bluuebunny/crossref_metadata_2025 huggingface dataset:
Abatement of CF4 by atmospheric-pressure microwave plasma torch
Authors: Hong Yong C. , Uhm Han S.
Published: July 2003

An atmospheric microwave plasma torch is presented for post-pump destruction of perfluorocompound gases (PFCs), which are used widely in the semiconductor industry and are emitted with nitrogen gas for vacuum pump purges. Discharges of the microwave plasma torch are well suited for abatement of PFC contaminants discharged at a typical flow rate. The abatement was carried out using oxygen or air as additive gases. Analytical results are systematically compared to quadrupole mass spectroscopy and Fourier transform infrared (FTIR) data in the laboratory. Destruction and removal efficiency of more than 99% in FTIR data was achieved for carbon tetrafluoride.
Determination of the indium content and layer thicknesses in InGaN/GaN quantum wells by x-ray scattering
Authors: Vickers M. E. , Kappers M. J. , Smeeton T. M. , Thrush E. J. , Barnard J. S. , Humphreys C. J.
Published: July 2003

We have determined the indium content and the layer thicknesses in an InGaN epilayer and InGaN/GaN quantum well structures by high-resolution x-ray diffraction (XRD) using the (002) reflection. The thickness of the total repeat (an InGaN well plus a GaN barrier) in the superlattice is easily determined from the spacing between the satellite peaks in an omega/2theta scan. Measurement of the individual thickness of InGaN and GaN layers and the indium content is less straightforward, since for multilayer structures the peak positions are influenced by both the indium content and the thickness ratio of the GaN to the InGaN layer. Thus, several different models may give reasonable fits to data collected over a limited range (about 1° omega/2theta either side of the (002)) showing only lower-order (−3 to +3) satellite peaks. Whenever possible, we have collected data over a wide range (about 4° omega/2theta) and determined the thickness ratio by examination of the relative intensities of weak higher-order satellite peaks (−7 to +5). An alternative but less sensitive method is to use relative intensities from x-ray reflectivity measurements to give the thickness ratio. Once the thickness of both the InGaN and GaN layers has been established, the InGaN composition can be determined from the peak positions. If the quality of the samples is poor, because of inhomogeneities or wide diffuse interfaces, there are insufficient data to characterize the structures. There is good agreement between the composition of the epilayer as determined by XRD and secondary ion mass spectroscopy and good agreement between x-ray and electron microscopy results for the quantum well structures. We find no variation from Vegard’s rule for In contents less than 0.20. This article shows that structural parameters of high-quality InGaN/GaN superlattices with 10 and 5 repeats can be determined reliably by x-ray techniques: The InGaN and GaN thicknesses to ±1 Å and the In content to ±0.01.
Conductivity of nanometer-sized ion tracks in diamond-like carbon films
Authors: Krauser J. , Zollondz J.-H. , Weidinger A. , Trautmann C.
Published: July 2003

Amorphous diamond-like carbon films were irradiated with high-energy heavy ions like 1 GeV uranium or 340 MeV gold. The large energy deposition along the ion track leads to a transformation of the material from insulating diamond-like to conducting graphite-like carbon. The ion tracks form thin straight channels (nanowires) with a diameter of approximately 8 nm and a conductivity of the order of 1–10 S/cm. The conductivity in the tracks is up to eight orders of magnitude larger than in the surrounding material. The surface topography and the conduction properties of these channels were studied with an atomic force microscope.
Emissive properties of xenon ions from a laser-produced plasma in the 100–140 Å spectral range: Atomic-physics analysis of the experimental data
Authors: Gilleron F. , Poirier M. , Blenski T. , Schmidt M. , Ceccotti T.
Published: July 2003

In order to design extreme ultraviolet (EUV) sources for nanolithography, xenon EUV emission has been experimentally studied in a plasma generated by the interaction of a high-power laser with a droplet jet. A theoretical model assuming that the resulting plasma is optically thick allows one to find the distribution of the relevant ions and transitions involved in the emission process. Atomic physics computations are performed using the HULLAC code to give a detailed account of the transitions involved. The importance of 4p–4d, 4d–4f, and 4d–5p transitions is stressed, as well as the need for configuration-interaction treatment of the Δn=0 transitions. Comparisons of a modeled local thermodynamical equilibrium spectrum with experiment provides qualitative agreement and permits an estimate of the plasma temperature, density, and dimensions.
Negative magnetocaloric effect at the antiferromagnetic to ferromagnetic transition of Mn3GaC
Authors: Tohei T. , Wada H. , Kanomata T.
Published: July 2003

The magnetocaloric effect of Mn3GaC, which shows an antiferromagnetic to ferromagnetic transition at 165 K has been investigated. In this compound, magnetocaloric effect obtained at the transition is opposite to that of ordinary ferromagnetic systems, namely, negative magnetocaloric effect. It was found that a large magnetic entropy change, ΔSmag, of 15 J/kg K is obtained under an applied field of 2 T. The adiabatic temperature change, ΔTad, reaches 5.4 K in a field change of 2 T. At higher magnetic fields, both ΔSmag and ΔTad retain a large value over wide temperature range, exhibiting characteristic temperature dependence of a trapezoidal shape. These features are attributed to a sharp first-order transition retained in high magnetic fields as well as small magnetocrystalline anisotropy.
Influence of sputtering mechanisms on the preferred orientation of aluminum nitride thin films
Authors: Clement M. , Iborra E. , Sangrador J. , Sanz-Hervás A. , Vergara L. , Aguilar M.
Published: July 2003

