bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical turbulence suppression by a strong magnetic field: A Feynman path integral argument
Authors: Botelho Luiz C. L.
Published: July 2002

We show the suppression of the randomness of the magnetohydrodynamic flux (modeled by the Navier–Stokes equation) in the presence of a random stirring by a Feynman path integral one-loop and low viscosity argument. Additionally, we show exactly the above supressing turbulence phenomenon in the context of Beltrami fluxes.
Coherent magnetization reversal of nanoparticles with crystal and shape anisotropy
Authors: Schmidt H. , Ram R. J.
Published: July 2002

We discuss coherent magnetization reversal in single-domain particles with cubic or uniaxial crystal anisotropy and present the derivation and simulation of the nucleation field in the important case of particles with shape anisotropy and a random orientation of a cubic crystal anisotropy axis. We analyze the interplay of shape and crystal anisotropy as a function of crystal orientation with respect to the applied field. In addition, we present simulations for particles with uniaxial crystal anisotropy and derive the values for remanence and coercive field of an ensemble of particles from calculated hysteresis loops in each case.
A novel resistance measurement technique in the field of directional solidification
Authors: Salvi C. , Garandet J. P.
Published: July 2002

A brief review of existing diagnostic methods in directional solidification is first presented. We then discuss the potential of global and differential resistance measurements to determine the interface position and velocity in liquid metals and semiconductors. An electronic design allowing the measurements to be carried out with maximum accuracy is proposed. This design is characterized both at low and high temperatures and it is shown that the peak to peak noise on the resistance signal can be kept of the order of 0.1 μ Ω. In the tin based system used in our investigations, this amounts to an accuracy in terms of solid–liquid interface position in the micrometer range, which represents a significant improvement with respect to existing techniques. We found that such an accuracy allows us to characterize in detail the heat transfer phenomena within our directional solidification furnace, and to identify the thermal lag of the device in the translation and stabilization stages.
As-cast quasicrystalline phase in a Zr-based multicomponent bulk alloy
Authors: Kühn U. , Eckert J. , Mattern N. , Schultz L.
Published: July 2002

An icosahedral quasicrystalline phase is obtained directly from the melt by copper mold casting of a Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 alloy. On the other hand, rapid quenching of the alloy leads to an amorphous phase. Upon annealing, the amorphous structure precipitates quasicrystals in the first stage of crystallization. The microstructure of the quasicrystalline state is quite different for the two preparation routes, which is correlated with the asymmetry of the nucleation and growth rate upon cooling or heating. The quasicrystals formed upon slow cooling from the melt have a size of about 1 μm. In contrast, the quasicrystals formed by annealing do not exceed a size of 5–10 nm.
Dipole bound and valence state coupling in argon-solvated nitromethane anions
Authors: Lecomte F. , Carles S. , Desfrançois C. , Johnson M. A.
Published: July 2002

The coupling between the dipole bound and valence electronic states of the nitromethane anion has been investigated via Rydberg electron transfer spectroscopy and field detachment spectroscopy of the bare and argon-solvated anions. The unique aspects of the nitromethane system are highlighted by comparing the solvation behavior of nitromethane anion with that of acetonitrile, a system in which the dipole bound state is well-isolated from the resonance arising from excess electron occupation of its high lying lowest unoccupied molecular orbital.
Osmotic pressure of ring polymer solutions: A Monte Carlo study
Authors: Flikkema Edwin , ten Brinke Gerrit
Published: July 2002

Using the wall theorem, the osmotic pressure of ring polymers in solution has been determined using an off-lattice topology conserving Monte Carlo algorithm. The ring polymers are modeled as freely-jointed chains with point-like beads, i.e., under conditions corresponding to θ-conditions for the linear chains. The equation of state is determined. It is furthermore demonstrated that a similar equation of state is found for the corresponding linear chains taking beads of size 0.113 in units of bond length.
High power, high uniformity strip electron gun design, simulation and performance
Authors: Thakur K. B. , Sahu G. K. , Tamhankar R. V. , Patel Kartik
Published: July 2002

This article discusses the simulation, design and performance of an 80 kW strip electron gun. Electrons from an electrically heated, multisegmented tantalum filament are electrostatically focused to provide a sheet of electrons in the postanode region, that when bent through 270° along a circle of radius 120 mm by a uniform magnetic field, focuses on the target. This gun has been tested up to 80 kW (45 kV×1.8 A), and delivers power densities on the target in the range of 20 kW cm−2. The linear current density is uniform up to 20%, over a focal spot of dimension 80×5 mm2. Good correspondence between filament geometry and focal spot image of the electron beam on the target has been obtained, which is desirable in applications involving metal vapor coatings. This design is scaleable along the length of the focal spot without sacrificing uniformity, which is not possible in scanning or multiple pencil-type of electron guns. Experimental measurement of gun parameters such as position of filament in relation to the cathode opening, distribution of current along the length of the beam, focal spot size, extraction efficiency, etc., along with data concerning electron beam quality as a function of various parameters, are presented. The performance of this gun under different experimental conditions, as well as the simulation and modeling using an in-house three-dimensional beam dynamics code, is discussed.
High coercivity in SiO2-doped CoFe2O4 powders and thin films
Authors: Ding J. , Chen Y. J. , Shi Y. , Wang S.
Published: July 2002

