bluuebunny/crossref_metadata_2025 huggingface dataset:
Non-thermal nuclear magnetic resonance quantum computing using hyperpolarized xenon
Authors: Verhulst Anne S. , Liivak Oskar , Sherwood Mark H. , Vieth Hans-Martin , Chuang Isaac L.
Published: July 2002

Current experiments in liquid-state nuclear magnetic resonance (NMR) quantum computing are limited by low initial polarization. To address this problem, we have investigated the use of optical pumping techniques to enhance the polarization of a 2-qubit NMR quantum computer (C13 and H1 in CHCl313). We have generalized the procedure for effective pure state preparation in order to efficiently use the increased polarization. With this more flexible scheme, an effective pure state is prepared with polarization enhancement of a factor of 10 compared to the thermal state. An implementation of Grover’s quantum search algorithm with a polarization-enhanced spin system is demonstrated.
Application and development of multiconfigurational localized perturbation theory
Authors: Dunietz Barry D. , Friesner Richard A.
Published: July 2002

Generalization of localized perturbation theory, which results with a method able to span the spin space correctly, is presented. This generalization is achieved by using a multiconfigurational (MC) wave function as the reference. This is the most comprehensive expansion used within MC–LMP2 approach to date, with, however, low computational cost [computational scaling with system size (N) of the new method is O(N3)]. Recently, we have reported the successful Jaguar2 (J2) model for calculating atomization energies. Within the MC–LMP2 framework, the J2 model for calculating heats of formation is based on the generalized valence bond–perfect pairing (GVB–PP) wave function. The J2 model was applied only to closed shell cases because of the perfect pairing (PP) restriction in the reference function. In order to describe other systems, the PP restriction needs to be lifted. This work describes efforts in that direction. The PP restriction can be lifted by a restricted configuration interaction (RCI) procedure applied to the GVB–PP wave function. In this paper, the equations describing the application of LMP2 theory to self-consistent RCI wave function are derived and explained. The RCI wave function is a “true” MC expansion as opposed to the GVB–PP, which uses only a single spin eigenfunction (SEF). We also present the self-consistent (SC) optimization of the RCI wave function. The SC–RCI–LMP2 is the first MC–LMP2 method where the spin space is spanned in the reference. This is important for describing the nondynamical correlation (near degeneracy) effects associated, for example, with bond breaking processes. The SC–RCI–LMP2 is an efficient method applicable to large systems; it is shown to reproduce the potential energy surfaces calculated by the complete active space–second order perturbation (CAS–SCF–PT2) method. This is demonstrated, for the first time, on some widely used test cases.
Hafnium interdiffusion studies from hafnium silicate into silicon
Authors: Quevedo-Lopez M. , El-Bouanani M. , Addepalli S. , Duggan J. L. , Gnade B. E. , Wallace R. M. , Visokay M. R. , Douglas M. , Colombo L.
Published: July 2002

The interdiffusion of Hf and Si from high-κ gate dielectric candidate (HfO2)1−x(SiO2)x thin films deposited on Si (100) was studied using x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, high resolution transmission electron microscopy, and Rutherford backscattering spectrometry in combination with chemical etching. After extreme rapid and conventional furnace thermal annealing treatments, Hf incorporation into Si is limited to less than 0.5–1 nm from the interface. Implications for high-κ gate dielectric applications are also discussed.
The effect of uniaxial quartic anisotropy on thermal stability of magnetic nanograins
Authors: Bertram H. Neal , Safonov Vladimir L.
Published: July 2002

The effect of quartic anisotropy (K2) for nanograins with hexagonal crystalline structure on thermal stability has been studied. For reversal field parallel to the anisotropy axis, the presence of K2 increases the energy barrier in small fields but does not change the nucleation field. The mean first passage time was calculated from the stochastic differential equation. Reversal field at a finite temperature versus field pulse time showed very little effect at short times but thermal stability at long times was significantly extended. This effect was shown to be appreciable only for fields oriented within about 15° to the anisotropy axis.
Ballistic magnetoresistance in a magnetic nanometer sized contact: An effective gate for spintronics
Authors: Garcı́a N. , Muñoz M. , Qian G. G. , Rohrer H. , Saveliev I. G. , Zhao Y.-W.
Published: July 2002

We present experimental results of unprecedented large magnetoresistance obtained in stable electrodeposited Ni–Ni nanocontacts 10–30 nm in diameter. The contacts exhibit magnetoresistance of up to 700% at room temperature and low applied fields and, therefore, act as very effective spin filters. These large values of the magnetoresistance are attributed to spin ballistic transport through a magnetic “dead layer” at the contact of width of about 1 nm or smaller. Nanometer sized, high sensitive magnetoresistive sensors could become key elements for magnetic storage in the terabit/in.2 range and in high density magnetic random access memories.
Theoretical and experimental investigation of straight defect waveguides in AlGaAs-based air-bridge-type two-dimensional photonic crystal slabs
Authors: Sugimoto Y. , Ikeda N. , Carlsson N. , Asakawa K. , Kawai N. , Inoue K.
Published: July 2002

A detailed investigation was undertaken from both theoretical and experimental perspectives of straight defect waveguides in an AlGaAs-based air-bridge-type two-dimensional photonic crystal slab. Optical propagation properties were obtained by two methods: measurement of transmission spectra at wavelengths ranging from 850 to 1100 nm, and with plan-view observations of the optical beam trace along the waveguide measured with an infrared-vidicon camera. Three-dimensional finite-difference time-domain simulations for the band structure and transmission spectra in the air-bridge slab with and without defect waveguides resulted in the appearance of defect propagation modes specific to the defect waveguide, between two slab modes for the defect-free photonic crystal slab. These defect modes were experimentally identified in the measured transmission spectra.
Singular current density in the planar superconductor/normal metal/superconductor junction
Authors: Spal Richard D.
Published: July 2002