This article presents a study of the influence of the sputtering parameters on the preferred orientation of polycrystalline aluminum nitride thin films. Aluminum nitride films were grown by rf reactive sputtering of an aluminum target in an N2/Ar gas mixture for different values of the deposition parameters: total pressure, nitrogen content in the discharge gas, and substrate bias voltage. The preferred orientation of the films was analyzed by x-ray diffraction. Films with different preferred orientations were obtained, ranging from c-axis oriented films to films with the c axis tilted by up to 61.6° from the substrate normal. The different mechanisms influencing the preferred orientation of the films have been considered, especially the transfer of energy to the adatoms on the substrate by particle bombardment. An analysis of the relation between the deposition parameters and the crystal orientation has allowed us to determine the relative importance of the different particles in the supply of energy to the substrate. We have found that Ar ion bombardment of the film during growth is the most influential mechanism on the preferred orientation of the films. As bombardment becomes more energetic, microcrystals in the film tend to grow with the c axis along the surface normal. The energy of Ar bombardment can be best controlled through the substrate bias voltage, a characteristic that we have employed to obtain AlN films exhibiting pure (00.2) preferred orientation and rocking curves with a full width at half maximum as low as 4.2°.
Large negative magnetoresistance and two-dimensional weak localization in carbon nanofiber fabricated using electrospinning
Authors: Wang Yu , Santiago-Avilés Jorge J.
Published: July 2003

The carbon nanofibers used in this work were derived from polyacrylonitrile/N,N-dimethyl formamide precursor solution using electrospinning and vacuum pyrolysis techniques. Their conductivity σ was measured at temperatures between 1.9 and 300 K and transverse magnetic field between −9 and 9 T. Zero magnetic field conductivity σ(0, T) was found to increase monotonically with the temperature with a convex σ(0, T) vs T curve. The conductivity increases with the external transverse magnetic field, revealing negative magnetoresistance at temperatures between 1.9 and 10 K, with a maximum magnetoresistance of −75% at 1.9 K and 9 T. The magnetic field dependence of the conductivity can be explained using the two-dimensional (2D) weak localization effect. The temperature dependence of σ(0, T) can be explained using a modified simple two-band model with temperature-dependent carrier mobility, and a correction introduced by 2D weak localization.
Photoluminescence of Cl-doped ZnTe epitaxial layer grown by atmospheric pressure metalorganic vapor phase epitaxy
Authors: Tanaka Tooru , Hayashida Kazuki , Nishio Mitsuhiro , Guo Qixin , Ogawa Hiroshi
Published: July 2003

Effects of Cl doping on the photoluminescence (PL) of ZnTe homoepitaxial layers grown by atmospheric pressure metalorganic vapor phase epitaxy using n-butylchloride (n-BuCl) have been investigated. A quite strong donor bound excitonic emission was observed by increasing the n-BuCl transport rate, implying that Cl is incorporated into ZnTe layer as a donor. From the temperature dependence of PL spectra, the thermal activation energy of donor bound excitonic emission was found to be 5.9 meV. Two luminescence bands due to donor-acceptor pair emission appeared at 2.34 and 2.22 eV in the layer grown at an n-BuCl transport rate as high as 0.2 μmol/min. The donor and acceptor levels corresponding to the PL peak at 2.34 eV were estimated to be 22 and 51 meV from the excitation power dependence of PL peak energy, respectively.
Simulation and theory of vibrational phase relaxation in the critical and supercritical nitrogen: Origin of observed anomalies
Authors: Roychowdhury Swapan , Bagchi Biman
Published: July 2003

We present results of extensive computer simulations and theoretical analysis of vibrational phase relaxation of a nitrogen molecule along the critical isochore and also along the gas-liquid coexistence. The simulation includes all the different contributions [atom-atom (AA), vibration- rotation (VR), and resonant transfer] and their cross-correlations. Following Everitt and Skinner, we have included the vibrational coordinate (q) dependence of the interatomic potential. It is found that the latter makes an important contribution. The simulated results are in good agreement with the experiments. Dephasing time (τv) and the root mean square frequency fluctuation (Δ) in the supercritical region are calculated. The principal important results are: (a) a crossover from a Lorentzian-type to a Gaussian lineshape is observed as the critical point is approached along the isochore (from above), (b) the root mean square frequency fluctuation shows nonmonotonic dependence on the temperature along critical isochore, (c) along the coexistence line and the critical isochore the temperature dependent linewidth shows a divergence-like λ-shape behavior, and (d) the value of the critical exponents along the coexistence and along the isochore are obtained by fitting. It is found that the linewidths (directly proportional to the rate of vibrational phase relaxation) calculated from the time integral of the normal coordinate time correlation function [CQ(t)] are in good agreement with the known experimental results. The origin of the anomalous temperature dependence of linewidth can be traced to simultaneous occurrence of several factors, (i) the enhancement of negative cross-correlations between AA and VR contributions and (ii) the large density fluctuations as the critical point (CP) is approached. The former makes the decay faster so that local density fluctuations are probed on a femtosecond time scale. The reason for the negative cross-correlation between AA and VR is explored in detail. A mode coupling theory (MCT) analysis shows the slow decay of the enhanced density fluctuations near critical point. The MCT analysis demonstrates that the large enhancement of VR coupling near CP arises from the non-Gaussian behavior of density fluctuation and this enters through a nonzero value of the triplet direct correlation function.
Microscopic fluctuations and pattern formation in a supercritical oscillatory chemical system
Authors: Hastings Harold M. , Field Richard J. , Sobel Sabrina G.
Published: July 2003