Mössbauer and magnetic studies have shown that 1–2 wt % of SiO2 could be dissolved in the CoFe2O4 structure. Changes in magnetization and Curie temperature were observed. The presence of SiO2 might enhance magnetic anisotropy. Coercivity values of up to 3.5 kOe were measured for mechanically alloyed CoFe2O4/SiO2 powders. High coercivities were also achieved in SiO2-doped Co–ferrite thin films prepared by sputtering technique. The Co–ferrite thin film deposited on a silicon wafer using a 5 wt % SiO2/Co–ferrite target possessed a coercivity of 7.4 kOe. Spring-magnet behavior was observed, indicating the possible presence of remanence enhancement due to exchange coupling because of nanocrystalline structure.
Infrared optical properties of Ba0.8Sr0.2TiO3 ferroelectric thin films
Authors: Huang Zhiming , Zhang Zhanhong , Jiang Chuping , Yu Jian , Sun Jinlan , Chu Junhao
Published: July 2002

Infrared optical properties of barium strontium titanate Ba0.8Sr0.2TiO3 thin films grown on Pt/Ti/SiO2/Si substrates are studied by infrared spectroscopic ellipsometry (IRSE) in the spectral range of 2.5–12.6 μm. The refractive index and extinction coefficient of Ba0.8Sr0.2TiO3 are determined by fitting the IRSE data using a classical dielectric function formula. As the wavelength increases, the refractive index decreases, but on the contrary, the extinction coefficient increases. The absorption coefficient is greater than 1000 cm−1 for the wavelength over 12.1 μm, but very small and less than 100 cm−1 under 5.3 μm. The effective static ionic charge has also been derived, which is smaller than it would be in a purely ionic material for Ba0.8Sr0.2TiO3 thin films.
Development of cross-hatch morphology during growth of lattice mismatched layers
Authors: Andrews A. M. , Romanov A. E. , Speck J. S. , Bobeth M. , Pompe W.
Published: July 2002

An approach for understanding the cross-hatch morphology in lattice mismatched heteroepitaxial film growth is developed. It is argued that both strain relaxation associated with misfit dislocation formation and lateral surface step flow are required for the appearance of mesoscopic scale surface undulations during layer growth. The results of Monte Carlo simulations for dislocation assisted strain relaxation and consequent film growth predict the development of cross-hatch patterns with a characteristic surface undulation magnitude ∼50 Å in an approximately 70% relaxed In0.25Ga0.75As layers. This is supported by atomic force microscopy observations of cross-hatch morphology in the same composition samples grown well beyond the critical thickness for misfit dislocation formation.
Electron tunneling detected by electrostatic force
Authors: Klein L. J. , Williams C. C. , Kim J.
Published: July 2002

A method is introduced for measuring the tunneling of electrons between a specially fabricated scanning probe microscope tip and a surface. The technique is based upon electrostatic force detection of charge as it is transferred to and from a small (10−17 F) electrically isolated metallic dot on the scanning probe tip. The methods for dot fabrication, charging, and discharging are described and electron tunneling to a sample surface is demonstrated.
Photoresist etching in an inductively coupled, traveling wave driven, large area plasma source
Authors: Takechi K. , Lieberman M. A.
Published: July 2002

We report on experimental and modeling results for photoresist etching with oxygen gas in an inductively coupled large area plasma source (LAPS). The source is driven by a 13.56 MHz traveling wave launched along a serpentine antenna embedded in the plasma and has a processing area as large as 40 cm×50 cm. We describe a new series-parallel antenna coil configuration, and we present experimental observations for oxygen plasma density profiles, photoresist etch rates, and etch profiles. We introduce a simplified spatially varying oxygen discharge model corresponding to a two-dimensional LAPS geometry in order to account for the generation and loss of both etchant atoms (O atoms) and bombarding ions. The model and experimental results on plasma density are compared. A simplified photoresist etch kinetics model combined with the spatially varying oxygen discharge model is then described. The ion flux and O-atom density predicted by the model are compared with the etch rate data. In order to gain insight into the underlying physical mechanism, we also compare the data with the scaling behavior of the etch rates predicted by a volume-averaged (global) oxygen discharge model. The models and experimental results are generally in good agreement.
Tuning the detection wavelength of quantum-well infrared photodetectors by single high-energy implantation
Authors: Fu L. , Tan H. H. , Jagadish C. , Li Na , Li Ning , Liu Xingquan , Lu Wei , Shen S. C.
Published: July 2002

Single high-energy (0.9 MeV) proton implantation and rapid thermal annealing was used to tune the spectral response of the quantum-well infrared photodetectors (QWIPs). In addition to the large redshift of the QWIPs’ response wavelength after implantation, either narrowed or broadened spectrum was obtained at different interdiffusion extent. In general, the overall device performance for the low-dose implantation was not significantly degraded. In comparison with the other implantation schemes, this single high-energy implantation is the most effective and simple technique in tuning the wavelength of QWIPs, thus, to achieve the fabrication of multicolor detectors.
Chemical and electrical properties of interfaces between magnesium and aluminum and tris-(8-hydroxy quinoline) aluminum
Authors: Shen C. , Kahn A. , Schwartz J.
Published: July 2002

