bluuebunny/crossref_metadata_2025 huggingface dataset:
Second harmonic generation in transparent surface crystallized glasses with stillwellite-type LaBGeO5
Authors: Takahashi Y. , Benino Y. , Fujiwara T. , Komatsu T.
Published: July 2002

Transparent optical nonlinear crystallized glasses with the composition of 25La2O3 25B2O3 50GeO2, stoichiometric to ferroelectric stillwellite-type LaBGeO5 crystalline phase, have been prepared by a two-step heat-treatment (first heat treatment: T1=670 °C, t1=10 h, second heat-treatment: T2, t2), and their second harmonic (SH) intensities have been examined using the Maker fringe method. The samples obtained by heat treatments at T2=720∼725 °C for t2=3 h show only surface crystallization and exhibit clear and fine (narrow) fringe patterns. The samples heat treated at T2=740 and 750 °C exhibit relatively strong SH intensities, but the fringe patterns in such samples are broad. It is proposed that SH waves generated from surface LaBGeO5 crystalline layers scatter at LaBGeO5 crystals formed in the interior of glass, causing the disappearance of fine fringe patterns.
Growth, structural, and magnetic properties of high coercivity Co/Pt multilayers
Authors: Weller D. , Folks L. , Best M. , Fullerton E. E. , Terris B. D. , Kusinski G. J. , Krishnan K. M. , Thomas G.
Published: July 2002

Electron beam evaporated Co/Pt multilayers {[Co(tCo nm)/Pt(1 nm)]10, 0.2<tCo<2 nm} with perpendicular magnetic anisotropy and room temperature coercivities Hc=2–15 kOe are studied as a function of growth temperature TG. Hysteresis loops and magnetic force microscopy (MFM) indicate changes in the magnetization reversal mechanism along with a sharp increase in coercivity for TG≳230–250 °C. Films grown at TG<230 °C (tCo=0.2–0.4 nm) show micrometer size magnetic domains and rectangular hysteresis indicating magnetization reversal dominated by rapid domain wall motion following nucleation at Hn∼Hc. Films grown at TG>250 °C show fine-grained MFM features on the sub-100-nm length scale indicating reversal dominated by localized switching of small clusters. High-resolution cross-sectional transmission electron microscopy (TEM) with elemental analysis shows columnar grains extending throughout the multilayer stack. Co depletion and structural defects at the grain boundaries provide a mechanism for exchange decoupling of adjacent grains, which may result in the high coercivities observed.
Magnetic properties lithium-doped manganite single crystals
Authors: Barilo S. N. , Bychkov G. L. , Gatalskaya V. I. , Kurochkin L. A. , Sokol V. P. , Szymczak H. , Szymczak R. , Baran M.
Published: July 2002

Magnetic and resistivity properties of La1−xLixMnO3 single crystals with the perovskite structure at lithium concentrations x=0.06 and 0.2 are investigated over a broad temperature range at fields of up to 50 kOe. The Curie temperature is TC=90 K (x=0.06) and 300 K (x=0.2), respectively. The electrical resistance of the single crystal with x=0.06 increases steadily with temperature decrease, while the crystal with x=0.2 exhibits a magnetoresistance effect of ∼8% at an external field of 9 kOe. The different dependence of the magnetizations MFC and MZFC of the sample with lower lithium content in the temperature range below TC is associated with the formation of a cluster glass and is caused by high coercivity and magnetic anisotropy of the crystal.
Excimer laser-induced expansion in hydrogen-loaded silica
Authors: Smith C. M. , Borrelli N. F. , Price J. J. , Allan D. C.
Published: July 2002

High-purity silica that contains a high concentration of dissolved molecular hydrogen is found to undergo expansion, rather than densification, as a consequence of exposure to deep ultraviolet irradiation. Concurrent with the decreased density, we suggest that a positive photorefractive effect occurs, which increases the index. The contributions of the density change and the photorefractive effect can be observed by comparison of the birefringence and the wave front change obtained through interferometry. Expansion and the photorefractive effect are interpreted in terms of the photolytic formation of SiOH.
Fast imaging polarimeter for magneto-optical investigations
Authors: Wijngaarden Rinke J. , Heeck K. , Welling M. , Limburg R. , Pannetier M. , van Zetten K. , Roorda V. L. G. , Voorwinden A. R.
Published: July 2002

A new imaging polarimeter for magneto-optical investigations is described. Improvements over conventional magneto-optics are: it (i) is insensitive to uneven illumination, (ii) determines also the sign of the magnetic field, and (iii) has significantly improved sensitivity at small magnetic fields. The typical root-mean-square (rms) noise level is 0.7 mT Hz−1/2 for a single pixel, corresponding to a polarization rotation of 0.03 deg Hz−1/2. With limited temporal and spatial averaging, the rms error in magnetization profiles can be reduced to <10 μT, corresponding to 4×10−4 deg. Time resolution is 12 frames per second. Demonstration of the performance of the polarimeter is given for measurements of the local field above superconductors and for measurements of the rotation angle of sugar dissolved in water.
HF inversion in the 2,5-dihydrofuran⋯HF complex
Authors: López Juan C. , Blanco Susana , Lesarri Alberto , Sanz M. Eugenia , Lorenzo Felipe J. , Alonso José L.
Published: July 2002

