bluuebunny/crossref_metadata_2025 huggingface dataset:
Photothermal inspection of rough steel surfaces
Authors: Walther H. G.
Published: July 2002

Surface roughness effects have a remarkable influence on photothermal measurements from steel samples and seriously distort hardness depth profiling. The thermal response of rough surfaces is described by an effective layer whose properties are defined by global roughness numbers like root- mean square roughness and peak-to-valley-roughness. Calculations of roughness effects as well as measurements from rough steel samples were performed and compared to each other. A good correspondence between the model and experiment was found which demonstrates the suitability of the effective layer model for a description of roughness.
Scattering of hyperthermal reactive and nonreactive ions from Pt(111)
Authors: Herrmann Gero , Okada Michio , Murata Yoshitada
Published: July 2002

Reactive (N+) and nonreactive (Ne+) ions scattered from Pt(111) at an incident energy of 30 eV were detected through an energy analyzer at various scattering angles using a newly developed ion scattering apparatus. The surviving ions of Ne+ are mainly scattered via single collision, while N+ ions are dominantly scattered via double collision. The angular distribution of scattered Ne+ ions shows a broad scattering lobe with the maximum intensity around specular reflection. For N+, on the other hand, the angular distribution around specular reflection is so sharp that it should be considered to be caused by resonancelike scattering, although the maximum of the main broad lobe is shifted to the surface plane compared to the specular direction. The difference between N+ and Ne+ in the scattering process for the survival ions is due to the difference in the interaction potential including the neutralization process.
Luminescence quenching in InAs quantum dots
Authors: Haft Dirk , Warburton Richard J. , Karrai Khaled , Huant Serge , Medeiros-Ribeiro Gilberto , Garcia Jorge M. , Schoenfeld Winston , Petroff Pierre M.
Published: July 2002

We report how photoluminescence from self-assembled InAs quantum dots depend on pumping power and vertical electric field. The InAs dots, which are embedded in a capacitor-like structure, act as efficient trapping centers for excitons. At a high enough electric field, however, the photoexcited electrons tunnel out of the dots fast enough to quench the emission. For samples with two adjacent layers of vertically aligned dots, we find that the threshold voltage for quenching depends very strongly on the optical pumping power. In total contrast to this, we find no comparable effect for samples grown with a single layer of dots. We explain this in terms of efficient storage of electrons and holes in the double-layer samples.
Hypersonic frequency and damping anomalies in relaxor ferroelectric Pb(Zn1/3Nb2/3)O3
Authors: Kuok M. H. , Ng S. C. , Fan H. J. , Iwata M. , Ishibashi Y.
Published: July 2002

Brillouin scattering from relaxor ferroelectric Pb(Zn1/3Nb2/3)O3 has been measured between 296 and 780 K. Heating of the virgin Pb(Zn1/3Nb2/3)O3 crystal produced results that are different from those of subsequent heating or cooling processes. An unusually broad softening of the longitudinal acoustic-mode frequency, due to the diffuse ferroelectric-phase transition, is observed. The attendant phonon damping is attributed to order-parameter fluctuations. A huge temperature hysteresis, which eluded observation by other techniques, has been found.
Interfacial reactions between thin rare-earth-metal oxide films and Si substrates
Authors: Ono Haruhiko , Katsumata Tooru
Published: July 2002

Rare-earth-metal oxide films (Ln2O3; Ln=Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, and Yb) between 20 and 30 nm thick were grown on Si substrates by using a pyrolysis method. We found that a silicate (LnSiO) layer and a silicon oxide layer were formed at the interface between oxides and substrate after postannealing. The infrared absorption of the Si–O–Ln bonds increased as the postannealing temperature rose. The Si–O–Ln bond formation strongly depended on the ion radii of the rare-earth elements. We conclude that an interfacial silicate layer can easily be formed by a reaction with Si atoms diffusing from the substrate for oxides with larger ion radii. This is because such oxides may have a larger space between atoms. The quantity of Si–O–Si bonds also increased after postannealing. The increase in the Si–O–Si bonds for Ln2O3 was independent of the elements, and almost the same as the increases for Ta2O5 and ZrO2.
Taylor dispersion of a solute in a microfluidic channel
Authors: Beard Daniel A.
Published: July 2002

The dispersion of a solute, traveling in a laminar-flow microfluidic channel, along the major axis of the channel is described by Taylor dispersion. It is shown that although diffusion of solute along the major axis is often neglected in constructing a governing transport equation, axial dispersion due to the laminar flow profile influences the distribution of solute in the channel. An expression for the effective Taylor dispersion coefficient due to the laminar velocity profile is derived and verified by comparing the predicted dispersion to results from a three-dimensional finite-element representation of the fluid dynamics and solute transport. Since the dispersion coefficient can be several orders of magnitude greater than the molecular diffusion coefficient, axial dispersion in microfluidic channels should not be casually neglected.
Spin-dependent scattering in transition metals
Authors: Drouhin Henri-Jean
Published: July 2002