Interfacial resistivity (ρ) is crucial in limiting the peak current density in the planar superconductor/normal metal/superconductor junction. Counterintuitively, as ρ→0, the maximum current density normal to the interface (Jnmax) keeps rising until ultimately limited by the superconductor’s critical current density. This little known effect, which results from a singularly perturbed mixed boundary value problem, is studied theoretically by analytical and numerical methods, assuming Josephson coupling to be negligible. An approximate formula for Jnmax is found, and graphs of Jnmax and the junction resistance are computed. Three applications are considered, involving ρ measurement, overlap wire joints, and high temperature superconductor composite tape.
Strain and electronic interactions in InAs/GaAs quantum dot multilayers for 1300 nm emission
Authors: Le Ru E. C. , Bennett A. J. , Roberts C. , Murray R.
Published: July 2002

We have investigated the emission properties of low-growth-rate InAs/GaAs self-assembled quantum-dot multilayer samples with spacer layers of different thicknesses. For two layers with the same InAs coverage and a spacer of 40 nm, emission from the two layers is shown to be at different wavelengths. This is discussed in terms of local strain and surface undulation caused by the first layer influencing the growth of the second layer. We show that by annealing the surface before the growth of each subsequent layer this effect can be avoided for spacers of 40 nm and above. Furthermore, it is shown by collecting photoluminescence over a limited area of an N-layer sample, grown with surface annealing, that this sample exhibits a maximum modal gain N times that of a single layer. For spacers below 10 nm, only one emission peak is observed. However, we show that the observation of a single peak is due, in this case, to tunneling between adjacent layers, and that the maximum modal gain at 1300 nm of such electronically coupled layers is comparable to that of a single layer.
High-efficiency spin polarimetry by very-low-energy electron scattering from Fe(100) for spin-resolved photoemission
Authors: Hillebrecht F. U. , Jungblut R. M. , Wiebusch L. , Roth Ch. , Rose H. B. , Knabben D. , Bethke C. , Weber N. B. , Manderla St. , Rosowski U. , Kisker E.
Published: July 2002

We describe concept, design, and performance of a novel spin polarimeter based on spin-dependent specular reflection of electrons from a Fe(100) surface. The Fe surface is prepared as an ultrathin film on Ag(100). By tuning the energy of the electrons to a critical point in the Fe band structure, a large spin asymmetry combined with a large scattering efficiency is achieved. The polarimeter yields a figure of merit up to 10−2 for the best Fe(100) surfaces.
Chemical reaction of sympathetically laser-cooled molecular ions
Authors: Baba Takashi , Waki Izumi
Published: July 2002

We report an ion/neutral reaction of sympathetically laser-cooled molecular ions. Using laser-cooled fluorescence mass spectrometry, a reaction of about 100 sympathetically cooled H3O+ ions with NH3 were traced at 4 min interval as the reacting ions were kept within a radiofrequency-quadrupole ion trap. To obtain the reaction rate of the reaction molecular dynamical calculation was used to relate the fluorescence mass spectra to the quantity of the molecular ions.
Near-field scanning optical microscopy investigation of immiscibility effects in In1−xGaxP films grown by liquid phase epitaxy
Authors: Paulson C. A. , Ellis A. B. , Moran P. D. , Kuech T. F.
Published: July 2002

Near-field scanning optical microscopy (NSOM) and electron probe microanalysis (EPMA) were used to study the topographic and microscopic optical properties of indium–gallium–phosphide (In1−xGaxP) samples grown by liquid phase epitaxy on gallium–arsenide substrates. NSOM imaging found strong and highly localized variations in the photoluminescence (PL) intensity for samples that were highly lattice mismatched with the substrate. The topography and optical features were roughly spatially correlated for these samples. Shifts in the PL peak energy position (by as much as 27 meV) were found during scans across highly mismatched samples, whereas no shifts were seen for In1−xGaxP films with a nearly lattice matched composition. Compositional fluctuations were determined to be the cause of these PL peak energy shifts. EPMA provided corroborating evidence that compositional fluctuations are spatially correlated with the topography. These composition fluctuations arise from the known solid–solid miscibility gap in the In1−xGaxP system at temperatures used for the growth of these samples.
Elastic properties of chemically ordered Co3Pt thin films
Authors: Bandhu R. S. , Sooryakumar R. , Farrow R. F. C , Weller D. , Toney M. F. , Rabedeau T. A.
Published: July 2002

We report room temperature Brillouin light scattering measurements designed to probe the elastic properties of chemically ordered Co3Pt alloy films. A series of 100 nm thick films were deposited by molecular beam epitaxy at different growth temperatures (Tg) between 450 and 950 K. As the deposition temperature increases the lattice structure is observed to transform from a compositionally disordered, mixed fcc/hexagonal phase to a compositionally ordered, purely hexagonal phase, and finally to a compositionally disordered fcc phase. These structural transitions are accompanied by large changes in magnetic anisotropy, new features in the magneto-optical Kerr effect, and changes in the magnetic domain widths which all occur within about a 100 K temperature window centered around Tg=650 K. However, the independent elastic constants show no significant changes in the different films. These findings suggest that macroscopic strain effects are unlikely to underlie the diverse magnetic signatures observed within the chemically ordered phase.
The configuration of Cu centers in electroluminescent SrS:Cu phosphors: an x-ray absorption fine structure and optical study
Authors: Wruck D. , Boyn R. , Wienecke M. , Henneberger F. , Troppenz U. , Hüttl B. , Bohne W. , Reinhold B. , Mahnke H.-E.
Published: July 2002

