bluuebunny/crossref_metadata_2025 huggingface dataset:
Epitaxially grown YMnO3 film: New candidate for nonvolatile memory devices
Authors: Fujimura Norifumi , Ishida Tadashi , Yoshimura Takeshi , Ito Taichiro
Published: July 2002

We have proposed ReMnO3 (Re:rare earth) thin films as a new candidate for nonvolatile memory devices. In this letter, we report on fabrication of (0001) YMnO3 films on (111)MgO, (0001)ZnO:Al/(0001) sapphire, and (111)Pt/(111)MgO using rf magnetron sputtering. We succeeded in obtaining (0001) epitaxial YMnO3 films on (111) MgO and (0001)ZnO:Al/(0001) sapphire substrate, and polycrystalline films on (111)Pt/(111)MgO. The dielectric properties of the epitaxial and polycrystalline YMnO3 films are almost the same. The dielectric permittivities of both films are smaller than those reported for YMnO3 single crystal.
High-temperature operation of an electroluminescent device fabricated using a novel triphenylamine derivative
Authors: Tokito Shizuo , Tanaka Hiromitsu , Okada Akane , Taga Yasunori
Published: July 2002

We have fabricated thermally stable organic electroluminescent (EL) devices using a hole transporting material, a tetramer of triphenylamine, and a typical emitting material, tris (8-quinolinolato) aluminum (Alq). The organic EL devices show uniform light emission in a continuous operation up to 140 °C without breakdown. A lowering of turn-on voltage for light emission and an increase of luminous efficiency with increasing temperature are found; the significantly low turn-on voltage of 2.1 V and the high luminous efficiency of 1.25 lm/W are obtained at 130 °C. Excellent durability of continuous operation is also achieved at the high temperature.
Specific volumes of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy in the liquid, glass, and crystalline states
Authors: Ohsaka K. , Chung S. K. , Rhim W. K. , Peker A. , Scruggs D. , Johnson W. L.
Published: July 2002

The specific volumes of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy as a function of temperature, T, are determined by employing an image digitizing technique and numerical calculation methods applied to the electrostatically levitated spherical alloy. The linear fitting of the volumes of the alloy in the liquid, Vl, glass, Vg, and crystalline Vc, states in the temperature ranges shown in parentheses are Vl(T)=0.1583+8.877×10−6T(cm3/g) (700–1300 K);Vg(T)=0.1603+5.528×10−6T (400–550 K);Vc(T)=0.1583+6.211×10−6T(400–850 K). The average volume thermal expansion coefficients within the temperature ranges are determined to be 5.32, 3.39, and 3.83×10−5 (1/K) for the liquid, glass, and crystalline states, respectively.
Smectic layer rotation by dc field in ferroelectric liquid crystal
Authors: Nakayama K. , Ozaki M. , Yoshino K.
Published: July 2002

The smectic layer rotation of a ferroelectric liquid crystal (FLC) has been investigated by applying a dc electric field. The smectic layer of FLC doped with a small amount of an ionic impurity can be rotated around the axis perpendicular to the glass plates upon application of dc voltage. This smectic layer rotation is interpreted in terms of the electrohydrodynamic flow due to an ionic impurity.
Urbach–Martienssen tails in a wurtzite GaN epilayer
Authors: Chichibu S. , Mizutani T. , Shioda T. , Nakanishi H. , Deguchi T. , Azuhata T. , Sota T. , Nakamura S.
Published: July 2002

Effects of an interaction between the electronic and lattice systems on the optical spectra of a wurtzite GaN epilayer were investigated. The exponentially increasing absorption tail was well explained as an Urbach–Martienssen tail, giving the characteristic phonon energy of 30 meV. The result indicates that few longitudinal optical phonons contribute to the exciton-phonon coupling even at room temperature.
Reduction of light-scattering loss in silica glass by the structural relaxation of “frozen-in” density fluctuations
Authors: Saito K. , Ikushima A. J.
Published: July 2002

Temperature dependences of light-scattering intensity in silica glasses were measured over a wide temperature range from 300 to 1 900 K. It was found that in OH-free silica glass, the diffusive density fluctuations are completely “frozen-in” at the glass transition temperature, while in OH-containing glass, OH gives rise to the structural relaxation of these “frozen-in” density fluctuations over 400 K below the glass transition temperature. This structural relaxation, expected to be caused by the introduction of some dopants with the same role of OH, can be used for reduction of the light-scattering loss in silica fibers.
Reduction mechanism of surface oxide in aluminum alloy powders containing magnesium studied by x-ray photoelectron spectroscopy using synchrotron radiation
Authors: Kimura Atsushi , Shibata Masahiro , Kondoh Katsuyoshi , Takeda Yoshinobu , Katayama Makoto , Kanie Tomohiko , Takada Hiroshi
Published: July 2002

We investigated the reduction mechanism of surface oxide on aluminum alloy powders containing magnesium, by x-ray photoelectron spectroscopy using synchrotron radiation (SR-XPS). The reduction is the initial reaction in a new aluminum nitridation method developed by one of the authors. In heating the powders to 823 K, magnesium soluted in the powders moves from the inner region to the surface at temperatures below 573 K, and finally, above 773 K, the magnesium reduces the aluminum oxide of powder surfaces by chemical reaction, which breaks the surface oxide films, and metallic aluminum appears on the topmost surface of the powders. These results suggest that the SR-XPS system is very useful for dynamic chemical reaction analysis of the surface via in situ measurement.
Ultrafast hole–phonon interactions in GaAs
Authors: Del Fatti N. , Langot P. , Tommasi R. , Vallée F.
Published: July 2002