The chemistry, electronic structure, and electron injection characteristics at interfaces formed between tris(8-hydroxy quinoline) aluminum (Alq3) and magnesium (Mg) and aluminum (Al) are studied via x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and current–voltage (I–V) measurements. Both metal-on-Alq3 and Alq3-on-metal interfaces are investigated. All interfaces are fabricated and tested in ultrahigh vacuum in order to eliminate extrinsic effects related to interface contamination. The propensity for Mg and Al to form covalent metal–carbon bonds leads to broad and heavily reacted interfaces when the metal is deposited on the organic film. For this deposition sequence, we propose the formation of an organometallic structure where a single metal atom attaches to the pyridyl side of the quinolate ligand of the molecule and coordinates with an oxygen atom of another ligand or of a neighboring molecule. The other deposition sequence leads to significantly more abrupt interfaces where the chemical reaction is limited to the first molecular layer in contact with the metal surface. Both types of interface exhibit chemistry-induced electronic gap states, the position of which depends on the chemical structure of the interface. The interface width, chemical structure, and electronic states appear to play no significant role in electron injection in metal/Alq3/metal sandwich structures, the I–V characteristics for top and bottom injection being identical over several decades of current.
Step towards sum frequency generation spectromicroscopy at a submicronic spatial resolution
Authors: Humbert B. , Grausem J. , Burneau A. , Spajer M. , Tadjeddine A.
Published: July 2002

A near-field optical device has been developed to collect sum frequency signal, generated in an AsGa single crystal sample by two pulsed lasers, one fixed at a wavelength of 532 nm and the other tunable in midinfrared. The sum frequency signal was collected with an uncoated silica tip. When the tip sample distance was increased, a strong decrease of the collected sum frequency signal was observed: the signal was divided by 2 for an increase of about 100 nm of the distance between the nanoprobe end and the sample surface. Without demonstrating submicronic lateral spatial resolution, this letter is, however, a first step towards a microscopic device of sum frequency generation spectroscopy.
Time resolved ultraviolet absorption spectroscopy of pulsed fluorocarbon plasmas
Authors: Cruden Brett A. , Gleason Karen K. , Sawin Herbert H.
Published: July 2002

Ultraviolet absorption spectroscopy has been used to quantitatively measure CF2 transients in 1 Torr capacitively coupled pulsed plasmas. Time resolved concentrations were obtained for both tetrafluoroethylene (TFE) and hexafluoropropylene oxide (HFPO) feed gases. In the TFE plasma, the CF2 production kinetics follow a first order rise to concentrations of ∼1014/cm3. In the plasma afterglow, a net production of CF2 is observed for a few milliseconds before the transient becomes dominated by a second order recombination process. In the HFPO plasma, three distinct regimes are observed in the plasma on time. Two production regimes exist, one presumably due to HFPO dissociation and the second due to an unknown source, beginning about 5 ms into the on time. Finally, the CF2 concentration passes through a maximum and decreases to steady state. The afterglow processes are similar to those observed for TFE. The production observed in the off time is believed to be related to surface processes.
Direct observation of self-focusing near the diffraction limit in polycrystalline silicon film
Authors: Choi Yunjin , Park June-H. , Kim Myong R. , Jhe Wonho , Rhee Bum Ku
Published: July 2002

We present direct observation of self-focusing near the diffraction limit by measuring the beam-spot size with a scanning fiber probe tip. We have used the polycrystalline silicon film, which exhibits a reverse-saturation (Im χ(3)≈8×10−3 esu) and self-focusing (Re χ(3)≈2×10−2 esu), as measured by the conventional z-scan method with He–Ne laser. It is observed that the beam radius of about its wavelength becomes smaller as the input laser intensity is increased, which indicates that the self-focusing effect dominates over the reverse saturation in the 300-nm-thick sample.
Multiscale modeling of stress-mediated diffusion in silicon: Ab initio to continuum
Authors: Laudon Matthew , Carlson Neil N. , Masquelier Michael P. , Daw Murray S. , Windl Wolfgang
Published: July 2002

In this letter, we present the development of a complete methodology to simulate the effects of general anisotropic nonuniform stress on dopant diffusion in silicon. The macroscopic diffusion equation is derived from microscopic transition-state theory; the microscopic parameters are calculated from first principles; a feature-scale stress-prediction methodology based on stress measurements in the relevant materials as a function of temperature has been developed. The developed methodology, implemented in a continuum solver, is used to investigate a TiN metal gate system. A compressive stress field is predicted in the substrate, resulting in an enhancement in lateral boron diffusion. This enhancement, which our model attributes mostly to solubility effects, is in good agreement with experiment.
Deep level transient spectroscopy characterization of InAs self-assembled quantum dots
Authors: Ilchenko V. V. , Lin S. D. , Lee C. P. , Tretyak O. V.
Published: July 2002

Deep level transient spectroscopy (DLTS) was used to obtain the energy level and the capture characteristics of InAs self-assembled quantum dots embedded in GaAs. A specially designed structure was used for the DLTS measurement for enhanced resolution. This structure allows us to detect capacitance relaxation signal from a single layer of quantum dots and to separate this signal from those of the DX centers inside the structure. The DLTS spectra with different filling pulses and different rate windows provide clear information on the energy level and the capture characteristics of the quantum dots.
Spin-state transition in La1−xSmxCoO3 perovskites
Authors: Sun J. R. , Li R. W. , Shen B. G.
Published: July 2002

