bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and predicted near edge x-ray absorption fine structure spectra for the surface of liquid formamide from molecular-dynamics simulation
Authors: Wojcik Mark C. , Hermansson Kersti , Siegbahn Hans O. G.
Published: July 2002

A modification of an existing model for formamide interactions in the liquid phase makes it possible to study the bulk liquid and its free surface at conditions near the triple point, i.e., 300 K and essentially zero pressure, by (MD) molecular-dynamics simulation. Density profiles and orientation profiles in the surface region show the depth and gradual loss of the molecular ordering in the transition from bulk vapor to the bulk liquid. Near edge x-ray absorption fine structure (NEXAFS) resonance intensity spectra are computed from the orientation profiles, so that comparison should be possible with experimental depth resolved NEXAFS spectra.
Clebsch–Gordan coefficients of SU(3) in SU(2) and SO(3) bases
Authors: Rowe D. J. , Bahri C.
Published: July 2002

New algorithms are developed for the purpose of optimizing the efficient calculation of SU(3) Clebsch–Gordan coefficients in both SU(2)- and SO(3)-coupled bases. The new algorithms make use of the fact that highest weight states in a tensor product space are easily identified by vector coherent state methods. The methods are developed for SU(3) but apply to other compact semi-simple Lie groups.
Taylor relaxation and λ decay of unbounded, freely expanding spheromaks
Authors: Yee J. , Bellan P. M.
Published: July 2002

A magnetized coaxial gun is discharged into a much larger vacuum chamber and the subsequent evolution of the plasma is observed using high speed cameras and a magnetic probe array. Photographic results indicate four distinct regimes of operation, labeled I–IV, each possessing qualitatively different dynamics, with the parameter λgun=μ0Igun/Φbias determining the operative regime. Plasmas produced in Regime II are identified as detached spheromak configurations. Images depict a donut-like shape, while magnetic data demonstrate that a closed toroidal flux-surface topology is present. Poloidal flux amplification shows that Taylor relaxation mechanisms are at work. The spatial and temporal variation of plasma λ=μ0Jφ/Bφ indicate that the spheromak is decaying and expanding in a manner analogous to a self-similar expansion model proposed for interplanetary magnetic clouds. In Regime III, the plasma is unable to detach from the gun due to excess bias flux. Analysis of toroidal and poloidal flux as well as the λ profile shows that magnetic flux and helicity are confined within the gun for this regime.
Short optical solitons in fibers
Authors: Gromov E. M. , Talanov V. I.
Published: July 2002

The dynamics of short (of the order of a few wave periods) intense optical pulses and interaction of short optical solitons in fibers are considered within the framework of the third-order nonlinear Schrödinger equation. It is shown that an initial pulse tends to one or a few short solitons plus a linear quasiperiodic wave when the third-order linear dispersion and nonlinear dispersion have parameters of the same sign. The number and parameters of the solitons depend on the magnitudes of initial pulse parameters. Interaction of short optical solitons having different amplitudes is accompanied by radiation of part of the wave field from the area of interaction, by an increase of the soliton with larger amplitude, and a decrease of the soliton with a smaller one.
Optical properties of CuAlSe2
Authors: Alonso M. I. , Pascual J. , Garriga M. , Kikuno Y. , Yamamoto N. , Wakita K.
Published: July 2002

We have determined the complex dielectric tensor components of the chalcopyrite semiconductor CuAlSe2 in the energy range between 1.4 and 5.2 eV, at room temperature, using spectroscopic ellipsometry. We present results obtained on two single crystals grown by the traveling-heater method using In solvent. Values of refractive indices n, extinction coefficients k and normal-incidence reflectivity R in the two independent polarizations are reported. The structures observed in the energy region studied are analyzed by fitting the second-derivative spectra d2ε(ω)/dω2 to analytic critical-point line shapes. The obtained energies are assigned to certain electronic interband transitions by comparison with existing band structure calculations.
Effects of tensile and compressive strain on the luminescence properties of AlInGaN/InGaN quantum well structures
Authors: Aumer M. E. , LeBoeuf S. F. , Bedair S. M. , Smith M. , Lin J. Y. , Jiang H. X.
Published: July 2002

We report on the luminescence properties of AlInGaN/In0.08Ga0.92N quantum wells (QWs) subjected to a variable amount of lattice mismatch induced strain, including wells with zero strain, compressive strain, and tensile strain. The primary peak emission energy of a 3 nm In0.08Ga0.92N QW was redshifted by 236 meV as the stress in the well was changed from −0.86% (compressive) to 0.25% (tensile). It was also found that the photoluminescence intensity of quantum wells decreased with increasing strain. A lattice matched 9 nm QW exhibited a luminescence intensity that is three times greater than its highly strained counterpart. The potential applications of this strain engineering will be discussed.
Large-eddy simulations of turbulent flow in a rotating square duct
Authors: Pallares J. , Davidson L.
Published: July 2002

