bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Al substitution on the structure and Co-sublattice magnetocrystalline anisotropy of Gd2Co17 compounds
Authors: Cheng Zhao-hua , Shen Bao-gen , Zhang Jun-xian , Liang Bing , Guo Hui-qun , Kronmüller H.
Published: July 2002

The structure and magnetic properties of Gd2Co17−xAlx compounds were investigated. All samples have a rhombohedral Th2Zn17-type structure and the replacement of Co by Al results in an approximately linear increase in the unit cell volumes at a rate of 8.2 Å3/Al. The Curie temperature decreases monotically with the increase of Al concentration. On the basis of magnetization curves at the compensation temperature, the intersublattice-molecular-field coefficient, nRT, and the RT exchange-coupling constant JRT have been determined. It is noteworthy that the substitution of Al has a significant effect on the magnetocrystalline anisotropy of the Co sublattice, and changes the easy magnetization direction of Gd2Co17−xAlx compounds from the basal plane to the c axis.
Observation of the polarization of domains in ferroelectric thin films using x-ray interference
Authors: Thompson Carol , Foster Christopher M. , Eastman Jeffrey A. , Stephenson G. Brian
Published: July 2002

We report that the sign of the polarization of an epitaxial ferroelectric film can be determined from the interference between the x-ray scattering from the film and the substrate. X-ray scattering measurements of a 10 nm epitaxial PbTiO3 film grown by metal-organic chemical vapor deposition on a SrTiO3 substrate are presented. The scattering profile near the 001 peaks of the film and substrate shows clear evidence of the interference effects. Analysis indicates that this film is a single domain of specific polarity.
Free-standing ZnSe/ZnS quantum wires with high luminescence efficiency
Authors: Rinaldi R. , Turco C. , Lovergine N. , Cingolani R. , Vasanelli L. , DiFabrizio E. , Grella L. , Gentili M. , DeCaro L. , Tapfer L.
Published: July 2002

We have fabricated ZnSe/ZnS quantum wires emitting at the shortest wavelength ever reported for a one-dimensional system. We show that inhomogeneous strain relaxation and Stark effect due to the internal piezoelectric field influence the ground level energy of the wires and overcome the quantum size effect.
X-ray photoelectron spectroscopy studies on Pd doped SnO2 liquid petroleum gas sensor
Authors: Phani A. R.
Published: July 2002

The present investigation deals with the electrical response of palladium doped tin oxide, as a means of improving the selectivity for liquid petroleum gas (LPG) in the presence of CO, CH4. The sensor element with the composition of Pd(1.5 wt %) in the base material SnO2 sintered at 800 °C, has shown a high sensitivity towards LPG with a negligible cross interference of CO and CH4at an operating temperature of 350 °C. This greatly suggests the possibility of utilizing the sensor for the detection of LPG. X-ray photoelectron spectroscopy studies have been carried out to determine the possible chemical species involved in the gas-solid interaction and the enhancing mechanism of the Pd doped SnO2 sensor element, towards LPG sensitivity.
Effect of crystal phases on refractive index profiles of annealed proton-exchanged waveguides in X-cut LiTaO3
Authors: Maring D. B. , Tavlykaev R. F. , Ramaswamy R. V. , Korkishko Yu. N. , Fedorov V. A. , Zavada J. M.
Published: July 2002

A complete structural phase diagram of annealed proton-exchanged X-cut LiTaO3 is presented, to relate refractive index change to proton-induced lattice deformation. From this diagram, explanations for the previously observed phenomena of index increase upon annealing and index decrease with increasing proton concentration (above a certain value) are derived. We have analyzed the evolution of index profiles during postexchange annealing, describing how the presence of multiple phases within a waveguide can lead to large index variations and buried index profiles. Finally, we identify at least two high-concentration phases that exhibit large temporal instabilities.
Occurrence of cubic GaN and strain relaxation in GaN buffer layers grown by low-pressure metalorganic vapor phase epitaxy on (0001) sapphire substrates
Authors: Cheng Lisen , Zhou Kuan , Zhang Ze , Zhang Guoyi , Yang Zhijian , Tong Yuzhen
Published: July 2002

Investigations on GaN buffer layers grown by low-pressure metalorganic vapor phase epitaxy on (0001) sapphire substrates indicated that the mechanisms by way of which GaN buffer layers relax stresses introduced by the lattice mismatch and thermal expansion coefficient difference between GaN epilayer and sapphire substrate are related to both the crystallographic structure of GaN and thickness of the buffer layers. Beside forming misfit dislocations, mismatch-induced stresses can also be relaxed by forming stacking faults and microtwin boundaries parallel to (11-1) of GaN near the interface between GaN and sapphire substrate in cubic GaN buffer layers. It was found that, in cubic GaN buffer layers, there exists a critical thickness within which the stacking faults and/or microtwin boundaries parallel to (11-1) of GaN can be formed. This critical value is determined to be 50 nm.
Evidence for relaxed and high-quality growth of GaN on SiC(0001)
Authors: Boscherini F. , Lantier R. , Rizzi A. , D’Acapito F. , Mobilio S.
Published: July 2002

