bluuebunny/crossref_metadata_2025 huggingface dataset:
An atmospheric pressure plasma source
Authors: Park Jaeyoung , Henins I. , Herrmann H. W. , Selwyn G. S. , Jeong J. Y. , Hicks R. F. , Shim D. , Chang C. S.
Published: July 2002

An atmospheric pressure plasma source operated by radio frequency power has been developed. This source produces a unique discharge that is volumetric and homogeneous at atmospheric pressure with a gas temperature below 300 °C. It also produces a large quantity of oxygen atoms, ∼5×1015 cm−3, which has important value for materials applications. A theoretical model shows electron densities of 0.2–2×1011 cm−3 and characteristic electron energies of 2–4 eV for helium discharges at a power level of 3–30 W cm−3.
Plasma fluidized bed imaging and possible strong coupling effects
Authors: Snyder Hans R. , Currier Robert P. , Murillo Michael S.
Published: July 2002

A plasma fluidized bed was constructed to study the physics of fluidized plasmas. The plasma fluidized bed is a unique system involving contact of hydrodynamically suspended micron-sized (and larger) particles with the current-carrying portion of a plasma. These particles become negatively charged and, under certain conditions, could possibly form a strongly coupled fluidized plasma. Using laser light scattering measurements, we have imaged particles and calculated a coupling parameter showing what could be strongly coupled effects superimposed on the hydrodynamic behavior of the fluidized bed systems. This system may provide an ideal means to experimentally explore the properties and dynamics of strongly coupled plasmas subjected to hydrodynamic shear forces.
An optically controllable terahertz filter
Authors: Libon I. H. , Baumgärtner S. , Hempel M. , Hecker N. E. , Feldmann J. , Koch M. , Dawson P.
Published: July 2002

Using a mixed type-I/type-II GaAs/AlAs multiple-quantum-well sample, we have demonstrated an optically controllable and tunable terahertz (THz) filter. Long-lived electron–hole pairs in the quantum wells allow for efficient THz attenuation over a large THz spot size (2 mm) for extremely low optical cw power. This sample can also be used as an optically tunable THz phase shifter. The optically induced change of the GaAs quantum wells from a dielectric to a conducting material leads to the observed attenuation and the shifting of the THz wave forms.
Low-threshold interband cascade lasers with power efficiency exceeding 9%
Authors: Bruno J. D. , Bradshaw J. L. , Yang Rui Q. , Pham J. T. , Wortman D. E.
Published: July 2002

Midinfrared (3.6–3.8 μm) interband cascade lasers based on InAs/GaInSb type-II quantum wells have been demonstrated in the continuous-wave (cw) mode with low-threshold current densities (e.g., ∼56 A/cm2 at 80 K) and power efficiencies exceeding 9%. At a relatively low current of 0.4 A, we observed ∼100 mW/facet of optical power out at 80 K (124 mW at 60 K) from lasers mounted epilayer-side up with uncoated facets. These lasers were able to operate in the cw mode at temperatures up to 127 K. Also, in the pulsed mode, devices lased at temperatures up to 250 K and displayed, at 80 K, a peak power efficiency exceeding 11%.
Growth of zinc blende MgS/ZnSe single quantum wells by molecular-beam epitaxy using ZnS as a sulphur source
Authors: Bradford C. , O’Donnell C. B. , Urbaszek B. , Balocchi A. , Morhain C. , Prior K. A. , Cavenett B. C.
Published: July 2002

Zinc blende MgS has been grown on GaAs by molecular beam epitaxy using a novel method where the sources were Mg and ZnS. A reaction at the surface results in the formation of MgS layers with a Zn content estimated by secondary ion mass spectrometry and Auger spectroscopy to be between 0.5% and 2%. Double crystal x-ray rocking curve measurements of ZnSe/MgS/ZnSe layers show layers with good crystallinity. Using this growth technique layers up to 67 nm thick have been grown. Photoluminescence measurements of MgS/ZnSe/MgS single-quantum-well structures show that the confinement of the heavy hole excitons can be as large as 430 meV for a 1.7 nm well.
Extension of frequency spectrum methods for phase velocity measurements in ultrasonic testing
Authors: Lang M. J. , Duarte-Dominguez M. , Arnold W.
Published: July 2002

The analysis of ultrasonic signals in the frequency domain offers advantages over processing in the time domain. The principal methods are discussed to derive phase velocity and attenuation data from the Fourier spectra of the measured signals. An extension of these methods is pointed out. It yields a higher resolution and accuracy and is particularly useful for thin specimens where separate echoes are difficult to obtain. The method also allows corrections for coupling layers. The underlying theory is easily implemented in a software code for automated signal processing systems. Good agreement of theory and experimental data is obtained.
Coherent structures, self-similarity, and universal roll wave coarsening dynamics
Authors: Chang Hsueh-Chia , Demekhin Evgeny A. , Kalaidin Evgeny
Published: July 2002