The turbulent flow at low Reynolds numbers in a rotating straight square duct was simulated using the large-eddy simulation technique. The rotation axis is parallel to two opposite walls of the duct, and the pressure-driven flow is assumed to be fully developed, isothermal and incompressible. The Reynolds number based on the friction velocity (Reτ=300) was kept constant in the range of the rotational numbers studied (0⩽Roτ⩽1.5) Computations were carried out using a second-order finite volume code with a localized one-equation dynamic subgrid scale model. Simulations of rotating channel flows were initially carried out and were seen to be in agreement with experiments and direct numerical simulations reported in the literature. The study of the flow in a rotating square duct revealed the influence of the Coriolis force on the spatial distribution of the average velocity fields and Reynolds stresses. At low rotation rates, turbulence-driven secondary flows developed near the corners convect the rotation-generated cross-stream currents. At moderate and high rotation rates, the mean secondary flow structure consists essentially of two large counter-rotating cells convecting low/high momentum fluid from the stable/unstable side to the unstable/stable side. Inspection of the terms of the transport equations of the average axial velocity and vorticity components shows the mechanisms responsible for the changes in the average flow structure. Spatial distributions of the Reynolds stresses are mainly influenced by the changes that rotation induces in the main strain rates. It has been found that, globally, at the low Reynolds number studied, rotation tends to significantly reduce the overall turbulence level of the flow.
Direct observation of local ferroelectric phase transitions in BaxSr1−xTiO3 thin films
Authors: Tikhomirov Oleg , Jiang Hua , Levy Jeremy
Published: July 2002

The temperature dependence of the local ferroelectric polarization of BaxSr1−xTiO3 thin films is measured with submicrometer resolution using confocal scanning optical microscopy. The local ferroelectric hysteresis loop changes shape and disappears as the temperature is increased over a narrow range of temperatures (ΔT∼10 K). The local Curie temperature TC(x,y) varies significantly with position, and the distribution of TC is consistent with the broad temperature dependence of the dielectric permittivity measured through conventional methods. These results directly confirm the hypothesis that the ferroelectric phase transition in these materials is inhomogeneously broadened.
Complementary model for intrinsic time-dependent dielectric breakdown in SiO2 dielectrics
Authors: McPherson J. W. , Khamankar R. B. , Shanware A.
Published: July 2002

A molecular physics-based complementary model, which includes both field and current, is introduced to help resolve the E versus 1/E-model controversy that has existed for many years as to the true physics behind time-dependent dielectric breakdown (TDDB). It is shown here that either TDDB model can be valid for certain specified field, temperature, and molecular bonding-energy ranges. For bond strengths <3 eV, the bond breakage rate is generally dominated by field-enhanced thermal processes and the E model is valid. For bond strengths >3 eV, the bond breakage must be hole catalyzed by current-induced hole injection and capture. Under these conditions, the TDDB physics is described well by the 1/E model.
Enhanced quantized current driven by surface acoustic waves
Authors: Ebbecke J. , Bastian G. , Blöcker M. , Pierz K. , Ahlers F. J.
Published: July 2002

We present the experimental realization of different approaches to increase the amount of quantized current which is driven by surface acoustic waves through split-gate structures in a two-dimensional electron gas. Samples with driving frequencies of up to 4.7 GHz have been fabricated without a deterioration of the precision of the current steps, and a parallelization of two channels with correspondingly doubled current values have been achieved. We discuss theoretical and technological limitations of these approaches for metrological applications as well as for quantum logics.
Single superconducting quantum interference device multiplexer for arrays of low-temperature sensors
Authors: Yoon Jongsoo , Clarke John , Gildemeister J. M. , Lee Adrian T. , Myers M. J. , Richards P. L. , Skidmore J. T.
Published: July 2002

We present the design and experimental evaluation of a superconducting quantum interference device (SQUID) multiplexer for an array of low-temperature sensors. Each sensor is inductively coupled to a superconducting summing loop which, in turn, is inductively coupled to the readout SQUID. The flux-locked loop of the SQUID is used to null the current in the summing loop and thus cancel crosstalk. The sensors are biased with an alternating current, each with a separate frequency, and the individual sensor signals are separated by lock-in detection at the SQUID output. We have fabricated a prototype 8 channel multiplexer and discuss the application to a larger array.
Satellite drops: Unexpected dynamics and change of scaling during pinch-off
Authors: Notz Patrick K. , Chen Alvin U. , Basaran Osman A.
Published: July 2002

During drop formation from a tube, a thin liquid thread—the precursor to satellites—connects an about-to-form primary drop to the remainder of the liquid hanging from the tube at the incipience of breakup. Whether the thread, once it detaches from the primary and pendant drops, evolves into a sphere or breaks into several subsatellites has heretofore been inadequately explored due to experimental and theoretical difficulties. These challenges are resolved here with an ultrafast digital imaging system and a novel computational algorithm. New findings range from the discovery of unexpected dynamics to the first demonstration of the transition from one scaling law governing interface rupture to another.
Neutron emission from a deuterated solid target irradiated by an ultraintense laser pulse
Authors: Toupin C. , Lefebvre E. , Bonnaud G.
Published: July 2002

Neutron production from a deuterated solid target irradiated by an ultraintense laser pulse is studied by means of kinetic numerical simulations. A two-dimensional particle-in-cell code is used to compute the velocity distribution of the deuterium ions accelerated during the interaction of the laser pulse with a hot deuterium plasma. A postprocessor has been designed to evaluate the slowing down of these ions in the solid unionized target, and the amount of neutrons produced by nuclear reactions between the accelerated and target ions. The energy and angle distributions of these neutrons are computed and compared to recent experimental results.
An add-on secondary electron energy spectrometer for scanning electron microscopes
Authors: Khursheed A. , Karuppiah N.
Published: July 2002

