bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-symmetric bubbles in Rayleigh–Taylor instability
Authors: Abarzhi S. I.
Published: July 2002

We report a multimode analysis of the 3D–2D dimensional crossover for the nonlinear structure, which occurs in the nonlinear regime of the Rayleigh–Taylor instability (RTI). This structure is an array of bubbles and spikes periodic in the plane normal to the direction of gravity. The flow is assumed to be anisotropic in the plane and to have low rectangular symmetry. For regular bubbles, there is a two-parameter family of steady solutions, and we analyze stability of these nonlinear solutions. It is shown that 3D bubbles in RTI conserve a near-circular contour, and cannot be transformed into 2D bubbles continuously. We discuss the mechanism of secondary instabilities of anisotropic RT flow.
Optically pumped integrated absorber 3.4 μm laser with InAs-to-InGaAsSb type-II transition
Authors: Kaspi R. , Ongstad A. , Moeller C. , Dente G. C. , Chavez J. , Tilton M. L. , Gianardi D.
Published: July 2002

We report optically pumped lasing at λ∼3.4 μm from an integrated absorber structure in which the electrons confined in the InAs quantum wells recombine with holes in adjacent InGaAsSb layers to provide the gain. This type-II laser exhibits an estimated photon-to-photon conversion rate of ∼24% at 85 K. The self-consistent empirical pseudopotential method calculations suggest that Coulomb attraction can lead to a strong enhancement in carrier overlap, and the resulting small shift in transition energy is consistent with that observed.
Off-axis electron holography of exchange-biased CoFe/FeMn patterned nanostructures
Authors: Dunin-Borkowski R. E. , McCartney M. R. , Kardynal B. , Scheinfein M. R. , Smith David J. , Parkin S. S. P.
Published: July 2002

Off-axis electron holography and micromagnetic simulations have been used to investigate magnetization reversal mechanisms and remanent states in exchange-biased submicron Co84Fe16/Fe54Mn46 patterned elements. Domain structures within the elements were characterized despite the narrow thickness (∼3 nm) of the ferromagnetic layer relative to the total element thickness (∼42 nm). Individual elements were able to support different remanent states and their magnetic microstructure was sensitive to their size. The simulations confirmed that the coercivities of the elements and their domain structures were highly sensitive to the strength and orientation of the pinning field. A good fit to the experimental data was provided by using an interface exchange field that had a fixed amplitude and direction in the simulations, and small disagreements were attributed to structural imperfections. These differences emphasize the value of an experimental technique such as electron holography for probing local micromagnetic structure in individual nanostructured elements.
Effects of morphology on the low-energy electron stimulated desorption of O− from O2 deposited on benzene and water ices
Authors: Bass A. D. , Parenteau L. , Weik F. , Sanche L.
Published: July 2002

We investigate the effects of the geometrical structure (phase and porosity) of multilayer benzene films on the desorption of O− induced by 2–20 eV electron impact on varying quantities of absorbed O2. Differences in the yield of O− from O2 doped amorphous and crystalline benzene films are attributed to the ability of O2 to diffuse into the amorphous solid via pores and defects formed during its deposition at 20 K. In contrast, diffusion into crystalline benzene is limited and deposited O2 molecules remain at the surface of the film. Thermal desorption measurements support this analysis. The data are also compared with results of similar experiments for O2 on water. While it is apparent that some of the variation in O− yield observed from ice films is similarly related to morphology, a substantial suppression of the O− yield is likely to result from energy loss by electrons prior to dissociation. Quenching of intermediate O2− states by water ice may also contribute to this suppression in the range 5–12 eV.
Novel nonlinear optical organic materials: Dithienylethylenes
Authors: Sahraoui B. , Kityk I. V. , Fuks I. , Paci B. , Baldeck P. , Nunzi J.-M. , Frère P. , Roncali J.
Published: July 2002

In this paper, we report the measurements of the third-order susceptibilities of new dithienylethylenes in solutions using the degenerate four-wave mixing technique at λ=532 nm in the picosecond regime. From these measurements we deduced the second-order hyperpolarizabilities γ values, which are about 103 larger than the γ value of CS2 and are comparable with the corresponding values for the new synthesized diphenylpolyene derivatives. We also determined the optical power limiting properties of the mentioned compounds using nanosecond laser pulses in the visible range. The dispersion of nonlinear absorption coefficients is obtained in the framework of Gaussian collimated beam propagation in sample. Kerr ellipsometry measurements are responsible for the excited absorption cross section and are spectrally shifted comparing with the optical limiting processes (up to 50 nm). The appropriate quantum chemical calculations are done. The contributions of electronic and vibration subsystems as well as contribution of the particular parts of each molecule into the output third nonlinear optical response are given.
Reaction mechanism and isotope effects derived from centroid transition state theory in intramolecular proton transfer reactions
Authors: Iftimie Radu , Schofield Jeremy
Published: July 2002

