bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectrum analyzer with noise reduction by cross-correlation technique on two channels
Authors: Sampietro M. , Fasoli L. , Ferrari G.
Published: July 2002

A spectrum analyzer with a sensitivity better than few pV/Hz in voltage noise measurements and better than 1 fA/Hz in current noise measurements is presented. It has two distinct and independent input amplifiers in parallel, connected to the same device under test (DUT) and is based on the suppression of their uncorrelated noises. The instrument is modular with different front-end amplifiers conceived to optimize the measurement of low impedance or high impedance DUTs. The instrument can cover 8 decades of frequency span, from 10 mHz to 1 MHz. The improvement of sensitivity with respect to a traditional system and the simplicity in the connection and biasing of the DUT makes it perfectly suited to measure ultralow noise levels in semiconductor devices, like trapping noise, shot noise associated with tunneling in fractional quantum Hall systems, 1/f and channel noise in metal–oxide–semiconductor field effect transistors operated below threshold.
Design and operation of different optical fiber sensors for displacement measurements
Authors: Golnabi Hossein
Published: July 2002

Design and operation of different intensity modulated extrinsic optical-fiber sensors are reported. A cantilever fiber-to-fiber, as well as a fiber-lens-fiber design is arranged and studied experimentally. The third scheme introduces a fiber-lens-double fiber design in which a double fiber is used for the light detection. In each case, a stepper motor driven micrometer system is used for the small displacement variations of the cantilever fiber or the ball lens. The results obtained for these sensors are checked against each other, advantages and disadvantages are explored, and the optimum sensor design for such a measurement is proposed.
Defect-free band-edge photoluminescence and band gap measurement of pseudomorphic Si1−x−yGexCy alloy layers on Si (100)
Authors: St. Amour A. , Liu C. W. , Sturm J. C. , Lacroix Y. , Thewalt M. L. W.
Published: July 2002

Pseudomorphic Si1−x−yGexCy alloy layers on Si (100) with band-edge photoluminescence and without defect-related luminescence have been achieved. The photoluminescence was measured from 2 to 77 K and was used to make a direct measurement of the band gap shift as a function of strain reduction as C was added. Compared to the effect of just reducing Ge content, results show that as C is added, strain is reduced more efficiently than the band gap is increased. Furthermore, results imply that a fully strain-compensated Si1−x−yGexCy layer on Si (100) would have a band gap much less than that of Si, and suggest that initial C incorporation reduces the band gap of relaxed, unstrained Si1−x−yGexCy alloys.
Ridge-geometry InGaN multi-quantum-well-structure laser diodes
Authors: Nakamura Shuji , Senoh Masayuki , Nagahama Shin-ichi , Iwasa Naruhito , Yamada Takao , Matsushita Toshio , Sugimoto Yasunobu , Kiyoku Hiroyuki
Published: July 2002

Stripe- and ridge-geometry in InGaN multi-quantum-well (MQW)-structure laser diodes (LDs) were fabricated on sapphire substrates with (112̄0) orientation (A face). The ridge-geometry InGaN MQW LDs showed strong stimulated emission at a wavelength of 411.3 nm under a pulsed current injection of 199 mA at room temperature. The differential quantum efficiency per facet and the threshold current of ridge-geometry LDs were 30% and 180 mA, respectively. The laser threshold current density was 3 kA/cm2. These values were greatly improved in comparison to those of stripe-geometry LDs. The characteristic temperature of the threshold current was around 185 K.
Ionic and neutral growth of dust in plasmas
Authors: Haaland P. , Garscadden A. , Ganguly B.
Published: July 2002

Microscopic dust particles may be formed by accretion of neutral or positively charged precursors in laboratory plasmas. When the concentration of neutral precursors is large, as when a mixture of He and CO is discharged, the radius of grains increases linearly with time and ex situ electron microscopy shows polydisperse particle sizes. By contrast, when the particle acquires multiple negative charges to satisfy the floating potential and the grain size is less than the plasma’s linearized Debye length, ionic growth increases the radii of the particles as t1/3. Monodisperse particle size distributions consistent with ionic growth kinetics are observed in low pressure He plasmas excited by graphite electrodes.
Microfabrication of an electroluminescent polymer light emitting diode pixel array
Authors: Noach Salman , Faraggi Erez Z. , Cohen Gil , Avny Yair , Neumann Ronny , Davidov Dan , Lewis Aaron
Published: July 2002

We describe a method to microfabricate a light emitting diode array with pixels based on conjugated electroluminescent polymers sandwiched between appropriate electrodes. This method, based on direct photoablation with the 193 nm emission of an excimer laser, maintains the properties of these unique polymers. The technique as described here has already achieved an array of 20 μm×20 μm pixels with enhanced electroluminescence (EL) from these pixels and possible spectral tuning of the EL by the application of varying external field. This method can be extended to achieve nanometer dimensionalities using near-field nanolithography.
A noninvasive bunch length monitor for femtosecond electron bunches
Authors: Wang D. X. , Kraft G. A. , Price E. , Wood P. A. D. , Porterfield D. W. , Crowe T. W.
Published: July 2002