This article presents an add-on secondary electron energy spectrometer for scanning electron microscopes (SEMs). The add-on unit fits on to the specimen stage of a conventional SEM, and the SEM is operated as normal. As an objective lens, the add-on unit improves the SEMs spatial resolution by around a factor of 4 at a primary beam energy of 1 keV. The add-on unit functions as a multichannel open-loop voltage contrast spectrometer by employing a bandpass deflector/filter unit whose pass energy is ramped with time. The unit is designed to deflect the secondary electrons while leaving the primary beam unaffected. Initial experimental results show that significant voltage and material contrast can be obtained. In a data acquisition time of 0.32 s, a minimum detectable voltage difference of around 54 mV was obtained. Regions of copper and brass that were indistinguishable via the normal secondary electron image, were distinguished with a signal to noise ratio of around 12 by using the add-on spectrometer.
III-nitride blue microdisplays
Authors: Jiang H. X. , Jin S. X. , Li J. , Shakya J. , Lin J. Y.
Published: July 2002

Prototype blue microdisplays have been fabricated from InGaN/GaN quantum wells. The device has a dimension of 0.5×0.5 mm2 and consists of 10×10 pixels 12 μm in diameter. Emission properties such as electroluminescence spectra, output power versus forward current (L–I) characteristic, viewing angle, and uniformity have been measured. Due to the unique properties of III-nitride wide-band-gap semiconductors, microdisplays fabricated from III nitrides can potentially provide unsurpassed performance, including high-brightness/resolution/contrast, high-temperature/high-power operation, high shock resistance, wide viewing angles, full-color spectrum capability, long life, high speed, and low-power consumption, thus providing an enhancement and benefit to the present capabilities of miniature display systems.
Franck–Condon spectra and electron-libration coupling in para-polyphenyls
Authors: Karabunarliev Stoyan , Bittner Eric R. , Baumgarten Martin
Published: July 2002

Proceeding from quantum-chemical potential energy surfaces, we compute the absorption and fluorescence spectra of conventional and ladder-type para-phenylene oligomers (OPP and OLPP) with up to 7 benzene rings. Electronically excited states are addressed by means of extended configuration interaction within a standard molecular all-valence-electron semiempirical Hamiltonian. Adiabatic excitation energies, interstate distortions and normal modes are used to compute Franck–Condon band shapes with rigorous consideration of vibrational structure. Theoretical spectra agree with the experiment and rationalize the striking disparities in the linear optical response of OPP and OLPP. Whereas electron–phonon coupling in OLPP is essentially restricted to the carbon–carbon bond-stretching modes, photoexcitation, and emission processes in OPP are followed by significant relaxations in ring-torsional degrees of freedom. The broadening of spectra of OPP, especially pronounced in absorption, and the large Stokes shift between absorption and emission are traced back to the strong coupling of electronic excitations and low-frequency libration motions. The results highlight the importance of ring-torsional flexibility in conjugated polymers.
Resistive states of superconducting channels in an alternating electromagnetic field (Review)
Authors: Dmitriev V. M. , Zolochevskiı̆ I. V. , Khristenko E. V.
Published: July 2002

The processes that determine the structure and properties of the resistive state of superconducting channels having a uniform distribution of the current and order parameter over their transverse cross section are discussed. Particular attention is devoted to the resistive state arising when an external alternating electromagnetic field is applied to the superconductor. The current, temperature, frequency, and power characteristics of the resistive state are examined. The necessary information about the NS boundary and phase-slip centers is given, and the known results for high-Tc superconducting materials are presented.
Symmetry breaking effects in NO3−: Raman spectra of nitrate salts and ab initio resonance Raman spectra of nitrate–water complexes
Authors: Waterland Mark R. , Stockwell David , Kelley Anne Myers
Published: July 2002

Ground-state structures and vibrational frequencies are calculated for complexes of the nitrate anion with one and two water molecules at the ab initio Hartree–Fock level with a basis set including diffuse and polarization functions. Two local minimum geometries are found for each complex. Calculations of the electronically excited states at the CIS level are then used to find the forces on each of the atoms upon vertical excitation to the two lowest-lying (near-degenerate) strongly allowed electronic transitions. These forces are converted to gradients of the excited-state potential surfaces along the ground-state normal modes and compared with the parameters obtained previously from empirical simulations of the experimental resonance Raman intensities of NO3− in dilute aqueous solution. The calculations on two-water clusters agree well with the experimental excited-state geometry changes along the totally symmetric N–O stretch. The calculations underestimate the frequency splitting of the antisymmetric stretching vibration (degenerate in the isolated D3h ion) and the resonance Raman intensity in this mode, suggesting that bulk solvent polarization enhances the asymmetry of the local environment for NO3− in water. Comparison of the ground-state vibrational frequency splitting of the antisymmetric stretch with the corresponding values for the nitrate ion in salts having known crystal structures suggests that the rms difference among the three N–O bond lengths for nitrate anion in water probably exceeds 0.01 Å.
Role of the Eu3+ ions in the formation of transparent oxyfluoride glass ceramics
Authors: Méndez-Ramos J. , Lavı́n V. , Martı́n I. R. , Rodrı́guez-Mendoza U. R. , Rodrı́guez V. D. , Lozano-Gorrı́n A. D. , Núñez P.
Published: July 2002