In this article the tautomerization reaction of the enol form of malonaldehyde is used to investigate the magnitude and origin of changes in centroid transition state theory proton transfer reaction rate predictions caused by the quantum dispersion of heavy nuclei. Using an empirical valence bond method to construct the potential energy surface, it is found that quantization of the nuclear degrees of freedom of the carbon atoms significantly influences the centroid potential of mean force used to describe the proton transfer reaction. In contrast, an ab initio simulation carried out using a recently developed molecular mechanics based importance sampling method [J. Chem. Phys. 114, 6763 (2001)] in combination with an accurate density functional theory evaluation of the electronic energies shows a substantially smaller influence of the quantum nuclear degrees of freedom of the secondary atoms on the centroid potential of mean force. A detailed analysis of the different influence of quantization of the nuclear degrees of freedom of secondary atoms observed in the ab initio and empirical valence bond centroid potential of mean force was carried out. It is shown that for the empirical valence bond potential, a significant decrease of the centroid potential of mean force arises through the quantum tunneling of carbon atoms in the molecular backbone. Furthermore, it is demonstrated that in molecular mechanics potentials aimed to describe intramolecular proton transfer reactions, the functional form of the potential energy terms coupling the primary and secondary atom motions as the reaction proceeds as well as the mass of the primary particle can significantly influence the centroid transition state theory predictions of secondary kinetic isotope effects. Finally, the dependence of the reaction rate predictions and isotope effects on the choice of reaction coordinate is investigated and the validity of calculating kinetic isotope effects using the centroid transition state theory formalism is discussed.
Generalized Yang–Mills actions from Dirac operator determinants
Authors: Langmann Edwin
Published: July 2002

We consider the quantum effective action of Dirac fermions on four-dimensional flat Euclidean space coupled to external vector- and axial Yang–Mills fields, i.e., the logarithm of the (regularized) determinant of a Dirac operator on flat R4 twisted by generalized Yang–Mills fields. According to physics folklore, the logarithmic divergent part of this effective action in the pure vector case is proportional to the Yang–Mills action. We present a simple explicit computation proving this fact and extending it to the chiral case. We use an efficient computation method for quantum effective actions which is based on calculation rules for pseudo-differential operators and which yields an expansion of the logarithm of Dirac operators in local and quasi-gauge invariant polynomials of decreasing scaling dimension.
Improvement in ferroelectric properties of SrBi2Ta2O9 thin films with Bi2O3 buffer layers by liquid-delivery metalorganic chemical-vapor deposition
Authors: Shin Woong-Chul , Yoon Soon-Gil
Published: July 2002

Ferroelectric SrBi2Ta2O9 (SBT) thin films were deposited onto the Bi2O3 buffered Pt/Ti/SiO2/Si substrates using liquid-delivery metalorganic chemical-vapor deposition. The SBT films with a 5-nm-thick Bi2O3 buffer layer on a Pt bottom electrode showed (115) orientation stronger than those without the Bi2O3 buffer layer after annealing at 750 °C. The Bi2O3 buffer layer in Pt/Bi2O3/SBT/Bi2O3/Pt/Ti/SiO2/Si capacitors effectively prevents the evaporation of Bi through the Pt top electrode and diffusion of Bi into the Pt bottom electrode. The remanent polarization and leakage current densities of SBT films with Bi2O3 buffer layers were improved significantly in comparison with those for the films without the Bi2O3 buffer layer. The remanent polarization (2Pr) and coercive field (Ec) of SBT films without and with the Bi2O3 buffer layer annealed at 750 °C were 12 and 23 μC/cm2, 57 and 38 kV/cm at an applied voltage of 5 V, respectively.
Transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers
Authors: Mun Junho , Yoon Choon Sup , Kim Hyun-Wuk , Choi Su-An , Kim Jong-Duk
Published: July 2002

The transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers was investigated systematically. The transport of the photocharges is explained in terms of current paths that are formed along the bright sites of an interference pattern. Our study shows clearly that charge trapping occurs predominantly in the photoconductive poly(N-vinylcarbazole) layers and not in the insulating poly(vinyl alcohol) layers, contrary to a previous report.
Electrical properties of metal/porous silicon/p-Si structures with thin porous silicon layer
Authors: Balagurov L. A. , Bayliss S. C. , Orlov A. F. , Petrova E. A. , Unal B. , Yarkin D. G.
Published: July 2002

Porous silicon (PS) based metal/PS/p-Si structures with PS layer of different thickness were prepared on moderate- and high-resistivity substrates. Measurements of current–voltage (I–V) characteristics and impedance at various temperatures were used for the investigation of the electrical properties of these structures. Electrical properties of the structures with relatively thin (1 μm) PS layer significantly differ from those of thick structures. The exponential forward bias I–V dependencies for thin structures spread over several orders of magnitude with a low value of quality factor (close to 2) and have activation temperature dependencies with an activation energy equal to half the c-Si band gap. The reverse current has a square root dependence on the reverse bias voltage and the activation energy is equal to half the c-Si band gap. Therefore, it was concluded that the reverse and forward currents in thin PS-based device structures were determined by the generation and recombination of carriers in the depletion region of the c-Si substrate. It was shown that a large area spreading current exists in structures made on highly resistive substrates, which appears to be due to a highly conductive inverse (n-type) layer formed in the p-Si substrate at the PS/p-Si heterojunction. The spreading effect leads to high reverse currents and high capacitance of the device structures made on highly resistive substrates.
Ionization and dissociation mechanisms of ketene using resonance-enhanced multiphoton ionization mass spectrometer: (2+2) versus (2+1) schemes
Authors: Lee Wei-Yen , Lee Wei-Bin , Fu Hunghsin , Pan Ching-Chwan , Lin King-Chuen
Published: July 2002