The spin-dependent electron inelastic mean free path (IMFP) in transition metals is studied in the energy range 5–50 eV above the Fermi level. It is shown that the spin-dependent IMFP is simply related to the number of holes in both d spin subbands, whatever the detail of the d-band structure. This analysis allows us to disentangle the different scattering channels. Many experimental determinations of the spin-dependent part of the electron scattering cross section, from several teams, are analyzed in the framework of this model. The strong energy dependence of the exchange matrix element is clearly evidenced.
Generalized forward–backward initial value representation for the calculation of correlation functions in complex systems
Authors: Thoss Michael , Wang Haobin , Miller William H.
Published: July 2002

The capability of two different, recently proposed semiclassical (SC) forward–backward (FB) initial value representations (IVR) to describe quantum interference and coherence effects is investigated. It is shown that depending on the way the observable under consideration is represented by unitary operators one can obtain rather different results. Although the FB-IVR based on an integral representation as a rule is capable of describing quantum interference, a closer analysis reveals that it depends on the observable under consideration if all interference that can be described semiclassically is actually included in the calculation. To overcome this problem a new, generalized FB-IVR method (GFB-IVR) is proposed, which combines the capability of the SC-IVR to describe quantum interference effects independent of the observable and the better convergence properties of the FB-IVR. The performance of this new approach is studied in some detail. In particular, it is shown that the GFB-IVR can describe both the coherent and incoherent regime in the dynamics of an anharmonic vibration coupled to a harmonic bath.
Approximate description of the two-dimensional director field in a liquid crystal display
Authors: Panasyuk G. , Allender D. W. , Kelly J.
Published: July 2002

A model for describing properties of a liquid crystal display which combines the concepts of in-plane switching and vertical alignment is proposed. There is good agreement between the results of this model and direct computer calculation of the director and the light transmittance. The usefulness of the model lies in the faster speed of calculations compared to direct computer solution of the Euler–Lagrange equations.
Electrical properties of three-terminal ballistic junctions
Authors: Xu H. Q.
Published: July 2002

Based on the ballistic nature of electron transport, exploitable nonlinear transport phenomena are predicted for three-terminal ballistic junctions (TBJs). For a symmetric TBJ, it is shown that when finite voltages Vl and Vr are applied in push-pull fashion, with Vl=V and Vr=−V, to the left and right branches, the voltage output Vc from the central branch will always be negative. This characteristic appears even when the device symmetry is broken, provided that |V| is greater than a certain threshold. It is also shown that the TBJs exhibit parabolic behavior for Vc vs V, in the weak nonlinear response regime. Potential applications of these devices in nanoelectronics are discussed.
Conservation of flatband conditions for DySi2 monolayers on n-type Si(111)
Authors: Vandré S. , Preinesberger C. , Busse W. , Dähne M.
Published: July 2002

We propose a low-resistivity metal/Si/DySi2/Si multilayer Ohmic contact to n-type Si. For a DySi2 monolayer on n-type Si(111), the Fermi level has been found to be located only 0.08 eV below the conduction-band minimum of Si, corresponding to flatband conditions. Here, we demonstrate that this Fermi-level position is conserved to a large extent upon Si overgrowth of the monolayer, allowing us to exploit the flatband conditions for device applications under ambient conditions.
Power series expansion of the roots of a secular equation containing symbolic elements: Computer algebra and Moseley’s law
Authors: Barnett Michael P. , Decker Thomas , Krandick Werner
Published: July 2002

We use computer algebra to expand the Pekeris secular determinant for two-electron atoms symbolically, to produce an explicit polynomial in the energy parameter ε, with coefficients that are polynomials in the nuclear charge Z. Repeated differentiation of the polynomial, followed by a simple transformation, gives a series for ε in decreasing powers of Z. The leading term is linear, consistent with well-known behavior that corresponds to the approximate quadratic dependence of ionization potential on atomic number (Moseley’s law). Evaluating the 12-term series for individual Z gives the roots to a precision of 10 or more digits for Z⩾2. This suggests the use of similar tactics to construct formulas for roots vs atomic, molecular, and variational parameters in other eigenvalue problems, in accordance with the general objectives of gradient theory. Matrix elements can be represented by symbols in the secular determinants, enabling the use of analytical expressions for the molecular integrals in the differentiation of the explicit polynomials. The mathematical and computational techniques include modular arithmetic to handle matrix and polynomial operations, and unrestricted precision arithmetic to overcome severe digital erosion. These are likely to find many further applications in computational chemistry.
Condensed phase growth of single-wall carbon nanotubes from laser annealed nanoparticulates
Authors: Geohegan D. B. , Schittenhelm H. , Fan X. , Pennycook S. J. , Puretzky A. A. , Guillorn M. A. , Blom D. A. , Joy D. C.
Published: July 2002

Single-wall carbon nanotubes (SWNT) were grown to micron lengths by laser-annealing nanoparticulate soot containing short (∼50 nm long) nanotube “seeds.” The “seeded” nanoparticulate soot was produced by restricting the time spent by an ablation plume inside an 800 °C oven following laser vaporization of a C–Ni–Co target. The soot collected from the laser vaporization apparatus was placed inside graphite crucibles under argon, and heated by a CO2 laser. In situ pyrometry was used to estimate the sample temperature. Length distributions of SWNT bundles in the unannealed and annealed samples were measured by transmission electron microscopy and field emission scanning electron microscopy. Annealing treatments exceeding 1600 °C produced no increase in nanotube length, while lower temperatures in the 1000–1300 °C range were optimal for growth. These experiments indicate that SWNT grow by the conversion of condensed phase nanomaterial during annealing, a similar mechanism to that proposed for growth during normal laser–vaporization production.
An efficient tabulated collisional radiative equilibrium radiation transport model suitable for multidimensional hydrodynamics calculations
Authors: Thornhill J. W. , Apruzese J. P. , Davis J. , Clark R. W. , Velikovich A. L. , Giuliani J. L. , Chong Y. K. , Whitney K. G. , Deeney C. , Coverdale C. A. , Cochran F. L.
Published: July 2002

