bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-leakage current and high-breakdown voltage GaAs-based heterostructure field-effect transistor with In0.5(Al0.66Ga0.34)0.5P Schottky layer
Authors: Lin Y. S. , Hsu W. C. , Yang C. S.
Published: July 2002

An In0.5(Al0.66Ga0.34)0.5P/GaAs heterostructure field-effect transistor has been fabricated by metal-organic chemical vapor deposition. A turn-on voltage as high as 3.2 V along with an extremely low gate reverse leakage current of 69 μA/mm at VGD=−40 V are achieved. In addition, it is found that the device can be operated with gate voltage up to 1.5 V without significant drain current compression. These characteristics are attributed to the use of high Schottky barrier height, high band gap of In0.5(Al0.66Ga0.34)0.5P Schottky layer, and to the large conduction-band discontinuity at the In0.5(Al0.66Ga0.34)0.5P/GaAs heterojunction.
Recombination balance in green-light-emitting GaN/InGaN/AlGaN quantum wells
Authors: Eliseev Petr G. , Osin’ski Marek , Li Hua , Akimova Irina V.
Published: July 2002

Recombination balance parameters for GaN/InGaN/AlGaN single-quantum-well green-lightemitting diodes are extracted from optical power and carrier lifetime measurements. The radiative recombination coefficient B is found to depend on two-dimensional carrier density N, with a low-carrier-density limit of B0=1.2×10−4 cm2/s. Sublinearity of the light–current characteristic at temperatures ⩾300 K is associated with a nonradiative process whose rate is proportional to ∼N4.8. The external quantum efficiency of 5.5% at 20 mA results from the internal quantum yield of 63% and the photon extraction efficiency of 8.7%. At low temperatures, a nonradiative loss term proportional to ∼N9 is also identified.
Low-noise submillimeter-wave NbTiN superconducting tunnel junction mixers
Authors: Kawamura Jonathan , Chen Jian , Miller David , Kooi Jacob , Zmuidzinas Jonas , Bumble Bruce , LeDuc Henry G. , Stern Jeff A.
Published: July 2002

We have developed a low-noise 850 GHz superconductor–insulator–superconductor quasiparticle mixer with NbTiN thin-film microstrip tuning circuits and hybrid Nb/AlN/NbTiN tunnel junctions. The mixer uses a quasioptical configuration with a planar twin-slot antenna feeding a two-junction tuning circuit. At 798 GHz, we measured an uncorrected double-sideband receiver noise temperature of TRX=260 K at 4.2 K bath temperature. This mixer outperforms current Nb SIS mixers by a factor of nearly 2 near 800 GHz. The high-gap frequency and low loss at 800 GHz make NbTiN an attractive material with which to fabricate tuning circuits for SIS mixers. NbTiN mixers can potentially operate up to the gap frequency, 2Δ/h∼1.2 THz.
Magnetic configurations in exchange-biased double superlattices
Authors: te Velthuis S. G. E. , Felcher G. P. , Jiang J. S. , Inomata A. , Nelson C. S. , Berger A. , Bader S. D.
Published: July 2002

The layer-by-layer magnetization of a “double-superlattice” Fe/Cr(211) exchange-bias junction was determined by polarized neutron reflectometry. An n-layered [Fe/Cr]n antiferromagnetic (AF) superlattice is coupled with an m-layered [Fe/Cr]m ferromagnetic (F) superlattice, to provide a controlled exchange bias. In low magnetic fields, the magnetizations of the two superlattices are collinear. The two magnetized states (along or opposite to the bias field) differ only in the relative orientation of the F and adjacent AF layer. At higher fields, the AF moments flop to the direction perpendicular to the applied field. The structure, thus determined, explains the magnitude of the bias field.
Focused-ion-beam writing of electrical connections into platinum oxide films
Authors: Machalett F. , Edinger K. , Ye L. , Melngailis J. , Venkatesan T. , Diegel M. , Steenbeck K.
Published: July 2002

A focused Ga+ ion-beam (FIB) writing system has been used to directly change an insulating platinum oxide film into a conducting film with a dose in the order of 1014 Ga+/cm2 at 30 keV ion energy. The sheet resistance of a PtO2 film, which is prepared by magnetron sputtering, was reduced from 4×109 Ω/□ to approximately 5×102 Ω/□. Electron microprobe measurements indicate that oxygen loss in the irradiated regions causes the large decrease in resistivity. Scanning electron microscope pictures show that the film quality after ion irradiation is more homogeneous than after laser irradiation, which has been used to pattern these materials by thermal processes. Compared with laser patterning, the resolution of FIB patterning is more than one order of magnitude higher and is suitable for possible applications in nanotechnology.
Bi surfactant effects on ordering in GaInP grown by organometallic vapor-phase epitaxy
Authors: Jun S. W. , Fetzer C. M. , Lee R. T. , Shurtleff J. K. , Stringfellow G. B.
Published: July 2002

The effect of the isoelectronic surfactant Bi on surface structure and ordering has been studied for GaInP semiconductor alloys grown by organometallic vapor-phase epitaxy. A small amount of Bi (trimethylbismuth) added during growth is found to result in disordering for layers grown using conditions that would otherwise produce highly ordered materials. An order of magnitude increase in the step velocity was observed by atomic-force microscopy. Bi completely eliminates three-dimensional islands on the singular (001) surface.
Mid-infrared angled-grating distributed feedback laser
Authors: Bartolo R. E. , Bewley W. W. , Vurgaftman I. , Felix C. L. , Meyer J. R. , Yang M. J.
Published: July 2002