By using a resonance-enhanced multiphoton ionization (REMPI) technique, we have studied ionization and photodissociation mechanisms of ketene. Prior to ionization, the jet-cooled ketene is first excited at a wavelength 355 nm to a 3py,00 Rydberg state through a two-photon absorption. The (2+2) and (2+1) REMPI schemes may be distinguished depending on the impinging laser energy. If the (2+2) REMPI process dominates, the ketene ion is produced by the autoionization of a superexcited state, which lies in a rovibrationally excited Rydberg state. The autoionization then occurs due to energy transfer from nuclear to electronic degrees of freedom. The CH2+ is fragmented following two schemes. One is a consecutive process, i.e., the fragment ion is produced from the autoionized ketene. This conclusion is supported by a series of measurements of pulse field and pressure effects in this work. The factors of pulse field and collisions may enhance the autoionization rate significantly, imposing the same influence on the ketene ion and CH2+. The second is a dissociative ionization, i.e., the CH2+ ion is fragmented from the superexcited state in competition with the autoionization, as reported previously. These two schemes exclude the probability of ionic ladder or neutral ladder dissociation mechanisms. On the other hand, if the (2+1) REMPI scheme dominates under a low ionizing laser energy, the ketene ions are led by a direct photoionization. The increase of either pulse field intensity or interacting duration simply shortens the arrival time of the ketene ion on the detector. Finally, a time-resolved ketene ion spectrum is measured to characterize temporal behaviors for the autoionization and direct photodissociation. The relaxation lifetime for the autoionized ketene is found to be much faster than that for the directly photoionized source.
The Ar–HCl potential energy surface from a global map-facilitated inversion of state-to-state rotationally resolved differential scattering cross sections and rovibrational spectral data
Authors: Geremia J. M. , Rabitz H.
Published: July 2002

A recently developed global, nonlinear map-facilitated quantum inversion procedure is used to obtain the interaction potential for Ar–HCl(v=0) based on the rotationally resolved state-to-state inelastic cross sections of Lorenz, Westley, and Chandler [Phys. Chem. Chem. Phys. 2, 481 (2000)] as well as rovibrational spectral data. The algorithm adopted here makes use of nonlinear potential→observable maps to reveal the complete family of surfaces that reproduce the observed scattering and spectral data to within its experimental error. A nonlinear analysis is performed on the error propagation from the measured data to the recovered family of potentials. The family of potentials extracted from the inversion data is compared to the Hutson H6(4,3,0) surface [Phys. Chem. 96, 4237 (1992)], which was unable to fully account for the inelastic scattering data [Phys. Chem. Chem. Phys. 2, 481 (2000)]. There is excellent agreement with H6(4,3,0) in the attractive well, where Hutson’s surface is considered most reliable. There is also good long-range agreement. However, it is shown that H6(4,3,0) predicts too soft a wall for the linear Ar–HCl configuration and significantly too steep a wall for linear Ar–ClH. These differences account for the systematically backscattered inelastic cross sections computed using the H6(4,3,0) surface. The new, nonlinear inversion results provide a global Ar–HCl interaction potential with reliable error bars that are consistent with all of the experimental data.
Lattice constant variation and complex formation in zincblende gallium manganese arsenide
Authors: Schott G. M. , Faschinger W. , Molenkamp L. W.
Published: July 2002

We perform high resolution x-ray diffraction on GaMnAs mixed crystals as well as on GaMnAs/GaAs and GaAs/MnAs superlattices for samples grown by low-temperature molecular-beam epitaxy under different growth conditions. Although all samples are of high crystalline quality and show narrow rocking curve widths and pronounced finite thickness fringes, the lattice constant variation with increasing manganese concentration depends strongly on the growth conditions: For samples grown at substrate temperatures of 220 and 270 °C, the extrapolated relaxed lattice constant of Zincblende MnAs is 0.590 nm and 0.598 nm, respectively. This is in contrast to low-temperature GaAs, for which the lattice constant decreases with increasing substrate temperature.
Sound absorption in nonelectrolyte aqueous solutions
Authors: Endo Harumi , Honda Kazuyuki
Published: July 2002

We indicate that the curve fitting for Debye-type relaxation spectrum of sound absorption (SA) cannot sufficiently account for the observed data of nonelectrolyte aqueous solutions (NEAS). To solve these problems, we introduce the distribution function of relaxation time [DFRT, F(τ)] from a diffusion equation of concentration fluctuations using the fluctuation dispersion theory. The SA expression is described by four-adjustable parameters. By use of the mixtures of 1-propanol, t-butanol, and monobutyl triethylene glycol with water, our calculation of SA shows the best fit between the observed and calculated curves, compared with other models. It was found that at lower frequencies the SA behaves as the square root of frequency. The approximate expression of DFRT was expressed in terms of a power law of relaxation time, F(τ)∝τ−γ, which is the same as the expression of dielectric relaxation by Matsumoto and Higashi. Our exponent (γ) of relaxation time is varied from 5/2 in hydrophilic solutes to 3/2 in hydrophobic solutes. The power (γ) of relaxation time was regarded as a parameter to explain the hydrophobic and hydrophilic in the dissolved states of a solute. Our SA expression of γ=5/2 for solutes of a small correlation length leads to that of Romanov–Solov’ev, where the value of 5/2 is that of the Debye distribution for the relaxation time in the Romanov–Solov’ev model.
Fast and accurate thermodynamics of square-well systems from umbrella-sampling simulations of hard-sphere systems
Authors: Zhou Yaoqi
Published: July 2002