The rotational spectrum of the hydrogen bond heterodimer 2,5-dihydrofuran⋯HF has been investigated in the frequency region 6–18.5 GHz using molecular beam Fourier transform microwave spectroscopy. The R-branch μa-type spectra of the parent, 2,5-dihydrofuran⋯DF, Cα13-2,5-dihydrofuran⋯HF and Cβ13-2,5-dihydrofuran⋯HF species have been measured. All the observed lines are split into two components due to the tunneling of HF between the two equivalent hydrogen bonding sites at the oxygen atom. A value for the barrier to HF inversion of 100 cm−1 has been calculated from the analysis of the rotational constants of both tunneling states on the basis of a simple one-dimensional model. Ab initio calculations at the MP2/6-311+G(d,p) and MP2/6-311+G(2df,p) levels of theory have been carried out to complement the experimental data. The results on the HF inversion barrier and hydrogen bond angular geometry of 2,5-dihydrofuran⋯HF have been compared to those previously obtained for the H2O⋯HF complex.
Photo-enhanced negative differential resistance and photo-accelerated time-dependent dielectric breakdown in thin nitride-oxide dielectric film
Authors: Chen Fen , Li Baozhen , Jammy Rajarao , Dufresne Roger A. , Strong Alvin W.
Published: July 2002

Photo-enhanced negative differential resistance (NDR) and photo-accelerated time-dependent dielectric breakdown (TDDB) were observed in thin nitride–oxide (N–O) dielectric film biased with gate negative under tungsten lamp illumination. The photo-induced leakage current and photo-accelerated TDDB show dramatic asymmetry under negative and positive gate bias with constant photo-illumination. Our experiments suggest a unique current conduction mechanism in this nitride thin film. A two-carrier conduction induced positive feedback transport process under negative gate bias, and a two-carrier conduction induced self-limiting transport process under positive gate bias are proposed to qualitatively explain the experimental data. The nitride thin film device possessing a light-enhanced NDR can be employed to develop Si-based optoelectronic devices such as switching and logic control.
Ultrasonic trapping in capillaries for trace-amount biomedical analysis
Authors: Wiklund M. , Nilsson S. , Hertz H. M.
Published: July 2002

A longitudinal hemispherical standing-wave ultrasonic trap for size-selective separation of microspheres in small-diameter capillaries is described. The trap utilizes the competition between acoustic radiation forces and viscous drag forces on spheres suspended in a liquid inside 20–75-μm-diam fused silica capillaries. Experiments performed on 3.0- and 4.7-μm-diam latex spheres demonstrate the principles of trapping and verify the theoretically calculated size-dependent forces on the spheres. The spheres are detected by the use of laser-induced fluorescence. The goal is to use the trap for separation and ultrahigh-sensitivity detection of trace amounts of proteins and other macromolecules containing two antigenic sites, by binding the target molecule with high specificity to antibody-coated latex spheres.
Charge-separation effects in 1.3 μm GaAsSb type-II quantum-well laser gain
Authors: Chow W. W. , Schneider H. C.
Published: July 2002

A microscopic laser theory is used to investigate gain and threshold properties in a GaAsSb quantum-well laser. Depending on the geometry of the type-II quantum-well gain region, there may be appreciable band distortions due to electron–hole charge separation. The charge separation and accompanying band distortions lead to interesting optical behaviors, such as excitation-dependent oscillator strength and band edge energies. Implications to laser operation include significant blueshift of the gain peak with increasing injection current, and inhibition of spontaneous emission, which may result in threshold current reduction.
Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer
Authors: Ikai Masamichi , Tokito Shizuo , Sakamoto Youichi , Suzuki Toshiyasu , Taga Yasunori
Published: July 2002

One of the keys to highly efficient phosphorescent emission in organic light-emitting devices is to confine triplet excitons generated within the emitting layer. We employ “starburst” perfluorinated phenylenes (C60F42) as a both hole- and exciton-block layer, and a hole-transport material 4,4′,4″-tri(N-carbazolyl) triphenylamine as a host for the phosphorescent dopant dye in the emitting layer. A maximum external quantum efficiency reaches to 19.2%, and keeps over 15% even at high current densities of 10–20 mA/cm2, providing several times the brightness of fluorescent tubes for lighting. The onset voltage of the electroluminescence is as low as 2.4 V and the peak power efficiency is 70–72 lm/W, promising for low-power display devices.
Strong optical transitions via surface states on Si(001)2×1:H ultrathin films: A theoretical study
Authors: Nishida Masahiko
Published: July 2002

Optical transitions via localized surface states on Si(001) ultrathin films terminated by monohydride dimers on both surfaces with 2×1 periodicity are studied by use of the extended Hückel-type nonorthogonal tight-binding method. The calculated oscillator strength between the bottom of the empty surface states and the valence-band maximum is much larger than that for direct band-to-band transitions in films with an ideal dihydride termination on both surfaces. An analysis shows that this strong optical coupling is attributed to a significant s-like character introduced into the surface-state bottom by the formation of the monohydride dimers on the film surface.
Light-induced self-written three-dimensional optical waveguide
Authors: Kagami Manabu , Yamashita Tatsuya , Ito Hiroshi
Published: July 2002