Beyond their inception region from wide-banded inlet noise, roll waves on an inclined plane increase their amplitude and separation downstream in a scale-invariant linear manner by as much as one order of magnitude. Analysis of a set of dissipative hydraulic equations shows that this self-similar coarsening dynamics is driven by irreversible coalescence events between localized roll waves. By exploiting the localized structure of the roll waves and certain symmetries of the hydraulic equations to simple affine coordinate stretching by the substrate thickness, the time-averaged wave properties at every station and wave texture is shown to be self-similar. Moreover, due to weak variations in the substrate thickness and the sensitivity of the coarsening dynamics to it, the downstream evolution along this self-similar family can be captured by a generic cascade coalescence model for all wave separations. The linear coarsening rate of the average wave period is shown to be roughly (√−1) times the fraction of “excited” waves, which is only a weak function of a modified Froude number G based on the channel inclination angle.
Molecular wires, solenoids, and capacitors by sidewall functionalization of carbon nanotubes
Authors: Seifert Gotthard , Köhler Thomas , Frauenheim Thomas
Published: July 2002

Theoretical investigations (density-functional tight binding) about the changes of the properties of single-wall carbon nanotubes (SWNTs) by fluorination are reported. It is shown that a sidewall functionalization of SWNTs may lead to tubes with a wide variety of electronic properties, ranging from insulating over semiconducting to a metallic-like behavior. Therefore, a chemical derivatization of SWNTs may open possibilities for tailoring the electronic properties of nanotubes, providing nanoscale wires, capacitors, and solenoids.
Grain-size-dependent thermal conductivity of nanocrystalline yttria-stabilized zirconia films grown by metal-organic chemical vapor deposition
Authors: Soyez G. , Eastman J. A. , Thompson L. J. , Bai G.-R. , Baldo P. M. , McCormick A. W. , DiMelfi R. J. , Elmustafa A. A. , Tambwe M. F. , Stone D. S.
Published: July 2002

A grain-size-dependent reduction in the room-temperature thermal conductivity of nanocrystalline yttria-stabilized zirconia is reported for the first time. Films were grown by metal-organic chemical vapor deposition with controlled grain sizes from 10 to 100 nm. For grain sizes smaller than approximately 30 nm, a substantial reduction in thermal conductivity was observed, reaching a value of less than one-third the bulk value at the smallest grain sizes measured. The observed behavior is consistent with expectations based on an estimation of the phonon mean-free path in zirconia.
Waveguiding in planar photonic crystals
Authors: Lončar Marko , Nedeljković Dušan , Doll Theodor , Vučković Jelena , Scherer Axel , Pearsall Thomas P.
Published: July 2002

Photonic crystal planar circuits designed and fabricated in silicon on silicon dioxide are demonstrated. Our structures are based on two-dimensional confinement by photonic crystals in the plane of propagation, and total internal reflection to achieve confinement in the third dimension. These circuits are shown to guide light at 1550 nm around sharp corners where the radius of curvature is similar to the wavelength of light.
Femtosecond dynamics and absorbance of self-organized InAs quantum dots emitting near 1.3 μm at room temperature
Authors: Birkedal D. , Bloch J. , Shah J. , Pfeiffer L. N. , West K.
Published: July 2002

High-sensitivity, femtosecond differential transmission measurements on self-organized InAs quantum dots at room temperature allow us to determine the dynamics of resonantly excited electron-hole pairs, as well as the absorbance (α0d) of quantum dots. The room temperature differential transmission signal decreases with a time constant of 65 ps, determined by the rate of excitation from the initial photoexcited states to higher excited states. From the magnitude of the differential transmission, we deduce that the absorbance, and hence the maximum gain/pass in the direction normal to the layers, is ≈2.1×10−3 at the spectral peak for our sample with six layers of quantum dots.
Polymer phosphorescent light-emitting devices doped with tris(2-phenylpyridine) iridium as a triplet emitter
Authors: Lee Chang-Lyoul , Lee Kyung Bok , Kim Jang-Joo
Published: July 2002

We have fabricated phosphorescent polymer light-emitting devices with tris(2-phenylpyridine) iridium [Ir(ppy)3] as a triplet emissive dopant in poly(vinylcarbazole) (PVK) host. The device with 8% doping concentration of [Ir(ppy)3] in PVK showed the external quantum efficiency of 1.9% and the peak luminance of 2,500 cd/m2. The emission spectrum of the device exhibited no emission from PVK, indicating that the energy transfer from PVK to [Ir(ppy)3] is efficient. This work demonstrates that efficient electrophosphorescent light-emitting devices can be realized with polymers.
Stabilization of the response time in photorefractive polymers
Authors: Herlocker J. A. , Fuentes-Hernandez C. , Ferrio K. B. , Hendrickx E. , Blanche P.-A. , Peyghambarian N. , Kippelen B. , Zhang Y. , Wang J. F. , Marder S. R.
Published: July 2002

The optical and photoconductive fatigue of fast photorefractive polymers have been studied in a family of C60-sensitized polymer composites containing styrene-based chromophores with varying ionization potential. Changes in response time and in photoconductivity were studied for exposures up to 104 J/cm2. Increasing the chromophore ionization potential beyond that of the polyvinylcarbazole host was found to stabilize the response time. Studies of the electric-field dependence of the steady-state diffraction efficiency in various samples confirm the role of C60 anions as possible traps.
Supercurrent distribution in high-TC superconducting YBa2Cu3O7−y thin films by scanning superconducting quantum interference device microscopy
Authors: Sugimoto Akira , Yamaguchi Tetsuji , Iguchi Ienari
Published: July 2002