A new method for calculating thermodynamics of any square-well systems by umbrella sampling of the corresponding hard-sphere systems is proposed. The removal of energetic barriers leads to an efficient configurational sampling. As an illustrative example, the method is applied to a square-well chain of freely-jointed 64 beads. Results indicate that it produces fast and more accurate thermodynamic results than either the direct molecular dynamics simulation or the umbrella sampling of the square-well chain. The new method, which provides a new route for accurate heat-capacity and free energy calculations, can be easily extended to a system interacting with the Lennard-Jones or any other continuous potentials.
Comment on “Mixture model description of the T-, P dependence of the refractive index of water” [J. Chem. Phys. 114, 3157 (2001)]
Authors: Harvey Allan H.
Published: July 2002

In a recent paper modeling the refractive index of water, some data taken at the National Bureau of Standards in the 1930s were not handled correctly. The problems, which do not invalidate the model, concern the range of the data, their uncertainty, and the temperature scale used. These problems also distort the comparison in the paper to the standard for water’s refractive index adopted by the International Association for the Properties of Water and Steam.
Role of bond coordination and molecular volume on the dielectric constant of mixed-oxide compounds
Authors: Kurtz H. A. , Devine R. A. B.
Published: July 2002

First-principles calculations have been employed to study clusters of Zr embedded in SiO2. Stable complexes are found with four, six, and seven oxygens coordinated to the Zr atom. Consistent with experiment, the higher coordinated complexes are the most stable. These also have a higher density, and hence, smaller molar volume. This smaller molar volume provides an explanation of the increased dielectric constant of ZrxSi1−xO2 mixed-oxide systems for small amounts of Zr (x<0.3). An unusual sevenfold coordinated structure is described.
Dependence of carrier doping level on the photo control of (La, Sr)MnO3/SrTiO3 functional heterojunction
Authors: Katsu Hayato , Tanaka Hidekazu , Kawai Tomoji
Published: July 2002

In a junction system which consists of ferromagnetic- (La1−xSrx)MnO3 and band insulating-SrTiO3, the double exchange ferromagnetism in the (La, Sr)MnO3 layer was controlled by light irradiation through photocarrier injection form SrTiO3. We investigated the dependence of hole doping level (Curie temperature) on the photo control of the (La, Sr)MnO3/SrTiO3 heterojunction. Whereas magnetization and conductivity of the heterojunction with TC of 240 K is strongly reduced, including from metal–insulator transition, by light irradiation, that with TC of 280 K cannot be modulated. This result indicates that phase boundary, between the metallic and insulating region on the (La1−xSrx)MnO3 phase diagram, plays an important role in photo carrier control of the functional heterojunction.
Schottky diodes of Ni/Au on n-GaN grown on sapphire and SiC substrates
Authors: Zhang B. J. , Egawa T. , Zhao G. Y. , Ishikawa H. , Umeno M. , Jimbo T.
Published: July 2002

GaN films were grown on sapphire and SiC substrates. The crystal qualities of GaN films were investigated by photoluminescence, atomic force microscopy, and electron-beam-induced current measurements, etc. It was found that the crystal quality of GaN on SiC is better than the one on sapphire. Ni/Au Schottky contacts were formed on the both samples. The electronic characteristics were obtained by current–voltage and capacitance–voltage measurements. Schottky diodes on sapphire substrate show breakdown voltage of −80 V. While for SiC substrate, the strong breakdown was not observed even at −100 V. The reverse leakage current of diodes based on SiC is over three orders of magnitude lower than that of sapphire substrate when the reverse voltage is above 50 V, which is due to the presence of low dislocation density and high thermal conductivity.
High-temperature metastability of cubic spinel Si3N4
Authors: Sekine T. , Mitsuhashi T.
Published: July 2002

The metastability of cubic spinel Si3N4 has been investigated at high temperature, up to about 1800 K, and in Ar atmosphere by quench experiments and differential thermal analyses coupled with thermogravimetry. The results indicate that the enthalpy change from spinel-type Si3N4 to β-Si3N4 is −29.2±3.5 kJ/mol and that the transformation starts at 1670 K. The high-temperature metastability of spinel Si3N4 may provide various directions for industrial applications.
Novel technique for the extraction of ionization profiles from spatial density measurements
Authors: Deegan C. M. , Markham C. E. , Turner M. , Vender D.
Published: July 2002

A method has been devised to extract the two-dimensional ionization profiles from spatially resolved ion density measurements made in a capacitively coupled argon rf plasma. This technique is valid if the production process is ionization and the loss process is ambipolar diffusion. As this procedure calculates the total production (ionization) term, processes such as two-step ionization and photoionization are included. The main benefit of this new technique is that it extracts ionization rates that pertain to the total ionization in the discharge. It follows that this technique could be used to determine when other ionization processes (e.g., photo- and two-step ionization) are important. On testing with experimental data, the inversion process was observed to reproduce the original source term, thereby verifying that the iteration process converges to a real solution. The calculated ionization rate using the integral method is in good agreement with the ionization profiles obtained from the density measurements. Hence, there is now a method of determining the ionization rate that is independent of electron energy probability function measurement and that includes all ionization processes, not just ground state ionization by electron–neutral collisions.
Oxygen-related defects in low-dose separation-by-implanted oxygen wafers probed by monoenergetic positron beams
Authors: Uedono Akira , Chen Zhi Quan , Ogura Atsushi , Ono Haruhiko , Suzuki Ryoichi , Ohdaira Toshiyuki , Mikado Tomohisa
Published: July 2002

