bluuebunny/crossref_metadata_2025 huggingface dataset:
A pseudorandom pink noise for the computer-based measurements of linear responses
Authors: Kinugawa Tohru , Sakurai Katsumi , Mitsui Takahisa
Published: July 2002

We propose a nonbinary pseudorandom sequence for the measurement of linear responses. Compared with a maximal length shift register sequence (m sequence), this sequence is more suitable for digital processing based on computers; with the use of discrete Fourier transforms, the response functions are reproduced without approximations from digitally sampled data because the input power spectrum is pink, i.e., completely flat in a given frequency range and 0 otherwise. In practice, the new sequence is simply the sum of harmonics with arbitrary phases and is produced readily with digital wave form generators. For reducing the peak power of this sequence, the amplitude distribution is better to be bimodal rather than Gaussian. For demonstrating its feasibility with common digital hardware, the magnetic resonance of Rb atoms in a sub-MHz region was measured successfully. With the use of the fast Fourier transform algorithm, our software task was only 0.4% of that for the cross-correlational calculation based on an m sequence.
Photorefractive coherence-gated interferometry
Authors: Shiratori Akira , Obara Minoru
Published: July 2002

We have demonstrated a novel optical interferometric technique for a highly scattering environment, using photorefractive coherence gating. A two-wave mixing process in a photorefractive barium titanate crystal effectively reduces scattering noise, and we have successfully detected interferometric signals through scattering media of up to 17 mfp, with a high visibility. We have also demonstrated a two-dimensional thickness measurement of an optically scattering thin film using our new interferometry.
Automatic coupling control system for radio frequency in vivo electron paramagnetic resonance based on a piezoelectric controlled capacitor
Authors: McCallum Stephen , Resmer Frank
Published: July 2002

Electron paramagnetic resonance (EPR) experiments on whole animals present a number of challenging problems. With the animal placed inside the resonant structure, respiratory and cardiac motion cause the resonant frequency and coupling to change dynamically. We have developed an automatic coupling control (ACC) system designed to reduce motion artifacts using an electronically controlled variable capacitor based on a piezoelectric actuator. The capacitor is varied between 11 and 21 pF. We have also developed a novel method for detecting coupling mismatches. The ACC system operates by applying a small modulation signal to the piezo-capacitor control voltage, which in turn modulates the coupling and the reflected signal from the resonator. The phase of the modulation signal, at the output of the diode detector, depends on whether the resonator is under- or overcoupled. We have tested the system with a phantom designed to simulate respiratory motion. The results reported here show that the use of the ACC system in combination with an automatic frequency control can dramatically reduce motion-induced artifacts on the EPR spectrum.
Detection of single macromolecules using a cryogenic particle detector coupled to a biopolymer mass spectrometer
Authors: Twerenbold Damian , Vuilleumier Jean-Luc , Gerber Daniel , Tadsen Almut , van den Brandt Ben , Gillevet Patrick M.
Published: July 2002

Macromolecules with masses up to 50 kDa have been detected with a cryogenic particle detector in a MALDI time-of-flight biopolymer mass spectrometer. The cryogenic particle detector was a Sn/Sn-ox/Sn tunnel junction operated at a temperature of 0.4 K. A calibration with 6 keV single photons inferred that the delayed detector pulses corresponded to the absorption of the kinetic energy of a single macromolecule. Time-of-flight spectra of lysozyme proteins are presented. The mass resolution is 100 Da at 14 300 Da. The energy sensitive detection mechanism suggests that cryogenic particle detectors have a high and mass independent detection efficiency for macromolecules.
Second-harmonic generation in a LiTaO3 waveguide domain-inverted by proton exchange and masked heat treatment
Authors: Yi Sang-Yun , Shin Sang-Yung , Jin Yong-Sung , Son Yung-Sung
Published: July 2002

Second-harmonic generation in the proton-exchanged LiTaO3 waveguide is greatly enhanced by reducing the amount of proton exchange in fabricating the domain grating and the waveguide. To reduce the amount of proton exchange, a Ta-SiO2 mask is used for the periodic domain inversion and a SiO2 mask for the proton-exchanged waveguide. The fabricated device generates second-harmonic blue light of 1 mW at 429 nm with the fundamental wave power of 10.3 mW. Its normalized efficiency is 1500 %/Wcm2, which is the highest value for LiTaO3 waveguide devices reported to date.
The role of Si3N4 layers in determining the texture of sputter deposited LiNbO3 thin films
Authors: Tan S. , Schlesinger T. E. , Migliuolo M.
Published: July 2002

We report on the role of amorphous silicon nitride layers in determining the crystallographic orientation of lithium niobate sputter deposited on a layered substrate. In a structure consisting of LiNbO3/SiNx/SiO2/Si we show that details of the sputtering conditions of the SiNx, in particular, the nitrogen flow rate determine not only the deposition rate and index of refraction of the SiNx but also the orientation of the LiNbO3 subsequently deposited on it. Conditions that yield entirely c-axis LiNbO3 are reported and reasons for these observations are discussed.
Formation of quasicrystals in bulk glass forming Zr–Cu–Ni–Al alloys
Authors: Köster U. , Meinhardt J. , Roos S. , Liebertz H.
Published: July 2002

