bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical formulation for electron transfer coupled to multiple protons: Application to amidinium–carboxylate interfaces
Authors: Rostov Ivan , Hammes-Schiffer Sharon
Published: July 2002

This paper presents a theoretical formulation for electron transfer coupled to the motion of multiple protons. This theory is applied to proton-coupled electron transfer (PCET) through amidinium–carboxylate salt bridges, where the electron transfer reaction is coupled to the motion of two protons at the proton transfer interface. The rate for the donor–(amidinium–carboxylate)–acceptor system is found to be substantially slower than the rate for the switched interface donor–(carboxylate–amidinium)–acceptor system. This trend is consistent with experimental data for photoinduced PCET in analogous systems. The calculations indicate that this difference in rates is due mainly to the opposite dipole moments at the proton transfer interfaces for the two systems, leading to an endothermic reaction for the donor–(amidinium–carboxylate)–acceptor system and an exothermic reaction for the donor–(carboxylate–amidinium)–acceptor system. The deuterium kinetic isotope effects are found to be moderate (i.e., kH/kD<3) for both types of systems. These moderate kinetic isotope effects are due to the dominance of vibrationally excited product states, leading to significant overlap between the reactant and product proton vibrational wave functions.
Additive noise in noise-induced nonequilibrium transitions
Authors: Zaikin A. , Kurths J.
Published: July 2002

We study different nonlinear systems which possess noise-induced nonequlibrium transitions and shed light on the role of additive noise in these effects. We find that the influence of additive noise can be very nontrivial: it can induce first- and second-order phase transitions, can change properties of on–off intermittency, or stabilize oscillations. For the Swift–Hohenberg coupling, that is a paradigm in the study of pattern formation, we show that additive noise can cause the formation of ordered spatial patterns in distributed systems. We show also the effect of doubly stochastic resonance, which differs from stochastic resonance, because the influence of noise is twofold: multiplicative noise and coupling induce a bistability of a system, and additive noise changes a response of this noise-induced structure to the periodic driving. Despite the close similarity, we point out several important distinctions between conventional stochastic resonance and doubly stochastic resonance. Finally, we discuss open questions and possible experimental implementations.
Ultrathin fluorinated silicon nitride gate dielectric films formed by remote plasma enhanced chemical vapor deposition employing NH3 and SiF4
Authors: Ohta Hiroyuki , Hori Masaru , Goto Toshio
Published: July 2002

Ultrathin fluorinated silicon nitride (SiNx) films of 4 nm in thickness were formed on a Si substrate at 350 °C in the downflow of electron cyclotron resonance plasma-enhanced chemical vapor deposition employing ammonia and tetrafluorosilane (NH3/SiF4) gases. Ultrathin fluorinated SiNx film was evaluated for use as a gate dielectric film. The observed properties indicated an extremely low leakage current, one order of magnitude lower than thermal SiO2 of identical equivalent oxide thickness, as well as an excellent hysteresis loop (20 mV) and interface trap density (Dit=4×1011 cm−2) in the capacitance–voltage characteristics. The film structures and the surface reactions for the fluorinated SiNx film formation were examined via in situ x-ray photoelectron spectroscopy. in situ Fourier-transform infrared reflection absorption spectroscopy, in situ atomic force microscopy, and thermal desorption mass spectroscopy. The control of the fluorine concentration in the SiNx films was found to be a key factor in the formation of fluorinated SiNx films of high quality at low temperatures. Fluorinated SiNx is the effective material for application in ultrathin gate dielectric film in ultralarge-scale integrated circuits.
Translation-covariant Markovian master equation for a test particle in a quantum fluid
Authors: Vacchini Bassano
Published: July 2002

A recently proposed master equation in the Lindblad form is studied with respect to covariance properties and existence of a stationary solution. The master equation describes the interaction of a test particle with a quantum fluid, the so-called Rayleigh gas, and is characterized by the appearance of a two-point correlation function known as the dynamic structure factor, which reflects symmetry and statistical mechanics properties of the fluid. In the case of a free gas, all relevant physical parameters such as fugacity, ratio between the masses, momentum transfer, and energy transfer are put into evidence, giving an exact expansion of the dynamic structure factor. The limit in which these quantities are small is then considered. In particular, in the Brownian limit a Fokker–Planck equation is obtained in which the corrections due to quantum statistics can be explicitly evaluated and are given in terms of the Bose function g0(z) and the Fermi function f0(z).
Electrical and optical properties of TiO2 in accumulation and of lithium titanate Li0.5TiO2
Authors: van de Krol Roel , Goossens Albert , Meulenkamp Eric A.
Published: July 2002

Changes in the optical absorption and electrical conductivity of dense and mesoporous anatase TiO2 films were measured in situ as a function of electrode potential during electrochemical lithium intercalation. A special two-electrode geometry was used for the conductivity measurements, in which the contacts were separated by a small gap bridged by the TiO2. When electrons are injected, an accumulation layer is formed and the conductivity increases several orders of magnitude. A monotonic increase of the optical absorption with wavelength confirms the presence of (partially) delocalized electrons. Insertion of lithium ions results in the formation of the Li0.5TiO2 phase and a decrease of the overall conductance. The specific conductivity of the Li0.5TiO2 phase is (9.1±0.2) S/cm, significantly lower than that of Li-doped anatase TiO2. This is corroborated by the absorption spectrum of Li0.5TiO2, which shows two pronounced peaks around 440 and 725 nm and no characteristic free-electron features. At potentials below 1.7 V vs Li additional lithium could be inserted into Li0.5TiO2, and the conductance was found to increase again. The results are discussed in terms of possible changes in the electronic structure that are induced by the insertion of lithium ions.
Hybrid and particle simulations of an interface expansion and of collisionless shock: A comparative and quantitative study
Authors: Lembege B. , Simonet F.
Published: July 2002