X-ray absorption fine structure and photoluminescence measurements are combined to analyze the structure of Cu centers in SrS:Cu thin-film and powder samples. We find that all the data can be understood on the basis of a model involving three kinds of Cu centers: (i) dumbbell-type Cu–Cu complexes (which are the dominant centers), (ii) substitutional centers (CuSr), and (iii) interstitial centers (Cui). The bond lengths in the dumbbell centers are determined from the x-ray absorption data. Two characteristic emission bands peaking at about 460 and 510 nm are attributed to Cui and dumbbell centers, respectively. The dependences of the emission spectrum on temperature and Cu concentration are explained by a dissociation–association reaction of the type Cu–Cu↔CuSr+Cui, assuming center-dependent excitation efficiencies and thermal quenching rates.
SrBi
4
Ti
4
O
15
thin films and their ferroelectric fatigue behaviors under varying switching pulse widths and frequencies
Authors: Zhang Shan-Tao , Yang Bin , Chen Yan-Feng , Liu Zhi-Guo , Yin Xiao-Bo , Wang Yuan , Wang Mu , Ming Nai-Ben
Published: July 2002

Polycrystalline ferroelectric SrBi4Ti4O15 thin films were prepared on Pt-coated silicon substrates by pulsed laser deposition. Structures of the films were analyzed by x-ray diffraction, atomic force microscopy (AFM) and scanning electron microscopy. At an applied field of 275 kV/cm, the films showed good hysteresis loops with remnant polarization (Pr), saturated polarization (Ps) and coercive field (Ec) of 3.11 μC/cm2, 7.81 μC/cm2 and 68 kV/cm, respectively. At 120 kV/cm switching pulse field, fatigue tests were carried out systematically by varying the switching pulsewidth with a fixed duty cycle and with a fixed switching period, respectively. A gradual increase followed by an abrupt increase of the fatigue rate was observed with the increase of the switching pulsewidth. Field-induced defect diffusion was used to explain the results.
X-ray diffraction patterns from samples in the laser-heated diamond anvil cell
Authors: Panero Wendy R. , Jeanloz Raymond
Published: July 2002

Thermal pressure and thermal expansion have competing effects on x-ray diffraction patterns obtained from polycrystalline samples at high pressures (10–100 GPa) and temperatures (300–4000 K) within the laser-heated diamond cell. Modeling shows that realistic temperature and pressure variations within the sample cause systematic shifts in diffraction-line positions and shapes, predicting that inferred values of pressure and thermal expansion coefficient can be off by 0.5%–20% and up to 50%–100%, respectively. Peak splitting due solely to temperature variations within the sample can be spuriously ascribed to the occurrence of a phase transition. The Debye–Waller factor has a systematic effect on diffraction-pattern intensities, but a negligible effect (<0.1%) on line positions except in extreme cases.
Experimental determination of positron-related surface characteristics of 6H-SiC
Authors: Nangia A. , Kim J. H. , Weiss A. H. , Brauer G.
Published: July 2002

The positron work function of 6H-SiC was determined to be −2.1±0.1 eV from an analysis of the energy spectrum of positrons reemitted from the surface. The positron reemission yield, highest in the sample inserted into vacuum after atmospheric exposure and cleaning with ethanol, was significantly reduced after sputtering with 3 keV, 125 μA min Ne+ ions. The yield was not recovered even after annealing at 900 °C, presumably due to the stability of sputter induced defects. Sputtering at lower energies caused a smaller decrease in the reemission yield that was largely recovered after annealing at 850 °C. Analysis using electron induced Auger electron spectroscopy and positron-annihilation-induced Auger electron spectroscopy indicated that the surface was Si enriched after sputtering and C enriched after subsequent annealing. Values of positron diffusion length and mobility in the unsputtered material were extracted from the dependence of the reemission yield on the beam energy. The application of SiC as a field-assisted positron moderator is discussed.
Steady state optical gain in small semiconductor quantum dots
Authors: Andrews J. Thomas , Sen Pratima
Published: July 2002

The optical gain phenomena in a single semiconductor quantum dot as well as in an inhomogeneous distribution of quantum dots have been studied. The analytical results obtained using the density matrix technique and numerical analysis for CdS quantum dots show an enhancement of gain in the presence of biexcitons. The numerical results show larger gain in bigger quantum dots. A redshift of gain maxima occurs when the distribution width is large. The realization of high gain quantum dot lasers and photodetectors appears to be a distinct possibility.
Effect of nitridation on magnetic properties of Fe53Pt47 thin film
Authors: Hsiao H. H. , Panda R. N. , Shih J. C. , Chin T. S.
Published: July 2002

