bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical approach to photochromism of aromatic Schiff bases: A minimal chromophore salicylidene methylamine
Authors: Zgierski Marek Z. , Grabowska Anna
Published: July 2002

The ground and excited electronic states conformations of various tautomers of salicylidene methylamine (SMA) are calculated by ab initio methods. It is demonstrated that the cis-keto form, more stable in the ππ* S1 state, can rapidly decay to a strongly nonplanar configuration of the nπ* state, which subsequently undergoes very fast internal conversion to one of the trans-keto tautomers. This we identify with the photochromic species of SMA. The calculated photochromic absorption spectrum reproduces very well the observed spectrum. The rates of the proton transfer processes in the ground and excited states of SMA are calculated with the help of the ab initio geometries, force fields, and the instanton method. It is shown that the excited state proton transfer occurs faster than 50 fs for the CIS barrier of 1.6 kcal/mol. The ground state backproton transfer between the cis-keto and enol tautomers is predicted to take ∼30 ps and to exhibit a large deuterium effect.
Room temperature synthesis of c-AlN thin films by nitrogen-ion-assisted pulsed laser deposition
Authors: Ren Z. M. , Lu Y. F. , Ni H. Q. , Liew T. Y. F. , Cheong B. A. , Chow S. K. , Ng M. L. , Wang J. P.
Published: July 2002

Cubic aluminum nitride (c-AlN) thin films have been deposited at room temperature on silicon substrates by nitrogen-ion-assisted pulsed laser ablation of a hexagonal AlN target. The deposited thin films exhibit good crystal properties with sharp x-ray diffraction peaks. The influences of the nitrogen ion energy on the morphological, compositional, and electronic properties of the AlN thin films have been studied. The nitrogen ions can effectively promote the formation of Al–N bonds and improve the crystal properties of the deposited thin films. A nitrogen ion energy of 400 eV is proposed to deposit high quality c-AlN thin films.
In situ scanning tunneling microscopic study of polymerization of C60 clusters induced by electron injection from the probe tips
Authors: Nakamura Y. , Mera Y. , Maeda K.
Published: July 2002

Polymerization of C60 clusters epitaxially grown on Si(111)-(7×7) substrates was found to be induced by electron injection from the probe tips of scanning tunneling microscopes (STM) as the sample bias was increased from +4.0 to +5.5 V, exhibiting an evolution behavior characterized by an incubation, a linear growth, and a saturation. The incubation time and the growth rate are dependent greatly on the sample site, which is explained by a model taking into account the pre-existing stress as the driving force of the polymerization and the internal stress built up as a consequence of polymerization producing a stress for backward reactions.
Effect of poloidal density variation on parallel viscosity for large Mach numbers
Authors: Kiviniemi T. P. , Heikkinen J. A. , Peeters A. G.
Published: July 2002

Parallel viscosity 〈B⋅∇⋅Πi〉 as a function of radial electric field Er (or, equivalently, nonequilibrium poloidal rotation) in a tokamak is studied using a five-dimensional (5D) (3D in configuration space and 2D in velocity space) Monte Carlo particle following code. It is shown that including the poloidal density variation changes the qualitative behavior of the solution causing the parallel viscosity to change sign when the Mach number Mp=|Er/vtBθ| is of the order of unity. Here, vt is the thermal velocity and Bθ is the poloidal magnetic field.
Thickness measurements for ultrathin-film insulator metal–oxide–semiconductor structures using Fowler–Nordheim tunneling current oscillations
Authors: Mao Lingfeng , Tan Changhua , Xu Mingzhen
Published: July 2002

An interference method is introduced to analyze tunneling current oscillations, and a fresh way to extrapolate the oxide thickness in ultrathin-film insulator metal–oxide–semiconductor structures by using the oscillations in the Fowler–Nordheim tunneling currents is presented. A comparison between this extrapolation algorithm and a previous algorithm using tunneling current oscillations shows that the new extrapolation algorithm provides a more accurate and convenient solution to a first principles calculation especially for ultrathin oxide. Another important feature of the proposed method is that it can be applied to various shapes of potential barriers and wells.
Diffusional creep in damascene Cu lines
Authors: Kobrinsky Mauro J. , Thompson Carl V. , Gross Mihal E.
Published: July 2002

Damascene Cu lines are increasingly being used as electrical interconnects in microelectronic integrated circuits. During the manufacturing process, Cu interconnects are subjected to thermal cycling that leads to complex stress states, which affect their reliability. While a realistic assessment of the impact of thermal stresses on the reliability should take into account the inelastic behavior of the damascene Cu lines, unfortunately, little is known about their mechanical behavior. The goal of this work was to identify the physical mechanisms responsible for the inelastic behavior of damascene Cu lines, and to assess the effects of the varied dimensions and passivation layers. The curvature changes caused by thermal cycling of wafers with damascene Cu lines were measured in the temperature range from −80 to 450 °C. At low temperatures, the deformation in the lines is predominantly elastic, but becomes inelastic above 200 °C. Our experimental results indicate that diffusion-controlled creep is the dominant inelastic mechanism at high temperatures. A TaN capping layer was found to suppress inelastic deformation, presumably by preventing diffusion of Cu between the free surface and the grain boundaries. This effect suggests a simple method for comparison of the effectiveness of different capping layers (e.g., TaN, Ta, Si3N4) in inhibiting Cu interfacial diffusion.
Nanoscale composition analysis of atomically flat SrTiO3(001) by friction force microscopy
Authors: Iwahori Koichiro , Watanabe Shunji , Kawai Maki , Mizuno Keisuke , Sasaki Kenji , Yoshimoto Mamoru
Published: July 2002