We report near-diffraction-limited output from an angled-grating distributed feedback type-II W laser emitting near 3.4 μm. For pulsed optical pumping of a 50-μm-wide stripe at 78 K, the far-field beam divergence angle was only 1.4°. The slope efficiency was 64% of that for a conventional Fabry–Pérot laser on the same bar. However, the spectral linewidth decreased by only a factor of 2. The beam quality was substantially better than that for the Fabry–Pérot laser out to stripe widths of at least 800 μm.
All-optical wavelength converter and switch based on electromagnetically induced transparency
Authors: Schmidt H. , Ram R. J.
Published: July 2002

We propose a method for all-optical switching and wavelength conversion using electromagnetically induced transparency (EIT). We discuss the mechanism for the transfer of information in this scheme and determine the conditions for which N×N wavelength conversion can be realized. We compare the properties and limits of an EIT-based switch to conventional wavelength converters.
Effect of interfacial strain on critical temperature of YBa2Cu3O7−δ thin films
Authors: Zhai H. Y. , Chu W. K.
Published: July 2002

Ultrathin YBa2Cu3O7−δ (YBCO) films down to two unit cells thick have been prepared on LaAlO3 (LAO) and SrTiO3 (STO) substrates for the study of substrate-induced strain effects on critical transition temperature (Tc). The YBCO on LAO has a higher Tc than that on STO for very thin films, and this Tc difference increases with reduction of film thickness. X-ray diffraction experiments reveal that compressive strain exists in the a–b plane for YBCO thin films on LAO, while tensile strain occurs for films on STO. The different stresses in these films account for the Tc difference.
Efficient, high-bandwidth organic multilayer photodetectors
Authors: Peumans P. , Bulović V. , Forrest S. R.
Published: July 2002

Organic photodetectors incorporating an ultrathin (⩾5 Å) donor–acceptor alternating multilayer stack as the optically active region exhibit external quantum efficiencies of 75% across the visible spectrum, and have subnanosecond response times. Photogenerated excitons efficiently dissociate into free electrons and holes by rapid charge-transfer across the several closely spaced organic-layer interfaces. The dependence of the quantum efficiency on applied voltage and layer thickness suggests that escape of photogenerated carriers from potential wells formed by the multilayers due to tunneling prior to recombination leads to the high efficiencies observed. The impulse response of the highest-bandwidth devices is characterized by a full width at half maximum of (720±70) ps. The performance of these devices makes them a useful building block for molecular organic photonic integrated circuits.
Dispersion of metal nanoparticles for aligned carbon nanotube arrays
Authors: Ago Hiroki , Komatsu Toshiki , Ohshima Satoshi , Kuriki Yasunori , Yumura Motoo
Published: July 2002

We report that Co metal nanoparticles (an average diameter of 4 nm) chemically synthesized by a reverse micelle method catalyzes the growth of multiwall carbon nanotubes (MWNTs) aligned perpendicular to a substrate. The surface of the nanoparticles is covered with surfactants so that the nanoparticles can be dispersed in organic solvent. The dispersion of the nanoparticles was cast directly onto a plane Si substrate for thermal pyrolysis of acetylene. We have found that the pretreatment of the metal nanoparticles with hydrogen sulfide before the pyrolysis straightens the MWNTs, suggesting sulfurization of the nanoparticle catalyst plays an important role in regular growth of the MWNTs. The dispersion of the nanoparticles offers a conventional and processible approach to synthesize large area aligned MWNT arrays.
Dielectric properties of pulsed-laser deposited SrTiO3 films at microwave frequency ranges
Authors: Hong J. P. , Kwak J. S. , Kim C. O. , Park S. J. , Sok J. H. , Lee E. H.
Published: July 2002

The dielectric constant and loss tangent of SrTiO3 thin films were characterized under the influence of an applied dc voltage at about 3.64 GHz. The measurement was carried out utilizing a gold resonator with a flip-chip capacitor at cryogenic temperatures. The analysis of the experimentally observed capacitance and quality factor served to give a measure of the dielectric constants and the loss tangents of the SrTiO3 film at microwave ranges, respectively. A dielectric constant of 830 and a low loss tangent of 6×10−3 at 3.64 GHz were observed at 90 K and 100 V. The dielectric loss decreases as the bias voltage increases. In addition, the quality of the SrTiO3 film is presented in terms of fractional frequency under the bias voltages and cryogenic temperatures.
Investigation of chemisorbed molecular states for oxygen on rhodium (111)
Authors: Walter Eric J. , Lewis Steven P. , Rappe Andrew M.
Published: July 2002

Using density-functional theory, we provide the first conclusive evidence of the existence of a molecularly chemisorbed state for oxygen on the Rh (111) surface. Four species are identified: a paramagnetic state above the bridge site with a binding energy of 1.95 eV, a more weakly bound paramagnetic state above the top site with a binding energy of 0.95 eV, and two nonmagnetic states above the face-centered-cubic (fcc) and hexagonal-close-packed (hcp) hollow sites each with a binding energy of 1.98 eV. We compare these results with our calculations of the binding energy for atomic oxygen on the fcc and hcp hollow sites and an upper bound on the dissociation barrier to understand major portions of the dissociation reaction coordinate. Combining our data with the experimental and theoretical results for oxygen dissociation on many other fcc (111) metal surfaces, we conclude that all these metal surfaces possess similar minima corresponding to physisorption, molecular chemisorption, and dissociative chemisorption. Despite these similarities, the differing binding energies and barrier heights account for the varying mechanisms of oxygen dissociation on different fcc (111) surfaces.
Generation of zonal flows by drift waves in a dusty plasma
Authors: Shukla P. K.
Published: July 2002