The Fe53Pt47 thin films with varying thickness were prepared using dc magnetron sputtering at various substrate temperatures (i.e., from 250 to 600 °C) on to CrMo seeded glass substrates. The powder x-ray diffraction studies reveal that the ordered fct γ2-FePt phase begins to appear starting from the substrate temperature of 250 °C. The estimated ordering parameters show that the films prepared at 300 °C are well ordered and ordering parameters increase slowly as increasing substrate temperatures. On nitridation of the Fe53Pt47 thin films, expansion of the face centered tetragonal crystal lattice along the c axis is observed. The saturation magnetization is decreased with decreasing film thickness. This has been explained mainly on the basis of size and surface effects of nanocrystals in the films including intergranular interactions. Maximum coercivity of 8.7 kOe is observed for the film thickness of 350 nm. The drastic decrease in magnetization with the increase in nitridation time duration has been attributed to the spin pairing effects as a result of electron supply from interstitial nitrogen atoms to the 3d bands of iron atoms. We observed an increase of Hc from 7.8 kOe for non-nitrided Fe53Pt47 film to 10.8 kOe for a 3 h nitrided film.
Effects of excluded volume and bond length on the dynamics of dense bead-spring polymer melts
Authors: Abrams Cameron F. , Kremer Kurt
Published: July 2002

We consider the effects of the ratio of average bond length, l0, to excluded volume bead diameter, d0, on the dynamics of dense liquids of freely jointed bead-spring polymer chains. Monodisperse (N=50) systems for which 0.73<l0/d0<1.34 were simulated using molecular dynamics in the NVT ensemble at a monomer number density ρ=0.85 σ−3. The monomer friction coefficient, ζ, increased by a factor of ∼20 for a factor ∼2 increase in l0/d0. This is due to the enhancement of sphere-packing homogeneity in the melts for l0/d0>1, which in turn increases the volume density and optimizes packing. Friction coefficients depend on local density (i.e., density of nearest neighbors around any monomer) in a manner akin to the empirical relationship between diffusivity and total number density in simple Lennard-Jones fluids [Levesque and Verlet, Phys. Rev. A 2, 2514 (1970)]. This work demonstrates the degree of interplay between structure and dynamics in macromolecular liquids.
Determination of the eigenfunctions of arbitrary nanostructures using time domain simulation
Authors: Sullivan Dennis M. , Citrin D. S.
Published: July 2002

With the present interest in nanostructures, such as quantum dots, there is a need to have a flexible method with which to be able to determine eigenvalues and eigenstates for those structures that do not lend themselves to existing analytical methods. In this article we present a method that accomplishes this by using a simulation of the Schrödinger equation based on the finite-difference time-domain method. This method is capable of simulating any structure within the limits of discretization. By initializing a simulation with a test function, the eigenfrequencies are determined through a Fourier transform of the resulting time-domain data collected at a sample point. Another simulation implements a discrete Fourier transform at the eigenfrequencies at every cell in the problem space, from which the eigenfunctions can be constructed.
Confirmation of the pyroelectric coefficient of strained InxGa1−xAs/GaAs quantum well structures grown on (111)B GaAs by differential photocurrent spectroscopy
Authors: Sánchez J. J. , Izpura J. I. , Tijero J. M. G. , Muñoz E. , Cho Soohaeng , Majerfeld A.
Published: July 2002

In this work we used the differential photocurrent technique to measure the strain-induced piezoelectric field in pseudomorphic InxGa1−xAs/GaAs heterostructures grown by molecular beam epitaxy on (111)B GaAs substrates. Single and multiple quantum well p–i–n diodes with two different In fractions in the well were analyzed in the temperature range of 25–300 K. Our results for a sample with a 17% In fraction confirm the previously reported value of the pyroelectric coefficient for a similar sample obtained by photoreflectance spectroscopy, hence, the equivalence of the differential photocurrent and photoreflectance techniques is also demonstrated. For a sample with 21% In, we report experimental determination of the temperature dependence of the piezoelectric constant and, therefore, of the strain-induced component of the pyroelectric coefficient.
Transition of ferromagnetism to superparamagnetism in Fe/Tb multilayers
Authors: Yang F. , He T. , Chen J. B. , Pan F.
Published: July 2002

Fe/Tb (1.2 nm) multilayer films with various Fe layer thicknesses were prepared by vapor deposition. Magnetic and microstructure of the films were characterized by various experimental methods. The results show that the films evolve from ferromagnetic to superparamagnetic, and Fe in the multilayers transforms from bcc crystalline to amorphous state. The transition of magnetic property and structure is found to be related to the rough interface associated with the amorphous phase formation in Fe/Tb interface. In the [Tb(1.2 nm)/Fe(0.5 nm)]30 multilayer, Fe layers are separated into discontinuous superparamagnetic clusters by three-dimensional Tb crystal aggregates, which are responsible for paramagnetic-like spins at the interface that strengthen the spin-dependent scattering.
Multiple subband transitions and evidence for population inversion in InAs/In(As,Sb) heterostructures
Authors: Heber J. D. , Hardaway H. R. , Li X. , Pullin M. J. , Phillips C. C.
Published: July 2002

We report on improved InAs/InAs1−xSbx heterostructure single quantum well emitters for the mid-infrared wavelength region. An InAlAs barrier layer has been incorporated into the active region of the structures for improved electron confinement. With room temperature pulsed power outputs of 140 μW at a wavelength of 4.3 μm, an improvement of more than a factor of 6 has been achieved compared to similar structures without the barrier layer. Magneto-electroluminescence measurements at 4 K reveal the presence of multiple subband transitions in the spectra. Based on a k⋅p model assuming a type-IIa band offset, these transitions are identified as (e1,hh1) and (e1,lh1). The observation of these multiple transitions is shown to be strong evidence for population inversion in the structures.
Controlling the electron tunneling through InAs self-assembled dots
Authors: Hill R. J. A. , Patanè A. , Main P. C. , Henini M. , Eaves L. , Tarucha S. , Austing D. G.
Published: July 2002