The release of a hot and dense drifted plasma into a background ambient magnetoplasma is analyzed with the help of one-dimensional hybrid and full particle electromagnetic simulations, in conditions approaching those commonly met in laboratory experiments. The overall study is mainly focused on the early times of the interaction between both plasmas and for a strictly perpendicular expansion in the magnetic field. Two successive regimes are evidenced: a “transitory” regime, where a characteristic interface builds up, followed by a second regime, where a shock forms. Our purpose in this study is to analyze in detail the differences inherent to each code, and to quantify their impact on the corresponding simulation results. Several difficulties are identified throughout this comparison, since different assumptions are used within each code; a method based both on the best time/space scale lengths fit and on a judicious choice of plasma and numerical parameters is proposed in order to overcome these difficulties. The present results and a similar comparative method may be easily extended when analyzing the dynamics of the interface expansion building up in laboratory plasmas (during experiments of laser–target interactions), in space plasma (active experiments), and when applied to open questions related to collisionless shocks.
In situ multiphase fluid experiments in hydrothermal carbon nanotubes
Authors: Gogotsi Yury , Libera Joseph A. , Güvenç-Yazicioglu Almila , Megaridis Constantine M.
Published: July 2002

Hydrothermal multiwall closed carbon nanotubes are shown to contain an encapsulated multiphase aqueous fluid, thus offering an attractive test platform for unique in situ nanofluidic experiments in the vacuum of a transmission electron microscope. The excellent wettability of the graphitic inner tube walls by the aqueous liquid and the mobility of this liquid in the nanotube channels are observed. Complex interface dynamic behavior is induced by means of electron irradiation. Strong atomic-scale interactions between the entrapped liquid phase and the wetted terminated graphite layers are revealed by means of high-resolution electron microscopy. The documented phenomena in this study demonstrate the potential of implementing such tubes in future nanofluidic devices.
Semiclassical modeling of infrared pressure-broadened linewidths: A comparative analysis in CO2–Ar at various temperatures
Authors: Buldyreva J. , Chrysos M.
Published: July 2002

A novel semiclassical approach, which makes use of the exact trajectory implemented within the Robert–Bonamy formalism, is employed for modeling infrared pressure-broadened linewidths. As a prototype, the carbon dioxide molecule perturbed by argon is examined in the temperature range 160–760 K, for which various measurements and computations are available. For a meaningful comparison with previous theoretical works done with both semiclassical and quantum approaches, the ab initio intermolecular potential surface of Parker et al. [J. Chem. Phys. 64, 1668 (1976)] is used. Our values are found to be in agreement with up-to-date experimental data at all temperatures studied.
Nonlinear transport properties in multiwall carbon nanotube heterojunctions
Authors: Lee Jeong-O , Oh Hwangyou , Kim Jae-Ryoung , Kang Kicheon , Kim Ju-Jin , Kim Jinhee , Yoo Kyung-Hwa
Published: July 2002

Electronic transport properties of a heterostructure junction made of two different multiwall carbon nanotubes are studied. Independent measurement of the current–voltage characteristics of each nanotube revealed that both of them were lightly p-doped semiconductors with energy gaps whose magnitudes are widely different from each other. The current–voltage characteristics measured across the heterojunction show reproducible rectifying diode behavior. The forward bias current across the heterojunction increases rapidly with the application of a positive gate bias voltage, implying an n-type gate response.
Single-crystal aluminum nitride nanomechanical resonators
Authors: Cleland A. N. , Pophristic M. , Ferguson I.
Published: July 2002

Aluminum nitride is a light, stiff, piezoelectrically active material that can be epitaxially grown on single-crystal Si. AlN is beginning to play a role in the integration of semiconducting electronic and surface acoustic wave devices, and may prove useful for the integration of other types of mechanical devices as well. We describe the growth and subsequent electron-beam patterning and etching of epitaxial AlN-on-silicon films into nanomechanical flexural resonators. We have measured resonators with fundamental mechanical resonance frequencies above 80 MHz, and quality factors in excess of 20 000.
Model for superconductivity in optimally doped electron cuprates
Authors: Geballe T. H. , Moyzhes B. Y.
Published: July 2002

The very anomalous normal state properties and superconductivity of electron-doped superconducting cuprates with the T* structure are discussed. The high values of the electron and hole mobilities in the literature are explained by the electrons and holes existing in different layers. The annealing procedures necessary for superconductivity produce oxygen-vacancy negative-U pairing centers. The relative insensitivity of Tc to the annealing condition is attributed to stabilization of the chemical potential near the middle of the narrow oxygen band formed by oxygen vacancies, where the electronic entropy is large.
High-frequency mechanical spectroscopy with an atomic force microscope
Authors: Dupas E. , Gremaud G. , Kulik A. , Loubet J.-L.
Published: July 2002

