bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal stability of spin valve sensors using artificial CoFe/Ir based ferrimagnets
Authors: Colis S. , Guth M. , Arabski J. , Dinia A. , Muller D.
Published: July 2002

Hard–soft spin valve structures have been grown by molecular beam epitaxy on MgO(001) substrates. The hard magnetic layer consists of (Co50Fe50)/Ir/(Co50Fe50) artificial ferrimagnetic (AFi) system, while a Fe/Co bilayer integrated in the buffer, has been used as a soft detection layer. The Fe has been grown at 500 °C giving rise to a monocrystalline layer with a body centered cubic structure. The spin valve structure presents a progressive evolution after successive annealing steps up to 350 °C. The total giant magnetoresistance (GMR) reaches its maximum (5.3%) after annealing at 250 °C, together with a good rigidity of the hard layer and a sharp switch of the magnetic moments. Such characteristics are reduced, but still interesting, after annealing at 300 °C. For annealing at higher temperature (350 °C) the total GMR signal and the coercive field of the AFi decrease dramatically and all the stack behaves like a single magnetic layer. Rutherford backscattering measurements were performed in order to investigate the changes in the morphology of CoFe/Ir interfaces and to correlate them to the magnetotransport properties.
Electrical transport properties of NiSi2 layers synthesized by metal vapor vacuum-arc ion implantation: Temperature dependence and two-band model
Authors: Zhang X. W. , Wong S. P. , Cheung W. Y. , Zhang F.
Published: July 2002

Nickel disilicide (NiSi2) layers were synthesized by nickel ion implantation into silicon substrates using a metal vapor vacuum-arc ion source. The electrical transport properties were studied before and after annealing using sheet resistivity and Hall effect measurements from 30 to 400 K, and the results showed surprising features very different from those generally reported NiSi2 layers. The temperature dependence of sheet resistivity Rs(T) and Hall mobility μH(T) in the as-implanted samples showed peculiar peak and valley features varying from sample to sample, depending on the preparation conditions. However, after annealing, the differences between samples seemed to have diminished in that they all showed similar shapes in both the Rs(T) and the μH(T) curves. A two-band model was proposed to explain the observed electrical transport properties.
Structural disorder in ion-implanted AlxGa1−xN
Authors: Kucheyev S. O. , Williams J. S. , Zou J. , Li G. , Jagadish C. , Manasreh M. O. , Pophristic M. , Guo S. , Ferguson I. T.
Published: July 2002

The accumulation of structural damage in AlxGa1−xN films (with x=0.05–0.60) under heavy-ion bombardment at room temperature is studied by a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy (XTEM). Results show that an increase in Al concentration strongly enhances dynamic annealing processes in AlGaN and suppresses ion-beam-induced amorphization. All AlGaN wafers studied show damage saturation in the bulk for high ion doses. Interestingly, the disorder level in the saturation regime is essentially independent of Al content. In contrast to the case of GaN, no preferential surface disordering is observed in AlGaN during heavy-ion bombardment. XTEM reveals similar implantation-produced defect structures in both GaN and AlGaN.
High resolution imaging and lithography with hard x rays using parabolic compound refractive lenses
Authors: Schroer C. G. , Benner B. , Günzler T. F. , Kuhlmann M. , Zimprich C. , Lengeler B. , Rau C. , Weitkamp T. , Snigirev A. , Snigireva I. , Appenzeller J.
Published: July 2002

Parabolic compound refractive lenses are high quality optical components for hard x rays. They are particularly suited for full field imaging, with applications in microscopy and x-ray lithography. Taking advantage of the large penetration depth of hard x rays, the interior of opaque samples can be imaged with submicrometer resolution. To obtain the three-dimensional structure of a sample, microscopy is combined with tomographic techniques. In a first hard x-ray lithography experiment, parabolic compound refractive lenses have been used to project the reduced image of a lithography mask onto a resist. Future developments are discussed.
Coherent states on spheres
Authors: Hall Brian C. , Mitchell Jeffrey J.
Published: July 2002

We describe a family of coherent states and an associated resolution of the identity for a quantum particle whose classical configuration space is the d-dimensional sphere Sd. The coherent states are labeled by points in the associated phase space T*(Sd). These coherent states are not of Perelomov type but rather are constructed as the eigenvectors of suitably defined annihilation operators. We describe as well the Segal–Bargmann representation for the system, the associated unitary Segal–Bargmann transform, and a natural inversion formula. Although many of these results are in principle special cases of the results of Hall and Stenzel, we give here a substantially different description based on ideas of Thiemann and of Kowalski and Rembieliński. All of these results can be generalized to a system whose configuration space is an arbitrary compact symmetric space. We focus on the sphere case in order to carry out the calculations in a self-contained and explicit way.
Volume and interface magnetic anisotropy of Fe1−xCox thin films on GaAs(001)
Authors: Dumm M. , Uhl B. , Zölfl M. , Kipferl W. , Bayreuther G.
Published: July 2002