The spontaneous formation of ordered spatial concentration patterns in an unstirred chemical medium, supported by dissipation of chemical free energy, has been considered often since a pioneering suggestion by Turing and early work by Prigogine et al. and more recent work by Ross et al. involving nonequilibrium thermodynamics. The prototype experimental example is the oscillatory Belousov–Zhabotinsky reaction, in which target patterns of outward-moving concentric rings are readily observed. One widely-studied question is whether “microscopic” fluctuations can nucleate these target centers, or whether a catalytic, nucleating heterogeneous center is required. Vidal and Pagola observed spontaneous initiation with no nucleating particles visible at 6-micron resolution; however Zhang, Förster, and Ross argued theoretically that this is impossible in regimes far from Hopf bifurcations. We describe here an explicit mechanism in a “supercritical regime,” following and near to the low-f Hopf bifurcation in a generalized Oregonator model, by which microscopic fluctuations can nucleate activity, and reconcile these results with Zhang et al. Concentrations remain very close to the unstable steady-state values after the system slowly passes through the bifurcation point but before occurrence of the inevitable transition to large-amplitude limit cycle oscillations. Suitably timed small (even microscopic) fluctuations about this supercritical state can sharply accelerate the inevitable onset of large-amplitude limit cycle oscillations, potentially nucleating targets.
Chemical-shift γ-encoding nuclear magnetic resonance for uniaxially oriented matter under sample spinning
Authors: Nishiyama Yusuke , Kubo Atsushi , Terao Takehiko
Published: July 2003

Two approaches are proposed by which chemical shift anisotropies (CSAs) in uniaxially oriented matter can be obtained from the positions of sharp peaks under sample spinning. Both rely on the removal of broadening due to the γ dependence of the resonance frequency, where γ is the third angle of the Euler angles describing the orientation of the chemical shift tensor in the sample symmetry-axis frame. In one approach, an inversion pulse train recovers the γ-dependent CSAs, and the free-induction decay is transformed into sharp lines by γ-encoding transformation. In the other, the free-induction decay signal due to γ-dependent anisotropic chemical shifts is observed by rf-field inhomogeneity-compensated rotary resonance, and the CSAs are represented as the positions of sharp lines by Fourier transformation. The effects of finite pulse width, rf-amplitude missetting, rf-field inhomogeneity, and resonance offset are theoretically and experimentally investigated for both approaches. When the orientation of the sample symmetry axis in the CSA principal axis system distributes around the central orientation to some extent, peaks in γ-encoded two-dimensional (2D) spectra under off-magic-angle spinning show two-dimensionally broadened line shapes. It is theoretically shown that the orientational distribution can be directly obtained by transforming the 2D spectrum on the basis of the one-to-one correspondence between the peak position and the symmetry axis orientation. The C13 CSA in uniaxially drawn polyethylene and the C13 CSA with the orientational distribution in uniaxially drawn polypropylene are obtained from the γ-encoded 2D spectra.
Trends in radiation production from dynamic-hohlraums driven by single and nested wire arrays
Authors: Sanford T. W. L. , Lemke R. W. , Mock R. C. , Peterson D. L.
Published: July 2003

The axial radiation developed primarily from the interior of an imploding dynamic hohlraum (DH) target within a Z pinch using a single array with a large number of tungsten wires is compared with that generated using a standard [Phys. Plasmas 9, 3573 (2002)] nested (outer plus inner) array on Z. Measurements indicate that a single-array with a mass (2.5 mg) near that of the combined mass of the nested-array maximizes the DH axial power. At this mass, the DH utilizing the standard nested array generates (23±15)% more axial power than that of a single array. Measurements over a range of single-array masses (2–6 mg) show a decrease in radiation power for masses above 3.5 mg. Two-dimensional radiation magnetohydrodynamic simulations, which have successfully described radial emission from targetless implosions for both single and nested-array configurations [Phys. Plasmas 6, 2178 (1999)], however, do not follow the observed trends well. This lack of tracking implies that current 2D models, which take into account the development of the magnetic Rayleigh–Taylor (RT) instability in the r–z plane, require improvements in order to provide a self-consistent description of the implosion dynamics and radiation production for DH experiments.
Experimental study for angular distribution of the hot electrons generated by femtosecond laser interaction with solid targets
Authors: Cai D. F. , Gu Y. Q. , Zheng Z. J. , Wen T. S. , Chunyu S. T. , Wang Z. B. , Yang X. D.
Published: July 2003