A computationally efficient method for transporting radiation in multidimensional plasmas has been developed and evaluated. The basis of this method is a uniform plasma approximation that allows one to utilize existing escape probability techniques that are successfully used in one-dimensional (1D) calculations to approximately solve the multidimensional radiation transport problem. This method is superior to diffusion methods because (1) the probability of escape technique insures that the plasma goes to the correct optically thin and thick limits, (2) the effects of line absorption due to photoexcitations are modeled, and (3) this method uses source functions that are based on a self-consistent nonlocal thermodynamic equilibrium calculation, not an ad hoc assumption that the source functions are Planckian. This method is highly efficient because equation of state information from 1D calculations is tabulated as a function of plasma internal energy, ion density, and the line probability of escape from a uniform plasma, and then used in multidimensional calculations. Given the internal energy and ion density, and by calculating the line probability of escape from a zone of the multidimensional plasma, the equation of state, including emissivities and absorption coefficients, of the zone is determined from the table. Total radiative power, K-shell radiative power, total radiative yield, K-shell radiative yield, and plasma density and temperature profiles obtained from 1D Z-pinch calculations employing this method are in good agreement with the same powers, yields, and profiles calculated using a full radiation transport model. This method has been implemented in the 2D plasma radiating imploding source model code [F. L. Cochran et al., Phys. Plasmas 2, 2765 (1995)] to determine the influence of radiation transport in argon Z-pinch experiments performed on the Z machine [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories.
Multipole-cancellation mechanism for high-Q cavities in the absence of a complete photonic band gap
Authors: Johnson Steven G. , Fan Shanhui , Mekis Attila , Joannopoulos J. D.
Published: July 2002

We describe and demonstrate a new mechanism for low radiation losses in structures lacking a complete band gap, and show how resonant cavities with Q>103 can be achieved without sacrificing strong localization in 3d. This involves a forced cancellation in the lowest-order term(s) of the multipole far-field radiation expansion. We focus on the system of photonic-crystal slabs, one- to two-dimensionally periodic dielectric structures of finite height with vertical index guiding. Simulations and analytical results in 2d and 3d are presented.
Quasicoherent fluctuations associated with a transport barrier in the sandpile model
Authors: Hicks H. R. , Carreras B. A.
Published: July 2002

A transport barrier is introduced in the one-dimensional sandpile model by reducing the local flux in a barrier region. This reduction causes a modification in the structure of the avalanches and triggers quasiperiodic fluctuations of the particle fluxes. The frequency of these fluctuations is inversely proportional to the transport barrier width.
Thermal instability theory analysis of multifaceted asymmetric radiation from the edge (MARFE) in Tokamak Experiment for Technology Oriented Research (TEXTOR)
Authors: Kelly F. A. , Stacey W. M. , Rapp J. , Brix M.
Published: July 2002

The density limits for a series of shots in TEXTOR [Tokamak Experiment for Technology Oriented Research, E. Hintz, P. Bogen, H. A. Claaßen et al., in Contributions to High-Temperature Plasma Physics, edited by K. H. Spatschek and J. Uhlenbusch (Akademie Verlag, Berlin, 1994, p. 373)], over a range of heating powers, that ended in multifaceted asymmetric radiation from the edge (MARFE) have been analyzed within the context of thermal instability theory. The prediction of MARFE onset agrees with observation to within the experimental uncertainty.
Semiclassical Coulomb differential excitation function: Asymptotic expansions
Authors: Thorsley Michael D. , Chidichimo Marita C.
Published: July 2002

We have obtained asymptotic expansions of the electric dipole (E1) differential excitation function for large values of the adiabaticity parameter ξ and for all values of the eccentricity (ε) of the projectile orbit. To accomplish this, we have developed a new asymptotic power series of exponential integrals related to the Airy integrals, introduced originally in a paper by Brussaard et al. [Ann. Phys. (N.Y.) 7, 47 (1962)], in which asymptotic expansions of the total excitation function were derived.
Electrical conductivities of methane, benzene, and polybutene shock compressed to 60 GPa (600 kbar)
Authors: Nellis W. J. , Hamilton D. C. , Mitchell A. C.
Published: July 2002