By using polarization-dependent x-ray absorption spectroscopy at the Ga edge, we study the growth of GaN on SiC(0001) in the thickness range 0.7–150 nm. We find that the growth is always relaxed (i.e., nonpseudomorphic) even for the thinnest epilayers, i.e., below the expected critical thickness. No evidence is found for a mixed Ga/Si interface plane, while a C/N mixed interface plane cannot be ruled out. The results are discussed with reference to the electronic structure of the SiC/GaN heterojunction and in particular to band offsets and strain-induced piezoelectric polarization.
Spiral growth of InGaN/InGaN quantum wells due to Si doping in the barrier layers
Authors: Uchida Kenji , Tang Tao , Goto Shigeo , Mishima Tomoyoshi , Niwa Atsuko , Gotoh Jun
Published: July 2002

We have examined Si-doping effects in InGaN/InGaN quantum-well (QW) structures, especially the influence of Si-doped InGaN barrier layers on the growth mechanism of QW structures, by atomic force microscopy (AFM) and by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. Our AFM observations revealed that Si-doped InGaN barriers strongly affect the growth mode of overlying InGaN QW layers. This effect leads to the formation of nanoscale islands (with a density of 108 cm−2) due to spiral growth of the QW layers. The spirally grown nanoscale islands significantly increase the PL intensity. Through spatially resolved CL observations, we found that the number of dot-like CL bright spots increased dramatically when the barrier layers were Si doped, and the increased density of the spots was in good agreement with the increased density of the nanoscale islands observed by AFM. By combining these results, we show that the spirally grown QW structures produced by Si doping of the barriers effectively reduce the fluctuation of the band-gap potential in InGaN QW layers.
Polymer gel light-emitting devices
Authors: Chang Shun-Chi , Yang Yang
Published: July 2002

Polymers can be processed into several forms such as solid films, solutions, and gels. Traditional conjugated polymer electroluminescent devices are thin film, solid-state devices consisting of a thin polymer film sandwiched between two electrodes. Recently, a light-emitting device based on polymer solutions has been demonstrated. In this letter, we report the generation of luminescence from a polymer gel in a compact cell configuration. This unique polymer gel light-emitting device (GLED) consists of a thin layer of a polymer gel sandwiched between two electrode/glass substrates. The emission spectrum is consistent with the photoluminescence spectrum obtained from the polymer gel. The device shows an operating mechanism similar to the electrogenerated chemiluminescence.
Band offset determination in the strained-layer InSb/AlxIn1−xSb system
Authors: Dai N. , Khodaparast G. A. , Brown F. , Doezema R. E. , Chung S. J. , Santos M. B.
Published: July 2002

We use interband exciton transitions in parabolically graded quantum wells to measure the band offset at the InSb/AlxIn1−xSb interface. The method we use is based on similar studies in the GaAs/AlxGa1−xAs system but modified to reflect the strong nonparabolicity and strain of the InSb/AlxIn1−xSb system. We find a conduction band offset ratio of 0.62±0.04 for Al concentrations in the range 2%–12%. The observed lack of variation of the offset with Al concentration suggests a lack of strain dependence in the InSb/AlxIn1−xSb system for practical Al concentrations.
Surface passivation by sulfur treatment of undoped p-CdTe(100)
Authors: Kang C. K. , Yuldashev Sh. U. , Leem J. H. , Ryu Y. S. , Hyun J. K. , Jung H. S. , Kim H. J. , Kang T. W. , Lee H. I. , Woo Y. D. , Kim T. W.
Published: July 2002

The effect of surface passivation of undoped p-CdTe(100) by (NH4)2Sx treatment was investigated by using photoluminescence (PL), photoconductivity (PC), and x-ray photoelectron spectroscopy (XPS). After sulfur treatment for 2 min, the acceptor bound exciton (A0, X) peak increases greatly in the PL spectrum, and the minority-carrier lifetime of CdTe becomes the longest value in the PC measurement. The XPS spectrum for untreated CdTe shows the additional peaks on the right side of two main Te peaks corresponding to the Te 3d core levels, and these additional peaks are related to TeO3 with binding energies of 576.2 and 586.5 eV. After sulfur treatment, while the intensities of the Te 3d core levels decreased gradually, those of the TeO3 peaks disappear. In addition, the S 2p core-level spectra for sulfur-treated CdTe show the peaks at the 161.7 and 162.8 eV, which are attributed to a CdS formation at the surface of CdTe. These results indicate the sulfur effectively dissociates the native oxides from and neutralizes the dangling bonds at the surface of CdTe.
Solvation dynamics of nile blue in ethanol confined in porous sol–gel glasses
Authors: Baumann R. , Ferrante C. , Deeg F. W. , Bräuchle C.
Published: July 2002

We report on solvation dynamics measurements of the chromophore nile blue in ethanol confined to sol–gel glasses with 50 Å and 75 Å average pore size and compare them with the dynamics of the respective bulk solution. Both the amplitude of the dynamic Stokes shift as well as the dynamics of the solvation process are drastically changed upon confinement. In both confined solutions the dynamic Stokes shift is reduced by a factor of about 2. As the large majority of the chromophores is adsorbed at the inner pore surfaces the solute molecules interact with only a “half-space” of solvent molecules. In a first approximation this decreases the stabilization energy by a factor of 2. The solvation dynamics in the confined solutions show nonexponential behavior comparable to the bulk. However, the whole solvation process slows down and the single decay time constants characterizing it increase with decreasing pore size of the sol–gel glass. We have introduced two phenomenological models to rationalize this behavior. The enhanced polarization field model takes into account the confinement and predicts a strengthening of the solvent’s polarization field in an extended solvation cage induced by the electrical field of the chromophore. The steric hindrance model focuses on the interaction of the liquid with the surface reducing molecular mobility resulting in longer relaxation times. Probably both effects are of relevance in the confined solutions investigated. Confinement affects the steady-state spectra as well leading to a redshift of the absorption and a blueshift of the fluorescence. Additional data on the reorientational dynamics of the chromophore are consistent with the fact that the molecules are predominantly adsorbed at the pore surfaces.
Solid–liquid interface velocity and diffusivity in laser-melt amorphous silicon
Authors: Brambilla Luigi , Colombo Luciano , Rosato Vittorio , Cleri Fabrizio
Published: July 2002