In this article we further develop local mechanical spectroscopy and extend the frequency range over which it can be used. Using a heterodyne method to measure the deflection of the cantilever enables one to measure the probe vibration at any frequency. Since the detection sensitivity of force gradients follows a f2 dependence, extending the frequency range from 1 to more than 5 MHz increases the sensitivity by over an order of magnitude. This setup is combined with a realistic model of the cantilever taking into account the geometry of the cantilever. The model is presented and discussed, and compared with experimental behavior measured on WC–Co and NiTi–epoxy samples. Experimental moduli of 730±50 and 260±40 GPa are obtained for WC and Co, respectively.
Improved evolution equations for magnetic island chains in toroidal pinch plasmas subject to externally applied resonant magnetic perturbations
Authors: Fitzpatrick Richard , Rossi Enrico , Yu Edmund P.
Published: July 2002

An improved set of island evolution equations is derived that incorporates the latest advances in MHD (magnetohydrodynamical) theory. These equations describe the resistive/viscous-MHD dynamics of a nonlinear magnetic island chain, embedded in a toroidal pinch plasma, in the presence of a programmable, externally applied, resonant magnetic perturbation. A number of interesting example calculations are performed using the new equations. In particular, an investigation is made of a recently discovered class of multiharmonic resonant magnetic perturbations that have the novel property that they can lock resonant island chains in a stabilizing phase.
Fiber optic catalytic probe for weakly ionized oxygen plasma characterization
Authors: Babič D. , Poberaj I. , Mozetič M.
Published: July 2002

Construction and operation of a novel fiber optic catalytic probe is presented. The probe is intended for measurements of atomic oxygen density in plasma postglows. The operation of the probe is based on a catalytic recombination of oxygen atoms and remote temperature sensing via optical fiber. Compared to the classical catalytic probes, the new approach results in smaller dimensions, better sensitivity, and higher immunity to the electromagnetic interference. Comparative measurements of atomic oxygen density performed simultaneously with both types of probes demonstrated a superior performance of the novel probes.
Tris(dibenzoylmethane)(monophenanthroline)europium(III) based red emitting organic light emitting diodes
Authors: Heil H. , Steiger J. , Schmechel R. , von Seggern H.
Published: July 2002

The concept of a blue light emitting diode containing a hole blocking layer (HBL) is generalized to efficiently excite dye molecules introduced in the form of an emission layer on the hole transporting side. The concept allows the excitation of an emitting molecule even if the molecule does not act primarily as a recombination center. On the basis of this concept pure red emitting organic light emitting diodes were produced utilizing tris(dibenzoylmethane)(monophenanthroline)europium(III) [Eu(dbm)3phen] as the emitting layer in an Al/LiF/Aluminum tris(8-hydroxyquinoline) (Alq3)/HBL/Eu(dbm)3phen/N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD)/copper-phthalo- cyanine (CuPc)/indium tin oxide structure. Additionally, the luminance output of such a device was improved by partially adding a hole transport material to the emitting layer in order to increase the penetration of holes into the emission region. A maximum luminance output of 200 cd/m2 at 15 V was achieved without optimizing the layer thickness. Furthermore, the effect of saturation of the Eu3+ emission is studied and discussed. The basic device architecture presented here is expected to work with other transport and emitting materials also.
Lifetime and decomposition pathways of a chemically bound helium compound
Authors: Chaban Galina M. , Lundell Jan , Gerber R. Benny
Published: July 2002

HHeF, a first predicted chemically-bound helium compound, is a metastable species that disintegrates by tunneling through energy barriers into He+HF and H+He+F. The reaction paths for these decomposition processes are calculated with single-configurational Møller-Plesset (MP2) and multiconfigurational quasidegenerate MCQDPT2/MCSCF(10,6) electronic structure methods. The lifetime of HHeF, estimated using a one-dimensional model along the minimum energy path and the semiclassical WKB approximation, is more than 120 ps, that of DHeF is 14 ns. The relatively long lifetimes are encouraging for the preparation prospects of this helium compound.
High-current field emission from a vertically aligned carbon nanotube field emitter array
Authors: Thong J. T. L. , Oon C. H. , Eng W. K. , Zhang W. D. , Gan L. M.
Published: July 2002

Arrays of vertically aligned carbon nanotubes (CNTs) were grown on a patterned sputtered cobalt film by chemical vapor deposition from ethylenediamine at 900 °C. Each square array comprises a moderate density of nanotubes with an average height of 90 μm covering a total area of 3.6×10−3 cm2. Field emission measurements were carried out on individual arrays at pressures below 10−8 mbar. The spacing between the anode and the top of the CNT array is 935 μm and a total current of 2 mA could be obtained at 2.5 kV. A Fowler–Nordheim plot of the I–V data shows an unusually high field enhancement factor at lower fields. At an average field strength of 1.925 V/μm, the corresponding emission current density is 130 mA/cm2. This emission current was found to be very stable, with short-term fluctuations (5 Hz measurement bandwidth) of no more than ±1.5%, while the current drifted less than 1.5% over a test period of 20 h.
Orientational order study of monolayers at the air–water interface by Maxwell-displacement current and optical second harmonic generation
Authors: Tojima Atsushi , Manaka Takaaki , Iwamoto Mitsumasa
Published: July 2002