We investigate electron tunneling through GaAs/(AlGa)As/GaAs, single-barrier tunnel diodes in which a layer of self-assembled InAs quantum dots is incorporated onto the center plane of the (AlGa)As tunnel barrier. We study the effect on the electrical conduction of substrate orientation and of the inclusion of thin GaAs cladding layers on either side of the dot layer. We find that the presence of the cladding layers increases the conduction. We attribute this to a raising of the energy of the electron states, probably due to the diffusion of Ga into the dots. Photoluminescence measurements confirm this hypothesis. We show that this effect can be used to tailor the transport characteristics for specific applications.
Bound state of the quantum dot formed at intersection of L- or T-shaped quantum wire in inhomogeneous magnetic field
Authors: Lin Yuh-Kae , Chen Yueh-Nan , Chuu Der-San
Published: July 2002

A quantum dot (QD) can be formed at the intersection of the symmetric or asymmetric L-shaped (LQW) or T-shaped quantum wire (TQW). The bound state energies in such QD systems surrounded by inhomogeneous magnetic fields are found to depend strongly on the asymmetric parameter α=W2/W1, i.e., the ratio of the arm widths and magnetic field applied on the wire arms. Two effects of the magnetic field on the bound state energy of the electron can be obtained. One is the depletion effect which purges the electron out of the QD system. The other is to create an effective potential due to the quantized Landau levels of the magnetic field. Depletion effect is found to be more prominent in weak field region. Our results show the bound state energy of the electron in such QD system depends quadratically (linearly) on the magnetic field in the weak (strong) field region. It is also found that the bound state energy of the electron depends on the magnetic field strength only and not on its direction. A simple model is proposed to explain the behavior of the magnetic dependence of the bound state energy of the electron both in weak and strong magnetic field regions. The contour plots of the relative probability of the bound state in LQW or TQW in magnetic field are also presented.
Stability limits for a liquid of Einstein glasses
Authors: Speedy Robin J.
Published: July 2002

The thermal properties of a model liquid are expressed in terms of the properties of the glasses that it samples. Each glass is modeled as an Einstein solid with enthalpy H0 at absolute zero and an Einstein temperature that decreases linearly with H0. The number of glasses with enthalpy H0 is represented by a Gaussian distribution. The range over which the liquid exists is bounded below by a Kauzmann temperature, where it has the same entropy as an individual glass, and bounded above by a heat capacity divergence.
Vibrational motions and nuclear spin relaxation in paramagnetic complexes: Hexaaquonickel(II) as an example
Authors: Kruk Danuta , Kowalewski Jozef
Published: July 2002

The proton paramagnetic relaxation enhancement (PRE) in an aqueous solution of nickel(II) is described in terms of electron spin relaxation caused by damped vibrational motions of Eg and T2g symmetry. The damped vibrations generate a transient zero-field splitting (ZFS), of variable amplitude and variable principal direction in the molecular frame, and are modeled by the Smoluchowski equation. The parameters of the model are obtained from a combination of two approaches: first, quantum-chemical calculations of the ZFS as a function of the geometry of the coordination shell of the nickel(II) ion and, second, molecular-dynamic simulations generating a trajectory of water positions around the metal. The description of the electron spin dynamics is included in the calculations of the PRE in two ways: Using the traditional Solomon–Bloembergen–Morgan approach and also by means of the more general slow-motion theory. The calculated PRE as a function of the magnetic field, free of any adjustable parameters, is compared with the experimental data. The two methods of calculating the PRE agree with each other—and with the experimental data—at high magnetic field. At low field, the models predict very different PRE, and only the general model is in reasonable agreement with the experiments.
Single to triple ionization of propane due to electron impact: Cross sections for the different dissociation channels
Authors: Wang Pengqian , Vidal C. R.
Published: July 2002

Electron impact single to triple ionization and subsequent dissociation of propane have been studied at an electron energy of 200 eV using a covariance mapping technique employing a focusing time-of-flight mass spectrometer. The dissociation channels of up to triply ionized propane have been identified and the corresponding absolute cross sections for the different dissociation channels have been obtained. Results show that 88% of the singly ionized propane molecules are unstable and dissociate through more than 20 channels, among which 50% of the C3H8+ ions dissociate into C2H5+ or C2H4+. All the doubly ionized propane molecules are unstable and dissociate through more than 80 channels, among which 97% of the C3H82+ ions dissociate into ion pairs. The major dissociation channels for C3H82+ are C2H3++CH3+, H++H+, and C2H4++CH3+, while the major ionic products from the dissociation of C3H82+ are H+, CH3+, and C2H3+.
Real-time photoluminescence evaluation of low-temperature photodarkening effect in amorphous SeGe
Authors: Lee Hyun-Yong , Song Joon-Suk , Yao Takafumi
Published: July 2002