Structural and magnetic properties of La1−xSmxCoO3 with x=0, 0.02, 0.04, 0.06, 0.08, 0.12, 0.16, and 0.2 have been systematically studied. LaCoO3 exhibits a rhombohedral structure of R3̄C symmetry. The lattice parameters decrease progressively with the incorporation of Sm, and a second orthorhombic phase of Pbnm symmetry appears for x⩾0.12. The volume fraction of the orthorhombic phase is a sensitive function of the Sm content in the compounds, and increases from 16% for x=0.12 to 75% for x=0.2. Accordingly, the low-spin-to-high-spin transition of the trivalent Co ions, characterized by a steep rise of the magnetization, shifts to higher temperatures. A molecular field analysis indicates an increased energy gap, from 317 K for x=0 to 500 K for x=0.2, between the low-spin and the high-spin state, implying a stabilization of the low-spin trivalent Co ions by the presence of Sm. The enhanced compressive chemical pressure due to the introduction of Sm is suggested to be responsible for the present observations.
Growth of buried silicon oxide in Si–Si bonded wafers upon annealing
Authors: Himcinschi C. , Milekhin A. , Friedrich M. , Hiller K. , Wiemer M. , Gessner T. , Schulze S. , Zahn D. R. T.
Published: July 2002

Properties of the buried silicon oxide layer in Si–Si bonded wafers upon annealing were studied using Infrared (IR) spectroscopy and high resolution transmission electron microscopy (HRTEM). IR spectra of chemically etched Si–Si bonded wafers allow the thickness of the buried oxide layers to be evaluated. The increasing thickness of the buried oxide layer with annealing temperature is determined via a curve fitting procedure of IR spectra measured in the spectral range of longitudinal optical and transversal optical phonons in silicon oxide. The behavior observed is in very good agreement with that obtained from HRTEM measurements.
A model for the segregation and pileup of boron at the SiO2/Si interface during the formation of ultrashallow p+ junctions
Authors: Shima Akio , Jinbo Tomoko , Natsuaki Nobuyoshi , Ushio Jiro , Oh J.-H. , Ono Kanta , Oshima Masaharu
Published: July 2002

We have quantitatively investigated how boron segregates to regions close to the surface, and what controls this phenomenon, using x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and backside secondary ion mass spectrometry measurement techniques. We found that, contrary to the equilibrium segregation, the pileup of boron is mainly on and within 0.6 nm of the Si side of the interface, and that there is no difference between the kind of encapsulation. This also suggests that the pileup of boron is mainly on the Si side, and implies that the main factor in this segregation is the existence of the Si surface. From the viewpoint of device fabrication, this result seems to be useful in terms of the fabrication of sidewalls. The possibility of boron pileup to occurring in the interstitial state was also shown. Our results suggested a way of looking at dopant profiles by predictive computer modeling.
Influence of secondary electron emission on breakdown voltage in a plasma display panel
Authors: Uhm Han S. , Choi Eun H. , Cho Guang S.
Published: July 2002

A measurement of the secondary electron-emission coefficient γ from MgO film in a plasma display panel is carried out. The influence of the secondary electron emission on the breakdown voltage in microdischarges on the plasma display panel is investigated by making use of the Townsend sparking criterion. Experimental data agree well with theoretical results, verifying the previous theoretical model.
The role of carbon on the electrical properties of polycrystalline Si1−yCy and Si0.82−yGe0.18Cy films
Authors: Anteney I. M. , Parker G. J. , Ashburn P. , Kemhadjian H. A.
Published: July 2002

A comparison is made of the electrical effects of carbon in n- and p-type in situ doped polycrystalline Si1−yCy and Si0.82−yGe0.18Cy layers. Values of resistivity as a function of temperature, effective carrier concentration and Hall mobility are reported. The n-type polycrystalline Si1−yCy and Si0.82−yGe0.18Cy films show dramatic increases in resistivity with carbon content, rising from 0.044 Ω cm to 450 Ω cm (0 and 0.8% C) and 0.01 Ω cm to 2.4 Ω cm (0 and 0.6% C), respectively. In contrast, the increase in B-doped films is much less severe, rising from 0.001 Ω cm to 0.939 Ω cm (0 and 7.9% C) and 0.003 Ω cm to 0.015 Ω cm (0 and 4% C) for the Si1−yCy and Si0.82−yGe0.18Cy layers, respectively. The grain boundary energy barrier, determined from the temperature dependence of the resistivity, is found to vary as the square of the C content in the n-type polycrystalline Si1−yCy and Si0.82−yGe0.18Cy layers, but linearly in the p-type Si1−yCy layers. The square law dependence seen in the n-type layers for C contents up to 0.9% is explained by an increase in the grain boundary trap density due to the presence of carbon, whereas the linear relationship seen in the p-type layers for C contents between 2% and 8% is explained by a shift in the grain boundary trap energy toward the valence band. Finally, lower values of grain boundary energy barrier are obtained in p-type Si0.82−yGe0.18Cy layers with a C content of 4% than in equivalent Si1−yCy layers, which could be explained by a larger shift in trap energy toward the valence band.
Atomistic simulations of extrinsic defects evolution and transient enhanced diffusion in silicon
Authors: Colombeau B. , Cristiano F. , Altibelli A. , Bonafos C. , Assayag G. Ben , Claverie A.
Published: July 2002