Electrical conductivities were measured for methane, benzene, and polybutene shock compressed to pressures in the range 20 to 60 GPa (600 kbar) and temperatures in the range 2000 to 4000 K achieved with a two-stage light-gas gun. The data for methane and benzene are interpreted simply in terms of chemical decomposition into diamondlike, defected C nanoparticles and fluid H2 and their relative abundances (C:H2), 1:2 for methane and 2:1 for benzene. The measured conductivities suggest that conduction flows predominately through the majority species, H2 for methane and C for benzene. These data also suggest that methane is in a range of shock pressures in which dissociation increases continuously from a system which is mostly methane to one which has a substantial concentration of H2. Thermal activation of benzene conductivities at 20–40 GPa is probably caused by thermal activation of nucleation, growth, and connectivity of diamondlike, defected C nanoparticles. At 40 GPa the concentration of these C nanoparticles reaches a critical density, such that further increase in density does not have a significant affect on the cross-sectional area of conduction and, thus, conductivity saturates. The electrical conductivity of polybutene (1:1) is very low. While the mechanism is unknown, one possibility is that the electronic bandgap of whatever species are present is large compared to the temperature. Electrical conductivity measurements are proposed as a way to determine the melting curve of diamondlike C nanoparticles at 100 GPa pressures.
The electronic structure and properties of group 8 oxides MO4, where M=Ru, Os, and Element 108, Hs
Authors: Pershina V. , Bastug T. , Fricke B. , Varga S.
Published: July 2002

Fully relativistic density functional calculations have been performed for group 8 tetroxides MO4, where M=Ru, Os, and element 108, Hs. The electronic structure analysis has shown HsO4 to be very similar to OsO4, with the covalence and stability increasing from OsO4 to HsO4. Using models of atom-slab interactions, adsorption enthalpies of RuO4 and HsO4 on the quartz surface have been calculated using some models of physisorption. The volatility of the single species was shown to have the following trend, RuO4<OsO4⩽HsO4, with differences in the adsorption enthalpies between the species being almost within the experimental uncertainty of ±1.5 kJ/mol.
New inversion coordinate for ammonia: Application to a CCSD(T) bidimensional potential energy surface
Authors: Pesonen Janne , Miani Andrea , Halonen Lauri
Published: July 2002

A new inversion coordinate is defined for ammonia as a function of the valence angles. Its square is similar to the often used totally symmetric bending displacement coordinate for the pyramidal XY3–type molecules. We have used this in a two-dimensional calculation including the totally symmetric stretching and the inversion mode. A conventional symmetrized internal coordinate is employed for the symmetric stretch. A two-dimensional potential energy surface is calculated using the ab initio CCSD(T) method together with the aug-cc-pVTZ, cc-pVQZ, and aug-cc-pVQZ basis sets. The corresponding eigenvalues are calculated variationally using a Morse oscillator basis set for the stretch and a harmonic oscillator basis set for the inversion. A good agreement is obtained between the calculated and 22 experimental inversion levels, 9 of NH314 and the others involving 4 other isotopomers (14ND3, NH315, ND315, and NT314). With the aug-cc-pVTZ basis, a mean absolute error of 5.0 cm−1 is obtained whereas with the aug-cc-pVQZ basis set the error becomes 7.9 cm−1.
Ethylene adsorption on Ge(100)-(2×1): A combined angle-resolved photoemission and thermal desorption spectroscopy study
Authors: Fink A. , Huber R. , Widdra W.
Published: July 2002

Ethylene adsorption on vicinal, single-domain Ge(100)-(2×1) has been investigated by thermal desorption spectroscopy (TPD) and angle-resolved photoemission (ARUPS) using linearly polarized synchrotron radiation. Thermal desorption experiments show that chemisorbed C2H4 desorbs from Ge(100) nondissociatively around 393 K with a high temperature shoulder which is tentatively assigned to step site desorption. The ethylene saturation coverage is strongly temperature dependent. Adsorption at 90 K saturates at 0.38 monolayer (ML), whereas adsorption at 170 K leads to a saturation coverage of approximately 1 ML. This behavior is explained by an adsorption barrier for coverages exceeding 0.38 ML. ARUP spectra for a dilute and the saturated ethylene monolayer reveal clear differences. Using photoemission selection rules a highly (C2v) symmetric adsorption geometry with a C–C bond axis parallel to the Ge–Ge dimer axis is found for the dilute layer; whereas a reduced C2 adsorption symmetry is found for the saturated ethylene layer. The comparison of photoemission spectra for C2H4 on Ge(100) and Si(100) shows that C2H4 is di-σ bound to the dangling bonds of a single Ge–Ge dimer. For two molecular orbitals, 1b3u and 1b2g, one-dimensional band structures with dispersion widths of 0.5 and 0.39 eV, respectively, along the Ge–Ge dimer rows are found which present a straightforward explanation for the observed symmetry reduction and adsorption behavior.
Lorentzian regularization and the problem of point-like particles in general relativity
Authors: Blanchet Luc , Faye Guillaume
Published: July 2002

The two purposes of the article are (1) to present a regularization of the self-field of point-like particles, based on Hadamard’s concept of “partie finie,” that permits in principle to maintain the Lorentz covariance of a relativistic field theory, and (2) to use this regularization for defining a model of stress-energy tensor that describes point-particles in post-Newtonian expansions (e.g., 3PN) of general relativity. We consider specifically the case of a system of two point-particles. We first perform a Lorentz transformation of the system’s variables which carries one of the particles to its rest frame, next implement the Hadamard regularization within that frame, and finally come back to the original variables with the help of the inverse Lorentz transformation. The Lorentzian regularization is defined in this way up to any order in the relativistic parameter 1/c2. Following a previous work of ours, we then construct the delta-pseudo-functions associated with this regularization. Using an action principle, we derive the stress-energy tensor, made of delta-pseudo-functions, of point-like particles. The equations of motion take the same form as the geodesic equations of test particles on a fixed background, but the role of the background is now played by the regularized metric.
Room temperature persistent spectral hole burning in x-ray irradiated Eu3+-doped borate glasses
Authors: Chung Woon Jin , Heo Jong
Published: July 2002

