bluuebunny/crossref_metadata_2025 huggingface dataset:
Room-temperature visible luminescence from silicon nanocrystals in silicon implanted SiO2 layers
Authors: Mutti P. , Ghislotti G. , Bertoni S. , Bonoldi L. , Cerofolini G. F. , Meda L. , Grilli E. , Guzzi M.
Published: July 2002

We report the observation of visible-light emission at room temperature from high fluence (0.3–3×1017 cm−2) Si+ implanted thermal SiO2 layers grown on silicon substrates. Significant blue-light emission and an intense broad luminescent band with a peak beyond 750 nm are observed after annealing at high temperature (T≥1000 °C). The red-light emission, present only in the highest fluence implant, is attributed to the luminescence emitted from silicon nanocrystals produced by silicon precipitation. The presence of silicon nanocrystals is confirmed by transmission electron microscopy. Significant blue-light emission is visible after thermal annealing in the 1×1017 cm−2 fluence implant. The peak position shifts from 490 to 540 nm by increasing the annealing cycles temperature.
Electron paramagnetic resonance hyperfine spectrum of the Si E′ defect associated with weakly bonded hydrogen molecules in synthetic silica optical fibers
Authors: Li J. , Kannan S. , Lehman R. L. , Sigel G. H.
Published: July 2002

Electron paramagnetic resonance (EPR) studies of low hydroxyl synthetic silica optical fibers have revealed the presence of a 13 G hyperfine splitting of the Si E′ center which is associated with weakly bonded hydrogen molecules in the silica structure. The fibers were previously exposed to hydrogen at room temperature. During an annealing of the silica fibers at 100 °C in air, the 13 G doublet was observed to decline and disappear while the 74 G doublet and the Si E′ signal were growing. The result correlates with the redistribution of hydrogen molecules and is relevant to the environmental aging that is encountered in fiber optical waveguides under certain conditions.
Separation of fine particles using rotating tube with alternate flow
Authors: Aoki Ichiro , Shirane Kotaro , Tokimoto Takayuki , Nakagawa Koichi
Published: July 2002

A new method for selective separation of fine particles was developed by using a rotating tube with alternate flow (RAF). The separation is performed in the condition of negligible small of the centrifugal force of a particle compared to gravity. In the horizontal tube with stable flow, each particle moves along the flow drawing a helical path. The locus of each particle depends on a parameter Aω/V0, where A is a radius of the tube, ω is its angular velocity, and V0 is the sedimentation velocity in a liquid at fixed temperature. The velocity along the flow is determined by the locus and the distribution of the flow speed in the tube. The ratio V/Umax (where V is the particle velocity and Umax is the flow speed at the tube center) was obtained theoretically and experimentally. The value took the maximum at Aω/V0≂2 and decreased near exponentially within the range of Aω/V0≳2.5 and precipitously decreased within Aω/V0≲2. By using an alternate flow method with suitable rotation for each directional flow, large particles were taken out successively in descending order of the size from one end and small ones in ascending order from the other end.
Noise measurements on silicon J-FETs at low temperature using a very high Q superconducting resonator
Authors: Ayela F. , Bret J. L. , Chaussy J.
Published: July 2002

Noise voltage and noise current measurements have been carried out at different temperatures on two kinds of low-noise silicon J-FET at moderately high frequencies up to 100 kHz. We have made careful noise current measurements by constructing a very low-loss superconducting resonator working in the kHz frequency range, whose quality factor reaches 3×105. At 10 kHz, the measured noise energy lies between 1.1 and 1.8×10−24 W/Hz for both types of transistors, but the ratio between the noise voltage and the noise current exhibits pronounced differences depending on the device under test.
Nanometer scale lithography at high scanning speeds with the atomic force microscope using spin on glass
Authors: Park S. W. , Soh H. T. , Quate C. F. , Park S.-I.
Published: July 2002

We have identified a resist material that is suitable for high-speed, nanometer-scale scanning probe lithography (SPL) using the atomic force microscope (AFM). The material is siloxene, commonly known as spin on glass (SOG). The SOG film is deposited on a silicon sample and exposed with a voltage applied between the AFM tip (negative) and the silicon substrate (positive). Voltages of 70 V and currents of 1 nA are typical. It is a positive resist where the etch selectivity between the exposed and unexposed areas is greater than 20. We have recorded line widths as narrow as 40 nm. The writing speed is greater than 1 mm/s, which we believe to be an important attribute in future systems for SPL.
Passive mode locking of a Nd3+:SrLaGa3O7 laser
Authors: Terentiev A. V. , Prokoshin P. V. , Yumashev K. V. , Mikhailov V. P. , Ryba-Romanowski W. , Golab S. , Pisarski W.
Published: July 2002

A Nd3+:SrLaGa3O7 laser with a pentacarbocyanine dye cell produced regular 25–30 ps pulses over the period of the flash lamp. Output energy as a function of mirror reflectivity and lamp energy was also determined.
Micro-fabricated piezoelectric cantilever for atomic force microscopy
Authors: Watanabe Shunji , Fujii Toru
Published: July 2002