It is well known that the spectra of electrostatic drift waves and convective cells are modified by the presence of stationary charged dust grains in a nonuniform dusty magnetoplasma. While modification of the drift wave frequency is caused by the modification of the equilibrium quasineutrality condition, purely damped convective cells of an electron–ion plasma obtain a real frequency due to the finite E×B0 currents in a nonuniform dusty magnetoplasma, where E is the wave electric field and B0 is the external magnetic field. It is shown that the ponderomotive force of electrostatic drift waves in a dusty plasma can generate zonal flows or dust convective cells. Explicit expressions for the growth rates of parametrically excited zonal flows are presented. The growth rates strongly depend on the dust parameters. Linearly unstable zonal flows can self-organize in the form of long-living vortices. Such nonlinear zonal flow structures constitute a dynamical paradigm for intermittency in drift wave driven long wavelength turbulence in a nonuniform dusty magnetoplasma.
Reduced density matrix and combined dynamics of electrons and nuclei
Authors: Zhao Yang , Yokojima Satoshi , Chen GuanHua
Published: July 2002

Nuclear dynamics is incorporated into an efficient density matrix formalism of electronic dynamics which has been applied to molecular systems containing thousands of atoms. The formalism for the combined dynamics of electrons and nuclei is derived from the Dirac–Frenkel variational principle. The single electron reduced density matrices and the Glauber coherent states are used for the electronic and nuclear degrees of freedom, respectively. The new formalism is applicable to simulate the dynamics of large molecular systems. As an illustration of its validity, the formalism is employed to calculate the electron and nuclei dynamics of hydrogen molecules.
Theory of coupled phase transitions: Phase separation and abnormal variation of order parameter
Authors: Zhong Fan , Jiang M. , Xing D. Y. , Dong Jinming
Published: July 2002

A simplified Ginzburg–Landau theory is developed to study coupling of a first-order gas–liquid phase transition (FOPT) with a second-order orientational phase transition (SOPT). We show analytically that, due to the coupling between the two phase transitions, the SOPT may exhibit a FOPT-like phase separation in which a partially ordered liquid phase is separated from a disordered gaseous phase. This inhomogeneous phase separation leads to an abnormal variation of the order parameter of the SOPT, including a distinct mode of growth and an apparent mean-field critical exponent qualitatively different from the normal one. © 2000 American Institute of Physics.
Field-dependent chemisorption of carbon monoxide and nitric oxide on platinum-group (111) surfaces: Quantum chemical calculations compared with infrared spectroscopy at electrochemical and vacuum-based interfaces
Authors: Koper Marc T. M. , van Santen Rutger A. , Wasileski Sally A. , Weaver Michael J.
Published: July 2002

Density Functional Theory (DFT) is utilized to compute field-dependent binding energies and intramolecular vibrational frequencies for carbon monoxide and nitric oxide chemisorbed on five hexagonal Pt-group metal surfaces, Pt, Ir, Pd, Rh, and Ru. The results are compared with corresponding binding geometries and vibrational frequencies obtained chiefly from infrared spectroscopy in electrochemical and ultrahigh vacuum environments in order to elucidate the broad-based quantum-chemical factors responsible for the observed metal- and potential-dependent surface bonding in these benchmark diatomic chemisorbate systems. The surfaces are modeled chiefly as 13-atom metal clusters in a variable external field, enabling examination of potential-dependent CO and NO bonding at low coverages in atop and threefold-hollow geometries. The calculated trends in the CO binding-site preferences are in accordance with spectral data: Pt and Rh switch from atop to multifold coordination at negative fields, whereas Ir and Ru exhibit uniformly atop, and Pd hollow-site binding, throughout the experimentally accessible interfacial fields. These trends are analyzed with reference to metal d-band parameters by decomposing the field-dependent DFT binding energies into steric (electrostatic plus Pauli) repulsion, and donation and back-donation orbital components. The increasing tendency towards multifold CO coordination seen at more negative fields is due primarily to enhanced back-donation. The decreasing propensity for atop vs multifold CO binding seen in moving from the lower-left to the upper-right Periodic corner of the Pt-group elements is due to the combined effects of weaker donation, stronger back-donation, and weaker steric repulsion. The uniformly hollow-site binding seen for NO arises from markedly stronger back-donation and weaker donation than for CO. The metal-dependent zero-field DFT vibrational frequencies are in uniformly good agreement with experiment; a semiquantitative concordance is found between the DFT and experimental frequency-field (“Stark-tuning”) slopes. Decomposition of the DFT bond frequencies shows that the redshifts observed upon chemisorption are due to donation as well as back-donation interactions; the metal-dependent trends, however, are due to a combination of several factors. While the observed positive Stark-tuning slopes are due predominantly to field-dependent back-donation, their observed sensitivity to the binding site and metal again reflect the interplay of several interaction components.
PH
3
/H
2
plasma passivation of metal-organic chemical vapor deposition grown GaAs on Si
Authors: Wang Gang , Ogawa Takashi , Soga Tetsuo , Jimbo Takashi , Umeno Masayoshi
Published: July 2002