The two-dimensional vector mapping of current distributions in high-TC superconducting YBa2Cu3O7−y thin films obtained by converting magnetic-field data measured by scanning superconducting quantum interference device (SQUID) microscopy is reported. The current distribution contains the contributions from both transport supercurrent and vortex current. The transport supercurrent is found to flow mainly along the edge of a stripline, and the numerical calculation based on the simple London model by taking the specific sample-detector geometry into account is given. The agreement between the experimental data and the calculated results is good, demonstrating that the scanning SQUID microscope provides a useful tool for studying the current distribution in a superconductor.
On the classification of irreducible finite-dimensional representations of Uq′(so3) algebra
Authors: Havlı́ček M. , Pošta S.
Published: July 2002

In an earlier work [M. Havlı́ček et al., J. Math. Phys. 40, 2135 (1999)] we defined for any finite dimension five nonequivalent irreducible representations of the nonstandard deformation Uq′(so3) of the Lie algebra so3 where q is not a root of unity [for each dimension only one of them (called classical) admits limit q→1]. In the first part of this paper we show that any finite-dimensional irreducible representation is equivalent to some of these representations. In the case qn=1 we derive new Casimir elements of Uq′(so3) and show that a dimension of any irreducible representation is not higher than n. These elements are Casimir elements of Uq′(som) for all m and even of Uq′(isom+1) due to Inönu–Wigner contraction. According to the spectrum of one of the generators, the representations are found to belong to two main disjoint sets. We give full classification and explicit formulas for all representations from the first set (we call them nonsingular representations). If n is odd, we have full classification also for the remaining singular case with the exception of a finite number of representations.
Synthesis and characterization of pure C40 TiSi2
Authors: Chen S. Y. , Shen Z. X. , Li K. , See A. K. , Chan L. H.
Published: July 2002

In this letter, we report on the synthesis of a third phase of Ti disilicide, the C40 TiSi2 on Si substrate with pulsed laser annealing. This is achieved without doping the samples with foreign metals. We also show that with this C40 TiSi2, the technologically important C54 TiSi2 is achieved directly, completely bypassing the undesirable C49 phase. The C40 phase was identified using convergent beam electron diffraction. Raman spectrum of pure C40 TiSi2 was also obtained. The synthesis of the C40 phase without the additional refractory metal and its promotion effect on the C54 phase formation has important implications for the integrated circuit industry in 0.13 μm technology and beyond.
Some questions in the theory of a superfluid Fermi liquid with triplet pairing in a magnetic field
Authors: Tarasov A. N.
Published: July 2002

General expressions are obtained in explicit form for the anomalous and normal distribution functions of quasiparticles in nonunitary phases of a superfluid paramagnetic Fermi liquid consisting of electrically neutral fermions with triplet pairing (the pairs have spin s=1 and an arbitrary odd value of the orbital angular momentum l) in a uniform static magnetic field. A generalized Fermi-liquid approach is used which takes into account the exchange Fermi-liquid interaction but does not specify the form of the energy functional of the superfluid Fermi liquid. The results are valid at any temperature in the interval 0⩽T⩽Tc, where Tc is the temperature of the phase transition from the normal to the superfluid state. When an explicit form of the energy functional is specified, the general formulas for the distribution function can be used to describe the various nonunitary phases of a superfluid Fermi liquid of the He3 type in a high magnetic field. In particular, for He3-A1, He3-A2, and the nonunitary two-dimensional phase of He3 in a high magnetic field at 0⩽T⩽Tc, an energy functional quadratic in the distribution function is used to find a system of coupled equations for the order parameter and the effective magnetic field and to obtain an expression is obtained for the nonlinear magnetic susceptibility.
Threshold gain and gain-enhancement due to distributed-feedback in two-dimensional photonic-crystal lasers
Authors: Susa Nobuhiko
Published: July 2002

The threshold gain (gth) of a two-dimensional (2D) photonic crystal distributed-feedback (DFB) laser composed of 8×8 dielectric cylinders was one order of magnitude smaller than that of an 8-pair 1D DFB laser with the identical refractive indexes. In the 2D finite-width photonic crystal laser, gth using the 1st photonic band was smaller than that using the higher photonic band, contrary to the expectation from the flatness of the photonic band structures. This unexpected gth is probably due to the longer optical path caused by the reflection at the side boundary. Moreover, gth using the 1st photonic band was the smallest in the Γ–X direction of the square-lattice photonic crystal. The gain-enhancement using the 1st–3rd photonic bands were 10–30 in the 2D photonic crystal and that using the 3rd band was the largest. The gain-enhancement using the 1st photonic band of the 2D photonic crystal consisting of dielectric cylinders was larger than that of air cylinders.
Critical layer thickness determination of GaN/InGaN/GaN double heterostructures
Authors: Reed M. J. , El-Masry N. A. , Parker C. A. , Roberts J. C. , Bedair S. M.
Published: July 2002

We report on the critical layer thickness of GaN/InxGa1−xN/GaN double heterostructures in the composition range 0<x<0.16. The evolution of the photoluminescence spectra and the electrical properties of the InxGa1−xN well were monitored as its thickness was increased for a given % InN. Due to compressive stress and possible quantum-size effects, the emission energy from thin InGaN wells is blueshifted relative to thicker wells of a given % InN. The transition from the blueshifted emission of strained InGaN to redshifted emission of relaxed InGaN is also accompanied by dramatic changes in film conductivity and mobility. The thickness at which the onset of relaxation occurs is deemed the critical layer thickness of the InxGa1−xN film.
Lattice constant and coefficient of linear thermal expansion of the silicon crystal. Influence of isotopic composition
Authors: Zhernov A. P.
Published: July 2002