We successfully developed an atomic force microscope (AFM) with a batch-fabricated silicon cantilever with a pyramidal stylus. The high quality lead zirconate titanate (PZT) piezoelectric thin film allows simultaneously displacement sensing and actuating. The PZT thin film with a dielectric constant of 1000, a remanent polarization of 30 μC/cm2, and a piezoelectric constant of −100 pC/N, which are as high as those of bulk ceramics PZT, has been formed by sputtering at 400 °C and subsequent annealing at 650 °C. A Si(111) mono-atomic step whose height is 4 Å has been observed clearly in an AFM cyclic contact mode by a PZT-AFM lever as a displacement sensor. A PZT-AFM lever has been used as an actuator for z feedback positioning for AFM imaging of a compact disk pit.
Atomic force microscopy for high speed imaging using cantilevers with an integrated actuator and sensor
Authors: Manalis S. R. , Minne S. C. , Quate C. F.
Published: July 2002

A cantilever with an integrated ZnO piezoelectric actuator in feedback with a piezoresistive sensor is utilized in an atomic force microscope (AFM) to achieve a new high speed imaging technique. The imaging bandwidth is increased from 0.6 to 6 kHz by bending the cantilever over sample topography with the actuator rather than moving the sample with a 2 in. piezotube. Images taken in the constant force mode with a 3 mm/s tip velocity of a sample containing 2 μm vertical steps are presented. The effects of electrical coupling from the actuator were eliminated by measuring the piezoresistor sensor with a lock-in amplifier.
Nanocomposite resist system
Authors: Ishii Tetsuyoshi , Nozawa Hiroshi , Tamamura Toshiaki
Published: July 2002

We propose a nanocomposite resist system that incorporates sub-nm carbon particles into a resist film to enable an ultrathin film resist process for nanometer pattern fabrication. Fullerene (C60) is found to be an excellent material for incorporation in view of its etching resistance, dissolution inhibiting effect, molecular size, and composite preparation. A nanocomposite system of C60 and an electron-beam positive resist, ZEP520, show enhancements in both pattern contrast and etching resistance and provide 50 nm patterns in a 50-nm-thick film with a sensitivity of ∼50 μC/cm2. Furthermore, a C60-incorporated chemically amplified resist, SAL601, shows strong environmental stabilization in postexposure delay (<10% after five days) presumably due to the reduction of free volume in the closely packed nanocomposite film.
Parallel fabrication and single-electron charging of devices based on ordered, two-dimensional phases of organically functionalized metal nanocrystals
Authors: Markovich Gil , Leff Daniel V. , Chung Sung-Wook , Soyez Hermes M. , Dunn Bruce , Heath James R.
Published: July 2002

A parallel technique for fabricating single-electron, solid-state capacitance devices from ordered, two-dimensional closest-packed phases of organically functionalized metal nanocrystals is presented. The nanocrystal phases were prepared as Langmuir monolayers and subsequently transferred onto Al-electrode patterned glass substrates for device construction. Alternating current impedance measurements were carried out to probe the single-electron charging characteristics of the devices under both ambient and 77 K conditions. Evidence of a Coulomb blockade and step structure reminiscent of a Coulomb staircase is presented.
Low pressure organic vapor phase deposition of small molecular weight organic light emitting device structures
Authors: Baldo M. A. , Kozlov V. G. , Burrows P. E. , Forrest S. R. , Ban V. S. , Koene B. , Thompson M. E.
Published: July 2002

A new technique for the deposition of amorphous organic thin films, low pressure organic vapor phase deposition (LP-OVPD), was used to fabricate organic light emitting devices (OLEDs) consisting of a film of aluminum tris-(8 hydroxyquinoline) (Alq3) grown on the surface of a film of N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′biphenyl-4-4′diamine. The resulting heterojunction OLED was found to have a performance similar to conventional, small molecular weight OLEDs grown using thermal evaporation in vacuum. The LP-OVPD grown device has an external quantum efficiency of 0.40±0.05% and a turn-on voltage of approximately 6 V. The rapid throughput demonstrated with LP-OVPD has the potential to facilitate low cost mass production of conventional small molecule based OLEDs, and its use of low vacuum in a horizontal reactor lends itself to roll-to-roll deposition of organic films for many photonic device applications.
Automated parallel high-speed atomic force microscopy
Authors: Minne S. C. , Yaralioglu G. , Manalis S. R. , Adams J. D. , Zesch J. , Atalar A. , Quate C. F.
Published: July 2002

An expandable system has been developed to operate multiple probes for the atomic force microscope in parallel at high speeds. The combined improvements from parallelism and enhanced tip speed in this system represent an increase in throughput by over two orders of magnitude. A modular cantilever design has been replicated to produce an array of 50 cantilevers with a 200 μm pitch. This design contains a dedicated integrated sensor and integrated actuator where the cells can be repeated indefinitely. Electrical shielding within the array virtually eliminates coupling between the actuators and sensors. The reduced coupling simplifies the control electronics, facilitating the design of a computer system to automate the parallel high-speed arrays. This automated system has been applied to four cantilevers within the array of 50 cantilevers, with a 20 kHz bandwidth and a noise level of less than 50 Å. For typical samples, this bandwidth allows us to scan the probes at 4 mm/s.
Fabrication and optical characterization of Pb(Mg1/3Nb2/3)O3-PbTiO3 planar thin film optical waveguides
Authors: Lu Yalin , Jin G.-H. , Cronin-Golomb M. , Liu S.-W. , Jiang H. , Wang F.-L. , Zhao J. , Wang S.-Q. , Drehman A. J.
Published: July 2002