The depth distributions of oxygen-related defects in separation-by-implanted oxygen wafers were determined from measurements of Doppler broadening spectra of the annihilation radiation. Vacany–oxygen complexes were introduced by implanting 180-keV oxygen at (2–6)×1017 cm−2 into Si substrates. Their sizes decreased below the subsurface region (<100 nm) because an agglomeration of vacancy-type defects was suppressed by the interaction between vacancies and oxygen atoms. As the dosage was increased, in the region near the projected range of oxygen, atomic rearrangement of vacancy-type defects occurred, and this rearrangement is considered to introduce SiOx (x<2) that is stable at high annealing temperatures. Oxygen-related defects were presented in the superficial Si layer even after annealing at 1350 °C. The concentration of such defects was low when the substrate was implanted with a dose of 4×1017 cm−2.
Observation of nitrogen vacancy in proton-irradiated AlxGa1−xN
Authors: Zhou Qiaoying , Manasreh M. O. , Pophristic M. , Guo S. , Ferguson I. T.
Published: July 2002

The optical absorption spectra of nitrogen vacancy (VN) in proton-irradiated AlxGa1−xN samples are observed. The spectra obtained for samples with 0.55⩽x⩽1 exhibit a peak and a shoulder with their energy positions dependent on the Al mole fraction. The peak and shoulder in the optical absorption spectra were interpreted as transitions from the valence band to the VN energy levels located below the conduction band in samples with x>0.55. The results were used to extrapolate the VN energy level positions in samples with 0⩽x⩽0.55. A linear fit of the observed VN energy level yields E(VN)=4.230+0.680x (eV) for all values of x. Thermal annealing of various samples shows that the total integrated area, which is directly proportional to the defect density, of the absorption band attributed to the nitrogen vacancy is decreased as the annealing temperature is increased.
Quantum effect of solvent on molecular vibrational energy relaxation of solute based upon path integral influence functional theory
Authors: Mikami Taiji , Shiga Motoyuki , Okazaki Susumu
Published: July 2002

Path integral influence functional theory has been applied to the investigation of the quantum effect of the solvent on vibrational relaxation of the solute. A classical bath approximation was attained by taking the ℏ→0 limit with respect to the solvent degrees of freedom. A comparison of the calculated relaxation time for the quantum solvent with that for the classical one showed that the quantum effect is very large and, at the same time, it depends much upon the process, i.e., single-phonon process, two-phonon process, or three-phonon process. This indicates that the so-called quantum correction does not work since the relaxation is usually a mixture of these multiphonon processes. A numerical example for CN− in water also demonstrates that, although the classical approximation for the solvent significantly overestimates the relaxation time, it presents reliable energy transfer pathways or relaxation mechanism.
Absolute calibration of space- and time-resolving flat-field vacuum ultraviolet spectrograph under both P and S polarized light conditions for plasma diagnostics
Authors: Okamoto Y. , Yoshikawa M. , Yamaguchi N. , Watabe C. , Tamano T. , Kawamori E. , Watanabe Y. , Furukawa T. , Kubota Y. , Sedo K. , Yatsu K.
Published: July 2002

Measurement of spectra in the wavelength range from vacuum ultraviolet (VUV) to soft x ray is an important means to diagnose impurity behavior in magnetically confined plasmas used in fusion plasmas. Recently, a space- and time-resolving flat-field grazing-incidence VUV spectrograph was constructed for the simultaneous observation of spatial, temporal, and spectral distributions of plasma radiation in the wavelength range of 150–1050 Å. Absolute calibration experiments were performed at beamline 11C and beamline 12A in the Photon Factory at the High Energy Acceleration Research Organization. The absolute sensitivity of the VUV spectrograph was measured under both P and S polarized light conditions.
Determining liquid substrate cleanliness using infrared imaging
Authors: Saylor J. R.
Published: July 2002

Experiments conducted in a modified Langmuir trough are described. Measurements of surface tension were obtained using a Wilhelmy plate, while infrared (IR) images of the water surface adjacent to the Wilhelmy plate were simultaneously recorded. A contaminating surfactant film was allowed to form on the surface while the experiments were being conducted. The data reveal a substantial change in the IR imagery due to the film in all cases. The difference between the appearance of the clean and surfactant-covered regions of the surface is clear and distinct. For some of these experiments the change in the IR imagery is accompanied by no observable change in surface tension. This, along with several other aspects of this work, suggest that IR imagery may be a superior tool, compared to traditional surface tension measurements, for ascertaining the cleanliness of a liquid substrate.
Temperature control of a liquid helium cooled Eigler-style scanning tunneling microscope
Authors: Rust H.-P. , Doering M. , Pascual J. I. , Pearl T. P. , Weiss P. S.
Published: July 2002