The features of the temperature behavior of the lattice constant and the coefficient of thermal expansion of silicon crystals are analyzed in the bond-charge model. The coefficient of thermal expansion and the Grüneisen factor for the natural isotopic composition are described to reasonable quantitative accuracy. The influence of the isotopic composition on the value of the lattice constant is discussed in detail.
Refractive-index patterning using near-field scanning optical microscopy
Authors: Takahashi Satoshi , Samata Katsutoshi , Muta Hirokazu , Machida Shinjiro , Horie Kazuyuki
Published: July 2002

By using near-field scanning optical microscopy (NSOM), patterning of pure refractive-index contrast on a transparent polymer film has been demonstrated. A thin film of poly(methyl methacrylate) doped with 3-phenyl-2,5-norbornadiene-2-carboxylic acid is irradiated with 325 nm UV light from an NSOM probe, resulting in a refractive-index decrease without changes in complete transparency in the visible region and surface flatness. The transmission NSOM image was obtained by the same probe at an incident wavelength of 442 nm. The relative change of the transmission signal is in agreement with the value deduced from a simple model based on the Rayleigh scattering theory of dielectric media.
Complex micromachines produced and driven by light
Authors: Galajda Péter , Ormos Pál
Published: July 2002

A method is introduced to build microscopic light driven rotors that are created and manipulated in laser tweezers. Laser light-induced two-photon polymerization of light curing resins is applied to generate effective rotating particles several microns in size. The dynamics of rotation are evaluated. Mechanical devices consisting of multiple moving parts driven by these rotors are produced. It is shown that the combination of the techniques of microfabrication by two-photon polymerization by light and the light-induced trapping and rotation offers a powerful tool with which to build complex mechanical machines of micrometer size.
Modeling of hydrophilic wafer bonding by molecular dynamics simulations
Authors: Litton David A. , Garofalini Stephen H.
Published: July 2002

The role of moisture in hydrophilic wafer bonding was modeled using molecular dynamics computer simulations of interface formation between amorphous silica surfaces. Three different surface treatments were used in order to determine the effect of moisture on the formation of siloxane (Si–O–Si) bridges across the interface at two temperatures. The three surface conditions that were studied were: (a) wet interfaces containing 1 monolayer of water adsorbed at the interface (based on the room temperature bulk density of water), (b) hydroxylated interfaces with concentrations of 3–5 silanols/nm2 on each surface and no excess water molecules initially in the system, and (c) pristine interfaces that had only Si and O and no water or H present. The surfaces were slowly brought together and siloxane bond formation was monitored. In the pristine interfaces, siloxane bridges formed across the interface by the coalescence of various defect species in each surface. A bimodal distribution of siloxane bond angles formed during the first 2.5 Å of approach after the first siloxane bond was formed. These bond angles were much lower than and higher than the bulk average, indicating the formation of less stable bonds. The hydroxylated (with no excess water) and wet surfaces showed a more uniform distribution of siloxane bond angles, with no highly reactive small bond angles forming. The presence of water molecules enhanced H-bond formation across the interface, but trapped water molecules inhibited formation of the strong siloxane bridges across the interface. In real systems, high temperatures are required to remove this trapped moisture.
Hydrogen bond cooperativity and electron delocalization in hydrogen fluoride clusters
Authors: Rincón Luis , Almeida Rafael , Garcı́a-Aldea David , Diez y Riega Helena
Published: July 2002

We investigate the energetic, structural, electronic and thermodynamics properties of hydrogen fluoride cluster, (HF)n, in the range n=2–8, by ab initio methods and density functional theory (DFT). The ab initio methods chosen were Hartree–Fock (RHF) and second-order Mo/ller–Plesset perturbation theory (MP2). The DFT calculations were based on Becke’s hybrid functional and the Lee–Yang–Parr correlation functional (B3LYP). We found that symmetric cyclic clusters are the most stable structure, and that large cooperative effects, particularly from trimer to tetramer are present, in binding energy, and hydrogen bond distance. An analysis of the topology of the electron density reveals a linear correlation between the binding energy per hydrogen bond and the density at the hydrogen bond critical point and the Cioslowski covalent bond order. Based on these correlations, hydrogen bond cooperativity is associated with the electronic delocalization between monomers units. Analysis of the thermodynamics properties shows that the enthalpy changes are determined by the electronic cooperative effects, while the entropic statistical factors are fundamental in the relative stability of these clusters. Finally, for the trimer and tetramer, nonstable linear zigzag chains where found in a detailed analysis of the potential energy surfaces.
Lithium doping of semiconducting organic charge transport materials
Authors: Parthasarathy G. , Shen C. , Kahn A. , Forrest S. R.
Published: July 2002

We study the effects of lithium (Li) incorporation in the cathodes of organic light-emitting devices. A thermally evaporated surface layer of metallic Li is found to diffuse through, and subsequently dope, the electron transporting organic semiconducting thin films immediately below the cathode, forming an Ohmic contact. A diffusion length of ∼700 Å is inferred from analyses of the current–voltage and secondary ion mass spectrometry data. The conductivity of the Li-doped organic films is ∼3×10−5 S/cm. Photoemission spectroscopy suggests that Li lowers the barrier to injection at the organic/cathode interface, introduces gap states in the bulk of the organic semiconductor, and dopes the bulk to facilitate efficient charge transport.
Elimination of endpoint-discontinuity artifacts in the analysis of spectra in reciprocal space
Authors: Yoo S. D. , Aspnes D. E.
Published: July 2002