Highly (110)-oriented and (100) nearly epitaxial 0.7PMN-0.3PT thin films were deposited on (1012) sapphire and (100) LaAlO3 substrates, respectively, using a dip-coating method. Optical waveguide characterization and electro-optic effect measurements of the film on sapphire substrate were demonstrated. Low propagation loss of 4.1 dB/cm and high quadratic electro-optic coefficient of 0.75×10−16 (m/V)2 were obtained at wavelength of 632.8 nm. Epitaxial PMN-PT thin films will be suitable for integrated optic devices.
Hole limited recombination in polymer light-emitting diodes
Authors: Scott J. C. , Malliaras G. G. , Chen W. D. , Breach J.-C. , Salem J. R. , Brock P. J. , Sachs S. B. , Chidsey C. E. D.
Published: July 2002

By comparing the quantum efficiencies of light emission in a series of poly[2-methoxy-5(2′ethyl)hexoxy-phenylenevinylene] diodes with calcium cathodes and various anode metals, we show that, in all cases electrons are the majority carrier and recombination is limited by hole injection. These conclusions are confirmed by the examination of a second series of samples in which alkanethiol barrier layers of varying thickness, are deposited on a gold anode. The highest external quantum efficiency was achieved in these experiments using a clean, semitransparent gold anode. We suggest that electron and hole injection rates play the primary role in determining current balance and that mobilities play a minor role.
Voltage modulation scanned probe oxidation
Authors: Pérez-Murano Francesc , Birkelund Karen , Morimoto Kiyoshi , Dagata John A.
Published: July 2002

Scanned probe microscope (SPM) oxidation with voltage modulation leads to a significant enhancement of the oxide growth rate, improvement of the aspect ratio of oxide features, and control of the structural and electrical properties of the SPM oxide. Variation of the voltage-pulse parameters confirms that the oxide dimensions can be controlled sensitively over a wide range of pulse parameters and that voltage modulation overcomes the self-limiting character of SPM oxidation by reducing the buildup of space charge within the oxide during growth. The enhancement can be used to increase the writing speed or lower the voltage, both beneficial for practical nanoelectronics fabrication.
High speed patterning of a metal silicide using scanned probe lithography
Authors: Snow E. S. , Campbell P. M. , Perkins F. K.
Published: July 2002

We describe a simple high-speed process for patterning metal silicides with an atomic force microscope (AFM). The process uses a thin AFM-generated oxide to block Pt diffusion during the formation of Pt silicide. Because the process requires only ∼1 nm of oxide, high write speeds and fine lateral resolution are achieved. We find that by maintaining ambient moisture at the tip–sample interface we can under optimal tip conditions achieve a minimum exposure time of ∼300 ns for a 30 nm size pixel which corresponds to a maximum write speed of ∼10 cm/s.
Peculiar noise properties of phonons generated by femtosecond laser pulses in antimony
Authors: Misochko O. V. , Kisoda K. , Sakai K. , Nakashima S.
Published: July 2002

In femtosecond pump-probe experiments on antimony, we have detected coherent oscillations that exhibit phase-dependent noise. The Fourier transforms of the coherent amplitude and its variance show that the two fluctuate at different frequencies, suggesting that the phonons created in the pump-probe experiment are of a squeezed nature.
Selective growth of high quality GaN on Si(111) substrates
Authors: Seon M. , Prokofyeva T. , Holtz M. , Nikishin S. A. , Faleev N. N. , Temkin H.
Published: July 2002

We demonstrate selective growth of high-quality GaN by gas-source molecular beam epitaxy on Si(111) wafers patterned with SiO2. GaN was grown on wafers having two different buffer layers. The first buffer layer contains two AlGaN/GaN superlattices, separated by GaN spacer, grown on AlN, with a total thickness of 400 nm. The second is a thin AlN (1.5 nm) buffer layer. X-ray diffraction confirms (0001) growth orientation, smooth interfaces, and coherence lengths comparable to the layer thickness in both samples. In the case of the thin AlN buffer layer, the tensile stress measured by the E2 Raman line shift is attributed to the mismatch in the thermal expansion coefficients of GaN and Si. However, when the AlGaN/GaN superlattice buffer layer is grown first, a reduced stress is measured. High carrier concentrations (≈1018 cm−3) are seen in the GaN grown on the thin AlN buffer layer, which we attribute to the incorporation of silicon from the substrate during the growth process. The superlattice buffer layer is seen to inhibit this diffusion, resulting in a carrier concentration of <1017 cm−3 in the GaN.
Behavior of grain boundary resistivity in metals predicted by a two-dimensional model
Authors: Dannenberg Rand , King Alexander H.
Published: July 2002

The behavior of a model for the specific grain boundary resistivity in metallic bamboo conductor lines is developed and compared to other theoretical treatments, and to experiment. The grain boundary is modeled as an array of scatterers on a plane. The scatterers are called “vacancy-ion” complexes, in which the vacancy represents the boundary free volume, and the ion is an atom adjacent to the vacancy. Three cases are investigated, that of noninterfering scatterers, a continuum of interfering scatterers, and discrete interfering scatterers. The approximations used lead to a specific grain boundary resistivity ∼10−16 Ω m2 for aluminum, in agreement with experiment, for the first two cases. In the noninterfering case, the specific resistivity is independent of the grain boundary area. For the continuum interfering case it is found that the grain boundary resistivity is only weakly dependent on the grain boundary area, and that the grain boundary has a high probability of perfect reflection or transmission of incident electrons. The source of resistivity is from reflection of electrons. This behavior is independent of the exact interaction potential between the incident electrons and the defects which comprise the grain boundary free volume. The discrete interfering case produces specific resistivities several of orders of magnitude too large, and a strong dependence on the grain boundary area. A connection is established between the grain boundary resistivity and the electromigration wind force.
Optical strain characteristics of holographically formed polymer-dispersed liquid crystal films
Authors: Cairns Darran R. , Bowley Christopher C. , Danworaphong Sorasak , Fontecchio Adam K. , Crawford Gregory P. , Li Le , Faris Sadag M.
Published: July 2002

