bluuebunny/crossref_metadata_2025 huggingface dataset:
Long-implosion plasma radiation sources using “solid-fill” nozzles
Authors: Levine J. S. , Coleman P. L. , Failor B. H. , Riordan J. C. , Song Y. , Sze H. M. , Coverdale C. A. , Deeney C. , McGurn J. S. , Apruzese J. P. , Davis J. , Moosman B. , Thornhill J. W. , Velikovich A. L. , Weber B. V. , Bell D. , Schneider R.
Published: July 2002

Solid-fill nozzles for long-implosion Z-pinch experiments to produce argon K-shell x rays (photon energy >3.1 keV) have been developed. With a 7 cm diam nozzle, which is appropriate for a 200 ns driver, stable implosions at 180 ns and 4 MA have produced peak argon K-shell yields exceeding 15 kJ. As previously seen with short (∼100 ns) implosion times, the K-shell yield scales as the fourth power of peak current, I4. Limited testing with a 10 cm nozzle, which is appropriate for a >250 ns driver, has also achieved a stable implosion.
Spin correlation function of benzene and naphthalene from spin-coupled wave functions
Authors: Bomfleur B. , Voitländer J. , Sironi M. , Raimondi M.
Published: July 2002

In this paper we report on the calculation of the spin correlation function describing the dipolar spin–spin interaction for benzene and naphthalene in their lowest excited triplet state. Spin-coupled wave functions were used. The spin correlation function measures the contributions to the 〈3Sz2–S2〉 expectation value arising from electrons located at different points. An alternation of positive and negative correlation dependent on the separation of the unpaired electrons is found. The results are used to analyze and interpret the zero-field splitting parameter D of these molecules in terms of different contributions. The relationship between the spin correlation function and the sequence of the zero-field levels as well as the value of the parameter D and its dependence from molecular size is discussed.
Combined electron diffraction/microanalysis investigation of crystallography and cation distributions in the transparent conductive oxide Cd1+xIn2−2xSnxO4
Authors: Brewer L. N. , Kammler D. R. , Mason T. O. , Dravid V. P.
Published: July 2002

The distribution of Cd, In, and Sn cations on the spinel lattice was investigated across the solid solution, Cd1+xIn2−2xSnxO4. Convergent beam electron diffraction was used to establish the presence of the spinel crystallography throughout the solid solution. Atom location by channeling enhanced microanalysis was employed to determine the distributions of cations on the octahedral and tetrahedral lattice sites. CdIn2O4 was observed to be a normal-type spinel. As x was increased, the cation distribution became more random as Cd and Sn filled the octahedral sublattice. These observations may correlate with previously observed changes in optical gap and conductivity across the solution.
Blowout bifurcations and the onset of magnetic dynamo action
Authors: Sweet David , Ott Edward , Antonsen Thomas M. , Lathrop Daniel P. , Finn John M.
Published: July 2002

This paper numerically investigates the magnetohydrodynamic equations in three dimensions with periodic boundary conditions in a parameter range where a forced fluid flow is chaotic. It is found that the transition to dynamo action, whereby the magnetic field is sustained by interaction with the forced flow, is a blowout bifurcation. The blowout bifurcation is typified by bursting behavior, or “on-off intermittency.” In particular, near the transition there are short, intermittently occurring bursts of strong magnetic field activity where the total magnetic energy is comparable to the total flow kinetic energy. Between these bursts the magnetic energy is very small. As one approaches the transition from the dynamo-active side, the time between bursts becomes longer and longer, approaching infinity at the transition. Numerical verification is given for the presence of signature scaling laws in numerical computations utilizing a pseudospectral model with triply periodic boundary conditions. This work implies specific testable predictions for experimental dynamos.
Piezoelectrically driven rotator for use in high magnetic fields at low temperatures
Authors: Ohmichi E. , Nagai S. , Maeno Y. , Ishiguro T. , Mizuno H. , Nagamura T.
Published: July 2002

We report the development of a piezoelectrically driven rotator for use in high magnetic fields at low temperatures. This rotator, controllable with two lead wires instead of a driving shaft, is designed to fit into a narrow sample space with a diameter of 13 mm and is capable of making rotations with a high angular resolution of ∼0.01° in magnetic fields reaching 33 T. In addition, estimated heat dissipation of less than 100 μW accompanied by rotation allows continuous rotations in a dilution refrigerator (T<100 mK).
Superconducting niobium tip for scanning tunneling microscope light emission spectroscopy
Authors: Uehara Y. , Fujita T. , Iwami M. , Ushioda S.
Published: July 2002

Niobium tips for use in scanning tunneling microscope (STM) light emission measurements have been fabricated by an electrochemical etching method. The typical radius of curvature of the tip is less than 100 nm. The Nb tip has been demonstrated to be capable of producing STM images of highly oriented pyrolytic graphite and Au(110)-(2×1) surfaces with atomic spatial resolution. It has been confirmed by STM light emission spectroscopy that the front of the Nb tip becomes superconducting for temperatures below the superconducting transition temperature of Nb.
Scanning noninvasive voltage probe operating at 4.2 K
Authors: Sellwood A. T. , Smith C. G. , Linfield E. H. , Simmons M. Y. , Ritchie D. A.
Published: July 2002