Ultrafast heating of cold holes is investigated in bulk GaAs using a high-sensitivity two-color absorption saturation technique. Measurements performed as a function of the lattice temperature and of the carrier excess energy show that absorption of optical phonons is the main hole heating mechanism for the investigated temperatures in the range 100–300 K. Using a numerical model for carrier dynamics, the optical deformation potential is estimated to be d0∼40 eV.
Unoccupied molecular orbital states of tris (8-hydroxy quinoline) aluminum: Observation and dynamics
Authors: Probst M. , Haight R.
Published: July 2002

Electrons photoexcited into the normally unoccupied states of vacuum deposited thin films of tris (8-hydroxy quinoline) aluminum (Alq) have been studied with excite-probe harmonic laser photoemission. Both valence and transiently excited empty states are observed. A surface recombination velocity of 75±30 cm/s has been determined.
Epitaxial carbon nanotube film self-organized by sublimation decomposition of silicon carbide
Authors: Kusunoki Michiko , Rokkaku Masumi , Suzuki Toshiyuki
Published: July 2002

A film of well-oriented carbon nanotubes was produced by sublimation decomposition of silicon carbide at 1700 °C by using YAG laser heating in a transmission electron microscope (TEM). The processes of SiC decomposition and the formation of carbon nanotubes were observed successively by high-resolution electron microscopy (HREM). Carbon nanotubes were mostly oriented along the [111] direction on the (111) surface plane of β-SiC single crystal. The interface between them was observed by HREM.
Size reduction of gold particles in aqueous solution by pulsed laser irradiation
Authors: Kurita Hideaki , Takami Akinori , Koda Seiichiro
Published: July 2002

When the gold particles produced by the chemical reduction of hydrogen chloroaurate solution were irradiated by the second harmonic of a pulsed Nd:Yag laser (532 nm), their diameter was reduced from more than several deca-nm to less than 20 nm. At the same time, the shape of the particles, originally nonspherical, became spherical. This phenomenon was limited to pulsed laser, and is caused by the heating of the particles within the very short time of laser irradiation; the very large absorption cross section of the particles at the plasmon absorption and the thermal insulation of the irradiated particles in water should be responsible.
Semiconducting polymer distributed feedback lasers
Authors: McGehee M. D. , Dı́az-Garcı́a M. A. , Hide F. , Gupta R. , Miller E. K. , Moses D. , Heeger A. J.
Published: July 2002

We have fabricated photopumped distributed feedback lasers by spin-casting thin films of the semiconducting polymer poly(2-butyl, 5-(2′-ethyl-hexyl)-1,4-phenylenevinylene) over gratings in silicon oxide. The lasers have two modes that each have a linewidth of 0.2 nm. The lasing wavelength was tuned from 540 to 583 nm by adjusting the period of the gratings.
Thickness dependence of the effective dielectric constant in a thin film capacitor
Authors: Natori Kenji , Otani Daijiro , Sano Nobuyuki
Published: July 2002

The static value of the effective dielectric constant in a thin film capacitor is simulated by means of the local field theory. The value of it shows a sharp decrease as the film thickness is decreased in an ultrathin film geometry. This phenomenon is due to the size effect intrinsic to a thin film structure and has nothing to do with the material aspect. The decrease is more remarkable for larger values of the bulk dielectric constant. It is recovered by inserting interface layers with larger atomic polarizability between the film and the capacitor electrode.
Dielectric breakdown of ferromagnetic tunnel junctions
Authors: Oepts W. , Verhagen H. J. , de Jonge W. J. M. , Coehoorn R.
Published: July 2002

The time-dependent dielectric breakdown of Co/Al2O3/Co(-Fe) magnetic tunnel junctions is investigated. At voltages larger than 1.2 V, almost immediate breakdown of the junction is observed, leading to a decreased (magneto)resistance. The shorts, which are local hot spots, were visualized by making use of a liquid crystal film on top of the junction. The breakdown voltages of a series of nominally identical tunnel junctions measured in a voltage-ramp experiment are shown to increase with increasing ramp speed. The results are analyzed in the framework of several models for the voltage dependent breakdown probability.
Photoluminescence of GaN: Effect of electron irradiation
Authors: Buyanova I. A. , Wagner Mt. , Chen W. M. , Monemar B. , Lindström J. L. , Amano H. , Akasaki I.
Published: July 2002

The effect of electron irradiation on the optical properties of GaN material with various electrical conductivity (i.e., n type, compensated, and p type) is studied in detail by photoluminescence (PL) spectroscopy. Electron irradiation with a dose <1017 cm−2 is found to have a minor effect on photoluminescence, indicating a high radiation resistance of GaN. For higher doses, two major effects of electron irradiation on PL properties can be distinguished, i.e., radiation-induced quenching of the PL, likely caused by a radiation-induced formation of competing recombination channels, and radiation-induced formation/activation of new optically active centers.
Thermal diffusivity measurements of sub-micron organic dye thin films using a high temperature superconductor bolometer
Authors: Savoy Steven M. , Wells Cyndi A. , McDevitt John T. , Rhodes Timothy A.
Published: July 2002