The experimental results of angular distribution of hot electrons in the interaction of a 60 fs, 125 mJ, 800 nm, ∼1017 W cm−2 laser pulse with Al targets are reported. Three obvious peaks of hot electrons emission have been observed, as there is a weak normal component of the laser electric field. These emission peaks are located in the directions of the specular reflection of the laser, the target normal, and the backreflection of the laser, respectively. In the case of the P-polarized laser pulse, which has a strong normal component of the laser electric field, the peak in the backreflection of the laser disappeared, and only two obvious peaks of hot electron emissions existed. It shows that the different directions of hot electrons emission are dominated by different absorption or acceleration mechanisms. The experimental result of the hot electrons energy spectrum at the target normal shows that the effective temperature of hot electrons is about 190 keV, which is consistent with a scaling law of the resonance absorption.
Electrical and nonlinear optical properties of KTiOPO4 single crystals doped by Nb or Sb
Authors: Voronkova V. I. , Yanovskii V. K. , Losevskaya T. Yu. , Stefanovich S. Yu.
Published: July 2003

Single crystals of K1−xTi1−xNbxOPO4 and K1−xTi1−xSbxOPO4 solid solutions are grown and their dielectric properties, electrical conductivity, and nonlinear optical properties are studied. Maximal content of these dopants x is found to be 0.11 for Nb and 0.23 for Sb. This doping leads to the creation of the additional potassium vacancies and the high ionic conductivity σ33. Ferroelectric phase transitions become more diffuse and their temperature decreases. All the crystals show intensive broad maxima of the dielectric permeability attributed with the relaxation phenomena.
Additional symmetries and solutions of the dispersionless KP hierarchy
Authors: Alonso Luis Martı́nez , Mañas Manuel
Published: July 2003

The dispersionless KP hierarchy is considered from the point of view of the twistor formalism. A set of explicit additional symmetries is characterized and its action on the solutions of the twistor equations is studied. A method for dealing with the twistor equations by taking advantage of hodograph type equations is proposed. This method is applied for determining the orbits of solutions satisfying reduction constraints of Gelfand–Dikii type under the action of additional symmetries.
Extended resolvent and inverse scattering with an application to KPI
Authors: Boiti M. , Pempinelli F. , Pogrebkov A. K. , Prinari B.
Published: July 2003

We present in detail an extended resolvent approach for investigating linear problems associated to 2+1 dimensional integrable equations. Our presentation is based as an example on the nonstationary Schrödinger equation with potential being a perturbation of the one-soliton potential by means of a decaying two-dimensional function. Modification of the inverse scattering theory as well as properties of the Jost solutions and spectral data as follows from the resolvent approach are given.
Products and ratios of characteristic polynomials of random Hermitian matrices
Authors: Baik Jinho , Deift Percy , Strahov Eugene
Published: July 2003

We present new and streamlined proofs of various formulas for products and ratios of characteristic polynomials of random Hermitian matrices that have appeared recently in the literature.
Si nanocrystals obtained through polymer pyrolysis
Authors: Sorarù Gian Domenico , Modena Stefano , Bettotti Paolo , Das Gobind , Mariotto Gino , Pavesi Lorenzo
Published: July 2003

In this letter, we report the formation of bulk samples of silica-based glass containing Si nanocrystals (Si-ncs) by pyrolysis of a preceramic precursor. The starting precursor is a sol–gel-derived polysiloxane containing only Si–H groups which leads, after annealing in a controlled atmosphere in the range 1000–1200 °C, to the precipitation of Si-ncs. Characterization of the nanostructure was performed by x-ray diffraction and Raman scattering analyses. Room-temperature luminescence experiments show the interesting optical properties of the Si-ncs/SiO2 material.
Epitaxy of Fe3O4 on Si(001) by pulsed laser deposition using a TiN/MgO buffer layer
Authors: Reisinger D. , Schonecke M. , Brenninger T. , Opel M. , Erb A. , Alff L. , Gross R.
Published: July 2003

Epitaxy of oxide materials on silicon (Si) substrates is of great interest for future functional devices using the large variety of physical properties of the oxides as ferroelectricity, ferromagnetism, or superconductivity. Recently, materials with high spin polarization of the charge carriers have become interesting for semiconductor-oxide hybrid devices in spin electronics. Here, we report on pulsed laser deposition of magnetite (Fe3O4) on Si(001) substrates cleaned by an in situ laser beam high temperature treatment. After depositing a double buffer layer of titanium nitride and magnesium oxide (MgO), a high quality epitaxial magnetite layer can be grown as verified by reflection high energy electron diffraction intensity oscillations and high resolution x-ray diffraction.
Heat conduction of a spheroidal inhomogeneity with imperfectly bonded interface
Authors: Duschlbauer D. , Pettermann H. E. , Böhm H. J.
Published: July 2003

An analytical approach is presented for solving the steady state thermal conductivity problem for the following configuration. An infinite matrix material contains a single inhomogeneous spheroidal inclusion and a thermal resistance is present at the pertinent interface. The matrix shows isotropic conductivity while the inclusion is transversely isotropic, the principal material axes being aligned with the spheroid. Analytical expressions are derived for the local gradient fields in the matrix and in the inclusion as well as for the temperature mismatch along the interface. An analytical method is developed which enables replacement of the original imperfectly bonded inclusion by a less conductive but perfectly bonded inclusion. For the specific case of confocal distributions of the interface resistance the present approach yields the exact solution, i.e., the replacement operation leaves the matrix fields unchanged. For general spatial distributions of the interface properties (consistent with the spheroidal symmetry properties) an approximate solution strategy is introduced providing estimates. The further application of the present method for investigations of nondilute composites is discussed in terms of homogenization and localization. The effect of the inclusion size in combination with an interface resistance on the conductivity of nonhomogeneous systems is addressed, as well as critical inclusion dimensions. The proposed method is compared with existing approaches in the literature. Additionally, cross links to established models of coated inclusions are provided. As an example, the material system of diamond inclusions in a zinc sulfide matrix is considered. Predictions are compared with results from analytical approaches as well as numerical results from finite element analyses.
Modeling of magnetically enhanced capacitively coupled plasma sources: Ar discharges
Authors: Kushner Mark J.
Published: July 2003