In this letter, a physically based model describing the kinetic evolution of extrinsic defects during annealing is presented. The fundamental concepts of Ostwald ripening and formation energy of extrinsic defects are combined in this model, which has been tested against some classical experiments concerning (i) transient enhanced diffusion (TED) of dopants in conjunction with the dissolution of {113} defects and (ii) the “pulsed” TED observed in the case of ultralow energy implants where the surface acts as a strong sink for the silicon interstitial atoms. We show that a full understanding of the formation and the evolution of extended defects leads to a correct prediction of dopant enhanced diffusion in all experimental conditions.
Epitaxial NiO hillocks on truncated octahedral nanoparticles of passivated Ni
Authors: Kwok Y. S. , Zhang X. X. , Qin Boxiong , Fung K. K.
Published: July 2002

Epitaxial NiO hillocks on the {111} and {001} facets of truncated octahedral nanoparticles of Ni have been directly observed by high-resolution transmission electron microscopy. These nanometer hillocks form a rough shell enclosing the Ni nanoparticle. The epitaxial relationships of NiO on nanoparticles of Ni are the same as those of NiO on bulk Ni {111} and {001} surfaces. The formation of hillocks is related to the relaxation of the compressive stress in NiO arising from the very large lattice mismatch between NiO and Ni. The compressively stressed epitaxial NiO shell provides effective protection to the nanoparticles of Ni against further oxidation.
Disordering and anomalous surface erosion of GaN during ion bombardment at elevated temperatures
Authors: Kucheyev S. O. , Williams J. S. , Zou J. , Jagadish C. , Li G.
Published: July 2002

Ion-produced lattice disorder—often an undesirable effect of ion implantation—can be reduced if implantation is carried out at an elevated temperature. We present here a study of the structural characteristics of wurtzite GaN bombarded with Au+197 ions at 550 °C over a wide dose range. Results show that disorder buildup and amorphization are suppressed at elevated temperatures, as compared to implantation at room temperature and below. With increasing ion dose, the evolution of damage proceeds via the formation of point-defect complexes and some planar defects, which are parallel to the basal plane of the GaN film. However, high-dose ion bombardment of GaN at elevated temperatures is complicated by anomalous surface erosion. Such an erosion is attributed to a three-step process of (i) the accumulation of implantation disorder with increasing ion dose, (ii) thermally and ion-beam-induced decomposition of a heavily damaged near-surface layer, and (iii) ion-beam-stimulated erosion of such a highly N-deficient layer.
Interaction of vacancies with interstitial oxygen in silicon
Authors: Casali R. A. , Rücker H. , Methfessel M.
Published: July 2002

Based on first-principle total-energy calculations, we show that the majority of vacancies are trapped by interstitial oxygen in silicon wafers with a typical oxygen concentration of about 1018 cm−3. Vacancies and interstitial oxygen form so called A centers with a binding energy of 1.7±0.4 eV. As a consequence, the density of bound vacancies exceeds the equilibrium density of free vacancies by several orders of magnitude and effective vacancy diffusion coefficients in Si are reduced in comparison to the diffusion coefficient of free vacancies. However, we find that trapping of vacancies alone cannot account for the large discrepancies between previously reported diffusion coefficients for vacancies.
Effects of contact resistance and film thickness on current crowding and the critical product of electromigration in Blech structures
Authors: Yeh Everett C. C. , Tu K. N.
Published: July 2002

In using Blech structures to study electromigration, the current density has been assumed to be the same in similar sets of short strips deposited on an underlayer. But this is not true owing to the existence of contact resistance between the strips and the underlayer. The contact resistivity may change due to different methods of sample preparation. High contact resistivity reduces the maximum current in the strip. Also, it diffuses the currents at the strip/underlayer interface from a narrow crowding to a wide spreading. In this work, we simulate these phenomena of current reduction and spreading in terms of contact resistivity and film thickness. We explore their effects on back stress and incubation time of void formation in short strips. We demonstrate these effects by the discrepancy in the critical product measurements found in experiments using similar Blech structures.
Curie temperature of Nd0.7(Ca, Sr, Ba)0.3MnO3, as determined by electron-spin resonance
Authors: Zhou S. M. , Yoshizawa M. , Lou P. , Jin S. Z. , Zhang Y. H.
Published: July 2002

A paramagnetic–ferromagnetic transition has been observed in detail by electron-spin resonance (ESR) around the Curie temperature in a series of Nd0.7(Ca, Sr, Ba)0.3MnO3 with a fixed A-site mean radius 〈rA〉=1.21 Å. As A-site cation disorder σ2 increases, the ESR linewidth increases and the activation energy obtained from the ESR decreases, suggesting an increase of the local lattice distortions and a weakening of the ferromagnetic correlations with increasing σ2. ESR signals reveal that the A-site cation disorder causes the local microscopic inhomogeneity of the Mn oxidation state, which leads to the broad paramagnetic–ferromagnetic transition.
Electron spin manipulation using semimagnetic resonant tunneling diodes
Authors: Gruber Th. , Keim M. , Fiederling R. , Reuscher G. , Ossau W. , Schmidt G. , Molenkamp L. W. , Waag A.
Published: July 2002

One major challenge for the development of spintronic devices is the control of the spin polarization of an electron current. We propose and demonstrate the use of a BeTe/Zn1−xSe/BeTe double barrier resonant tunneling diode for the injection of a spin-polarized electron current into GaAs and the manipulation of the spin orientation of the injected carriers via an external voltage. A spin polarization of up to 80% can be observed with a semimagnetic layer of only 3.5 nm thickness. By changing the resonance condition via the external voltage, the degree of spin polarization can be varied, though a complete spin switching has not yet been accomplished.
High gain beam compression in new-generation thin-film x-ray waveguides
Authors: Jark W. , Cedola A. , Di Fonzo S. , Fiordelisi M. , Lagomarsino S. , Kovalenko N. V. , Chernov V. A.
Published: July 2002

