bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron emission mechanism from cubic boron nitride-coated molybdenum emitters
Authors: McCarson B. L. , Schlesser R. , McClure M. T. , Sitar Z.
Published: July 2002

The energy distribution of field-emitted electrons from Mo tips coated with intrinsic cubic boron nitride (c-BN) was studied in an effort to determine the origin of the emitted electrons. Voltage-dependent field-emission energy distribution (V-FEED) spectra were collected from the Mo emitters under ultra-high-vacuum conditions both before and after being coated. Emission current at a given voltage increased by as much as two orders of magnitude for the c-BN-coated emitters relative to bare emitters. The energy of field-emitted electrons from the c-BN-coated emitters was linearly dependent upon the applied voltage. Extrapolation of V-FEED data from c-BN-coated emitters to the flatband condition evidenced that the electrons were emitted from the conduction-band minimum of the c-BN coating at the c-BN/vacuum interface.
Near single crystal-level dielectric loss and nonlinearity in pulsed laser deposited SrTiO3 thin films
Authors: Li Hong-Cheng , Si Weidong , West Alexander D. , Xi X. X.
Published: July 2002

We present low-frequency dielectric loss and nonlinearity measurements in SrTiO3 thin films grown by pulsed laser deposition on SrRuO3 electrode layers. A low loss tangent in the order of 10−4, close to the level found in SrTiO3 single crystals, was observed. Combined with a large tunability, this resulted in a figure of merit for frequency and phase agile materials that can rival that observed in single crystals. The result is potentially significant for tunable microwave device applications, and it points to stress and interface effects as the possible causes for higher dielectric losses in thin films.
High brightness and efficiency blue light-emitting polymer diodes
Authors: Grice A. W. , Bradley D. D. C. , Bernius M. T. , Inbasekaran M. , Wu W. W. , Woo E. P.
Published: July 2002

Efficient blue electroluminescence, peaked at 436 nm, is demonstrated from polymer light-emitting diodes operating at high brightness. A dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer. The luminance reaches 600 cd/m2 at a current density of 150 mA/cm2 for a bias voltage of 20 V, corresponding to an efficiency of 0.25 cd/A and a luminosity of 0.04 lm/W. These values are optimized at a critical emissive layer thickness.
Multicolored light emitters on silicon substrates
Authors: Guha Supratik , Bojarczuk Nestor A.
Published: July 2002

We demonstrate the operation of multicolored light emitting diodes (LEDs) on a silicon wafer using ultraviolet/violet GaN LEDs grown by molecular beam epitaxy on Si(111) in conjunction with organic dye based color converters. These organic converters, applied on top of the nitride diodes, absorb the nitride LED electroluminescence at ∼360 nm, and fluoresce in the green (∼530 nm), and orange (∼600 nm). In this fashion, multicolored light emitters may be made to operate side by side and monolithically on the same Si wafer in an approach that may be attractive for miniature GaN based full color displays.
Schottky barrier engineering in III–V nitrides via the piezoelectric effect
Authors: Yu E. T. , Dang X. Z. , Yu L. S. , Qiao D. , Asbeck P. M. , Lau S. S. , Sullivan G. J. , Boutros K. S. , Redwing J. M.
Published: July 2002

A method for enhancing effective Schottky barrier heights in III–V nitride heterostructures based on the piezoelectric effect is proposed, demonstrated, and analyzed. Two-layer GaN/AlxGa1−xN barriers within heterostructure field-effect transistor epitaxial layer structures are shown to possess significantly larger effective barrier heights than those for AlxGa1−xN, and the influence of composition, doping, and layer thicknesses is assessed. A GaN/Al0.25Ga0.75N barrier structure optimized for heterojunction field-effect transistors is shown to yield a barrier height enhancement of 0.37 V over that for Al0.25Ga0.75N. Corresponding reductions in forward-bias current and reverse-bias leakage are observed in current–voltage measurements performed on Schottky diodes.
Vacuum tunneling of superconducting quasiparticles from atomically sharp scanning tunneling microscope tips
Authors: Pan S. H. , Hudson E. W. , Davis J. C.
Published: July 2002

We report on the study of atomically sharp superconducting tips for scanning tunneling microscopy and spectroscopy. The results clearly show vacuum tunneling of superconducting quasiparticles from atomically sharp tips. Observed deviations of the energy gap of the superconducting tip from its bulk value are attributed to the proximity effect. We show that a combination of a superconducting tip and an atomic resolution scanning tunneling microscope provides a means of achieving very high resolution local spectroscopy. We also discuss how this combination paves the way for a number of important applications.
Electric circuit model for MgO-doped ZrO2–TiO2 ceramic humidity sensor
Authors: Jain M. K. , Bhatnagar M. C. , Sharma G. L.
Published: July 2002

The MgO-doped ZrO2–TiO2 ceramic pellets were studied for its humidity-sensitive electrical conduction. An equivalent circuit model has been proposed to define the humidity-sensitive electrical properties. This model is in agreement with the experimental findings. The electrical conduction is largely controlled by the intergranular impedance except at very high humidities. The impedance of the pellets showed inductive behavior in high-humidity region. This behavior can be attributed to the spherical paths adopted by charge carrier because conduction is mainly through the spherical grain surface.
Excited-state absorption and optical-limiting properties of organometallic fullerene–C60 derivatives
Authors: Song Yinglin , Fang Guangyu , Wang Yuxiao , Liu Shutian , Li Chunfei , Song Licheng , Zhu Yinghuai , Hu Qingmei
Published: July 2002