A bunch length monitor for ultrashort (90 fs to 1 ps) electron bunches using a coherent synchrotron radiation detection techniques has been developed in a collaboration between the Thomas Jefferson National accelerator Facility (Jefferson Lab) and the University of Virginia. The noninvasive, high-resolution, high-sensitivity, low-noise monitor employs a state-of-the-art “bandpass” GaAs Schottky whisker diode operated at room temperature. This letter presents the monitor’s performance.
Trapping of photogenerated carriers by InAs quantum dots and persistent photoconductivity in novel GaAs/n-AlGaAs field-effect transistor structures
Authors: Yusa G. , Sakaki H.
Published: July 2002

The trapping of photogenerated carriers by embedded InAs quantum dots (QDs) has been studied at 77 K in novel GaAs/n-AlGaAs structures. It is found that the concentration Ns of two dimensional electrons at a given gate voltage Vg is persistently increased by light illumination, because of the trapping of holes by QDs. By the interplay of the gate voltage and photocarrier generation, a distinct hysteresis is observed in the Ns-Vg characteristics. A drastic change of electron mobility by a factor of 19 is achieved by light illumination. The applications of this device for a novel light-controllable floating dot memory is suggested.
Lattice expansion of Ca and Ar ion implanted GaN
Authors: Liu C. , Mensching B. , Volz K. , Rauschenbach B.
Published: July 2002

The 180 keV Ca+ and Ar+ ions were homogeneously implanted in GaN at temperature of liquid nitrogen. High resolution x-ray diffraction was used to monitor the change of GaN (0002) peak with the dose ranging from 5×1012 to 1×1016 cm−2. It has been found that with increasing dose a new peak beside the GaN (0002) peak appears, grows up, and gradually shrinks until disappearance with arising of the amorphous peak, accompanied with a shift towards smaller angles. The difference between Ca+ and Ar+ implantation is discussed. Expansion of GaN crystal lattice due to Ca+ and Ar+ implantation accounts for this phenomenon and is confirmed by TEM results.
Oxygen participation in the formation of the photoluminescence W center and the center’s origin in ion-implanted silicon crystals
Authors: Nakamura M. , Nagai S. , Aoki Y. , Naramoto H.
Published: July 2002

The relationship between the photoluminescence (PL) intensity due to the W (or I1) center and the oxygen concentration in implanted silicon crystals was studied. The PL intensity of the W center decreased consistently with increasing oxygen concentration for carbon-implanted samples with a wide range of carbon fluences, while it showed no dependence on the oxygen concentration for samples implanted with elements such as hydrogen, silicon, and heavy metals. Based on these results and considerations of the reactions of intrinsic defects generated by implantation, the origin of the PL W center was attributed to a defect composed of silicon self-interstitials.
Ultrafast dynamics of holes in GaAs probed by two-color femtosecond spectroscopy
Authors: Ganikhanov F. , Burr K. C. , Tang C. L.
Published: July 2002

Ultrafast relaxation dynamics of light and heavy holes in GaAs following femtosecond valence-to-conduction-band excitation are measured by probing the light- and heavy-to-split-off hole transitions at different midinfrared wavelengths using the recently developed broadly tunable femtosecond optical parametric oscillator. The initial relaxation times are less than 75 fs, and a spectral hole-burning effect is seen. The results suggest that carrier–carrier and optical-phonon scattering, in particular, polar optical-phonon scattering, are the primary processes leading to the initial redistribution of heavy and light holes.
Suppressed diffusion of boron and carbon in carbon-rich silicon
Authors: Rücker H. , Heinemann B. , Röpke W. , Kurps R. , Krüger D. , Lippert G. , Osten H. J.
Published: July 2002

Transient enhanced diffusion of boron in silicon can be suppressed by substitutional carbon. We show here that diffusion of boron and carbon is strongly reduced in carbon-rich silicon, even when no supersaturation of interstitials due to implantation is present. Pronounced non-Fickian diffusion behavior was found for epitaxially grown-in carbon at concentrations well above its solid solubility. The experimentally observed suppression of B and C diffusion at high C concentrations is explained in terms of a recently proposed model that predicts an undersaturation of Si self-interstitials caused by diffusion of C out of C-rich regions.
Electron emission from amorphous carbon nitride films
Authors: Modinos A. , Xanthakis J. P.
Published: July 2002

We propose a model of field electron emission from amorphous carbon nitride films. According to this model, emission occurs at protrusions of the film surface. Most of the emitted electrons come from states with energy within a rather narrow region about the Fermi level, which shifts with nitrogen concentration but remains within the mobility gap of the film. The model reproduces reasonably well the observed variation of the emitted current with the externally applied electric field and with the nitrogen concentration.
Gettering of Cu and Ni in mega-electron-volt ion-implanted epitaxial silicon
Authors: Koveshnikov Sergei , Kononchuk Oleg
Published: July 2002

Gettering of Cu and Ni by 2.3 MeV Si ion-implantation-induced defects has been investigated in epitaxial silicon as a function of annealing temperature, time, and cooling rate. Secondary ion mass spectrometry revealed two distinct gettering regions, the position of which correlated with the ion projected range Rp and approximately half of Rp. Gettering experiments performed on samples with low metal impurity concentration have shown that capture of Cu and Ni in the two gettering regions occurred during high-temperature annealing, indicating a segregation-induced gettering mechanism. The binding energies of Cu and Ni are higher in the shallow Rp/2 region than in the Rp region.
Electronic structure of 8-hydroxyquinoline aluminum/LiF/Al interface for organic electroluminescent device studied by ultraviolet photoelectron spectroscopy
Authors: Mori T. , Fujikawa H. , Tokito S. , Taga Y.
Published: July 2002