A noninvasive voltage probe, consisting of a one-dimensional channel, has been fabricated within a modulation doped GaAs/AlGaAs heterostructure. With precision piezoelectric scanning equipment this probe has been brought to within 14 nm of a semiconductor device which includes surface gates and a 300 nm deep two-dimensional electron gas. Measurements of voltages applied to these conducting layers have been made at room temperature and at 4.2 K using the sensitive conductance of the one-dimensional channel. A voltage resolution of 0.48 mV has been observed at 4.2 K. Probe conductance measurements have also been made as a function of probe–sample separation. The conductance–separation data were fitted using a simple parallel plate capacitor model and a height resolution of 4 nm was calculated. Images of the sample layers have been obtained and a lateral resolution of 760 nm observed.
Permanent magnet objective lenses for multicolumn electron-beam systems
Authors: Khursheed A. , Yan Z. , Karuppiah N.
Published: July 2002

This article presents permanent magnetic objective lenses suitable for use in multicolumn electron-beam systems. Two types of lens arrays are presented, a single-pole lens tip array, and a multibore immersion lens array. Prototype lenses were tested as add-on lens attachments on an existing scanning electron microscopy (SEM), by mounting them on to the specimen stage of a conventional SEM, and successively moving each tip/bore on to the optical axis. SEM images were obtained from 2 by 2 tip/bore arrays. The images have demagnification strengths varying from 1000 to 7000 and are obtained from specimen regions up to 16 mm apart. These results demonstrate that permanent magnet multibeam objective lenses are in principle feasible. The axial field distribution created by each tip/bore in these multibeam lenses is similar to the axial field distribution of a single tip/bore high-resolution immersion lens.
Time resolved optical Kerr effect analysis of urea–water system
Authors: Idrissi A. , Bartolini P. , Ricci M. , Righini R.
Published: July 2002

The nuclear dynamics of urea aqueous solution was analyzed by time resolved optical Kerr effect (OKE). The data analysis was achieved in time and in frequency domains. Three relaxation times characterize the time decay of the OKE signal at high mole fractions of urea, while only two relaxation times characterize this decay for the low mole fractions. The observed slowest relaxation time increases with increasing the mole fraction of urea. The comparison between this relaxation time and the ones determined by Raman and nuclear magnetic resonance spectroscopies suggests that the slow relaxation time is related to the reorientation of an axis lying in the plane of the urea molecule. At high mole fractions, the power spectra derived from the Fourier transform of the OKE signal are characterized by one broad peak at around 70 cm−1 and by a shoulder at around 160 cm−1 in the high frequency part of the former peak. This shoulder is related to the hydrogen bond interactions which involve urea molecules. Molecular dynamics simulation results on urea/water system suggest that the power spectra derived from OKE data could be interpreted in terms of translational motions (caging effect) and in terms of rotational motion (libration) of urea molecules.
Near-infrared acousto-optic tunable filter based instrumentation for the measurement of dynamic spectra of polymers
Authors: Sweat Joseph A. , Wetzel David L.
Published: July 2002

The use of an acousto-optic tunable filter spectrometer can simplify the instrumental requirements for the collection of time resolved spectra to measure the difference in absorbance of polarized radiation due to chemical functional group orientation of a polymer as it undergoes repetitive oscillatory strain. The rapid data collection achieved in the near-infrared spectral region rivals that of current interferometric instruments. Achieving a good signal to noise ratio requires only a singly modulated optical signal, and the use of digital filtering eliminates the requirement of phase sensitive detection. The usefulness of this instrument for routine testing of polymers is demonstrated by differentiating polymers of the same chemical composition but different rheology and by determining the effect of varying the concentration of monomers in a copolymer.
Multiphoton ionization and photoelectron spectroscopy of 1,3-trans-butadiene via its 3dπ Rydberg state
Authors: Liu Jianbo , Anderson Scott L.
Published: July 2002

Resonance-enhanced multiphoton ionization (REMPI) and photoelectron spectroscopy (PES), have been used to study the Ag1(3dπ) Rydberg state of 1,3-trans-butadiene in the two photon energy range from 61 000 to 66 400 cm−1. The Ag1(3dπ) spectrum is dominated by the ν4′, ν6′, and ν9′ vibrational modes, with some excitation of the ν8′ mode, as well. Photoelectron spectroscopy shows that the dominant ionization pathways are diagonal, i.e., they produce cations in the same vibrational level that was populated in the Rydberg state. Weaker off-diagonal ionization is also observed, with excitation of the ν4+, ν6+, and ν9+ modes. The relative intensities of diagonal and off-diagonal PES bands are observed to be strongly dependent on the angle between the laser polarization and the detection axis. It is possible to use REMPI to generate state-selected cations, however, the nascent ions are quite efficiently photodissociated by the REMPI laser.
On the nonlinear turbulent dynamics of shear-flow decorrelation and zonal flow generation
Authors: Tynan G. R. , Moyer R. A. , Burin M. J. , Holland C.
Published: July 2002