Three-dimensional (3D) optical waveguides were fabricated in a photopolymerizing resin mixture solution by using a multimode optical fiber, without any moving parts. The core portion has formed by the selective photopolymerization of a higher refractive index monomer by Ar+ laser irradiation through the optical fiber. A continuous, straight waveguide was grown by the self-trapping of a guided laser beam. We demonstrated automatic 3D optical circuit formation that enables regrowth after passing through thick transparent glass plates. This growth mechanism also enables automation of the optical interconnection and packaging process, and could potentially contribute to future expansion of optical fiber communications networks.
Dislocation-free GaAsyP1−x−yNx/GaP0.98N0.02 quantum-well structure lattice- matched to a Si substrate
Authors: Fujimoto Yasuhiro , Yonezu Hiroo , Utsumi Atsushi , Momose Kenji , Furukawa Yuzo
Published: July 2002

We report that a lattice-matched and dislocation-free GaAsyP1−x−yNx/GaP0.98N0.02 quantum-well (QW) structure can be grown on a Si substrate. A two-dimensional growth mode was maintained during growth of all the layers. It was confirmed that the QW structure was lattice-matched to the Si substrate from the lattice constant measured by x-ray diffraction. A cross-sectional image taken by transmission electron microscopy revealed that no threading dislocations or misfit dislocations were observed at the QW structure.
Interface ferromagnetism in oxide superlattices of CaMnO3/CaRuO3
Authors: Takahashi K. S. , Kawasaki M. , Tokura Y.
Published: July 2002

Oxide superlattices composed of antiferromagnetic insulator layers of CaMnO3 (10 unit cells) and paramagnetic metal layers of CaRuO3 (N unit cells) were fabricated on LaAlO3 substrates by pulsed-laser deposition. All the superlattices show ferromagnetic transitions at an almost identical temperature (TC∼95 K) and negative magnetoresistance below TC. Each magnetization and magnetoconductance of the whole superlattice at 5 K is constant and independent of CaRuO3 layer thickness when normalized by the number of the interfaces between CaMnO3 and CaRuO3. These results indicate that the ferromagnetism shows up only at the interface and is responsible for the magnetoresistance.
Structural relaxation kinetics of antimony borate glasses with covalent bonding character
Authors: Honma T. , Benino Y. , Komatsu T. , Sato R. , Dimitrov V.
Published: July 2002

The structural relaxation kinetics in the glass transition for xSb2O3⋅(100−x)B2O3 glasses with covalent bonding character has been examined from viscosity and heat capacity measurements. These binary glasses have low glass transition temperatures, Tg=250–290 °C, but show a high thermal resistance against crystallization. The degree of fragility m estimated from the activation energy for viscous flow (Eη=290–531 kJ mol−1) is m=29–51. The activation energy for enthalpy relaxation, ΔH=297–602 kJ mol−1, is evaluated from the cooling rate dependence of the limiting fictive temperature. The ΔH values are very close to the Eη values, meaning that the decoupling between enthalpy relaxation and viscous flow is small. The values of Kovacs–Aklonis–Huchinson–Ramos (KAHR) parameter θ estimated from ΔH/RTg2 are 0.13–0.25, where R is the gas constant. The glasses with 30–60 mol % Sb2O3 have very similar Eη, m, ΔH, and θ values. It has been demonstrated that the structural relaxation kinetics of the binary antimony borate glasses is affected by the boron coordination numbers (i.e., BO3 and BO4) and the covalent bonding character of Sb–O bonds, and consequently these glasses are regarded as a highly strong glass-forming system in the fragile/strong classification concept.
Large enhancement in dissociative electron attachment to HCl adsorbed on H2O ice via transfer of presolvated electrons
Authors: Lu Q.-B. , Sanche L.
Published: July 2002

We report that dissociative electron attachment (DEA) to HCl is strongly enhanced by adsorption on the surface of H2O ice. The absolute DEA cross section at ∼0 eV for HCl adsorbed on ice is measured to be ∼4.0×10−15 cm2, which is two orders of magnitude higher than in the gas phase. This enhancement is essentially due to electron transfer from precursor states of the solvated electron in ice to an unfilled molecular orbital of HCl followed by its dissociation. This study indicates that electron-induced dissociation may be a significant process leading to HCl dissociation on ice surfaces in polar stratospheric clouds due to ionization by cosmic rays.
Paramagnetism and antiferromagnetic d–d coupling in GaMnN magnetic semiconductor
Authors: Zając M. , Gosk J. , Kamińska M. , Twardowski A. , Szyszko T. , Podsiadło S.
Published: July 2002

The magnetization of Ga1−xMnxN (x<0.1) crystals was measured as a function of the magnetic field and temperature. Paramagnetic behavior typical of spin S=5/2 expected for Mn2+ (d5) magnetic centers was observed in the temperature range of 2 K<T<300 K. On the other hand, antiferromagnetic coupling between Mn ions was clearly visible. The nearest neighbor (NN) coupling constant JNN/kB=−1.9 K was estimated from the temperature dependence of the magnetization.
Inhomogeneous broadening of the zero phonon line of phthalocyanine in superfluid helium droplets
Authors: Slenczka Alkwin , Dick Bernhard , Hartmann Matthias , Peter Toennies J.
Published: July 2002

The electronic origin band of the S1←S0 transition of monomer phthalocyanine doped into liquid helium droplets consist of a single zero phonon line (ZPL) and a structured phonon wing. The latter reflects the low frequency modes of the helium droplet. At very high resolution (1 MHz) the asymmetric spectrum of the ZPL of phthalocyanine provides no indication of a rotational substructure. Changes in the asymmetry and the peak position of the ZPL with variation of the average droplet size are in very good agreement with an inhomogeneous model line shape.
A study on a lobed jet mixing flow by using stereoscopic particle image velocimetry technique
Authors: Hu Hui , Saga Tetsuo , Kobayashi Toshio , Taniguchi Nobuyuki
Published: July 2002