Optical properties of Eu3+ ions in oxyfluoride glass ceramics and in the precursor glasses (30 SiO2, 15 Al2O3, 29 CdF2, 22 PbF2, 1.5 YF3, and 2.5 EuF3, in mol %) have been analyzed and compared. The Eu3+ ions in the glass ceramics are incorporated into a crystalline environment with higher symmetry and lower energy coupled phonons. Emission measurements indicate that this crystalline phase is EuF3, instead of PbxCd1−xF2 as generally considered.
Growth of single-domain (Sm0.5Eu0.5)Ba2Cu3O7−δ with high Tc and Jc by employing a thermal gradient
Authors: Hu A. , Sakai N. , Murakami M.
Published: July 2002

Bulk (Sm0.5Eu0.5)Ba2Cu3O7−δ superconductors were grown isothermally and with a vertical thermal gradient of −5 °C/cm in air. The crystals grown at a large growth rate under the thermal gradient exhibited a high-Tc value of 95 K and a plateau in the Jc–B curve at 1–4 T at 77 K with postannealing in Ar. We found that the substitution of Ba by rare-earth (RE) elements could be suppressed with a large growth rate. The narrow range of RE–Ba solid solution was attributed to the fast crystallization rate of RE1+xBa2−xCu3O7−δ with low-x values.
Slow viscous flow in a partitioned channel
Authors: Jeong Jae-Tack
Published: July 2002

Two-dimensional slow viscous flow in a partitioned channel is investigated based on the Stokes approximation. The partitioned channel is composed of two infinite parallel planes and a semi-infinite plate located midway between the infinite planes. The flow is allowed to turn around the semi-infinite plate by a proper pressure gradient. By solving a Wiener–Hopf equation, an exact analytic solution for the stream function is obtained. From the streamline patterns shown, it is found that a Moffatt’s infinite sequence of viscous eddies develops in the channel. The pressure and shear stress distribution on the boundaries are illustrated, and a local analysis for the flow near the edge of the plate is performed. By superposing the symmetric and the anti-symmetric flow, general flows in this partitioned channel are obtained and typical flow patterns are shown.
Precessing vortex breakdown mode in an enclosed cylinder flow
Authors: Marques F. , Lopez J. M.
Published: July 2002

The flow in a cylinder driven by the rotation of one endwall for height to radius ratios around three is examined. Previous experimental observations suggest that the first mode of instability is a precession of the central vortex core, whereas a recent linear stability analysis to general three-dimensional perturbations suggests a Hopf bifurcation to a rotating wave at lower rotation rates than those where the precession mode was first detected. Here, this apparent discrepancy is resolved with the aid of fully nonlinear three-dimensional Navier–Stokes computations.
Universal n−5 spectrum of zonal flows on giant planets
Authors: Galperin Boris , Sukoriansky Semion , Huang Huei-Ping
Published: July 2002

The energy spectra of the observed zonal flows on Jupiter and Saturn are shown to obey the scaling law EZ(n)=CZ(Ω/R)2n−5 in the range of total wave numbers n not affected by large scale friction (here, Ω and R are the rotation rate and the radius of the planet, and CZ is an order-one constant). These spectra broadly resemble their counterpart in recent simulations of turbulent flows on the surface of a rotating sphere [Huang et al., Phys. Fluids 13, 225 (2001)] that represents a strongly anisotropic flow regime evoked by the planetary vorticity gradient. It is conjectured that this regime governs the large scale circulations and the multiple zonal jets on giant planets. The observed strong equatorial jets that were not produced in the nearly inviscid simulations by Huang et al. are attributed to the combined effect of the energy condensation in the lowest zonal modes and the large scale friction.
Charging and trapping of macroparticles in near-electrode regions of fluorocarbon plasmas with negative ions
Authors: Ostrikov K. N. , Kumar S. , Sugai H.
Published: July 2002

Charging and trapping of macroparticles in the near-electrode region of fluorocarbon etching plasmas with negative ions is considered. The equilibrium charge and forces on particles are computed as a function of the local position in the plasma presheath and sheath. The ionic composition of the plasma corresponds to the etching experiments in 2.45 GHz surface-wave sustained and 13.56 MHz inductively coupled C4F8+Ar plasmas. It is shown that despite negligible negative ion currents collected by the particles, the negative fluorine ions affect the charging and trapping of particulates through modification of the sheath/presheath structure.
The A 6Σ+-X 6Σ+ transition of CrH, Einstein coefficients, and an improved description of the A state
Authors: Bauschlicher Charles W. , Ram R. S. , Bernath Peter F. , Parsons C. G. , Galehouse D.
Published: July 2002

The spectrum of CrH has been reinvestigated in the 9000–15 000 cm−1 region using the Fourier transform spectrometer of the National Solar Observatory. The 1-0 and 1-1 bands of the A 6Σ+-X 6Σ+ transition have been measured and improved spectroscopic constants have been determined. A value for the 2-0 band origin has been obtained from the band head using estimated spectroscopic constants. These data provide a set of much improved equilibrium vibrational and rotational constants for the A 6Σ+ state. An accurate description of the A-X transition has been obtained using a multireference configuration interaction approach. The inclusion of both scalar relativity and Cr 3s3p correlation are required to obtain a good description of both states. The ab initio computed Einstein coefficients and radiative lifetimes are reported.
Evaluation of the computational methods for electron-impact total ionization cross sections: Fluoromethanes as benchmarks
Authors: Torres I. , Martı́nez R. , Sánchez Rayo M. N. , Castaño F.
Published: July 2002