Friction force microscopy (FFM) was used to distinguish the different surface terminations on the nanoscale on atomically flat SrTiO3(001) surfaces, obtained by a combination of ultrasonic agitation and subsequent annealing in air at 1000 °C. The surface exhibits atomically flat terraces and sharp steps whose height is 0.2 nm, corresponding to half the height of a unit cell of SrTiO3, or a single atomic layer, where strong friction contrast appears. A compositional analysis of the topmost surface by coaxial-impact-collision ion-scattering spectroscopy (CAICISS) indicated that the friction contrast corresponded to different surface terminations, namely, the SrO and TiO2 layer as a topmost surface, where the TiO2 domain was dominant. This is further supported by the correlation between the compositional Sr/Ti ratio and the area ratio which was observed by CAICISS and FFM, respectively.
Nb interaction with hydrogen plasma
Authors: Nakamura Y. , Busnyuk A. , Suzuki H. , Nakahara Y. , Ohyabu N. , Livshits A.
Published: July 2002

A niobium membrane was immersed in hydrogen plasma and could be electrically biased to vary the energy of bombarding ions in the range of 1–200 eV. The fluxes of plasma driven absorption and permeation were almost entirely governed by incident suprathermal neutrals (mostly, thermal atoms), whose energy does not depend on membrane bias, but the ions of controllable energy do affect the neutral-induced permeation through modifying the membrane surface. At the zero bias a high temperature-independent plasma driven permeation (superpermeation) was observed alongside of an enhanced absorption. Bombardment by ions of an energy higher than 50 eV resulted in a sharp decrease of the plasma driven permeation/retention and in an acceleration of boundary processes of absorption/reemission of thermal molecules. At ion energies below 50 eV, the effect of ion bombardment on the plasma driven permeation and the kinetic coefficients of boundary processes were nonmonotonic in ion energy, having a maximum at ∼10 eV. Both an in situ doping with O of the bulk of Nb and a membrane temperature increase reduced the effects of ion bombardment to their complete disappearance. Responsible for that was the replenishment by means of surface segregation of an oxygen monolayer sputtered by ion bombardment.
Direct observation of graphite layer edge states by scanning tunneling microscopy
Authors: Giunta P. L. , Kelty S. P.
Published: July 2002

The electronic and structural properties of two different edge terminations in normal graphite layer edges were investigated at atomic resolution using scanning tunneling microscopy (STM) and electronic band structure calculations. An emphasis is placed on distinguishing the electronic structure of [1,1,2̄,0] (“armchair”) and [1,0,1̄,0] (“zigzag”) edge terminations. Experimental STM investigations reveal that the [1,1,2̄,0] termination exhibits a (3×3)R30° superstructure in the local electron density of states profile superimposed on the normal graphite lattice. The [1,0,1̄,0] edge shows a different superstructure typified by two to three atomic lattice cells of high electron density parallel to the edge. The armchair edge superstructure persists for 2–3 nm (equivalent to about 10 unit cells) into the bulk layer. By modeling these edge types using extended Hückel tight binding calculations, we show that the results are consistent with an unreconstructed edge geometry. The details of the edge states do not appear to be particularly sensitive to the chemical composition of the edge. The experimental results are in general agreement with previously reported modeling studies of these edge states.
Nature of non-D and non-G bands in Raman spectra of a-C:H(N) films grown by reactive sputtering
Authors: Messina G. , Paoletti A. , Santangelo S. , Tagliaferro A. , Tucciarone A.
Published: July 2002

The Raman spectra of sputter-grown a-C:H(N) films are analyzed giving particular emphasis to the regions below and above that dominated by the well-known D and G bands, in order to deduce complementary information about the film physical properties and eventually clarify the nature of the features there detected. The conventionally studied evolution of the D and G bands evidences a sharp tendency toward graphitization, accompanied by an increasing level of structural disorder. The weak feature observed at about 700 cm−1 is assigned to the disorder-induced L band, originating from the formation, within the films, of graphitic microcrystallites, whose number and size are shown to change when the partial pressure of argon is varied. Thanks to the complementary infrared film characterization, indicating no appreciable hydrogen incorporation into the investigated samples, the broad asymmetrical Raman band centered at about 3000 cm−1 is suggested to represent the second order of the D and G bands, whose component detection and broadening are likely to represent a finite-crystal-size effect, due to the very small size of graphitic crystallites. A semiquantitative explanation of the observed dependence of the second- on the first-order integrated intensity is given.
Patterned selective growth of carbon nanotubes and large field emission from vertically well-aligned carbon nanotube field emitter arrays
Authors: Sohn Jung Inn , Lee Seonghoon , Song Yoon-Ho , Choi Sung-Yool , Cho Kyoung-Ik , Nam Kee-Soo
Published: July 2002