Reciprocal-space analysis offers several advantages for determining critical point parameters in optical and other spectra, for example the separation of baseline effects, information, and noise in low-, medium-, and high-index Fourier coefficients, respectively. However, endpoint-discontinuity artifacts can obscure much of the information when segments are isolated for analysis. We developed a procedure for eliminating these artifacts and recovering buried information by minimizing in the white-noise region the mean-square deviation between the Fourier coefficients of the data and those of low-order polynomials, then subtracting the resulting coefficients from the data over the entire range. We find that spectral analysis is optimized if no false data are used, i.e., when the number of points transformed equals the number of actual data points in the segment. Using fractional differentiation we develop a simple derivation of the variation of the reciprocal-space coefficients with index n for Lorentzian and Gaussian line shapes in direct space. More generally, we show that the definition of critical point energies in terms of phase coherence of the Fourier coefficients allows these energies to be determined for a broad class of line shapes even if the direct-space line shapes themselves are not known. Limitations for undersampled or highly broadened spectra are discussed, along with extensions to two- or higher-dimensional arrays of data.
Diffusion into a pair of reactive spheres with first-order reaction
Authors: Tsao Heng-Kwong
Published: July 2002

Exact series solutions for the reaction rates of two identical spherical sinks immersed in an infinite medium are presented for both reactant-impenetrable and -penetrable particles based on the method of twin spherical expansion. The reaction rates are explicitly expressed in terms of the interparticle distance from reaction- to diffusion-limited conditions. The effect of the ratio of the diffusivities inside and outside the sink is investigated. The existence of the second particle influences the reaction rate most substantially for the external diffusion-limited condition. The exact solutions are used to evaluate the accuracy of simple, analytical approximations obtained by including the first few multipole contributions only. The result by keeping terms up to quadrupole level gives values in excellent agreement with the exact solution.
Maximum modal gain of a self-assembled InAs/GaAs quantum-dot laser
Authors: Asryan L. V. , Grundmann M. , Ledentsov N. N. , Stier O. , Suris R. A. , Bimberg D.
Published: July 2002

Gain and threshold current of a self-assembled InAs/GaAs quantum-dot (QD) laser are simulated. A small overlap integral of the electron and hole wave functions in pyramidal QDs is shown to be a possible reason for the low single-layer modal gain, which limits lasing via the ground-state transition at short (under a millimeter) cavity lengths.
Tailoring of light emission properties of functionalized oligothiophenes
Authors: Caldas Marilia J. , Pettenati Emanuele , Goldoni Guido , Molinari Elisa
Published: July 2002

We investigate theoretically the light emission properties of short oligothiophenes containing a thienyl-S,S-dioxide moiety, which have recently been shown to exhibit strong photoluminescence efficiency and tunability. We find that the dioxide substitution tends to increase the torsion angle between rings and strongly affects the states at the optical band edges, in a way depending on the position of the substituted ring. We have identified a low-energy transition which is optically active only when the dioxide unit is inserted between two thiophene rings, as a result of increased inter-ring torsional angle also in the excited state. With the dioxide unit in a terminal position, planarity is favored in the excited state, and the transition is optically inactive. For short oligomers, this can lead to nonradiative decay quenching the luminescence.
Relaxation of disordered polymer networks: Regular lattice made up of small-world Rouse networks
Authors: Gurtovenko A. A. , Blumen A.
Published: July 2002

As models for inhomogeneous polymer networks, we investigate the Rouse dynamics of regular lattices built from subunits with arbitrary internal structure. We analyze as an example a two-dimensional lattice, consisting of small-world networks (SWNs). Using analytical and numerical calculations we study the stretching of such a structure under an external force. We find that the network shows interesting relaxation features and an unusual behavior in the intermediate time (frequency) domain, which lies in the region between the modes of the SWN subunits and those of the lattice. This behavior is related to the SWN-density of states, which leads to the appearance of a “pseudogap” between the highest lattice eigenvalue and the lowest SWN eigenvalue.
Reaction path for hydrogen adsorption and desorption on Si(100)-(2×1)
Authors: Tok E. S. , Kang H. Chuan
Published: July 2002

The intradimer pre-paired desorption mechanism for hydrogen on Si(100)-(2×1) has been studied with density functional calculations using 1-dimer and 3-dimer cluster models of the surface. We find that adsorption/desorption occurs in a two-step process through a metastable dihydridelike intermediate. Two transition states are identified in this pathway. We confirm that the transition states are at saddle points by computing the vibrational frequencies, and that the reaction path from the monohydride to the desorbed state goes through the intermediate by performing eigenvector-following calculations from the two transition states. The effects of cluster size and basis set on the energetics are investigated. It is observed that energetics from 3-dimer cluster B3LYP6-311G** calculations are in reasonable agreement with experimental data for both the adsorption and desorption barriers. We find an adsorption barrier of 0.65 eV and a desorption barrier of 2.94 eV. We obtain a distance of approximately 3.21 Å between the silicon dimer atoms in the intermediate structure, implying a broken dimer bond, which we confirm by considering the electron density plots for the structure. Thus our results suggest that during each adsorption or desorption event the dimer bond is broken and reformed. The forces acting on the silicon dimer atoms and the hydrogen atoms along the reaction path are also investigated and these provide a picture of the coupling of the adsorption/desorption process to surface vibrations, particularly to the dimer bond breaking and reforming. This occurrence of dimer bond breaking and reforming along the reaction path suggests an explanation for the experimentally observed large surface-temperature activation of the adsorption process.
Electronic structure of chromium oxides, CrOn− and CrOn (n=1–5) from photoelectron spectroscopy and density functional theory calculations
Authors: Gutsev G. L. , Jena P. , Zhai Hua-Jin , Wang Lai-Sheng
Published: July 2002