Electronic structures of the 8-hydroxyquinoline aluminum (Alq3)/LiF/Al and Alq3/Al interfaces were measured by ultraviolet photoelectron spectroscopy. Shifts of the highest occupied molecular orbital level and the vacuum level of the Alq3 layer due to insertion of a thin LiF layer were observed. This result indicates that the thin LiF layer at the Alq3/Al interface reduces barrier height for electron injection from the Al to Alq3. We, therefore, conclude that lowering of the driving voltage in an organic electroluminescent device with a thin LiF layer is attributable to the reduction of the barrier height.
Self-assembled chains of graphitized carbon nanoparticles
Authors: Bezryadin A. , Westervelt R. M. , Tinkham M.
Published: July 2002

We report a technique which allows self-assembly of conducting nanoparticles into long continuous chains. Transport properties of such chains have been studied at low temperatures. At low bias voltages, the charges are pinned and the chain resistance is exponentially high. Above a certain threshold VT, the system enters a conducting state. The threshold voltage is much bigger than the Coulomb gap voltage for a single particle and decreases linearly with increasing temperature. A sharp threshold was observed up to about 77 K. Such chains may be used as switchable links in Coulomb charge memories.
GaAs nanostructures and films deposited by a Cu-vapor laser
Authors: Dinh L. N. , Hayes S. , Saw C. K. , McLean W. , Balooch M. , Reimer J. A.
Published: July 2002

The properties of GaAs nanoclusters and films deposited on substrates by a Cu-vapor laser were investigated. Nanoclusters of GaAs were produced by laser ablating a GaAs target in an Ar background gas. X-ray diffraction and transmission electron microscopy revealed that these GaAs nanoclusters had randomly oriented crystalline cores and As-rich amorphous oxide outer shells. These clusters assembled, upon vacuum annealing, along step edges and at defects on substrates to form wire-like structures. Our results also showed that GaAs films, when deposited in vacuum, did not have crystalline cores and were rich in As. Postdeposition annealing in vacuum to between 400 and 500 °C drove off the excess As. The stoichiometry of the films was confirmed by both Auger electron spectroscopy and x-ray photoelectron spectroscopy.
Observation of Bragg reflection in photonic crystals synthesized from air spheres in a titania matrix
Authors: Richel A. , Johnson N. P. , McComb D. W.
Published: July 2002

Three-dimensional photonic crystals made of close-packed air spheres in an interconnected titania matrix have been fabricated using a self-organized template of polystyrene microspheres of 400 nm diameter. The matrix was obtained by vacuum-assisted infiltration of a precursor, with subsequent removal of the microspheres by calcination. Electron microscopy has confirmed the ordering of the structure and the presence of features likely to enlarge the photonic band gap. The reflection spectra of the crystal measured at different angles of incidence are consistent with Bragg’s law and with theoretical calculations, confirming the photonic nature of the material.
Mapping electrostatic potential across an AlGaN/InGaN/AlGaN diode by electron holography
Authors: McCartney M. R. , Ponce F. A. , Cai Juan , Bour D. P.
Published: July 2002

Electron holography has been used to image electrostatic potential variations across an AlGaN/InGaN/AlGaN heterojunction diode. Features in the energy profile have been interpreted in terms of the expected built-in voltage across the p-n junction of the diode as well as contributions from the spontaneous and piezoelectric polarization fields within the material. Additional profile features indicate the presence of two-dimensional electron gas at the upper interface of the InGaN well, and evidence for hole accumulation close to the lower interface.
Tailoring the colossal magnetoresistivity: La0.7(Pb0.63Sr0.37)0.3MnO3 thin-film uncooled bolometer
Authors: Lisauskas Alvydas , Khartsev S. I. , Grishin Alex
Published: July 2002

Epitaxial Ca, Sr, and Pb doped manganite films of various compositions and thickness have been prepared to tailor metal-to-semiconductor phase transition to room temperature. Continuous series of solid solutions La0.7(Pb1−xSrx)0.3MnO3 grown by the pulsed laser deposition technique were found to possess superior performance regarding the maximum of temperature coefficient of resistivity (TCR) @300 K. In these films phase transition temperature Tc ranges from 266 to 327 K. We have engineered the film of the optimum composition x=0.37 exhibiting the maximum of TCR=7.4% K−1 @295 K. Relatively low excess noise (normalized value γ/n of 3×10−21 cm3) has been achieved due to the epitaxial quality of the fabricated film. Using this film, infrared radiation bolometer demonstrator, operating at room temperature, has been built and tested. The bolometer resolves the noise equivalent temperature difference as low as 120 nK/Hz and shows signal-to-noise ratio SNR=8×106 Hz/K, responsivity R=0.6 V/W, detectivity D=0.9×107 cmHz/W, and noise equivalent power NEP=3×10−8 WHz at 30 Hz frame frequency. For micromachined thermally isolated La0.7(Pb0.63Sr0.37)0.3MnO3 thin-film bolometer one can expect to get responsivity about 4×103 V/W and detectivity higher than 109 cmHz/W @30 Hz.
A kinetic description for low frequency longitudinal waves in a dust plasma including charge fluctuations
Authors: Varma Ram K.
Published: July 2002