Magnetically enhanced capacitively coupled plasma sources use transverse static magnetic fields to modify the performance of low pressure radio frequency discharges. Magnetically enhanced reactive ion etching (MERIE) sources typically use magnetic fields of tens to hundreds of Gauss parallel to the substrate to increase the plasma density at a given pressure or to lower the operating pressure. In this article results from a two-dimensional hybrid-fluid computational investigation of MERIE reactors with plasmas sustained in argon are discussed for an industrially relevant geometry. The reduction in electron cross field mobility as the magnetic field increases produces a systematic decrease in the dc bias (becoming more positive). This decrease is accompanied by a decrease in the energy and increase in angular spread of the ion flux to the substrate. Similar trends are observed when decreasing pressure for a constant magnetic field. Although for constant power the magnitudes of ion fluxes to the substrate increase with moderate magnetic fields, the fluxes decreased at larger magnetic fields. These trends are due, in part, to a reduction in the contributions of more efficient multistep ionization.
Multielectronic conduction in La1−xSrxGa1/2Mn1/2O3−δ as solid oxide fuel cell cathode
Authors: Iguchi E. , Hashimoto Y. , Kurumada M. , Munakata F.
Published: July 2003

Four-probe dc conductivities, capacitances, and thermopower have been measured in the temperature range of 80–1123 K for La1−xSrxGa1/2Mn1/2O3−δ, which is a desirable cathode material for lanthanum–gallate electrolytes of solid oxide fuel cells. The dc conductivities in the specimens (0.1⩽x⩽0.3) are insensitive to x but the thermopower is very sensitive to x, although the x=0 specimen exhibits a somewhat different conduction behavior. At T<300 K, a relaxation process has shown in dielectric loss factor with the activation energy higher than that for dc conduction in every specimen. These results at T<300 K have been numerically analyzed within the framework of the multielectronic conduction consisting of the polaronic conduction of Mn 3d eg holes created by Sr doping, the band conduction of O 2p holes and the hopping conduction of Mn 3d eg electrons, where the O 2p holes and Mn 3d eg electrons are created by thermal excitation of electrons from O 2p bands to Mn 3d eg narrow bands. At T>500 K, the band conduction dominates the electronic transports. The ionic conduction due to O2− migration seems difficult to contribute directly to the dc conduction even at high temperature.
Ion collection by cylindrical probes in weakly collisional plasmas: Theory and experiment
Authors: Sternovsky Z. , Robertson S. , Lampe M.
Published: July 2003

A theoretical approach has recently been described [Z. Sternovsky, S. Robertson, and M. Lampe, Phys. Plasmas 10, 300 (2003)] for including the effect of ion collisions in the orbit motion limited theory for cylindrical Langmuir probes. In plasmas with a single ion species, ion-neutral charge exchange collisions are dominant and their first order effect is to increase the magnitude of the collected ion current. Measurements in Ar and Ne gas discharges at plasma densities <109 cm−3 show that the theory is accurate only for probes with radii less than approximately half the Debye length. For larger probes, absorption of ions at the probe surface reduces the ion density locally causing the sheath to expand. This increases the volume from which the charge exchange ions are collected and further increases the ion current. Poor agreement between measurements and theory is also found, when the probe is placed close to the ionization source.
Properties of Tb-doped vacuum-sintered Lu2O3 storage phosphor
Authors: Zych E. , Trojan-Piegza J. , Hreniak D. , Strek W.
Published: July 2003

Tb 3+ -doped Lu2O3 sintered ceramics were prepared in vacuum and in air. It was shown that the vacuum-sintered disks are able to store energy when irradiated with 300-nm or shorter photons. A small part of the stored energy could be recovered with 980-nm light. A much more significant amount of the stored energy could be released with red 647-nm photons. However, recovering the total stored energy could be accomplished only upon heating up to about 300 °C. Changes in absorption of the raw materials upon ultraviolet irradiation and subsequent IR (980 and 647 nm) treatments or upon heating at 300 °C are presented and discussed. A model for energy storing and recovering through the various IR irradiations or through heating is presented. At least two distinct ways of hole trapping as Tb4+ or Vk-center as well as creation of F and F+ is suggested.
Theoretical investigation of weak hydrogen bonds to sulfur
Authors: Wennmohs F. , Staemmler V. , Schindler M.
Published: July 2003