We studied the microscopic kinetics of the amorphous-liquid interface in supercooled laser-melt silicon by means of molecular dynamics computer simulations. The interface velocity was obtained as a function of temperature by direct simulation of the interface motion in an amorphous-liquid model system. The temperature dependence of the kinetic prefactor was extracted from the interface velocity function and compared to the values of self-diffusivity obtained from independent molecular dynamics simulations of bulk amorphous Si. The kinetic prefactor for interfacial diffusion shows a distinctly non-Arrhenius behavior which is attributed to Fulcher–Vogel kinetics in the supercooled liquid.
The adsorption conformation of chemisorbed pyridine on the Cu(110) surface
Authors: Lee J.-G. , Ahner J. , Yates J. T.
Published: July 2002

The adsorption conformation of pyridine on the Cu(110) surface has been studied using temperature programmed desorption, low energy electron diffraction, and electron stimulated desorption ion angular distribution techniques. Pyridine adsorbs on Cu(110) via the nitrogen lone pair orbital. The molecular symmetry axis is perpendicular to the surface. At low coverage, the aromatic ring plane of adsorbed pyridine is oriented parallel to the 〈001〉 plane, which is perpendicular to the close-packed copper atom rows on Cu(110). At high coverage, the aromatic ring plane of pyridine is azimuthally rotated by 25 (±5)° away from the 〈001〉 direction as a result of an attractive contribution from nonparallel aromatic ring–aromatic ring interactions superimposed on the overall repulsive adsorbate–adsorbate interactions. The zero coverage desorption activation energy of pyridine on Cu(110) is 0.97 eV and the repulsive adsorbate–adsorbate interaction energy is 0.16 eV/monolayer. At near saturation coverage, (5×3) and (4×3) long-range ordered structures were observed. It is proposed that the adsorbed pyridine on the Cu(110) surface forms a chain structure of nonparallel normally-oriented pyridine rings arranged in a chevron pattern at monolayer saturation coverage.
Multichordal charge exchange recombination spectroscopy on the JT-60U tokamak
Authors: Koide Y. , Sakasai A. , Sakamoto Y. , Kubo H. , Sugie T.
Published: July 2002

Recent development of the JT-60U charge exchange recombination spectroscopy (CXRS) diagnostic system is reported. The measurements of the radial profiles of the ion temperatures, the toroidal and poloidal rotation velocities, and the impurity density are based on CXR spectra of fully stripped carbon ions induced by neutral beam injection. Measurements are made simultaneously at 59 spatial points (23 toroidal and 36 poloidal) with a time resolution of 1/60 s throughout the full discharge pulse of 15 s. Considerable effort has been expended to ensure the automatic provision of reliable profile data, where the mechanical components of the system must cope with both a baked tokamak vessel (∼300 °C) and an expected disruption-induced acceleration of up to 60 g, where g denotes the gravitational acceleration, i.e., 9.8 m2/s.
Perturbative polydispersity: Phase equilibria of near-monodisperse systems
Authors: Evans R. M. L.
Published: July 2002

The conditions of multiphase equilibrium are solved for generic polydisperse systems. The case of multiple polydispersity is treated, where several properties (e.g., size, charge, shape) simultaneously vary from one particle to another. By developing a perturbative expansion in the width of the distribution of constituent species, it is possible to calculate the effects of polydispersity alone, avoiding difficulties associated with the underlying many-body problem. Explicit formulas are derived in detail, for the partitioning of species at coexistence and for the shift of phase boundaries due to polydispersity. Convective fractionation is quantified, whereby one property (e.g., charge) is partitioned between phases due to a driving force on another. To demonstrate the ease of use and versatility of the formulas, they are applied to models of a chemically polydisperse polymer blend, and of fluid–fluid coexistence in polydisperse colloid–polymer mixtures. In each case, the regime of coexistence is shown to be enlarged by polydispersity.
New results on the model problem of the diffusion of turbulence from a plane source
Authors: Cazalbou J.-B. , Chassaing P.
Published: July 2002

The problem of the diffusion of turbulence from a plane source is addressed in the context of two-equation eddy-viscosity models and Reynolds-stress-transport models. In the steady state, full analytic solutions are given. At second order, they provide the equilibrium value of the anisotropy level obtained with different combinations of return-to-isotropy and turbulent-diffusion schemes and confirm the results obtained by Straatman et al. [AIAA J. 36, 929 (1998)] in an approximate analysis. In addition, all the characteristics of the turbulence decrease can be determined and it is shown that a special constraint on the value of the modeling constants should hold if turbulence fills the whole surrounding space. In a second step, precise results can be given for the unsteady model problem at the first-order-closure level. The evolution cannot be described with a single set of characteristic scales and one has to distinguish the cases of short and large times. In the short-time regime, the flow is governed by the characteristic scales of turbulence at the source and contamination of the flow proceeds as t1/2. At large times, the flow is governed by time-dependent characteristic scales that correspond to the solution of the steady problem at the instantaneous location of the front. Contamination of the flow proceeds as a power of time that can be related to the value of the modeling constants. The role of a combination of these constants is emphasized whose value can be specified to produce a solution that matches simultaneously the experimental data for the decrease of turbulent kinetic energy in the steady state and the exponent of the propagation velocity in the transient regime.
Investigation of a strong titanium alloy KS15-5-3 and the application to a high pressure apparatus for magnetization measurements
Authors: Kamishima K. , Hagiwara M. , Yoshida H.
Published: July 2002