A procedure for operating an Eigler-style, low temperature scanning tunneling microscope (STM) at variable temperatures has been developed. A critical exchange gas pressure regime was found to allow for controlled variation of the STM temperature while it is encapsulated in a liquid helium Dewar. The sensitivity of various parameters to the ability to generate stable variable temperatures above 4 K is discussed.
Metalorganic vapor phase epitaxy growth of crack-free AlN on GaN and its application to high-mobility AlN/GaN superlattices
Authors: Yamaguchi Shigeo , Kosaki Masayoshi , Watanabe Yasuyukihiro , Yukawa Yohei , Nitta Shugo , Amano Hiroshi , Akasaki Isamu
Published: July 2002

We have succeeded in growing crack-free AlN of even 0.5 μm thickness on GaN by metalorganic vapor phase epitaxy. A (0001) sapphire substrate was used. Crack-free AlN was grown on GaN at 1000 °C with N2 carrier gas. An AlN layer was grown on GaN of 2 μm thickness grown at 1050 °C, following the low-temperature deposition of an AlN buffer layer of 30 nm. No cracks were observed in the microphotographs of AlN on GaN grown using N2. X-ray diffraction analysis revealed that AlN/GaN superlattices (SLs) were coherently grown on GaN, and satellite peaks up to the third order were observed. The structure of AlN/GaN SLs on GaN showed a maximum electron mobility of 1580 cm2/V s at room temperature and a nominal sheet carrier density of 8.4×1012 cm−2.
Electromagnetic surface waves on ultrashort pulse laser-generated plasma channels
Authors: Keskinen M. J. , Fernsler R. , Ladouceur H. D. , Baronavski A. P. , Grounds P. W. , Girardi P. G.
Published: July 2002

Using analytical and numerical techniques, the properties of electromagnetic surface waves on conducting plasma channels produced by ultrashort pulse lasers have been studied. Using a homogeneous slab approximation to model the conducting plasma channel, we solve for the surface wave dispersion and wave damping for both the symmetric and antisymmetric wave mode cases for both a thick and thin slab. For the dense plasma case, i.e., for wave frequencies less than the electron collision frequency, it is found that surface wave damping attenuation length can be large, on the order of the laser plasma channel propagation distance, for atmospheric pressure conditions for both the thick and thin slab approximation. In addition, it is shown that large amplitude surface waves can modify the plasma channel conductivity through nonlinear effects.
Rotational dynamics of neutral red in dimethylsulfoxide: How important is the solute’s charge in causing “additional friction?”
Authors: Dutt G. B. , Ghanty T. K. , Singh M. K.
Published: July 2002

Temperature dependent rotational relaxation study of neutral and cationic forms of neutral red has been carried out in dimethylsulfoxide (DMSO) in an attempt to find out how the charge on the solute influences its dynamics. Experimental results indicate that the cationic form rotates marginally slower (less than 20%) than the neutral form. The mechanical friction has been modeled using the Stokes–Einstein–Debye hydrodynamic theory with slip boundary condition and the dielectric friction using the extended charge distribution model of Alavi–Waldeck. The marginally slower reorientation times of the cationic form has been ascribed to the effect of dielectric friction. Alternatively, it has also been explained by invoking the concept of solute–solvent hydrogen bonding due to the presence of an additional hydrogen bonding site on the cation in the form of a hydrogen atom attached to the ring nitrogen. This result is different from that of the others in literature where cationic probes experience a lot more friction than their neutral counterparts in DMSO, the reason being that the cationic and neutral solutes used in almost all the previous studies possess different functional groups.
Potentiometric cell for measuring pH of supercritical aqueous solutions
Authors: Sue Kiwamu , Murata Kenji , Matsuura Yuuki , Tsukagoshi Masayuki , Adschiri Tadafumi , Arai Kunio
Published: July 2002

A flow-through electrochemical cell for subcritical and supercritical water systems was developed based on the cell of Lvov et al. [J. Electroanal. Chem. 463, 146 (1999)]. The cell consisted of two flow-through platinum hydrogen electrodes and was constructed from a high nickel alloy (Hastelloy-B2). The inner surface of electrochemical cell was coated with Al2O3 adhesive and covered with the Al2O3 tube for corrosion resistance. Orifices were placed near the center of the cell to reduce concentration fluctuations around the test electrode. With the cell, a flow apparatus was used to measure the potentials for HCl+NaCl aqueous solutions at supercritical conditions. The cell potentials were measured at temperatures ranging from 23.9 to 400.2 °C and at pressures ranging from 25.0 to 35.1 MPa and the standard deviations of measured potentials were less than 0.4 mV for all tests. Comparison of the experimentally determined and theoretically calculated pH differences are better than ±0.03 logarithmic units over the range of conditions studied.
Size dependent charge storage effect of the InAs nanodots in Al0.5Ga0.5As/GaAs field-effect diodes
Authors: Koike Kazuto , Komai Hisayoshi , Li Shuwei , Yano Mitsuaki
Published: July 2002