Singlet excited-state absorption properties and optical limiting behaviors of organochromium and organomolybdenum C60 derivatives (C89H24CrO3P2 and C89H24MoO3P2) were studied by using a double frequency ns/ps Nd:YAG pulse laser system at wavelength of 532 nm and pulse width 21 ps. The experimental results demonstrated that the optical limiting behavior of organochromium C60 derivative is better than both organomolybdenum C60 derivative and fullerene–C60 itself. The experimental results agree well with the singlet excited-state absorption theory.
Fast optical switching by a laser-manipulated microdroplet of liquid crystal
Authors: Juodkazis Saulius , Shikata Masaya , Takahashi Toshimasa , Matsuo Shigeki , Misawa Hiroaki
Published: July 2002

We demonstrate the feasibility of an all-optical liquid-crystal (LC) switch with submillisecond switching time. This switch is realized by optical manipulation of a three-dimensionally trapped microscopic nematic LC droplet using an “optical tweezers.” Transmission of a polarizer/LC droplet/analyzer system can be modulated at frequencies as high as 103 Hz, a faster response than that of a nematic LC demonstrated so far. The response corresponds to a submillisecond switching time when the LC droplet is rotated by an angle of π/4. The alignment of the LC droplet to the plane of the incident plane-polarized beam is also demonstrated.
Magnetoresistance of single-domain ferromagnetic particles
Authors: Aumentado J. , Chandrasekhar V.
Published: July 2002

We have performed magnetoresistance measurements on single-domain, submicron elliptical Ni particles using nonmagnetic probes in a four-probe geometry at liquid-helium temperatures. In the smallest particles, the magnetoresistance shows sharp jumps which are associated with the switching of individual domains. Using an anisotropic magnetoresistance model, we can reconstruct hysteresis loops of the normalized magnetization. The remanent magnetization in zero applied magnetic field is typically 15% less than the saturation magnetization. This relaxation of the magnetization may be due to surface effects or crystal grain structure in the particles.
Doppler broadening positron annihilation spectroscopy: A technique for measuring open-volume defects in silsesquioxane spin-on glass films
Authors: Petkov Mihail P. , Weber Marc H. , Lynn Kelvin G. , Rodbell Kenneth P. , Cohen Stephan A.
Published: July 2002

Doppler broadening positron annihilation spectroscopy is used to measure the concentration, spatial distribution, and size of open-volume defects in low dielectric constant (low-k) hydrogen- and methyl-silsesquioxane thin films. A simple correlation between the number of open-volume defects and the dielectric constant is obtained. In addition, the depth-resolving capability enables profiling of the local electronic environment of open-volume defects as a function of depth. The potential for using this technique for measuring k as a function of film depth is also discussed.
Picosecond time-resolved cyclotron resonance in semiconductors
Authors: Kono J. , Chin A. H. , Mitchell A. P. , Takahashi T. , Akiyama H.
Published: July 2002

A promising method for studying intraband carrier dynamics in semiconductors is monitoring the evolution of far-infrared (FIR) absorption induced by photoexcited carriers. By monitoring the photoinduced FIR absorption as a function of magnetic field, we performed time-resolved cyclotron resonance of photocreated electrons in InSb with picosecond resolution.
Field-emission studies of smooth and nanostructured carbon films
Authors: Merkulov Vladimir I. , Lowndes Douglas H. , Baylor Larry R.
Published: July 2002

Electron field-emission measurements of carbon films prepared by pulsed-laser deposition (PLD) and hot-filament chemical-vapor deposition (HF-CVD) are reported. Smooth PLD carbon films, with both high- and low-sp3 contents, appear to be poor field emitters. In contrast, HF-CVD carbon exhibits very good field-emission properties, including the emission turn-on field as low as 9 V/μm, high emission site density, and excellent durability. In addition, HF-CVD was carried out at temperatures below 600 °C, compatible with the use of glass substrates. The promising field-emission properties of HF-CVD carbon are attributed to the nanostructured nature of this material.
On field emission from a semiconducting substrate
Authors: Waters Richard , Van Zeghbroeck Bart
Published: July 2002

A theoretical examination of field emission from the conduction band of a semiconducting substrate is reported. The analysis includes a comparison with Fowler–Nordheim theory, and it is concluded that the formalism of the Fowler–Nordheim theory is incorrect when applied to carriers originating from a semiconducting substrate. The use of a Fowler–Nordheim analysis results in an error in the extraction of the barrier height that is dependent upon the applied electric field across the oxide, conduction band offset, and temperature. A lower limit of the error was calculated to be between 5% and 15%. An analytical expression is developed to describe the field emission of electrons from the conduction band of a semiconductor.
Field-emission characteristics of SiC nanowires prepared by chemical-vapor deposition
Authors: Wong K. W. , Zhou X. T. , Au Frederick C. K. , Lai H. L. , Lee C. S. , Lee S. T.
Published: July 2002

Silicon carbide (SiC) nanowires on a silicon substrate were prepared using hot-filament-assisted chemical-vapor deposition with a solid silicon and carbon source. The SiC nanowires show good field-emitting properties as revealed by the current–voltage characteristics. Together with its ease of preparation, these SiC nanowires are shown to have great potential in the area of electron field-emitting devices.
Space-charge-limited electron currents in 8-hydroxyquinoline aluminum
Authors: Stößel M. , Staudigel J. , Steuber F. , Blässing J. , Simmerer J. , Winnacker A.
Published: July 2002