Sheared flows, thought to be generated by turbulence in magnetized fusion plasmas, are predicted to mediate the transport of mass, momentum, and heat across the shear flow region. In this paper we show that an examination of three-wave coupling processes using the bispectrum and bicoherence of turbulent fields provides an experimentally accessible test of the turbulence-generated shear flow hypothesis. Results from the Continuous Current Tokamak (CCT), Princeton Beta Experiment–Modified (PBX–M), and DIII–D tokamaks indicate that the relative strength of three-wave coupling increases during low-mode to high-mode (L–H) transitions and that this increase is localized to the region of strong flow and strong flow shear. These results appear to be qualitatively consistent with the turbulence-generated shear flow hypothesis.
Detailed description of a compact cryogenic magnetic resonance force microscope
Authors: Smith Doran D. , Marohn John A. , Harrell Lee E.
Published: July 2002

We describe the design and operation of a cryogenic magnetic resonance force microscope for detecting nuclear magnetic resonance. Instrument-critical details are enumerated, including fabrication of a positionable radio-frequency coil, detection of angstrom-level microcantilever oscillations using an optical fiber interferometer, design of a compact fiber/cantilever alignment system, temperature compensation of the fiber/cantilever gap, control of sample temperature, and vibration isolation. Additionally, experimental protocols and sample specific considerations such as spin relaxation times are addressed. F19 nuclear magnetic resonance data obtained from a Nd-doped CaF2 sample are presented.
An ab initio direct-trajectory study of the kinetic isotope effect on the bifurcating reaction
Authors: Taketsugu Tetsuya , Kumeda Yuko
Published: July 2002

The isotope effects on the bifurcating reaction path of the isomerization reaction, H3CO→H2COH (stereoisomers of P and P*), have been investigated recently [J. Chem. Phys. 113, 477 (2000)], and it was found that the valley-ridge inflection point appears in the totally symmetric direction relative to the intrinsic reaction coordinate in the deuterium-substituted system. In the present report, the kinetic isotope effect on this reaction has been studied using an ab initio direct-trajectory method at the computational level of the complete active space self-consistent field method with 6-31G(d,p) basis sets. The branching ratio to the respective stereoisomers has been investigated in terms of distributions of several coordinates along the trajectories. The energy landscape for H3CO is also described in terms of several structures and the respective permutational isomers.
Spin-stand imaging of overwritten data and its comparison with magnetic force microscopy
Authors: Mayergoyz I. D. , Tse C. , Krafft C. , Gomez R. D.
Published: July 2002

A new technique of magnetic imaging on a spin-stand [Mayergoyz et al., J. Appl. Phys. 87, 6824 (2000)] is further developed and extensively tested. The results of successful imaging of digital patterns overwritten with misregistration ranging from 0.3 to 0.07 μm are reported. The results are compared with magnetic force microscopy (MFM) images and the conclusion is reached that the spin-stand imaging technique can provide (at least) the same level of resolution and accuracy as the MFM imaging technique.
Modifying the microstructure and morphology of film surface layers by manipulating chemical vapor deposition reactor conditions
Authors: Paritosh F. , Srolovitz D. J.
Published: July 2002

Two spatial dimension front tracking simulations have been performed to study the growth of polycrystalline, faceted films from randomly oriented nuclei. We present strategies to optimize the microstructure, morphology, and texture of thin films during chemical vapor deposition. In particular, we examine how changes in reactor conditions can be used to modify the mean grain size, surface roughness, crystallographic texture, and growth zones. Changing growth conditions once the target bulk film structure is established can be used to establish a thin surface region with much different structural characteristics. Analytical models are provided to aid in choosing the appropriate changes in reactor conditions and surface layer thickness to achieve optimal properties.
State-resolved translation energy distributions for NCO photodissociation
Authors: Hoops Alexandra A. , Bise Ryan T. , Gascooke Jason R. , Neumark Daniel M.
Published: July 2002

The photodissociation dynamics of NCO have been examined using fast beam photofragment translational spectroscopy. Excitation of the 102, 301, and 102302 transitions of the B̃ 2Π←X̃ 2Π band produces N(4S)+CO photofragments exclusively, while excitation of the 103303 transition yields primarily N(2D)+CO photoproducts. The translational energy [P(ET)] distributions yield D0(N–CO)=2.34±0.03 eV, and ΔHf,00(NCO)=1.36±0.03 eV. The P(ET) distributions exhibit vibrationally resolved structure reflecting the vibrational and rotational distributions of the CO product. The N(2D)+CO distribution can be fit by phase space theory (PST), while the higher degree of CO rotational excitation for N(4S)+CO products implies that NCO passes through a bent geometry upon dissociation. The P(ET) distributions suggest that when the B̃ 2Π←X̃ 2Π band is excited, NCO undergoes internal conversion to its ground electronic state prior to dissociation. Excitation of NCO at 193 nm clearly leads to the production of N(2D)+CO fragments. While conclusive evidence for the higher energy O(3P)+CN(X 2Σ+) channel was not observed, the presence of this dissociation pathway could not be excluded.
Structural and magnetic properties of GaMnAs layers with high Mn-content grown by migration-enhanced epitaxy on GaAs(100) substrates
Authors: Sadowski J. , Mathieu R. , Svedlindh P. , Domagała J. Z. , Bak-Misiuk J. , Światek K. , Karlsteen M. , Kanski J. , Ilver L. , Åsklund H. , Södervall U.
Published: July 2002