Epitaxial Fe1−xCox alloy films with x=0.22 and x=0.33 were grown on GaAs(001) by molecular-beam epitaxy in a thickness range of 3 to 80 monolayers (ML). The magnetic properties were investigated by magneto-optic Kerr effect. Ferromagnetic order at room temperature was observed for thicknesses above 4 ML. The in-plane magnetic anisotropy of all films is a superposition of a uniaxial component with the easy axis along [110], which is not discussed here, and a four-fold contribution. The effective uniaxial and four-fold anisotropy constants, KUeff and K1eff, were determined by fits to the hard axis magnetization loops. K1eff contains an interface and a volume term which lead to a linear variation of K1eff with the inverse film thickness. It turns out that the surface and volume anisotropy constants are of opposite sign for all alloy compositions causing a sign reversal at a critical thickness, tcrit. This critical thickness seems to be a universal value, which is caused by a general proportionality between the volume and the interface anisotropy constants with the same negative constant of proportionality for Fe1−xCox/GaAs(001), Fe/Au(001), and Fe/Ag(001). This behavior of the four-fold anisotropy constants is consistently explained within Néel’s pair energy model for a body-centered-cubic ferromagnet.
Exchange coupling in FeTaN/IrMn/FeTaN and NiFe/IrMn/NiFe trilayer films
Authors: Jung H. S. , Doyle W. D. , Fujiwara H. , Wittig J. E. , Al-Sharab J. F. , Bentley J. , Evans N. D.
Published: July 2002

The dependence of the exchange bias field Heb in FeTaN/IrMn and NiFe/IrMn systems on the microstructure has been investigated. Bilayer and trilayer films of 50 nm thick FeTaN and NiFe and 10 nm thick IrMn were prepared by dc magnetron sputtering. The glass/FeTaN/IrMn/FeTaN trilayer showed that the top FeTaN did not influence Heb in the bottom FeTaN, closest to the substrate, during deposition and annealing. High-resolution transmission electron microscopy results showed limited evidence of epitaxial growth with both columnar single IrMn grains and multiple IrMn grains. In contrast, in the glass/NiFe/IrMn/NiFe trilayer, the top NiFe significantly influenced Heb in the bottom NiFe during deposition and annealing. X-ray diffraction data for both systems showed no detectable changes in either the crystallinity or (111) texture of the IrMn layer during annealing. In the NiFe system, the trend in Heb in the as-deposited and annealed states may be explained by assuming single columnar grains in the IrMn which couple the top and bottom NiFe layers. In the FeTaN system, the trend in Heb in the as-deposited state may be explained by assuming multiple IrMn grains which decouple the top and bottom FeTaN layers. However, the behavior on annealing is a mystery.
Anderson localization of ballooning modes, quantum chaos and the stability of compact quasiaxially symmetric stellarators
Authors: Redi M. H. , Johnson J. L. , Klasky S. , Canik J. , Dewar R. L. , Cooper W. A.
Published: July 2002

The radially local magnetohydrodynamic (MHD) ballooning stability of a compact, quasiaxially symmetric stellarator (QAS), is examined just above the ballooning beta limit with a method that can lead to estimates of global stability. Here MHD stability is analyzed through the calculation and examination of the ballooning mode eigenvalue isosurfaces in the 3-space (s,α,θk); s is the edge normalized toroidal flux, α is the field line variable, and θk is the perpendicular wave vector or ballooning parameter. Broken symmetry, i.e., deviations from axisymmetry, in the stellarator magnetic field geometry causes localization of the ballooning mode eigenfunction, and gives rise to new types of nonsymmetric eigenvalue isosurfaces in both the stable and unstable spectrum. For eigenvalues far above the marginal point, isosurfaces are topologically spherical, indicative of strong “quantum chaos.” The complexity of QAS marginal isosurfaces suggests that finite Larmor radius stabilization estimates will be difficult and that fully three-dimensional, high-n MHD computations are required to predict the beta limit.
From atomistic lattice-gas models for surface reactions to hydrodynamic reaction-diffusion equations
Authors: Evans J. W. , Liu Da-Jiang , Tammaro M.
Published: July 2002

Atomistic lattice-gas models for surface reactions can accurately describe spatial correlations and ordering in chemisorbed layers due to adspecies interactions or due to limited mobility of some adspecies. The primary challenge in such modeling is to describe spatiotemporal behavior in the physically relevant “hydrodynamic” regime of rapid diffusion of (at least some) reactant adspecies. For such models, we discuss the development of exact reaction-diffusion equations (RDEs) describing mesoscale spatial pattern formation in surface reactions. Formulation and implementation of these RDEs requires detailed analysis of chemical diffusion in mixed reactant adlayers, as well as development of novel hybrid and parallel simulation techniques.
Structure and magnetic properties of the MnBi low temperature phase
Authors: Yang J. B. , Yelon W. B. , James W. J. , Cai Q. , Roy S. , Ali N.
Published: July 2002

High purity MnBi low temperature phase has been prepared and analyzed using magnetic measurements and neutron diffraction. The low-temperature phase of the MnBi alloy has a coercivity μ0iHc of 2.0 T at 400 K, and exhibits a positive temperature coefficient from 0 to at least 400 K. The neutron data refinement indicated that the Mn atom changes its spin direction from c axis above room temperature to nearly perpendicular to the c axis at 50 K. A canted magnetic structure has been observed below 200 K. The anisotropy field increases with increasing temperature which gives rise to a high coercivity at the higher temperatures. The anisotropic bonded magnets have maximum energy products (BH)max of 7.7 and 4.6 MGOe at room temperature and 400 K, respectively.
Front initiation on microdesigned composite catalysts
Authors: Li Xiujiang , Kevrekidis Ioannis G. , Pollmann Michael , Papathanasiou Athanasios G. , Rotermund Harm Hinrich
Published: July 2002