The thermal diffusivity of a thin organic dye layer deposited atop thin films of the high temperature superconductor YBa2Cu3O7−δ is measured using a pulsed laser flash method. Here, the underlying superconductor acts as a highly sensitive temperature transducer after appropriate conversion of the transient voltage response from 7 ns optical pulses. Film surface temperature decays for several thicknesses of the dye layers were evaluated; these decays exhibited a linear dependence of the time at half temperature maximum versus thickness squared. Three dimensional finite difference modeling was used to study and extract the thermal diffusivity values of the thin organic layers as well as to investigate the transient temperature distributions within the dye and superconductor areas.
Characterization of dynamic optical nonlinearities with pulse trains
Authors: Misoguti L. , Mendonça C. R. , Zilio S. C.
Published: July 2002

We report a simple extension of the Z-scan technique where pulse trains characteristic of a Q-switched and mode-locked Nd:YAG laser are employed to investigate the dynamics of third-order optical nonlinearities. The method allows discriminating between fast and accumulative nonlinearities, and was applied to the investigation of liquid samples, namely chloroform, benzene, and a solution of Disperse Red 1 dye.
Free-carrier screening of polarization fields in wurtzite GaN/InGaN laser structures
Authors: Della Sala Fabio , Di Carlo Aldo , Lugli Paolo , Bernardini Fabio , Fiorentini Vincenzo , Scholz Reinhard , Jancu Jean-Marc
Published: July 2002

The free-carrier screening of macroscopic polarization fields in wurtzite GaN/InGaN quantum well lasers is investigated via a self-consistent tight-binding approach. We show that the high carrier concentrations found experimentally in nitride laser structures effectively screen the built-in spontaneous and piezoelectric polarization fields, thus inducing a “field-free” band profile. Our results explain some heretofore puzzling experimental data on nitride lasers, such as the unusually high lasing excitation thresholds and emission blue shifts for increasing excitation levels.
Electric conduction in bending electrostriction of polyurethanes
Authors: Watanabe Masashi , Hirai Toshihiro , Suzuki Makoto , Amaike Yoichi
Published: July 2002

Polyurethane films coated with thin gold electrodes bent by the application of an electric field. Because the degree of the bending deformation was proportional to the square of the applied electric field, this phenomenon was considered to be bending electrostriction. The mechanism of the bending electrostriction was also investigated by use of “blocking electrode” and “contacting electrode.” As the blocking electrode, and insulating thin film of polyethylene was inserted between the polyurethane and the gold electrode. As a result, the polyurethane film bent only when both electrodes were contacting electrodes. This shows that the bending electrostriction is not caused by dielectric polarization but is related to electric conduction through the polyurethane. The bending deformation is, thus, suggested to result from an asymmetric distribution of space charge formed by a space-charge-limited current.
Lasing in quantum-dot ensembles with sharp adjustable electronic shells
Authors: Fafard S. , Wasilewski Z. R. , Allen C. Nı̀. , Hinzer K. , McCaffrey J. P. , Feng Y.
Published: July 2002

Quantum-dot laser diodes with up to five well-defined electronic shells are fabricated using self-assembled quantum dots (QDs) grown by molecular-beam epitaxy. Shape-engineered stacks of self-aligned QDs with improved uniformity are used to increase the gain in the active region. Lasing is observed in the upper QD shells for small-gain media, and progresses towards the QD ground states for longer cavity lengths. We obtained at 77 K thresholds of Jth=15 A/cm2 for a 2 mm cavity lasing in the first excited state (p shell), and Jth=125 A/cm2 for a 1 mm cavity lasing in n=3 (d shell). At 300 K for a 1 mm cavity, Jth is 490 A/cm2 with lasing in n=4 (f shell).
Photoluminescence study of Cu diffusion and electromigration in CdTe
Authors: Grecu D. , Compaan A. D.
Published: July 2002

We report changes in the photoluminescence (PL) spectra associated with the diffusion of Cu in CdTe thin films used in CdTe/CdS solar cells. We studied films grown by vapor transport deposition and radio-frequency sputtering as well as single-crystal CdTe. The main effects of Cu diffusion appear to be the quenching of a donor-acceptor transition associated with Cd vacancies and the increase in intensity of a lower energy band due to deep acceptor states. The changes in junction PL are consistent with the movement of Cu+ ions in the electric fields near the CdS/CdTe junction.
Accumulation of positive charges in organic light-emitting diodes with a double-layer structure
Authors: Matsumura Michio , Ito Atsushi , Miyamae Yuichiro
Published: July 2002

The current–voltage properties of organic light-emitting diodes were analyzed based on the assumptions that: (1) they are determined by the carrier injection processes, and (2) the charges are accumulated near the organic/organic interface. From the results, the amount of accumulated charges was estimated. The amount was also determined from the analyses of transient currents observed by applying pulsed voltage to the devices. By deconvoluting the current components, the discharge of accumulated charges from the bulk of the organic layers was evaluated. The amounts determined by these two methods showed good agreement. All the results indicated that positive charges are accumulated near the organic/organic interface and that the amount increases as the electron injection ability of the cathode is lowered.
Magnetotransport and magnetic domain structure in compressively strained colossal magnetoresistance films
Authors: Wu Yan , Suzuki Y. , Rüdiger U. , Yu J. , Kent A. D. , Nath T. K. , Eom C. B.
Published: July 2002