Effects of PH3/H2 (PH3/H2=10%) plasma passivation of GaAs grown on Si substrate have been investigated in detail. It is observed that both the surface phosphidization and defect hydrogenation can be realized simultaneously with a reduced plasma-induced damage. The optical and electrical properties of GaAs on Si are effectively improved by PH3/H2 plasma exposure due to the passivation of bulk and surface defects-related nonradiative recombination centers by incorporation of hydrogen (H) and phosphorous (P) atoms. As a result, the PH3/H2 plasma exposed GaAs Schottky diodes on Si show an increase in the reverse breakdown voltage by a factor of about 1.6, and the as-passivated GaAs solar cell grown on Si shows an increase in the conversion efficiency from 15.9% to 18.6% compared to that of the as-grown samples. The passivated GaAs devices on Si show outstanding thermal stability, which is probably due to the active participation of both H and P atoms in the PH3/H2 plasma passivation process.
Thermodynamic properties of H2O and D2O in the critical region
Authors: Wyczalkowska A. Kostrowicka , Abdulkadirova Kh. S. , Anisimov M. A. , Sengers J. V.
Published: July 2002

An analysis is presented of the thermodynamic properties of H2O and D2O in the critical region in terms of a six-term Landau expansion for the Helmholtz-energy density transformed so as to incorporate crossover from Ising-type to mean-field critical behavior. It is shown that the effects of the critical fluctuations on the thermodynamic properties of H2O and D2O satisfy the principle of corresponding states, so that the amplitudes of the asymptotic scaling laws, as well as the parameters that govern the crossover from Ising-type to mean-field critical behavior, are identical for H2O and D2O. For H2O, our results provide information supplementing a formulation for calculating thermodynamic properties of H2O for general and scientific use adopted by the International Association for the Properties of Water and Steam.
Silicon-induced faceting of vicinal GaAs(001)
Authors: Brennan S. , Stephenson G. B. , Fuoss P. H. , Kisker D. W. , Lavoie C. , Evans-Lutterodt K. L.
Published: July 2002

This article presents in situ x-ray diffraction studies of the evolution of the morphology of 0.5°-miscut vicinal GaAs(001) surfaces during and following undoped and silicon-doped growth using organometallic vapor-phase epitaxy. Undoped growth leads to ordered monolayer steps. However, growth in the presence of silicon destabilizes this surface morphology and triggers faceting. Coarsening of the facet size proceeds even after the growth has stopped and results in large singular regions of GaAs(001) surface separated by step bunches. Dosing the surface with silicon without growing material does not trigger faceting. Growth of undoped GaAs on faceted surfaces recovers the initial state of ordered monolayer steps.
Influence of oxygen on band alignment at the organic/aluminum interface
Authors: Blyth R. I. R. , Sardar S. A. , Netzer F. P. , Ramsey M. G.
Published: July 2002

The presence of adventitious oxygen is inevitable when organic/metal interfaces are formed by evaporation in high vacuum (10−6 mbar.). In this letter, we highlight the importance of this oxygen for band alignment, and hence, performance, in organic-based devices. The influence of controlled amounts of oxygen on band alignment in benzene/aluminum model cathode interfaces has been studied using ultraviolet photoemission in ultrahigh vacuum. We show that even small amounts of oxygen significantly lower the aluminum work function with concomitant improvement in band alignment.
Gate bias instability in hydrogenated polycrystalline silicon thin-film transistors
Authors: Dimitriadis C. A.
Published: July 2002

The instability of n-channel hydrogenated polycrystalline silicon thin-film transistors has been investigated with respect to gate biasing. The hydrogenation was performed by hydrogen ion implantation through the gate oxide. The conduction mechanism in the gate oxide was studied for positive and negative gate bias, showing that the electron tunneling is much higher for positive gate bias. The oxide conduction follows the Fowler–Nordheim (FN) tunneling mechanism for electron tunneling from the channel and Poole–Frenkel for electron tunneling from the gate polysilicon. After constant FN stressing for short duration (<10 min), the evolution of the transfer characteristics with stress time indicate passivation of the grain boundary dangling bonds by the H+ positive ions introduced into the gate during hydrogenation with simultaneous electron injection into the gate oxide and interface states generation. For longer FN stress duration, the transfer characteristics are degraded due to enhancement of the donor-like interface states generation.
Canonical Noether symmetries and commutativity properties for gauge systems
Authors: Gràcia Xavier , Pons Josep M.
Published: July 2002

For a dynamical system defined by a singular Lagrangian, canonical Noether symmetries are characterized in terms of their commutation relations with the evolution operators of Lagrangian and Hamiltonian formalisms. Separate characterizations are given in phase space, in velocity space, and through an evolution operator that links both spaces.
Shape memory alloy activated high-pressure optical cell for biophysical studies
Authors: Zhou Jack G. , Koulas Spiros , Chong Parkson Lee-Gau
Published: July 2002