We have grown well-aligned carbon nanotube arrays by thermal chemical vapor deposition at 800 °C on Fe nanoparticles deposited by a pulsed laser on a porous Si substrate. We also attain a selective growth of carbon nanotubes on a patterned Fe film on Si substrates in terms of pulsed laser deposition and a liftoff patterning method. Field emission measurement has been made on the carbon nanotube (CNT)-cathode diode device at room temperature and in a vacuum chamber below 10−6 Torr. The distance between the CNT cathode and the anode is 60 μm and is kept through an insulating spacer of polyvinyl film. The measured field emitting area is 4.0×10−5 cm2. Our vertically well-aligned carbon nanotube field emitter arrays on the Si-wafer substrate emit a large current density as high as 80 mA/cm2 at 3 V/μm. The transmission electron microscope image shows that they are multiwalled and bamboolike structures and that the tips of some of the carbon nanotube emitters are open. The open tip structure of our CNTs and their good adhesion via Fe nanoparticles to the Si substrate are part of the reason why we can attain a large field emission current density within a low field.
Thermal fluctuation noise in a voltage biased superconducting transition edge thermometer
Authors: Hoevers H. F. C. , Bento A. C. , Bruijn M. P. , Gottardi L. , Korevaar M. A. N. , Mels W. A. , de Korte P. A. J.
Published: July 2002

The current noise at the output of a microcalorimeter with a voltage biased superconducting transition edge thermometer is studied in detail. In addition to the two well-known noise sources: thermal fluctuation noise from the heat link to the bath and Johnson noise from the resistive thermometer, a third noise source strongly correlated with the steepness of the thermometer is required to fit the measured noise spectra. Thermal fluctuation noise, originating in the thermometer itself, fully explains the additional noise. A simple model provides quantitative agreement between the observed and calculated noise spectra for all bias points in the superconducting transition.
Effects of rapid thermal annealing on strain-compensated GaInNAs/GaAsP quantum well structures and lasers
Authors: Li Wei , Turpeinen Jani , Melanen Petri , Savolainen Pekka , Uusimaa Petteri , Pessa Markus
Published: July 2002

Strain-compensated GaInNAs/GaAsP quantum well structures and lasers were grown by gas-source molecular beam epitaxy using a rf-plasma nitrogen radical beam source. Effects of rapid thermal annealing on the optical properties of GaInNAs/GaAsP quantum well structures as well as laser diodes were examined. It was found to significantly increase the photoluminescence from the quantum wells and reduce the threshold current density of the lasers, mainly due to a removal of N-induced nonradiative centers from GaInNAs wells.
Variable separation for natural Hamiltonians with scalar and vector potentials on Riemannian manifolds
Authors: Benenti S. , Chanu C. , Rastelli G.
Published: July 2002

The additive variable separation in the Hamilton–Jacobi equation is studied for a natural Hamiltonian with scalar and vector potentials on a Riemannian manifold with positive–definite metric. The separation of this Hamiltonian is related to the separation of a suitable geodesic Hamiltonian over an extended Riemannian manifold. Thus the geometrical theory of the geodesic separation is applied and the geometrical characterization of the separation is given in terms of Killing webs, Killing tensors, and Killing vectors. The results are applicable to the case of a nondegenarate separation on a manifold with indefinite metric, where no null essential separable coordinates occur.
Comparison of field effect transistor characteristics between space-grown and earth-grown gallium arsenide single crystal substrates
Authors: Chen Nuo Fu , Zhong Xingru , Lin Lanying , Zhang Mian , Wang Yunsheng , Bai Xiwei , Zhao Jing
Published: July 2002

Semi-insulating gallium arsenide single crystal grown in space has been used in fabricating low noise field effect transistors and analog switch integrated circuits by the direct ion-implantation technique. All key electrical properties of these transistors and integrated circuits have surpassed those made from conventional earth-grown gallium arsenide. This result shows that device-grade space-grown semiconducting single crystal has surpassed the best terrestrial counterparts.
Mechanism of dynamic nuclear polarization in high magnetic fields
Authors: Farrar C. T. , Hall D. A. , Gerfen G. J. , Inati S. J. , Griffin R. G.
Published: July 2002

Solid-state NMR signal enhancements of about two orders of magnitude (100–400) have been observed in dynamic nuclear polarization (DNP) experiments performed at high magnetic field (5 T) and low temperature (10 K) using the nitroxide radical 4-amino TEMPO as the source of electron polarization. Since the breadth of the 4-amino TEMPO EPR spectrum is large compared to the nuclear Larmor frequency, it has been assumed that thermal mixing (TM) is the dominate mechanism by which polarization is transferred from electron to nuclear spins. However, theoretical explanations of TM generally assume a homogeneously broadened EPR line and, since the 4-amino TEMPO line at 5 T is inhomogeneously broadened, they do not explain the observed DNP enhancements. Accordingly, we have developed a treatment of DNP that explicitly uses electron–electron cross-relaxation to mediate electron–nuclear polarization transfer. The process proceeds via spin flip–flops between pairs of electronic spin packets whose Zeeman temperatures differ from one another. To confirm the essential features of the model we have studied the field dependence of electron–electron double resonance (ELDOR) data and DNP enhancement data. Both are well simulated using a simple model of electron cross-relaxation in the inhomogeneously broadened 4-amino TEMPO EPR line.
Degraded axial buckling strain of multiwalled carbon nanotubes due to interlayer slips
Authors: Ru C. Q.
Published: July 2002