The optical strain characteristics of a reflective holographic-polymer dispersed liquid crystal are investigated. A spectral blueshift and induced polarization dependence are observed to occur with increasing strain. The spectral dependence on strain is explained by the Poisson contraction of the material and the polarization dependence is largely attributed to elongation of liquid crystal droplets. The material has potential use as an optical strain gauge.
Nucleation and growth in cluster dynamics: A quantitative test of the classical kinetic approach
Authors: Gránásy László , James Peter F.
Published: July 2002

Nucleation and size dependent growth of nanometer sized crystalline particles in glassy media have been studied by numerically solving the Turnbull–Fisher master equations that describe the time evolution of cluster population. Time dependencies of the formation rate and number density are determined for large clusters (built of up to 2×105 formula units, containing 1.8×106 atoms). We demonstrate that the formation rate and number density of such clusters are well approximated by Shneidman’s asymptotically exact analytical solution. A quantitative test of the kinetic Turnbull–Fisher model has been performed: Evaluating the kinetic coefficients and interfacial parameters from the transient time and steady-state nucleation rates measured on six stoichiometric oxide glass compositions (lithium–disilicate, barium–disilicate, lithium–diborate, wollastonite, 1:2:3 and 2:1:3 soda–lime–silica glass compositions), we calculated the macroscopic growth rates and compared with experiments. For wollastonite, lithium–diborate and the 1:2:3 soda–lime–silica glass, differences of 2 to 4 orders of magnitude have been observed between theory and experiment. This inadequacy of the microscopic kinetic parameters in describing macroscopic growth cannot be explained by either the curvature effect on the interfacial free energy or the self-consistency correction for the cluster free energy. The origin of the discrepancy is discussed.
Dependence of ion implantation: Induced defects on substrate doping
Authors: Kanemoto Kei , Imaizumi Fuminobu , Hamada Tatsufumi , Tamai Yukio , Nakada Akira , Ohmi Tadahiro
Published: July 2002

The characteristics of an ion implantation-induced defect in a silicon substrate are investigated. This defect is considered to be a complex of a point defect and a substrate dopant atom. The experiments are conducted by focusing on the dependence of the substrate dopant species (phosphorus and boron) on defect formation. The characteristics of the defect are investigated by measuring the bulk generation lifetime (τg) of metal–oxide–semiconductor capacitors, in which Si+ has been implanted to form the dopant-related defects in the substrate (damaging implantation) after gate oxide formation. As a result, it is found that the τg of the boron-doped substrate is one to two orders of magnitude smaller than that of the phosphorus-doped substrate for the same Nsub under the same implantation conditions. The temperature dependence of τg shows that the energy level of the defect is located at the intrinsic Fermi level, independent of the substrate dopant species and the concentration. By measuring the dependence of τg on the temperature of postdamaging implantation annealing, it is shown that the defects vanish at about 800 °C for both types of substrate. Also, it is found that the amount of dopant-related defects depends on the implantation ion species. BF2+ implantation forms more defects than As+ implantation.
A design of mammography units using a quasimonochromatic x-ray source
Authors: Piestrup M. A. , Wu Xizeng , Kaplan V. V. , Uglov S. R. , Cremer J. T. , Rule D. W. , Fiorito R. B.
Published: July 2002

In this article we present a mammography unit design using a parametric x-radiation (PXR) source. We show that PXR can provide a fanned quasimonochromatic x-ray beam that can be used to obtain mammography images of higher contrast and lower dose than those obtained from a conventional x-ray system. Changing the Bragg angle of the PXR crystal with respect to the electron beam changes the photon energy, improves image quality, and minimizes dose. Monte Carlo computer simulations are given that show that the PXR source with a 5% bandwidth gives a figure of merit close to that of the ideal monoenergetic source and significantly higher than that of the filtered-x-ray-tube sources. In order to simultaneously obtain adequate flux and achieve bandwidths below 5%, we utilized an electron-beam energy of 35 MeV and an average current of 300 μA to 1 mA for 3 s (depending upon breast thickness and density). Slits after the PX radiator are used to define both the spatial distribution and the spectral bandwidth of the x-ray beam, which is scanned over the breast in approximately 3 s. A graphite crystal C (002) in the Laue geometry is utilized as the PX radiator. Lower electron-beam currents might be possible as higher efficiency PX or hybrid radiators become available.
The structure of a weakly bound ionic trimer: Calculations for the He24H− complex
Authors: Gianturco F. A. , Paesani F. , Baccarelli I. , Delgado-Barrio G. , Gonzalez-Lezana T. , Miret-Artés S. , Villarreal P. , Bendazzoli G. B. , Evangelisti S.
Published: July 2002