We first briefly review the subject of spatiotemporal pattern formation on microdesigned composite catalysts. One of the most significant interaction mechanisms between different reacting domains (consisting of different metal catalysts such as Pt and Rh, coupled through surface diffusion) is the initiation of reaction fronts at the interface between them. We then explore in some detail the effect of two-dimensional composite geometry on this basic building block of composite catalyst dynamics.
Performing current versus voltage measurements of single-walled carbon nanotubes using scanning force microscopy
Authors: de Pablo P. J. , Gómez-Navarro C. , Martı́nez M. T. , Benito A. M. , Maser W. K. , Colchero J. , Gómez-Herrero J. , Baró A. M.
Published: July 2002

In this letter, a method is presented to perform current versus voltage measurements on carbon nanotubes using the tip of a scanning force microscope as an electrode that can be positioned along the molecule. This method allows current versus voltage measurements to be carried out at any spot along a nanotube. By using this method, we present indications of ballistic transport in carbon nanotubes, as well as effects in the electrical properties due to the mechanical deformation. This is a general technique that can be applied to any conducting nanowire.
Structure and dynamics of ice Ih films upon HCl adsorption between 190 and 270 K. I. Neutron diffraction and quasielastic neutron scattering experiments
Authors: Demirdjian B. , Ferry D. , Suzanne J. , Toubin C. , Picaud S. , Hoang P. N. M. , Girardet C.
Published: July 2002

Neutron diffraction and quasielastic neutron experiments are performed to investigate the effect of HCl adsorption on the structure and dynamics of an ultra-thin ice Ih film (5 H2O bilayers thick) deposited on a crystalline MgO(001) substrate. Three HCl coverages have been studied 0.3, 0.6, and 1 monolayer (ML) in the temperature range 190–270 K. At 0.3 and 0.6 HCl monolayer, no mobility is measured at T⩽220 K. A translational mobility, which is a signature of a liquid phase, is observed at T=250 K. This phase occurs 15 K below the surface melting temperature of the bare ice film. The fraction of mobile molecules represents 30% (0.3 ML HCl) and 45% (0.6 ML HCl) of the film. At 1 HCl monolayer and T=220 K, HCl–dihydrate coexists with ice Ih, whereas at T=250 K the ice film becomes amorphous and only 9% of the film is mobile. The results are discussed within the context of atmospheric chemistry.
Fractal cluster modeling of the fatigue behavior of lead zirconate titanate
Authors: Priya Shashank , Kim Hyeoung Woo , Ryu Jungho , Uchino Kenji , Viehland Dwight
Published: July 2002

The fatigue behavior of lead zirconate titanate ceramics (PZT) has been studied under electrical and mechanical drives. Piezoelectric fatigue was studied using a mechanical method. Under ac mechanical drive, hard and soft PZTs showed an increase in the longitudinal piezoelectric constant at short times, reaching a maximum at intermediate times. Systematic investigations were performed to characterize the electrical fatigue behavior. A decrease in the magnitude of the remanent polarization was observed, which was relatively high for soft PZT. The dynamics of fatigue have been shown to scale to a hierarchical relaxation process, and this is typical of random field systems.
Broadband absorption changes and sensitization of near-infrared photorefractivity induced by ultraviolet light in LiNbO3:Mg
Authors: Zhang Guoquan , Tomita Yasuo
Published: July 2002

We observe ultraviolet-light-induced absorption changes in spectral regions from the near-ultraviolet to the near-infrared in as-grown LiNbO3:Mg. Such a broad light-induced absorption band is unstable at room temperature and decays nonmonoexponentially in the dark with a time constant on the order of tens of seconds. We attribute the appearance of the broadband light-induced absorption to the creation of the intermediate state O− in which holes excited from deep defect centers are trapped at O2− sites near cation vacancies. It is also found that near-infrared photorefractive sensitivities increase by several orders of magnitude by pre-exposing the crystal to incoherent ultraviolet light. These properties may be useful for an application of LiNbO3:Mg to nonvolatile two-color holography.
Vibrational energy relaxation in classical fluids. I. High-frequency spectra in gases
Authors: Schwarzer Dirk , Teubner Max
Published: July 2002

We study high-frequency spectra of classical gases interacting via Lennard-Jones and similar potentials with applications to the problem of vibrational energy relaxation. We derive expressions for the spectra which are asymptotically exact at high frequencies and compare these with classical trajectory and molecular dynamics simulations. Trajectory calculations are in quantitative agreement with the theory, but conventional NVE molecular dynamics simulations are qualitatively in error at high frequencies due to insufficient simulation time.
Na effects on CuInSe2: Distinguishing bulk from surface phenomena
Authors: Lyahovitskaya V. , Feldman Y. , Gartsman K. , Cohen H. , Cytermann C. , Cahen David
Published: July 2002