This article describes size dependent charge storage effect of the InAs nanodots in the barrier layers of Al0.5Ga0.5As/GaAs field-effect diodes. This charging effect is analyzed by a capacitance–voltage (C–V) measurement at 77 K, and results in a clockwise hysteresis loop due to the electron storage at nanodot potentials. It is revealed that the number of stored electrons is nearly independent of dot size, and the amount of stored charge increases proportionally with dot density. The retention time of the stored charge, however, is deteriorated dramatically by the inclusion of coalesced dots in the storage nodes. These C–V characteristics are in good agreement with their photoluminescence properties.
Demonstration of scaled (⩾0.12 μm2) Pb(Zr,Ti)O3 capacitors on W plugs with Al interconnect
Authors: Summerfelt S. R. , Moise T. S. , Xing G. , Colombo L. , Sakoda T. , Gilbert S. R. , Loke A. L. S. , Ma S. , Wills L. A. , Kavari R. , Hsu T. , Amano J. , Johnson S. T. , Vestcyk D. J. , Russell M. W. , Bilodeau S. M. , van Buskirk P.
Published: July 2002

The measured switched polarization properties of integrated Pb(Zr,Ti)O3 (PZT) capacitors arrays have been found to show a small dependence on individual capacitor size in the range from 0.17 and 100 μm2. These thin (90 nm) PZT capacitors have low voltage switching properties with polarization saturation of <1.8 V with switched polarization for the smallest capacitors (0.17 μm2) still larger than 25 μC/cm2. The capacitor stack consisted of TiAlN hardmask/Ir/IrOx/PZT/Ir/TiAlN on either SiO2 dielectric or W plugs. The capacitor was patterned using 248 nm lithography and etched using only one mask. For wafers without W plugs, the Ir bottom electrode was not etched. For wafers with W plugs, the entire capacitor stack was etched and electrical connection to the bottom electrode was through the W plugs. The capacitors were integrated using SiO2 dielectrics and one level of Al metallization. These data suggest that high-density, ferroelectric capacitor-based memories may be feasible.
Polarization spectroscopy using short-pulse lasers: Theoretical analysis
Authors: Roy Sukesh , Lucht Robert P. , Reichardt Thomas A.
Published: July 2002

The physics of short-pulse polarization spectroscopy (PS) and the diagnostic potential for quantitative measurements of species concentration are investigated by direct numerical integration (DNI) of the time-dependent density matrix equations for a multistate system. The effects of laser power, collision rates, and Doppler broadening on the short-pulse PS signal generation process are investigated by systematically varying these parameters in the numerical calculations. It is found that the use of a short-pulse laser (laser pulse width τLτC), even for a nonsaturating pump beam. For a saturating pump beam, the short-pulse PS signal is found to be nearly independent of collision rate. Increasing the collision rate by a factor of 100 (from 108 to 1010 s−1), the calculated PS signal strength decreases by only a factor of 2 for a 100-ps pump laser at high intensity. This insensitivity of the PS signal to the collision rate in the medium enhances greatly the potential for quantitative application of the technique for concentration measurements in reacting flows. The underlying physics of the short-pulse PS is explored by studying the effects of collision rate, Doppler broadening, and the pump laser intensity on the temporal profile of the Zeeman state populations and the coherences between the Zeeman states.
Interactions between gallium and nitrogen dopants in ZnO films grown by radical-source molecular-beam epitaxy
Authors: Nakahara K. , Takasu H. , Fons P. , Yamada A. , Iwata K. , Matsubara K. , Hunger R. , Niki S.
Published: July 2002

It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4.
Soft phonon modes in Ba0.5Sr0.5TiO3 thin films studied by Raman spectroscopy
Authors: Tenne D. A. , Clark A. M. , James A. R. , Chen K. , Xi X. X.
Published: July 2002

We have studied vibrational properties of Ba0.5Sr0.5TiO3 thin films grown by pulsed laser deposition on SrTiO3 and LaAlO3 substrates with SrRuO3 buffer layers by Raman spectroscopy over the temperature range from 5 to 300 K. The soft phonon modes were observed. The temperature dependence of the soft phonon frequency and the splitting of the triply degenerated soft mode into two components of A and E symmetries indicate the ferroelectric phase transition. The E soft mode is overdamped over a broad range of temperatures, and the A soft mode intensity decreases gradually in this range. This indicates a broad ferroelectric phase transition in the thin films. A comparison of results obtained from films deposited on SrTiO3 and LaAlO3 substrates shows that strain shifts the ferroelectric phase transition temperatures.
Polarization-graded ferroelectrics: Transpacitor energy gain
Authors: Mantese Joseph V. , Schubring Norman W. , Micheli Adolph L.
Published: July 2002

The energy gain of polarization-graded ferroelectric devices, configured as active transcapacitive elements, have been measured. Gain factors in excess of 1000 were obtained for small signal, static inputs; yielding charge gain amplifiers with gain factors of ∼150, remarkably similar to what is typically found for transistor current gain devices.
Near-field optics: Direct observation of the field enhancement below an apertureless probe using a photosensitive polymer
Authors: H’dhili Fekhra , Bachelot Renaud , Lerondel Gilles , Barchiesi Dominique , Royer Pascal
Published: July 2002

We report the direct observation of the optical near-field enhancement at the nanometric extremity of a metallic probe for apertureless scanning near-field optical microscopy. Our approach consists in making the “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end via a photosensitive polymer. This distribution is coded by polymer surface topography which is characterized in situ by atomic force microscopy using the same probe. Results clearly reveal nanometric dots corresponding to local field enhancement below the tip end. The field enhancement is shown to be crucially dependent on the polarization state of the incident laser beam as well as the tip material and geometry. The experimental results are found to agree with the results of preliminary calculations. This experiment both constitutes a useful tool for investigating field enhancement below apertureless probes and has potential applications in nanophotolithography.
Vibrational properties and microstructure of reactively sputtered hydrogenated carbon nitrides
Authors: Fanchini G. , Tagliaferro A. , Messina G. , Santangelo S. , Paoletti A. , Tucciarone A.
Published: July 2002