Real-time photoluminescence (PL) evaluation of the low-temperature photodarkening effect has been performed in amorphous (a-) Se75Ge25 chalcogenide thin films using He–Cd laser light. At room temperature, the optical energy gaps Eop of the as-deposited and photodarkened films obtained from a conventional extrapolation method utilizing the plot of (αhv)1/2 versus hv are about 1.86 and 1.66 eV, respectively, i.e., ΔEop∼0.20 eV, where α is the absorption coefficient. At 10 K, the values of Eop for the as-deposited and photodarkened films are indirectly evaluated from the PL spectra. That is, the near band-edge (NB) luminescence occurs at about 2.11 and 1.85 eV for the as-deposited and photodarkened films, respectively. All the PL spectra of the low-temperature photodarkened films show the five main peaks at 1.530, 1.575, 1.651, 1.709, and 1.850 eV. In particular, the peaks at 1.751 and 1.793 eV began to appear in samples illuminated for more than 2.5 h. The only peak observed in the as-deposited film is the NB-like luminescence. In this case, the excitation power was reduced to 10% in order to minimize an effect of illumination. We believe that low-temperature illumination by a He–Cd laser causes a photodarkening effect in a-Se75Ge25, resulting in an increase of luminescence centries at shallow regions rather than at the deep levels.
Anomalous variation of coercivity with annealing in nanocrystalline NiZn ferrite films
Authors: Desai Mrugesh , Prasad Shiva , Venkataramani N. , Samajdar Indradev , Nigam A. K. , Keller N. , Krishnan R. , Baggio-Saitovitch E. M. , Pujada B. R. , Rossi A.
Published: July 2002

The sputter deposited NiZn ferrite thin films were studied as a function of annealing temperature. The magnetization showed a monotonic increase with increasing annealing temperature. The coercivity shows a minimum at annealing temperature of 400 °C and shows a value of 14 Oe. Transmission electron microscopy study indicated that the grain size increases from ∼3 nm for the as-deposited case to ∼15 nm for the film annealed at 800 °C. The observed coercivity behavior could be attributed to the defects present in the films, the change in cation distribution, and the grain growth.
Enhanced room temperature magnetoresistance in La0.7Sr0.3MnO3/Sm0.7Sr0.3MnO3 nanocomposites
Authors: Yan Chun-Hua , Luo Feng , Huang Yun-Hui , Li Xiao-Hang , Wang Zhe-Ming , Liao Chun-Sheng , Zhao Hong-Wu , Shen Bao-Gen
Published: July 2002

La 0.7 Sr 0.3 MnO 3 / Sm 0.7 Sr 0.3 MnO 3 (LSMO/SSMO) nanocomposites were prepared by a sol–gel coating method. The microstructural, magnetic, and transport properties of the composites were investigated. A significant enhancement in magnetoresistance (MR) under an applied field of 5 T is observed from low temperature up to room temperature. In the vicinity of room temperature, the MR values of the composites are larger than those of both LSMO and SSMO. Magnetic measurement reveals that the ferromagnetic order of LSMO is weakened by addition of SSMO. We suggest that the improved MR should be arisen from the enhanced spin-polarized tunneling caused by the magnetic disorder.
Structure and magnetic properties of Co grown on yttria-stabilized cubic zirconia substrates
Authors: Yu C. C. , Lee S. F. , Yao Y. D. , Cheng W. C. , Chen D. C. , Liou Y.
Published: July 2002

Cobalt films have been grown on yttria-stabilized cubic zirconia (YSZ) (100) and (110) substrates by molecular beam epitaxy. Both hcp(0001) and fcc(111) twin structures and fcc(110) films have been successfully fabricated on the YSZ(100) and (110), respectively. For the Co on YSZ(100) case, the Co films possess either hcp(0001) or fcc(111) crystals with in-plane 30° rotation. For the Co on YSZ(110) case, the structural relationship is YSZ(110)[100] ‖ Co(110)[1–10]. All the films display an isotropiclike magnetic anisotropy with the coercivity increasing abruptly above its martensitic transition temperature. The coercivity decreases with increasing the thickness of Co films from 100 Å to 500 Å; and increases as the deposited temperature above 500 °C. Co films grown on YSZ(100) are in favor of the layer by layer growth, and Co films grown on YSZ(110) are in favor of the island growth.
Simulations of intense heavy ion beams propagating through a gaseous fusion target chamber
Authors: Welch D. R. , Rose D. V. , Oliver B. V. , Genoni T. C. , Clark R. E. , Olson C. L. , Yu S. S.
Published: July 2002

In heavy-ion inertial confinement fusion (HIF), an ion beam is transported several meters through the reactor chamber to the target. This standoff distance mitigates damage to the accelerator from the target explosion. For the high perveance beams and millimeter-scale targets under consideration, the transport method is largely determined by the degree of ion charge and current neutralization in the chamber. This neutralization becomes increasingly difficult as the beam interacts with the ambient chamber environment and strips to higher charge states. Nearly complete neutralization permits neutralized-ballistic transport (main-line HIF transport method), where the ion beam enters the chamber at roughly 3-cm radius and focuses onto the target. In the backup pinched-transport schemes, the beam is first focused outside the chamber before propagating at small radius to the target. With nearly complete charge neutralization, the large beam divergence is contained by a strong magnetic field resulting from roughly 50-kA net current. In assisted-pinched transport, a preformed discharge channel provides the net current and the discharge plasma provides nearly complete charge and current neutralization of the beam. In self-pinched transport, the residual net current results solely from the beam-driven breakdown of the ambient gas. Using hybrid particle-in-cell simulation codes, the behavior of HIF driver-scale beams in these three transport modes is examined. Simulations of neutralized ballistic transport, at a few-mTorr flibe pressure, show excellent neutralization given a preformed or photoionized (from the heated target) plasma. Two- and three-dimensional simulations of assisted-pinch transport in roughly 1-Torr Xe show the importance of attaining >1-μs magnetic diffusion time to limit self-field effects and achieve good transport efficiency. For Xe gas pressures ranging from 10–150 mTorr, calculations predict a robust self-magnetic force sufficient for self-pinched transport. The latest simulation results are presented and the important remaining issues for each transport scheme are discussed.
Photo- and low-voltage cathodoluminescence in lithium zinc gallate blue and green thin-film phosphors
Authors: Lee Yong Eui , Norton David P. , Budai John D. , Rack Philip D. , Peterson Jeff , Potter Michael D.
Published: July 2002