The high-pressure optical cell (HPOC) originally designed by A. A. Paladini and G. Weber [Rev. Sci. Instrum. 52, 419 (1981)] has been widely used in biophysical research. However, the original HPOC only allows for the observations of optical properties of single samples, thus limiting the data which researchers can obtain. The objective of this article is to demonstrate a new HPOC design, which enables researchers to introduce an additional component to the original sample for optical observation of biochemical reactions in real time, while all the components are under pressure. The core of this new design is the introduction of the shape memory alloy activated stopper/mixing mechanism, which separates different biological samples until the experimenter is ready to mix them. A urethane stopper separates the two samples, which are sealed in a dual chamber cuvette. The cuvette is suspended in the mid chamber, so that the pressure can be transmitted to both sides of the cuvette equally. Once an electric current is transmitted through the electrical plugs, the shape memory alloy is heated and moves the urethane stopper away from the neck of the cuvette. In addition, the shape memory alloy achieves a pumping action, which facilitates the mixing of the two samples. A multiple-chamber cuvette capable of mixing three components is also designed. Some preliminary feasibility testing has been done, and a stainless steel HPOC prototype has been built to validate the design.
Ornstein–Uhlenbeck–Cauchy process
Authors: Garbaczewski Piotr , Olkiewicz Robert
Published: July 2002

We combine earlier investigations of linear systems subject to Lévy fluctuations with recent attempts to give meaning to so-called Lévy flights in external force fields. We give a complete construction of the Ornstein–Uhlenbeck–Cauchy process as a fully computable paradigm example of Doob’s stable noise-supported Ornstein–Uhlenbeck process. Despite the nonexistence of all moments, we determine local characteristics (forward drift) of the process, generators of forward and backward dynamics, and relevant (pseudodifferential) evolution equations. The induced nonstationary spatial process is proved to be Markovian and quite apart from its inherent discontinuity defines an associated velocity process in a probabilistic sense.
Self-consistent integral equation theory for semiflexible chain polyelectrolyte solutions
Authors: Shew Chwen-Yang , Yethiraj Arun
Published: July 2002

A self-consistent integral equation theory is presented for the conformational behavior and static structure of polyelectrolyte solutions composed of semiflexible chain molecules. The free energy of the solution is expressed as the sum of an elastic, a Coulombic, and a medium-induced contribution, and is minimized with respect to the single chain structure factor. In the flexible limit, the theory is accurate for the chain conformations and static structure factor when compared to Monte Carlo simulations of flexible chains. For semiflexible chains near the flexible limit, the chain conformations are sensitive to the bare persistence length in dilute solutions but not in the semidilute regime. In contrast, the effect of the bare persistence length on the static structure factor is not as significant. The theory predicts that charged flexible chains exhibit more pronounced liquidlike order in solution than their rigid-rod counterparts, and the length scale of these liquidlike correlations is also shorter in flexible chains.
A method to improve the agreement between calculated and observed vibrational frequencies after scaling of a quantum mechanical force field
Authors: Arenas Juan F. , Centeno Silvia P. , Marcos Juan I. , Otero Juan C. , Soto Juan
Published: July 2002

A systematic method to fit calculated to observed vibrational frequencies has been developed and implemented in a computer program. The procedure consists of the refinement of a scaled quantum mechanical force field (SQMFF) previously obtained according to Pulay’s method. The key step in the process is the generation of an intermediate matrix, CΛCT, which is then refined. The above step produces only small corrections to the scaled force constants, yielding a considerable improvement of the fitted frequencies. This scheme of refinement can be carried out using any kind of coordinates. To show the reliability and performance of the proposed method, the force fields of two very different systems, as benzene and tetranitromethane, have been chosen as example tests.
High-speed high spatial resolution magneto-optic Kerr effect polarimeter/microscope for studies of ultrathin magnetic structures
Authors: Heidkamp Marcus , Erskine J. L.
Published: July 2002

A magneto-optic Kerr effect polarimeter designed to study the dynamics of magnetization reversal in ultrathin films, multilayer films, and microstructures is described. The polarimeter is integrated into a long focal-length charge coupled device (CCD) camera based Kerr microscope that permits viewing domain structures and facilitates positioning of the focused polarimeter beam on microstructures in ultrahigh vacuum. Diffraction-limited spatial resolution, based on the f-number of the respective objective lenses, is achieved by the microscope (∼1 μm) and polarimeter (∼5 μm). The polarimeter is capable of measuring continuous wave or repetitive transient ultrathin film magnetic response at sampling rates of 40 million samples/s (MS/s) over a micron-scale region defined by the illuminating spot. Hysteresis loops generated by ultrathin (monolayer) films and microstructures can be measured at high signal-to-noise ratio over a nine-decade range of drive frequencies.
Transition between two next-nearest-neighbor phases in a mixed Langmuir monolayer. A study by grazing-incidence x-ray diffraction and Brewster-angle microscopy
Authors: Teer Ellis , Knobler Charles M. , Braslau Alan , Daillant Jean , Blot Christian , Luzet Daniel , Goldmann Michel , Fontiane Phillipe
Published: July 2002

Grazing incidence x-ray diffraction and Brewster-angle microscopy measurements have been undertaken for a mixed Langmuir monolayer of octadecanoic acid and methyl octadecanoate. For the composition studied (49.7 mol % ester) there are two noncrystalline tilted phases, one at low pressure in which the chains point to their nearest neighbors (L2 phase) and a higher-pressure phase in which the tilt is toward next-nearest neighbors. The higher-pressure phase arises from a merger of the L2′ and Ov phases, which are separated in the pure acid. A continuous change between the two regions is observed rather than a first-order transition, as had been suggested. The results are discussed with reference to different proposals for the origin of two distinct next-nearest neighbor tilted mesophases: the presence of one-dimensional chain backbone order [V. M. Kaganer and E. B. Loginov, Phys. Rev. E 51, 2237 (1995)] and a coupling between tilt and distortion [E. Sirota, Langmuir 13, 3849 (1997)].
Strain-induced material intermixing of InAs quantum dots in GaAs
Authors: Lipinski M. O. , Schuler H. , Schmidt O. G. , Eberl K. , Jin-Phillipp N. Y.
Published: July 2002