A β titanium alloy of Ti–15Mo–5Zr–3Al (KS15-5-3) is presented as a new material for experimental apparatus to be used at extreme conditions of high pressure, low temperature, and high magnetic field. The material has a tensile strength of 1760 MPa at room temperature and is found to be a Pauli paramagnet with nearly temperature-independent magnetization M/B∼3×10−2 (mJ/T g)/T, except for T<TC∼2.5 K. Using this material, we developed a high-pressure clamp cell for measuring the magnetization in a commercial superconducting quantum interference device magnetometer (Quantum Design MPMS2). This pressure cell can be used in the pressure range 0<P<1.0 GPa and temperatures T>TC. To illustrate the performance of this cell, we present experimental results on a one-dimensional alternating bond antiferromagnet with S=1:Ni(333-tet)(μ-NO2)(ClO4) [333-tet denotes N,N′-bis(3-aminopropyl)-1,3-propanediamine].
Behavior of laser-induced ultrasonic waves radiated from a wet surface. Part I. Theory
Authors: Wooh Shi-Chang , Zhou Quanlin
Published: July 2002

This article deals with the directivity analysis of laser-generated ultrasound irradiated from a wet surface. A mathematical model is developed to take into account the wet surface condition, which is described by nonuniform load distribution on the illuminated surface. In particular, the free surface of an elastic half-space loaded by a time-harmonic normal traction distributed elliptically on a circular source area is considered. The solutions are obtained by using the Bessel–Fourier transformation and evaluating the transform integrals asymptotically. The results are presented for various laser beam diameters and ultrasonic frequencies.
Erratum: “Vibrational predissociation of the ND3–Ar Van der Waals complex: Comparison with NH3–Ar” [J. Chem. Phys. 106, 9141 (1997)]
Authors: Millan J. , Halberstadt N. , van der Sanden G. , van der Avoird A.
Published: July 2002

A programming error, which affects many of the numbers, tables, and figures of the dissociation part of this paper, is corrected. It does not affect the results concerning the quasibound states. The main conclusions are unchanged, except that the correlation between final rotational distributions originating from the two components of the same tunneling doublet has become rather high.
Ferroelectric and electromechanical properties of poly(vinylidene-fluoride–trifluoroethylene–chlorotrifluoroethylene) terpolymer
Authors: Xu Haisheng , Cheng Z.-Y. , Olson Dana , Mai T. , Zhang Q. M. , Kavarnos G.
Published: July 2002

This letter reports the ferroelectric and electromechanical properties of a class of ferroelectric polymer, poly(vinylidene-fluoride–trifluoroethylene–chlorotrifluoroethylene) terpolymer, which exhibits a slim polarization hysteresis loop and a high electrostrictive strain at room temperature. The dielectric and polarization behaviors of this terpolymer are typical of the ferroelectric relaxor. The x-ray and Fourier transform infrared results reveal that the random incorporation of bulky chlorotrifluoroethylene (CTFE) ter-monomers into polymer chains causes disordering of the ferroelectric phase. Furthermore, CTFE also acts as random defect fields which randomize the inter- and intrachain polar coupling, resulting in the observed ferroelectric relaxor behavior.
Resonance Raman intensity analysis of chlorine dioxide dissolved in chloroform: The role of nonpolar solvation
Authors: Foster Catherine E. , Barham Bethany P. , Reid Philip J.
Published: July 2002

Absolute resonance Raman cross sections for chlorine dioxide (OClO) dissolved in chloroform are obtained at several excitation wavelengths spanning the photochemically relevant B12–A22 optical transition. The absolute scattering cross sections of OClO are determined by reference to the 666 cm−1 transition of chloroform whose absolute scattering cross sections are reported here. The time-dependent theory for Raman and absorption are used to develop a mode-specific description of the A22 excited state surface. This description demonstrates that photoexcitation of OClO leads to significant structural evolution along the symmetric stretch and bend coordinates, with only limited evolution occurring along the asymmetric stretch. This description is similar to that determined for OClO dissolved in cyclohexane and water demonstrating that the excited-state structural evolution of OClO is similar in these solvents. Analysis of the OClO absolute scattering cross sections establishes that the homogeneous linewidth is 95±15 cm−1 in chloroform, essentially identical to the linewidths in cyclohexane and water. To establish the origin of this linewidth, the fluorescence cross section for OClO dissolved in cyclohexane is measured and found to be consistent with an excited-state lifetime of ∼200 fs. Comparison of this lifetime to the homogeneous linewidth establishes that the homogeneous broadening is dominated by solvent-induced pure dephasing. It is proposed that the apparent solvent independence of the homogeneous linewidth reflects the mechanical response of the solvent to the photoinitiated change in solute geometry. In support of this hypothesis, the homogeneous linewidth is reproduced using the viscoelastic continuum model of nonpolar solvation. Finally, it is argued that the restricted evolution along the asymmetric-stretch coordinate is due to dielectric solvent–solute interactions consistent with the increase in inhomogeneous linewidth with an increase in solvent polarity.
NIST-JANAF Thermochemical Tables. I. Ten Organic Molecules Related to Atmospheric Chemistry
Authors: Dorofeeva O. V. , Novikov Vladimir P. , Neumann David B.
Published: July 2002