In a continuing effort to study the mixing enhancement by large-scale streamwise vortices generated by a lobed nozzle, a high-resolution stereoscopic particle image velocimetry (PIV) system was used in the present study to conduct three-dimensional measurements of air jet flows exhausted from a lobed nozzle and a conventional circular nozzle. The three-dimensional instantaneous and ensemble-averaged velocity fields, instantaneous and ensemble-averaged streamwise vorticity distributions and turbulent kinetic energy distributions were used to analyze the characteristics of the mixing process in the lobed jet flow compared with conventional circular jet flow. The existence of large-scale streamwise vortices in the lobed jet mixing flow is revealed clearly from the stereoscopic PIV measurement results. The instantaneous streamwise vorticity distributions revealed that the large-scale streamwise vortices generated by the corrugated trailing edge of the lobed nozzle break into smaller, but not weaker streamwise vortices gradually as they travel downstream. This is the proposed reason why a lobed nozzle would enhance both large-scale mixing and small-scale mixing reported by other researchers. The overall effect of the lobed nozzle on the mixing process in the lobed jet mixing flow was evaluated by analyzing the ensemble-averaged streamwise vorticity distributions. It was found that ensemble-averaged streamwise vortices in the lobed jet mixing flow grew up and expanded radially over the first diameter of the test nozzle, then began to break down into smaller and weaker vortices further downstream. The averaged turbulent kinetic energy profile indicated that most of the intensive mixing between the core jet flow and ambient flow, due to the special geometry of the lobed nozzle, occurred within the first two diameters of the lobed nozzle, which corresponds to the upstream region where the ensemble-averaged streamwise vortices break down.
Large χ(3) of squarylium dye J aggregates measured using the Z-scan technique
Authors: Tatsuura Satoshi , Wada Osamu , Tian Minquan , Furuki Makoto , Sato Yasuhiro , Iwasa Izumi , Pu Lyong Sun , Kawashima Hitoshi
Published: July 2002

Third-order nonlinear optical coefficients χ(3) were measured for the J aggregates of two types of squarylium dye derivatives at resonant and near-resonant wavelengths by using the Z-scan technique. The maximum χ(3) value evaluated at one-photon resonance was 2.9×10−6 e.s.u., which was greater than that of phthalocyanines by 4 orders of magnitude. χ(3) for one squarylium derivative was approximately two times as large as that of the other. This can be attributed to the difference of the number of molecules contributing to a coherent state in each J aggregate.
Simple, discriminative measurement technique for radon and thoron concentrations with a single scintillation cell
Authors: Tokonami Shinji , Yang Mingli , Yonehara Hidenori , Yamada Yuji
Published: July 2002

A simple, discriminative measurement technique for radon and thoron concentrations is discussed. In this technique, a single scintillation cell is used for both radon and thoron measurements. It consists of two measurements that use the difference of the half life between the two isotopes. Alpha counting efficiencies for their associated radionuclides were estimated by a Monte Carlo calculation. When evaluating the conversion factor for concentration on two types of scintillation cells, both agreed well with experimental values. Optimum measurement conditions on the timetable are also discussed. This technique can provide two concentrations promptly. Although it is not highly sensitive, it is applicable to performance tests for radon/thoron monitors and simultaneous exhalation rate measurements for both radon and thoron.
Stress-induced depolarization of (Pb, La)TiO3 ferroelectric thin films by nanoindentation
Authors: Algueró M. , Bushby A. J. , Reece M. J. , Poyato R. , Ricote J. , Calzada M. L. , Pardo L.
Published: July 2002

Electrical depolarization has been observed in lanthanum-modified lead titanate ferroelectric thin films stressed by nanoindentation. A spherical metallic indenter was used as a top electrode to locally pole the films and then to measure the depolarization current intensity. The current intensity had distinctive maxima at given indentation forces. These are related to the stress thresholds for the depolarization mechanism, which is probably 90° domain wall movements. Knowledge of the depolarization stresses is necessary for the design of microelectromechanical systems that include a ferroelectric layer.
Atomistic evolution of Si1–x–yGexCy thin films on Si(001) surfaces
Authors: Sakai Akira , Torige Yuji , Okada Masahisa , Ikeda Hiroya , Yasuda Yukio , Zaima Shigeaki
Published: July 2002

The initial growth process of Si1−x−yGexCy thin films on Si(001) surfaces is examined by scanning tunneling microscopy. The surface morphology of the film critically depends on the C fraction in the film. Evidence is presented on an atomic scale that the epitaxial growth of Si1−x−yGexCy films with large C fractions is dominated by phase separation between Si–C and Si–Ge, concomitant with C condensation on the surface of the growing films. We find that the addition of a thin (1–2 ML) SiGe interlayer between the Si1−x−yGexCy film and the Si substrate drastically improves the film structure, leading to a planar morphology even with large C fractions present in the film.
From two-dimensional heterogeneous nucleation-induced kinetic roughening to needle-like pattern formation
Authors: Liu Xiang Y.
Published: July 2002