A kinetic description is presented here for the study of low frequency electrostatic waves in a dusty plasma including the effect of charge fluctuations on the dust grains. The kinetic description uses the charge on the grains as a dynamical variable as a generalization of the usual kinetic model to describe the charge fluctuation effects. When properly carried out it leads to the appearance of a new term in the dispersion relation arising out of the kinetic treatment of charge fluctuations. This new term strongly modifies the known dust plasma modes such as the dust acoustic modes, dust-Coulomb mode, etc., as well as leads to new modes of oscillation in the zero wave number limit both in the tenuous and dense plasma regimes (determined entirely by charging and discharging frequencies of the grain as a capacitance), which may be called as “grain capacitance oscillations.” Because of the presence of such a frequency of oscillation, the possibility is also pointed out of a reflection of dust-acoustic wave from the region of increasing plasma density very much analogous to the reflection of a plasma wave in a region of increasing plasma density. There is also found the existence in the tenuous plasma regime of a new purely oscillatory and purely damped “dust hybrid” mode with a frequency as a geometric mean of the new term—modified dust acoustic wave frequency and discharging frequency.
Electromechanical magnetization measurements at ultralow temperatures and high magnetic fields
Authors: Sakon Takuo , Motokawa Mitsuhiro
Published: July 2002

A new magnetization measurement system, which is designed to be used in ultralow temperatures and high magnetic fields, has been developed. In this equipment, a small capacitor is used and its capacitance changes by the displacement of an electrode due to force acting on a magnetic material placed on the electrode. The most significant point of this equipment is that a Ni plate is placed just below the capacitor to make a gradient field at the sample position instead of usual field-gradient coils installed in a magnet. Using this system, we have studied precise magnetization measurements on rare-earth monopnictides Ce0.9La0.1Sb and a uranium heavy Fermion superconductor UPd2Al3 at temperature down to 0.5 K in high magnetic fields provided by a superconducting magnet and a hybrid magnet.
Method to produce high-resolution scanning near-field optical microscope probes by beveling optical fibers
Authors: Held T. , Emonin S. , Marti O. , Hollricher O.
Published: July 2002

A new two-step method to fabricate scanning near-field optical microscope (SNOM) probes with an aperture size clearly below 100 nm has been developed. For the first step, a chemical etching process is used in which an optical fiber is dipped with its acrylate jacket into hydrofluoric acid to get a suitable tapered shape of the fiber. The second step consists of beveling the etched fiber using a modified micropipette beveler to obtain a tip diameter in the nanometer range as well as a smooth surface to allow a good aluminum metallization by evaporation. By varying the beveling angle tapered shapes with different cone angles can be obtained. First transmission experiments with our probes show an optical resolution below 80 nm. In comparison to fiber tips obtained by a standard heating and pulling method, the transmission efficiency of these tips is up to three orders of magnitude higher due to the optimized tapered shape.
Structure of carbon nanotubes grown by microwave-plasma-enhanced chemical vapor deposition
Authors: Okai M. , Muneyoshi T. , Yaguchi T. , Sasaki S.
Published: July 2002

Carbon nanotubes grown on a Ni substrate and an Fe–Ni–Cr alloy substrate by plasma-enhanced chemical vapor deposition were investigated by transmission electron microscope (TEM) and energy dispersive x-ray (EDX) analysis. TEM showed that the nanotubes on both substrates have a piled-cone structure with metal particles on top which determine the diameter of the nanotubes. Their diameter ranges from 60 to 80 nm. Moreover, EDX showed that the metal particles are composed of Ni when the nanotubes are grown on Ni substrate and of Fe and Ni in the case of the Fe–Ni–Cr alloy substrate.
Vibrational dynamics up to the dissociation threshold: A case study of two-dimensional HOCl
Authors: Joyeux Marc , Sugny Dominique , Lombardi Maurice , Jost Remy , Schinke Reinhard , Skokov Sergei , Bowman Joël
Published: July 2002

This work is aimed at extending recent studies dealing with the highly excited vibrational dynamics of HOCl [J. Chem. Phys. 111, 6807 (1999); J. Chem. Phys. 112, 77 (2000)], by taking advantage of the fact that the OH-stretch remains largely decoupled from the two other degrees of freedom up to and above the dissociation threshold. The molecule is thus reduced to a two-dimensional (2D) system by freezing the OH bond length to its equilibrium value. All of the calculated bound states of the 2D system, as well as the first 40 resonances, can be assigned with a Fermi polyad quantum number. The bifurcation diagram of the principal families of periodic orbits (POs) is extended to higher energies compared to 3D studies. In particular, the birth of “inversion” states (states exploring two equivalent wells connected through the linear HOCl configuration) is related to a period-doubling bifurcation of the families of bending POs, while “dissociation” states (states for which the energy flows back and forth along the dissociation pathway) are shown to lie on top of three successive families of POs born at saddle-node bifurcations. Based on the derivation of a classical analogue of the quantum Fermi polyad number, the energies of particular quantum states and classical POs are plotted on the same diagram for the 2D ab initio surface and are shown to agree perfectly. In contrast, comparison of classical Poincaré surfaces of section and quantum Husimi distributions suggests that the classical dynamics of 2D HOCl is much more chaotic than the quantum dynamics. This observation is discussed in terms of the quantum/classical correspondence, and particularly of the vague tori introduced by Reinhardt. It is nevertheless shown that quantum and classical mechanics agree in predicting a slow intramolecular vibrational energy redistribution (IVR) between the OCl stretch and the bend degrees of freedom.
Eley–Rideal reactions between H atoms on metal and graphite surfaces: The variation of reactivity with substrate
Authors: Jackson Bret , Lemoine Didier
Published: July 2002