A multiple-shell model is presented for infinitesimal axially compressed buckling of a multiwalled carbon nanotube embedded within an elastic matrix. In contrast to an existing single-shell model which treats the entire multiwalled nanotube as a singlelayer elastic shell, the present model assumes that each of the nested concentric tubes is an individual elastic shell and the deflections of all shells are coupled through the van der Waals interaction between adjacent nanotubes. By examining a doublewalled carbon nanotube, it is found that the change in interlayer spacing has a negligible effect on the axial buckling strain provided that the innermost radius is at least a few nanometers. Under this condition, a single equation is derived which determines the deflection of the multiwalled carbon nanotube, and it is shown that infinitesimal axial buckling of a N-walled carbon nanotubes is equivalent to that of a single layer elastic shell whose bending stiffness is approximately N times the effective bending stiffness of a single walled carbon nanotube. As a result, the axial buckling strain of a N-walled carbon nanotube is about 5 N times lower than that predicted by the existing single-shell model. The degraded axial buckling strain is attributed to the interlayer slips between adjacent nanotubes, which represents an essential feature of mechanical behavior of multiwalled carbon nanotubes.
Luminescence of epitaxial GaN laterally overgrown on (0001) sapphire substrate: Spectroscopic characterization and dislocation contrasts
Authors: Dassonneville S. , Amokrane A. , Sieber B. , Farvacque J.-L. , Beaumont B. , Gibart P.
Published: July 2002

Photoluminescence and cathodoluminescence spectra are recorded on epitaxial GaN laterally overgrown on (0001) sapphire. Photon recycling, which influences the position of the near band edge transition, is evidenced in cathodoluminescence (CL) spectra by changing the accelerating voltage. CL monochromatic images recorded at different wavelengths show that dislocations act as efficient nonradiative recombination centers, and that they are not responsible for the yellow band.
The new ultraviolet spectral responsivity scale based on cryogenic radiometry at Synchrotron Ultraviolet Radiation Facility III
Authors: Shaw Ping-Shine , Larason Thomas C. , Gupta Rajeev , Brown Steven W. , Vest Robert E. , Lykke Keith R.
Published: July 2002

The recently completed upgrade of the Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and Technology (NIST) has improved the accuracy of radiometric measurements over a broad spectral range from the infrared to the soft x ray. The beamline 4 at SURF III is a cryogenic-radiometer based radiometric facility for the ultraviolet (UV) spectral range. The upgrade of SURF III has allowed us to use beamline 4 to improve the detector spectral power responsivity scales in the wavelength range from 125 to 320 nm. The achieved combined relative standard uncertainty is better than 0.5% over most of this spectral range. This is a significant improvement over the more than 6% relative standard uncertainty in this spectral range of the current scales maintained at the Spectral Comparator Facility (SCF) in the Optical Technology Division and the Far UV Calibration Facility in the Electron and Optical Physics Division. The new UV scale of beamline 4 was subsequently intercompared and transferred to the SCF and to the Far UV Calibration Facility to improve their UV scales and ensure consistency within NIST. The new scale established at beamline 4 improves NIST’s calibration capabilities for environmental monitoring, astrophysics, and the UV industry. The new scale also includes wavelengths such as 193 and 157 nm excimer laser wavelengths, which are of particular interest to the semiconductor photolithography industry.
Magnetoresistance and interlayer diffusion in PtMn spin valves upon postdeposition annealing
Authors: Kim Young Keun , Lee Seong-Rae , Song Se Ahn , Park Gyeong-Su , Yang Hyuck Soo , Min K.-I.
Published: July 2002

We report annealing time effects on the microstructural evolution and resultant magneto-transport property changes in Ta/NiFe/CoFe/Cu/CoFe/PtMn/Ta spin valves comprising PtMn layer thicknesses ranging from 10 to 30 nm. Postdeposition annealing was performed at 270 °C up to 35 h. The blocking temperatures of samples with 20 nm PtMn and 30 nm PtMn layers were found to be 350 °C and 400 °C, respectively. The magnetoresistance and interlayer coupling field changes became large as annealing time increased, in particular, for samples with relatively thicker PtMn layers. The main cause of microstructural changes and property degradation was due to interlayer diffusion of atomic constituents such as Mn, most likely through grain boundaries. Light B doping (1 at. %) in both free and pinned CoFe layers was proven effective in terms of blocking diffusion processes.
Structure of ammonia clusters from n=3 to 18
Authors: Beu Titus A. , Buck Udo
Published: July 2002

Optimized structures and bonding energies have been calculated for ammonia clusters from n=3 to n=18 using a pairwise additive model potential. The trimer and tetramer are stable cyclic configurations. From the pentamer onward the structures are three dimensional with an increasing tendency to amorphous behavior. The exceptions are the heptamer with a Cs axis, the hexadecamer with a central atom, and the very stable and completely symmetric dodecamer with the D6h point group. Here each ammonia molecule is bound by two covalent and two hydrogen bonds. In general, the coordination number increases from 2.0 for the rings over 4.0 for n=12 to 4.2 for n=18. The structures agree where available with previously obtained results for a more elaborate potential.
High-performance blue electroluminescent devices based on hydroxyphenyl-pyridine beryllium complex
Authors: Liu Yu , Guo Jianhua , Feng Jing , Zhang Huidong , Li Yanqin , Wang Yue
Published: July 2002

Blue light organic electroluminescent (EL) devices with high-performance were fabricated by using a highly fluorescent material, bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2). The double layer devices with a structure of [indium tin oxide/N, N′-di(α-naphthyl)-N-N′-diphenyl(1, 1′-biphenyl)-4, 4′-diamine (600 Å)/Bepp2 (500 Å)/LiF (10 Å)/Al (2000 Å)] exhibited a maximum luminance of 15 000 cd/m2 and a maximum electroluminescent efficiency of 3.43 lm/w (3.8 cd/A).
Elastic constants of yttria (Y2O3) monocrystals to high temperatures
Authors: Palko James W. , Kriven Waltraud M. , Sinogeikin Stanislav V. , Bass Jay D. , Sayir Ali
Published: July 2002