We have studied the magnetoresistance (MR) of compressively strained La0.7Sr0.3MnO3 (LSMO) films in various magnetic states in order to understand the role of magnetic domain structure on magnetotransport. In thin films of LSMO on (100) LaAlO3, the perpendicular magnetic anisotropy results in perpendicularly magnetized domains with fine scale ∼200 nm domain subdivision, which we image directly at room temperature using magnetic force microscopy. The main MR effects can be understood in terms of bulk colossal MR and anisotropic MR. We also find evidence for a small domain wall contribution to the MR, which is an order of magnitude larger than expected from a double exchange model.
Site-controlled self-organization of individual InAs quantum dots by scanning tunneling probe-assisted nanolithography
Authors: Kohmoto Shigeru , Nakamura Hitoshi , Ishikawa Tomonori , Asakawa Kiyoshi
Published: July 2002

A nanometer-scale site-control technique for individual InAs quantum dots (QDs) has been developed by using scanning tunneling microscope (STM) -assisted nanolithography and self-organizing molecular-beam epitaxy. We find that nanometer-scale deposits can be created on a GaAs surface by applying voltage and current pulses between the surface and a tungsten probe of the STM, and that they act as “nanomasks” on which GaAs does not grow directly. Accordingly, subsequent thin GaAs growth produces GaAs nanoholes above the deposits. By supplying 1.1 ML InAs on this surface, QDs are self-organized at the hole sites, while hardly any undesirable Stranski–Krastanov QDs are formed in the flat surface region. Using this technique with nanometer precision, a QD pair with 45 nm pitch is fabricated.
Photoluminescence of Cu-doped CdTe and related stability issues in CdS/CdTe solar cells
Authors: Grecu D. , Compaan A. D. , Young D. , Jayamaha U. , Rose D. H.
Published: July 2002

We explore Cu electronic states in CdTe using photoluminescence as the main investigative method. Our results are consistent with some Cu atoms occupying substitutional positions on the Cd sublattice and with others forming Frenkel pairs of the type Cui+–VCd− involving an interstitial Cu and a Cd vacancy. In addition, we find that Cu-doped CdTe samples exhibit a significant “aging” behavior, attributable to the instability of Cu acceptor states as verified by our Hall measurements. The aging appears to be reversible by a 150–200 °C anneal. Our results are used to explain efficiency degradation of some CdTe solar-cell devices which use Cu for the formation of a backcontact.
Study of interface effects in thermoelectric microrefrigerators
Authors: Sungtaek Ju Y. , Ghoshal Uttam
Published: July 2002

Interface phenomena play a vital role in thermoelectric (TE) microrefrigerators. The present study employs a phenomenological model to examine the behavior of TE refrigerators as a function of thermal and electrical contact resistance, boundary Seebeck coefficient, and heat sink conductance. We modify the conventional definition of the figure of merit to capture the interface effects. A finite temperature drop across the interface between a metal electrode and a thermoelement is found to strongly influence the boundary Seebeck effect. Interface engineering can potentially improve the overall performance of TE microrefrigerators.
Monte Carlo simulation of liquid water in a magnetic field
Authors: Zhou K. X. , Lu G. W. , Zhou Q. C. , Song J. H. , Jiang S. T. , Xia H. R.
Published: July 2002

A Monte Carlo simulation of liquid water was carried out in order to study the influence of an external magnetic field on the internal energy and heat capacity of pure water. Analysis of the structure of water was made by calculating the radial distribution function of water molecules. A multipeak function of the magnetic effect versus magnetic flux density was found. A significant change in the internal energy and heat capacity occurred at a magnetic flux density of about 0.2 T. In particular, when the magnetic flux density increases, the second peak of the water–water radial distribution function increases markedly. This has been demonstrated by calculations for both 64- and 125-molecule systems.
Subsecond relaxation of internal field after polarization reversal in congruent LiNbO3 and LiTaO3 crystals
Authors: Ro Jung Hoon , Cha Myoungsik
Published: July 2002

We developed an experimental method for measuring subsecond temporal evolution of the internal field after polarization reversal in ferroelectric crystals, congruent LiNbO3 and LiTaO3 crystals. In each crystal the internal field exhibited two distinct relaxations, suggesting that they originate from two different types of defects. The fast one decays in subsecond range while the slow one lasts over days at room temperature. The subsecond relaxation could be fit to a stretched exponential. This method can be applied to other ferroelectric crystals to investigate the properties of defects.
Energy resolution of 12 eV at 5.9 keV from Al-superconducting tunnel junction detectors
Authors: Angloher G. , Hettl P. , Huber M. , Jochum J. , Feilitzsch F. v. , Mößbauer R. L.
Published: July 2002

An energy resolution of ΔE=12 eV [full width at half maximum (FWHM)] has been measured for the Mn55 Kα1 line (E=5.9 keV) using a superconducting lead absorber (90×90×1.3 μm3) readout by a single aluminum superconducting tunnel junction (100×100 μm2). The total detector area has been illuminated, while a slit mask of 150 μm width was used to partially shadow the detector leads and substrate area from impinging x rays. The total electronic noise contribution was measured as ΔEelec=4 eV (FWHM). The superconducting tunnel junction is located on a Si3N4 membrane of thickness 0.3 μm, the lead absorber is separated from the superconducting tunnel junction’s top layer by a thin layer of natural aluminum oxide. No deviations from linear energy response were observed in the energy range between E=1.74 keV and E=6.49 keV. The same resolution was obtained for a single aluminum superconducting tunnel junction, where the influence of substrate events was suppressed by a metallic buffer layer between tunnel junction and substrate. The escape of recombination phonons into the substrate causes small nonlinearities in the tunnel junction’s energy response.
A wideband lag correlator for heterodyne spectroscopy of broad astronomical and atmospheric spectral lines
Authors: Harris A. I. , Zmuidzinas J.
Published: July 2002