Results are presented for the reaction of gas-phase H atoms with H atoms adsorbed onto a variety of substrates. Time-dependent quantum methods are used to compute reaction cross sections and product H2 rotational and vibrational distributions for a large number of model potential energy surfaces. The potentials which model reactions on metals exhibit a wide range of reactivity. In addition, the single-collision Eley–Rideal reaction cross sections are generally small, suggesting that hot-atom processes should in general play an important role in H2 formation on metal surfaces. These observations are consistent with recent experiments. Eley–Rideal reactivity is shown to increase as the strength of the H-substrate bond decreases, and H atom trapping becomes less favorable. The cross sections for the reaction of H(g) with H adsorbed onto model graphite surfaces are generally large (5–10 Å2).
Particle “swarm” dynamics in triboelectric systems
Authors: Vinay Stephen J. , Jhon Myung S.
Published: July 2002

Using state-of-the-art flow/particle visualization and animation techniques, the time-dependent statistical distributions of charged-particle “swarms” exposed to external fields (both electrostatic and flow) are examined. We found that interparticle interaction and drag forces mainly influenced swarm dispersion in a Lagrangian reference frame, whereas the average particle trajectory was affected primarily by the external electric and flow fields.
Blue photoluminescence activated by surface states in GaN grown by molecular beam epitaxy
Authors: Reshchikov M. A. , Visconti P. , Morkoç H.
Published: July 2002

We have studied the broad blue band, which emerges in the photoluminescence (PL) spectrum of c-plane GaN layers after etching in hot H3PO4 and subsequent exposure to air. This band exhibited a 100 meV blueshift with increasing excitation intensity and a thermal quenching with activation energies of 12 and 100 meV. These observations led us to suggest that surface states may be formed on etched surfaces and cause bandbending, which leads to a shift in transition energy with excitation. The blue PL is related to transitions from the shallow donors filled with nonequilibrium electrons to the surface states, which capture the photogenerated holes. The observed irreversible bleaching of the blue luminescence may be attributed to the metastable nature of the surface states or to the oxygen desorption.
Three-quark exchange operators, crossing matrices and Fierz transformations in SU(2) and SU(3)
Authors: Dmitrašinović V.
Published: July 2002

We give explicit expressions for the three-quark exchange operators, crossing matrices and Fierz transforms for the SU(2) and SU(3) groups. We identify the invariant terms in these operators and express them in terms of Casimir operators.
Photoionization study of CH3SCH2Cl formed in the reaction system Cl/Cl2/CH3SCH3
Authors: Cheng Bing-Ming , Chew Eh Piew , Yu Jen-Shiang K. , Yu Chin-hui
Published: July 2002

A photoionization-efficiency spectrum of CH3SCH2Cl was measured over the wavelength range 108–142 nm by means of a photoionization mass spectrometer coupled to a synchrotron as the source of radiation. Gaseous CH3SCH2Cl was generated in a discharge-flow reactor involving Cl, Cl2, and CH3SCH3 at room temperature via these sequential reactions: Cl+CH3SCH3→CH3SCH2+HCl; CH3SCH2+Cl2→CH3SCH2Cl+Cl. According to the PIE spectrum of CH3SCH2Cl thus obtained, the ionization energy is (9.077±0.007) eV. Based on GAUSSIAN-2 calculations, the observed ionization of CH3SCH2Cl near the threshold region is likely to form from singlet CH3SCH2Cl ionizing to doublet CH3SCH2Cl+; the calculated ionization energy 9.064 eV agrees with the experimental value. The adiabatic ionization energy of CH3SCH2 and appearance energy of CH3SCH2+ from CH3SCH2Cl were determined to be (6.884±0.008) eV and (10.007±0.016) eV, respectively; the dissociation energy of the CH3SCH2–Cl bond is thus derived to be (72.0±0.6) kcal mol−1.
Electronic structure of molecular crystals containing edge dislocations
Authors: Kuklja Maija M. , Kunz A. Barry
Published: July 2002

An attempt to model the electronic structure of molecular crystals containing an edge dislocation at the ab initio Hartree–Fock level is performed. The experimentally determined configurations for edge-type dislocations with the Burgers vector [001] in crystalline cyclotrimethylene trinitramine (RDX) and pentaetythritol tetranitrate (PETN) are theoretically simulated. It is shown that a shear stress, induced by the dislocations, produces local electronic states in the fundamental band gap of the crystal. These states are mainly formed by molecular orbitals of critical bonds (which are the N–NO2 group in RDX and the O–NO2 group in PETN) responsible for the stability of the materials. Optical absorption attributed to these electronic states is predicted and compared to the available experimental data. Properties of the defective solids are compared with those of the perfect crystals. Correlation of the electronic structure and sensitivity of the materials to initiation of a chemical reaction as well as some practical applications of the obtained results are discussed.
Erratum: “Time evolution of reactants, intermediates and products in the vibrational predissociation of Br2⋅⋅⋅Ne: A theoretical study” [J. Chem. Phys. 113, 10130 (2000)]
Authors: Miguel Beatriz , Bastida Adolfo , Zúñiga José , Requena Alberto , Halberstadt Nadine
Published: July 2002