The structural, spectroscopic, and thermodynamic properties of 10 gas phase organic molecules related to atmospheric chemistry, including three peroxides and four carboxylic acids, are reviewed. The calculation of the thermochemical tables involved the critical evaluation of new spectroscopic data, enthalpy of formation determinations, and the use of recent internal rotation data. Since insufficient information to characterize all 10 molecules exists, estimation schemes were used to provide the missing experimental and theoretical data.
Modification of the short range order in amorphous alloys induced by the swift heavy ion irradiation
Authors: Kopcewicz M. , Dunlop A.
Published: July 2002

Experimental evidence of the changes of the short range order (SRO) in amorphous alloys due to high energy deposition in electronic processes related to the slowing down of swift heavy ions is presented. Modification of the local structure was studied by the specialized “rf-Mössbauer” technique in amorphous Fe40Ni35Si10B15, Fe73.5Nb3Cu1Si13.5B9, Fe73.5Nb4.5Cr5Cu1B16, and Fe80.5Nb7B12.5 alloys irradiated at low temperature with 6 GeV Pb ions. The changes of SRO were studied as a function of ion fluence (from 1×1011 to 2.4×1013ions/cm2) and electronic energy deposition rate for the amorphous Fe40Ni35Si10B15 alloy. The effects observed were interpreted in relation to the anisotropic growth of the amorphous samples. A novel effect consisting of partial surface crystallization induced by swift heavy ion irradiation was observed in the Fe90Zr7B3 alloy by conversion electron Mössbauer spectroscopy.
High temperature semiconducting characteristics of magnesium-doped α-Al2O3 single crystals
Authors: Tardı́o M. , Ramı́rez R. , González R. , Chen Y. , Kokta M. R.
Published: July 2002

In the temperature interval 300–773 K, ac and dc electrical measurements show a phenomenally large enhancement of the electrical conductivity of otherwise excellent insulator Al2O3 crystals when doped with magnesium impurities. The conductivity increases linearly with the concentration of hole-trapped [Mg]0 centers and is four times higher in the direction parallel to the crystallographic c axis than in the perpendicular direction. The conductivity activation energy is 0.68 eV, independent of both [Mg]0 content and crystallographic orientation. Electroluminescence experiments indicate that holes are the majority carriers. These results favor the small-polaron-motion mechanism.
Change in electrical resistivity due to icosahedral phase precipitation in Zr70Pd20Ni10 and Zr65Al7.5Cu7.5Ni10Ag10 glasses
Authors: Haruyama O. , Miyazawa T. , Saida J. , Inoue A.
Published: July 2002

The glass-to-icosahedral phase transformation in Zr70Pd20Ni10 and Zr65Al7.5Cu7.5Ni10Ag10 glasses was examined by the electrical resistivity measurement performed with a heating rate of 0.67 K/s. The resistivity increased with the promotion of icosahedral precipitation in Zr70Pd20Ni10 glass. On the other hand, Zr65Al7.5Cu7.5Ni10Ag10 glass exhibited the decrement of the resistivity according to the evolution of icosahedral phase. The latter was qualitatively explained by the drop of the resistivity of supercooled liquid phase due to the transfer of oxide atoms into the icosahedral phase. Also, the low temperature resistivity experiment showed that the conductivity of glassy and icosahedral phases might obey the weak localization model of conduction electrons.
Bulk synthesis of silicon nanowires using a low-temperature vapor–liquid–solid method
Authors: Sunkara M. K. , Sharma S. , Miranda R. , Lian G. , Dickey E. C.
Published: July 2002

Silicon nanowires will find applications in nanoscale electronics and optoelectronics both as active and passive components. Here, we demonstrate a low-temperature vapor–liquid–solid synthesis method that uses liquid-metal solvents with low solubility for silicon and other elemental semiconductor materials. This method eliminates the usual requirement of quantum-sized droplets in order to obtain quantum-scale one-dimensional structures. Specifically, we synthesized silicon nanowires with uniform diameters distributed around 6 nm using gallium as the molten solvent, at temperatures less than 400 °C in hydrogen plasma. The potential exists for bulk synthesis of silicon nanowires at temperatures significantly lower than 400 °C. Gallium forms a eutectic with silicon near room temperature and offers a wide temperature range for bulk synthesis of nanowires. These properties are important for creating monodispersed one-dimensional structures capable of yielding sharp hetero- or homointerfaces.
The potential energy curve and dipole polarizability tensor of mercury dimer
Authors: Schwerdtfeger Peter , Wesendrup Ralf , Moyano Gloria E. , Sadlej Andrzej J. , Greif Jörn , Hensel Friedrich
Published: July 2002