Ferromagnetic GaMnAs containing up to 10% Mn has been grown by migration-enhanced epitaxy at a substrate temperature of 150 °C. The lattice constant of hypothetical zinc-blende structure MnAs is determined to be 5.90 Å, which deviates somewhat from previously reported values. This deviation is ascribed to growth-condition-dependent density of point defects. Magnetization measurements showed an onset of ferromagnetic ordering around 75 K for the GaMnAs layer with 10% Mn. This means that the trend of falling Curie temperatures with increasing Mn concentrations above 5.3% is broken.
Network analysis based interface electronics for quartz crystal microbalance
Authors: Schröder Jens , Borngräber Ralf , Lucklum Ralf , Hauptmann Peter
Published: July 2002

The application of resonant sensors such as quartz crystal microbalance (QCM) resonators requires interface electronics to measure parameters that characterize sufficiently the resonant behavior of the sensor due to effects under investigation. Common oscillators as sensor electronics have two major disadvantages. Since they measure the resonant frequency shift as the only parameter their employment is restricted to mass sensing of thin and rigid films in terms of QCM. Because of spurious phase shifts the measured resonant frequency may be erratic. In order to characterize the sensor itself or a material on the sensor surface more parameters such as damping are to be measured. Therefore, a sensor electronics was developed that precisely acquires the impedance spectrum of the resonant sensor in a small frequency range which reflects properties like thickness of a sensor coating, its density, and shear moduli or the density-viscosity product when measuring in liquids. Originally developed as a sensor interface electronics this novel device can help researchers to investigate any effects or material parameters affecting the impedance spectrum of the sensor. In this article the measurement concept of the network analysis based device will be explained followed by details of realization on single printed cardboard. Measurement errors are estimated by simulation and by a comparison of measurement results from a commercially available network analyzer as a reference. Examples of laboratory and industrial applications will be given.
Nonfreezing water confined in water layer of multilamellar L-α, distearoyl phosphatidylcholine in temperature range between 0 °C and −190 °C
Authors: Utoh Shigenori
Published: July 2002

Where multilamellar phospholipid water exists, e.g., inside or outside of the multilamellar phospholipid, depends largely on the percentage of water content. For example, the multilamellar L-α, distearoyl phosphatidylcholine (DSPC) with 44 wt % water content carries water in excess water regions as well as in the water layer inside of the multilamellar DSPC. This excess water freezes at about −20 °C in forms ranging from super-cooled water to homogeneous ice with a hexagonal symmetry. On the other hand, when no excess water exists, e.g., in cases of multilamellar DSPC containing 15 wt % or 11 wt % water, water is contained entirely in the water layer inside of the multilamellar DSPC. When multilamellar DSPC contains 15 wt % water, the state of super-cooled water in the water layer is maintained at about −45 °C and then changes into heterogeneous ice or into many ice nuclei with a hexagonal symmetry in the temperature region of less than −45 °C. In the case of multilamellar DSPC with 11 wt % water content, however, the state of super-cooled water in the water layer remains unchanged even down to −190 °C. X-ray diffraction results prove that the super-cooled water in the water layer of multilamellar DSPC containing 11 wt % water does not freeze even down to −190 °C.
Tantalum oxide as an alternative low height tunnel barrier in magnetic junctions
Authors: Rottländer P. , Hehn M. , Lenoble O. , Schuhl A.
Published: July 2002

Magnetic tunnel junctions with a barrier of tantalum oxide were prepared by plasma oxidation of sputter-deposited tantalum. They show magnetoresistance ratios of 2.5% at room temperature and 4% at low temperatures. The material exhibits low barrier heights of ∼0.4 eV. This makes it possible to substantially increase the barrier thickness, compared to a barrier of aluminum oxide. The resulting decrease of coupling between the ferromagnetic layers is easily seen. Tantalum oxide appears to be a candidate for use as a tunnel barrier of spin-dependent tunneling devices.
Experimental determination of phenol (CH3F)1 complex binding energies in the S, S1, and I states and comparison with ab initio calculations
Authors: Longarte Asier , Fernández José A. , Unamuno Iñigo , Basterrechea Francisco , Castaño Fernando
Published: July 2002

Weakly bound complexes of phenol (Ph) and fluoromethane (CH3F) formed in a supersonic expansion have been identified by one- and two-color mass-resolved and hole burning spectroscopies. Only one isomer has been observed for the 1:1 complex. Threshold fragmentation has been employed to determine the binding energies of the complex in its ground, S0, and first electronic, S1, states, as well as in the ion ground state, I0, yielding the following results: D0(S0)=1540±50 cm−1, D0(S1)=1713±50 cm−1, and D0(I0)=3932±50 cm−1, respectively. In a complementary study, calculations on the complex geometries and binding energies were conducted at the B3LYP/6-31+G* and the MP2/6-31+G* levels. It has been shown that the binding energies computed at the MP2/6-31+G* level are in excellent agreement with the experimental values, whilst those calculated at the B3LYP/6-31+G* level underestimate them by nearly 30%, probably due to the poor description of the dispersion forces.
An M-theory flop as a large N duality
Authors: Atiyah Michael , Maldacena Juan , Vafa Cumrun
Published: July 2002

We show how a recently proposed large N duality in the context of type IIA strings with N=1 supersymmetry in 4 dimensions can be derived from purely geometric considerations by embedding type IIA strings in M-theory. The phase structure of M-theory on G2 holonomy manifolds and an S3 flop are the key ingredients in this derivation.
Electronic structure of a copper(III) compound
Authors: Klimkāns Agris , Larsson Sven
Published: July 2002