Irradiation of x-rays has induced room-temperature persistent spectral hole burning (PSHB) in Eu3+-doped borate glasses melted under an inert atmosphere. Defects were formed by x-ray irradiation and these defects, especially electron trapping centers near rare-earth ions in glasses, were responsible for the PSHB. Electrons were released from these defects upon irradiation of a burning light. Photoreduction of Eu3+ to Eu2+ by trapping these electrons resulted in the formation of persistent spectral holes.
Optical transitions and radiative lifetime in GaN/AlN self-organized quantum dots
Authors: Andreev Aleksey D. , O’Reilly Eoin P.
Published: July 2002

We present a theoretical study of the optical matrix elements and radiative lifetime for the ground state optical transitions in GaN/AlN quantum dots (QD). An efficient plane-wave expansion method is used to calculate the energy levels, wave functions, and optical matrix elements in the framework of a multiband k⋅p model taking account of the three-dimensional strain and built-in electric field distributions for QDs with a hexagonal truncated-pyramid shape. We demonstrate that the built-in electric field determines the energy spectrum of GaN/AlN QDs and leads to a dramatic decrease in the optical matrix element with increasing QD size. As a result, the radiative lifetime for the ground state optical transition increases strongly with QD size. The theoretical results obtained are in good agreement with available experimental data.
High current gains obtained by InGaN/GaN double heterojunction bipolar transistors with p-InGaN base
Authors: Makimoto Toshiki , Kumakura Kazuhide , Kobayashi Naoki
Published: July 2002

InGaN/GaN double heterojunction bipolar transistors have been fabricated using p-type InGaN as a base layer. The structures were grown on SiC substrates by metalorganic vapor phase expitaxy and defined by electron cyclotron resonance plasma etching. The In mole fraction in the base layer and its thickness were 0.06 and 100 nm, respectively. The Mg doping concentration in the base layer was 1×1019 cm−3 corresponding to a hole concentration of 5×1018 cm−3 at room temperature. From their common-emitter current–voltage characteristics, the maximum current gain of 20 was obtained at room temperature.
Ordered indium-oxide nanowire arrays and their photoluminescence properties
Authors: Zheng M. J. , Zhang L. D. , Li G. H. , Zhang X. Y. , Wang X. F.
Published: July 2002

Ordered semiconductor In2O3 nanowire arrays are uniformly assembled into hexagonally ordered nanochannels of anodic alumina membranes (AAMs) by electrodeposition and oxidizing methods. Their microstructures were characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. A blue-green photoluminescence (PL) band in the wavelength range of 300–650 nm was observed in the In2O3/AAM assembly system. The PL intensity and peak position depend on the annealing temperature, which is mainly attributed to the singly ionized oxygen vacancy in the In2O3 nanowire array system.
Spatially resolved photoluminescence in InGaN/GaN quantum wells by near-field scanning optical microscopy
Authors: Jeong M. S. , Kim J. Y. , Kim Y.-W. , White J. O. , Suh E.-K. , Hong C.-H. , Lee H. J.
Published: July 2002

Spatially and spectrally resolved photoluminescence (PL) from InGaN/GaN quantum wells is obtained using near-field scanning optical microscopy (NSOM). Samples displaying high macroscopic PL intensity revealed nonuniform intensity and linewidth but nearly uniform peak position. It suggests that the contrast in the NSOM image reflects nonuniform distribution of dislocations or defects which act as nonradiative recombination centers. The formation of quantum dots with size of 30±25 nm and their size-dependent interaction with dislocations were observed in plan-view transmission electron microscopy. It is likely that the high luminescence efficiency is due to the efficient localization of excitons in high-density quantum dots located in regions with fewer dislocations.
Spatial distribution of space charge in conjugated polymers
Authors: Feller F. , Geschke D. , Monkman A. P.
Published: July 2002

We demonstrate the pyroelectric effect in a conjugated polymer, poly(2,5-pyridinediyl) (PPY), and we use the laser intensity modulation method (LIMM) to resolve the spatial distribution of electric field and space charges inside a 5 μm Au/PPY/Au sandwich device. The pyroelectric signal shows hysteresis behavior with respect to the applied bias indicating permanent storage of injected charges. From the analysis of the LIMM spectra we conclude that application of a bias leads to the accumulation of space charges near the electrodes, while a zone of opposite space charge may establish in a distance of about 1 μm from it. The charged state retains after removing the bias and can lead to an internal electric field that is opposite to the external poling field in the bulk of the polymer film.
H-bonding in amorphous acetamide CH3CONH2 as studied by x-ray scattering
Authors: Nasr Salah
Published: July 2002