We report in this letter a special type of kinetic roughening at a growing crystal face induced by foreign particles. At relatively low supercoolings, the faceted growth of the {110} faces of naphthalene crystals from its melt reveals a transition from smooth to rough growth when foreign particles are in contact with the crystal surface, and followed by an increase in the growth rate. This is due to the vanishing of the two-dimensional nucleation barrier at the particle contact area, which gives rise to the formation of a needle-like pattern formation at the contact point. At high supercoolings, the particle-induced kinetic roughening is suppressed. The particle-induced kinetic roughening was also observed at the {110} faces of n-C21H44 crystals.
Effect of Landau damping on ion-acoustic solitary waves in a magnetized nonthermal plasma with warm ions
Authors: Bandyopadhyay Anup , Das K. P.
Published: July 2002

An evolution equation describing weakly nonlinear and weakly dispersive ion-acoustic waves in magnetized nonthermal plasma with warm ions, including the effect of Landau damping, has been derived. It is found that the coefficient of the nonlinear term of this equation vanishes along a particular curve in βσ-plane, where β is a parameter that determines the proportion of fast electrons and σ is the ratio of electron to ion temperature. In this case, the modified evolution equation has also been derived. The solitary wave solutions of these equations propagating obliquely to the external magnetic field have been obtained by the multiple time scale method. It has been found that in either case the amplitude of the solitary waves slowly decreases with time.
Collision-induced electronic energy transfer from v=0 of the E(0g+) ion-pair state in I2: Collisions with He and Ar
Authors: Fecko Christopher J. , Freedman Miriam A. , Stephenson Thomas A.
Published: July 2002

The electronic energy transfer pathways that occur following collisions between I2 in the E ion-pair electronic state (v=0, J=55) and He and Ar atoms have been determined. The nearby D, D′, and β ion-pair states are populated, but with relative branching ratios that vary with the rare gas collision partner. In He/I2 collisions, the D state is preferentially populated, while Ar/I2 collisions preferentially populate the β electronic state. Bimolecular rate constants and effective hard sphere collision cross sections have been determined for each channel; the cross sections range from 7.0±1.0 Å2 for populating the β state with Ar collisions to 0.9±0.2 Å2 for populating the D′ state with He collisions. For both rare gas collision partners, and all three final electronic states, low vibrational levels are populated, in rough accord with the relevant Franck–Condon factors. There is little propensity observed for population of vibrational levels that are in near resonance with the initially prepared level in the E state.
Influence of strain-induced indium clustering on characteristics of InGaN/GaN multiple quantum wells with high indium composition
Authors: Cho H. K. , Lee J. Y. , Song J. H. , Yu P. W. , Yang G. M. , Kim C. S.
Published: July 2002

We report the effect of strain-induced indium clustering on the emission properties of InGaN/GaN multiple quantum wells grown with high indium composition by metalorganic chemical vapor deposition. Indium clustering confirmed by high-resolution transmission electron microscopy results in the redshift of the emission peak and the increase of the integrated photoluminescence (PL) intensity. We found that strong carrier localization in indium clustering induces the increases of the activation energy of PL integrated intensity, the temperature independence of PL decay profiles, and the intensity fluctuation of the cathodoluminescence images. All these observations suggest structurally and optically that the improved emission properties in the InGaN/GaN multiple quantum well with high indium composition are associated with the localized states in the strain-induced indium cluster.
Predicting unimolecular chemical reactions: Chemical flooding
Authors: Müller E. Matthias , de Meijere Armin , Grubmüller Helmut
Published: July 2002

We present a method to predict products, transition states, and reaction paths of unimolecular chemical reactions such as dissociation or rearrangement reactions of small to medium sized molecules. The method thus provides the necessary input for established procedures to compute barrier heights and reaction rates, which conventionally have to be assumed heuristically. The method is an extension of the force field based conformational flooding procedure, but here aims at an accelerated barrier crossing of chemical reactions rather than conformational motions. Accordingly, it is now coupled to density functional molecular dynamics, such that the chemical reaction under study takes place at the picoseconds time scale set by todays computer technology. Barrier crossings are accelerated by means of an additional energy term (flooding potential) that locally destabilizes the educt conformation without affecting possible transition states or product states. The method is applied to two test systems, bicyclopropylidene and methylenecyclopropane, for which the reaction paths are predicted correctly. New details of reaction pathways are found, such as a transient concerted, but asynchronous rotation of the two methylene groups for the bicyclopropylidene→methylenespiropentane reaction. Our method can be applied to simulations in the gas phase as well as in solution and can be combined with force field simulations, e.g., in hybrid density functional/force field (QM/MM) computations.
Monte Carlo calculation of electron drift characteristics and avalanche noise in bulk InAs
Authors: Satyanadh G. , Joshi R. P. , Abedin N. , Singh U.
Published: July 2002

Field dependent drift velocity results are presented for electron transport in bulk indium arsenide (InAs) material based on a Monte Carlo model, which includes an analytical treatment of band-to-band impact ionization. Avalanche multiplication and related excess noise factor (F) are computed as a function of device length and applied voltage. A decrease in F with increases in device length is obtained. The results suggest an inherent utility for InAs-based single-photon avalanche detectors, particularly around the 2 μm region of interest for atmospheric remote sensing applications.
Growth of low-stress cubic boron nitride films by simultaneous medium-energy ion implantation
Authors: Fitz C. , Kolitsch A. , Möller W. , Fukarek W.
Published: July 2002