We investigate electron injection and transport in single-layer devices of 8-hydroxyquinoline aluminum sandwiched between two electrodes. Electrodes comprising a thin lithium fluoride layer are compared with co-evaporated magnesium–silver cathodes and with pure aluminum cathodes. By employing both transient and quasistatic current measurements, the impact of the LiF-layer thickness on electron injection is investigated. It is demonstrated that contacts comprising 0.1–0.2 nm LiF and an aluminum capping layer are able to sustain space-charge-limited currents in 8-hydroxyquinoline aluminum. Further, steady-state current–voltage measurements as a function of temperature are discussed with respect to trap distributions in 8-hydroxyquinoline aluminum.
High electron mobility AlGaN/GaN heterostructure on (111) Si
Authors: Schremer A. T. , Smart J. A. , Wang Y. , Ambacher O. , MacDonald N. C. , Shealy J. R.
Published: July 2002

Room-temperature Hall mobilities exceeding 900 cm2/V s are obtained for AlGaN/GaN heterostructures on (111) Si by single-temperature flow modulation organometallic vapor phase epitaxy. Thin pseudomorphic AlGaN top layers exhibit a 1.5 nm surface roughness and induces a two-dimensional electron gas sheet carrier concentration of 1.0×1013 cm−2. The GaN buffer layer has a background carrier concentration of 1.0×1015 cm−3, 130 arcsec x-ray diffraction full width at half maximum, and a low-temperature photoluminescence linewidth of 10 meV. An AlN nucleation layer provides static electrical isolation between the AlGaN/GaN and the conducting Si substrate. Large crack-free areas of high-crystalline-quality epitaxial material are obtained and have been successfully used for transistor fabrication.
Organic light-emitting device with an ordered monolayer of silica microspheres as a scattering medium
Authors: Yamasaki Takashi , Sumioka Kazuhiro , Tsutsui Tetsuo
Published: July 2002

Periodic dielectric structures, consisting of hexagonally closed-packed arrays of silica microspheres with the diameter of 550 nm, were incorporated into organic light-emitting devices with a conventional two-layer structure made with vacuum-sublimation. The arrays acted as a two-dimensional diffraction lattice which behaved as a light scattering medium for the light propagated in waveguiding modes within the device. Strongly scattered light emission through the front surface of the devices was observed. An increase in the device coupling-out factor for electroluminescent efficiency by using the scattering structure is demonstrated.
Growth of vertically self-organized InGaAs quantum dots with narrow inhomogeneous broadening
Authors: Xie Qianghua , Brown J. L. , Jones R. L. , Van Nostrand J. E. , Leedy K. D.
Published: July 2002

We have fabricated vertically self-organized multiple sets of In0.6Ga0.4As quantum dots (QDs) on GaAs (001) that combines the concepts of variable amount deposition and shape stabilization and size equalization of QDs. The inhomogeneous broadening of optical emission from these dots reached a record low value of 18.4 meV at a wavelength of ∼1185 nm (4 K). The seed layer and the second dot layer have essentially the same dot density of ∼250 μm−2 due to the high degree of dot vertical alignment. The deposition amount for the second dot layer was selected to be 9 monolayers, which resulted in dots with convergent lateral size (∼62 nm) and stabilized facets, close to {011}. The third layer, with the same amount of InGaAs as the second layer, had a dot density of ∼350 μm−2, an average lateral dot size of ∼71 nm, an average dot height of ∼11 nm, and shallower side facets close to {023}.
Dynamical analysis of the buildup process near resonance
Authors: Villavicencio Jorge , Romo Roberto
Published: July 2002

The time evolution of the buildup process inside a double-barrier system for off-resonance incidence energies is studied by considering the analytic solution of the time-dependent Schrödinger equation with cutoff plane wave initial conditions. We show that the buildup process exhibits invariances under arbitrary changes on the system parameters, which can be successfully described by a simple and easy-to-use one-level formula. We find that the buildup of the off-resonant probability density is characterized by an oscillatory pattern modulated by the resonant case which governs the duration of the transient regime. This is evidence that off-resonant and resonant tunneling are two correlated processes, whose transient regime is characterized by the same transient time constant of two lifetimes.
Characterization of acoustic Lamb wave propagation in polycrystalline diamond films by laser ultrasonics
Authors: Whitfield Michael D. , Audic Barbara , Flannery Colm M. , Kehoe Liam P. , Crean Gabriel M. , Jackman Richard B.
Published: July 2002

The propagation of acoustic Lamb waves in free standing chemical vapor deposited polycrystalline diamond has been studied using a laser ultrasonic technique. The influence of film morphology, quality, and thickness on the waves has been assessed. Acoustic waves with high velocities in the range 8700–12 200 ms−1 were observed; the lowest values were recorded for films with the lowest crystal quality and highest nondiamond content. High quality films with differing crystal textures or thickness show little variation. The influence of temperature on the dispersion characteristics of Lamb wave propagating in a 50 mm diam polycrystalline diamond wafer were also investigated. Little variation was apparent across the range studied (30–250 °C). Material parameters extracted from the dispersion chracteristics of the acoustic signal together with scanning electron microscopy studies suggested that void, microcrack, and grain boundary density most influences the propagation of low frequency Lamb waves in free standing CVD diamond films.
Modeling of the microdischarges in plasma addressed liquid crystal displays
Authors: Hagelaar G. J. M. , Kroesen G. M. W. , van Slooten U. , Schreuders H.
Published: July 2002