Scalar relativistic coupled cluster calculations for the potential energy curve and the distance dependence of the static dipole polarizability tensor of Hg2 are presented and compared with current experimental work. The role of the basis set superposition error for the potential energy curve and the dipole polarizability is discussed in detail. Our recently optimized correlation consistent valence basis sets together with energy adjusted pseudopotentials are well suited to accurately describe the van der Waals system Hg2. The vibrational–rotational analysis of the best spin–orbit corrected potential energy curve yields re=3.74 Å, D0=328 cm−1, ωe=18.4 cm−1, and ωexe=0.28 cm−1 in reasonable agreement with experimental data (re=3.69±0.01 Å, De=380±25 cm−1, ωe=19.6±0.3 cm−1 and ωexe=0.25±0.05 cm−1). We finally present a scaled potential energy curve of the form ∑ja2jr−2j which fits the experimental fundamental vibrational transition of 19.1 cm−1 and the form of our calculated potential energy curve best (re=3.69 Å, D0=365 cm−1, ωe=19.7 cm−1, and ωexe=0.29 cm−1). We recommend these accurate two-body potentials as the starting point for the construction of many-body potentials in dynamic simulations of mercury clusters.
Optical, electronic, and transport properties of nanocrystalline titanium nitride thin films
Authors: Patsalas P. , Logothetidis S.
Published: July 2002

Spectroscopic ellipsometry (SE) was employed to get insights on the optical, electronic, and transport properties of nanocrystalline titanium nitride (TiNx) films with respect to their microstructure and stoichiometry. The films’ properties can be tailored by varying the energy of bombarding ions during sputter deposition and the substrate temperature (Td). The best metallic behavior of TiNx (resistivity 40 μΩ cm and conduction density 5.5×1022 electrons/cm3) has been observed in films developed with energy above 100 eV and Td⩾400 °C. A redshift of the optical gaps has been observed for overstoichiometric films, suggesting it as a sensitive probe to investigate the TiNx stoichiometry. The energy, strength, and broadening of the interband transitions were studied with respect to the energy of ions and Td and they were explicitly correlated with the TiNx crystal cell size and grain orientation. On the other hand, the study of intraband absorption has provided the conduction electron density with respect to ion energy and Td, which promotes the densification of TiNx films due to different mechanisms. Combined SE and x-ray analysis was used to identify the electron scattering mechanisms, showing that the main electron scattering sites are the grain boundaries and the Ti vacancies for stoichiometric (x=1) and overstoichiometric (x∼1.1) films, respectively.
The Step and Slide method for finding saddle points on multidimensional potential surfaces
Authors: Miron Radu A. , Fichthorn Kristen A.
Published: July 2002

We present the Step and Slide method for finding saddle points between two potential-energy minima. The method is applicable when both initial and final states are known. The potential-energy surface is probed by two replicas of the system that converge to the saddle point by following isoenergetic surfaces. The value of the transition-state potential is bracketed in the process, such that a convergence criterion based on the potential can be used. We applied the method to study diffusion mechanisms of a small Ag cluster on a Ag(111) surface using an embedded-atom method potential. Our approach is comparable in efficiency to other commonly used methods.
Quantum-well effect in magnetic tunnel junctions with ultrathin single-crystal Fe(100) electrodes
Authors: Nagahama T. , Yuasa S. , Suzuki Y. , Tamura E.
Published: July 2002

We studied the tunnel spectra of magnetic tunnel junctions with a single-crystal ultrathin Fe(100) electrode. The tunnel spectra show oscillations of the differential conductivity and the differential tunnel magnetoresistance. The positions of the maxima of the oscillations move systematically with the change in the Fe(100) electrode’s thickness, indicating that the oscillations originate from the quantum-well states in the ultrathin Fe(100) electrode. This effect provides us with an opportunity to create voltage-controlled spin functional devices.
Structural characterization of zinc stannate thin films
Authors: Young D. L. , Williamson D. L. , Coutts T. J.
Published: July 2002

Polycrystalline thin films of Zn2SnO4 were deposited by rf magnetron sputtering onto glass substrates. Films were characterized by θ–2θ x-ray diffraction and by Sn119 conversion electron Mössbauer spectroscopy. The films were randomly oriented in a cubic spinel structure. Comparison of x-ray diffraction peak intensities with structure-factor-calculated peak intensities confirmed that the films were in an inverse spinel configuration. Mössbauer studies detected two distinct Sn4+ octahedral sites. These distinct sites may be induced by distortions in the lattice associated with equally distinct Zn2+ octahedral sites. A model is suggested to explain that the relatively low electron mobility of Zn2SnO4 may be associated with disorder on the cation octahedral sites. This may disrupt transport between edge-sharing d10s0 electronically configured cations.
Effects of temperature-dependent viscosity in channels with porous walls
Authors: Ferro Sergio , Gnavi Graciela
Published: July 2002

The influence of temperature-dependent viscosity on the flow along a channel with porous walls at different temperatures was analyzed. Numerical schemes were developed to solve the Navier–Stokes equation and the energy equation, coupled through the dependence of viscosity on temperature. Bifurcation diagrams are presented. Local analysis of the behavior of these bifurcations was also performed. The temporal and spatial stability of stationary solutions was studied by solving eigenvalue problems. The parameter space was explored and the regions where different stationary and periodic solutions appear are described. Periodic solutions obtained by solving the time-dependent problem are also presented.
Finite orbit analysis in tokamak plasmas
Authors: Chiu S. C. , Chan V. S. , Omelchenko Y. A.
Published: July 2002

The orbits of particles in the core region of a rotating tokamak plasma are analyzed. It is found that the phase space topologies of finite orbits differ significantly from those of thin orbit considerations. Specifically, in the absence of rotations, the trapped-passing boundary of co-moving particles has a finite extent in energies and disappears above a critical energy; and the pitch angle of the boundary has a rather strong energy dependence. Rotation and rotation shear cause significant shifts of this boundary in phase space so that above certain thresholds this boundary disappears. The finite orbit analysis also leads to improved analytic formulas for orbit averaged quantities of interest.
Lattice symmetry and magnetization reversal in micron-size antidot arrays in Permalloy film
Authors: Vavassori P. , Gubbiotti G. , Zangari G. , Yu C. T. , Yin H. , Jiang H. , Mankey G. J.
Published: July 2002