It is demonstrated that the intrinsic stress in cubic boron nitride films can be significantly relaxed during growth by simultaneous medium-energy ion implantation. The stress in the growing film has been studied in situ using cantilever curvature measurements and has been reduced to below 2 GPa by simultaneous Ar+ or N+ ion implantation with an energy of 70 and 35 keV, respectively. The resulting cubic boron nitride films show an increased long-term stability. The results reveal that the stress in cBN is not reduced due to segregation of boron at grain boundaries.
Structure of DySi2 nanowires on Si(001)
Authors: Preinesberger C. , Becker S. K. , Vandré S. , Kalka T. , Dähne M.
Published: July 2002

Free-standing dysprosium–silicide nanowires can be formed on Si(001) by self assembly. It is shown that the wires consist of anisotropically strained hexagonal DySi2 with the c axis aligned perpendicular to the wires. The surface is characterized by a 2×1 reconstruction due to the formation of Si dimer chains.
Exploring the Bermuda triangle of homonuclear diatomic spectroscopy: The electronic spectrum and structure of Ge2
Authors: Hostutler David A. , Li Haiyang , Clouthier Dennis J. , Wannous Ghassan
Published: July 2002

The optical spectrum of jet-cooled Ge2 has been observed for the first time. Laser-induced fluorescence (LIF) and wavelength resolved emission spectra were recorded using the pulsed discharge technique with a tetramethylgermane precursor. Analysis of the spectra yielded the vibrational constants ωe″=287.9(47), ωexe″=0.81(55), ωeye″=0.0037(18), ωe′=189.0(15), ωexe′=6.41(30), and Te′=20 610.8(16) cm−1. High-resolution rotationally resolved spectra of several bands of Ge274 show two strong P and R branches and two very weak Q branches. We have assigned the band system as a Hund’s case (c) Ω′=1−Ω″=1 transition from the ground Σg−3 state to a Σu−3 excited state. The bond lengths derived from the rotational constants are r0″=2.3680(1) Å and re′=2.5244(18) Å, an ∼0.16 Å increase on electronic excitation. Arguments are presented for assigning the transition to a σg2πu2→σg2πuπg electron promotion, although the observed increase in the bond length is much less than predicted by previous ab initio calculations. The absence of the 0u+–0g+ component in the spectra has been attributed to an excited state predissociation.
Polar diffraction gratings made by spatially periodic photopoling Langmuir–Blodgett films
Authors: Blinov L. M. , Palto S. P. , Yudin S. G. , De Santo M. P. , Cipparrone G. , Mazzulla A. , Barberi R.
Published: July 2002

Polar diffraction gratings have been prepared by spatially periodic photopoling Langmuir–Blodgett films consisting of azobenzene compounds. The gratings were made by film irradiation with two left circularly polarized Ar-ion laser beams under strong electric field from a corona discharge. Therefore, simultaneously with the spatial modulation of the linear refractive index, the local polar order of the film becomes periodically modulated (through symmetry, the latter is related to the modulation of second-order optical nonlinearity χ(2)). The local polarity is shown by an electrostatic force microscopy technique: an image of the polar grating is recorded without any trace of periodicity in the film topography.
Development of large radio frequency negative-ion sources for nuclear fusion
Authors: Kraus W. , McNeely P. , Speth E. , Heinemann B. , Vollmer O. , Wilhelm R.
Published: July 2002

The development of a large-area radio frequency (rf) source for the negative-ion beam production has been continued using a prototype source of 37 1 with a rf power up to 160 kW at 0.93 MHz. The design is based on a small rf driven cylindrical driver source on a large magnetically confined expansion volume (60×31×20 cm3) and is in principle scalable to any size. Although the H− yields reported earlier have not been exceeded, progress has been made in several areas. To protect the ceramic insulator of the driver from thermal stress an internal copper Faraday screen has been added. This has resulted in reliable long pulse operation with a duration up to 15 s. Various different grid materials have been investigated as potential options for cesium-free operation. No clear-cut conclusion can yet be drawn from those experiments due to unexpected copper sputtering from the Faraday shield onto the plasma grid. The dependence of the H− yield on gas pressure and filter strength have been studied. The effect of noble gases addition to the discharge has been investigated using a Langmuir probe. These measurements indicate that the enhancement of H− current may be caused by a change in plasma density brought about by the addition of a noble gas.
High energy ion beam studies of ion exchange in a Na2O–Al2O3–SiO2 glass
Authors: Shutthanandan V. , Baer D. R. , Thevuthasan S. , Adams E. M. , Maheswaran S. , Engelhard M. H. , Icenhower J. P. , McGrail B. P.
Published: July 2002

As part of understanding the processes leading to sodium release and ion exchange, the surface and near surface reaction regions on several specimens of a Na2O–Al2O3–SiO2 glass have been examined after exposures to isotopically labeled aqueous solutions. The majority of the analyses described here have been carried out using energetic ion beam analysis. Rutherford backscattering spectrometry (RBS) has been used to measure the overall glass composition and to determine the profiles and amounts of Na released from the surface. An important part of the ion exchange process is the uptake and incorporation of hydrogen and oxygen in the glass from the solution. To facilitate this analysis, the glasses were exposed to a solution containing O18 and D and analyzed by accelerator based nuclear reaction analysis. To confirm some of the RBS depth profile data very near the surface, x-ray photoelectron spectroscopy depth profiles were collected on some samples. Although the Na concentration is decreased in the near surface region, it is not totally removed from the outer surface. In this same region, there is also a significant amount of O18 incorporated demonstrating considerable interaction between the water and the glass. Deeper into the material the amounts of D and O18 are more consistent with water or D3O+ diffusion. These results suggest that there exist an outer reaction layer and an inner ion-exchange layer in the surface region of the reacted glass.
Multifractal spectra and the relaxation of model polymer networks
Authors: Blumen A. , Jurjiu A.
Published: July 2002