Plasma addressed liquid crystal (PALC) is a promising technology for large size flat display devices, which uses gas discharges as electrical switches for the addressing of a liquid crystal (LC) layer. This work presents a comprehensive two-dimensional fluid model, that we developed for the simulation of the microdischarges occurring in PALC displays. The model comprises continuity equations and drift-diffusion equations for plasma particle species, a balance equation for the electron energy, and Poisson’s equation for the electric field. Using this model, we succeeded in simulating the full PALC operation, reproducing a series of discharge pulses and afterglows in three consecutive PALC discharge channels. Results are presented for helium and helium–hydrogen mixtures. The results include: calculated particle densities, current–voltage curves, plasma decay times, surface charges, and LC transmission profiles. The influence of electrical crosstalk between adjacent channels is demonstrated.
An ab initio two-component relativistic method including spin–orbit coupling using the regular approximation
Authors: Faas S. , van Lenthe J. H. , Hennum A. C. , Snijders J. G.
Published: July 2002

In this paper we present the implementation of the two-component scaled zeroth-order regular approximation (ZORA) method in the molecular electronic structure package GAMESS-UK. It is the first application of this method, which was earlier investigated in the context of density functional theory, in molecular ab initio basis set calculations. The performance of the method is tested in atomic calculations, which we can compare with numerical results, on xenon and radon and in molecular calculations on the molecules AgH, HI, I2, AuH, TlH, and Bi2. In calculations on the I2 molecule we investigated the effect of the different approaches regarding the internal Coulomb matrix used in the ZORA method. For the remaining molecules we computed harmonic frequencies and bond lengths. It is shown that the scaled ZORA approach is a cost-effective alternative to the Dirac–Fock method.
Six-degree-of-freedom displacement measurement system using a diffraction grating
Authors: Kim Jong-Ahn , Kim Kyung-Chan , Bae Eui Won , Kim Soohyun , Kwak Yoon Keun
Published: July 2002

Six-degree-of-freedom displacement measurement systems are applicable in many fields: precision machine control, precision assembly, vibration analysis, and so on. This article presents a new six-degree-of-freedom displacement measurement system utilizing typical features of a diffraction grating. It is composed of a laser source, three position sensitive detectors, a diffraction grating target, and several optical components. Six-degree-of-freedom displacement is calculated from the coordinates of diffracted rays on the detectors. A forward and an inverse problem were solved to compute the full pose of an object through kinematic analysis. The experimental results show that the measurement system had a maximum error of ±10 μm for translation and ±0.012° for rotation. The repeatability is about 10 μm for translation and 0.01° for rotation.
On the mechanisms of modifying the structure of turbulent homogeneous shear flows by dispersed particles
Authors: Ahmed A. M. , Elghobashi S.
Published: July 2002

The present study is concerned with answering the question: What are the physical mechanisms responsible for the modification of the turbulence structure by solid particles dispersed in a homogeneous shear flow? We employ DNS (direct numerical simulations) to examine the effects of the two-way interaction between the two phases on the turbulence structure. Our results indicate that particles affect the rate of production of turbulence energy via modifying the vorticity dynamics. It is known that regions of large production rate of turbulence energy are sandwiched between counter rotating vortices whose vorticity, ωs, is aligned with the axes of the longitudinal vortex tubes. These longitudinal vortex tubes are strongly inclined toward the streamwise direction due to the imposed mean shear. The stronger ωs is, the larger the production rate. The dispersed solid particles modify the alignment of the local vorticity vector, ω, with the axis of the longitudinal vortex tube. Increasing this alignment increases ωs, which in turn augments the turbulence production rate, and vice versa. In addition, due to the enhanced strain rate of the carrier fluid by the particles, the dissipation rate of turbulence energy is always increased. The particles also reduce the alignment of the vorticity vector with the intermediate eigenvector (β) of the strain rate tensor. This reduction in alignment is due to an increase in the rotational term and particle-source term in the governing equation of the cosine of the angle between the vorticity vector and the intermediate strain eigenvector.
Sharp green electroluminescence from 1H-pyrazolo[3,4-b]quinoline-based light-emitting diodes
Authors: Tao Y. T. , Balasubramaniam E. , Danel A. , Jarosz B. , Tomasik P.
Published: July 2002

A multilayer organic light-emitting diode was fabricated using a fluorescent compound {6-N,N-diethylamino-1-methyl-3-phenyl-1H-pyrazolo[3,4-b]quinoline} (PAQ–NEt2) doped into the hole-transporting layer of NPB {4,4′-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl}, with the TPBI {2,2′,2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole]} as an electrontransporting material. At 16% PAQ–NEt2 doping concentration, the device gave a sharp, bright, and efficient green electroluminescence (EL) peaked at around 530 nm. The full width at half maximum of the EL is 60 nm, which is 60% of the green emission from typical NPB/AlQ [where AlQ=tris(8-hydroxyquinoline) aluminum] device. For the same concentration, a maximum luminance of 37 000 cd/m2 was obtained at 10.0 V and the maximum power, luminescence, and external quantum efficiencies were obtained 4.2 lm/W, 6.0 cd/A, and 1.6%, respectively, at 5.0 V.
Piezoelectric response of epitaxial Pb(Zr0.2Ti0.8)O3 films measured by scanning tunneling microscopy
Authors: Kuffer O. , Maggio-Aprile I. , Triscone J.-M. , Fischer O/. , Renner Ch.
Published: July 2002