The dielectric polarization of organic monolayers at the air–water interface has been analyzed, assuming that the monolayers have C∞ symmetry. The formula of polarization of organic monolayers is derived and it is expressed using orientational order parameters. It is shown that Maxwell-displacement current measurement coupled with optical second harmonic generation measurement is helpful for the determination of these orientational order parameters. Monolayers of 4′-n-pentyl-4-cyanobiphenyl on the air–water interface were examined during monolayer compression and the orientational order parameters were determined.
Coupled-cluster theory, pseudo-Jahn–Teller effects and conical intersections
Authors: Stanton John F.
Published: July 2002

A detailed analysis of the strengths and weaknesses of coupled-cluster and many-body perturbation theories in treating strongly interacting potential energy surfaces is presented. Standard coupled cluster theory is shown to provide a qualitative treatment of Herzberg–Teller coupling that is vastly superior to that associated with perturbation theory. However, it also predicts unphysical effects that will always cause it to fail in describing the topology of potential energy surfaces in the immediate vicinity of conical intersections. To treat problems involving strong interstate coupling (notably those involving radicals subject to pseudo-Jahn–Teller effects), methods based on equation-of-motion (linear response) coupled-cluster theory appear to be considerably more suitable. In particular, they provide a description of intersecting surfaces that is qualitatively correct in all respects. It is also shown that there is no reason to believe that the noniterative inclusion of triple excitation contributions to the correlation energy should provide for any systematic improvement in describing this class of phenomena.
Electron pulse broadening due to space charge effects in a photoelectron gun for electron diffraction and streak camera systems
Authors: Qian Bao-Liang , Elsayed-Ali Hani E.
Published: July 2002

The electron pulse broadening and energy spread, caused by space charge effects, in a photoelectron gun are studied analytically using a fluid model. The model is applicable in both the photocathode-to-mesh region and the postanode electron drift region. It is found that space charge effects in the photocathode-to-mesh region are generally unimportant even for subpicosecond pulses. However, because of the long drift distance, electron pulse broadening due to space charge effects in the drift region is usually significant and could be much larger than the initial electron pulse duration for a subpicosecond electron pulse. Space charge effects can also lead to a considerable electron energy spread in the drift region. Temporal broadening is calculated for an initial electron pulse as short as 50 fs with different electron densities, final electron energies, and drift distances. The results can be used to design electron guns producing subpicosecond pulses for streak cameras as well as for time resolved electron diffraction.
Suppression of leakage current in Schottky barrier metal–oxide–semiconductor field-effect transistors
Authors: Calvet L. E. , Luebben H. , Reed M. A. , Wang C. , Snyder J. P. , Tucker J. R.
Published: July 2002

In this article we investigate the subthreshold behavior of PtSi source/drain Schottky barrier metal–oxide–semiconductor field-effect transistors. We demonstrate very large on/off ratios on bulk silicon devices and show that slight process variations can result in anomalous leakage paths that degrade the subthreshold swing and complicate investigations of device scaling.
Bright white organic light-emitting diode
Authors: Ko C. W. , Tao Y. T.
Published: July 2002

A very bright white organic light-emitting diode (OLED) was realized by using a bright blue-emitting layer, 1,7-diphenyl-4-biphenyl-3,5-dimethyl-1,7-dihydrodipyrazolo[3,4-b;4′,3′-e]pyridine (PAP-Ph), together with a 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyry)-4H-pyran (DCM)-doped Alq [tris(8-hydroxyquinolinato) aluminum (III)] layer to provide blue, red, and green emission for color mixing. By appropriately controlling the layer thickness, the white light OLED achieved good performance of 24 700 cd/m2 at 15 V, 1.93 lm/W at 6.5 V, and >300 cd/m2 at 7.7 mA/cm2. The Commission Internationale de l’Eclairage coordinates of the emitted light are quite stable at voltages from 6 to 12 V, ranging from (0.35, 0.34) to (0.34, 0.35).
Nonreciprocal phase shift of TE modes induced by a compensation wall in a magneto–optic rib waveguide
Authors: Wilkens L. , Träger D. , Dötsch H. , Popkov A. F. , Alekseev A. M.
Published: July 2002

Garnet films of composition (Gd, Bi)3(Fe, Ga)5O12 are grown by liquid–phase epitaxy on (111) oriented substrates of gadolinium gallium garnet. The films are close to compensation. Using laser annealing straight vertical compensation walls (CW) are generated. Monomode rib waveguides are fabricated by ion beam etching so that a CW is located close to a rib center. The CW induces a nonreciprocal phase shift of the TE0 mode. The dependence of this phase shift on the position of the CW within the rib is calculated. Experimentally a phase shift of 0.7 cm−1 at 1.3 μm wavelength is observed. The deviation from the theoretical value is discussed. Such waveguides are essential to realize polarization–independent optical isolators.
Ultrafast photodissociation dynamics and energetics of the electronically excited H atom transfer state of the ammonia dimer and trimer
Authors: Farmanara P. , Ritze H.-H. , Stert V. , Radloff W. , Hertel I. V.
Published: July 2002