The photo- and cathodoluminescence properties of epitaxial lithium-doped zinc gallate thin-film phosphors grown on (100) MgO single crystal substrates using pulsed laser ablation were investigated. The addition of lithium to both undoped and Mn-doped ZnGa2O4 films significantly enhanced the photo- and low-voltage cathodoluminescent intensity. Cathodoluminescent efficiencies for as-deposited undoped and Mn-doped lithium zinc gallate films as high as 0.21 lm/W and 0.29 lm/W, respectively, were achieved for 1 kV and 2.26 μA/cm2. Based on these results, lithium-doped zinc gallate thin-film phosphors appear to be attractive for applications in low-voltage field emission display devices.
Large-scale computer investigations of finite-temperature nucleation and growth phenomena in magnetization reversal and hysteresis (invited)
Authors: Novotny M. A. , Brown G. , Rikvold P. A.
Published: July 2002

An overview of some of the results obtained from long-time dynamic simulations of models of magnetic nanoparticles and thin magnetic films is presented. The simulation methods include dynamic Monte Carlo simulations and micromagnetic simulations. The effects of nucleation and growth due to the finite temperature display similar behaviors for various models of magnetic materials and computational approaches. These behaviors can only be seen computationally with the use of advanced algorithms and massively parallel computers. Several different modes of reversal are seen at finite temperature, and crossovers from one mode of reversal to another for the same system can be seen as the temperature or applied field is changed. Statistical interpretations of both field-reversal experiments and hysteresis experiments are shown to be necessary to understand the finite-temperature behavior of magnetic nanoparticles and thin magnetic films.
Dual vibrational high frequency magnetic force microscopy
Authors: Li Shaoping , Stokes Scott , Liu Y. , Foss-Schroder Sheryl , Zhu W. , Palmer Dean
Published: July 2002

The ability to evaluate high frequency writer performance is crucial to the write-head designs for high data rate application. To quantify the write-head performance at high frequency (∼1 GHz), a new high frequency detection approach has been implemented. The essence of the method is to utilize a two-step signal mixing process to produce the force harmonics, which are not only proportional to the amplitude of the deep-gap field but also fall within the active bandwidth of the probe spectrum. Furthermore, a dual-vibration detection scheme is also described, which can provide superior high spatial resolution and enhance the sensitivity of attractive mode force microscopy as well at high frequencies.
A photometric imaging solar telescope, tunable in the extreme ultraviolet, utilizing multilayer x-ray optics
Authors: Golub L. , Deluca E. , Hamilton P. , Nystrom G. , Windt D. L. , Schmidt W. K. H. , Dannenberg A.
Published: July 2002

We present a new instrument for space-based observational solar physics, recently flown successfully on a sounding rocket, designed to provide high spatial resolution, time-resolved images of the solar corona at specific wavelengths in the extreme ultraviolet (XUV). The primary instrument employs multilayer x-ray mirrors in a novel geometry that affords quasi-monochromatic imaging at wavelengths tunable continuously over the spectral range from 17.1 to 21.1 nm. The secondary instrument also uses multilayer x-ray mirrors to provide high-resolution imaging at three fixed XUV wavelength bands. Both instruments use charge coupled device detectors and thin A1 filters for rejection of unwanted wavelengths. We describe the design, construction, and performance of the instrument and discuss prospects for the future.
Growth of ferromagnetic semiconductor: (Ga, Cr)As
Authors: Yamada M. , Ono K. , Mizuguchi M. , Okabayashi J. , Oshima M. , Yuri M. , Lin H. J. , Hsieh H. H. , Chen C. T. , Akinaga H.
Published: July 2002

A type of GaAs-based ferromagnetic semiconductor, (Ga, Cr)As, was successfully prepared by low-temperature molecular-beam epitaxy. The (Ga, Cr)As thin film shows a flat surface up to the Cr content x=0.18. However, when Cr content x exceeded 0.20, the roughness of the film became larger. The ferromagnetic ordering of the semiconductor sample with the Cr content of x=0.11 was observed. The electronic structures of (Ga, Cr)As were characterized by x-ray absorption spectroscopy (XAS). The XAS spectra show different line shapes in different Cr content.
Noise imaging using magneto-optical sampling techniques (invited)
Authors: Freeman M. R. , Steeves G. M. , Ballentine G. E. , Krichevsky A.
Published: July 2002

The presumption in most ultrafast stroboscopic time-resolved experiments is that the dynamics under investigation are entirely deterministic. Complex magnetic systems offer several mechanisms whereby this presumption could fail. The challenge for the experimentalist is to detect if an admixture of stochastic response is occurring, and if so, to extract as much information about it as possible. We describe a method for observing random sample behavior within the context of stroboscopically averaged time-resolved imaging experiments, and discuss the range of conditions over which it may be applied. Examples are presented in which random magnetic switching is found within otherwise deterministic time-resolved scanning Kerr imaging measurements. Results from micromagnetic modeling of thermally assisted switching in a small platelet are presented to help elucidate the procedure and its interpretation.
Landau resonant heating using standing waves excited by a distributed circuit for electron velocity distribution control
Authors: Bowers K. J. , Birdsall C. K.
Published: July 2002