We report on scanning tunneling microscopy measurements of the piezoelectric response in ferroelectric heterostructures grown by off-axis rf magnetron sputtering. The samples are composed of a single-crystalline ferroelectric film of Pb(Zr0.2Ti0.8)O3 deposited on a conducting substrate and covered with an ultrathin metallic film of gold. The high quality of the c-axis oriented ferroelectric layer is evidenced by sharp polarization hysteresis loops. By applying a voltage to the bilayer and recording the inverse piezoelectric effect with the scanning tunneling microscope, we demonstrate the ability to measure the phase response as well as the ferroelectric switching. We obtained strain-field plots with a butterfly loop shape, and a quantitative measurement of the longitudinal piezoelectric coefficient (d33).
Free energy of the Lennard-Jones solid
Authors: van der Hoef Martin A.
Published: July 2002

We have determined a simple expression for the absolute Helmholtz free energy of the fcc Lennard-Jones solid from molecular dynamics simulations. The pressure and energy data from these simulations have been fitted to a simple functional form (18 parameters) for densities ranging from around 0.94–1.20, and temperatures ranging from 0.1 to 2.0 (values in reduced Lennard-Jones units). The absolute free energy at an arbitrary state point in this range is obtained by integrating over density and temperature from the triple-point. Our result for the free energy is in very good agreement with the values reported in literature previously. Also the melting line obtained from our free energy expression, in combination with an equation of state for the liquid phase, is in excellent agreement with results by Agrawal and Kofke [Mol. Phys. 85, 43 (1995)] obtained via the Gibbs–Duhem integration method.
Néel “orange-peel” coupling in magnetic tunneling junction devices
Authors: Schrag B. D. , Anguelouch A. , Ingvarsson S. , Xiao Gang , Lu Yu , Trouilloud P. L. , Gupta A. , Wanner R. A. , Gallagher W. J. , Rice P. M. , Parkin S. S. P.
Published: July 2002

We present measurements of the magnitude of Néel “orange-peel” coupling due to interface roughness in a series of magnetic tunneling junction devices. Results from magnetometry and transport measurements are shown to be in good agreement with the theoretical model of Néel. In addition, we have used transmission electron microscopy to directly probe the sample interface roughness and obtain results consistent with the values obtained by magnetometry and transport methods.
Investigation of the thermal conductivity of the mixed pentatellurides Hf1−xZrxTe5
Authors: Zawilski B. M. , Littleton R. T. , Tritt Terry M.
Published: July 2002

Transition-metal pentatellurides (HfTe5 and ZrTe5) exhibit a promising power factor (electronic properties) for possible use as a thermoelectric material. For complete characterization of these crystals, thermal conductivity measurements are necessary. In this letter, we report measurements of the thermal conductivity for this group of materials using the parallel thermal conductance technique which is well adapted for needle-like samples. Thermal conductivity is presented as a function of temperature and composition of the pentatelluride solid solution HfxZr1−xTe5 with 0⩽x⩽1 in which the magnitude of the room temperature thermal conductivity varies from 5 to 8 W/(m K). Dependence on the cross-sectional area and possible size effects (or sample quality) is also presented and discussed. These results also indicate the importance of sample quality on the low-temperature thermal conductance maximum λmax.
The spectroscopy of AgF: CASSCF+CASPT2 calculations on the lowest Σ+3, Σ+1, Π1, Π1, Δ3, and Δ1 excited states
Authors: Ramı́rez-Solı́s A. , Daudey J. P.
Published: July 2002

The spectroscopic properties of the three lowest-lying (X,2 and 3)1Σ+, the first Σ+3, the two lowest-lying (1 and 2)3Π, the first Π1, and the Δ3,1 states of the AgF molecule have been studied through extensive CASSCF (complete active space self-consistent field)+CASPT2 (complete active space second-order perturbational) calculations, using a 19-active-electron relativistic effective core potential for Ag and large Gaussian basis sets for both atoms. Strong mixtures of the Ag+(4d95s1)F−(2s22p6) ionic and Ag(4d95s2)F(2s22p5) or Ag(4d105s1)F(2s22p5) neutral configurations were found for the Σ+3, 2 1Σ+, and 1 3Π states between 4.0 and 4.4 a.u., while for the higher lying states no evident neutral-ionic crossings were found. This leads to curves that present local maxima at 4.3 a.u. for the 2 1Σ+ and Σ+3 states as well as for the 1 3Π state at 4.0 a.u. The 2 3Π excited state shows the lowest ionic character of all the states. The calculated spectroscopic constants for all the studied states are reported and found in good accordance with available experimental data. The question of the nature of the electronic parent state of the observed B0+ state, responsible for the most intense transition and which is the shortest lived excited state of AgF, is thoroughly addressed in the light of the present results. They clearly indicate that the B0+ state is not correlated with the Rydberg Ag+(4d95p1)+F−(2s22p6) ionic structure, as previously proposed [J. Chem. Phys. 102, 4482 (1995)]. Since the 2 1Σ+ state has been shown to be the ΛSΣ electronic parent state of the fine-structure A0+ state (these results confirm this idea), and given the difference between the calculated Te (1513 cm−1) of the 2 1Σ+ and 1 3Π states, these calculations point to this latter state as the ΛSΣ parent of the experimental B0+ state. At this level of calculation, the next higher lying state that could contribute (3 1Σ+) through spin–orbit couplings to this B0+ state lies more than 8000 cm−1 away. This, however, is not consistent with the accurately measured radiative lifetimes of 7.1 μs (A′Ω1), 9.1 μs (aΩ1), 240 ns (A0+), 21 ns (B0+) for the four observed excited states, which seem to indicate that the two Ω=0+ excited states are of singlet character. Therefore, only a theoretical study including a substantially more accurate and complete account of the electronic+spin–orbit interactions will yield a reliable answer to this complex problem in the spectroscopy of AgF.
Particle-in-cell simulations of high-power cylindrical electron beam diodes
Authors: Swanekamp S. B. , Commisso R. J. , Cooperstein G. , Ottinger P. F. , Schumer J. W.
Published: July 2002