WASP2 is a lag correlation spectrometer that is optimized for astronomical heterodyne observations of spectral lines from mid-infrared through centimeter wavelengths. Since the instrument is a wideband microwave spectrum analyzer, its basic technology is well suited to radio astronomy, atmospheric remote sensing, and any other application which requires observations of broad spectral lines. In this specific instrument, eight 16 lag correlator boards form a 128 lag spectrometer with 33 MHz spectral resolution. The spectrometer’s present microwave components dominate its gain shape, yielding a spectrometer which covers a useful (6 dB bandwidth) single band of 3600 MHz, from 280 to 3880 MHz. The correlator boards have a 3 dB bandwidth of 4000 MHz, from below 100 MHz to near their aliasing limit of 4200 MHz. WASP2 obtains its wide bandwidth with fully analog high frequency components: the voltage multipliers are transistor circuits and the delays are lengths of microstrip transmission line. WASP2 is simple, compact, requires little power, and integrates stably for many hours.
Performance of CCSDT for first row AB/AB− diatomics: Dissociation energies and electron affinities
Authors: Sordo J. A.
Published: July 2002

CCSDT/aug-cc-pVXZ (X=D,T,Q) calculations were performed on the C2, C2−, CN, CN−, O2, and O2− first row diatomic molecules. The inclusion of diffuse functions improves the dissociation energies of the anionic systems by 2.0–3.4 kcal/mol, which is relevant bearing in mind the goal of achieving chemical accuracy. The contribution of the diffuse functions in the case of neutral O2 (0.6 kcal/mol) is by no means negligible in this context. A serious discrepancy between the theoretical prediction and the experimental values available for the dissociation energy of C2− was found. Since the theoretical deficiences commonly ascribed to the CCSDT method (single-reference and spin contamination when using UHF zeroth-order wave functions) cannot be invoked in this case, further experimental work is required to throw some light on the origin of such a discrepancy. The performance of CCSDT for adiabatic electron affinities is excellent in the case of O2/O2− and CN/CN−. For C2/C2−, the observed discrepancies can be explained in terms of the well-known multiconfigurational nature of the ground state of the C2 molecule.
Simulations of ice and liquid water over a range of temperatures using the fluctuating charge model
Authors: Rick Steven W.
Published: July 2002

The temperature dependence of the thermodynamic and dynamical properties of liquid water using the polarizable fluctuating charge (FQ) model is presented. The properties of ice Ih, both for a perfect lattice with no thermal disorder and at a temperature of 273 K, are also presented. In contrast to nonpolarizable models, the FQ model has a density maximum of water near 277 K. For ice, the model has a dipole moment of the perfect lattice of 3.05 Debye, in good agreement with a recent induction model calculation. The simulations at 273 K and the correct density find that thermal motion decreases the average dipole moment to 2.96 D. The liquid state dipole moment is less than the ice value and decreases with temperature.
Simultaneous generation of red, green, and blue continuous-wave laser radiation in Nd3+-doped aperiodically poled lithium niobate
Authors: Capmany J.
Published: July 2002

Simultaneous generation of red, green, and blue continuous-wave laser radiation in a Nd3+-doped aperiodically poled lithium-niobate crystal is reported. Red (686 nm) and green (542 nm) are obtained by self-frequency-doubling fundamental infrared laser lines at 1084 and 1372 nm oscillating simultaneously. Blue (441 nm) is obtained by self-sum-frequency mixing of end-pump laser radiation at 744 nm and the fundamental laser line at 1084 nm. Other laser lines at 605 nm (orange) and 482 nm (blue) are simultaneously obtained by self-sum-frequency mixing of both infrared fundamentals and by self-sum-frequency mixing of pump laser radiation with the remaining fundamental line at 1372 nm, respectively. A weaker violet (372 nm) laser emission results from frequency doubling of the pump radiation.
A method for ab initio nonlinear electron-density evolution
Authors: Baer Roi , Gould Recca
Published: July 2002

A numerical method is given for effecting nonlinear local density functional evolution. Within a given time interval, Chebyshev quadrature points are used to sample the evolving orbitals. An implicit equation coupling wave functions at the different time points is then set up. The equation is solved iteratively using the “direct inversion in iterative space” acceleration technique. Spatially, the orbitals are represented on a Fourier grid combined with soft pseudopotentials. The method is first applied to the computation of the Πg3 adiabatic potential energy curves of Al2. Next, the electronic dynamics of a toy molecular wire is studied. The wire consists of a C2H4 molecule connected via sulfur atoms to two gold atoms, the “electrodes.” The molecule is placed in a homogeneous electric field and a dynamical process of charge transfer is observed. By comparing the transient with that of a resistance-capacitance circuit, an effective Ohmic resistance and capacitance is estimated for the system.
Features of the nonlinear dynamics of domain walls in ferrimagnets
Authors: Gerasimchuk V. S. , Shitov A. A.
Published: July 2002