The mechanical and dielectric relaxation of polymer networks depends (especially in simple Gaussian-type approaches which extend the Rouse model) on the eigenvalues of the corresponding connectivity matrices. We use this to evaluate explicitly experimentally accessible relaxation forms for finite Sierpinski-type networks, whose eigenvalue spectra are multifractal. It turns out that the observable quantities are by far less singular than the eigenvalue spectra, since the underlying spectral structures get smoothed out. Our results establish unequivocally the spectral dimension as fundamental relaxation parameter; to see this, however, the finite fractal networks have to be sufficiently large.
High pressure reactivity of solid benzene probed by infrared spectroscopy
Authors: Ciabini Lucia , Santoro Mario , Bini Roberto , Schettino Vincenzo
Published: July 2002

The chemical transformation of benzene under pressure is investigated, at room temperature and at 100 K, by means of infrared spectroscopy. Pressurization-decompression cycles in the 0–50 GPa pressure range have been performed to achieve the complete transformation of the monomer. The yellow-brownish recovered sample has been identified as an amorphous hydrogenated carbon (a-C:H). A correlation has been established between the pressure behavior of the frequencies of both Raman and infrared internal modes, and the corresponding vibrational energies in the S1 excited state (1B2u). From this comparison we conclude that pressure induces a mixing between the ground and the S1 electronic states. The increased ring flexibility enhances the interactions among nearest-neighbor molecules inducing the formation of a network of interconnected benzene units where the aromatic character is lost. The bond breaking mainly occurs during the decompression cycle favored by the density decrease. Radical species form in this stage and rapidly propagate to give the denser a-C:H final product.
Impact of Ga/N flux ratio on trap states in n-GaN grown by plasma-assisted molecular-beam epitaxy
Authors: Hierro A. , Arehart A. R. , Heying B. , Hansen M. , Mishra U. K. , DenBaars S. P. , Speck J. S. , Ringel S. A.
Published: July 2002

The effect of growth regime on the deep level spectrum of n-GaN using molecular-beam epitaxy (MBE) was investigated. As the Ga/N flux ratio was decreased towards Ga-lean conditions, the concentration of two acceptor-like levels, at Ec−3.04 and 3.28 eV, increased from 1015 to 1016 cm−3 causing carrier compensation in these films. Thus, these two traps behaved as the dominant compensating centers in MBE n-GaN. Furthermore, the increase in trap concentration also strongly correlated with the degradation of both surface morphology and bulk electron mobility towards Ga-lean conditions, where higher pit densities and lower mobility were observed. These results show that the growth regime directly impacts all morphology, bulk transport, and trap states in n-GaN.
High-resolution inner-shell photoabsorption of the OH and OD free radicals
Authors: Stranges Stefano , Richter Robert , Alagia Michele
Published: July 2002

The O 1s photoabsorption (ion-yield) spectra of OH (2Πi,v″=0) and OD (2Πi,v″=0) radicals have been studied in the gas phase for the first time. Spectroscopic parameters for the O 1s→π excited Σ+2 state of OH and OD, observed at ∼525.8 eV, have been obtained from a Franck–Condon analysis comparing the spectra of both molecules. The lifetime width of this state has been measured as Γ=147±11 meV and Γ=156±7 meV for OH and OD, respectively. Excellent quantitative agreement is found between the experimental spectroscopic parameters re, ωe, and T0, measured in this work for the Σ+2 state and recent theoretical results obtained for OH by high-level ab initio calculations. The spectroscopic parameters predicted by the equivalent-core model are close to the experimental values within a few percent. This model therefore provides an accurate description of the lowest-lying excitation process in both OH and OD molecules.
Spectroscopic ellipsometry characterization of high-k dielectric HfO2 thin films and the high-temperature annealing effects on their optical properties
Authors: Cho Yong Jai , Nguyen N. V. , Richter C. A. , Ehrstein J. R. , Lee Byoung Hun , Lee Jack C.
Published: July 2002

The optical properties of a set of high-k dielectric HfO2 films annealed at various high temperatures were determined by spectroscopic ellipsometry. The results show that the characteristics of the dielectric functions of these films are strongly affected by high temperature annealing. For a sample annealed at 600 °C, the film becomes polycrystalline, and its dielectric function displays a distinctive peak at 5.9 eV. On the other hand, the film remains amorphous without the 5.9 eV feature after 500 °C annealing. To model the dielectric functions, the Tauc–Lorentz dispersion was successfully adopted for these amorphous and polycrystalline films. The absorption edge was observed to shift to a higher energy at a high temperature annealing. Defects in the films were shown to relate to the appearance of a band tail above the absorption edge, and they appear to diminish with high temperature annealing.
Understanding the sensitivity of nucleation kinetics: A case study on water
Authors: Kathmann Shawn M. , Schenter Gregory K. , Garrett Bruce C.
Published: July 2002