The singlet ground state of a potassium cuprate system (KCuO2), represented by clusters of copper(III) and oxygen, consistent of one and three copper atoms embedded in the Madelung potential of the remainder of the crystal, is calculated using the complete active space-self-consistent field method. The ground state is found to be a 3d8 state with high σ covalency. The electron structure of hole doped high-Tc superconductors with similar copper ligation is shortly discussed on the basis of our results, which do not support the “hole on oxygen” model.
Low frequency fishbone mode induced by circulating particles in spherical tori
Authors: Kolesnichenko Ya. I. , Marchenko V. S. , White R. B.
Published: July 2002

It is found that high β in low-aspect-ratio tori tends to stabilize the fishbone instability in a plasma with energetic circulating ions. The stabilization results from enhancement of the toroidal drift motion by large Shafranov shift, which makes it difficult to reconcile the condition of considerable energy exchange between the ions and the internal kink perturbation with the condition of the resonant wave–particle interaction.
Ornstein–Zernike equation for convex molecule fluids
Authors: Boublı́k Tomáš
Published: July 2002

Structure of fluids is suitably characterized by distribution functions from which the most important is the pair correlation function. Theoretical approaches to get the pair distribution function are based mainly on the solution of the Ornstein–Zernike (OZ) integral equation. In this paper, the OZ equation for molecular fluids is modified to yield the average correlation function for systems of convex molecules. In our approach we employed the previously proposed method to separate the shape effect of molecular cores from that due to the variable surface–surface distances among three pairs of convex cores. The effect of nonspherical shape of hard cores in the convolution integral is expressed through the derivative with respect to three surface–surface distances of the expression for the hard convex body third virial coefficient. For simple fluids (with the pointwise cores) the derived expression reduces to the standard OZ equation. The modified OZ equation is solved numerically for the Percus–Yevick-type closure and the average correlation functions in the systems of hard spherocylinders with l/σ=0.4, 0,6 and 1 were determined. The obtained dependencies of the average correlation functions on the reduced distances calculated from the modified OZ equation agree well with the simulation data for the above systems at relatively high densities.
Nanometer-scale electrical characterization of stressed ultrathin SiO2 films using conducting atomic force microscopy
Authors: Porti M. , Nafrı́a M. , Aymerich X. , Olbrich A. , Ebersberger B.
Published: July 2002

A conductive atomic force microscope has been used to electrically stress and to investigate the effects of degradation in the conduction properties of ultrathin (<6 nm) SiO2 films on a nanometer scale (areas of ≈100 nm2). Before oxide breakdown, switching between two states of well-defined conductivity and sudden changes of conductivity were observed, which are attributed to the capture/release of single charges in the defects generated during stress.
Continuous-wave self-pumped optical parametric oscillator based on Yb3+-doped bulk periodically poled LiNbO3 (MgO)
Authors: Capmany J. , Callejo D. , Bermúdez V. , Diéguez E. , Artigas D. , Torner L.
Published: July 2002

We present experimental evidence of continuous-wave self-pumped parametric oscillation in Yb3+-doped periodically poled lithium niobate co-doped with MgO. A single bulk crystal of periodically poled lithium niobate containing Yb3+ laser-active ions performs as a singly resonant parametric oscillator generating a signal wave at 1360 nm. The optical parametric oscillator is intracavity pumped by the laser emission at 1063 nm produced by the Yb3+ ions in the crystal. The whole system is end pumped at 980 nm.
Effective action for QED4 through ζ function regularization
Authors: Beneventano C. G. , Santangelo E. M.
Published: July 2002

We obtain, through ζ function methods, the one-loop effective action for massive Dirac fields in the presence of a uniform, but otherwise general, electromagnetic background. After discussing renormalization, we compare our ζ function result with Schwinger’s proper-time approach.
Fabrication of strain-balanced Si/Si1−xGex multiple quantum wells on Si1−yGey virtual substrates and their optical properties
Authors: Kawaguchi K. , Shiraki Y. , Usami N. , Zhang J. , Woods N. J. , Breton G. , Parry G.
Published: July 2002

Strain-balanced Si/SiGe multiple quantum wells (MQWs), which are designed to overcome the limitation of the number of wells coming from the strain accumulation, were fabricated, and their optical properties were investigated. X-ray diffraction spectra and cross-sectional transmission-electron-microscope images showed a high-crystalline quality of samples and excellent uniformity of the well width. Well-resolved no-phonon and TO-phonon-assisted transitions from strain-balanced MQWs were observed by low-temperature photoluminescence spectroscopy, and both their temperature and excitation power dependence showed blueshifts due to the delocalization of excitons, the band bending, and/or the band-filling effect.
The HD–He complex: Interaction-induced dipole surface and infrared absorption spectra
Authors: Gustafsson Magnus , Frommhold Lothar
Published: July 2002