Noncrystalline solid acetamide (CH3CONH2) can be prepared by vapor deposition onto precooled substrates at 77 K. The structure factor SM(Q) derived from x-ray diffraction patterns is compared to that recently obtained for liquid acetamide near the melting point. The data are analyzed to yield the molecular structure factor F1(Q) and the distinct pair correlation function gL(r). Our measurements clearly show that the hydrogen bonded network occurs in amorphous acetamide between C=O and N–H groups. Two H-bonds per molecule are found in average. The mean N⋯O distance is about 3.01 Å. The results could be explained in terms of an open-chain structure which resembles preferentially the one existing in the metastable crystalline form. The crystallization of the glassy film which occurs on heating is also studied.
Interpretation of the molten BeF2 viscosity anomaly in terms of a high temperature density maximum, and other waterlike features
Authors: Hemmati Mahin , Moynihan Cornelius T. , Angell C. Austen
Published: July 2002

In an effort to understand the anomalous behavior of the viscosity of liquid beryllium fluoride relative to other liquids in the strong/fragile classification we have carried out ion dynamics computer simulations of BeF2 over a temperature range which overlaps with the experimental viscosity data. Using the simple rigid ion potentials which seem to be suitable for the nonpolarizable ions of this substance, we obtain diffusivity data which are in good agreement with values obtained from the experimental viscosities when converted to diffusivities using the Eyring equation for jump transport processes. The diffusivity data show a highly anomalous fragile region of behavior at temperatures just above the limits of laboratory measurement, which reconciles the observed viscosity with that of other liquids. This strongly curved region is interpreted, using the Adams–Gibbs equation, in terms of a strongly negative liquid expansivity regime associated with a large heat capacity (hence strongly temperature-dependent entropy) regime. The negative expansivity regime ends in a density maximum at 2000 K, beyond the reach of experiment, but a related density minimum expected at about 1250 K may be observable in sealed vessel experiments. In particular, a sudden ∼30% rise in heat capacity, accessible to high temperature differential scanning calorimetry experiments, is predicted. The confirmation of this anomaly by laboratory experiments will bring much credibility to current speculations on the origin of the anomalous behavior of supercooled water. The reason for the displacement of the anomaly to high temperature relative to water is found in the parameters of the cooperative “bond lattice” model and is physically identified with contrasting changes in the low frequency density of vibrational states as temperature increases above the glass transition. Finally we show that the fragility of the BeF2, and also of the analogous SiO2, greatly exceed that of Lennard-Jones liquids in the computationally accessible regime, and then utilize this anomalous fragility to demonstrate the existence of nonlinear relaxation, behavior typical of fragile liquids, for BeF2. The fragile-liquid-to-strong-liquid crossover occurring at the limit of our computational range is a consequence of the thermodynamic anomaly in the liquid state. It is analogous to—but more pronounced than—that suggested earlier for liquid SiO2 in which it occurs in an experimentally quite inaccessible temperature range.
Existence of multivortex solutions in the self-dual Chern–Simons–Higgs theory in a background metric
Authors: Choe Kwangseok
Published: July 2002

In this paper we show the existence of the multiple multivortex solutions of the self-duality equations of (2+1) dimensional Chern–Simons–Higgs model in a background metric of the form gμν=diag(1,−b(x),−b(x)) with b(x)=O(|x|−l) as |x|→∞ for some l>2.
Statistical angular correlation coefficients and second electron-pair moments for atoms
Authors: Koga Toshikatsu
Published: July 2002

Statistical correlation coefficients τ[f] introduced by Kutzelnigg et al. [Phys. Rev. 172, 49 (1968)] provide overall measures of the difference between the electron-pair density and the product of single-electron densities, where f=f(r) is a probe function. It is shown that the angular correlation coefficient τ[r], a particular case of τ[f] for f(r)=r, is simply expressible in terms of the generalized electron-pair moments 〈q2〉(a,b) with two real-valued parameters a and b. Especially, the relation for a=b=1 means that τ[r] is nothing but the difference between the center-of-mass motion and relative motion contributions in the second electron-pair moments. Conversely, the electron-pair moments 〈q2〉(a,b) are obtained from the single-electron moment 〈∑iri2〉 and the correlation coefficient τ[r]. The same is also true in momentum space.
Enhancement of magnetoresistance in the intermediate state of Pr0.5Sr0.3Ca0.2MnO3
Authors: Chang C. W. , Lin J. G.
Published: July 2002

Current–voltage (I–V) characteristics of Pr0.5Sr0.3Ca0.2MnO3 are measured at different temperatures and different fields. Our data of I–V curves show a crossover from linear to nonlinear behavior at the temperature lower than the charge ordering temperature. New findings in this work include (i) abrupt jumps/drops of voltage in each I–V curve below 100 K and (ii) an enhancement of magnetoresistance (MR) ratio from 15% to 20% which is associated with the unusual voltage jumps/drops in I–V curves. The enhancement of MR ratio evidences the existence of an intermediate state between the charge ordering state and the charge ordering breakdown state. Most importantly, the existence of such intermediate state provides a new route to achieve a low field colossal magnetoresistance for future applications of spintronics.
Pressure-dependent studies on hydration of the C–H group in formic acid
Authors: Chang Hai-Chou , Jiang Jyh-Chiang , Chao Ming-Chi , Lin Ming-Shan , Lin Sheng Hsien , Chen Hsin-Yen , Hsueh Hung-Chung
Published: July 2002