Small atomic or molecular clusters provide the bridge between vapor and liquid phases. Nucleation is a rare event process by which clusters of a new phase are produced. This process is inherently dynamic and as such the new phase cannot exist until an activation barrier is surmounted. Dynamical nucleation theory (DNT) utilizes variational transition state theory to provide a framework in which cluster evaporation and condensation rate constants can be determined directly. To date, the fundamental nature regarding the intrinsic instability of the kinetics of the nucleation process has eluded theoretical efforts. In this paper we present a sensitivity analysis of the homogeneous nucleation rate on kinetic parameters used in DNT. Moreover, several classical interaction potentials for water exist, most of which have been parametrized to reproduce some bulk properties of water at ambient conditions. Thus, an analysis was undertaken to explore what effects different water potentials have on the dynamical quantities relevant to nucleation. The implication of these results on future work will be discussed.
Characterization of avalanche-like events in a confined plasma
Authors: Politzer P. A. , Austin M. E. , Gilmore M. , McKee G. R. , Rhodes T. L. , Yu C. X. , Doyle E. J. , Evans T. E. , Moyere R. A.
Published: July 2002

One mechanism for transport of energy and particles in a plasma is by discrete, intermittent, uncorrelated events, often called avalanches. This paper reports observations and quantitative characterization of avalanche events in a magnetically confined plasma. The observations are primarily of electron temperature fluctuations. Avalanches are identified by their large spatial scale, up to the system size, by self-similar behavior in the frequency spectrum and the autocorrelation function and by propagation. The two-point cross-correlation function allows determination of a characteristic velocity, which typically varies from several hundred meters per second in the outer part of the plasma to zero or even inward near the axis. This can be interpreted as resulting from the prevalence of negative avalanches (i.e., holes) near the axis. The presence of a long-tailed probability distribution is indicated by a Hurst parameter (H) in the range 0.80 to 0.95, which becomes smaller in the outer quarter of the plasma radius. Density fluctuation spectra from the plasma core also show self-similar behavior. Power transport estimates show that about half of the heat flux is carried by the avalanche events under conditions with no magnetohydrodynamic activity. These observations are qualitatively similar to results of modeling calculations based on drift wave turbulence. It is reasonable to infer that avalanches are the macroscopic manifestation of turbulence which inherently has a small spatial scale and, thus, allow a local, gyro-Bohm scaling process to show global Bohm-type behavior.
Structural, thermal, and magnetic properties of Ni2MnGa
Authors: Enkovaara J. , Ayuela A. , Nordström L. , Nieminen R. M.
Published: July 2002

The two main effects underlying the magnetic shape memory effect in Ni2MnGa are martensitic transformations and magnetic anisotropy energies. Both issues are addressed here with first-principles calculations. First, we examine how the tetragonality in the martensitic phase varies with the composition. Then, the actual transformation is investigated by comparing the free energies of different phases. The transition from the cubic structure to the tetragonal structure with c/a=1.27 is driven by the vibrational free energy and occurs at a temperature of 200 K which is in the experimental range. Finally, we focus on the magnetic anisotropy energy for the tetragonal structure with c/a=0.94. It is shown to be a magnetically nearly ideal uniaxial system determined by the first-order anisotropy constant. However, it is estimated that the twinned microstructure can cause higher-order anisotropies to show up in the measured anisotropy.
Robust wave function optimization procedures in quantum Monte Carlo methods
Authors: Bressanini Dario , Morosi Gabriele , Mella Massimo
Published: July 2002

The energy variance optimization algorithm over a fixed ensemble of configurations in variational Monte Carlo often encounters problems of convergence. Being formally identical to a problem of fitting data, we re-examine it from a statistical maximum-likelihood point of view. We show that the assumption of an underlying Gaussian distribution of the local energy, implicit in the standard variance minimization scheme, is not theoretically nor practically justified, and frequently generates convergence problems. We propose alternative procedures for optimization of trial wave functions in quantum Monte Carlo and successfully test them by optimizing a trial wave function for the helium trimer.
Nanoparticle structure of MgB2 with ultrathin TiB2 grain boundaries
Authors: Zhao Y. , Huang D. X. , Feng Y. , Cheng C. H. , Machi T. , Koshizuka N. , Murakami M.
Published: July 2002

The microstructure of the Ti-doped MgB2 which shows a significantly improved critical current density, Jc [Appl. Phys. Lett. 79, 1154 (2001)], is investigated. It is found that Ti does not occupy the atomic site in the MgB2 crystal structure, but forms a thin TiB2 layer (with a thickness about one unit cell of TiB2) in the grain boundaries of MgB2. Besides, MgB2 grains are greatly refined by Ti doping, forming a strongly coupled nanoparticle structure. It is argued that the unique microstructure of the MgB2 nanoparticles with TiB2 nanograin boundaries may take responsibility for the enhancement of Jc in the Ti-doped MgB2 bulk superconductor.
Raman study of collective plasmon-longitudinal optical phonon excitations in cubic GaN and AlxGa1−xN epitaxial layers
Authors: Pusep Yu. A. , Silva M. T. O. , Fernandez J. R. L. , Chitta V. A. , Leite J. R. , Frey T. , As D. J. , Schikora D. , Lischka K.
Published: July 2002

The plasmon-longitudinal optical (LO) phonon collective excitations were studied by Raman scattering in cubic GaN doped with Si and in intrinsically doped cubic AlxGa1−xN alloys. The specific asymmetry of the corresponding Raman lines, associated with the dispersion of the coupled excitations was clearly detected in GaN. The spatial coherence of the relevant coupled oscillations was determined from the comparison of the experimental Raman spectra with the calculated ones. A broad line, which was previously attributed to the overdamped plasmon-LO phonon oscillations, was also observed in the GaN and AlxGa1−xN samples in the range between the transversal optical and LO phonon frequencies.