The single-crystal elastic moduli of yttria have been measured by Brillouin spectroscopy up to 1200 °C. The room temperature values obtained are C11=223.7±0.6 GPa, C44=74.6±0.7 GPa, and C12=112.4±1.1 GPa. The resulting bulk and (Voigt–Reuss–Hill) shear moduli are K=149.5±1.0 GPa and GVRH=66.3±0.8 GPa, respectively. These agree much more closely with experimental values reported for polycrystalline samples than do previous single-crystal measurements. Linear least squares regressions to the variation of bulk and shear moduli with temperature result in derivatives of dK/dT=−17±2 MPa/°C and dGVRH/dT=−8±2 MPa/°C. Elastic anisotropy was found to remain essentially constant over the temperature range studied.
Kinetic Monte Carlo simulation of titin unfolding
Authors: Makarov Dmitrii E. , Hansma Paul K. , Metiu Horia
Published: July 2002

Recently, it has become possible to unfold a single protein molecule titin, by pulling it with an atomic-force-microscope tip. In this paper, we propose and study a stochastic kinetic model of this unfolding process. Our model assumes that each immunoglobulin domain of titin is held together by six hydrogen bonds. The external force pulls on these bonds and lowers the energy barrier that prevents the hydrogen bond from breaking; this increases the rate of bond breaking and decreases the rate of bond healing. When all six bonds are broken, the domain unfolds. Since the experiment controls the pulling rate, not the force, the latter is calculated from a wormlike chain model for the protein. In the limit of high pulling rate, this kinetic model is solved by a novel simulation method. In the limit of low pulling rate, we develop a quasiequilibrium rate theory, which is tested by simulations. The results are in agreement with the experiments: the distribution of the unfolding force and the dependence of the mean unfolding force on the pulling rate are similar to those measured. The simulations also explain why the work of the force to break bonds is less than the bond energy and why the breaking-force distribution varies from sample to sample. We suggest that one can synthesize polymers that are well described by our model and that they may have unusual mechanical properties.
Active atomic force microscopy cantilevers for imaging in liquids
Authors: Buguin A. , Du Roure O. , Silberzan P.
Published: July 2002

We make an atomic force microscopy (AFM) cantilever oscillate by having an ac current circulating along it. When interacting with a permanent magnet, a normal force acts on the magnetic loop formed this way and induces its vibration. By choosing the current frequency at the resonance of the spring, this effect can be used for imaging in fluids in intermittent contact mode as demonstrated by an image of actin filaments in buffer. These active cantilevers can also be used in dc mode to reach a static position; they should be suitable for a parallel control of multiple probes.
Low-temperature activation of Mg-doped GaN using Ni films
Authors: Waki I. , Fujioka H. , Oshima M. , Miki H. , Fukizawa A.
Published: July 2002

The activation of metalorganic chemical vapor deposition-grown Mg-doped GaN by N2 annealing with a thin Ni film has been investigated. p-type conduction in GaN has been obtained at an annealing temperature as low as 200 °C using the proposed technique. A hole concentration of 2×1017 cm−3 has been achieved by the annealing at 400 °C. Secondary ion mass spectroscopy measurements have revealed that hydrogen is effectively removed from the Mg-doped GaN layer. These results suggest that the Ni film significantly enhances hydrogen desorption from the GaN film, which results in the activation of Mg-doped GaN at quite low temperatures.
Titanium sample holder for small-angle x-ray scattering measurements of supercritical aqueous solutions
Authors: Morita Takeshi , Kusano Kouhei , Nishikawa Keiko , Miyagi Hiroshi , Shimokawa Yuji , Matsuo Hitoshi
Published: July 2002

A titanium high-temperature sample holder for small-angle x-ray scattering (SAXS) was newly constructed. It is applicable to aqueous solutions in the supercritical state up to 750 K and 50 MPa. The use of high-tension titanium for assemblies and high-purity titanium for gaskets enables us to apply the holder to fluids in extreme conditions such as supercritical water, supercritical aqueous solutions, and other corrosive hydrothermal aqueous solutions. Details are presented for the diamond window sealed by a flange set made of titanium. The seal is superior to the conventional unsupported-area-seal by a screw cap and plug for the titanium high-temperature sample holder. As a test of the instrument, the SAXS experiments for supercritical water were made at the isothermal condition of T=662 K with pressures from 22.8 to 29.3 MPa. The first SAXS measurements for supercritical aqueous solution were also carried out.
X-ray absorption near-edge structure in alpha-quartz and stishovite: Ab initio calculation with core–hole interaction
Authors: Mo Shang-Di , Ching W. Y.
Published: July 2002

Ab initio calculation of the XANSE/ELNES spectra for α quartz and stishovite were carried out using a large-supercell approach that includes the electron–core–hole interaction. Excellent agreements with experimental spectra were obtained for Si–K, Si–L2,3, and O–K edges. The usual interpretation using orbital-resolved local density of states in the conduction band is unsatisfactory.
Sharp photoluminescence of CdS nanocrystals in Al2O3 matrices formed by sequential ion implantation
Authors: Ando M. , Kanemitsu Y. , Kushida T. , Matsuda K. , Saiki T. , White C. W.
Published: July 2002