The weakly bound diatomic systems He24 and HeH−4 have been found able to support only one bound state for J=0, although the latter also supports an additional bound state for J=1. In the present paper we, therefore, study the structure of the bound states which might exist for the weakly bound triatomic He24H−, in its J=0 state, by describing the full potential as a simple addition of two-body (2B) interactions. We carry out bound state calculations using both Jacobi coordinates within a discrete variable representation (DVR) and pair coordinates with a distributed Gaussian function (DGF) expansion. The system is shown to possess two bound states with respect to its lower dissociation threshold and two further “ghost” states before the complete break-up threshold. The spatial structures of such states and of the floppy complex are analyzed in detail, as is the possibility of detecting Efimov-type states in such a weakly bound aggregate. Finally, the inclusion of three-body (3B) forces in the description of the full interaction and its effect on the number of possible bound states is also discussed.
Fabrication of <5 nm width lines in poly(methylmethacrylate) resist using a water:isopropyl alcohol developer and ultrasonically-assisted development
Authors: Yasin Shazia , Hasko D. G. , Ahmed H.
Published: July 2002

We report on the fabrication of 3–4 nm wide continuous lines in a positive tone electron beam resist poly(methylmethacrylate) on a solid substrate. This narrow linewidth was made possible through the use of a nonsolvent-based developer system, water:isopropyl alcohol, together with ultrasonically-assisted development, which reduced the effective development time thus limiting the swelling of the unexposed resist. This combination of solvent system and development technique results in a smaller radius of gyration in the developing polymer molecules and in a wider exposure dose latitude compared to conventional processing and so allows ultrasmall features to be reproduced.
On the stability of stationary shock waves in nozzle flows with homogeneous condensation
Authors: Delale C. F. , Lamanna G. , van Dongen M. E. H.
Published: July 2002

The stability limit of stationary normal shock waves in supercritical nozzle flows with homogeneous condensation is investigated by the singularity theory of the quasi-one-dimensional steady-state differential equations of motion. In this case it is shown that a catastrophic change in the phase portraits of the flow variables, where the spiral point turns into a nodal point, occurs when the initial relative humidity exceeds a critical value, resulting in the alteration of the quasi-one-dimensional steady-state condensation structures. In particular, a criterion for the limit of stability, based on the break-up of structural stability of the steady equations of motion in the quasi-one-dimensional approximation, is established by variational analysis and a correlation for the critical initial relative humidity is derived for fixed nozzle geometry keeping appropriate reservoir conditions fixed in the same approximation. A comparison of the values of the critical initial relative humidity, calculated by this correlation, shows excellent agreement with those of experiments and/or numerical simulations for moist air expansions in various slender nozzles under different reservoir conditions.
Attractive-mode force microscope for investigations of biomolecules under ambient conditions
Authors: King G. M. , Nunes G.
Published: July 2002

We report on the design and performance of a quartz tuning-fork-based dynamic mode atomic force microscope for the imaging of biological samples under ambient conditions. The instrument uses a stiff cantilever that maintains stable oscillations at low amplitudes even in the presence of capillary and other long range forces that can overwhelm more flexible cantilevers. The performance of the microscope is demonstrated with high resolution measurements of double-stranded DNA on cleaved mica substrates using Si and Si3N4 tips.
On the biparametric quantum deformation of GL(2)⊗GL(1)
Authors: Parashar Deepak
Published: July 2002

We study the biparametric quantum deformation of GL(2)⊗GL(1) and exhibit its cross-product structure. We derive explicitly the associated dual algebra, i.e., the quantized universal enveloping algebra employing the R-matrix procedure. This facilitates construction of a bicovariant differential calculus which is also shown to have a cross-product structure. Finally, a Jordanian analog of the deformation is presented as a cross-product algebra.
Quantum-classical dynamics including continuum states using quantum trajectories
Authors: Gindensperger E. , Meier C. , Beswick J. A.
Published: July 2002

We apply the MQCB (mixed quantum/classical mechanics based on Bohmian trajectories) [E. Gindensperger, C. Meier, and J. A. Beswick, J. Chem. Phys. 113, 9369 (2000)] to the case where the quantum degree of freedom contains both bound and continuum states. The quantum degree of freedom is treated by a wave packet that is propagated on a spatial grid with a Hamiltonian that depends parametrically on the classical degrees of freedom, while the classical degrees of freedom themselves are coupled to the wave function via quantum trajectories. The method is applied to a simple model of a light particle colliding with a heavy (classical) particle absorbed on a surface, which has been used by other authors to develop methods to combine quantum degrees of freedom including continuum states with classical mechanics. The results are compared to the Ehrenfest mean-field method as well as to full quantum results.
Improvement of critical current density in MgB2 superconductors by Zr doping at ambient pressure
Authors: Feng Y. , Zhao Y. , Sun Y. P. , Liu F. C. , Fu B. Q. , Zhou L. , Cheng C. H. , Koshizuka N. , Murakami M.
Published: July 2002

We present the superconducting properties and phase compositions of Mg1−xZrxB2 bulk samples fabricated by a solid-state reaction at ambient pressure. It is found that a small amount of Zr atoms may be introduced into the lattice of MgB2, while the majority of them forms ZrB2 phase. The Mg0.9Zr0.1B2 sample shows the highest JC of 2.1×106 A/cm2 in 0.56 T at 5 K and 1.83×106 A/cm2 in self-field at 20 K, higher irreversibility field and larger upper critical field in MgB2 bulk samples. The combination of good grain connection, the reduction of grain size and small ZrB2 particles in the sample may be responsible for the significant enhancement of JC in Zr-doped samples. This technique has a great potential to prepare high performance MgB2 bulk samples and wires on an industrial scale.
Outer- and inner-valence ionization spectra of NH3, PH3, and AsH3: symmetry-adapted cluster configuration interaction general-R study
Authors: Ishida Mayumi , Ehara Masahiro , Nakatsuji Hiroshi
Published: July 2002