The effect of sodium on the performance of CuInSe2-based solar cells has been under discussion for already a decade. We present experimental evidence using secondary ion mass spectroscopy, x-ray photoelectron spectroscopy (XPS) and other, complementary physical characterization methods, which indicate that, after exposure to an external Na source, no significant amounts of sodium, beyond the residual amount, found in as-grown samples, enter intact crystals, except via defects such as grain boundaries. However, after such exposure, sodium is found in significant concentrations on crystal surfaces, something that is accompanied by an increase in oxygen concentration, as judged by XPS. As expected metallic Na attacks the crystals and can destroy them or at least introduce significant defect densities. Adding Se0 is found, via Na2Se formation, to temper Na activity specifically its effects on crystal disintegration. This is different from the effect of Se0 along where annealing (of n-type) crystals results in n to p type conversion by Cu outdiffusion.
A newly developed high-pressure cell by using modified Bridgman anvils for precise measurements in magnetic fields at low temperatures
Authors: Nakanishi T. , Takeshita N. , Môri N.
Published: July 2002

We have succeeded in developing a new high-pressure cell designed for measuring the absolute value of the electrical resistivity in magnetic fields at low temperatures. This pressure cell, based on a technique using modified Bridgman anvils with a Teflon capsule, can generate nearly hydrostatic pressure at least up to 6 GPa. The most important feature of this pressure cell is its compact body which can be easily combined with a dilution refrigerator in a superconducting magnet. One of its applications is for precise magneto-transport measurements under nearly hydrostatic pressure, which is demonstrated by the investigation into the pressure-induced superconducting state of the spin-ladder Sr14−xCaxCu24O41 (x=12) single crystal.
Determination of the ionization and acceleration zones in a stationary plasma thruster by optical spectroscopy study: Experiments and model
Authors: Dorval N. , Bonnet J. , Marque J. P. , Rosencher E. , Chable S. , Rogier F. , Lasgorceix P.
Published: July 2002

A stationary plasma thruster is experimentally studied using different optical spectroscopies of xenon ions. Doppler shift in laser induced fluorescence is used for velocity determination while the ion density is determined by emission spectroscopy. These experiments show unambiguously that the ionization and the acceleration zones are spatially distinct inside the thruster channel. Moreover, it is shown that these results can be easily taken into account with a very simple quasineutral stationary one-dimensional model.
Gradient vector field and properties of the experimental electrostatic potential: Application to ibuprofen drug molecule
Authors: Bouhmaida Nouzha , Dutheil Michael , Ghermani Nour Eddine , Becker Pierre
Published: July 2002

The present study focuses on the electric field features and related physical properties which can be derived from the topology of the experimental electrostatic potential. These properties were retrieved from the electron density multipole refinement of high-resolution x-ray data collected on a racemic crystal of ibuprofen drug. The electric field lines are depicted around the molecule revealing gradient vector zero flux atomic basins and critical points (CP’s) having a different significance than that brought out by the topology of the electron density. This method emphasizes a partioning of the molecular system mainly governed by the nuclear–electron interaction. The concept of Slater’s nuclear screening is here explored from the inspection of the gradient field zero flux surface separating the atoms in the molecule. Moreover, empirical parameters like covalent or atomic bond radii are accurately estimated from CP–atom distances in the molecular heteroatomic bonds. The local minima of the electrostatic potential are searched around the ibuprofen molecule in order to locate the binding sites for further molecular interactions with biological targets or with excipients in pharmaceutical preparations. Ibuprofen dipole moment is also estimated by a method based upon the fit to the experimental electrostatic potential values generated around the molecule.
Recursive construction for a class of radial functions. II. Superspace
Authors: Guhr Thomas , Kohler Heiner
Published: July 2002

We extend the recursion formula for matrix Bessel functions, which we obtained previously, to superspace. It is sufficient to do this for the unitary orthosymplectic supergroup. By direct computations, we show that fairly explicit results can be obtained, at least up to dimension 8×8 for the supermatrices. Since we introduce a new technique, we discuss various of its aspects in some detail.
Thermal annealing effect on nitrogen vacancy in proton-irradiated AlxGa1−xN
Authors: Zhou Qiaoying , Manasreh M. O.
Published: July 2002

The optical absorption band associated with the nitrogen vacancy in proton-irradiated AlxGa1−xN, for x=0.61 and 1, is investigated under isothermal furnace annealing in the temperature range of 150–350 °C. The results show that the enthalpy associated with the thermal migration process is 0.410±0.023 and 0.445±0.037 eV for Al0.61Ga0.39N and AlN, respectively. The variation of the full width at half maximum (FWHM) of the nitrogen vacancy absorption band in AlN as a function of temperature indicates that the electronic transition associated with the defect is coupled to a phonon with an energy of 47.8 meV. This phonon energy is in good agreement with the E2 phonon mode reported for AlN. A Frank–Condon shift of 1.142 eV is also estimated from the FWHM measured as a function of temperature, which is indicative of a large lattice distortion in the vicinity of the defect.
Design of strongly modulating pulses to implement precise effective Hamiltonians for quantum information processing
Authors: Fortunato Evan M. , Pravia Marco A. , Boulant Nicolas , Teklemariam Grum , Havel Timothy F. , Cory David G.
Published: July 2002