The lower part of Fig. 6 was erroneous, due to a transcription error. The MDQT and experimental results now show a much better agreement with the quantum mechanical ones for the Δv=−2 channel of the Br2⋅⋅⋅Ne(v=27) vibrational predissociation, as was the case for the other channels and the other initial vibrational levels. The rest of the paper is unchanged.
Reduction of islands in full-pressure stellarator equilibria
Authors: Hudson S. R. , Monticello D. A. , Reiman A. H.
Published: July 2002

The control of magnetic islands is a crucial issue in designing stellarators. Islands are associated with resonant radial magnetic fields at rational rotational-transform surfaces and can lead to chaos and poor plasma confinement. In this article it is shown that variations in the resonant fields of a full pressure stellarator equilibrium can be related to variations in the boundary via a coupling matrix, and that inversion of this matrix determines a boundary modification for which the island content is significantly reduced. The numerical procedure is described and the results of island optimization are presented. Equilibria with islands are computed using the Princeton Iterative Equilibrium Solver, and resonant radial fields are calculated via construction of quadratic-flux-minimizing surfaces. A design candidate for the National Compact Stellarator Experiment [A. Reimann, L. Ku, D. Monticello et al., Phys. Plasmas 8, 2083 (2001)], which has a large island, is used to illustrate the technique. Small variations in the boundary shape are used to reduce island size and to reverse the phase of a major island chain.
Nature of small-polaron hopping conduction and the effect of Cr doping on the transport properties of rare-earth manganite La0.5Pb0.5Mn1−xCrxO3
Authors: Banerjee Aritra , Pal S. , Chaudhuri B. K.
Published: July 2002

The conductivity and magnetoresistance of La0.5Pb0.5Mn1−xCrxO3 (0.0⩽x⩽0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158–276 K, depending on x. In contrast to the behavior of a similar sample La0.7Ca0.3Mn1−xCrxO3 showing no (MIT) for x⩾0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (Tp) shifts towards lower temperature with increasing x while magnetic field shifts Tp to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (ρ) curve (below Tp) is well fitted with ρ(T)=ρ0+ρ2.5T2.5 indicating the importance of electron–magnon interaction (second term). We have successfully fitted the high temperature (T>θD/2, θD is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the samples showing MIT while nonadiabatic hopping conduction mechanism is obeyed by the samples showing no MIT. The lower temperature (between Tp and θD/2) conductivity data of all the samples can be well fitted to the variable range hopping (VRH) model similar to the case of many semiconducting transition metal oxides. Temperature dependent Seebeck coefficient data also support the small polaron hopping conduction mechanism above Tp.
A rarefied gas flow caused by a discontinuous wall temperature
Authors: Aoki Kazuo , Takata Shigeru , Aikawa Hidefumi , Golse François
Published: July 2002

A flow of a rarefied gas caused by a discontinuous wall temperature is investigated on the basis of kinetic theory in the following situation. The gas is confined in a two-dimensional square container, and the left and right halves of the wall of the container are kept at different uniform temperatures, so that the temperatures of the top and bottom walls are discontinuous at their respective middle points. External forces are assumed to be absent. The steady flow of the gas induced in the container by the effect of the discontinuities is analyzed numerically on the basis of the Bhatnagar–Gross–Krook model of the Boltzmann equation and the diffuse reflection boundary condition by means of an accurate finite-difference method. The features of the flow are clarified for a wide range of the Knudsen number. In particular, it is shown that, as the Knudsen number becomes small (i.e., as the system approaches the continuum limit), the maximum flow speed tends to approach a finite value, but the region with appreciable flow shrinks to the points of discontinuity; thus, the overall flow in the container vanishes nonuniformly in the continuum limit. The behavior of the molecular velocity distribution function is also investigated in detail.
Optical and electrical properties of direct-current magnetron sputtered ZnO:Al films
Authors: Pei Z. L. , Sun C. , Tan M. H. , Xiao J. Q. , Guan D. H. , Huang R. F. , Wen L. S.
Published: July 2002

In this study, high-quality ZnO:Al (ZAO) films were prepared by using dc reaction magnetron sputtering technology. The effect of Al doped in ZnO films on electrical and optical properties and its scattering mechanism were discussed in detail. The results showed that Al2O3 could be effectively removed by controlling oxygen flow and Al-doped concentration during deposition of ZnO:Al films. Zn, Al, and oxygen elements were well distributed through the films. For highly degenerated ZnO:Al semiconductor thin films, it was revealed that ionized impurity scattering dominated the Hall mobility of the films in the low-temperature range; while the lattice vibration became a major scattering mechanism in the high-temperature range. The grain-boundary scattering only played a major role in the ZAO films with small grain size (as compared to the electron mean-free path). The photoelectric properties of ZAO films showed that the lower resistivity (∼5×10−4 Ω cm) was obtained, and transmittance in the visible range and reflectance in the IR region were above 80% and 60%, respectively.
The critical current of superconductors: an historical review
Authors: Dew-Hughes D.
Published: July 2002