The present study is focused on the vibrational and structural characterization of a set of disordered hydrogenated carbon nitride (a-CN:H) thin films grown by reactive sputtering. A comparative analysis of the experimental results as achieved by Raman and infrared (IR) spectroscopies is made. The disorder-induced features of IR and Raman spectra are discussed as well in light of the current assessment on the vibrational properties of carbon-based materials. Some differences between a-CN:H and nonnitrogenated a-C:H materials are evidenced and attributed to the effects of charge redistribution and bond polarization due to the presence of nitrogen. In order to justify such a hypothesis, the dielectric constant, the dynamic effective charge, and the IR cross section determined by the charge transfer effects are calculated and found to be in agreement with the corresponding experimental values.
Chemical ordering in lanthanum-doped lead magnesium niobate relaxor ferroelectrics probed by A1g Raman mode
Authors: Jiang Fuming , Kojima Seiji , Zhao Changlei , Feng Chude
Published: July 2002

Chemical ordering of the B-site cations in undoped, 5, 10, 20, and 30 mol % La3+-doped Pb(Mg1/3Nb2/3)O3 (PMN) ceramics with perovskite structure, has been examined by Raman scattering. Two B-site chemical ordering models, the space-charge model, and the random layer model assume a space group of Fm3̄m for the ordered nanoregions. It is found that A1g Raman mode around 780 cm−1 from the B-site ordered nanoregions with a space group Fm3̄m, is very sensitive to the delicate changes of the chemical order at B site. Frequency changes of this A1g characteristic mode with La3+ doping, are analyzed according to the lattice parameters and chemical order effects. The present results provide stronge evidence supporting the random layer model in La-doped PMN.
Shape transition of coherent three-dimensional (In,Ga)As islands on GaAs(100)
Authors: Ma Wenquan , Nötzel Richard , Schönherr Hans-Peter , Ploog Klaus H.
Published: July 2002

The shape transition of coherent three-dimensional (3D) islands is observed experimentally in the (In,Ga)As/GaAs(100) material system. In the molecular-beam epitaxy of a 1.8-nm-thick In0.35Ga0.65As single layer, we find that the shape of the coherent 3D islands transforms from round to elongated when increasing the growth temperature. A quantitative agreement of our experimental data with the theoretical work of Tersoff and Tromp is achieved.
Ultrafast cis-trans photoswitching: A model study
Authors: Hahn Susanne , Stock Gerhard
Published: July 2002

A quantum-mechanical model description of a molecular photoswitch is developed. It takes into account (i) the electronic curve crossing arising from the cis-trans twisting of a double bond, resulting in an ultrafast internal-conversion process of the system and (ii) the coupling of the initially excited chromophore (the “system”) to the remaining degrees of freedom (the “bath”), affecting a vibrational cooling of the hot photoproducts. The latter mechanism is responsible for the localization of the molecule in the cis and trans configuration, respectively, thus determining the quantum yield of the photoreaction. Following a discussion of the validity and the numerical implementation of the Redfield formulation employed, detailed numerical studies of the time-dependent dissipative photoisomerization dynamics are presented. While the short-time dynamics (≲1 ps) is dominated by the coherent wave-packet motion of the system, the time evolution at larger times mainly reflects the interaction between system and bath. The quantum yield of the cis-trans forward reaction (Yc→t) and the trans-cis backward reaction (Yt→c) is shown to depend on the energy storage of the photoreaction and, in particular, on the form of the system–bath coupling. On the other hand, it is found that Yt→c=1−Yc→t, that is the population probabilities of the cis and trans configuration at long times do not depend on the initial preparation of the system.
Multiple-tunneling noise in superconducting tunnel junctions from partial current integration
Authors: Hiller L. J. , van den Berg M. L. , Labov S. E.
Published: July 2002

Superconducting tunnel junctions can be used as high-resolution particle or photon energy spectrometers. A photon absorbed in a superconductor breaks Cooper pairs into quasiparticles. These quasiparticles tunnel through the junction barrier and are detected as a pulse of excess current. Many junction designs allow the quasiparticles to tunnel more than once, an exponentially mixed Poisson process. However, multiple tunneling increases the fluctuation in the measured charge. We calculate the significance of these fluctuations algebraically as a function of time during the current pulse. We also calculate the finite integration window that minimizes the contribution of this noise. In addition, we calculate the effects of a low-pass amplifier and a Gaussian-shaping amplifier on the tunneling noise. With certain filtering time constants, the tunneling noise can be reduced while still providing some gain.
Generalized coherent and intelligent states for exact solvable quantum systems
Authors: El Kinani A. H. , Daoud M.
Published: July 2002

The so-called Gazeau–Klauder and Perelomov coherent states are introduced for an arbitrary quantum system. We give also the general framework to construct the generalized intelligent states which minimize the Robertson–Schrödinger uncertainty relation. As illustration, the Pöschl–Teller potentials of trigonometric type will be chosen. We show the advantage of the analytical representations of Gazeau–Klauder and Perelomov coherent states in obtaining the generalized intelligent states in analytical way.