The infrared spectroscopic profiles of HCOOD/D2O mixtures were measured as a function of pressure and concentration. The C–H bond of HCOOD shortens as the pressure is elevated, while the increase in C–H bond length upon diluting HCOOD with D2O was observed. Based on the experimental results, the shift in frequency of C–H stretching band is concluded to relate to the mechanism of the hydration of the C–H group and the water structure in the vicinity of the C–H group. The pressure-dependent results can be attributed to the strengthening of C–H---O electrostatic/dispersion interaction upon increasing pressure. The observations are in accord with ab initio calculation forecasting a blueshift of the C–H stretching mode via C–H---O interaction in HCOOD-water/(HCOOD)2-(D2O) complexes relative to the noninteracting monomer/dimer. Hydrogen-bonding nonadditivity and the size of water clusters are suggested to be responsible to cause the redshift in C–H stretching mode upon dilution HCOOD with D2O.
Induced strain mechanism of current collapse in AlGaN/GaN heterostructure field-effect transistors
Authors: Simin G. , Koudymov A. , Tarakji A. , Hu X. , Yang J. , Khan M. Asif , Shur M. S. , Gaska R.
Published: July 2002

Gated transmission line model pattern measurements of the transient current–voltage characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) and metal–oxide–semiconductor HFETs were made to develop a phenomenological model for current collapse. Our measurements show that, under pulsed gate bias, the current collapse results from increased source–gate and gate–drain resistances but not from the channel resistance under the gate. We propose a model linking this increase in series resistances (and, therefore, the current collapse) to a decrease in piezoelectric charge resulting from the gate bias-induced nonuniform strain in the AlGaN barrier layer.
High-resolution infrared spectroscopic determination of the nν5±n vibrational energies of HCCN and DCCN
Authors: Hung P. Y. , Sun F. , Hunt N. T. , Burns L. A. , Curl R. F.
Published: July 2002

The high-resolution infrared kinetic spectra of HCCN and DCCN have been recorded, the former between 3395 and 3481 cm−1 and the latter in the 2517–2593 cm−1 region. A total of four dominant Q branches were observed and have been subsequently assigned to the ν1+2ν5±2←ν5±1, ν1+3ν5±3←2ν5±2 bands of the two isotopomers. The band origins have been determined to be 3420.666(1), 2544.743(2), 3460.912(2), and 2573.480(1) cm−1 respectively. These new values have been used in conjunction with the previously obtained ν1+2ν5±2←2ν5±2 band origins of HCCN and DCCN to obtain 2ν5±2←ν5±1 vibrational energies of 212.822(1) and 133.106(1) cm−1 for HCCN and DCCN, respectively. Semirigid bender calculations of the energy levels of these molecules have been carried out. By adjusting the height of the barrier to linearity, excellent agreement with experimental data has been attained.
Enhanced hysteresis in the semiconductor-to-metal phase transition of VO2 precipitates formed in SiO2 by ion implantation
Authors: Lopez R. , Boatner L. A. , Haynes T. E. , Haglund R. F. , Feldman L. C.
Published: July 2002

A strongly enhanced hysteresis with a width of >34 °C has been observed in the semiconductor-to-metal phase transition of submicron-scale VO2 precipitates formed in the near-surface region of amorphous SiO2 by the stoichiometric coimplantation of vanadium and oxygen and subsequent thermal processing. This width is approximately an order of magnitude larger than that reported previously for the phase transition of VO2 particles formed in Al2O3 by a similar technique. The phase transition is accompanied by a significant change in infrared transmission. The anomalously wide hysteresis loop observed here for the VO2/SiO2 system can be exploited in optical data storage and switching applications in the infrared region.
Very large magnetoimpedance effect in FeCoNi ferromagnetic tubes with high order magnetic anisotropy
Authors: Kurlyandskaya G. V. , Yakabchuk H. , Kisker E. , Bebenin N. G. , Garcı́a-Miquel H. , Vázquez M. , Vas’kovskiy V. O.
Published: July 2002

The extraordinarily high (up to 800% magnetoimpedance ratio) and sensitive magnetoimpedance effect has been found and studied in FeCoNi magnetic tubes electroplated onto CuBe nonmagnetic wire at frequency of about 1 MHz order. Special annealing was done in order to induce magnetic anisotropy. The high harmonic response has been studied, and the harmonics show larger variations with the external magnetic field than the fundamental. This huge sensitivity of the harmonics (up to an order of the tens of thousands %/Oe) is promising in regard to the increase of the sensitivity of giant magnetoimpedance sensors. To explain the experiment results, we calculated the high frequency transverse susceptibility taking into account the magnetic anisotropy of first and second orders. The susceptibility is extremely high at the points of orientational phase transitions in the magnetic layer which gives rise to strong nonlinear effects.
On the local structure of Eu3+ ions in oxyfluoride glasses. Comparison with fluoride and oxide glasses
Authors: Lavı́n V. , Babu P. , Jayasankar C. K. , Martı́n I. R. , Rodrı́guez V. D.
Published: July 2002