The collision-induced dipole surface of an HD molecule interacting with an He atom is derived from the ab initio dipole data of the H2–He complex [Gustafsson et al., J. Chem. Phys. 113, 3641, (2000)]. Collision-induced absorption spectra of gaseous mixtures of deuterium hydride and helium in the rotational and fundamental bands of HD are calculated, for comparison with an existing measurement taken at a temperature of 77 K. To that end, we integrate the close-coupled, radial Schrödinger equations, accounting for the anisotropy of the HD–He interaction potential. The computed absorption spectra generally agree reasonably well with the observed spectral profiles and intensities of the collision-induced spectra. We also consider the interference phenomena of the HD permanent dipole with the interaction-induced, supramolecular dipole by computing the wings of various R(j) lines and of the P1(1) line in single, binary collision limit. For comparison, the line broadening and shift for the P and R line shape parameters are also computed using the impact approximation. The close-coupled treatment of our calculation of the spectral profiles accounts for rotational level mixing in a nonperturbative manner. The treatment is fully quantum-mechanical and takes into account single binary collisions of HD and He.
Laser frequency converter for continuous-wave tunable Ti:sapphire lasers based on aperiodically poled LiNbO3:Nd3+
Authors: Capmany J. , Pereda J. A. , Bermúdez V. , Callejo D. , Diéguez E.
Published: July 2002

We present a laser device that shifts the frequencies contained in the 750–850 nm emission band interval of a continuous wave Ti:sapphire tunable laser and produces continuous-wave tunable laser radiation between 440 and 475 and 485 and 505 nm. The device is based on self-sum-frequency mixing of the tunable emission of a Ti:sapphire laser injected in the cavity of a diode-pumped aperiodically poled Nd3+:LiNbO3 laser oscillating in continuous-wave at 1084 or 1372 nm.
Current crowding and 1/f noise in polycrystalline silicon thin film transistors
Authors: Mercha A. , Vandamme L. K. J. , Pichon L. , Carin R. , Bonnaud O.
Published: July 2002

Here we investigate dc characteristics, impedance versus frequency, and low frequency noise. The effect of current distribution on 1/f noise in polycrystalline silicon thin film transistors (TFTs) is discussed. We show that the channel impedance versus frequency roll induces spectra that could be misinterpreted above a corner frequency in terms of a frequency index γ in 1/fγ with γ higher than 2. Moreover, ignoring the inhomogeneous current distribution leads to overestimation of the noise parameter. Whereas it seemed evident that the noise in TFTs can be interpreted in terms of carrier number fluctuation, the noise parameter variation versus the gate bias can be also understood in terms of mobility fluctuations by including the potential barrier evolution.
Infrared spectroscopy studies of iodoethane on Si(100)-2×1: Adsorption and thermal decomposition leading to adsorbate ordering
Authors: Bulanin Kirill M. , Shah Ankit G. , Teplyakov Andrew V.
Published: July 2002

The adsorption and chemical transformation of iodoethane were studied on a Si(100)-2×1 surface using multiple-internal reflection Fourier-transform infrared spectroscopy (MIR-FTIR). The C–H and Si–H stretch vibrations served as fingerprints of all surface transformations. Although ethyl groups are stable on the Si(100)-2×1 surface at room temperature, thermal annealing studies suggest the reaction-limited formation of ethylene, a major hydrocarbon reaction product, accompanied by the loss of hydrogen, which is left on the surface until the temperature of recombinative H2 desorption is reached. Variable temperature studies indicate that ethyl groups are the only hydrocarbon entities on a surface up until the hydrogen elimination temperature. The positions of Si–H stretching bands suggest that a mixture of surface sites is formed at temperatures between 300 K and 600 K. However, the majority of surface hydrogen forms SiH–SiI surface species as the surface temperature reaches 700 K.
Proton friction and diffusion coefficients in hydrated polymer electrolyte membranes: Computations with a non-equilibrium statistical mechanical model
Authors: Paddison Stephen J. , Paul Reginald , Zawodzinski Thomas A.
Published: July 2002

A recently derived mathematical model to compute the effective friction and diffusion coefficients of hydronium ions in hydrated polymer electrolyte membranes is described and tested for dependence on membrane-specific parameters. Contributions to the friction coefficient due to water–polymer, water–hydronium, and hydronium–polymer interactions are determined through computation of force–force correlation functions. The conventional Stokes law friction coefficient of the hydronium ion in bulk water is then “corrected” with these statistically derived contributions and the corresponding diffusion coefficient calculated. For a Nafion® membrane pore with an hydration level of six water molecules per sulfonic acid functional, the model was used to compute friction coefficients for various distributions of the fixed sites, and for different side chain lengths. The model showed substantial sensitivity to these parameters and predicted that for pores of fixed volume and a constant total number of sulfonate groups, the friction on the hydrated proton is the greatest for distributions with high local anionic charge density. In a second series of computations where the radius and length of the pore were varied, the model demonstrated that the proton diffusion increases with increasing channel diameter. These calculations, therefore, demonstrate the important predictive capability of this molecular-based, nonequilibrium statistical mechanical model.
Spatial discrimination of Rydberg tagged molecular photofragments in an inhomogeneous electric field
Authors: Monti O. L. A. , Cruse H. A. , Softley T. P. , Mackenzie S. R.
Published: July 2002