The electronic structure of CrOn− and CrOn (n=1–5) was investigated using anion photoelectron spectroscopy and density functional theory. Photoelectron spectra of CrOn− were obtained at several photon energies and yielded electron affinities, vibrational and electronic structure information about the neutral CrOn species. Density functional theory calculations were carried out for both the neutrals and anions and were used to interpret the experimental spectra. Several low-lying electronic states of CrO were observed and assigned from photodetachment of the CrO− ground state (6∑+) and an excited state (4∏), which is only 0.1 eV higher. The main spectral features of CrO2− were interpreted based on a C2v CrO2− (4B1). A very weak Cr(O2)− isomer was also observed with lower electron binding energies. Relatively simple and vibrationally resolved spectra were observed for CrO3−, which was determined to be D3h. The CrO3 neutral was calculated to be C3v with the Cr atom slightly out of the plane of the three O atoms. The spectrum of CrO4− revealed a very high electron binding energy. Several isomers of CrO4− were predicted and the ground state has a distorted tetrahedral structure (C2) without any O–O bonding. Only one stable structure was predicted for CrO5− with a superoxo O2 bonded to a C3v CrO3.
Magnetoimpedance effect in amorphous and nanocrystalline ribbons
Authors: Hernando B. , Sánchez M. L. , Prida V. M. , Tejedor M. , Vázquez M.
Published: July 2002

The magnetoimpedance effect in several Co-rich amorphous ribbons is overviewed. Results are classified in the following sections: influence of anisotropies induced by stress annealing, dependence on applied stress, its dependence on stress or stress-impedance, and the appearance of hysteresis. The influence of nanocrystallization of given Fe-rich ribbons is also analyzed.
Applications of higher order composite factorization schemes in imaginary time path integral simulations
Authors: Jang Seogjoo , Jang Soonmin , Voth Gregory A.
Published: July 2002

Suzuki’s higher order composite factorization which involves both the potential and the force is applied to imaginary time path integral simulation. The expression is more general than the original version and involves a free parameter α in the range of [0, 1]. Formal expressions are derived for statistical averages, based on both thermodynamic and quantum operator identities. The derived expressions are then tested for one-dimensional model systems using the numerical matrix multiplication method, which involves no statistical error. When an optimum choice of α is made, the higher order factorization approach is shown to be more efficient than primitive factorization by about a factor of 4 and better than other existing higher order algorithms with similar character. Actual path integral simulation tests are then made for an excess electron in supercritical helium and for bulk water, and these generally demonstrate the efficiency of the higher order factorization approach.
Investigation of the effects of different annealing ambients on Ag/Al bilayers: Electrical properties and morphology
Authors: Malgas Gerald F. , Adams Daniel , Nguyen Phucanh , Wang Yu , Alford T. L. , Mayer J. W.
Published: July 2002

Ag (200 nm)/Al (8 nm) bilayer structures on SiO2 substrates were annealed at temperatures ranging from 300 °C–700 °C and for 30 min in He–H, Ar, or NH3 ambient. Upon annealing, Al segregates to the surface where it reacts with residual oxygen to form an Al-oxide passivation layer. Rutherford backscattering spectrometry and Auger spectroscopy showed the formation of an AlxOy diffusion barrier at the Ag/SiO2 interface. Apart from the surface passivation that resulted from annealing, the Ag/Al/SiO2 in different ambients, the adhesion of Ag to SiO2 was also improved. A strong correlation between the resistivity and the residual Al in the Ag film was observed. Resistivity values equivalent to that of the as-deposited sample were obtained only for temperatures at 700 °C or above. Scanning electron microscopy (SEM) combined with energy dispersive spectroscopy was used to study the effect of the different ambients on the observed surface morphology. The SEM studies showed that the passivated surfaces were not uniform but have hillocks and holes randomly distributed. In contrast to Ag on SiO2, no agglomeration was observed in the Ag/Al/SiO2 system.
Three- versus four-coordinate phosphorus in the gas phase and in solution: Treacherous relative energies for phosphine oxide and phosphinous acid
Authors: Wesolowski Steven S. , Brinkmann Nicole R. , Valeev Edward F. , Schaefer Henry F. , Repasky Matthew P. , Jorgensen William L.
Published: July 2002