The magnetization reversal in four arrays of micron-size circular holes (antidots) in a Permalloy film has been studied by means of quantitative magneto-optic Kerr vector magnetometry and magnetic force microscopy. The primitive antidot meshes of the arrays investigated here can be classified as square, rectangular, hexagonal, and oblique. The vector magnetometry data show that the hole arrays induce a magnetic anisotropy completely different from that of the unpatterned film, with new hard axes along the directions connecting nearest neighboring holes. Also the coercive field is strongly affected by the pattern. The results of the vector magnetometry analysis indicate that the reversal process takes place through a collective and periodic domain nucleation and expansion process. The domain structure in the remanent state has been investigated by magnetic force microscopy imaging. The images display well-defined domain structures, which are periodic and commensurate with the holes array.
Differentiating between elastically bent rectangular beams and plates
Authors: Kaldor S. K. , Noyan I. C.
Published: July 2002

We report x-ray microdiffraction curvature measurements of a (100)-type Si single crystal loaded in four-point bending and provide experimental verification of a procedure for differentiating between anisotropic, elastically bent beams and plates. In general usage, beam and plate components are distinguished by dimensions alone. In mechanics, however, beams and plates are differentiated based on their flexural rigidity and stress state. Since current textbooks do not provide a quantitative technique for selecting the proper constitutive equations for these two types of structures, we suggest the extension of an analysis for isotropic materials originated by Searle [G. F. C. Searle, Experimental Elasticity (Cambridge University Press, Cambridge, 1908), pp. 40–58] and expanded on by Ashwell [D. G. Ashwell, J. R. Aeronaut. Soc. 54, 708 (1950)]. We demonstrate that, by varying the degree of bending of an anisotropic strip, a single specimen can behave as both a beam and a plate, as is predicted by this analysis.
Stability of mixed convection in an anisotropic vertical porous channel
Authors: Bera P. , Khalili A.
Published: July 2002

This paper addresses the stability of mixed convective buoyancy assisted flow due to external pressure gradient and buoyancy force in a vertical fluid saturated porous channel with linearly varying wall temperature. The porous medium is assumed to be both hydrodynamically and thermally anisotropic. Two different types of temperature perturbations, (i) zero temperature and (ii) zero heat flux, have been considered to study the effect of anisotropic permeability and thermal diffusivity on the flow stability. The stability analysis indicated that the least stable mode is two-dimensional. Furthermore, the results show that for the same Reynolds number, the fully developed base flow is highly unstable (stable) for high (low) permeable porous media as well as for a porous medium with small (large) thermal diffusivity ratio. Depending on the magnitude of all parameters studied, three types of instabilities (shear, thermal, and mixed instability) occurred. The transition of instability from one type to another took place smoothly, except when the permeability ratio exceeded 6. Based on the value of the permeability ratio, the flow in an anisotropic medium for a specific Reynolds number may be either more or less stable than the flow in an isotropic medium. In addition, the fully developed flow is more stable for relatively small values of the modified Darcy number than for larger values. The effect of Brinkman as well as Forchheimer terms are negligible for the set of other parameters studied here. In contrast to a pure viscous fluid or an isotropic porous medium, which are characterized by unicellular convective cells, in anisotropic porous media convective cells may be unicellular or bicellular. The stability analysis of mixed convection in channels filled either with a viscous fluid or with an isotropic saturated porous medium may be obtained as special cases of the general study presented here.
Interdependence of absorber composition and recombination mechanism in Cu(In,Ga)(Se,S)2 heterojunction solar cells
Authors: Turcu M. , Pakma O. , Rau U.
Published: July 2002

Temperature-dependent current-voltage measurements are used to determine the dominant recombination path in thin-film heterojunction solar cells based on a variety of Cu(In,Ga)(Se,S)2 alloys. The activation energy of recombination follows the band gap energy of the respective Cu(In,Ga)(Se,S)2 alloy as long as the films are grown with a Cu-poor final composition. Thus, electronic loss in these devices is dominated by bulk recombination. In contrast, all devices based on absorber alloys with a Cu-rich composition prior to heterojunction formation are dominated by recombination at the heterointerface, with activation energies smaller than the band gap energy of the absorber material. These activation energies are independent from the S/Se ratio but increase with increasing Ga/In ratio.
Improving “difficult” reaction barriers with self-interaction corrected density functional theory
Authors: Patchkovskii Serguei , Ziegler Tom
Published: July 2002