A new approach to detecting the spatial and energetic distribution of photodissociation products is demonstrated, in which an inhomogeneous electric field is used for spatially selective field ionization of Rydberg molecules. State-selected NO fragments from NO2 photodissociation in a supersonic beam are excited to high-n Rydberg states and allowed to expand through the inhomogeneous field provided by an octopole. The field in the octopole results in the field ionization of those fragments that have moved to large radial distances (∼5 mm) from the beam axis, and therefore this device transmits only those Rydberg molecules with a low velocity component in the direction perpendicular to the beam. A detailed characterization of the properties of this “steradiancy analyzer” is carried out and factors limiting the energy resolution (currently approximately 100 cm−1 at an excess energy of 250 cm−1), along with possible improvements, are discussed in detail.
High temperature photoluminescence efficiency and thermal stability of (InGa)(AsN)/GaAs quantum wells
Authors: Bissiri M. , Gaspari V. , Polimeni A. , Baldassarri Höger von Högersthal G. , Capizzi M. , Frova A. , Fischer M. , Reinhardt M. , Forchel A.
Published: July 2002

The temperature dependence of the photoluminescence (PL) efficiency of (InGa)(AsN)/GaAs single quantum wells (QWs) has been studied from 10 to 500 K. The PL intensity of N-containing samples is almost constant from room temperature to 500 K, in contrast to what is observed in (InGa)As QWs grown under the same conditions. This thermal stability increases for an increase in nitrogen content. We discuss these effects in terms of strain compensation at high nitrogen concentrations.
Trap influenced properties of the delayed luminescence in thin solid films of the conjugated polymer Poly (9,9-di(ethylhexyl)fluorene)
Authors: Rothe C. , Guentner R. , Scherf U. , Monkman A. P.
Published: July 2002

Spectra, kinetics, temperature, and excitation-dose dependencies of phosphorescence and delayed fluorescence intensities in thin films of the conjugated polymer poly(9,9-di(ethylhexyl)fluorene) [PF2/6] have been investigated via time-resolved spectroscopy. The results reflect a limited number of available energy sites (traps) for triplet-excited states in the long time region after laser excitation. Triplets captured in these traps decay mono-exponential with a time constant of several 100 ms. In the first few milliseconds after laser excitation, pronounced delayed fluorescence is observed due to triplet–triplet annihilation. The decay kinetics of the delayed fluorescence obeys a power law at all times in accordance with the framework in which triplets find each other after performing a random walk (hopping) in an energetic disordered media, which is represented by variations of the conjugation length in the polymer.
Structural phase transition in early growth of Bi2Sr2CaCu2O8+x films on SrTiO3 substrates
Authors: Abrecht M. , Ariosa D. , Onellion M. , Margaritondo G. , Pavuna D.
Published: July 2002

We used pulsed laser deposition, with a Bi2Sr2CaCu2O8+x target, to grow films ranging from (1/4) to 10 unit cells thick. We studied these films, and reference Bi2Sr2CaCu2O8+x single crystal samples, using angle-integrated photoemission, core level photoemission, and x-ray diffraction. The data indicate that all films exhibit a metallic-like Fermi edge in the photoemission data. More strikingly, a structural phase transition occurs at a nominal Bi2Sr2CaCu2O8+x thickness of approximately one unit cell, converting the precursor Bi2O2.33 highly coherent thin film into a Bi2Sr2CaCu2O8+x structure.
A 75 keV, 140 mA proton injector
Authors: Sherman Joseph D. , Figueroa Terry L. , Hansborough Lash D. , Kerstiens Debora M. , Schneider J. David , Smith H. Vernon , Stettler Matthew W. , Stevens Ralph R. , Thuot Michael E. , Warren David S. , Zaugg Thomas J. , Arvin Adrian A. , Bolt Alvin S. , Boers Jack E.
Published: July 2002

A dc and pulsed-mode 75 keV proton injector has been developed and is used in characterization of a continuous-wave 6.7 MeV, 100 mA radio-frequency quadrupole (RFQ). The injector is used frequently at the full RFQ design power (100 mA, 6.7 MeV) where the RFQ admittance (1 rms, normalized) is 0.23 (π mm mrad). The injector includes a 2.45 GHz microwave proton source and a beam space-charge-neutralized, two magnetic-solenoid, low-energy beam-transport system. The design RFQ beam transmission of 95% has been demonstrated at 100 mA RFQ output current.
Polarization reversal in a perovskite ferroelectric by molecular-dynamics simulation
Authors: Sepliarsky M. , Phillpot S. R. , Streiffer S. K. , Stachiotti M. G. , Migoni R. L.
Published: July 2002

We characterize the microscopic processes that take place during polarization reversal in the tetragonal phase of a monodomain order–disorder perovskite ferroelectric using molecular-dynamics simulation. Microscopically the polarization of each unit cell reorients from an [001] orientation to an [001̄] orientation through intermediate states with polarization parallel to [011] and [011̄]. For low electric field and low temperature, chains of polarization reverse in a spatially coherent manner, resulting in macroscopic intermediate states with orthorhombic symmetry. At high electric field and high temperature the process is completely incoherent and there is no well-defined macroscopic intermediate state.
The role of computational physics in the design of ECRIS
Authors: Girard A. , Serebrennikov K. , Melin G. , Vallcorba R. , Lécot C.
Published: July 2002