Previous ab initio studies have consistently predicted phosphine oxide (H3PO) to be less stable than its nearly isoenergetic cis- and trans-phosphinous acid isomers (H2POH). However, complete basis set extrapolations employing the coupled-cluster series show that phosphine oxide is actually ca. 1.0 kcal/mol more stable than its acid forms in the gas phase. Incorporation of tight d functions via Dunning’s core-valence (cc-pCVXZ) or newly constructed “plus d” [cc-pV(X+d)Z] basis sets is essential for rapid convergence of core polarization effects which are evident even at the SCF level. The precision to which the phosphorus hybridization is described in the three- and four-coordinate environments ultimately determines the predicted gas-phase relative energy orderings. Focal-point analyses demonstrate that this system represents a disturbing case where use of a conventional valence quadruple-ζ quality basis set (cc-pVQZ)—even at the CCSD(T) level—fails to provide the correct relative energy ordering for simple closed-shell species which do not exhibit appreciable multireference character. Thus, we underscore the importance of using phosphorus basis sets which have the flexibility to describe core polarization adequately. In addition, Monte Carlo (MC) free-energy perturbation simulations in solution clearly demonstrate that the small energy gap significantly increases in favor of the oxide (10.0 kcal/mol) upon solvation due to stronger hydrogen bonding with the highly polar Pδ+→Oδ− bond.
Abnormal temperature dependence of low-field magnetization of FeCr2−xAlxS4
Authors: Yang Z. R. , Tan S. , Zhang Y. H.
Published: July 2002

In this letter, the abnormal temperature dependence of low-field magnetization of FeCr2−xAlxS4 (0⩽x⩽0.7) is reported. In addition to a large irreversibility between zero-field-cooling (ZFC) and field-cooling (FC) magnetization for all samples, a cusp is also observed in both FC and ZFC curves for x⩽0.2. The cusp and the irreversibility are suppressed under an applied field of 0.5 T. The study of magnetic hysteresis loop reveals that this abnormal low-field magnetic behavior is closely related to coercitivity of the system. Accordingly, the abnormal magnetic behavior is explained qualitatively by taking account of large magnetic anisotropy and domain wall pinning of nonmagnetic ions.
Fluctuation operators and spontaneous symmetry breaking
Authors: Requardt Manfred
Published: July 2002

We develop an alternative approach to this field, which was to a large extent developed by Verbeure et al. It is meant to complement their approach, which is largely based on a noncommutative central limit theorem and coordinate space estimates. In contrast to that we deal directly with the limits of l-point truncated correlation functions and show that they typically vanish for l⩾3 provided that the respective scaling exponents of the fluctuation observables are appropriately chosen. This direct approach is greatly simplified by the introduction of a smooth version of spatial averaging, which has a much nicer scaling behavior and the systematic development of Fourier space and energy-momentum spectral methods. We both analyze the regime of normal fluctuations, the various regimes of poor clustering and the case of spontaneous symmetry breaking or Goldstone phenomenon.
High-resolution three-dimensional scanning optical image system for intrinsic and extrinsic contrast agents in tissue
Authors: Gu Yueqing , Qian Zhiyu , Chen Jinxian , Blessington Dana , Ramanujam Nimmi , Chance Britton
Published: July 2002

This article presents the theory and development of a three-dimensional (3D) imaging instrument capable of determining the biochemical properties of tissue by measuring the absorption or fluorescence of different intrinsic and extrinsic agents simultaneously. A bifurcated optical fiber bundle, serving to deliver the excitation light and collect the emission or reflection light, scans over the flat tissue surface retrieving optical signals in each pixel. Two-dimensional (2D) images of a series of subsequent sections are obtained after signal conversion and processing to yield a 3D image. Manipulation of the scanning step and diameter size of the fibers within the bundle, the spatial resolution of the instrument attains a maximum of 40 × 40 × 10 μm3. The wavelength range is extended from ultraviolet to the near infrared (NIR) through specialized optical design, typically employed for the NIR extrinsic contrast agents study. The instrument is most applicable in situations involving the measurement of fluorescence or absorption at any specific wavelength within the spectrum range. Flavoprotein and nicotinamide adeine dinucleotide are the two typical intrinsic agents indicating the oxidization and reduction status of the tissue sample, with their fluorescence detected at wavelengths of 540 and 440 nm, respectively. Oxy and deoxy hemoglobin are two other significant intrinsic agents for evaluating the blood oxygenation saturation by recording their absorptions at two different wavelengths of 577 and 546 nm. These intrinsic agents were measured in this study for comparison of biochemical properties of rat liver in different gas inhalation treatments. Indocyanine green, a NIR extrinsic contrast agent measured at wavelengths of 780 nm/830 nm as excitation/emission can indicate blood pooling by displaying the distribution of blood vessels within a 9 L tumor. The advantage of high sensitivity, spatial resolution, and broad applied potentiality were demonstrated by the instrument during these experiments.
Highly directive light sources using two-dimensional photonic crystal slabs
Authors: Fehrembach Anne-Laure , Enoch Stefan , Sentenac Anne
Published: July 2002

We have designed a microcavity with periodic microstructure that extracts nearly all the power emitted by a luminescent source and confines 80% of the energy radiated in the superstrate in a cone of half width 0.2° about the normal of the device.
Direct comparison of mechanical and electro-optic responses of a low Tg photorefractive doped polymer
Authors: Ribierre J.-C. , Cheval G. , Huber F. , Mager L. , Fort A. , Muller R. , Méry S. , Nicoud J. F.
Published: July 2002