The use of waves applied by a periodic electrode to controllably modify the particle velocity distribution in a plasma is explored. The applied waves induce a strong wave–particle interaction analogous to Landau damping. However, as the waves are powered by an external circuit, the interaction continues until a new plasma equilibrium is established. At saturation, the velocity distribution is enhanced above the phase velocity of the applied wave and suppressed below. The use of this technique to selectively heat the electron high-energy tail and to enhance the electron-impact ionization rate is demonstrated using particle-in-cell simulation.
Two-dimensional potential distribution measurement of surface discharge with subnanosecond resolution
Authors: Kumada Akiko , Sugihara Toshio , Chiba Masakuni , Hidaka Kunihiko
Published: July 2002

The Pockels sensing system has been developed for directly measuring the potential and electric field distribution on a dielectric material. The system consists of an Ar ion laser, a Nd:YAG laser, a streak camera, and charge coupled device cameras. With this system, both the transient change in the potential distribution along a linear path of 50 mm in maximum length and the instantaneous two-dimensional potential distribution on a 25 mm square area can be measured simultaneously. The minimum resolution of this system reaches up to 0.2 ns. This article describes the transient change in potential distribution with a surface discharge in atmospheric air. The transient change of potential distribution with a propagating negative surface discharge in atmospheric air is measured with this system. The propagating direction component of the electric field Ex near the tip of the propagating streamer exceeds 3 MV/m.
An accurate description of the ground and excited states of SiH
Authors: Kalemos Apostolos , Mavridis Aristides , Metropoulos Aristophanes
Published: July 2002

The astrophysical importance of the SiH radical has motivated significant experimental and theoretical work. However, only the X 2Π and A 2Δ states of SiH have been extensively investigated experimentally, while the study of higher excited states is rather limited. From a theoretical point of view, most of the studies have been focused on spectroscopic and thermochemical quantities of the ground state. The lack of accurate spectroscopic parameters (re,De,ωe,ωexe,αe,D̄e,Te) pertaining to higher excited states was the driving force of the present work, in line with our previous study of the isovalent CH molecule [A. Kalemos, A. Mavridis, and A. Metropoulos, J. Chem. Phys. 111, 9536 (1999)]. Using the multireference configuration interaction approach coupled with very large correlation-consistent basis sets, we have constructed potential energy curves for 18 molecular states correlating to Si(3P,1D,1S,5S,3P,1P)+H(2S). At the same level, the potential energy curve of the ground SiH+ state (X 1Σ+) has also been constructed. We report total energies, dissociation energies, and the usual spectroscopic constants for Si28–1,2H and for all states studied. Most of our results are in excellent agreement with existing experimental values. In particular, we believe that our dissociation energy for the X state, De=73.28 kcal/mol, is the most reliable reported so far in the literature.
Negative ion production in multicusp sources
Authors: Belchenko Yu. I. , Oka Y. , Kaneko O. , Takeiri Y. , Tsumori K.
Published: July 2002

The study of negative ion production in the multicusp negative ion sources (MS) was done by the directed deposition of well-defined amount of cesium into MS, by the cesium recovery from the polluted layers in the MS and by the plasma grid masking. The data obtained evidences the surface-plasma mechanism of negative ion production in the MS. It is shown that a dynamic cesium-tungsten coverage on a plasma grid surface provides the enhanced H− production in the MS. The Cs+W coverage is produced on the hot plasma grid surface due to coadsorption of cesium and of tungsten, evaporated from filaments. The permanent flux of cesium to plasma grid coverage is produced by the thick Cs+W reservoir with a high (75%) percentage of cesium on the cold anode surface. A relatively high cesium seed with a rate of about 20 mg/1 h/30 shots operation is necessary to support the increased H− production in the regular MS. Electrode processing by an additional discharge recovers the cesium from the aged coverage and enhances the H− production without an additional cesium seed. Recovery processing permits the use of deposited cesium more efficiently and to minimize cesium addition during the MS long-term operation. An essential decrease of cesium escape to accelerator and an improvement of injector high voltage operation is produced by introducing the shutter mask at plasma grid surface. The shutter mask experiment displayed, that >60% of H− beam current was born on the hot shutter mask surface. The source operation with a high negative shutter mask biasing and an increased hydrogen pressure resulted in a 35% higher H− yield, than of regular MS.
Modeling self-assembly in molecular fluids
Authors: Aranovich G. L. , Donohue M. D.
Published: July 2002

Equilibrium self-assembly in fluids is studied in the framework of the lattice density-functional theory (DFT). In particular, DFT is used to model the phase behavior of anisotropic monomers. Though anisotropic monomers are a highly idealized model system, the analysis presented here demonstrates a formalism that can be used to describe a wide variety of phase transitions, including processes referred to as self-assembly. In DFT, the free energy is represented as a functional of order parameters. Minimization of this functional allows modeling spontaneous nano-scale phase transitions and self-assembly of supramolecular structures. In particular, this theory predicts micellization, lamellization, fluid–glass phase transitions, crystallization, and more. A classification of phase transitions based on general differences in self-assembled structures is proposed. The roles of dimensionality and intermolecular interactions in different types of phase transitions are analyzed. The concept of primordial codes is discussed in terms of the structural variety of self-assembled systems.