results presented here provide new insight into the mechanism of ClOO formation following OClO photoexcitation.
Giant magnetocaloric effect of MnAs1−xSbx
Authors: Wada H. , Tanabe Y.
Published: July 2002

A giant magnetocaloric effect was found in MnAs, which undergoes a first-order ferromagnetic to paramagnetic transition at 318 K. The magnetic entropy change caused by a magnetic field of 5 T is as large as 30 J/K kg at the maximum value, which exceeds that of conventional magnetic refrigerant materials by a factor of 2–4. The adiabatic temperature change reaches 13 K in a field change of 5 T. The substitution of 10% Sb for As reduces the thermal hysteresis and lowers the Curie temperature to 280 K, while the giant magnetocaloric properties are retained.
Direct measurement of electron density and temperature distributions in a micro-discharge plasma for a plasma display panel
Authors: Noguchi Yasuyuki , Matsuoka Akira , Uchino Kiichiro , Muraoka Katsunori
Published: July 2002

Spatial distributions of electron density (ne) and electron temperature (Te) of a micro-discharge plasma for an alternating current plasma display panel cell were directly measured using the laser Thomson scattering method. The use of a triple-grating spectrometer was very successful in suppressing the strong stray laser light and allowed us to perform measurements at 0.1 mm above the surface of the electrode substrate. Values of ne and Te were (0.2–3)×1019 m−3 and (1.6–3.4) eV, respectively, depending on the time from the beginning of the pulsed discharge and the observation position. The structure of the micro-discharge is discussed in terms of the obtained distributions of ne and Te.
Density and binding forces in diatomics
Authors: Fernández Rico J. , López R. , Ema I. , Ramı́rez G.
Published: July 2002

In a recently reported method, the molecular density is partitioned in minimally deformed atomic contributions, which are expanded in spherical harmonics times radial factors. Here we use this representation to express the electrostatic potential of the molecule, the force on its nuclei, and the conformational variations of energy in terms of some simple integrals of the atomic radial factors. As a first application, we analyze the relationship between the density and the binding forces (and the bonding energy) in the diatomic molecules of the first row atoms. Two types of forces act on each nucleus: the self-pulling exerted by its own cloud and the external force due to the remaining atoms. The self-pulling comes only from the dipole type term of the atomic density. The external force comes from the other clouds and nuclei and is dominated by the effective charges which depend on the outermost region of the charge term. Analyzing the progressive deformations of the atoms when they approach each other, the forces associated with these deformations and their contributions to the energy, one has a detailed description of the chemical bond which is complementary, and in many aspects more appealing, than the conventional ones.
The dielectric self-consistent field method. I. Highways, byways, and illustrative results
Authors: Boresch Stefan , Steinhauser Othmar
Published: July 2002

The dielectric self-consistent field method (DSCF) provides a continuum electrostatics tool to study the effects which the use of modified electrostatic interactions has on the properties of a solute–solvent system in molecular dynamics or Monte Carlo simulations with explicit solvent. The theoretical foundation and the implementation of the method are described. Results of calculations for small solutes, water and a spherical ion, are presented. The properties studied include the electrostatic contribution to the solvation free energy and solute–solvent orientational correlation functions. The information obtained in this manner turns out to be complementary and provides a broader basis for a critical comparison of widely used modified electrostatic interactions. We also compute the potential-of-mean-force for a pair of spherical ions of opposite charge. Among the modified electrostatic interactions compared [spherical truncation, a shifted potential, a generalized reaction field, and Ewald summation (EW)], EW is found to give the most consistent results compared with the true 1/r Coulomb potential. The paper concludes with a comparison of DSCF to related continuum electrostatic based approaches, such as numerical solutions of the Poisson–Boltzmann equation and the method of Langevin dipoles.
Uniqueness of a convex sum of products of projectors
Authors: Kirkpatrick K. A.
Published: July 2002

Relative to a given factoring of the Hilbert space, the decomposition of an operator into a convex sum of correlated products of pairs of distinct 1-projectors, one set of projectors linearly independent, is unique.
Can optical spectroscopy directly elucidate the ground state of C20?
Authors: Castro Alberto , Marques Miguel A. L. , Alonso Julio A. , Bertsch George F. , Yabana K. , Rubio Angel
Published: July 2002

The optical response of the lowest energy isomers of the C20 family is calculated using time-dependent density functional theory within a real-space, real-time scheme. Significant differences are found among the spectra of the different isomers, and thus we propose optical spectroscopy as a tool for experimental investigation of the structure of these important clusters.
Optimization of laser-driven intramolecular hydrogen transfer in the presence of dephasing
Authors: Geva Eitan
Published: July 2002

The coherent control of laser-driven intramolecular hydrogen transfer is considered in the presence of pure dephasing. Emphasis is put on performing the optimization in the presence of dephasing. Simple analytical expressions are obtained for the optimal pulse shape and optimal yield as functions of the dephasing rate constant. It is found that dephasing is not always uncontrollable and destructive, and that the optimal pulse shape is dictated by the minimization of the destructive attributes of the dephasing processes, as much as by steering the coherent component of the dynamics towards the desired goal.
Effect of volatile solutes on sonoluminescence
Authors: Yasui Kyuichi
Published: July 2002

Computer simulations of bubble oscillations in aqueous methanol solution are performed under conditions of sonoluminescence (SL) in order to study the mechanism of SL quenching by methanol reported by Ashokkumar et al. [J. Phys. Chem. B 101, 10849 (1997); J. Phys. Chem. A 104, 8462 (2000)]. It is shown that methanol molecules evaporate into the bubble at the bubble expansion and almost all of them are dissociated inside a heated bubble at the collapse. The bubble temperature at the collapse decreases by the endothermal heat of the dissociation of methanol and by the increase of the molar heat of the gases inside a bubble, which results in the SL quenching. It is shown that SL from argon bubbles is more effectively quenched by methanol compared to SL from air bubbles.
Steady-state erosion of propagating ion beams
Authors: Rose D. V. , Genoni T. C. , Welch D. R.
Published: July 2002

A steady-state analytic model of beam erosion is presented and compared with two-dimensional hybrid particle-in-cell simulations of 100 MeV to 2 GeV proton beams propagating in a dense background gas. The analytic model accounts for nonzero beam erosion front velocities and the finite energies of beam particles radially exiting the beam through a single parameter. The model is in agreement with the simulation results for a single value of this parameter over the beam energy ranges considered.