Outer- and inner-valence ionization spectra of group V hydrides, NH3, PH3, and AsH3 were studied by the symmetry-adapted-cluster configuration-interaction (SAC-CI) general-R method. Fine details of the experimental spectra of these hydrides were reproduced and the quantitative assignments of the peaks were proposed. The inner-valence satellites were classified into those including the valence or Rydberg excitations. For NH3, we interpreted the spectrum using the relative intensity and proposed some unresolved bands. For PH3, bands 2 and 3, for which different assignments have been proposed, were attributed to the A12 inner-valence satellites. A detailed inner-valence satellite spectrum of AsH3 is theoretically proposed.
Ferroelectric phase transitions and dynamical behavior in KNbO3/KTaO3 superlattices by molecular-dynamics simulation
Authors: Sepliarsky M. , Phillpot S. R. , Stachiotti M. G. , Migoni R. L.
Published: July 2002

The phase transitions and dynamical behavior of superlattices consisting of equal-thickness layers of a perovskite ferroelectric (KNbO3) and a perovskite paraelectric (KTaO3) are explored using molecular-dynamics simulation. We find that the response in the plane and in the modulation direction are essentially decoupled. The Curie temperature for the transition from a polarized to unpolarized state in the modulation direction decreases approximately linearly with modulation length, Λ, for Λ>12; for smaller modulation lengths, it is essentially constant. The Curie temperature in the plane appears to be only weakly modulation-length dependent. We relate our results to experimental findings on the same system.
Annihilation-diffusion processes: An exactly solvable model
Authors: Roshani Farinaz , Khorrami Mohammad
Published: July 2002

A family of diffusion-annihilation processes is introduced, which is exactly solvable. This family contains parameters that control the diffusion and annihilation rates. The solution is based on the Bethe ansatz and using special boundary conditions to represent the reaction. The processes are investigated, both on the lattice and on the continuum. Special cases of this family of processes are the simple exclusion process and the drop-push model.
Improved superconducting properties in nanocrystalline bulk MgB2
Authors: Gümbel A. , Eckert J. , Fuchs G. , Nenkov K. , Müller K.-H. , Schultz L.
Published: July 2002

High density nanocrystalline MgB2 bulk superconductors with distinctly improved pinning were prepared by mechanical alloying of Mg and B powders at ambient temperatures followed by hot pressing. The nanocrystalline samples reveal high jc=105 A/cm2 at 20 K and 1 T together with an irreversibility line strongly shifted towards higher fields resulting in Hirr(T)∼0.8 Hc2(T), whereas typically Hirr(T)∼0.5 Hc2(T) is observed for untextured bulk samples. These values exceed those of all other reported bulk samples and are in the range of values for thin films. The improved pinning of this material, which mainly consists of spherical grains about 40–100 nm in size, is attributed to the large number of grain boundaries in the nanocrystalline state.
Quantum-classical description of rotational diffractive scattering using Bohmian trajectories: Comparison with full quantum wave packet results
Authors: Gindensperger E. , Meier C. , Beswick J. A. , Heitz M-C.
Published: July 2002

We apply the mixed quantum/classical method based on the Bohmian formulation of quantum mechanics [E. Gindensperger, C. Meier, and J. A. Beswick, J. Chem. Phys. 113, 9369 (2000)] to the case of rotational diffractive surface scattering of a diatomic molecule. The rotation as well as the normal translational degree of freedom are treated classically while the two parallel degrees of freedom that account for the diffraction are treated quantum mechanically. The effects of treating some degrees of freedom classically are discussed in detail by comparing our novel approximate method to quantum wave packet results obtained by the multiconfiguration time-dependent Hartree method.
Ab initio ground state potential energy surfaces for Rg–Br2 (Rg=He, Ne, Ar) complexes
Authors: Prosmiti Rita , Cunha Carlos , Villarreal Pablo , Delgado-Barrio Gerardo
Published: July 2002

High-level ab initio molecular electronic structure calculations are performed for Rg–Br2 (Rg=He, Ne, Ar) complexes at CCSD(T) (coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations) level of theory. Specific augmented correlation consistent basis sets are used for each noble atom (Rg), supplemented with an additional set of bond functions. Effective-core potentials (ECPs), augmented with diffusion (sp) and polarization (3df ) functions, have been employed for the bromine atoms. For all complexes, the CCSD(T) potential energy surfaces (PESs) show double-minimum topology, with wells at both linear and T-shaped configurations; the linear minimum is found to be deeper than the T-shaped one. Vibrational corrections are taken into account for all the complexes and their effects in the stability of the linear and T-shaped conformers are examined. For each complex and each configuration (linear and T-shaped), Re equilibrium intermolecular distances, De and D0 dissociation energies, are evaluated and compared with previous theoretical and/or experimental results.
Jets
Authors: Rhines Peter B.
Published: July 2002