The most important practical characteristic of a superconductor is its critical current density. This article traces the history of the experimental discoveries and of the development of the theoretical ideas that have lead to the understanding of those factors that control critical current densities. These include Silsbee’s hypothesis, the Meissner effect, the London, Ginsburg–Landau, and Abrikosov theories, flux pinning and the critical state, and the control of texture in high-temperature superconductors.
Optimization of a compact two-stage light-gas gun aiming at a velocity of 9 km/s
Authors: Moritoh Tatsumi , Kawai Nobuaki , Nakamura Kazutaka G. , Kondo Ken-ichi
Published: July 2002

We developed a compact two-stage light-gas gun and optimized several operating parameters for aiming at velocities higher than 9 km/s. A firing test achieved a velocity of 6.2 km/s using helium as driver gas with a projectile weighing 0.6 g and a piston weighing 430 g. In the case of firing tests using hydrogen, a velocity of 8.1 km/s was obtained under the same condition. Moreover, it was found that higher velocities could be achieved by using a lighter piston to increase piston velocities, and that the efficiency of acceleration strongly depended on the initial gas pressure for each piston. Finally, a projectile velocity of 8.9 km/s was achieved using a piston weighing 290 g.
Breakdown voltage in thin III–V avalanche photodiodes
Authors: Saleh Mohammad A. , Hayat Majeed M. , Kwon Oh-Hyun , Holmes Archie L. , Campbell Joe C. , Saleh Bahaa E. A. , Teich Malvin C.
Published: July 2002

The dead-space multiplication theory of Hayat and Saleh [J. Lightwave Technol. 10, 1415 (1992)], in conjunction with the multiplication-width-independent ionization-coefficient model developed by Saleh et al. [IEEE Trans. Electron Devices 47, 625 (2000)], are shown to accurately predict breakdown voltages for thin avalanche photodiodes of GaAs, InP, In0.52Al0.48As, and Al0.2Ga0.8As, over a broad range of device widths. The breakdown voltage is determined from the analytical expression for the impulse-response-function decay rate.
Stability of long field-reversed configurations
Authors: Barnes D. C.
Published: July 2002

The stability of very elongated field-reversed configurations is solved by an expansion in the small parameter ε (inverse elongation). It is first shown that all possible unstable modes have small growth rates (γ ∼ε). The internal tilt mode is considered in detail. An explicit form for δW in leading order is derived, and leads to a quadratic form including Hall terms. A sufficient condition for stability is obtained by minimizing δW, leading to a field-line ordinary differential equation. Sufficient stability conditions are obtained from this formulation, and indicate stability for S*/E<2 (where S* is the ratio of separatrix radius to collisionless ion skin depth and E the elongation of the separatrix), if the local criterion is used. It is argued that a volume-averaged condition is more appropriate when finite ion orbit effects are included. This leads to S*/E<3.5–4 for stability, independent of separatrix shape or xs (separatrix radius to wall radius at the midplane). This condition for stability compares favorably with experimental observations.
Dielectric behaviors under high electric field for Pb(Zr1/3Nb2/3)O3–PbTiO3–BaTiO3 relaxor ferroelectric ceramics
Authors: Wang Xiaoli , Chen Haydn
Published: July 2002

The dielectric behaviors of Pb(Zn1/3Nb2/3)O3–PbTiO3–BaTiO3 (PZN–PT–BT) relaxor ferroelectric ceramics of several compositions have been investigated under high alternating electric field. In the broad temperature range of the diffuse phase transition, they show highly induced polarization. An unusual linear relation of the polarization with electrical field has been observed near the central portion of the hysteresis loops of some PZN–PT–BT ceramics, which is both temperature and composition dependent. The corresponding dielectric constants are fairly high as well. Moreover, a strong annealing effect is found in its dielectric response. The observed dielectric and ferroelectric properties may be understood using a proposed model of implicit Ba(Zn1/3Nb2/3)O3 paraelectric nanoregions in PZN–PT–BT ceramics.
Exchange-only optimized effective potential for molecules from resolution-of-the-identity techniques: Comparison with the local density approximation, with and without asymptotic correction
Authors: Hamel Sébastien , Casida Mark E. , Salahub Dennis R.
Published: July 2002

The optimized effective potential (OEP) is (within a certain linear response approximation) the exact exchange-correlation potential of density-functional theory (DFT). In the program DEMON-DYNARHO, we have implemented the OEP at the exchange-only level without the evaluation of four-center integrals using resolution-of-the-identity techniques. We point out that great care must be taken in treating the asymptotic behavior of the OEP in finite basis set methods. Our results are compared with previous work using OEP-like potentials. Our OEP orbital energies are compared with experimental ionization potentials, Hartree–Fock (HF) orbital energies, and with orbital energies from the local density approximation (LDA) potential, with and without asymptotic correction (AC). We find that OEP orbital energies are a much better approximation to experimental ionization potentials than are HF orbital energies. LDA orbital energies also correlate well with OEP orbital energies, except for a molecule-dependent rigid shift, due to the well-known fact that the LDA potential falls off too rapidly at large distances. The resultant underbinding is largely corrected by the AC-LDA potential whose orbital energies correlate well with OEP orbital energies, with typical differences on the order of 0.5 eV. However larger differences between AC-LDA and OEP orbital energies are also observed, particularly for unoccupied orbitals and the reason for this is discussed. As an illustration of how the OEP might be used in practical calculations, we give an example from time-dependent DFT where use of the OEP instead of the AC-LDA potential leads to significant improvement in a key σ→π* excitation energy of ethylene.
On entropic barriers for diffusion in zeolites: A molecular dynamics study
Authors: Schüring Andreas , Auerbach Scott M. , Fritzsche Siegfried , Haberlandt Reinhold
Published: July 2002