The nonlinear motion of a 180° domain wall with two inequivalent sublattices in the field of a sound wave propagating orthogonally to the plane of the domain wall is investigated. The dependence of the drift velocity of the domain wall on the polarization and the parameters of the sound wave is found. An analysis of the long-wavelength and short-wavelength approximations is carried out. The conditions for drift of the stripe domain structure in ferrites are determined.
Self-consistency based control scheme for magnetization dynamics
Authors: Albuquerque G. , Miltat J. , Thiaville A.
Published: July 2002

A numerical framework is presented for the solution of the Landau–Lifshitz–Gilbert equation of magnetization motion using a semi-implicit Crank–Nicholson integration scheme. Along with the details of both space and time domain discretizations, we report on the development of a physically based self-consistency criterion that allows for a quantitative measurement of error in dynamic micromagnetic simulations. In essence, this criterion relies in recalculating from actual magnetization motion the imposed phenomenological damping constant. Test calculations were performed with special attention paid to the determination of suitable integration time steps.
The Enskog theory for transport coefficients of simple fluids with continuous potentials
Authors: Miyazaki Kunimasa , Srinivas Goundla , Bagchi Biman
Published: July 2002

The Enskog theory for the self-diffusion coefficient for fluids with continuous potentials, such as the Lennard-Jones, is developed. Starting from the Green–Kubo formula (rather than the conventional kinetic equation) and introducing the similar assumptions upon which the Boltzmann equation is based, we derived a general expression for the memory kernel and the self-diffusion coefficient. The numerical analysis is implemented for the Lennard-Jones fluid. The time-dependent memory kernel is calculated and compared with the latest molecular dynamics simulations. Excellent agreement is obtained at the low density. The self-diffusion coefficient is evaluated for various temperatures and densities. The ratio of the Enskog self-diffusion coefficient to the simulation value is plotted against density. Significant difference of this density dependence from that for the hard-sphere fluid is observed. In particular, the well-known maximum observed (in the diffusion versus density plot) for the hard sphere fluid is found to be completely absent in the Lennard-Jones fluid. Our results reduce to the conventional Chapman–Enskog expression in the low density limit and can be applicable to the systems with singular potentials such as the hard sphere.
Variable temperature measurements of the complex dielectric permittivity of lossy AlN–SiC composites from 26.5–40 GHz
Authors: Calame J. P. , Abe D. K. , Levush B. , Danly B. G.
Published: July 2002

The complex dielectric permittivity of electrically lossy AlN–SiC composites has been measured over the 26.5–40 GHz frequency range (Ka band), at temperatures from −180 to 200 °C. The dielectric permittivity is found to be strongly sensitive to temperature. Additional broadband, room-temperature dielectric measurements indicate that the Ka-band response is situated on the high-frequency tail of a broad Maxwell–Wagner relaxation. Analysis of the temperature-dependent losses at 30 GHz in a composite containing 20% SiC by weight, using an effective cluster model, indicates the existence of two conduction processes within the SiC, with activation energies of 0.0064 and 0.095 eV. The low-energy process is consistent with a hopping conductivity and the higher-energy process is attributed to a nitrogen donor ionization.
Jet spectroscopy of van der Waals dimers of 1-methoxynaphthalene: A laser induced fluorescence study
Authors: Das Aloke , Mahato K. K. , Chakraborty Tapas
Published: July 2002

Excited state dynamics following S1←S0 transition of jet-cooled van der Waals complexes of 1-methoxynaphthalene has been investigated by measuring the laser-induced fluorescence excitation, dispersed fluorescence and two-color hole-burning spectra. The results give evidence of formation of two isomeric dimers, which show only sharp absorption and emit excimer fluorescence for excitations to the origin region of the S1 state. Excimer formation in one of the dimers occurs from the zero-point level of the locally excited state. The other dimer shows vibrational barrier and the yield of the excimer emission increases as the successive higher quanta of a low-frequency intermolecular mode are excited. The barrier in the second case has been attributed to the reorganization of the dimer geometry from the locally excited state to the excimer state. The characteristic enhancement of the yield of excimer emission with increase in excitation energy indicates the involvement of a tunneling mechanism for crossing the reorganization barrier.
Evolution of small scale regular patterns generated by waves propagating over a sandy bottom
Authors: Faraci C. , Foti E.
Published: July 2002

In this paper the evolution of a sandy bottom subject to a wave generated flow has been analyzed using an image acquisition technique. In particular, ripple formation was observed starting from a flat bed till a stable configuration was attained. The experimental findings showed that rolling grain ripples never appeared as a stable configuration but only as a transition toward the equilibrium, represented by vortex ripples. The latter stage was reached after about 100 cycles if lightweight sediments were used, or about 400 cycles if quartz sediments were adopted. It was also observed that when the bedforms appear their wavelength is much smaller (about one half) than that at the equilibrium stage, this result being in contrast with most of the analytical models on bedform evolution. Finally, it was observed that ripples migrate as soon as they appear. The measured velocity of migration seems to confirm, from a qualitative point of view, the theoretical findings obtained for rolling grain ripples by Blondeaux et al. [Eur. J. Mech. B/Fluids 19, 285 (2000)].
Hybrid electrostatic–aerodynamic levitation furnace for the high-temperature processing of oxide materials on the ground
Authors: Paradis Paul-François , Ishikawa Takehiko , Yu Jianding , Yoda Shinichi
Published: July 2002