We describe a method for improving coherent control through the use of detailed knowledge of the system’s Hamiltonian. Precise unitary transformations were obtained by strongly modulating the system’s dynamics to average out unwanted evolution. With the aid of numerical search methods, pulsed irradiation schemes are obtained that perform accurate, arbitrary, selective gates on multiqubit systems. Compared to low power selective pulses, which cannot average out all unwanted evolution, these pulses are substantially shorter in time, thereby reducing the effects of relaxation. Liquid-state nuclear magnetic resonance techniques on homonuclear spin systems are used to demonstrate the accuracy of these gates both in simulation and experiment. Simulations of the coherent evolution of a three-qubit system show that the control sequences faithfully implement the unitary operations, typically yielding gate fidelities on the order of 0.999 and, for some sequences, up to 0.9997. The experimentally determined density matrices resulting from the application of different control sequences on a three-spin system have overlaps of up to 0.99 with the expected states, confirming the quality of the experimental implementation.
Stochastic Lagrangian models of velocity in homogeneous turbulent shear flow
Authors: Pope Stephen B.
Published: July 2002

Stochastic Lagrangian models for the velocity following a fluid particle are used both in studies of turbulent dispersion and in probability density function (PDF) modeling of turbulent flows. A general linear model is examined for the important case of homogeneous turbulent shear flow, for which there are recent direct numerical simulation (DNS) data on Lagrangian statistics. The model is defined by a drift coefficient tensor and a diffusion tensor, and it is shown that these are uniquely determined by the normalized Reynolds-stress and timescale tensors determined from DNS. With the coefficients thus determined, the model yields autocorrelation functions in good agreement with the DNS data. It is found that the diffusion tensor is significantly anisotropic—contrary to the Kolmogorov hypotheses and conventional modeling—which may be a low-Reynolds-number effect. The performance of two PDF models is also compared to the DNS data. These are the simplified Lagrangian model and the Lagrangian isotropization of production model. There are significant differences between the autocorrelation functions generated by these models and the DNS data.
Ab initio calculations of low-lying electronic states of vinyl chloride
Authors: Chang Jia-Lin , Chen Yit-Tsong
Published: July 2002

The equilibrium geometries, vibrational frequencies, excitation energies, and oscillator strengths of vinyl chloride in the ground and five lowest-lying excited singlet states have been calculated using MP2, CIS, CASSCF, and MRCI methods with the 6-311++G** basis set. The geometries and vibrational frequencies of the ground and excited states are utilized to compute Franck–Condon factors. Calculated vibronic spectra for the transitions from the ground state to these five excited states are in agreement with experiment at 52 500–60 000 cm−1, with major contributions from the Ã(1 1A″)←X̃(1 1A′) and C̃(2 1A′)←X̃(1 1A′) transitions. In this study, two spin-forbidden transitions of b̃(1 3A″)←X̃(1 1A′) and c̃(2 3A″)←X̃(1 1A′) are calculated to locate in 45 000–54 000 cm−1, and could be responsible for the observed one-photon absorption spectrum due to an intensity borrowing caused by the spin–orbit coupling of the Cl atom. Based on calculation, we speculate that upon the excitation of vinyl chloride at 193 nm the b̃(1 3A″) or c̃(2 3A″) excited state, instead of the (π,π*), is initially prepared prior to the subsequent photodissociation processes.
Vibronic transitions from coupled-cluster response theory: Theory and application to HSiF and H2O
Authors: Christiansen Ove , Ruden Torgeir A. , Ruud Kenneth , Helgaker Trygve
Published: July 2002

A scheme for calculating the vibrational structure of electronic spectra using coupled-cluster response theory is proposed. To calculate the vibrational structure of electronic transitions, the optimized geometries of the two electronic states, the molecular Hessians, the dipole transition moment and (for vibrationally induced transitions) the geometrical gradient of the dipole transition moment are used in conjunction with a recently developed method for the evaluation of Franck–Condon factors of multidimensional harmonic oscillators. Allowed and vibrationally induced transitions are both described. In this pilot implementation, the required geometrical derivatives are calculated by an automated finite-difference method. The scheme is applied to the 1 1A″←1 1A′ transition of monofluorosilylene (HSiF) and the vibrationally induced 1 1A2←1 1A1 transition of water.
Magnetic anisotropy of ferromagnetic La0.7Sr0.3MnO3 epitaxial thin films: Dependence on temperature and film thickness
Authors: Steenbeck K. , Habisreuther T. , Dubourdieu C. , Sénateur J. P.
Published: July 2002

The magnetic anisotropy of (001) oriented La0.7Sr0.3MnO3 films of thickness t=7–156 nm, deposited on LaAlO3 substrates, was measured by torque magnetometry in the temperature range T=10–300 K. For t⩾50 nm and H rotating out of plane the anisotropy Ku agrees well with shape anisotropy. For thinner films, Ku is reduced and its sign is reversed for t=7 nm and T<70 K; this can be explained by a perpendicular anisotropy Kuε due to lattice strain. The crystal anisotropy constant K1 was determined from the biaxial in-plane anisotropy. At T=100 K K1 differed by no more than 50% from the mean value −8 kJ/m3 in the thickness region investigated. K1 was much less dependent on the thickness t and strain relaxation in the films than Kuε.
On the classification of equilibrium superfluid states with scalar and tensor order parameters
Authors: Kovalevsky M. Yu. , Peletminsky S. V. , Chekanova N. N.
Published: July 2002