A systematic investigation of the stacking behavior of InAs quantum dots (QDs) with varying GaAs interlayer thickness d is presented. We compare two-fold stacks of large QDs (≈25 nm base width), which emit at 1.30 μm, to small QDs (≈20 nm base width) emitting at 1.14 μm. For large islands photoluminescence yields an energetic blueshift of the second layer islands with decreasing d, although transmission electron microscopy clearly reveals a ≈70% larger dot size in the second layer, whereas for small islands a similar size increase of the dots in the upper layer and an energetic redshift are observed. A detailed analysis of confinement and material intermixing effects suggests that for large QDs strain driven material intermixing is dominant. For small QDs the confinement effect plays the major role and causes the observed photoluminescence energy redshifts.
Analytical modeling of stress-induced leakage currents in 5.1–9.6-nm-thick silicon-dioxide films based on two-step inelastic trap-assisted tunneling
Authors: Lenski Markus , Endoh Tetsuo , Masuoka Fujio
Published: July 2002

The thickness dependence of stress-induced leakage currents (SILCs) has been investigated for silicon-dioxide films with thicknesses between 5.1 and 9.6 nm. Assuming a two-step trap-assisted tunneling process accompanied by an energy relaxation process of trapped electrons, a set of analytical equations is given, which describes quantitatively the SILC dependence on oxide electric field with trap site location, trapped sheet charge density, and trap state energy as characteristic trap site parameters. Applying this model to the SILC data of 5.1–9.6-nm-thick silicon-dioxide films, the best agreement between experimental and calculated I–V data is achieved by a constant trap state energy of 1.93 eV relative to the silicon-dioxide conduction-band edge. Trap sites are located at 4.24 nm from the gate interface for 6.8–9.6-nm-thick films, while the 5.1 nm film exhibits a slightly different trap site location of 4.08 nm. The trapped sheet charge density Qtrap increases linearly with oxide thickness from −0.34×10−6 to −1.29×10−6 C/cm2. As a result, the thickness dependence of Qtrap suppresses the local tunneling current between the gate injection interface and trap sites by a reduction of the local oxide electric field. This fact explains the decrease of SILC with an increase in oxide thickness.
Approaching the theoretical coercivity of Nd2Fe14B: Microstructural evaluation and interparticle interactions
Authors: Girt Er. , Krishnan Kannan M. , Thomas G. , Altounian Z. , Dikeakos M.
Published: July 2002

The microstructure of melt–spun rapidly quenched NdδFe13.1B (2.05⩽δ⩽147.6) and NdδFe14B (δ=40.6,151.7) ribbons was tailored by appropriate annealing from strongly interacting to magnetically isolated single domain Nd2Fe14B grains embedded in a nonmagnetic matrix of α-Nd and γ-Nd. This microstructure, as characterized by a variation in the magnetic interaction between Nd2Fe14B grains, was found to have a large impact on coercivity, μ0Hc, i.e., coercivity increases with an increase in the Nd concentration from 1.2 T in Nd2.05Fe13.1B to 2.75 T in Nd147.6Fe13.1B at 290 K. A detailed study of the microstructure of NdδFe13.1B (δ=38.1,148.7), carried out by conventional transmission electron microscopy, energy-filtered imaging, and energy dispersive x-ray microanalysis, showed that the majority of the Nd2Fe14B grains are completely isolated only in Nd147.6Fe13.1B. The Nd2Fe14B grains, in Nd147.6Fe13.1B, are found to be randomly oriented platelets with the c axis normal to the platelet and an average size of 100×40×25 nm. For these randomly oriented, noninteracting, single domain Nd2Fe14B grains, the coercivity was calculated using the Stoner–Wohlfarth model and including the shape anisotropy of the grains. The observed coercivity of Nd2Fe14B in Nd147.6Fe13.1B is ∼83% of this theoretical value and is the largest so far reported for Nd2Fe14B.
A cold-cathode source of low-energy low-divergent broad ion beams
Authors: Gavrilov N. V. , Emlin D. R. , Kuleshov S. V.
Published: July 2002

This article presents a study of the effect of a magnetic field (5–30 mT) and the potential difference between the discharge plasma and the screen electrode of the accelerating–decelerating ion optics on the performance of the ion plasma emitter and on the parameters of the ion beam in a glow-discharge-based ion source. Used in the experiment was a modified Penning ionization gauge electrode system in which ions are extracted along the magnetic field and the discharge operates stably with a current of ∼1 A at low gas pressures (∼10−2 Pa). It has been shown that the changes in the radial distribution of the ion beam current density (1–10 mA/cm2) with increasing magnetic field are due to an increase in the radial gradient of the electron temperature in the plasma. With an optimal B field strength of ∼15 mT, a nearly uniform (within ∼10%) current density distribution has been obtained over an area of ∼50 cm2. It has been demonstrated that the angular divergence of low-energy (∼0.5–1 keV) ion beams can be reduced with a simultaneous increase in the potential difference between the plasma and the screen electrode and in the electric field in the acceleration gap of the ion optics; however, the maximum beam current density, which is achieved thanks to the increase in the plasma density under conditions when the electric field in the accelerating gap approaches the breakdown value, drops with growing potential difference. The experimental results obtained are explained qualitatively based on the changes of the position and shape of the plasma boundary on varying the parameters of the ion space charge sheath between the plasma and the screen electrode of the ion optics.
Chemical vapor deposition diamond window as vacuum and tritium confinement barrier for fusion application
Authors: Takahashi K. , Sakamoto K. , Kasugai A. , Imai T. , Brandon J. R. , Sussmann R. S.
Published: July 2002