This article describes a hybrid electrostatic–aerodynamic levitation furnace for the containerless processing and study of oxide materials on the ground. Its operation principle relies on an aerodynamic levitator that allows sufficient electric charge to be accumulated on a sample, due to high-temperature heating, before electrostatic levitation can be effective. The article discusses the concept of this new levitator and presents the proof of the technical feasibility of electrostatically levitating and melting oxide material samples (BiFeO3, 49.5CaO–50.5Al2O3 mol %) in a pressurized atmosphere. In addition, superheating–undercooling cycles can be performed while maintaining an exceptional sample positioning stability along the three directions. Moreover, we report the first vitrification of dielectric oxide material samples (49.5CaO–50.5Al2O3 mol %) using an electrostatic levitation method. The article also discusses the advantages of this facility compared with other existing instruments for the containerless processing of oxide materials, in particular, with respect to molten sample position stability and hydrodynamic quietness, long duration levitation times, and open field of view of the sample. The facility is, in its current state, capable of novel glass synthesis. Moreover, it shows great promise for structural and thermophysical properties characterization and metastable phase studies when supplemented with the appropriate diagnostic tools. The demonstration of melting and vitrifying oxide material is also a corner stone for the design of a containerless research facility in microgravity.
Amplified spontaneous emission from a soluble thiophene-based oligomer
Authors: Anni M. , Gigli G. , Cingolani R. , Zavelani-Rossi M. , Gadermaier C. , Lanzani G. , Barbarella G. , Favaretto L.
Published: July 2002

Optical gain is demonstrated, through pump-probe measurement, in a spin-coated film of a quinquethiophene-S,S-dioxide. The temporal evolution of the gain as a function of the pump intensity shows a progressive shortening of the lifetime as the pump intensity increases. The photoluminescence spectra show line narrowing for excitation intensity higher than 960 μJ cm−2. From these measurements, we conclude that oligothiophene-S,S-dioxides are valuable candidates for thin-film organic light amplifier and lasers.
An exact prediction of N=4 supersymmetric Yang–Mills theory for string theory
Authors: Drukker Nadav , Gross David J.
Published: July 2002

We propose that the expectation value of a circular BPS-Wilson loop in N=4 supersymmetric Yang–Mills can be calculated exactly, to all orders in a 1/N expansion and to all orders in g2N. Using the AdS/CFT duality, this result yields a prediction of the value of the string amplitude with a circular boundary to all orders in α′ and to all orders in gs. We then compare this result with string theory. We find that the gauge theory calculation, for large g2N and to all orders in the 1/N2 expansion, does agree with the leading string theory calculation, to all orders in gs and to lowest order in α′. We also find a relation between the expectation value of any closed smooth Wilson loop and the loop related to it by an inversion that takes a point along the loop to infinity, and compare this result, again successfully, with string theory.
Structural change at the carbon-nanotube tip by field emission
Authors: Kuzumaki Toru , Takamura Yuzuru , Ichinose Hideki , Horiike Yasuhiro
Published: July 2002

Carbon-nanotube tips are plastically deformed during field emission. High-resolution transmission electron microscopy and structural simulations suggest that the deformed structure of the closed nanotube is explained by heterogeneous nucleation of the pentagonal and heptagonal carbon ring pairs, and that of the opened one is represented by sp3-like line defects in the hexagonal carbon network. It is considered that the changing of the inclination of the Fowler–Nordheim plots corresponds to the structural change in which a tip becomes sharp. The field ion microscope image and the corresponding field-emission pattern suggest that the electron emission from a closed nanotube is not necessarily from pentagonal carbon rings, but from the protrudent carbon network sites on the tip.
Can optimized effective potentials be determined uniquely?
Authors: Hirata So , Ivanov Stanislav , Grabowski Ireneusz , Bartlett Rodney J. , Burke Kieron , Talman James D.
Published: July 2002

Local (multiplicative) effective exchange potentials obtained from the linear-combination- of-atomic-orbital (LCAO) optimized effective potential (OEP) method are frequently unrealistic in that they tend to exhibit wrong asymptotic behavior (although formally they should have the correct asymptotic behavior) and also assume unphysical rapid oscillations around the nuclei. We give an algebraic proof that, with an infinity of orbitals, the kernel of the OEP integral equation has one and only one singularity associated with a constant and hence the OEP method determines a local exchange potential uniquely, provided that we impose some appropriate boundary condition upon the exchange potential. When the number of orbitals is finite, however, the OEP integral equation is ill-posed in that it has an infinite number of solutions. We circumvent this problem by projecting the equation and the exchange potential upon the function space accessible by the kernel and thereby making the exchange potential unique. The observed numerical problems are, therefore, primarily due to the slow convergence of the projected exchange potential with respect to the size of the expansion basis set for orbitals. Nonetheless, by making a judicious choice of the basis sets, we obtain accurate exchange potentials for atoms and molecules from an LCAO OEP procedure, which are significant improvements over local or gradient-corrected exchange functionals or the Slater potential. The Krieger–Li–Iafrate scheme offers better approximations to the OEP method.
Raman activation in disordered graphites of the A1′ symmetry forbidden k≠0 phonon: The origin of the D line
Authors: Castiglioni C. , Negri F. , Rigolio M. , Zerbi G.
Published: July 2002