A classification of the equilibrium states of superfluid liquids with scalar and tensor order parameters, based on the concept of quasi-averages, is presented. The condition of unbroken symmetry is generalized to inhomogeneous equilibrium states. The admissible conditions of spatial symmetry are found in terms of integrals of the motion. A connection between these symmetry conditions and the helicoidal structure of the spin and spatial anisotropy vectors is established. Under certain restrictions it is shown that the equilibrium structure of the order parameter can be represented as a product of a homogeneous part of the order parameter and a part that depends on the spatial coordinates.
Combining a scanning near-field optical microscope with a picosecond streak camera: Statistical analysis of exciton kinetics in GaAs single-quantum wells
Authors: Neuberth U. , Walter L. , von Freymann G. , Dal Don B. , Kalt H. , Wegener M. , Khitrova G. , Gibbs H. M.
Published: July 2002

Combining a low-temperature scanning near-field optical microscope with a picosecond streak camera allows us to measure the complete wavelength-time behavior at one spot on the sample within about 13 min at excitation powers of 100 nW. We use this instrument to measure the variation of relaxation times in disordered single-GaAs quantum wells with sample position.
Energy correction to simulation of volume polarization in reaction field theory
Authors: Chipman Daniel M.
Published: July 2002

Reaction field theory is useful in electronic structure calculations of solute properties to evaluate the effects of long-range electrostatic interactions with solvent. The reaction field is affected by quantum mechanical penetration of solute charge density outside the surface of the solvent cavity that nominally encloses it. Exact treatment of the effect of this charge penetration is possible, within the context of a simple dielectric continuum model, by invoking a certain volume polarization in addition to the usually treated surface polarization. However, with general nonspherical molecular cavities this is difficult and computationally demanding to handle, so more tractable means to optimally simulate or otherwise approximate the effect of volume polarization in terms of modified surface polarization are useful in practice. This work proposes and tests a simple formula for a correction term to estimate the energy difference between exact treatment of volume polarization and its optimal surface simulation.
Nuclear spin selection rule in the photochemical reaction of CH3 in solid parahydrogen
Authors: Fushitani Mizuho , Momose Takamasa
Published: July 2002

Photolysis of a methyl radical CH3 in solid parahydrogen produces a methane molecule CH4 via the reaction between an intermediate singlet methylene CH21 and a parahydrogen molecule H2. Conservation of nuclear spin during the reaction has been investigated by the intensity distribution of the rotation-vibration spectrum of methane produced by the reaction. It was found that the population of each nuclear spin state of methane just after the reaction was different from that of the statistical ratio, which indicates that a nuclear spin selection rule does exist in the reaction. However, the observed population was significantly different from the theoretically predicted ratio. The discrepancy between the experiment and the theory may indicate a breakdown of the nuclear spin conservation during the reaction, if the reaction mechanism in solid parahydrogen is the same as in the gas phase.
Varieties of spiral wave behavior: An experimentalist’s approach to the theory of excitable media
Authors: Winfree Arthur T.
Published: July 2002

Spiral waves in diverse excitable media exhibit strikingly variegated behavior. Mechanistic interpretations of excitability in laboratory systems are commonly tested by comparing the wavelength, period, and meander patterns of the model’s spiral waves with laboratory observations, but models seem seldom to be rejected by such tests. The reason may be that almost any excitable medium behaves in many respects like almost any other, if its parameters are properly adjusted within a reasonable range. What generalizations can be made about ‘‘excitable media’’ in the absence of more specifications? It would be useful to distinguish such generic features from idiosyncrasies of specific models. The range of behavioral flexibility of the FitzHugh–Nagumo excitable medium is explored by varying two of its parameters and comparing the results with other excitable media to suggest a generic pattern of parameter dependence. The results exhibit the remarkable diversity of rotor behavior in a single model and provide a database for quantitative testing of mathematical generalizations.
A decoding problem in dynamics and in number theory
Authors: Siegel Ralph M. , Tresser Charles , Zettler George
Published: July 2002

Given a homeomorphism f of the circle, any splitting of this circle in two semiopen arcs induces a coding process for the orbits of f, which can be determined by recording the successive arcs visited by the orbit. The problem of describing these codes has a two hundred year history (that we briefly recall) in the particular case when the arcs are limited by a point and its image; in modern language, it is the kneading theory of such maps, and as such is relevant for our understanding of dynamical problems involving oscillations. This paper deals with questions attached to the general case, a problem considered by many mathematicians in the 50’s and 60’s in the case where f is a rotation, and which has recently found some applications in physiology. We show that, except for trivial cases, any code determines the rotation number, up to the orientation, of the homeomorphism which generates it. In the case the code is periodic, we can also determine whether or not it can be generated in this way. An equivalent problem in arithmetic consists of finding ±p, knowing a collection of classes in Z/qZ of the form {m,m+p,...,m+(k−1)p}, where 2≤k≤q−2 and p and q are relatively prime. We describe this equivalence, and give simple solutions of the decoding problem both in the dynamical context and in the number theoretic context.
Effect of a magnetic flux line on the quantum beats in the Hénon–Heiles level density
Authors: Brack M. , Bhaduri R. K. , Law J. , Maier Ch. , Murthy M. V. N.
Published: July 2002