X-ray waveguides can compress an incident beam for microscopy applications above 8 keV photon energy to sizes smaller than 100 nm in one dimension, a range which is not routinely accessed with other x-ray optics (e.g., Fresnel zone plates). Beryllium, because of its low absorption, is expected to provide the highest intensity gain. Measured gains for a beryllium waveguide of 74 nm thickness exceed values of 100 at 13 and 20 keV photon energy, which is an improvement by an order of magnitude compared to previously reported performances. The same object works also at 8 keV with gain 43.
Ultracompact autocorrelator for femtosecond laser pulses
Authors: Wasylczyk Piotr
Published: July 2002

We present a novel optical and mechanical design of an autocorrelator which allows unmatched compactness. The device, capable of measuring laser pulses in the femtosecond regime, consists of the minimum number of elements, resulting in little distortion in pulse shape. A Mach–Zehnder interferometer provides operation for a wide range of input beam directions, making this autocorrelator a reliable tool for femtosecond lasers diagnostics.
Tailoring sphere density for high pressure physical property measurements on liquids
Authors: Secco R. A. , Tucker R. F. , Balog S. P. , Rutter M. D.
Published: July 2002

We present a new method of tailoring the density of a sphere for use as a probe in high pressure-temperature physical property experiments on liquids. The method consists of a composite sphere made of an inner, high density, metallic, spherical core and an exterior, low density, refractory, spherical shell or mantle. Micromechanical techniques are used to fabricate the composite sphere. We describe a relatively simple mechanical device that can grind hemispherical recesses as small as 200 μm in diameter in sapphire and as small as 500 μm in diameter in ruby hemispheres. Examples of composite spheres made with a Pt or WC core and Al2O3 shell used in metallic liquids pressurized to 16 GPa and 1900 K are shown.
A growth method for creating arrays of atomically flat mesas on silicon
Authors: Lee Doohan , Blakely Jack M. , Schroeder Todd W. , Engstrom J. R.
Published: July 2002

We describe a method for producing arrays of atomically flat mesa structures on Si(111) through the deposition of Si on a lithographically patterned substrate in the temperature range 750–850 °C. The Si source was a supersonic beam of Si2H6. With an appropriate choice of substrate temperature and deposition rate, the atomic steps initially present on each mesa surface move by a step flow process to the trenches, which define the mesa leaving the surface as one extensive exposed atomic plane or terrace. The rate of clearing is believed to be limited by the velocity of step bunches. When the terraces become sufficiently large, nucleation of stacks of monoatomic islands occurs. Comparison is made with a previously developed evaporation method for making step-free surfaces.
An asymptotic expansion for the hypergeometric function F12(a,b;c;x)
Authors: Thorsley Michael D. , Chidichimo Marita C.
Published: July 2002

Watson’s treatise expresses the ordinary Bessel function as a limit of a hypergeometric function, where the first two parameters go to infinity, while simultaneously the argument goes to zero. We have extended Watson’s method of proof to derive an asymptotic expression for the hypergeometric function to second order in the inverse of the first two parameters, with the ordinary Bessel function as its leading-order term. We show, as an example, that the use of this new result is pivotal in showing the correspondence between quantal and classical results of the differential cross section in Coulomb excitation.
Superhardness effects of heterostructure NbN/TaN nanostructured multilayers
Authors: Xu Junhua , Kamiko Masao , Zhou Yaomin , Yamamoto Ryoichi , Li Geyang , Gu Mingyuan
Published: July 2002

Although superhardness effects have been extensively investigated for epitaxial ceramic nanomultilayer films with the same crystal structures in the last decade, those for multilayers with different crystal structures have been seldom studied. In this article, NbN/TaN nanomultilayers have been designed and deposited by reactive magnetron sputtering. The results showed that the crystal structures of NbN and TaN are face-centered cubic and hexagonal in superlattice films, respectively, and the lattice plane (111) of NbN is coherent with the (110) of TaN, i.e., {111}fcc-NbN∥{110}h-TaN. The results of microhardness measurement showed that the superhardness effects of NbN/TaN multilayers exist in a wide range of modulation period from 2.3 to 17.0 nm. This phenomenon is different from that of epitaxial ceramic multilayers where the maximum hardness usually takes place at a modulation period of 5.0–10.0 nm. It is proposed that the coherent stresses and the structural barriers (fcc/hexagonal) to dislocation motion between NbN and TaN layers are the main reasons for the high-hardness value in a wide range of modulation periods.
Dynamics of chaotic magnetic lines: Intermittency and noble internal transport barriers in the tokamap
Authors: Misguich J. H.
Published: July 2002