The energetics and ultrafast dynamics in the H atom transfer configuration of ammonia dimer and trimer clusters have been studied. The clusters are first excited to the electronic Ã state with a 208 nm femtosecond laser pump pulse. This state is allowed to relax for about 1 ps during which the H-transfer state is formed which is then electronically excited by a time-delayed infrared control pulse at 832 nm and finally ionized with a third femtosecond probe pulse at 416 nm. We have also performed complementary theoretical studies elucidating the experimental findings. For the dimer in the excited NH4(3p)⋯NH2(X̃) state the time-dependent ion signals reveal an isotope-independent short lifetime of about τ6=(130±60) fs which can be explained by a curve crossing with the repulsive NH4(3s)⋯NH2(Ã) state, whereas the trimer signal persists on a time scale being more than one order of magnitude longer and exhibits a very large isotope effect. This is interpreted as being due to internal conversion from the excited state NH3NH4(3p)⋯NH2(X̃) back to the NH3NH4(3s)⋯NH2(X̃) ground state. The analysis of the corresponding photoelectron spectra also confirms the transition energies between the electronic states involved, e.g., ΔE[NH4(3s→3p)⋯NH2]=1.5 eV and ΔE[NH3NH4(3s→3p)⋯NH2]=1.2 eV, as determined by our ab initio calculations.
Hydrogen bonding and protonation in acid–base complexes: Methanesulfonic acid-pyridine
Authors: Lehtonen Olli , Hartikainen Juha , Rissanen Kari , Ikkala Olli , Pietilä Lars-Olof-
Published: July 2002

In order to study proton transfer and hydrogen bonding interactions in acid/base complexes, the equilibrium structure of a stoichiometric pyridine–methanesulfonic acid complex has been investigated using density functional theory with different functionals and compared with second-order Møller–Plesset perturbation theory and restricted Hartree–Fock calculations. The basis sets range from 6-31G to 6-31++G(d,p). Additionally, interaction energies and equilibrium geometries were calculated with 6-311++G(d,p) basis set, while aug-cc-pVDZ was used only with BLYP method. In the gas phase, the optimized geometry shows hydrogen bonding without proton transfer between the molecules. The polarized continuum model suggests that embedding the complex in a dielectric medium enhances the proton transfer. The role of added water was studied explicitly by optimizing the geometry in the presence of 1 or 2 water molecules. In such cases the proton is observed to transfer. The crystalline structure of the pyridine–methanesulfonic complex was also determined experimentally using x-ray diffraction, showing that proton transfer occurs. In this case, also geometry change which has been induced by the crystal packing may play a role. In conclusion, both the dielectric medium and the configuration of the complex have a pronounced effect on the potential energy surface experienced by the hydrogen in the bond between an aromatic base and a strong acid.
Deep multilayer gratings with adjustable bandpass for spectroscopy and monochromatization
Authors: Martynov V. V. , Platonov Yu.
Published: July 2002

We propose here a deep multilayer grating with an adjustable bandpass as an alternative dispersive element for use in x-ray spectroscopy and monochromatization. When making a multilayer grating, we remove part of the reflecting planes of a multilayer, allowing radiation to penetrate deeper. Since the resolution, or the bandpass, is reciprocally proportional to the penetration depth, we improve the resolution and reduce the bandpass of a simple multilayer mirror without affecting the reflectivity. By changing the grating land/period ratio, the amount of the removed material can be continuously varied and, thus, the extinction depth and the bandpass, or resolution, can be continuously varied. In this way we can find an optimum between required resolution and flux for a particular application.
Nonlinear optical response and yield in the femtosecond photodesorption of CO from the Cu(001) surface: A density matrix treatment
Authors: Micha D. A. , Santana A. , Salam A.
Published: July 2002

The dynamics of molecular photodesorption from a metal surface is described by a density matrix theory of the nonlinear optical response resulting from the interaction of a femtosecond pulsed laser with a metal surface. The extended system is divided into a primary region comprising the adsorbate species and the bonding substrate atoms and a secondary region consisting of the remaining substrate, that interact strongly and self-consistently with each other through an electric dipole-electric dipole coupling. The formalism uses the Liouville–von Neumann equation, with an effective Hamiltonian which includes the effects of energy dissipation into the metal. The nonlinear response of the substrate is studied by solving the optical Bloch equations with relaxation terms to account for the effects of energy dissipation, coupled to kinetics equations describing the excitation of the electron Fermi sea by the light pulse. A primary effective hamiltonian with a nonlinear dependence on the electric field strength of the laser is obtained as a result. The theory is applied to the CO/Cu(001) adsorbate–substrate complex and the nonlinear photodesorption yield of CO versus pulse fluence is evaluated through model calculations. The local electric field at the adsorbate, and the yields for several fluence values are obtained as functions of the desorption time.
In-plane bandgap control in porous GaN through electroless wet chemical etching
Authors: Li Xiuling , Kim Young-Woon , Bohn Paul W. , Adesida Ilesanmi
Published: July 2002

Nanoporous GaN structures were formed from crystalline GaN on conducting SiC substrate using metal-assisted electroless etching in HF/H2O2. Morphology varies as a function of etch time and solution parameters. The resulting porous GaN (PGaN) displays cathodoluminescence (CL) with two bands blue-shifted from the bulk bandgap energy by 103 meV and 352 meV, respectively. Appearance of the blueshifted emission is correlated with the development of highly anisotropic wire-like structures in the morphology, with the higher energy CL band arising from the portion of the structure with the smallest feature sizes. These observations suggest that the blueshifted emission arises from quantum confinement effects. CL imaging indicates that the blueshifted emission is spatially segregated from the band gap emission. Variations in morphology and light emission properties apparent between Pt-coated and uncoated areas likely arise from hole transport and access of solution reagents to the GaN interface.
Potential fluctuations in metal–oxide–semiconductor field-effect transistors generated by random impurities in the depletion layer
Authors: Slavcheva G. , Davies J. H. , Brown A. R. , Asenov A.
Published: July 2002