Broadband and fluorescence line narrowing optical spectroscopic studies have been used to investigate the local environments of Eu3+ ions in lithium fluoroborate glasses. From the vibronic spectra, different borate groups coupled with the Eu3+ ions have been identified. A pulsed tunable dye laser has been used to selectively excite the D05 level of the Eu3+ ion and the subsequent D05→F17 fluorescence spectra have been monitored as a function of the exciting wavelength. From these FLN studies, three F17 Stark levels have been identified and a C2v orthorhombic symmetry has been assumed in the subsequent calculation of the crystal-field parameters for the different environments occupied by the Eu3+ ions in the glass. The second rank crystal-field parameters have been systematically analyzed for the Eu3+:lithium fluoroborate glass from the site dependent behavior of the F17 level splitting. The importance of the J-mixing in the crystal-field analysis has been emphasized. An appropriate method for comparing the crystal-field interactions in different glasses has been proposed by analyzing the F17 level. Thus, results obtained for the Eu3+:fluoroborate have been compared with recalculated results in other Eu3+ doped fluoride, borate, silicate, and borosilicate glasses. An intermediate behavior between Eu3+:oxide and Eu3+:fluoride glasses is observed for the local structure of the Eu3+ ions in the fluoroborate glass, indicating the active participation of fluorine ions in the immediate environments of the lanthanide ion in this glass.
Theory of local optical spectroscopy of quantum wires with interface fluctuations
Authors: Di Stefano Omar , Savasta Salvatore , Girlanda Raffaello
Published: July 2002

We present a theory of local optical spectroscopy in quantum wires taking into account structural disorder. The calculated spatially resolved spectra show the individual spectral lines due to the exciton states localized by the disordered potential in agreement with experimental findings. We investigate systematically the influence of the potential profile and of the spatial resolution on the local optical spectra. Several line scans along the wire axis are obtained for different spatial correlations and strength of the disorder potential and for different spatial resolutions ranging from the subwavelength to the diffraction limit. Lowering the spatial resolution causes the disappearance of many spectral lines due to destructive spatial interference. However, our results show that information on the individual eigenstates of this quasi one-dimensional quantum system can be obtained at also resolutions significantly lower than the correlation length of interface fluctuations.
Thin betatron radiators for more efficient x-ray generation
Authors: Kaplin V. V. , Uglov S. R. , Bulaev O. F. , Goncharov V. J. , Voronin A. A. , Piestrup M. A. , Gary C. K.
Published: July 2002

We present experimental evidence that electrons of modest energy are making multiple passes through thin targets placed inside a betatron toroid, thus increasing their bremsstrahlung emission efficiency. Thin Cu, Be, W, and Mylar targets of thicknesses between 10−5 to 3.5×10−3 radiation lengths were used. The number of passes through the thin radiators were obtained using either the bremsstrahlung photon densities or angular distributions. The number of passes of 33-MeV electrons through the thin radiators was estimated to be 590 for a 1-μm-thick Cu foil, 171 for 5-μm Cu, 30 for 15-μm Cu, 200 for 20-μm Be, 51 for 60-μm Be, 20 for 200-μm Be, 460 for 3-μm-thick Mylar, and 123 for 2-μm W foils. The multiple-pass effect can be useful for increasing the efficiency of novel x-ray sources such as transition and parametric radiators.
Violet/blue emission from hydrogenated amorphous carbon films deposited from energetic CH3+ ions and ion bombardment
Authors: Liao Meiyong , Feng Zhihong , Chai Chunlin , Yang Shaoyan , Liu Zhikai , Wang Zhanguo
Published: July 2002

Considering the complexity of the general plasma techniques, pure single CH3+ ion beams were selected for the deposition of hydrogenated amorphous (a) carbon films with various ion energies and temperatures. Photoluminescence (PL) measurements have been performed on the films and violet/blue emission has been observed. The violet/blue emission is attributed to the small size distribution of sp2 clusters and is related to the intrinsic properties of CH3 terminals, which lead to a very high barrier for the photoexcited electrons. Ion bombardment plays an important role in the PL behavior. This would provide further insight into the growth dynamics of a-C:H films.
Electron work function: A parameter sensitive to the adhesion behavior of crystallographic surfaces
Authors: Li D. Y. , Li W.
Published: July 2002

The adhesive force (AF) and the electron work function (EWF) of different crystallographic planes of Cu were determined, with the aim of exploring the potential application of the Kelvin method in characterizing the adhesion of solid surfaces especially those in nano/microdevices. It was demonstrated that there was a close correlation between the EWF and AF, and both the parameters were dependent on the surface atomic arrangement. This study indicates that the EWF is a parameter that could be used to characterize the adhesion behavior of a surface.
Control of long-period orbits and arbitrary trajectories in chaotic systems using dynamic limiting
Authors: Corron Ned J. , Pethel Shawn D.
Published: July 2002

We demonstrate experimental control of long-period orbits and arbitrary chaotic trajectories using a new chaos control technique called dynamic limiting. Based on limiter control, dynamic limiting uses a predetermined sequence of limiter levels applied to the chaotic system to stabilize natural states of the system. The limiter sequence is clocked by the natural return time of the chaotic system such that the oscillator sees a new limiter level for each peak return. We demonstrate control of period-8 and period-34 unstable periodic orbits in a low-frequency circuit and provide evidence that the control perturbations are minimal. We also demonstrate control of an arbitrary waveform by replaying a sequence captured from the uncontrolled oscillator, achieving a form of delayed self-synchronization. Finally, we discuss the use of dynamic limiting for high-frequency chaos communications.