The interaction energies of the dimethylsulfide–methanol (I) and dimethylthiocarbonyl–methanol (II) complexes are calculated as a function of the S⋯H–O distances at various levels of theory and compared to those of their oxygen analogs. At the coupled cluster level the binding energy of (I) is −5.46 kcal/mol, only slightly smaller than the hydrogen bond energy of −5.97 kcal/mol for the corresponding oxygen analog, i.e., the dimethylether–methanol complex. It is also considerably larger than for dimethylether–methylthiol, where S and O of the parent complex are interchanged. Density functional theory is unable to describe these weak interactions properly. Choosing second-order Møller–Plesset perturbation theory, the interaction potential surfaces of both complexes with respect to the three relevant intermolecular coordinates are compared. The interactions in the hydrogen bonds involving sulfur are classified by Morokuma, atoms-in-molecules, and natural bond orbital analyses.
Computer simulations of a liquid crystalline dendrimer in liquid crystalline solvents
Authors: Wilson Mark R. , Ilnytskyi Jaroslav M. , Stimson Lorna M.
Published: July 2003

Molecular dynamics simulations have been carried out to study the structure of a model liquid crystalline dendrimer (LCDr) in solution. A simplified model is used for a third generation carbosilane LCDr in which united atom Lennard-Jones sites are used to represent all heavy atoms in the dendrimer with the exception of the terminal mesogenic groups, which are represented by Gay–Berne potentials. The model dendrimer is immersed in a mesogenic solvent composed of Gay–Berne particles, which can form nematic and smectic-A phases in addition to the isotropic liquid. Markedly different behavior results from simulations in the different phases, with the dendrimer changing shape from spherical to rodlike in moving from isotropic to nematic solvents. In the smectic-A phase the terminal mesogenic units are able to occupy five separate smectic layers. The change in structure of the dendrimer is mediated by conformational changes in the flexible chains, which link the terminal mesogenic moieties to the dendrimer core.
Multiple versus single pathways in electron transfer in proteins:  Influence of protein dynamics and hydrogen bonds
Authors: Kobayashi Chigusa , Baldridge Kim , Onuchic José N.
Published: July 2003

The present work addresses the question of how electron tunneling matrix elements can be quantitatively calculated. In particular, we demonstrate how to go beyond Pathways methods, which have been shown to be qualitatively appropriate to do such calculations. Utilizing a combination of molecular dynamics with semiempirical and ab initio (Hartree–Fock) quantum chemistry calculations, we quantify how the dynamics and quality of the electronic Hamiltonian affect the outcome of the tunneling matrix elements. We show that tunneling is dominated by either one or, at most, a few constructively interfering pathway tubes. Even when equilibrium structures have destructive interference, tunneling is dominated by structures reached by fluctuations, where this interference is destroyed. In this limit, when one or a few constructive pathway tubes dominate, properly selected semiempirical methods are sufficiently powerful to provide quantitative predictions of the tunneling matrix elements. This combined methodology allows us to investigate quite large protein complexes. Calculations involving the electron transfer processes in azurin are used to validate these conclusions.
Spectral properties of decaying turbulence in electron magnetohydrodynamics
Authors: Abdalla T. M. , Lakhin V. P. , Schep T. J. , Westerhof E.
Published: July 2003

The spectral properties of decaying turbulence in 212-dimensional electron magnetohydrodynamics are studied numerically. In the range kde<1 the energy exhibits a direct cascade while mean square momentum exhibits an inverse cascade. Their spectra are characterized by k−7/3 and k−13/3, respectively. The self-similar decay state of the turbulence is reached after an initial phase of fast exchange between the axial and poloidal magnetic energies. The time behavior t−2/3 of the total energy is found to be consistent with that obtained from selective decay. The maximum of the energy spectrum shifts towards low mode numbers and decays in time as t−1, in agreement with the infrared scaling of the turbulence. In the large de limit, both energy and mean square generalized momentum exhibit direct cascades. No stationary turbulent state could be found as long as the axial kinetic energy is large as compared to the poloidal kinetic energy initially. The global physical quantities decay well before turbulent macroscopic quantities have established similar space–time behavior, and the turbulence is infected by the lack of stationarity. The system decouples into a Navier–Stokes equation and a passive scalar equation only if the poloidal kinetic energy is larger than or equal to the axial kinetic energy. In this limit the k−5/3 and k−3 spectra of the poloidal kinetic energy are recovered.
Nonstationary nonlinear dust-acoustic waves in unmagnetized plasma
Authors: Chakrabarti Nikhil , Janaki M. S.
Published: July 2003

The nonstationary and nonlinear dust-acoustic wave is considered in the framework of the Lagrangian fluid description. A single nonlinear equation and its solutions are obtained for dust-acoustic waves with nontrivial space and time dependence in both the large and small amplitude limit. In the absence of linear dispersion solutions in the large amplitude limit, it is demonstrated that under well-defined initial and boundary conditions the amplitude of the solutions is decreasing and the spatial profile spreads. This is a new class of nonlinear solutions leading to short-lived nonlinear structures. In the small amplitude limit, the soliton-like solution indicates close association with Korteweg–de Vries solitons.
Etching organic low dielectric film in ultrahigh frequency plasma using N2/H2 and N2/NH3 gases
Authors: Nagai Hisao , Hiramatsu Mineo , Hori Masaru , Goto Toshio
Published: July 2003