The experimental electron-impact total ionization cross sections (TICSs, ICSs) of CF4, CHF3, CH2F2, and CH3F fluoromethanes reported so far and a new set of data obtained with a linear double focusing time-of-flight mass spectrometer have been compared with the ab initio and (semi)empirical based ICS available methods. TICSs computational methods include: two approximations of the binary-encounter dipole (BED) referred to hereafter as Kim (Kim-BEB) and Khare (Khare-BEB) methods, the Deutsch and Märk (DM) formalism, also requiring atomic and molecular ab initio information, the modified additivity rule (MAR), and the Harland and Vallance (HV) methods, both based on semiempirical or empirical correlations. The molecular ab initio information required by the Kim, Khare, and DM methods has been computed at a variety of quantum chemistry levels, with and without electron correlation and a comprehensive series of basis sets. The general conclusions are summarized as follows: the Kim method yields TICS in excellent agreement with the experimental method; the Khare method provides TICS very close to that of Kim at low electron-impact energies (<100 eV), but their Mott and Bethe contributions are noticeably different; in the Kim and Khare approximations the electron correlation methods improve the fittings to the experimental profiles in contrast with the large basis sets, that leads to poorer results; the DM formalism yields TICS profiles with shapes similar to the experimental and the BEB methods, but consistently lower and with the profiles maxima shifted towards lower incident electron energies; the MAR method supplies very good ICS profiles, between those of BEB and DM methods; finally, the empirical HV method provides rather poor fittings concomitant with the simplicity and the few empirical parameters used.
Stochastic estimation and flow sources associated with surface pressure events in a turbulent boundary layer
Authors: Naguib A. M. , Wark C. E. , Juckenhöfel O.
Published: July 2002

The ability of stochastic estimation to capture the conditional streamwise velocity field associated with wall-pressure events was investigated using simultaneous hot-wire and microphone measurements over the Reynolds number range: 1437⩽Reθ⩽5670. The velocity measurements were conducted at the same streamwise and spanwise location as the wall-mounted microphone but at various heights in the boundary layer, spanning the buffer and logarithmic regions. The results show that utilization of the wall-pressure as the event for the stochastic estimation requires the inclusion of both the linear and quadratic terms in order to provide an accurate representation of the conditional velocity field. The need for the inclusion of the quadratic term, which is unlike published results based on velocity and Reynolds stress events in wall-bounded flows, is attributed to the influences of the turbulent–turbulent pressure source term. Additionally, examination of the energy content of the quadratic component of the stochastic estimation and its Reynolds number scaling suggests that the quadratic term is associated with motions that scale with the boundary layer thickness.
Dynamic light scattering study on gelatin aqueous solutions and gels
Authors: Shibayama Mitsuhiro , Okamoto Masahiko
Published: July 2002

The sol–gel transition of gelatin aqueous solutions has been investigated by dynamic light scattering as a function of gelatin concentration, C, at 10 °C, by taking account of the nonergodic nature of gels. The sol–gel transition concentration, Cgel, was determined to be around 0.35 wt % in this particular case. The scattered intensity exhibited a speckle pattern for C>Cgel. The dynamic component of the scattered intensity, 〈IF〉T, was successfully extracted from the time-average scattered intensity, 〈I〉T. 〈IF〉T diverged at Cgel and decreased by further increasing C. The time-intensity correlation function exhibited a power-law behavior exclusively at C≈Cgel. The exponent was found to be independent of the scattering vector, as it should be as a critical exponent of connectivity.
Predicted maximum mobility in bulk GaN
Authors: Look D. C. , Sizelove J. R.
Published: July 2002

A 300 K bulk (three-dimensional) mobility of 1245 cm2/V s has been measured in free-standing GaN. Temperature-dependent Hall-effect data on this particular sample are fitted to obtain unknown lattice-scattering parameters, as well as shallow donor (ND) and acceptor (NA) concentrations, which are ND=6.7×1015 and NA=1.7×1015 cm−3. Realistic values of the maximum mobility attainable in bulk GaN are then obtained by assuming two-orders-of-magnitude lower values of ND and NA, leading to a maximum 300 K mobility of 1350 cm2/V s, and a maximum 77 K mobility of 19 200 cm2/V s.
Barriers to electron extraction in polymer light-emitting diodes
Authors: Murata K. , Cinà S. , Greenham N. C.
Published: July 2002

We have studied the performance of single-layer polymer light-emitting diodes based on a polyfluorene derivative. Hole-only devices show low currents; however, double-carrier devices show high currents and high efficiencies, implying that the presence of electrons in the device enhances hole injection. By numerical modeling, we show that this behavior is consistent with the presence of a barrier to electron extraction at the anode which causes an increased field for hole injection due to the buildup of electrons at the barrier.
Rate coefficients for the reactions of Si(3PJ) with C2H2 and C2H4: Experimental results down to 15 K
Authors: Canosa A. , Le Picard S. D. , Gougeon S. , Rebrion-Rowe C. , Travers D. , Rowe B. R.
Published: July 2002