The analysis of the tensors of polarizability derivatives, computed with quantum-chemical methods for several large polycyclic aromatic hydrocarbon molecules, allows to rationalize the origin of the Raman D line observed in disordered graphites as due to a relaxation of the phonon selection rules. It is shown that a minimal rearrangement of the atomic and electronic structure in a lattice different from that of a perfect graphite crystal causes a selective activation of a specific totally symmetric phonon whose frequency falls in the region of the D band and whose motion is dominated by the breathing vibration of aromatic rings. We numerically proof this conjecture on the basis of polarizability derivatives obtained with Density Functional Theory calculations.
Growth of aligned carbon nanotubes by plasma-enhanced chemical vapor deposition: Optimization of growth parameters
Authors: Tanemura M. , Iwata K. , Takahashi K. , Fujimoto Y. , Okuyama F. , Sugie H. , Filip V.
Published: July 2002

Direct-current plasma-enhanced chemical vapor deposition (CVD) with mixtures of acetylene and ammonia was optimized to synthesize aligned carbon nanotubes (CNTs) on Co- or Ni-covered W wires with regard to wire temperature, wire diameter, gas pressure, and sample bias. A phase diagram of CNT growth was established experimentally in this optimization process. It was revealed by transmission electron microscopy that Co-catalyzed CNTs encapsulated a Co carbide nanoparticle at their tip, disagreeing with a previous report that Co particles were located at the base of CNTs CVD grown on Co-covered Si substrates [C. Bower et al., Appl. Phys. Lett. 77, 2767 (2000)]. This leads to the conclusion that the formation mechanism of aligned CNTs depends significantly on the catalyst support material as well as the catalyst material itself. Since the sample bias strongly affected the morphology of CNTs, the selective supply of positive ions to CNT tips was possibly responsible for the alignment of growing CNTs.
All-optical diode in a periodically poled lithium niobate waveguide
Authors: Gallo Katia , Assanto Gaetano , Parameswaran Krishnan R. , Fejer Martin M.
Published: July 2002

We have demonstrated a guided-wave all-optical diode based on engineered quasiphase Matching in a LiNbO3 channel waveguide. For input peak powers beyond 1.5 W at 1.55 μm, the device exhibited a spatially nonreciprocal response, leading to optical isolation with contrasts as high as 90% at 3.1 W, in agreement with theoretical predictions.
Time-dependent automorphism inducing diffeomorphisms in vacuum Bianchi cosmologies and the complete closed form solutions for types II and V
Authors: Christodoulakis T. , Kofinas G. , Korfiatis E. , Papadopoulos G. O. , Paschos A.
Published: July 2002

We investigate the set of space–time general coordinate transformations (GCTs) which leave the line element of a generic Bianchi-type geometry quasiform invariant; i.e., preserve manifest spatial homogeneity. We find that these GCTs, induce special time-dependent automorphic changes, on the spatial scale factor matrix γαβ(t)—along with corresponding changes on the lapse function N(t) and the shift vector Nα(t). These changes, which are Bianchi-type dependent, form a group and are, in general, different from those induced by the group SAut(G) advocated in earlier investigations as the relevant symmetry group; they are used to simplify the form of the line element—and thus simplify Einstein’s equations as well, without losing generality. As far as this simplification procedure is concerned, the transformations found are proved to be essentially unique. For the case of Bianchi types II and V, where the most general solutions are known, Taub’s and Joseph’s, respectively, it is explicitly verified that our transformations and only those, suffice to reduce the generic line element to the previously known forms. It thus becomes possible—for these types—to give in closed form the most general solution, containing all the necessary ”gauge” freedom.
Numerical modeling of hole interband tunneling in wurtzite GaN and SiC
Authors: Nilsson H-E. , Martinez A. , Ghillino E. , Sannemo U. , Bellotti E. , Goano M.
Published: July 2002

The time evolution of Bloch electrons (holes) moving in a constant electric field has been studied for GaN and 2H-SiC using a numerical model based on realistic band structures. The large band gap of GaN and the SiC polytypes provide much larger critical fields than in conventional semiconductors, which allows device operation at very high electric fields. At sufficiently high electric fields the carriers may change band during drift due to tunneling. GaN has a direct band gap, while the hexagonal SiC polytypes have indirect band gaps. In spite of this difference the valence band structure is very similar due to the wurtzite symmetry. In this work the GaN and the 2H-SiC polytype are considered as wurtzite prototype semiconductors in order to study valence band to band tunneling in wurtzite semiconductors for electric fields directed along the c axis. A large valence band to band tunneling probability was found for both materials at electric fields above 400 kV/cm. This shows the importance of considering band to band tunneling in studies of high field hole transport in wide band-gap hexagonal semiconductor materials. The proposed numerical approach can be used to enhance the interband tunneling models used in Monte Carlo simulation of carrier transport in hexagonal semiconductors.
Interaction of hydrogen with nitrogen interstitials in wurtzite GaN
Authors: Wright A. F.
Published: July 2002

First-principles techniques are used to investigate the interaction of hydrogen with nitrogen interstitials in wurtzite GaN. The calculations reveal that hydrogen can either compensate an interstitial by donating an electron to an interstitial acceptor level, or passivate the interstitial by forming a hydrogen–interstitial complex. Nitrogen interstitials can bind up to three hydrogen atoms and hydrogen removal energies are computed as a function of the number of hydrogen atoms. Removal energies are found to depend strongly on the Fermi level, and hydrogen-interstitial complexes are predicted to be unstable in n-type GaN. Frequencies are computed for hydrogen vibrations and compared with previously reported infrared absorption measurements on hydrogen-implanted GaN.