The behavior of species in the gas phase and their effects on the etching characteristics of organic film with the lower dielectric constant (low-k) were investigated in 500 MHz ultrahigh frequency (UHF) plasma using N2/H2 and N2/NH3 gases. The absolute H and N radical densities and NH3 molecule density in etching plasmas were evaluated by the vacuum ultraviolet absorption spectroscopy and the infrared diode laser absorption spectroscopy technique, respectively. The effects of frequency that excited the plasma in the production of H and N radicals were clarified by comparing the behavior of radicals in the UHF plasma with that in the inductively coupled plasma (ICP) exciting at the frequency of 13.56 MHz reported in the previous study. H radicals were produced more efficiently from NH3 gas and N radicals more efficiently from N2 gas in the UHF plasma than in the ICP. H radicals were generated not only through the dissociation of the H2 molecule but also from NH3 molecules produced in the N2/H2 plasma. On the other hand, the N radical density was lower by one order of magnitude than the H radical density, and was seldom produced from the NH3 molecule. It was found that the etch rate and etched profile could be controlled with an internal plasma parameter of the ratio of the H and N radical densities under the constant electron density in the plasma gas chemistry of both N2/H2 and N2/NH3. The optimal conditions for the anisotropic profile and high etch rate were well determined by the ratio of the H and N radical densities and the substrate temperature. The results of this study are very useful not only for understanding the fundamental process of organic low-k film etching but also for precisely controlling the etching process.
Investigation of spatial current imaging in mesoscopic systems
Authors: Cresti Alessandro , Farchioni Riccardo , Grosso Giuseppe , Parravicini Giuseppe Pastori
Published: July 2003

We present a method for the numerical calculation of the spatial distribution of currents in mesoscopic quantum wires, based on the Keldysh–Green’s function formalism. We adopt a lattice model and exploit the real space renormalization procedure to numerically describe a quantum point contact with the connecting leads. The computed conductance maps shed light on recent experiments aimed to probe the current distribution using scanning microscopy and open perspectives to design optimized device geometries.
Optimum structure of deposited ultrathin silicon oxynitride film to minimize leakage current
Authors: Muraoka K. , Kurihara K. , Yasuda N. , Satake H.
Published: July 2003

The role of oxygen inside deposited silicon oxynitride (SiON) films with respect to leakage current was investigated. We controlled the composition of bulk SiON film in accordance with the alloy model (Si3N4)x(SiO2)1−x regarding the ultrathin composite SiON structure and stacked SiON/SiO2 structure and measured electrical properties of these SiON films. Furthermore, we studied the conduction mechanisms of the leakage current for these films by using a direct tunneling (DT) current simulation with Wentzel–Kramers–Brillouin approximation, taking the dielectric constant, band profile, and effective mass into consideration. From these experiments and simulations, it was found that the leakage current of composite structure is lower than that of the stacked structure, even if those structures have the same alloy ratio x. Furthermore, we found the optimum film composition to minimize the DT current. The optimum structure was determined by the balance between the decrease in the electron DT current and the increase in the hole DT current due to the physical thickness and the valence-band barrier height.
Lubricative coatings of copper oxide for aerospace applications
Authors: Goto Masahiro , Kasahara Akira , Oishi Tetsuo , Konishi Youko , Tosa Masahiro
Published: July 2003

Lubricative coatings of copper oxide on a stainless-steel substrate were synthesized by a rf magnetron sputter deposition method with a different gas ratio of oxygen and argon in sputter plasma. The crystal structure of the coatings was analyzed with x-ray diffraction spectroscopy. The friction force of the coating films in atmospheric pressure and in an ultrahigh vacuum (UHV) was evaluated by a vacuum pin-on-disk tribometer with different ball probes. Friction coefficients (μ) as low as 0.03 and 0.05 for a stainless-steel ball were achieved in atmospheric pressure and in an UHV, respectively. A value of μ=0.04 was also realized for a silicon nitride ball both in atmospheric pressure and in an UHV. Surface energy was analyzed by a contact angle measuring method which reveals that the behavior of the frictional properties correspond with that of the surface energy. Since the coatings consist of oxide, they will be applicable to a low-frictional coating with resistance to oxidation for aerospace applications in a low Earth orbit environment.
Size dependence of switching field of magnetic tunnel junctions down to 50 nm scale
Authors: Kubota H. , Ando Y. , Miyazaki T. , Reiss G. , Brückl H. , Schepper W. , Wecker J. , Gieres G.
Published: July 2003

Tunnel magnetoresistance curves were measured in very small tunnel junctions from scales of 1 μm to 50 nm using conductive atomic force microscopy. The junction arrays were prepared by a simple fabrication process using electron beam lithography. In large size junctions, the minor loops shifted in the negative field direction corresponding to ferromagnetic coupling between free and pinned layers. With decreasing size, the shift changed to the positive field direction corresponding to antiparallel coupling. The dependence of the shift was quantitatively explained by a model taking account of both Néel-type and dipole coupling. The minor loops showed asymmetric shape depending on field sweep directions.
Photobleaching and optical properties of organic crystal 4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate
Authors: Mutter Lukas , Jazbinšek Mojca , Zgonik Marko , Meier Urs , Bosshard Christian , Günter Peter
Published: July 2003

Photobleaching of organic optical materials can be used to structure the surface layer for integrated optics applications. Linear optical properties and absorption were studied in 4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) organic crystals within the absorption band from 260–700 nm in order to determine the depth-range of photobleaching. The results were obtained from reflectivity measurements and bleaching experiments. The depth range of photobleaching can be varied between 0.2 and 2.6 μm by selecting a suitable wavelength.
Forming gas annealing on physical characteristics and electrical properties of Sr0.8Bi2Ta2O9/Al2O3/Si capacitors
Authors: Lan Bang Chiang , Hsu Jung-Jui , Chen San-Yuan , Bow Jong-Shing
Published: July 2003