A purely magnetic approach is used to describe internal transport barriers (ITB’s) observed in tokamaks, generally “around” rational values of the safety factor q, and even in ohmic and positive shear situations. An Hamiltonian twist map “tokamap” is used to describe magnetic line section points in a poloidal (ψ,θ) plane in presence of a magnetic perturbation of strength L, inducing the appearance of magnetic islands and chaotic zones (“incomplete chaos”). For a low value of L<1, before breaking of the boundary circle on the plasma edge, a regular zone with robust magnetic surfaces is found in the plasma core, surrounded by two chaotic regions, separated by a transport barrier across which a magnetic line performs an intermittent motion. By using number theory and convergent rational island chains along Fibonacci series, the ITB is identified as composed of two permeable Cantori (broken Kolmogorov–Arnold–Moser surfaces) with “most noble” q values. No paradox however exists with experimental observations identifying the barriers as being “near” rational q values: this is also the case here for the two Cantori, but, more precisely, on neighboring most noble q values. Dispersion studies for an ensemble of magnetic lines in the large L regime where the boundary circle has been broken, shows an asymptotic radial subdiffusion, with different transient regimes. Comparison is performed with the flux diffusion in the standard map.
Polarization-dependent discharge in fibers of a semiconducting ladder-type polymer
Authors: Kumar N. , Narayan K. S.
Published: July 2002

We report results on polarization-dependent photoinduced discharge in oriented fibers and films of ladder-type, electron-transporting polymer poly(benzimidazobenzophenanthroline). The photocarrier generation efficiency (φ) in the fiber, which is indicated by the rate of discharge, is found to be distinctly higher for light polarized parallel to the fiber axis (P∥) compared to the radially perpendicular direction (P⊥). Similar results, with φ anisotropy ∼10, are obtained for oriented films. These observations are different from previously obtained results on polyparaphenylenevinylene. The results are compared with polarization-dependent steady-state photoconductivity measurements. We interpret these results on the basis of molecular and macroscopic features of the material.
Optical functions of the Drude model: transformation of the spectra over wide ranges of parameters
Authors: Galuza A. I. , Beznosov A. B.
Published: July 2002

The spectral features of the optical functions of the Drude model are investigated over wide ranges of parameters — the plasma frequency ωp of the current carriers, their transport relaxation frequency g, and the dielectric constant ε∞ due to high-energy electronic transitions in the system. The conditions are determined for: a) the square-root frequency dependence of the modulus and phase of the reflection; b) linearity of the phase θ=2ω/ωpε∞+θ0; c) the existence of a plasma reflection edge. Approximate relations are obtained which permit simplifying the analysis of optical reflection and electron characteristic energy loss data both in “good” metals and in materials with a strong temperature dependence of the electrical conductivity (semiconductors, doped and nonstoichiometric oxides, granular metal films, and amorphous alloys). The parameters of the systems of conduction electrons in Al and U2Zn17 are determined.
Control of size and density of InAs/(Al, Ga)As self-organized islands
Authors: Ballet P. , Smathers J. B. , Yang H. , Workman C. L. , Salamo G. J.
Published: July 2002

We report on the influence of the chemical composition of the (Al, Ga)As surface on the formation of strain induced three-dimensional (3D) InAs islands. The experiments have been carried out using a molecular beam epitaxy facility combined with a scanning tunneling microscope enabling in situ surface characterization. The evolution of the density and morphology of these islands is investigated as a function of the Al composition. The InAs deposition, substrate temperature, and annealing time effects on the island formation and morphology are studied. The morphologies of the (Al, Ga)As surface as well as that of the reconstructed InAs “wetting layer” are also described. Results indicate that there are major differences between the InAs/GaAs and the InAs/AlAs systems despite the same lattice mismatch. We observe these differences varying the aluminum content in the starting (Al, Ga)As surface. We show that control of the Al fraction leads to control of the size and density of the 3D islands. The control of island density and size as well as the growth mode of these islands is explained by considering the difference in surface mobility and cation intermixing between these two systems. Our observation is that strain energy is not the only parameter governing the formation of 3D islands but the chemical nature of the different layers involved is proved to significantly affect island properties.
Photodiode properties of epitaxial Pb(Ti, Zr)O3/SrTiO3 ferroelectric heterostructures
Authors: Watanabe Yukio , Okano Motochika
Published: July 2002

A substantial photovoltaic effect is found in heterostructures of typical ferroelectric oxides. Pb(Ti, Zr)O3/Nb-doped SrTiO3, especially, exhibits current–voltage characteristics of the photovoltaic effect of a typical pn junction (p: hole carrier type, n: electron carrier type). A preliminary nonoptimized device shows high performance such as open circuit voltage of 0.7–0.8 V, external conversion efficiency of 0.6%–0.8%, and response time faster than 20 μs for ultraviolet light at room temperature, suggesting the potential of this diode as a new class of photodiode. The results support the formation of a pn like junction by ferroelectric oxides. Additionally, the photovoltaic characteristics are tuned by the application of short pulse voltages and retained.
Anisotropic tunneling magnetoresistance in GaMnAs/AlAs/GaMnAs ferromagnetic semiconductor tunnel junctions
Authors: Higo Y. , Shimizu H. , Tanaka M.
Published: July 2002

We have observed very large tunneling magnetoresistance (TMR) in Ga1−xMnxAs/AlAs/Ga1−xMnxAs ferromagnetic semiconductor tunnel junctions. A TMR ratio as high as 75% was obtained in a junction with a thin (1.5 nm) AlAs tunnel barrier when the magnetic field was applied along the [100] axis in the film plane. The TMR ratio decreased when the applied magnetic field direction was along the [11̄0] and [110]. This anisotropic TMR was found to be explained by the single-domain theory assuming cubic magnetic anisotropy with the easy axis of 〈100〉, which is induced by the zincblende-type Ga1−xMnxAs crystal structure.
Effect of threading dislocations on AlGaN/GaN heterojunction bipolar transistors
Authors: McCarthy L. , Smorchkova I. , Xing H. , Fini P. , Keller S. , Speck J. , DenBaars S. P. , Rodwell M. J. W. , Mishra U. K.
Published: July 2002