The quantum density of states of the Hénon–Heiles potential displays a pronounced beating pattern. This has been explained by the interference of three isolated classical periodic orbits with nearby actions and periods. A singular magnetic flux line, passing through the origin, drastically alters the beats even though the classical Lagrangian equations of motion remain unchanged. Some of the changes can be easily understood in terms of the Aharonov–Bohm effect. However, we find that the standard periodic orbit theory does not reproduce the diffraction-like quantum effects on those classical orbits which intersect the singular flux line, and argue that corrections of relative order ℏ are necessary to describe these effects. We also discuss the changes in the distribution of nearest-neighbor spacings in the eigenvalue spectrum, brought about by the flux line.
Tests for nonlinearity in short stationary time series
Authors: Chang Taeun , Sauer Tim , Schiff Steven J.
Published: July 2002

To compare direct tests for detecting determinism in chaotic time series, data from Hénon, Lorenz, and Mackey–Glass equations were contaminated with various levels of additive colored noise. These data were analyzed with a variety of recently developed tests for determinism, and the results compared.
Single-head spin modes in frontal polymerization
Authors: Ilyashenko Victor M. , Pojman John A.
Published: July 2002

The single-head spin mode is the first two-dimensional pattern of self-propagating thermal reaction fronts observed after the planar front loses stability. A “hot spot” is observed to propagate around the periphery of the front. The dynamics of the single-head frontal polymerization regime were studied experimentally, and two novel results were found. The “hot spot” was measured and found to be superadiabatic. The relationship between the rotational velocity and the propagating velocity was found. Experimental data were also compared with the theoretical results based on the linear stability analysis and found to be in a reasonable agreement.
Generation of mask patterns for diffractive optical elements using MathematicaTM
Authors: O’Shea Donald C.
Published: July 2002

The generation of binary and grayscale masks used in the fabrication of diffractive optical elements is usually performed using a proprietary piece of software or a computer-aided drafting package. Once the pattern is computed or designed, it must be output to a plotting or imaging system that will produce a reticle plate. This article describes a number of short Mathematica modules that can be used to generate binary and grayscale patterns in a PostScript-compatible format. Approaches to ensure that the patterns are directly related to the function of the element and the design wavelength are discussed. A procedure to preserve the scale of the graphic output when it is transferred to another application is given. Examples of surfaces for a 100 mm effective focal length lens and an Alvarez surface are given. © 1996 American Institute of Physics.
Scanning capacitance microscopy
Authors: Matey J. R. , Blanc J.
Published: July 2002

Scanning capacitance microscopy is a new mechanically scanned microscopy. In its present configuration, the microscope has a demonstrated capability of detecting variations in surface topography of the order of 0.3 nm over areas of the order of 0.5 μm2. This great sensitivity to height changes is achieved by the placement of a capacitance probe some 20 nm above the sample surface. This paper describes the construction of the microscope and discusses its use in relating surface roughness of plastic parts to mold finish and material property variations.
Electrical properties of pure CdS
Authors: Boone Jack L. , Cantwell Gene
Published: July 2002

Electrical measurements on single-crystal CdS are used to evaluate the various material parameters of vapor-grown material. Hall measurement data is used to obtain a computer fit to a single donor level partially compensated model for the purpose of determining the donor density, acceptor density, and activation energy. Hall mobility data is fitted to a scattering model which includes lattice, neutral impurity, and ionized impurity scattering. The donor level in the low density doping limit is found to be approximately 32 meV below the conduction band. Over a temperature range of 100 k–300 K lattice scattering is predominate and the mobility variations have little impact on the electrical properties; the major variations are due to ionized donor variations and the band-gap variations with donor densities. At low temperatures, impurity densities and the type of impurity scattering have a significant impact on the mobility. Pure CdS displays a very high mobility, i.e., μ>10 000 cm2/V sec at low temperatures in the absence of ionized vacancy scattering. The absence of ionized vacancy scattering at low temperatures is found to be the result of vacancy pairing which results in a neutral scattering center. Low mobilities at low temperatures are found to result from a relatively high density of netural impurity scattering centers.
A gas puff soft x-ray laser target design
Authors: Maxon S. , Hagelstein P. , Reed K. , Scofield J.
Published: July 2002

An x-ray laser target to be driven by a Xe gas puff implosion powered by the Proto II machine at Sandia is proposed. The laser material is Sn pumped to the Ni-like ionization state. The main laser transition (4d-4p) is calculated to be near 110.5 eV.
High permeability iron-based metallic glasses for line frequency applications
Authors: Fish Gordon E.
Published: July 2002

The addition of 2–3 at. % of Cr or Mo to FeBSi-base metallic glasses results in a substantial improvement in line frequency magnetic properties. The saturation magnetization and magnetostriction are reduced by approximately 0.1 T and 5–7×10−6/at. %, respectively. When annealed in longitudinal field, these alloys exhibit dc coercivities of 0.3–0.7 A/m and maximum 60 Hz impedance permeabilities of 300 000–400 000, compared with 1 A/m and 200 000 typically seen in FeBSi glasses. By transversely annealing the alloys, the dc remanence ratio can be reduced to ∼0.3, resulting in nearly equal permeabilities under either bipolar or unipolar 60-Hz excitation. Loss separation measurements show that the improvement in 60 Hz properties resulting from addition of Cr or Mo is largely due to the decrease in the hysteresis component of loss. By annealing the Cr- or Mo-containing glasses at higher temperature to produce a slight precipitation of α-Fe particles, the eddy current component of loss is decreased at the expense of hysteresis loss. The resulting alloys thus show lower losses at 10–100 kHz but higher losses in the line frequency regime than alloys without the α-Fe precipitates.
Beam coupling in BaTiO3 at 442 nm
Authors: Klein M. B. , Valley George C.
Published: July 2002