The temperature dependence of the electro-optic responses in a low glass transition temperature (Tg) photorefractive polymer was investigated using an ellipsometric technique. The sample was composed of a carbazole functionalized polysiloxane doped with a push–pull chalcone derivative. The results provide information on the orientational dynamics of the chromophores doping the polymer host. For this purpose, the electro-optic response is directly compared, for different temperatures above Tg, to dynamic shear compliance measurements characterizing the mechanical macroscopic behavior of the material. We demonstrate here that these orientational processes are entirely ruled by the mechanical properties of the material.
Fabrication of highly balanced directly coupled YBa2Cu3Oy gradiometers and their noise properties in an unshielded environment
Authors: Tsukamoto A. , Fukazawa T. , Takagi K. , Yokosawa K. , Suzuki D. , Tsukada K.
Published: July 2002

We fabricated directly coupled gradiometers made of YBa2Cu3Oy thin films on bicrystal substrates. The parasitic effective area of the gradiometric pickup coil was found to be comparable with that of the superconducting quantum interference devices (SQUIDs) in each gradiometer. A gradiometer balance of below 10−3 was achieved by selecting the coupling direction of the SQUID so as to cancel the parasitic effective area of the pickup coil by the effective area of the SQUID itself. The noise of the gradiometers measured in an unshielded environment decreased in proportion to the improvement in the gradiometer balance; however, the noise of the gradiometer with a balance of 0.03% was limited by the gradient-field component of the environmental noise.
Scaling for ECR sources of multicharged ions with pumping at frequencies from 10 to 100 GHz
Authors: Semenov V. , Skalyga V. , Smirnov A. , Zorin V.
Published: July 2002

One of the major trends in the development of ECR sources of multicharged ions is an increase of the extracted ion current. The main factor affecting the current of an ion beam produced by such a source is, obviously, the density of the plasma from which ions are extracted. In the present article the mean ion charge, the ion current density, and the minimal microwave power required to sustain needed electron temperature are calculated for a broad range of plasma densities (up to 1014 cm−3, which corresponds to 90 GHz cutoff frequency of microwave pumping). The electron temperature is taken as granted and no effects due to instabilities are considered. The distinguishing future of the analysis performed is that it takes into account alteration of the regime of plasma confinement, which occurs as the plasma density is increased. If the plasma density exceeds a certain threshold, the classical Pastukhov’s regime of plasma confinement is replaced by the quasi-gas-dynamic regime. It is demonstrated, in particular, that within the framework of a common approach it’s possible to analyze qualitatively conventional ECR sources with pumping frequencies up to 18 GHz as well as experiments where microwave radiation with the frequency of 37.5 GHz was used.
Laser manipulation and fixation of single gold nanoparticles in solution at room temperature
Authors: Ito Syoji , Yoshikawa Hiroyuki , Masuhara Hiroshi
Published: July 2002

A method to fix single gold nanoparticles on glass substrate was demonstrated in solution at room temperature by utilizing focused intense laser beams. A single gold nanoparticle of 80 nm was optically trapped and manipulated to a surface of a glass substrate, and then fixed on it by irradiation with ultraviolet (UV) laser light. Suitable laser fluence range for the fixation was determined to be 32–60 mJ/cm2, above which the individual nanoparticles were fragmented to several smaller fragments of 10 to 40 nm. The fixation mechanism is discussed in view of pulsed-laser-induced transient temperature elevation.
Microwave measurements of the absolute London penetration depth in double-sided YBa2Cu3O7−x thin films on sapphire
Authors: Zaitsev A. G. , Schneider R. , Linker G. , Ratzel F. , Smithey R. , Schweiss P. , Geerk J. , Schwab R. , Heidinger R.
Published: July 2002

The absolute values of the London penetration depth λL, were measured in epitaxial (001) YBa2Cu3O7−x (YBCO) thin films prepared by simultaneous sputter deposition on both sides of 3 in. r-cut sapphire wafers buffered with thin (001) CeO2 layers. The measurements were performed by using a technique, which is based on the effect produced on the quality factor Q of a microwave disk resonator by a gold layer deposited on the YBCO electrodes. The observed change of Q can be transformed into λL value with an accuracy determined mainly by the uncertainty of the YBCO film thickness. The λL(T) data obtained by this technique revealed a good agreement with the variation of the resonant frequency with temperature, which is conventionally used for measurements of the variation of the effective London penetration depth ΔλL eff(T) with temperature. At temperatures above ∼60 K this λL(T) dependence was very close to the predictions of the Gorter–Casimir model. At lower temperatures a linear λL(T) behavior was observed with the slope higher for thinner YBCO films. The absolute λL values were higher than those reported in literature for λab (screening currents flow in the crystallographic a–b plane) of YBCO single crystals. For example, our measured λL (4.2 K) values were in the range of 195–220 nm compared to literature data for λab quoted around 140 nm.
Phase behaviors of dendrimer/solvent systems: Molecular thermodynamics approach
Authors: Jang Jeong Gyu , Bae Young Chan
Published: July 2002

We investigate vapor–liquid equilibria of dendrimer/solvent (benzyl ether dendrimer/toluene) systems by the combination of incompressible lattice cluster theory and atomistic simulation technique. We also examine the structure effect of dendritic polymer and the specific interaction due to the difference of interaction energies of endgroup at the periphery of dendrimer molecules. The interaction energy parameters are obtained by the pairs method including Monte Carlo simulation technique with excluded volume constraint. In the pairs method, we do not simulate the whole molecule as in molecular dynamics or molecular mechanics, but only monomer segments interacting with solvent molecules. In general, those parameters are determined by fitting experimental data. Our results show that the specific interactions between the endgroup and the solvent molecule play an important role in determining phase behaviors of the given systems.