Rate coefficients for the reaction of ground-state silicon atoms Si(3PJ) with acetylene and ethylene have been measured at temperatures down to 15 K. The experiments have been performed in a continuous flow CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus using pulsed laser photolysis of Si(CH3)4 to generate Si(3PJ) atoms and laser-induced fluorescence to observe the kinetic decay of the atoms and hence determine the rate coefficients. Both reactions are found to be fast, and the reaction rates show a very mild dependence on temperature. The rate coefficients match the expressions k(Si+C2H2)=(2.6±0.6)10−10(T/300)−(0.71±0.24) exp(−(29±10)/T) cm3 molecule−1 s−1 and k(Si+C2H4)=(3.7±0.3)10−10(T/300)−(0.34±0.10) exp(−(16±4)/T) cm3 molecule−1 s−1 in the temperature range 15–300 K. The nature of the products and the similarities of the carbon and silicon chemistry are discussed.
CoPt/Ag nanocomposites with (001) texture
Authors: Karanasos V. , Panagiotopoulos I. , Niarchos D. , Okumura H. , Hadjipanayis G. C.
Published: July 2002

CoPt/Ag nanocomposites with the tetragonal (L10) structure have been prepared by magnetron sputtering. The dependence of texture on film thickness, bilayer thickness, CoPt volume fraction, and annealing conditions is investigated. Films with a thickness below 15 nm consist of islands with (001) texture while as the thickness increases, the islands coalesce into a continuous film and the (111) texture appears. Microstrain is minimized in the range of film thickness where the (001) texturing is optimum indicating that strain energy provides the driving force of (001) growth texturing. The (001) texture improves with CoPt volume fraction for all annealing times but disappears above 95 vol % indicating that the existence of the Ag plays an important role in the development of the (001) texture.
Dielectric spectroscopy of Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals
Authors: James A. R. , Priya S. , Uchino K. , Srinivas K.
Published: July 2002

Complex impedance spectroscopic data were acquired on single crystals of the morphotropic phase boundary composition of 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 over a wide range of temperatures (25–525 °C) and frequencies 1 kHz–1 MHz. This study takes advantage of plotting ac data simultaneously in the form of impedance and modulus spectroscopic plots. This permits the easy interpretation of microscopic processes responsible for the measured ac response. Frequency explicit plots of imaginary components of impedance and modulus exhibit Debye-like peak shapes. The data for ac conductivity were computed from the impedance data and the activation energy for conduction at different frequencies was determined. Cole–Cole diagrams were plotted and these indicate the presence of a single relaxation process. The relaxation times determined from these plots followed an Arrhenius law, and the activation energy for relaxation was found to be 1.2 eV. The ac conductivity data was found to obey Jonscher’s universal power law and resulted in a value of the exponent “n”=0.95.
Damage buildup in Si under bombardment with MeV heavy atomic and molecular ions
Authors: Titov A. I. , Kucheyev S. O. , Belyakov V. S. , Azarov A. Yu.
Published: July 2002

Accumulation of structural disorder in Si bombarded at −196 °C with 0.5 MeV Bi1209 and 1 MeV Bi2209 ions (the so-called molecular effect) is studied by Rutherford backscattering/channeling spectrometry. Results show that the damage buildup is sigmodal even for such heavy-ion bombardment at liquid nitrogen temperature. This strongly suggests that, for the implant conditions of this study, the buildup of lattice damage cannot be considered as an accumulation of completely disordered regions. Instead, damage-dose curves are well described by a cascade-overlap model modified to take into account a catastrophic collapse of incompletely disordered regions into an amorphous phase after damage reaches some critical level. Results also show that Bi2 ions produce more lattice damage than Bi1 ions implanted to the same dose. The ratio of lattice disorder produced by Bi2 and Bi1 ions is 1.7 near the surface, decreases with depth, and finally becomes close to unity in the bulk defect peak region. Parameters of collision cascades obtained using ballistic calculations are in good agreement with experimental data. The molecular effect is attributed to a spatial overlap of (relatively dense) collision subcascades, which gives rise to (i) nonlinear energy spike processes and/or (ii) an increase in the defect clustering efficiency with an effective increase in the density of ion-beam-generated defects.
Hexapole magnet system for thermal energy He3 atom manipulation
Authors: Jardine A. P. , Fouquet P. , Ellis J. , Allison W.
Published: July 2002

We present design and construction details for a novel high field, small bore permanent hexapole magnet. The design is intended for focusing atomic beams of He3 at thermal energies. The magnet uses an optimized polepiece design which includes vacuum gaps to enable its use with high intensity atomic and molecular beams. The 0.3 m long, 1 mm internal radius magnet achieves a polepiece tip field of 1.1 T using NdFeB permanent magnets and Permendur 49 polepieces. The polepiece shanks are designed to saturate so that the hexapole properties are determined predominantly by the shape of the polepiece tip. The performance of the hexapole assembly is demonstrated with an 8 meV He3 beam in the beam source of the Cambridge spin echo spectrometer and the measured focused beam results show excellent agreement with theoretical predictions and negligible beam attenuation.
Structure and physical properties of paracrystalline atomistic models of amorphous silicon
Authors: Voyles P. M. , Zotov N. , Nakhmanson S. M. , Drabold D. A. , Gibson J. M. , Treacy M. M. J. , Keblinski P.
Published: July 2002