We demonstrate an AlGaN/GaN heterojunction bipolar transistor on a substrate grown using the lateral epitaxial overgrowth (LEO) technique. Common emitter characteristics show a current gain of 3. Active layers were grown by plasma-assisted molecular-beam epitaxy on metal–organic chemical-vapor-deposition-grown templates on sapphire. The collector–emitter leakage mechanism in these devices is found to be local punch-through associated with base layer compensation near the dislocations. LEO wing regions (nondislocated) were found to reduce the emitter–collector leakage by four orders of magnitude over adjacent window regions which had a dislocation density of 108 cm−2. Varying the doping profile through the base confirms that the mechanism for leakage is local punch-through due to compensation. This compensation mechanism is consistent with simulations which assume a donor-state line density of 107 cm−1. The implications of the emitter–collector leakage for dc device characterization are also discussed.
Magnetic properties of atomic boron in rare gas matrices: An electron paramagnetic resonance study with ab initio and diatomics-in-molecules molecular dynamics analysis
Authors: Kiljunen Toni , Eloranta Jussi , Ahokas Jussi , Kunttu Henrik
Published: July 2002

The anisotropic boron atom electron paramagnetic resonance spectra measured in rare gas matrices (Ar, Kr, Xe) are interpreted with the aid of highly correlated ab initio calculations including spin–orbit coupling and diatomics-in-molecules (DIM) molecular dynamics simulations. The heavy-element and crystal field effects are inspected as they contribute to the electron g-shift. The DIM-simulated p-orbital splittings and lattice perturbed hyperfine coupling values provide a good starting point for spectral fitting and show the correctness of the guidelines given by purely synthetic generation of the spectra. The present combination of experiment and theory resulted in improved accuracy of the parameters measured in Ar matrix, new values are extracted for Kr matrix, and tentative assignment is also provided for the Xe matrix case.
Electronic states and transitions of tellurium fluoride
Authors: Rai Vidisha , Liebermann Heinz-Peter , Alekseyev Aleksey B. , Buenker Robert J.
Published: July 2002

Ab initio multireference configuration interaction calculations including spin–orbit coupling are carried out for valence electronic states of the TeF molecule and compared to the results for the isovalent IO system obtained earlier at a similar level of theoretical treatment [S. Roszak et al., J. Phys. Chem. A 104, 2999 (2000)]. The calculated spectroscopic constants are in good agreement with available experimental data. It is shown that the X 2Π(σ2π4π*3) ground state is much more strongly bound in TeF (calc. De=25 480 cm−1) due to the greater ionic character of bonding in this system as compared to IO. The lowest excited states of TeF are found to be A 4Σ1/2,3/2− and B 2Σ1/2− which result from the π→σ* electronic excitation. In contrast to IO, the Π2(σ2π3π*4) excited state has a repulsive potential curve and is not expected to be a factor in the low-energy spectroscopy of TeF. Particular emphasis is placed on computation of the transition moments and radiative lifetimes of the TeF electronic states. Most transitions are found to be quite weak, with the strongest of them, B 2Σ1/2−→X1 2Π3/2, characterized by a τ value of 9.5 μs. The two C 2Δ spin components, Ω=5/2 and 3/2, not yet observed experimentally, are predicted to lie ∼1800 cm−1 above B 2Σ1/2− and have partial lifetimes for transitions to the X1 and X2 states which are only slightly longer than those calculated for the B→X1,2 transitions.
Towards metallic magnetic memory: How to interpret experimental results on magnetic switching induced by spin-polarized currents
Authors: Bazaliy Ya. B. , Jones B. A. , Zhang S. C.
Published: July 2002

In the field of magnetic memory the giant magnetoresistance effect provides a convenient way to read out the state of a spin-valve type device by passing a small current through it. It is now debated whether passing a large current through such a device could reliably switch its magnetic state between parallel and antiparallel configurations thus turning a spin valve into a complete read–write memory cell. Experiments with current induced switching were done by several groups, but their interpretation is not straightforward due to complicated interplay between the current-induced torques and those induced by anisotropy. We propose here an experimental setup where an exact solution for the switching behavior can be found in the framework of Slonczweski’s approach. Comparison with experiment would be especially interesting since alternative approaches are discussed in the literature.
Structures of fused-dimer fluids: A new closure based on the potential distribution theorems
Authors: Duda Yurko , Lee Lloyd L. , Kalyuzhnyi Yurij , Chapman Walter G. , Ting P. David
Published: July 2002

We carry out Monte Carlo (MC) simulation as well as solution of the Chandler–Silbey–Ladanyi (CSL) equations with self-consistent closures for symmetric hard dumbbells with bond lengths L=0.1, 0.4, 0.6, 0.8, and 1.0. The consistency conditions include the zero-separation theorems, the pressure consistency (Pv=Pc), and the overlap conditions. Conventional closures, such as the Percus–Yevick closure, give the wrong sign (negative values) for the unbonded bridge function B00(r) for small bond lengths L<0.8; the MC-determined behavior of B00(r) being >0. This explains the failure of the common liquid state closures for chain molecules. The CSL is solved with a set of new closures. The new closure forms are flexible, and are required to satisfy the consistencies imposed. The results are at the same time consistent and highly accurate.