We have studied the magnitude and length scale of potential fluctuations in the channel of metal–oxide–semiconductor field-effect transistors due to the random positions of ionized impurities in the depletion layer. These fluctuations effect the threshold voltage of deep submicron devices, impede their integration, and reduce yield and reliability. Our simple, analytic results complement numerical, atomistic simulations. The calculations are based on a model introduced by Brews to study fluctuations due to charges in the oxide. We find a typical standard deviation of 70 mV in the potential below threshold, where the channel is empty, falling to 40 mV above threshold due to screening by carriers in the channel. These figures can be reduced by a lightly doped epitaxial layer of a few nm thickness. The correlation function decays exponentially in an empty channel with a length scale of 9 nm, which screening by carriers reduces to about 5 nm. These calculations of the random potential provide a guide to fluctuations of the threshold voltage between devices because the length of the critical region in a well-scaled transistor near threshold is comparable to the correlation length of the fluctuations. The results agree reasonably well with atomistic simulations but detailed comparison is difficult because half of the total standard deviation comes from impurities within 1 nm of the silicon–oxide interface, which is a single layer of the grid used in the simulations.
How ions distribute in a drying porous medium: A simple model
Authors: Huinink H. P. , Pel L. , Michels M. A. J.
Published: July 2002

Salt crystallization at surfaces is an important problem for buildings and monuments. We do not consider the formation of salt crystals as such, but focus on transport properties of ions in a drying porous medium. We deal with the first phase of the drying process, where the water is still uniformly distributed throughout the medium. An approximate model is presented, which accounts for both convection and diffusion. It is shown that the key parameter is the Peclet number at the evaporating surface, Pe≡hL/εD, where h, L, ε, and D are the drying rate, sample size, porosity, and diffusion constant, respectively. When Pe≪1 (diffusion dominates over convection) the ions remain uniformly distributed throughout the system. Strong accumulation at the evaporating surface occurs for Pe≫1 (convection dominates over diffusion). Crossover behavior is found for Pe≈1. Therefore, it is likely that the first crystals will be formed both in the bulk and at the interfaces of the material when Pe≪1. For high values of Pe the density peak at the evaporating surface will reach the saturation concentration long before it is reached in the bulk of the material. As a consequence, the salt starts to crystallize at the interfaces.
Permanent magnet array for the magnetic refrigerator
Authors: Lee S. J. , Kenkel J. M. , Pecharsky V. K. , Jiles D. C.
Published: July 2002

Recent research into the development of magnetic refrigeration (MR) operating at room temperature has shown that it can provide a reliable, energy-efficient cooling system. To enhance the cooling power of the magnetic refrigerator, it is required to use a magnetic refrigerant material with large magnetocaloric effect (MCE) at the appropriate temperature. Most advanced magnetic refrigerant materials show largest MCE at high applied magnetic fields generated by a superconducting magnet. For application of MCE to air conditioners or household refrigerators, it is essential to develop a permanent magnet array to form a compact, strong, and energy-efficient magnetic field generator. Generating a magnetic field well above the remanence of a permanent magnet material is hard to achieve through conventional designs. A permanent magnet array based on a hollow cylindrical flux source is found to provide an appropriate geometry and magnetic field strength for MR applications.
High-gain coupled InGaAs quantum well InAs quantum dot AlGaAs–GaAs–InGaAs–InAs heterostructure diode laser operation
Authors: Walter G. , Chung T. , Holonyak N.
Published: July 2002

Data are presented showing that a single-layer InAs quantum dot (QD) laser in the AlGaAs–GaAs–InGaAs–InAs heterostructure system is improved in gain and continuous wave (cw) room temperature operation by coupling, via tunneling, auxiliary strained-layer InGaAs quantum wells (QWs) to the single InAs QD layer to assist carrier collection and thermalization. A QW-assisted single-layer InAs QD laser, a QD+QW laser, is demonstrated that operates cw (300 K), and at diode length 150 μm in pulsed operation exhibits gain as high as ∼100 cm−1.
Domain structure in magnetic dots prepared by nanoimprint and e-beam lithography
Authors: Moritz J. , Dieny B. , Nozières J. P. , Landis S. , Lebib A. , Chen Y.
Published: July 2002

Square arrays of silicon dots with feature sizes from 30 to 400 nm and periodicities from 60 to 500 nm were obtained by standard lithography or nanoimprint and reactive ion etching. Co/Pt multilayers with perpendicular anisotropy were subsequently deposited by magnetron sputtering onto these prepatterned wafers. The deposition was performed either at normal or oblique incidence. The magnetic domain structures were characterized by magnetic force microscopy after zero field cooling (ZFC) from the (Co/Pt) Curie temperature or by alternative demagnetization (AD). It appears that the angle of deposition of the Pt influences the existence of direct exchange coupling between neighboring dots due to the formation of a magnetic layer on the sidewalls of the dots.
Study of stacking faults in Co-alloy perpendicular media
Authors: Lu Bin , Klemmer Timothy , Wierman Kurt , Ju Ganping , Weller Dieter , Roy Anup G. , Laughlin David E. , Chang Chunghee , Ranjan Rajiv
Published: July 2002