We report on photoluminescence (PL) experiments in CdS nanocrystals fabricated by sequential ion implantation in Al2O3 matrices. The PL spectrum and the spatial image of the PL intensity have been studied at 8 K using a scanning near-field optical microscope. The PL spectrum at each bright spot has been found to consist of narrow lines of various energies, although the spectrum measured by conventional optics shows a single and broad band locating below the free-exciton absorption energy. The origin of the sharp PL lines in CdS nanocrystals will be discussed.
Stark effect and polarizability in a single CdSe/ZnSe quantum dot
Authors: Seufert J. , Obert M. , Scheibner M. , Gippius N. A. , Bacher G. , Forchel A. , Passow T. , Leonardi K. , Hommel D.
Published: July 2002

The quantum-confined Stark effect in a single self-assembled CdSe/ZnSe quantum dot was studied by means of highly spatially resolved photoluminescence spectroscopy. A nanotechnological approach making use of a capacitor-like geometry enabled us to apply a well-defined lateral electric field on the quantum dots. Stark shifts of up to 1.1 meV were obtained, which can be well fitted by a purely quadratic dependence on an electric field. In quite good agreement with theoretical calculations, an exciton polarizability of 4.9×10−3 meV/(kV/cm)2 can be extracted, while the permanent dipole moment in the lateral direction is found to be negligible.
Green function method for nonlinear elastic waves in layered media
Authors: Lobo A. E. , Tsoy E. N. , Martijn de Sterke C.
Published: July 2002

We construct the Green function for an elastic medium with a plane source. This Green function is particularly helpful in the analysis of weakly nonlinear problems, such as occur in nondestructive acoustic evaluation of bonded structures. By using a perturbative approach we describe the amplitude distribution of the second harmonic of the driving frequency by linear equations with sources. In deriving the Green function we include both longitudinal and transverse polarizations and consider an arbitrary direction of the wave vector with respect to the layer normal. We show how this Green function can be combined with the transfer matrix approach to calculate the fields in layered structures. The examples of second harmonic generation from half space and a single layer are considered.
Robust chaos in a model of the electroencephalogram: Implications for brain dynamics
Authors: Dafilis Mathew P. , Liley David T. J. , Cadusch Peter J.
Published: July 2002

Various techniques designed to extract nonlinear characteristics from experimental time series have provided no clear evidence as to whether the electroencephalogram (EEG) is chaotic. Compounding the lack of firm experimental evidence is the paucity of physiologically plausible theories of EEG that are capable of supporting nonlinear and chaotic dynamics. Here we provide evidence for the existence of chaotic dynamics in a neurophysiologically plausible continuum theory of electrocortical activity and show that the set of parameter values supporting chaos within parameter space has positive measure and exhibits fat fractal scaling.
Thermal diffusivities in DIII-D show evidence of critical gradients
Authors: Baker D. R. , Greenfield C. M. , Burrell K. H. , DeBoo J. C. , Doyle E. J. , Groebner R. J. , Luce T. C. , Petty C. C. , Stallard B. W. , Thomas D. M. , Wade M. R. ,   DIII-D Team
Published: July 2002

The ion thermal diffusivities (χi) in DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] discharges exhibit a strong nonlinear dependence on the measured temperature gradients. In low confinement mode (L-mode) discharges with low toroidal rotation, the ion thermal diffusivity, χi, has an approximately Heaviside function dependence on the major radius divided by the radial scale length of the ion temperature, R/LTi. When R/LTi is less than a critical value, the χi’s are very small. When R/LTi is about equal to the critical value, χi increases rapidly. Although the gradient profiles for high confinement (H-mode) have a different shape, they still show a critical gradient type of behavior. This type of dependence is consistent with the predictions for transport, which is dominated by ion temperature gradient modes and is a strong indicator that these modes are the main contributors toward L-mode transport in DIII-D and a major contributor to transport in a certain region of DIII-D H-mode discharges. When strong rotational shear is present, the thermal confinement is improved in regions of the plasma. In these regions, the dependence of the diffusivities on the gradients is changed. The type of change is consistent with the physical picture that when the E×B shearing frequency is greater than the maximum linear growth rate of the modes as calculated without shear, then the modes are stabilized and the transport is reduced.
Field emission from short and stubby vertically aligned carbon nanotubes
Authors: Chhowalla M. , Ducati C. , Rupesinghe N. L. , Teo K. B. K. , Amaratunga G. A. J.
Published: July 2002

Electron emission from vertically aligned carbon nanotubes grown by plasma enhanced chemical vapor deposition has been measured using a parallel plate anode and a 1 μm tungsten probe. The field emission characteristics were measured as a function of the nanotube diameter, length, and areal density. It was found that less densely populated “short and stubby” nanotubes with diameters of 200 nm and heights of 0.7 μm showed the best emission characteristics with a threshold voltage of 2 V/μm and saturation emission current density of 10 mA/cm2. A triple junction between nanotube, substrate, and vacuum is proposed to explain our results.
Low-resistance nonalloyed ohmic contact to p-type GaN using strained InGaN contact layer
Authors: Kumakura Kazuhide , Makimoto Toshiki , Kobayashi Naoki
Published: July 2002