Particle-in-cell (PIC) simulations are presented that characterize the electrical properties and charged-particle flows of cylindrical pinched-beam diodes. It is shown that there are three basic regimes of operation: A low-voltage, low-current regime characterized by space-charge-limited (SCL) flow, a high-voltage, high-current regime characterized by a strongly pinched magnetically limited (ML) flow, and an intermediate regime characterized by weakly pinched (WP) flow. The flow pattern in the SCL regime is mainly radial with a uniform current density on the anode. In the ML regime, electrons are strongly pinched by the self-magnetic field of the diode current resulting in a high-current-density pinch at the end of the anode rod. It is shown that the diode must first draw enough SCL current to reach the magnetic limit. The voltage at which this condition occurs depends strongly on the diode geometry and whether ions are produced at the anode. Analytic expressions are developed for the SCL and ML regimes and compared to simulations performed over a wide range of voltages and diode geometries. In the SCL regime, it is shown that many of the results from planar diodes provide reasonably good estimates for cylindrical diodes. In the ML regime, it is found that the critical current formula provides a better fit to the simulations than the parapotential and focused flow models. An empirical fit to the I–V characteristic was developed from the simulations that smoothly transitions from the SCL regime, through the WP regime, and into the ML regime.
Thermodynamic Properties of the NaNO3+H2O System
Authors: Archer Donald G.
Published: July 2002

New equations that describe the thermodynamic properties of the NaNO3+H2O system were obtained from previously published measurements for this system. The measured values included in the fitted equations spanned the range of temperatures of approximately 236–425 K for NaNO3(aq) and 16–548 K for NaNO3(cr). New equations and/or values for the following properties are given in the present work: (1) thermal properties of NaNO3(cr) from 0 K to near the lambda transition, 548.6 K, (2) the change in chemical potential for both NaNO3 and H2O in NaNO3(aq) as a function of temperature, and molality, valid from 236 to 425 K, and the molality range of 0 mol⋅kg−1 to the lesser of the saturation molality or 25 mol⋅kg−1, and (3) standard-state properties for the aqueous solution process.
Competition between protein folding and aggregation: A three-dimensional lattice-model simulation
Authors: Bratko D. , Blanch H. W.
Published: July 2002

Aggregation of protein molecules resulting in the loss of biological activity and the formation of insoluble deposits represents a serious problem for the biotechnology and pharmaceutical industries and in medicine. Considerable experimental and theoretical efforts are being made in order to improve our understanding of, and ability to control, the process. In the present work, we describe a Monte Carlo study of a multichain system of coarse-grained model proteins akin to lattice models developed for simulations of protein folding. The model is designed to examine the competition between intramolecular interactions leading to the native protein structure, and intermolecular association, resulting in the formation of aggregates of misfolded chains. Interactions between the segments are described by a variation of the Go potential [N. Go and H. Abe, Biopolymers 20, 1013 (1981)] that extends the recognition between attracting types of segments to pairs on distinct chains. For the particular model we adopt, the global free energy minimum of a pair of protein molecules corresponds to a dimer of native proteins. When three or more molecules interact, clusters of misfolded chains can be more stable than aggregates of native folds. A considerable fraction of native structure, however, is preserved in these cases. Rates of conformational changes rapidly decrease with the size of the protein cluster. Within the timescale accessible to computer simulations, the folding-aggregation balance is strongly affected by kinetic considerations. Both the native form and aggregates can persist in metastable states, even if conditions such as temperature or concentration favor a transition to an alternative form. Refolding yield can be affected by the presence of an additional polymer species mimicking the function of a molecular chaperone.
Patterned three-color ZnCdSe/ZnCdMgSe quantum-well structures for integrated full-color and white light emitters
Authors: Luo Y. , Guo S. P. , Maksimov O. , Tamargo M. C. , Asnin V. , Pollak F. H. , Chen Y. C.
Published: July 2002