Stacking faults (SFs) and fcc-phased grains are studied by electron diffraction in perpendicular media. Co-alloy compositions, such as CoCrPt, CoCrPtB, are compared on different underlayers and deposition conditions. It is found that CoCrPtB alloys tend to have greater amount of SFs and fcc grains than the CoCrPt alloy. Underlayers as well as low deposition rate and base pressure help to reduce the amount of SFs and fcc phase.
Anomalous switching behavior of antiparallel-coupled Co layers separated by a super thin Ru spacer
Authors: Gornakov V. S. , Nikitenko V. I. , Egelhoff W. F. , McMichael R. D. , Shapiro A. J. , Shull R. D.
Published: July 2002

The details of the magnetization reversal in coupled ferromagnetic Co/Ru/Co trilayers deposited on obliquely sputtered Ta underlayers were studied using the magneto-optical indicator film technique. The ground states of the sandwich are characterized by noncollinear magnetization orientations in the two Co layers, which can be remagnetized by the motion of 180° and non-180° domain walls. In the latter case, the angle between the magnetization vectors in the adjacent domains was revealed to be about ±100°, and an anomalous magnetization reversal was observed. The canted magnetization states and their mutual transformations are discussed in terms of the competition between ferromagnetic coupling through pinholes and antiferromagnetic coupling across the Ru layer.
A simulation of streaming flows associated with acoustic levitators
Authors: Rednikov A. , Riley N.
Published: July 2002

Steady-state acoustic streaming flow patterns have been observed by Trinh and Robey [Phys. Fluids 6, 3567 (1994)], during the operation of a variety of single axis ultrasonic levitators in a gaseous environment. Microstreaming around levitated samples is superimposed on the streaming flow which is observed in the levitator even in the absence of any particle therein. In this paper, by physical arguments, numerical and analytical simulations we provide entirely satisfactory interpretations of the observed flow patterns in both isothermal and nonisothermal situations.
Characterization of a high-intensity bipolar-mode pulsed ion source for surface modification of materials
Authors: Zhu X. P. , Lei M. K. , Ma T. C.
Published: July 2002

A high-intensity pulsed ion source of TEMP-type series, operating in bipolar mode, has been developed as a unique pulsed energy source to produce a high-intensity pulsed ion beam (HIPIB) for surface modification of materials. To generate the ion beam, a specially shaped bipolar pulse, consisting of a first negative pulse and a second delayed positive pulse both of nanosecond width, is formed by a double coaxial pulse-forming line (PFL) powered with a Marx generator and supplied to a magnetically insulated ion diode (MID) by a self-magnetic field. It is found that the efficient generation of a HIPIB is mainly dependent on the delay time of the bipolar pulse, adjusted by pressure ratio in the two gas switches of a PFL, and the anode–cathode (A–K) gap distance in the self-magnetic field MID. The delay time determines the effective area on the anode surface for plasma generation and the A–K gap distance ensures the stability of the process. A proper delay time and a proper A–K gap distance are obtained by a series of experimental investigations. Under delay time from 30 to 280 ns and several different A–K gap distances, the typical wave forms of the bipolar pulses at a dc charging voltage of 45 kV to Marx generator are illustrated to clarify the effects of delay time and A–K gap distance on the ion beam generation. The proper A–K gap distance is not uniform, varied from 6 to 8 mm, and the corresponding proper delay time is 250 ns. The most efficient plasma generation leads to a maximum output of HIPIB with a peak ion current density of 350 A cm−2 and a beam pulse width of 75 ns (full width at half maximum), at an accelerating pulse of 220 kV with a pulse width of 100 ns.
Interaction of a beam of fast electrons with solids
Authors: Tikhonchuk V. T.
Published: July 2002

The interaction of a high current relativistic electron beam with solid targets is analyzed. The conditions of the beam separation from the target are presented and the energy losses due to collective effects are evaluated for the cases of metals and dielectrics. The energy losses in dielectrics are higher because of the ionization of atoms in the beam self-consistent electric field. The problem of ion acceleration at the solid-vacuum interface is also considered.
Laser shock forming on coated metal sheets characterized by ultrahigh-strain-rate plastic deformation
Authors: Zhou Ming , Zhang Yongkang , Cai Lan
Published: July 2002

We report on a dynamic ultrahigh-strain-rate forming method driven by laser impact. This investigation reveals that laser shock forming is a mechanical, not a thermal process, and the strain rate can go up to 107–109 s−1, two or more orders higher than that of all the existing forming methods. By investigating the hardness and residual stress of the surfaces, we conclude that laser shock forming is a technique with combining laser shock strengthening and metal forming, which introduces strain hardening and compressive residual stresses. We also discover some nonlinear plastic deformation characters in laser shock forming.
Anion reactions in silane plasma
Authors: Gallagher Alan , Howling A. A. , Hollenstein Ch.
Published: July 2002

Previous measurements of SixHm− anions and electrons in the afterglow of silane discharges are analyzed here. A model of plasma anion chemistry includes two sources of anions, two possible causes of x→x+1 anion growth, and anion loss by mutual neutralization. Comparison to pulsed-discharge measurements for x=1–6, for a range of discharge times, clearly establishes radical reactions as the primary cause of anion growth; prior estimates generally suggested silane reactions. This comparison also indicates that electron attachment to radicals, not silane, is the largest source of SiHm− anions. Measured afterglow electron decay and SiHm− signals are also consistent with efficient, low energy electron attachment to radicals. Reasonable agreement with the observations is obtained using realistic estimates of the radical density, cation density, and the values of radical-anion (kx) and mutual neutralization rate coefficients. However, the x dependence of the kx required to fit the data is surprising. These conclusions have major consequences for silane-discharge, particle-nucleation models.
1-GHz-repetition-rate femtosecond optical parametric oscillator
Authors: Zhang X. P. , Hebling J. , Bartels A. , Nau D. , Kuhl J. , Rühle W. W. , Giessen H.
Published: July 2002