We examined 11 difficult reactions with self-interaction corrected density (SIC) functional theory. The data set includes dissociation of radicals into symmetric fragments (H2+→H+H+, He2+→He+He+), radical hydrogen abstraction (H+H2→H2+H, H+HCl→H2+Cl, H+N2H2→N2H+H2, CH3+H2→CH4+H), proton transfer [HC(OH)CHC(O)H→HC(O)CHC(OH)H], SN2 halogen exchange (X−+CH3X→CH3X+X−, X=F,Cl,Br), and closed-shell unimolecular dissociation of tetrasine (C2N4H2→N2+2HCN). Calculated self-interaction energies cancel, almost identically, for the reaction energies (ΔER), so that SIC functionals do not lead to a systematic improvement in ΔER. Self-interaction correction increases for reaction transition structures, leading to higher calculated activation barriers (ΔE≠). The average absolute deviation in ΔE≠, from ab initio and experimental barriers, is reduced from 14 kcal/mol for Vosko–Wilk–Nusair (VWN) or 12 kcal/mol for revised Perdew–Burke–Ernzerhof (revPBE) functionals to 5.4 (SIC-VWN) or 3.4 (SIC-revPBE) kcal/mol. Reorganization of the electron density, due to removal of self-interaction, appears to be important. When SIC is included as a perturbation, using self-consistent densities of the parent functional, the average absolute deviations for the barriers increase to 7.5 (VWN+SIC) or 5.3 (revPBE+SIC) kcal/mol. Gradient-corrected functionals (revPBE, BP86) reduce the magnitude of the total self-interaction correction, by improving the description of the core orbitals. For the valence orbitals, both the magnitudes of the self-interaction corrections, and their change between reagents and transition structures, are similar for VWN local density approximation, and generalized gradient approximation functionals. Reducing the magnitude of the self-interaction energy for valence electrons thus appears to be a promising direction for the development of chemically accurate exchange-correlation functionals.
Vector properties of S(3P) and S(1D) in the photodissociation of SH: Quantum interference and overlapping resonance
Authors: Lee Sungyul , Sun Hosung , Kim Bongsoo , Freed Karl F.
Published: July 2002

A quantal analysis is presented for the vector properties of the S(3P) and S(1D) fragments produced in the photodissociation of SH. We find that several different kinds of effects influence the angular distributions and the alignments of the sulfur atoms. At energies between the thresholds to the S(3P) and S(1D) terms, the vector properties are constant near the isolated Lorentzian resonances. The vector properties for the overlapping resonances, show rapid changes due to the difference of the geometrical factors characterizing the individual resonance. For the highest levels (v′=6,N′⩾6), quantum interference between the predissociative (via the bound A 2Σ+ state) and the direct (by the repulsive Σ−2 state) pathways produces strong variations in the values of vector properties across the asymmetric resonances, suggesting a convenient means of angular control of the different atomic terms or the fine structure components of a given atomic term. At energies above the threshold to the S(1D) term, the interference between the two direct dissociating routes [through the A Σ+2 and Σ-2 states for the S(3P) product, and through the Δ2 and 22Π states for the S(1D) product] is shown to yield highly oscillatory variations of the vector properties.
Counterion-induced entropic interactions in solutions of strongly stretched, osmotic polyelectrolyte stars
Authors: Jusufi A. , Likos C. N. , Löwen H.
Published: July 2002

We examine the conformations and effective interactions of star-branched polyelectrolytes with and without added salt, by employing monomer-resolved molecular dynamics simulations and an analytical theory. The simulations take into account the excluded-volume and Coulomb interactions between the individual monomers, as well as the counter- and coions. The theory is based on a variational free energy that is written as a sum of electrostatic, polymer, and entropic contributions of the counter- and coions. For the conformations of isolated polyelectrolyte stars, we find strong stretching of the chains, resulting in a linear scaling of the star radius with the degree of polymerization, as well as trapping and condensation of a large fraction of counterions. The effective interactions at arbitrarily strong overlaps between the stars are shown to be dominated by the entropic contributions of the trapped counterions, with the electrostatic contribution playing only a minor role due to an almost complete neutralization of the stars. In the case of added salt, we find a shrinking of the star size as well as a weakening of the effective force due to a generalized depletion mechanism. The good agreement between theory and simulations allows us to put forward analytic expressions for the effective interaction between polyelectrolyte stars at arbitrary separations.
Direct writing of fluorescent patterns on LiF films by x-ray microprobe
Authors: Larciprete R. , Gregoratti L. , Danailov M. , Montereali R. M. , Bonfigli F. , Kiskinova M.
Published: July 2002

Fluorescent patterns with submicron dimensions have been obtained by creating stable F3+ and F2 color centers in LiF films using a focused x-ray beam provided at the ELETTRA synchrotron radiation facility. The patterns were written by scanning the LiF specimen with respect to the x-ray microprobe. In these attempts, using an x-ray microspot with a diameter of 100 nm and a flux density ⩾109 photons/s, we generated ∼500-nm-wide lines efficiently emitting in the visible spectral region when excited by blue light at 458 nm. Preliminary results indicate that the spectral distribution of the emitted luminescence can be changed by varying the photon dose delivered to the sample.
Crystal orientation effects on electronic properties of wurtzite InGaN/GaN quantum wells
Authors: Park Seoung-Hwan
Published: July 2002

Crystal orientation effects on electronic properties of wurtzite InGaN/GaN quantum wells (QWs) with piezoelectric (PZ) and spontaneous (SP) polarization are investigated using the multiband effective-mass theory. With increasing crystal angle, the internal field of the InGaN/GaN QW structure changes its sign near the crystal angle of 55° while that of the GaN/AlGaN QW structure gradually decreases without changing its sign. The interband transition energy is redshifted for crystal angles near θ=0° and gradually increases with the crystal angle due to the reduced SP and PZ polarization effects. The y′-polarized optical matrix element largely increases with increasing crystal angle. This is mainly due to the fact that the states constituting the topmost valence subband near the band edge are predominantly |Y′〉-like for QW structures with larger crystal angle. Also, it is observed that the average hole effective mass is largely reduced with increasing crystal angle. In particular, the average hole effective mass of the (101̄0)-oriented QW structure are about four times smaller than that of the (0001)-oriented QW structure.