We report the growth and characterization of patterned ZnCdSe/ZnCdMgSe quantum-well (QW) structures grown adjacent to each other on a single InP substrate. Each structure emits at a different wavelength range spanning the visible range. Stripe and square-shaped QW structures of different emission wavelengths, with lateral dimensions between 15 and 60 μm, were deposited sequentially by shadow mask selective area epitaxy (SAE) steps. Conventional and microphotoluminescence measurements were used to characterize the patterned QWs. They exhibit well-defined excitonic emission in the red, yellow, and green regions of the visible spectrum. This result demonstrates the feasibility of fabricating integrated full-color light emitting diode and laser-based display elements and white light sources using the ZnCdMgSe material system and shadow mask SAE.
Deep-ultraviolet transparent conductive β-Ga2O3 thin films
Authors: Orita Masahiro , Ohta Hiromichi , Hirano Masahiro , Hosono Hideo
Published: July 2002

Thin films of β-Ga2O3 with an energy band gap of 4.9 eV were prepared on silica glass substrates by a pulsed-laser deposition method. N-type conductivity up to ∼1 S cm−1 was obtained by Sn-ion doping and deposition under low O2 partial pressure (∼10−5 Pa) at substrate temperatures above 800 °C. The resulting internal transmittance at the wavelength (248 nm) of the KrF excimer laser exceeded 50% for the 100-nm-thick film, making this the most ultraviolet-transparent conductive oxide thin film to date and opening up prospects for applications such as ultraviolet transparent antistatic electric films in ultraviolet lithography.
Multipurpose x-ray reflectometer optimized for the characterization of organic surface films on aqueous subphases
Authors: Krüger P. , Schalke M. , Linderholm J. , Lösche M.
Published: July 2002

A reflectometer based on a conventional sealed x-ray source for the study of molecular organic surface films (such as Langmuir monolayers) has been devised, which outperforms similar instruments using rotating anode generators and approaches even most advanced experimental stations attached to third generation synchrotron sources. Reflectivities of ∼5×10−9 are thus becoming available while the measurement of a full reflectivity curve takes approximately 4 h to complete. The instrument is operated under full digital control, permitting the automated recording of measurement programs. In an example of its performance we demonstrate that dipalmitoylphosphatidylglycerol monolayers on electrolytic subphases may be characterized to a level of detail which until recently was not even available at synchrotron sources. While conventional box models of lipid monolayers are inadequate for modeling experimental data at the high momentum transfer that has become accessible with the new instrument, a recently developed modeling technique based on volume-restricted distribution functions [Schalke et al., Biochim. Biophys. Acta 1464, 113 (2000)] enables studies of ion binding to the phospholipid in submolecular detail.
Effect of finite domain-wall width on the domain structures of epitaxial ferroelectric and ferroelastic thin films
Authors: Emelyanov A. Yu. , Pertsev N. A. , Salje E. K. H.
Published: July 2002

A theory of domain (twin) structures, which takes into account the finite width of domain walls, is developed for epitaxial ferroelectric and ferroelastic thin films. The theory is based on the dislocation–disclination modeling of the sources of mechanical stresses in polydomain films. Calculations are performed for an orthorhombic or tetragonal film grown on a dissimilar orthorhombic, tetragonal, or cubic substrate. The case of a laminar 90° domain structure with the walls inclined at 45° to the film/substrate interface (c/a/c/a structure in tetragonal films) is considered. A simple dislocation–disclination model is constructed for the junctions of thick domain walls with the film/substrate interface. Using this model, the stress fields in the film and substrate and the associated elastic energy are evaluated. By minimizing the total energy of the material system at a fixed domain-wall width, the equilibrium geometric parameters of a periodic 90° domain structure are calculated. Then the range of stability of this structure in epitaxial films is determined as a function of the wall width. The mechanical restoring forces, which hinder cooperative translational vibrations of thick 90° walls near their equilibrium positions, are also calculated. On this basis, the domain-wall contributions to the dielectric and piezoelectric responses of prepolarized ferroelectric films are evaluated at different wall widths. Finally, the influence of the film straining by the substrate on the equilibrium domain-wall width is analyzed. An increase of the wall width in an epitaxial thin film relative to that in a bulk crystal is predicted.
Quantized tunneling current in the metallic nanogaps formed by electrodeposition and etching
Authors: Li C. Z. , He H. X. , Tao N. J.
Published: July 2002

We have studied electron tunneling across the gap between two electrodes as the gap is varied by electrodeposition and etching. The tunneling current tends to change in a stepwise fashion, corresponding to a discrete change of the gap width. The stepwise change is due to the discrete nature of atoms and a series of structural relaxations of the atoms at the electrodes between stable configurations upon deposition and etching. By stabilizing the tunneling current on various steps using a feedback loop, we have demonstrated that stable molecular-scale gaps can be fabricated with subangstrom precision.
A physically transparent and transferable compressible ion model for oxides
Authors: Marks N. A. , Finnis M. W. , Harding J. H. , Pyper N. C.
Published: July 2002

A new compressible ion model for describing the energetic components of the cohesive energy of a fully ionic crystal is developed and tested using previous ab initio results for three cubic phases of MgO. This model is physically highly transparent and improves on previous compressible ion models in two ways. First, the short-range cation–anion interaction and the rearrangement energy needed to convert a free O− ion plus a free electron into an O−2 ion having a form optimal for its in-crystal environment are decomposed into the major contributions originating from the six outermost anion electrons plus smaller terms generated by the two 2s electrons. This model transfers to the B2 and B3 phases of MgO after parametrization on the ab initio data for the B1 phase even more accurately than previous compressible ion models. Second, the separate modeling of the repulsive (permutation) and attractive (penetration) components of the short-range anion–anion interactions enables the new model to describe their subtle dependencies on the in-crystal anion environment that lie beyond the scope of previous models. The new model is also used to illuminate the behavior of two fluorite oxides.
Measurements of energetic electrons from the high-intensity laser ionization of gases
Authors: Moore C. I. , Ting A. , Jones T. , Briscoe E. , Hafizi B. , Hubbard R. F. , Sprangle P.
Published: July 2002