A pressure resistant polycrystalline chemical vapor deposition (CVD) diamond disk for a microwave window is used for a vacuum and a tritium confinement boundary in fusion applications. A pressure test of a CVD diamond window disk (2.25 mm in thickness and 100 mm in diameter) was carried out. It was demonstrated that the diamond window tolerated 1.0 MPa (10 atm) in the plenum. The displacement of the window center for both the growth and the nucleation side on the unpressurized side is 40±1 and 41±1 μm, respectively, at the pressure of 1.0 MPa, and these values agree well with those calculated. No damage in the disk and the braze, and no vacuum leakage in the assembly was observed. This result demonstrates that the diamond window assembly could tolerate up to 1.45 MPa. It was experimentally proved that the diamond window satisfied the safety requirement of 0.5 MPa resistance for the vacuum and the tritium confinement boundary of an International Thermonuclear Experimental Reactor. The design prospect for the diamond window of the electron cyclotron heating and current drive system is also discussed, based on the stress analysis using the ABAQUS code.
Vacuum-ultraviolet resonant photoabsorption imaging of laser produced plasmas
Authors: Hirsch J. S. , Meighan O. , Mosnier J-P. , van Kampen P. , Whitty W. W. , Costello J. T. , Lewis C. L. S. , MacPhee A. G. , Hirst G. J. , Westhall J. , Shaikh W.
Published: July 2002

We present results from a vacuum-ultraviolet (VUV) “photoabsorption imaging” technique based on the measurement of the time and space resolved absorption of a quasimonochromatic VUV beam from a laser plasma light source. The use of VUV radiation as a probe beam permits direct access to resonance lines of (singly and more highly charged) ions and also to the resonant and nonresonant continua of atoms and ions. In this experiment we have confined ourselves to measurements using the 3p–3d resonances of Ca, Ca+, and Ca2+ as markers of the temporal and spatial distribution of ground state atoms and ions in an expanding laser plasma plume. We show how time resolved column density maps may be extracted from such images. In addition we have extracted plasma plume velocities from the data, which compare well with an analytical laser ablation model.
Investigation of local current flow in polycrystalline, anisotropic superconductors using finite element simulation
Authors: Zeimetz B. , Baranowski R. P. , Evetts J. E.
Published: July 2002

A finite element program was used to calculate current distributions in superconductors, assuming a nonlinear (power-law or percolation-type) local dependence of the electrical field on current density. A bicrystal geometry, which forms the basic building block of (Bi,Pb)2Sr2Ca2Cu3Ox powder-in-tube tapes and other polycrystalline conductors, was studied. Current–voltage curves and critical currents were calculated for different geometrical and electromagnetic parameters. Bicrystals of “brick-wall” and “railway-switch” geometry were compared, and it was found that in both cases anisotropy is the dominating factor determining the overall critical current. Strong anisotropy leads to current concentration around grain boundaries, thereby reducing the critical current. Addition of a grain boundary with finite resistance does not significantly change the current distribution within the grains.
The permanent electric dipole moment of yttrium dicarbide, YC2
Authors: Bousquet Robert R. , Steimle Timothy C.
Published: July 2002

Optical Stark measurements were performed on the (000)Ã 2A1–(000)X̃ 2A1 band system of yttrium dicarbide, YC2. A molecular beam sample was produced by skimming the output of a free jet supersonic expansion, which contained the reactive products of an ablated yttrium metal rod in the presence of a CH4/argon mixture. The permanent electric dipole moments, μ, for the (000) X̃ 2A1 and (000)Ã 2A1 states were determined to be 6.38(3) D and 7.09(6) D, respectively. A comparison with isovalent YO and YS, theoretical predictions, and a simple molecular orbital correlation diagram is presented.
X-ray photoelectron spectroscopy indication of decomposition species in the residue of shocked polytetrafluoroethylene powder
Authors: Holt W. H. , Mock W. , Santiago F.
Published: July 2002

Forty-eight percent porous polytetrafluoroethylene (PTFE) powder placed inside a steel closed container has been impact shock loaded with a gas gun and soft recovered. The initial stress in the powder is calculated to be 7.2 kbar. The residue in the container showed dark regions where the originally white powder had decomposed to form black soot. X-ray photoelectron spectroscopy (XPS) was used to analyze in situ a portion of a dark region. The resulting spectrum showed a large amorphous carbon peak that was not observed in the unshocked powder spectrum. In addition, the shocked material showed several peaks containing hydrogen and/or oxygen, suggesting reactions of dissociation products with ambient air and/or water vapor in the polymer pores or possibly with residual water in the polymer. (The residual gas analyzer in the XPS system detected water vapor in the unshocked specimen.) Both spectra showed peaks corresponding to the PTFE linear polymer chain F–C–F.
Coherent tunneling in ferromagnetic planar junctions: Role of thin layers at the barriers
Authors: Wilczynski M. , Barnas J.
Published: July 2002