Electron cyclotron resonance ion sources (ECRIS) are sophisticated plasma devices, with a complex magnetic structure. The designer of an ECRIS usually solves the following problems: (i) the conception of an efficient magnetic structure; (ii) the design of an efficient extraction system. To solve the first problem numerical codes are either free or commercially available; we briefly summarize the characteristics of these codes, with particular emphasis on cryogenic devices. The second problem is much more tricky, since it is not only an electrostatic problem, as it also involves the plasma. Different codes are, however, able to treat the problem, with some assumptions concerning the plasma; these assumptions are discussed. These codes can be very useful for the new ECRIS generation, where the current densities become very high. But today there is no program available for the calculation of the charge state distribution of the ions, the magnetic structure being given; only the experience of the designer and a certain knowledge of conception rules can lead to some (rough) estimate of the future performances. We present in this paper the first version of such a code, where the input parameters of the code are the technological parameters. These inputs being given, the code solves the self-consistent problem of the plasma creation (through the resonant interaction of the electrons with the wave), the production of the ions and their transport to the extraction zone. The first results of this code are presented and discussed. This article shows that it will be soon possible to have reliable predictions of the performances of an ECRIS.
Three-dimensional stability of periodic arrays of counter-rotating vortices
Authors: Julien S. , Chomaz J.-M. , Lasheras J.-C.
Published: July 2002

We study the temporally developing three-dimensional stability of a row of counter-rotating vortices defined by the exact solution of Euler’s equations proposed by Mallier and Maslowe [Phys. Fluids 5, 1074 (1993)]. On the basis of the symmetries of the base state, the instability modes are classified into two types, symmetric and anti-symmetric. We show that the row is unstable to two-dimensional symmetric perturbations leading to the formation of a staggered array of counter-rotating vortices. For long wavelengths, the anti-symmetric mode is shown to exhibit a maximum amplification rate at small wave numbers whose wavelengths scale mainly with the period of the row. This mode could be interpreted as due to the Crow-type of instability extended to the case of a periodic array of vortices. For short wavelengths, symmetric and anti-symmetric instability modes are shown to have comparable growth rates, and the shorter the wavelength, the more complex the structure of the eigenmode. We show that this short wavelength dynamic is due to the elliptic instability of the base flow vortices, and is well modeled by the asymptotic theory of Tsai and Widnall. The effect of varying the Reynolds number was also found to agree with theoretical predictions based on the elliptic instability.
Frequency up-conversion as a temperature probe of organic opto-electronic devices
Authors: Lupton John M.
Published: July 2002

Frequency up-conversion is demonstrated in a polyfluorene-based conjugated polymer. Up-converted emission is observed upon excitation to the red of the 0-0 luminescence band. The emission intensity depends strongly on temperature and provides an accurate probe of the operating temperature of organic light-emitting diodes. Temperature rises of up to 30 K are observed at standard operating current densities of 225 mA/cm2. Due to the low thermal conductivity and heat capacity of the polymer film, the temperature of the film is found to rise slowly on a time-scale of seconds. Upon termination of the current, the temperature decreases again on a time-scale of minutes.
Ion kinetics in a magnetized plasma source
Authors: Breizman Boris N. , Arefiev Alexey V.
Published: July 2002

There are regimes of plasma source operation in which the ion motion is controlled by the ambipolar electric field and ion–atom collisions. This paper presents a derivation of the corresponding ion distribution function and plasma density profiles in the source for a given electron temperature and a given gas ionization rate. Conditions are discussed under which the ion flux is predominantly radial or predominantly axial. It is found that the presence of plasma can substantially change the neutral gas pressure due to production of fast charge–exchange neutrals.
Submicron x-ray diffraction and its applications to problems in materials and environmental science
Authors: Tamura N. , Celestre R. S. , MacDowell A. A. , Padmore H. A. , Spolenak R. , Valek B. C. , Meier Chang N. , Manceau A. , Patel J. R.
Published: July 2002

The availability of high brilliance third generation synchrotron sources together with progress in achromatic focusing optics allows us to add submicron spatial resolution to the conventional century-old x-ray diffraction technique. The new capabilities include the possibility to map in situ, grain orientations, crystalline phase distribution, and full strain/stress tensors at a very local level, by combining white and monochromatic x-ray microbeam diffraction. This is particularly relevant for high technology industry where the understanding of material properties at a microstructural level becomes increasingly important. After describing the latest advances in the submicron x-ray diffraction techniques at the Advanced Light Source, we will give some examples of its application in material science for the measurement of strain/stress in metallic thin films and interconnects. Its use in the field of environmental science will also be discussed.
Model for the α→β1 phase transition in phthalocyanine thin films
Authors: Yim S. , Heutz S. , Jones T. S.
Published: July 2002

The α→β1 phase transition induced by annealing free-base phthalocyanine thin films grown by organic molecular-beam deposition has been studied by powder x-ray diffraction, electronic absorption spectroscopy, and Nomarski interference microscopy. The final β1 phase consists of two domains having a different orientation relative to the substrate surface. Starting with the α phase, by careful control of the annealing temperature, a series of intermediate states are observed consisting of coexisting α and β1 phases. At the beginning of the phase transition, only one domain is observed with the β1 crystal phase aligned with the (001) plane parallel to the substrate. As the transition proceeds, however, the amount of the other domain having the (201) plane parallel to the substrate increases rapidly. A mechanism is proposed for the transition that can account for all the experimental results.