We report a 1-GHz-repetition-rate, low-pump-threshold, all-solid-state femtosecond optical parametric oscillator (OPO) based on periodically poled LiNbO3 and pumped by a GHz repetition rate Ti:sapphire laser. The OPO provides nearly transform-limited 65-fs-signal pulses tunable from 1160 to 1320 nm. The pump threshold is about 580 mW, and 43 mW signal power is obtained with 950-mW-pump power. Higher-order quasi-phase-matched sum frequency generation and second harmonic generation processes produce 10 mW blue light at 486 nm and 15 mW red light at 620 nm.
Electrostatic degrees of freedom in non-Maxwellian plasma
Authors: Skiff F. , Gunell H. , Bhattacharjee A. , Ng C. S. , Noonan W. A.
Published: July 2002

Detailed measurements of the ion velocity distribution function are used to test representations of the electrostatic degrees of freedom of slightly non-Maxwellian plasmas. It is found that fluid theory does not describe the data very well because there exist multiple closely spaced kinetic electrostatic modes. New wave branches appear that theoretically should persist as weakly damped modes even with Te∼Ti. Both a sum over discrete dispersion relations and the Case–Van Kampen spectral representation can be used to provide working descriptions of the data, but the latter has certain advantages.
Vacancy-type defects in BaTiO3/SrTiO3 structures probed by monoenergetic positron beams
Authors: Uedono Akira , Shimoyama Kazuo , Kiyohara Masahiro , Chen Zhi Quan , Yamabe Kikuo , Ohdaira Toshiyuki , Suzuki Ryoichi , Mikado Tomohisa
Published: July 2002

Thin BaTiO3 films grown on SrTiO3 substrates were characterized by means of positron annihilation. The films were deposited by molecular-beam epitaxy without using oxygen source. We measured the Doppler broadening spectra of annihilation radiation and x-ray diffraction of the films and found that vacancy-type defects such as oxygen vacancies and other related defects caused lattice relaxation in the films. These defects disappeared after the films were annealed at 600 °C in an O2 atmosphere. Lattice relaxation in the films was also observed when the films were annealed at 1050 °C, but there was no direct relationship between the lattice relaxation that occurred at this temperature and vacancy-type defects. Vacancy-type defects were introduced into the SrTiO3 substrates by the growth of the BaTiO3 films. The species of these defects were identified as oxygen vacancies or defects related to oxygen vacancies, and they appeared as a result of diffusion of oxygen toward the BaTiO3 films. Almost all oxygen vacancies in the SrTiO3 substrates were annealed out at 500 °C.
Accurate molecular integrals and energies using combined plane wave and Gaussian basis sets in molecular electronic structure theory
Authors: Füsti-Molnar László , Pulay Peter
Published: July 2002

This paper introduces two developments for the application of plane wave basis sets for accurate molecular calculations. (1) An analytical formula is introduced for the momentum space representation of a Coulomb operator truncated to a finite range. Using this operator, interactions between the molecule and its periodic replicas can be exactly eliminated. Examples demonstrating the accuracy of our scheme are given. Calculations using a good-quality plane wave basis yield variational total SCF energies which are lower than those obtained with the cc-pvQZ basis for simple two-electron systems. (2) A new mixed-basis augmented plane wave all-electron method, the plane wave core Gaussian method has been developed which expands the valence part of the molecular orbitals in plane waves, and the corelike part in nonoverlapping compact Gaussians. Analytic equations have been derived for the necessary mixed Gaussian/plane wave electron repulsion integrals. Using such augmented basis set, we were able to reproduce the Gaussian-basis Hartree energies of small molecules to within a few μEh.
Visible emission from the vibrationally hot C2H radical following vacuum-ultraviolet photolysis of acetylene: Experiment and theory
Authors: Boyé S. , Campos A. , Douin S. , Fellows C. , Gauyacq D. , Shafizadeh N. , Halvick Ph. , Boggio-Pasqua M.
Published: July 2002

Photolysis of acetylene has been performed by vacuum-ultraviolet excitation with the synchrotron radiation via the Rydberg states converging to the first ionization potential (IP) at 11.4 eV. Only the visible fluorescence of the ethynyl radical was observed in the Ã 2Π–X̃ 2Σ+ system. Excitation of several Rydberg states of acetylene over a large energy range between 9 and 11.4 eV allowed us to observe for the first time the evolution of this continuum with increasing Rydberg excitation. Intensity calculations based on accurate ab initio potential energy surfaces of C2H were performed by using a one-dimensional model accounting for the large-amplitude motion of the H atom around the C–C bond and for the overall rotation of the radical. These calculations successfully reproduce the observed visible continuum (maximum at 500 nm and blue side cutoff at 400 nm) and bring new information on the distribution of the internal energy deposited in the fragment. For most excited Rydberg states, predissociation occurs in a rather low time scale, leaving the C2H fragment in the Ã state, vibrationally hot, mostly with significant excitation in the bending mode around the isomerization barrier.