A strained InGaN contact layer inserted between Pd/Au and p-type GaN resulted in low ohmic contact resistance without any special treatments. The thickness and In mole fraction of the p-type InGaN varied from 2 nm to 15 nm and from 0.14 to 0.23, respectively. Strained InGaN layers are effective in reducing the contact resistance. A contact layer of 2 nm thick strained In0.19Ga0.81N showed the lowest specific contact resistance of 1.1×10−6 Ω cm2. The mechanism for the lower contact resistance is ascribed to enhanced tunneling transport due to large polarization-induced band bending at the surface as well as to the high hole concentration in p-type InGaN.
560-nm-continuous wave laser emission from ZnSe-based laser diodes on GaAs
Authors: Klude M. , Hommel D.
Published: July 2002

Room-temperature continuous wave (cw)-lasing emission at wavelengths around 560 nm was obtained from ZnSe-based laser diodes grown on GaAs substrates. The wavelengths of the devices are most suitable as lightsources for plastic optical fibers. To achieve this emission wavelength, CdZnSSe quantum wells with high Cd content were employed as active region. The growth of such quantum wells requires Se-rich growth conditions.
Phase diagrams classification of thermoreversibly associating systems with due regard for mesoscopic cyclization effects
Authors: Erukhimovich Igor , Ermoshkin Alexander V.
Published: July 2002

We consider systems of f-functional monomers Af, capable of thermoreversible associating with an equilibrium Arrhenius association constant k=g0 exp(−E/T). Effects of the parameters f, g0, and E on the global phase behavior of these systems are analyzed within two theoretical approaches differing in the way to allow for the presence of closed trails of labile bonds (cycles). Within the Flory approximation, which takes into account the trails closed at infinity only (i) sol–gel transition (emergence of the infinite cluster of labile bonds) is only a geometric transition; (ii) the phase diagrams with one, two, or three critical solution temperatures could exist; (iii) there are no diagrams with three phase coexistence temperatures (triple points). On the contrary, as consistent within our new mesoscopic cyclization (MsC) approach (i) the sol–gel transition is a genuine first order phase transition accompanied by a heat effect and phase separation; (ii) the phase diagrams are more varied and could possess triple points as well as some other peculiarities. An explicit topological classification of all types of phase diagrams is given for both approaches via building (i) the phase portraits, i.e., separation of the plane (ln g0,E) into the regions corresponding to topologically similar phase diagrams, and (ii) typical phase diagrams on the (volume fraction, T) and (pressure, T) planes for all regions of these phase portraits. For MsC approach, the latent phase transition heats are also presented. Possible changes of the presented phase diagram classification for more complicated models (in particular, for systems with concurrent association) are discussed.
Electrode modification by electron-induced patterning of aromatic self-assembled monolayers
Authors: Felgenhauer T. , Yan C. , Geyer W. , Rong H.-T. , Gölzhäuser A. , Buck M.
Published: July 2002

Self-assembled monolayers of ω-(4′-methyl-biphenyl-4-yl)-dodecyl thiol [CH3–C6H4-C6H4–(CH2)12–SH,BP12] on gold were patterned via exposure to 300 eV electrons. Subsequent copper deposition in an electrochemical cell revealed behavior opposite to that of electron beam patterned monolayers of alkanethiols. Whereas alkanethiols act as a positive resist and lead to copper deposition only on irradiated parts, the biphenyl based thiol acts as a negative resist. At the irradiated areas the layer exhibits blocking behavior and copper deposition is observed only on the nonirradiated parts.
Three-dimensional codes to design stellarators
Authors: Garabedian Paul R.
Published: July 2002

Equilibrium, stability, and transport in stellarators can be studied theoretically by running large computer codes. Some of the codes have excellent resolution, and experimental data have been used to validate the results of numerical calculations. An analysis has been made of recent measurements from the Large Helical Device experiment in Japan to see how they compare with the theory [A. Komori, H. Yamada, O. Kaneko et al., Plasma Phys. Control. Fusion 42, 1165 (2000)]. Observations of confinement time at low collisionality like that in a reactor agree well with estimates using a quasineutrality algorithm to determine the electric potential. A nonlinear magnetohydrodynamic calculation of stability for various pressure profiles gives predictions of the beta limit that are consistent with the observations. Correlation of the theory with measurements justifies using the codes as a tool to design quasisymmetric configurations for a modular stellarator experiment promising better performance at reactor conditions.
Insulating behavior for DNA molecules between nanoelectrodes at the 100 nm length scale
Authors: Storm A. J. , van Noort J. , de Vries S. , Dekker C.
Published: July 2002

Electrical transport measurements are reported for double-stranded DNA molecules located between nanofabricated electrodes. We observe the absence of any electrical conduction through these DNA-based devices, both at the single-molecule level as well as for small bundles of DNA. We obtain a lower bound of 10 TΩ for the resistance of a DNA molecule at length scales larger than 40 nm. It is concluded that DNA is insulating. This conclusion is based on an extensive set of experiments in which we varied key parameters such as the base-pair sequence [mixed sequence and homogeneous poly(dG)⋅poly(dC)], length between contacts (40–500 nm), substrate (SiO2 or mica), electrode material (gold or platinum), and electrostatic doping fields. Discrepancies with other reports in the literature are discussed.
High-performance InAs quantum-dot lasers near 1.3 μm
Authors: Qiu Y. , Gogna P. , Forouhar S. , Stintz A. , Lester L. F.
Published: July 2002