This is a discussion of concentrated large-scale flows in planetary atmospheres and oceans, argued from the viewpoint of basic geophysical fluid dynamics. We give several elementary examples in which these flows form jets on rotating spheres. Jet formation occurs under a variety of circumstances: when flows driven by external stress have a rigid boundary which can balance the Coriolis force, and at which further concentration can be caused by the β effect; when there are singular lines like the line of vanishing windstress or windstress-curl, or the Equator; when compact sources of momentum, heat or mass radiate jet-like β plumes along latitude circles; when random external stirring of the fluid becomes organized by the β effect into jets; when internal instability of the mass field generates zonal flow which then is concentrated into jets; when bottom topographic obstacles radiate jets, and when frontogenesis leads to shallow jet formation. Essential to the process of jet formation in stratified fluids is the baroclinic life cycle described in geostrophic turbulence studies; there, conversion from potential to kinetic energy generates eddy motions, and these convert to quasibarotropic motions which then radiate and induce jet-like large-scale circulation. Ideas of potential vorticity stirring by eddies generalize the notion of Rossby-wave radiation, showing how jets embedded in an ambient potential vorticity gradient (typically due to the spherical geometry of the rotating planet) gain eastward momentum while promoting broader, weaker westward circulation. Homogenization of potential vorticity is an important limit point, which many geophysical circulations achieve. This well-mixed state is found in subdomains of the terrestrial midlatitude oceans, the high-latitude circumpolar ocean, and episodically in the middle atmosphere. Homogenization expels potential vorticity gradients vertically to the top and bottom of the fluid, and sideways to the edges of flow domains or gyres; in both these ways is jet formation enhanced.
Complex fluid-dynamical phenomena modeled by large-scale molecular-dynamics simulations
Authors: Alda Witold , Dzwinel Witold , Kitowski Jacek , Mościński Jacek , Pogoda Marek , Yuen David A.
Published: July 2002

We carried out large-scale molecular-dynamics simulations of the classical Rayleigh–Taylor (RT) phenomenon in a Lennard-Jones molecular liquid. We have observed from these simulations, involving 106–107 particles, the development of hydrodynamic instabilities from two different kinds of interacting particles. A free surface is introduced by deploying an overlying void. For a box with a dimension up to about 1 μm and two layers having different particle sizes, the fingering type of instability is observed as a result of oscillations caused by the gravitational field. In this gridless scheme, surface waves can be captured self-consistently. For equally sized particles, a spontaneous “fluctuation driven” mixing with a long start-up time is observed. These molecular- dynamics results suggest the possibilities of upscaling the RT phenomenon. For conducting these numerical experiments, which require at least ∼105 time steps, a single simulation would require 100–200 Tflops of massively parallel computer power. © 1998 American Institute of Physics.
Oriented growth of ultrathin tungsten films on sapphire substrates
Authors: Souk J. H. , Segmüller Armin , Angilello J.
Published: July 2002

Thin layers of tungsten∼30 and 300 Å thick were grown by electron-beam evaporation on (11̄02) sapphire substrates in a vacuum better than 10−8 Torr at a substrate temperature of 450 °C. After deposition, the films were characterized by x-ray diffraction in three ways: conventional Bragg diffraction, small-angle interferences, and grazing-incidence x-ray diffraction. In spite of the large lattice mismatch (∼10.5%) and the low deposition temperature, a well-oriented deposit with the orientation (001)W tilted ∼5° from (11̄02)Al2O3 towards (0001), and [110]W∥[112̄0]Al2O3 was obtained. Films with thicknesses ≳30 Å showed excellent planeness and smoothness. Films less than 30 Å thick are not continuous but consist of separate, oriented islands. In the films ≲30 Å thick, a small tensile strain of ≲0.6% was observed parallel to the interface. No strain was observed in the ∼300-Å-thick film.
Interactions between alumina and high lead glasses for hybrid components
Authors: Prudenziati M. , Morten B. , Cilloni F. , Ruffi G. , Sacchi M.
Published: July 2002

Kinetics of dissolution of alumina substrates in seven compositions of high lead glasses have been studied. The dissolution process is controlled by correlated diffusion of Al and Pb (and/or B), with interdiffusion coefficients D, at 850 °C, 10 min, in the range from 10−10 to 10−6 cm2 s−1, according to the different compositions. Al solubility ranges from about 10% to 20%. The diffusivity and solubility increase by increasing the content of B and Pb. The reactivity of alumina substrates with the glassy component of thick-film resistors greatly affects their microstructure and electrical properties. Some consequences of these interactions have been evidenced: change in electrical resistivity and its dependence on the resistor thickness, changes in the sintering process and grain growth of RuO2 grains, and a catalytic effect on phase transformations of pyrochlore ruthenates in rutile RuO2 in the glassy matrix of resistors. Moreover, it is shown that Al dissolution gives rise to a recession of the buried glass/substrate interface, which can easily be measured. When alumina is concerned, this measurement gives insight of the sum of two processes: aluminum dissolution in the glass layer and grain-boundary diffusion of glass into the substrate itself.
Perpendicular magnetic anisotropy and magnetostriction of sputtered Co/Pd and Co/Pt multilayered films
Authors: Hashimoto S. , Ochiai Y. , Aso K.
Published: July 2002

Magnetic properties of Co/Pd and Co/Pt multilayers were studied as a function of sputtering gas pressure. It was found that the magnetic properties of the films depended upon the sputtering gas pressure, and large coercivity (several kOe) and a perfect squareness of the perpendicular hysteresis loop were attained by the deposition at the high gas pressure. These multilayers are suitable for perpendicular magnetic recording media. In the Co/Pd, the origin of the perpendicular magnetic anisotropy is mainly an interfacial anisotropy, although there exists a stress-induced anisotropy for the films with the relatively short periodicity. The perpendicular magnetic anisotropy of the Co/Pt can be concerned with the Co-Pt mixture formed at the interface of the multilayer structure. The magnetostriction of the Co/Pd films with the relatively short periodicity is extraordinarily large with a negative sign, since Co atoms adjacent to Pd atoms at the interface could induce larger magnetostriction than that of pure Co. As the periodicity increased, the magnetostriction became small, but the sign remained negative. For the films with the long periodicity the magnetostriction may be caused by pure Co of the layer interior. On the contrary, the Co/Pt films have a positive magnetostriction even for the thick Co layers. The data indicate existence of the Co-Pt mixture at the interface.
Magnetic and structural studies in Sm-Fe-Ti magnets
Authors: Wang Y. , Hadjipanayis G. C. , Kim A. , Liu N. C. , Sellmyer D. J.
Published: July 2002