We have examined the structure and physical properties of paracrystalline molecular dynamics models of amorphous silicon. Simulations from these models show qualitative agreement with the results of recent mesoscale fluctuation electron microscopy experiments on amorphous silicon and germanium. Such agreement is not found in simulations from continuous random network models. The paracrystalline models consist of topologically crystalline grains which are strongly strained and a disordered matrix between them. We present extensive structural and topological characterization of the medium range order present in the paracrystalline models and examine their physical properties, such as the vibrational density of states, Raman spectra, and electron density of states. We show by direct simulation that the ratio of the transverse acoustic mode to transverse optical mode intensities ITA/ITO in the vibrational density of states and the Raman spectrum can provide a measure of medium range order. In general, we conclude that the current paracrystalline models are a good qualitative representation of the paracrystalline structures observed in the experiment and thus provide guidelines toward understanding structure and properties of medium-range-ordered structures of amorphous semiconductors as well as other amorphous materials.
Luminescence of oxidized porous silicon: Surface-induced emissions from disordered silica micro- to nanotextures
Authors: Fritsch E. , Mihut L. , Baibarac M. , Baltog I. , Ostrooumov M. , Lefrant S. , Wery J.
Published: July 2002

The luminescence spectra of fully oxidized porous silicon show two broad bands and one vibronic structure between 2 and 3.3 eV, all excited at 340 or 275 nm (3.65 and 4.51 eV). Two of these structures have not been described previously. Interestingly, these emissions are found in other forms of amorphous or disordered silica with high specific surfaces: frosted silica glass, silica gel, even various natural opals. These emissions disappear when the surface is passivated. They are, therefore, attributed to surface-related defects. The vibronic structure involves a nearly free SiO4 tetrahedron. Another vibronic structure found only in some opals is attributed to the uranyl group.
Growth of well-aligned carbon nanotube arrays on silicon substrates using porous alumina film as a nanotemplate
Authors: Hu Wenchong , Gong Dawei , Chen Zhi , Yuan Liming , Saito Kozo , Grimes Craig A. , Kichambare Padmakar
Published: July 2002

Aligned, open-tipped carbon nanotube arrays of high density and uniformity were synthesized via a flame method on silicon substrates using a nanoporous template of anodized aluminum oxide from which the nanotubes were grown. The diameter and length of the nanotubes are controlled by the geometry of the aluminum oxide template. These results show the feasibility of integration between carbon nanotube arrays and silicon microelectronics.
Character expansions for the orthogonal and symplectic groups
Authors: Balantekin A. B. , Cassak P.
Published: July 2002

Formulas for the expansion of arbitrary invariant group functions in terms of the characters for the Sp(2N), SO(2N+1), and SO(2N) groups are derived using a combinatorial method. The method is similar to one used by Balantekin to expand group functions over the characters of the U(N) group. All three expansions have been checked for all N by using them to calculate the known expansions of the generating function of the homogeneous symmetric functions. An expansion of the exponential of the traces of group elements, appearing in the finite-volume gauge field partition functions, is worked out for the orthogonal and symplectic groups.
Detection of particles of less than 5 nm in diameter formed in an argon–silane capacitively coupled radio-frequency discharge
Authors: Boufendi L. , Gaudin J. , Huet S. , Viera G. , Dudemaine M.
Published: July 2002

A method for the detection of dust particles occurring in silane–argon gas mixture plasmas is presented. It is based on the spectral analysis of the radio-frequency current. The amplitudes of the fundamental (13.56 MHz) and second harmonics (40.68 MHz) are very sensitive to the presence of the earlier nanoparticles when their size is in the range of 2–3 nm even if their influence on the capacitive character of the impedance is negligible. This method is nonperturbative, with a temporal resolution in the microsecond range, very easy to implement, and can thus be used for industrial reactors.
Local stresses induced by nanoscale As–Sb clusters in GaAs matrix
Authors: Chaldyshev V. V. , Bert N. A. , Romanov A. E. , Suvorova A. A. , Kolesnikova A. L. , Preobrazhenskii V. V. , Putyato M. A. , Semyagin B. R. , Werner P. , Zakharov N. D. , Claverie A.
Published: July 2002

Microstructure of GaAs films grown by molecular-beam epitaxy at low temperature and delta doped with Sb was studied by transmission electron microscopy. The material contained 0.5 at. % excess of arsenic that precipitated during post growth anneals. The Sb δ doping was found to strongly affect the microstructure of precipitates (clusters) and their ripening rate upon annealing. Segregation of Sb impurity in the clusters was revealed. In contrast to the well known pure As clusters, the As–Sb clusters induced strong local deformations in the surrounding GaAs matrix. Until a threshold diameter of 7–8 nm the clusters and surrounding matrix were coherently strained. Larger clusters were associated with dislocation loops of interstitial type. The cluster-loop orientation relationships were determined. Relaxation of local strains by formation of the dislocation loops was studied both experimentally and theoretically.
The effect of the matrix on film properties in matrix-assisted pulsed laser evaporation
Authors: Bubb D. M. , Wu P. K. , Horwitz J. S. , Callahan J. H. , Galicia M. , Vertes A. , McGill R. A. , Houser E. J. , Ringeisen B. R. , Chrisey D. B.
Published: July 2002