Sr 0.8 Bi 2 Ta 2 O 9 (SBT) ferroelectric film constructed on Al2O3/Si to form metal–ferroelectric–insulator–semiconductor (MFIS) was prepared to study the degradation behavior of SBT films under forming gas annealing (FGA). Although the diffusion of hydrogen ions has been detected during FGA treatment, no significant differences in the microstructure and crystalline phase are observed for the SBT film compared to that without FGA treatment. However, the diffusion of hydrogen ions leads to the rapid decrease in the dielectric constant but shows no apparent influence on the memory window of Sr0.8Bi2Ta2O9/Al2O3/Si capacitors. In addition, it was found that FGA shows a positive effect on the leakage current of MFIS in contrast to that of metal/ferroelectric/metal structure. The leakage current density of MFIS dramatically decreases as much as two orders of magnitude after FGA at 500 °C compared to that without FGA treatment that was attributed to the reduced defects at the interface of Al2O3/Si.
Strong dependence of the Fe thin-film magnetic anisotropy on the Si(111) substrate preparation
Authors: Cougo dos Santos M. , Geshev J. , Silva D. K. , Schmidt J. E. , Pereira L. G. , Hubler R. , Allongue P.
Published: July 2003

We report on the influence of the Si(111) surface preparation on both the structural and the magnetic properties of iron thin films grown onto them. Two different substrate preparation procedures were used, in one of which the substrate was purposely oxidized. The surface analysis was performed by using atomic force and scanning tunneling microscopies, and the magnetic behavior of the films was observed by magneto-optic Kerr effect polarimetry. A strong relationship between the substrate preparation procedure, morphology, and the magnetic response of the iron films was found. A phenomenological model was used to interpret the experimental magnetization data and to derive the anisotropy parameters for both types of Fe samples. It was found that the magnetic response of the film grown onto the oxidized substrate is dominated by the uniaxial anisotropy originated from the substrate topology (a terracelike structure); the behavior of the other film, grown onto the unoxidized substrate, is determined by both cubic and easy-plane anisotropies.
Generation of asymptotic solitons of the nonlinear Schrödinger equation by boundary data
Authors: Boutet de Monvel Anne , Kotlyarov Vladimir
Published: July 2003

This article is about the focusing nonlinear Schrödinger equation on the half-line. The initial function vanishes at infinity while boundary data are local perturbations of periodic or quasi-periodic (finite-gap) functions. We study the corresponding scattering problem for the Zakharov–Shabat compatible differential equations, the representation of the solution of the nonlinear Schrödinger equation in the quarter of the (x,t)-plane through functions, which satisfy Marchenko integral equations. We use this formalism to investigate the asymptotic behavior of the solution for large time. We prove that under certain conditions a periodic (quasi-periodic) behavior at infinity of boundary data generates an unbounded train of asymptotic solitons running away from the boundary. The asymptotics of the solution shows that boundary data with periodic behavior as time tends to infinity generates a train of such asymptotic solitons even in the case when the initial function is identically zero.
Twistor theory of hyper-Kähler metrics with hidden symmetries
Authors: Dunajski Maciej , Mason Lionel J.
Published: July 2003

We review the hierarchy for the hyper-Kähler equations and define a notion of symmetry for solutions of this hierarchy. A four-dimensional hyper-Kähler metric admits a hidden symmetry if it embeds into a hierarchy with a symmetry. It is shown that a hyper-Kähler metric admits a hidden symmetry if it admits a certain Killing spinor. We show that if the hidden symmetry is tri-holomorphic, then this is equivalent to requiring symmetry along a higher time and the hidden symmetry determines a “twistor group” action as introduced by Bielawski [Twistor Quotients of Hyper-Kähler Manifolds (World Scientific, River Edge, NJ, 2001)]. This leads to a construction for the solution to the hierarchy in terms of linear equations and variants of the generalized Legendre transform for the hyper-Kähler metric itself given by Ivanov and Rocek [Commun. Math. Phys. 182, 291 (1996)]. We show that the ALE spaces are examples of hyper-Kähler metrics admitting three tri-holomorphic Killing spinors. These metrics are in this sense analogous to the “finite gap” solutions in soliton theory. Finally we extend the concept of a hierarchy from that of our earlier work [Commun. Math. Phys. 213, 641 (2000)] for the four-dimensional hyper-Kähler equations to a generalization of the conformal anti-self-duality equations and briefly discuss hidden symmetries for these equations.
The vibrational spectrum of cyclic ozone
Authors: Siebert Rüdiger , Schinke Reinhard
Published: July 2003

We discuss the vibrational spectrum of rotationless cyclic ozone, i.e., ozone in the so-called ring minimum of the ground electronic state. The bound state calculations utilize the ab initio potential energy surface constructed by Siebert et al. on the basis of high-level electronic structure calculations [J. Chem. Phys. 116, 9749 (2002)]. The spectrum is qualitatively similar to that of H3+, except that the energy spacing between the states belonging to the same polyad [v1,v2] is much smaller; ν1 is the breathing mode and ν2 is the doubly degenerate bending mode.