Steady-state beam coupling is used to measure the concentration of empty traps, the sign of the dominant photocarrier, and the effective electro-optic coefficient in seven crystals of BaTiO3. The measured value of the effective electro-optic coefficient gives the product of the fractional poling of the crystal, the relative conductivity of the crystal, and the electro-optic coefficient. Anomalously low beam coupling gain measured in an eighth crystal suggests that this crystal is nearly perfectly compensated; i.e., the photoconductivities due to electrons and holes are nearly equal.
Effect of packing density on the coercivity of elongated Fe3O4 particles
Authors: Kuo P. C. , Chang C. Y.
Published: July 2002

The chain-of-spheres model is modified by taking the effect of particle interactions into account to interpret the packing density dependence of the coercivity of elongated particles. The relationship of coercivity versus packing density is derived from Lorentz mean field approximation. Coercivity of the particle assembly is treated in detail with regard to the particle axis orientation. Calculations on the coercivities of randomly oriented Fe3O4 particles at various packing densities are found in good agreement with experimental results of Davis.
Thermionic emission diffusion model of current conduction in polycrystalline silicon and temperature dependence of mobility
Authors: Singh S. N. , Kishore R. , Singh P. K.
Published: July 2002

In this paper a comprehensive model of current conduction in polycrystalline silicon (polysilicon) based on the thermionic-emission-diffusion (TED) theory is developed, and on the basis of this model an expression for the effective majority carrier mobility μeff is derived. This expression is quite general in nature and some thermionic emission (TE) theory based expressions for μeff can be obtained from it straightaway under certain simplifying assumptions. In addition, it helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results. Also, the experimental data on Hall mobility, which we obtained under an ohmic conduction regime in the 300–440 K temperature range for dark and illuminated conditions in lightly doped n-type polysilicon samples of different grain sizes, are presented and are interpreted on the basis of the TED model. Under strong illumination, the Hall mobility μHL was observed to vary with temperature T according to the relation μHL=aT−b, where a depended on grain size and was found to be smaller for the smaller grain size. The dark mobility data fitted well into the TED-based expression for μeff considering the interface states associated with grain boundaries to be localized at Ev+0.63 eV in the band gap. The analysis reveals that, generally, the scaling factor is needed if the effect of diffusion is neglected in comparison with the thermionic emission while in essence it is appreciable to be considered. However, in the TED model, as diffusion contribution in controlling the current transport across the grain-boundary potential barrier is well taken care of, the scaling factor is not required.
Optical properties of sputtered gold clusters
Authors: Parmigiani F. , Samoggia G. , Ferraris G. P.
Published: July 2002

Gold particles, sputtered at different substrate temperatures, have been analyzed by both optical absorption and transmission electron microscopy techniques. Changes in the absorption band depending on the preparation of the films was observed and related to the bulk properties of the material. However, the bandwidth was nearly independent of particle concentration. It is suggested that a high concentration of microscopic and macroscopic defects, not observed in clusters grown with other techniques, is probably responsible for this behavior.
Distribution of ion energies incident on electrodes in capacitively coupled rf discharges
Authors: Kushner M. J.
Published: July 2002

The energy and angular distribution of ions striking the electrodes in rf discharges are of interest with respect to the application of such discharges to the processing of semiconductor materials. The ability to fabricate small (< 1 μm) semiconductor features using the plasma etching process results, in part, from the energetic and anisotropic flux of ions which strike the semiconductor surface. In this paper the energy and angular distribution of ions striking the electrodes in low-pressure capacitively coupled rf discharges are studied using a Monte Carlo model for ion trajectories and a parametric model for the time-dependent electric field within the sheath. Energy and angular distributions are discussed as a function of rf frequency, ion mass, and the mean-free path between charge exchange collisions within the sheath. The ion energy distribution is found to be characterized by a scaling parameter proportional to (rf frequency × sheath thickness)2 × ion mass/(sheath voltage); small values of this parameter yield bimodal distributions, intermediate values yield distributions peaked at the maximum sheath potential, and high values yield distributions peaked at the average sheath potential. The ion energy distribution is also examined for different values of the dc and rf components of the sheath potential and for different models for the electric field within the sheaths. When the dc component of the sheath potential is small compared to the rf amplitude, a large thermal component to the ion energy distribution results. The implication of this result and that for the angular distribution of ions incident on the electrodes is discussed with respect to the isotropy of the etch obtained during plasma etching of semiconductor materials.
Effective masses in Sn-doped Ga1−xAlxAs (x<0.33) determined by the Shubnikov–de Haas effect
Authors: El Jani B. , Gibart P. , Portal J. C. , Aulombard R. L.
Published: July 2002

The first direct effective mass measurements of electrons in metalorganic vapor phase epitaxy (MOVPE) Sn-doped Ga1−xAlxAs (x<0.33) epilayers from Shubnikov–de Haas (SdH) oscillations are reported. The m*eΓ values of the conduction band were obtained up to x=0.3. These data, corrected for nonparabolicity, were compared to values calculated by the k ⋅ p model, taking into account the disorder bowing.