Thin films of polyethylene glycol of average molecular weight 1400 amu have been deposited by matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out in vacuum (∼10−6 Torr) with an ArF (λ=193 nm) laser at a fluence of 220–230 mJ/cm2. Films were deposited on NaCl plates and glass microscope slides. Both deionized water (H2O) and chloroform (CHCl3) were used as matrices. The physiochemical properties of the films are compared via Fourier transform infrared spectroscopy, and electrospray ionization mass spectrometry. The results show that the matrix used during MAPLE can greatly affect the chemical structure and molecular weight distribution of the deposited film. The infrared absorption spectrum shows evidence for C–Cl bond formation when CHCl3 is used as a matrix, while there is little evidence in the IR data for photochemical modification when H2O is used as a matrix. Time-of-flight analysis was performed using a quadrupole mass spectrometer to monitor evaporation of a frozen CHCl3 target during laser exposure. Using this approach, we determined that the TOF spectra for m/z=35 (Cl) and m/z=85 (CHCl2) differed significantly in both width and peak arrival time, indicating that neutral chlorine atoms were produced at the target surface. We attribute the reduction in molecular weight and structural modification of the film deposited using CHCl3 to the presence of these highly reactive species.
Annealing induced structural change in sputter deposited copper ferrite thin films and its impact on magnetic properties
Authors: Desai Mrugesh , Prasad Shiva , Venkataramani N. , Samajdar Indradev , Nigam A. K. , Krishnan R.
Published: July 2002

Copper ferrite (CuFe2O4) thin films have been prepared by rf sputtering on fused quartz and silicon (111) substrates at ambient temperature. The as-deposited film is found to be in cubic phase, which is stable only at higher temperatures in bulk. The films were annealed at temperatures ranging from 100 °C to 800 °C and slowly cooled. The films had tetragonal structure at annealing temperature above 200 °C. The c/a ratio was observed to increase with increasing annealing temperature. Transmission electron microscopy study confirmed the phase transformation from cubic to tetragonal as a function of annealing temperature. The magnetization values of the films were observed to show a maximum at the annealing temperature of 200 °C and a minimum at 500 °C. The coercivity increased monotonically from 70 Oe for the as-deposited film, to 1450 Oe for the film annealed at 800 °C. The results were explained on the basis of the phase transformation and the grain growth phenomenon.
Stabilization calculation of the energy and lifetime of metastable SO42−
Authors: Whitehead Amanda , Barrios Robyn , Simons Jack
Published: July 2002

It is known that SO42− is not electronically stable as an isolated species but can be rendered stable by solvation (e.g., by adding a few H2O molecules). Recently, our group introduced a Coulomb repulsion model that offers an approximation to the energy instability and lifetimes of such species. In order to achieve an independent and likely more reliable estimate of the instability of SO42−, we have undertaken a follow-up study of this dianion. Specifically, we apply a stabilization method to determine the vertical electronic energy difference between the metastable SO42− dianion and its SO4−1 daughter at several levels of theory. The particular variant of the stabilization method used here involves adding a partial positive charge to the central sulfur nucleus in order to confine the escaping electron. Our coupled-cluster data, which represent our highest level of theory, suggest that SO42− is unstable by 1.1 eV and has a lifetime with respect to electron loss of 1.6×10−10 s (our earlier estimates were 0.75 eV and 2.7×10−8 s).
Complete characterization of doubly doped PbWO4:Mo,Y scintillators
Authors: Nikl M. , Bohacek P. , Mihokova E. , Solovieva N. , Vedda A. , Martini M. , Pazzi G. P. , Fabeni P. , Kobayashi M.
Published: July 2002

We present correlated measurements of radioluminescence, photoluminescence, and thermoluminescence, as well as the measurements of decay kinetics both under ultraviolet and Na(51122 keV) photon excitations for Mo-doped and doubly (Mo,Y)-doped PbWO4 single crystals. A wide concentration range of Mo doping (200–50 000 ppm) is covered. Y codoping results in significant suppression of trapping states induced by the Mo doping. Consequently, an improvement of temporal characteristics, such as acceleration of the scintillation decay process, of doubly doped material with respect to Y-free crystals, is achieved. For the optimum Mo concentration (2750–10 000 ppm), the temporal characteristics are fully comparable to an undoped PbWO4, while the light yield is increased by a factor of 2.2.
Dependence of the excitonic transition energies and mosaicity on residual strain in ZnO thin films
Authors: Ong H. C. , Zhu A. X. E. , Du G. T.
Published: July 2002

The mosacity and optical properties of ZnO on (0001) Al2O3 grown by pulsed-laser deposition have been studied by x-ray diffraction and spectroscopic ellipsometry. Strong dependence has been found between the grain size and the residual strain along the c axis, εzz, as well as the film texture. In general, strain relieves and texture improves at larger grain size regardless of the growth conditions. The excitonic transition energies are also found to vary in the presence of strain field. It is observed that the transition energies increase with increasing strain and eventually they are resolved into two well-defined bands at the strain of 1.63%. By taking into account of the biaxial strain, the theoretical band structure of ZnO has been considered by solving the Luttinger–Kohn Hamiltonian. Reasonable agreement is found between the theory and experiment.
On the magnetic anisotropy of La2CuO4 above the Néel temperature
Authors: Loktev V. M.
Published: July 2002

It is conjectured that, because of the orthorhombicity of the lattice, which is preserved above the Néel temperature TN, and the quasi-two-dimensional character of the magnetic interactions, the destruction of the long-range antiferromagnetic order in La2CuO4 is brought about by the thermal generation of kink–antikink pairs. An attempt is made to interpret qualitatively the experiments of A. N. Lavrov et al., Phys. Rev. Lett. 87, 017007 (2001), on the observation of anisotropy of the magnetic susceptibility in the paraphase of this antiferromagnet.