High-performance quantum dot (QD) lasers near 1.3 μm were fabricated using four stacks of InAs QDs embedded within strained InGaAs quantum wells as an active region and a reactive-ion-etched 5-μm-ridge waveguide design. For a 1.5-mm-long cavity QD laser, ground-state continuous-wave (cw) lasing has been achieved with a single facet output power of 15 mW at temperatures as high as 100 °C, while at room temperature having a differential quantum efficiency of 55% and a single facet output power of 50 mW. The characteristic temperature T0 for ground-state cw lasing is 78 K up to our temperature measurement limit of 100 °C.
Influence of molecular weight on the photorefractivity of polymer/liquid crystal composites
Authors: Bai Yaowen , Chen Xiaofang , Wan Xinhua , Zhou Qi-feng , Liu Hui , Zhang Bo , Gong Qihuang
Published: July 2002

We report on photorefractive composites composed of poly(N-vinyl carbazole) (PVK)/liquid crystal/C60 with PVK of different molecular weight (MW). The influence of MW on the photorefractivity was studied. The results show that a higher MW leads to a larger two-beam coupling coefficient Γ and a larger effective electric–optical coefficient. Γ over 200 cm−1 is observed for the composite based on the highest MW PVK.
Phase separation of a binary liquid mixture in porous media studied by nuclear magnetic resonance cryoporometry
Authors: Valiullin Rustem , Furó István
Published: July 2002

H 1 nuclear magnetic resonance cryoporometry has been applied to study the low-temperature phase separation of the two components in a binary liquid imbibed in porous glass. The mixture of hexane and nitrobenzene was quenched below both its upper critical temperature and the freezing point of the nitrobenzene. The size distribution of phase-separated liquid domains was observed through their melting point suppression that reveals small droplets of nitrobenzene surrounded by hexane within the pores. If the bottlenecks of the porous network allow, some of these droplets coalesce and thereby completely fill parts of the network.
Multi-Lagrangians for integrable systems
Authors: Nutku Y. , Pavlov M. V.
Published: July 2002

We propose a general scheme to construct multiple Lagrangians for completely integrable nonlinear evolution equations that admit multi-Hamiltonian structure. The recursion operator plays a fundamental role in this construction. We use a conserved quantity higher/lower than the Hamiltonian in the potential part of the new Lagrangian and determine the corresponding kinetic terms by generating the appropriate momentum map. This leads to some remarkable new developments. We show that nonlinear evolutionary systems that admit N-fold first order local Hamiltonian structure can be cast into variational form with 2N−1 Lagrangians which will be local functionals of Clebsch potentials. This number increases to 3N−2 when the Miura transformation is invertible. Furthermore we construct a new Lagrangian for polytropic gas dynamics in 1+1 dimensions which is a free, local functional of the physical field variables, namely density and velocity, thus dispensing with the necessity of introducing Clebsch potentials entirely. This is a consequence of bi-Hamiltonian structure with a compatible pair of first and third order Hamiltonian operators derived from Sheftel’s recursion operator.
Scaling laws and intermittency in homogeneous shear flow
Authors: Gualtieri P. , Casciola C. M. , Benzi R. , Amati G. , Piva R.
Published: July 2002

In this article we discuss the dynamical features of intermittent fluctuations in homogeneous shear flow turbulence. In this flow the energy cascade is strongly modified by the production of turbulent kinetic energy related to the presence of vortical structures induced by the shear. As a consequence, the intermittency of velocity fluctuations increases with respect to homogeneous and isotropic turbulence. By using direct numerical simulations, we show that the refined Kolmogorov similarity is broken and a new form of similarity is observed, in agreement with previous results obtained in turbulent boundary layers. We find here that the statistical properties of the energy dissipation are practically unchanged with respect to homogeneous isotropic conditions, while the increased intermittency is entirely captured in terms of the new similarity law.
Optical transport of a single cell-sized liposome
Authors: Ichikawa Masatoshi , Yoshikawa Kenichi
Published: July 2002

A closed phospholipid membrane vesicle, or giant liposome, with a diameter of ∼10 μm, constitutes a model of a living cell. Since the phospholipid membrane is about 5 nm thick, it is extremely difficult to trap an individual giant liposome with a laser. We report here the intact transport of an individual cell-sized liposome using an infrared laser. It is shown that the optical attractive potential on a liposome with an inner solution of higher refractivity is about one order of magnitude greater than that of a liposome with the same solution both inside and outside. Since this cell-sized liposome is rather stable, the optical manipulation of liposomes could be useful for the realization of a μm-scaled laboratory for use in biochemistry and molecular biology.
A simple pulsed CO2 laser with long milliseconds pulse duration
Authors: Chung Hyun-Ju , Lee Dong-Hoon , Hong Jung-Hwan , Joung Jong-Han , Sung Youl-Moon , Park Sung-Joon , Kim Hee-Je
Published: July 2002

We demonstrate a simple pulsed CO2 laser with long pulse duration of millisecond order in a tube at a low pressure of less than 30 Torr. The power supply for our laser system switches the voltage of the ac power line (60 Hz) directly. The power supply does not need elements such as a rectifier bridge, energy-storage capacitors, or a current-limiting resistor in the discharge circuit. In order to control the laser output power, the pulse repetition rate is adjusted up to 60 Hz and the firing angle of the silicon controlled rectifier (SCR) gate is varied from 30° to 150°. The maximum laser output of 35 W is obtained at a total pressure of 18 Torr, a pulse repetition rate of 60 Hz, and a SCR gate firing angle of 90°. In addition, the resulting laser pulse width is approximately 3 ms (full width at half maximum). This is a relatively long pulse width, compared with other repetitively pulsed CO2 lasers.