The self-diffusion of ethane in cation-free Linde type A zeolite has been studied by molecular dynamics simulations for various temperatures. These simulations predict that the diffusivity decreases with increasing temperature between 150 K and 300 K for a low loading of one molecule per cage. The rate of cage-to-cage crossings shows the same temperature dependence. We explain this phenomenon based on an analysis of the activation entropy that controls motion through eight-ring windows separating adjacent cages. The diffusivity and the cage-to-cage rate constant both decrease with temperature because heating the system moves ethane away from eight-ring windows, on average, which increases the entropic barrier for cage-to-cage motion.
Vertical scaling of carbon nanotube field-effect transistors using top gate electrodes
Authors: Wind S. J. , Appenzeller J. , Martel R. , Derycke V. , Avouris Ph.
Published: July 2002

We have fabricated single-wall carbon nanotube field-effect transistors (CNFETs) in a conventional metal–oxide–semiconductor field-effect transistor (MOSFET) structure, with gate electrodes above the conduction channel separated from the channel by a thin dielectric. These top gate devices exhibit excellent electrical characteristics, including steep subthreshold slope and high transconductance, at gate voltages close to 1 V—a significant improvement relative to previously reported CNFETs which used the substrate as a gate and a thicker gate dielectric. Our measured device performance also compares very well to state-of-the-art silicon devices. These results are observed for both p- and n-type devices, and they suggest that CNFETs may be competitive with Si MOSFETs for future nanoelectronic applications.
Transition arrays in atomic spectroscopy with a commercial spreadsheet program
Authors: Reader Joseph
Published: July 2002

In deducing the experimental energy levels of an atom from its observed spectral lines, a two-dimensional transition array showing all the predicted and identified electric-dipole transitions can be of great help. A system for using a commercial spreadsheet program to construct these arrays is described. Use of such a program facilitates the generation of large arrays with little computer coding. The basic principles should be applicable to other areas where it is desired to organize experimental and theoretical data in a two-dimensional array. © 1997 American Institute of Physics.
A simple technique for the determination of mechanical strain in thin films with applications to polysilicon
Authors: Guckel H. , Randazzo T. , Burns D. W.
Published: July 2002

Free standing, doubly supported micromechanical beams which are fabricated from films with built-in compressive strain fields buckle at critical geometries. Experimental determination of the onset of buckling for known geometries leads to a direct measurement of the strain level in the films. This idea is supported by appropriate theory for experimental structures which form clamped, doubly supported beams with constant cross section and varying lengths. Application to low pressure chemical vapor deposition polysilicon leads to the conclusion that strain fields of 0.2% reduce to 0.05% during annealing.
Estimation of light-reflection-layer thickness by optical second-harmonic generation and Raman scattering from semiconductor superlattices
Authors: Kubota K.
Published: July 2002

The change in the refractive index at the surface of separation between two different media gives rise to the phenomena of reflection and refraction. Reflection requires that the boundary is not a geometrical surface but a thin transition layer. The depth of this layer was estimated by optical second-harmonic generation and Raman scattering from the surface of a semiconductor superlattice. The function of the secondary electromagnetic waves excited in the medium was discussed.
Hot electrons in one dimension
Authors: Mahan G. D.
Published: July 2002

An exact solution is given in one dimension for the electron distribution as limited by optical phonon scattering. The solution is valid for an arbitrary shape of potential energy, and current flow. Numerical results are presented for both constant field devices, and for the nonconstant fields of a typical metal-oxide-semiconductor-field-effect transistor (MOSFET) channel in silicon.
Excitonic transitions and exciton damping processes in InGaAs/InP
Authors: Zielinski E. , Schweizer H. , Streubel K. , Eisele H. , Weimann G.
Published: July 2002

Detailed absorption measurements and the analysis of the absorption spectra of In1−xGaxAs lattice matched to InP are reported. The lattice matching parameter Δa/a covered a range from +4×10−3 to −1×10−3. From the absorption data of material with small matching parameter we obtain the value of the interband matrix element ( P2=20.7 eV), the excitonic Rydberg (Ex =2.5 meV), and damping constant (Γ0=5.1 meV) in the temperature range from 1.5 to 340 K. From the temperature dependent band-gap shrinkage and exciton damping constant Γ, information on the carrier-phonon interaction is obtained. The effect of the biaxial stress in the epitaxial layers caused by the mismatch with the substrate is demonstrated by absorption spectra which directly reveal the valence band splitting due to stress. Absorption measurements on samples with and without substrate indicate that the strained expitaxial layers do not relax completely if the substrate is etched away. The remaining strain field is probably caused by misfit dislocations generated during the epitaxial growth. Taking into account these stress effects, a precise value of the band gap as a function of temperature is derived. At zero temperature, we obtain a value of Eg(0) =821.5±0.2 meV for the band gap of In0.53Ga0.47As. From the absorption spectra we further determine the value of the bimolecular recombination coefficient (B=0.96×10−10 cm3/s at room temperature). The comparison of material grown by molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE) shows that there is no difference between significant optical data of LPE and MBE high-quality layers.