Electrons ionized from tightly bound atomic states by a high-intensity laser pulse can gain energies from one to millions of electron volts dependent on the intensity of the pulse. We have currently been investigating hundreds of kilovolt to megavolt electrons produced by ionization of krypton and argon with terawatt laser pulses. Angular and energy distributions have been measured to determine the usability of this electron source as an injector for higher energy accelerators. Studies have included pressure dependence, angular ejection angle energy dependence, and polarization dependence. In particular, the energy-dependent ejection angle of electrons has been used to produce electron beams with energies peaked at 600 keV. Numerical simulations of these electrons show that 4 MV electron beams with excellent beam quality and femtosecond pulse widths can be produced from this electron source using higher power laser pulses.
Improved dielectric properties of lead zirconate titanate thin films deposited on metal foils with LaNiO3 buffer layers
Authors: Zou Q. , Ruda H. E. , Yacobi B. G.
Published: July 2002

Improved dielectric properties of lead zirconate titanate (PZT) films deposited on a variety of foils using buffer layers are reported. Foils include titanium, stainless steel, and nickel with LaNiO3(LNO) buffer layers which were prepared by sol–gel processing. High dielectric constant (330 for stainless steel, 420 for titanium, and 450 for nickel foils), low dielectric loss (<2.2% for titanium and 8% for stainless steel), symmetric ferroelectric C–V characteristics and P–E curves were obtained. The LNO layers are shown to provide an effective diffusion barrier for Ni and Cr and to restrict oxide layer formation (i.e., TiOx or NiOx) between the PZT film and the metallic foils during annealing in air.
Nonequilibrium theory of a hot-electron bolometer with normal metal-insulator-superconductor tunnel junction
Authors: Golubev Dmitri , Kuzmin Leonid
Published: July 2002

The operation of the hot-electron bolometer with normal metal-insulator-superconductor (NIS) tunnel junction as a temperature sensor is analyzed theoretically. The responsivity and the noise equivalent power (NEP) of the bolometer are obtained numerically for typical experimental parameters. Relatively simple approximate analytical expressions for these values are derived. The time constant of the device is also found. We demonstrate that the effect of the electron cooling by the NIS junction, which serves as a thermometer, can improve the sensitivity. This effect is also useful in the presence of the finite background power load. We discuss the effect of the correlation of the shot noise and the heat flow noise in the NIS junction.
Photoreflectance study of thermal degradation of n-InP/p+-InGaAs heterojunctions
Authors: Sugiyama Hiroki , Watanabe Noriyuki , Watanabe Kazuo , Kobayashi Takashi
Published: July 2002

The thermal stability of n-InP/p+-InGaAs heterojunctions was studied using photoreflectance (PR) spectroscopy. The PR signal amplitude from the n-InP depletion region decreased after postgrowth annealing. The reduction of the PR signal amplitude reflected an increase of recombination centers in the n-InP depletion region, which was attributed to the degradation of the adjacent carbon-doped p+-InGaAs layer. The reduction of the PR signal amplitude was more significant in samples with higher carbon doping concentration. We propose a model of the increase of the recombination centers that is consistent with our systematic experimental results, where the thermal degradation of the heavily carbon doped p+-InGaAs is taken into account. The present study shows that the PR method is quite helpful in the nondestructive diagnosis of the crystal quality of epitaxial wafers after thermal processing in heterojunction bipolar transistor device fabrication.
Recent developments in collisionless reconnection theory: Applications to laboratory and space plasmas
Authors: Bhattacharjee A. , Ma Z. W. , Wang Xiaogang
Published: July 2002

Recent developments in the theory and simulation of nonlinear collisionless reconnection hold the promise for providing solutions to some outstanding problems in laboratory and space plasma physics. Examples of such problems are sawtooth oscillations in tokamaks, magnetotail substorms, and impulsive solar flares. In each of these problems, a key issue is the identification of fast reconnection rates that are insensitive to the mechanism that breaks field lines (resistivity and/or electron inertia). The classical models of Sweet–Parker and Petschek sought to resolve this issue in the realm of resistive magnetohydrodynamics (MHD). However, the plasmas mentioned above are weakly collisional, and hence obey a generalized Ohm’s law in which the Hall current and electron pressure gradient terms play a crucial role. Recent theoretical models and simulations on impulsive (or triggered) as well as quasisteady reconnection governed by a generalized Ohm’s law are reviewed. In the impulsive reconnection problem, not only is the growth rate fast but the time derivative of the growth rate changes rapidly. In the steady-state reconnection problem, explicit analytical expressions are obtained for the geometric characteristics (that is, length and width) of the reconnection layer and the reconnection rate. Analytical results are tested by Hall MHD simulations. While some of the geometric features of the reconnection layer and the weak dependence of the reconnection rate on resistivity are reminiscent of Petschek’s classical model, the underlying wave and particle dynamics mediating the reconnection dynamics in the presence of the Hall current and electron pressure gradient are qualitatively different. Quantitative comparisons are made between theory and observations from laboratory as well as space plasmas.