The effects of vanadium substitution on the magnetic properties of Sm-Fe-Ti-V melt-spun ribbons and the magnetic properties and microstructure of sintered Sm-Fe-Ti based magnets are reported. The highest coercivity, Hc =10.65 kOe, was obtained in a heat-treated melt-spun Sm8 Fe75.5 Ga0.5 Ti8 V8 alloy. This is the highest value of the coercivities reported in alloys with the ThMn12 -type structure. Bulk magnets were prepared by the usual powder metallurgy technique. However, their highest coercivity (∼2 kOe) was much less than that of the corresponding ribbons. The low coercivity of sintered magnets has been attributed to their nonuniform microstructure.
X-ray optics of doubly curved diffractors
Authors: Wittry D. B. , Sun Songquan
Published: July 2002

Calculations of the x-ray optics of four types of doubly curved diffractors are described and the results are compared with similar results for singly curved diffractors (Johann and Johansson geometry). In these calculations the locus of points on a curved diffractor’s surface is determined for a given deviation from the Bragg angle when the diffractor is used at Bragg angles ranging from 15 to 75 degrees. The six cases discussed include cylindrical, spherical, and toroidal geometries. The calculations are used as a basis for comparing the collection efficiency and the spectral resolution when these diffractors are used in scanning monochromators. It is shown that doubly curved diffractors are superior to singly curved ones and that a diffractor with spherical planes and toroidal surface provides the best performance of all of the diffractors considered if a wide range of Bragg angles must be covered.
Analogous Sm-Ti-Fe high-coercivity phases in Sm-V-Fe and Sm-Zr-Fe sputtered film samples
Authors: Cadieu F. J. , Kamprath N. , Hegde H. , Chen K. , Navarathna A. , Rani R.
Published: July 2002

A phase with high room-temperature coercivity has been synthesized in sputtered film samples of Sm-Fe-V. The samples of SmFeV and SmFeZr have been processed as was SmFeTi to first produce a high room-temperature coercivity of 38.5 kOe. The observed intrinsic coercivity iHc for Sm:Fe:V=19.6:73.5:6.9 atomic was 15.8 kOe at 293 K with a rise to 31 kOe at 6 K. The 4πM at 293 K for an applied field of 90 kOe was 6.2 kG. When this sample was cooled in an applied field of 90 kOe, the 4πM rose smoothly to 7.9 kG at 6 K. The x-ray patterns for SmFeV were similar to the high iHc Sm-Fe-Ti which we have reported. All the observed x-ray-diffraction lines can be fit to the large-cell 5-17 Nd-Fe binary structure. In contrast to this, the Sm-Fe-Zr phase at a similar composition showed a readily indexed x-ray pattern of TbCu7 type with a=5.109 Å and c=4.074 Å with no unaccounted lines. The Sm:Fe:Zr atomic composition was 19.9:72.0:8.1. The Sm-Fe-Zr 4πM at 18 kOe was 6.2 kG and the coercivity was 6.7 kOe as measured at 293 K. Values of iHc for SmFe(Ti,V) intermediate compositions of 46 kOe have been measured at 293 K.
Properties of curved x-ray diffractors with stepped surfaces
Authors: Wittry D. B. , Sun Songquan
Published: July 2002

Problems in the practical fabrication of high-efficiency x-ray diffractors are discussed and a diffractor with a stepped surface is proposed to overcome these problems. The effects of the diffracting materials’ rocking curve width and x-ray penetration on the performance of diffractors with simple geometry are evaluated to determine when diffractor geometries involving doubly curved surface and planes are more advantageous than simple geometries. An example calculation of the performance of a stepped diffractor approximating the Wittry geometry (J. Appl. Phys. 68, 387, 1990) is given and experimental results are presented for the performance of a six-step diffractor using mica as the diffracting material and the fifth-order reflection of CuKα radiation produced by a microfocus x-ray source. It is concluded that the stepped diffractor geometry is a viable solution to the problem of fabricating high-efficiency x-ray diffractors.
Large particle size Sm2Fe17Nx compounds by hydrogen treatment
Authors: Fukuno A. , Ishizaka C. , Yoneyama T.
Published: July 2002

Large particle size Sm2Fe17Nx with a low oxygen content has been produced by treating Sm2Fe17 with hydrogen before nitrogenation. Large Sm2Fe17Nx particles of more than 2 mm include nearly 3.0-wt. % nitrogen and a magnetization level of 140 emu/g. The hydrogen decrepitation behavior was not observed for this alloy and the size of samples were not changed after hydrogen treatment and nitrogenation. It is thought that hydrogen absorption and desorption increase the gas-solid reaction area significantly. The oxygen content of these particles is less than 200 ppm because the fine pulverization step was skipped. The relation between the nitrogen content and magnetization of Sm2Fe17Nx will be presented.