We study theoretically coherent electron tunneling in single-barrier FM/NM/I/NM/FM and NM/FM/I/FM/NM systems, where FM (NM) denotes a ferromagnetic (nonmagnetic) metal and I stands for an insulating nonmagnetic barrier. The metallic films on both sides of the barrier are thin enough, so that partial confinement of electron states plays a significant role. Two different cases are analyzed in detail: (i) the case where the thickness of one of the thin films is constant while that of the second film is varied; and (ii) the case where both thin films are equally thick. The junction resistance depends on the relative orientation of magnetic moments of the ferromagnetic electrodes in FM/NM/I/NM/FM systems and on the relative orientation of magnetic moments of the thin ferromagnetic films in NM/FM/I/FM/NM systems. Tunneling current and magnetoresistance, calculated as a function of the thickness of thin films, show pronounced peaks related to resonant-type tunneling. The variation of the magnetoresistance with bias voltage is also studied and this variation is generally nonmonotonous.
Asymmetric Stark shifts in In0.18Ga0.82As/GaAs near-surface quantum wells: The image charge effect
Authors: Chang Kai , Peeters F. M.
Published: July 2002

We investigate theoretically the electronic structure of InxGa1−xAs/GaAs near-surface quantum wells within the framework of the effective mass theory. We found that the energies of electron and hole states depend sensitively on the capped layer thickness, and the Stark shift is asymmetric with respect to the applied electric field, i.e., the blueshifts or redshifts are determined by the direction and the strength of the electric field. The asymmetric Stark shift is due to the image charge effect and the Coulomb interaction. The spatial distribution of the electron and hole changes significantly when the thickness of the capped layer and the electric field is varied. The exciton binding energy is enhanced significantly by the image charge effect and decreases rapidly with increasing capped layer thickness.
A theoretical study of Si4H2 cluster with ab initio and density functional theory methods
Authors: Wang Wen-Ning , Tang Hai-Rong , Fan Kang-Nian , Iwata Suehiro
Published: July 2002

Various isomers of Si4H2 cluster have been investigated with ab initio molecular orbital and density functional theory (DFT) calculations. Nine local minimum isomers on the potential energy surface have been obtained with both Mo/ller–Plesset perturbation theory (MP2) and DFT methods. The Si4 frame is slightly distorted by the dissociative adsorption of H2 on it. The most stable isomer of Si4H2 is a classical structure with both hydrogen atoms bonded to a single silicon atom. The nonclassical H-bridged structures are also found in the calculations, but predicted to be less stable than the nonbridged structures energetically. The formation of the most stable isomer of Si4H2 from Si4 and H2 is proven to be a two-step process and exothermic. The first step is the dissociative adsorption of H2 on Si4 cluster by overcoming an energy barrier of 19.27 kcal/mol, and the second step of conversion from the intermediate to the product will readily proceed with a barrier of only 0.53 kcal/mol.
Depolarization of evanescent waves scattered by laser-trapped gold particles: Effect of particle size
Authors: Gu Min , Ke Puchun
Published: July 2002

Depolarization of evanescent waves scattered by laser-trapped gold particles of 0.1, 0.5 and 2 μm in diameter is experimentally characterized in order to reveal its dependence on the size of particles. It is found that the degree of polarization of scattered evanescent waves decreases with the size of gold particles, which is contrary to that previously observed for dielectric particles. This feature becomes advantageous in particle-trapped near-field microscopy since less depolarized photons carry more information of a sample. With the help of polarization gating, this property is demonstrated in images of the evanescent wave interference pattern as well as the surface of a glass prism.
Noise characterization of sputtered amorphous carbon films
Authors: Hastas N. A. , Dimitriadis C. A. , Panayiotatos Y. , Tassis D. H. , Patsalas P. , Logothetidis S.
Published: July 2002

Low-frequency noise measurements have been carried out at room temperature in amorphous carbon (α-C) thin films with the current I as the parameter. The α-C films, rich in sp2 bonds, were prepared by rf magnetron sputtering at room temperature. Hall measurements performed at room temperature show that the α-C films are p-type semiconductors with a hole concentration of about 2.8×1018 cm−3. In α-C film grown on oxidized silicon wafer, the current shows an ohmic behavior for low applied voltages, while the conduction mechanism is dominated by the Poole–Frenkel effect for high applied voltages. In the linear voltage region, the power spectral density of the current fluctuations exhibits 1/fγ (with γ<1) behavior and is proportional to I2. Using a noise model based on trapping–detrapping of holes of the valence band and the gap states of exponential energy distribution, the noise data can provide an assessment of the distribution of traps within the band gap of the α-C material.
Spin-dependent Coulomb blockade in a silicon-on-insulator-based single-electron transistor
Authors: Lee S. D. , Park K. S. , Park J. W. , Moon Y. M. , Choi Jung B. , Yoo K.-H. , Kim J.
Published: July 2002

We report low-temperature conductance measurement on a Coulomb-blockaded dot in a silicon-on-insulator-based single-electron transistor with in-plane side gates. The linear conductance for 4.2 K at zero magnetic field exhibits up to three paired peaks, indicating simple alternating odd (spin 1/2)-even(spin 0) filling. Three intrapair spacings are found to be nearly a constant value, corresponding to the single charging energy U, whereas two interpair spacings are different which are associated with U+ΔE1 and U+ΔE2, i.e., successive quantized level spacings added to U. The quantized level spacings were also revealed in the nonlinear current staircases.