Zr65Cu17.5Ni10Al7.5 as well as Zr69.5Cu12Ni11Al7.5 belong to the best glass forming alloys known. Glass transition temperatures of melt-spun ribbons are 372 and 360 °C, respectively. TEM and x-ray analysis of samples annealed above the glass transition temperature exhibit the formation of quasicrystalline microstructures with small amounts of crystalline phases. The metastable icosahedral phase is primitive with a quasilattice constant a=0.253 nm; its composition as determined by EDX is close to Zr69.5Cu12Ni11Al7.5. In both glasses, growth of the quasicrystals has been observed to be time-dependent (r∝t1/2), thus indicating a diffusion controlled transformation.
The effect of H2 on morphology evolution during GaN metalorganic chemical vapor deposition
Authors: Han J. , Ng T.-B. , Biefeld R. M. , Crawford M. H. , Follstaedt D. M.
Published: July 2002

In situ optical reflectance transients reveal that the morphology evolution of the initial low-temperature buffer layer strongly influences the structural and electrical quality of the high-temperature GaN films. Moreover, the morphology evolution of that buffer layer, specifically evolution of the spatial and orientational distributions of the nuclei, is strongly affected by H2. The growth conditions for which surface smoothness is maintained throughout the two-step growth do not necessarily produce the best quality final GaN films; instead, there may be an optimal roughness and incubation period en route to the best quality final films.
Channels of Cd diffusion and stoichiometry in CdTe grown by molecular beam epitaxy
Authors: Barcz A. , Karczewski G. , Wojtowicz T. , Sadlo M. , Kossut J.
Published: July 2002

By studying the diffusion of specially incorporated thin Mn markers in molecular beam epitaxy-grown CdTe, we can investigate quantitatively deviations from stoichiometry as well as the details of Cd diffusion in the crystal. In CdTe layers deficient in Cd, the diffusion proceeds through VCd vacancies, with the activation energy of 2.1 eV, characteristic for bulk CdTe. In CdTe grown in excess Cd flux, the evaluated activation energy of 1.4 eV for Cd self-diffusion is characteristic to Te self-diffusion in bulk CdTe, which implies that the flow of Cd atoms is mediated by VTe vacancies with formation of a virtual CdTe antisite defect. A striking correlation of the occurrence of the minimum of electrical resistivity in In-doped CdTe with nearly perfect stoichiometry with the minimum of the diffusivity of Mn provides further support of this interpretation.
Cylindrical microlasers and light emitting devices from conducting polymers
Authors: Frolov S. V. , Fujii A. , Chinn D. , Vardeny Z. V. , Yoshino K. , Gregory R. V.
Published: July 2002

Substantially improved, photopumped polymer lasers are demonstrated using microrings and microdisks of various diameters D ranging from 5 to 200 μm. Various cavity-dependent laser modes were observed, which for D<10 μm were dominated by a single longitudinal mode with linewidth of less than 1 Å. These microlasers were also characterized by Q of order 5000, low threshold excitation energy of order 100 pJ/pulse for pulse duration ranging from 100 ps to sub-μs, and an abrupt increase in the emission directionality and polarization degree. Light emitting diodes with cylindrical geometry, fully compatible with these microlasers are also demonstrated.
Thickness dependence of dielectric loss in SrTiO3 thin films
Authors: Li Hong-Cheng , Si Weidong , West Alexander D. , Xi X. X.
Published: July 2002

We have measured the dielectric loss in SrTiO3 thin films grown on SrRuO3 electrode layers with thickness ranging from 25 nm to 2.5 μm. The loss depends strongly on the thickness but differently above and below T≈80 K: as the thickness increases, the loss decreases at high temperatures but becomes higher at low temperatures. Our result suggests that, in the high temperature regime, the interfacial dead layer effect dominates while, in the low temperature regime, the losses related to the structural phase transition and quantum fluctuations are important.
Micromechanical properties of silica aerogels
Authors: Moner-Girona M. , Roig A. , Molins E. , Martı́nez E. , Esteve J.
Published: July 2002

Aerogels are nanostructured highly porous solids that present properties which are very different from other materials, i.e., extremely low densities. In this letter, the mechanical properties of aerogels have been studied with a nondestructive microindentation technique. This technique enables the continuous measurement of load-displacement curves during loading and unloading cycles by using very small indentation loads (∼1 mN), small enough to prevent cracking of the aerogels. The samples studied show two different types of mechanical behaviors; the low-density aerogels are elastic, while the denser aerogels behave as elastoplastic materials. Young’s modulus, hardness, and the elastic parameter have been evaluated for these aerogel samples. Power function relationships have been found between these mechanical parameters and the aerogel densities.
Room-temperature lasing via ground state of current-injected vertically aligned InP/GaInP quantum dots
Authors: Manz Y. M. , Schmidt O. G. , Eberl K.
Published: July 2002

We report room-temperature injection lasing of self-assembled InP/GaInP quantum dots. Stimulated emission occurs via the ground state at λ=728 nm for cavities as short as 0.5 mm. Threshold current densities of 2.3 kA/cm2 and external differential quantum efficiencies of 8.5% have been measured for 2 mm long devices. Light output power as high as 250 mW without saturation effects can be reached in pulsed excitation. Analysis of temperature-dependent laser emission indicates the thermal coupling of charge carriers in different quantum dots at higher temperatures.
Metastable states of negative carbon clusters: Cn−, n=2–6
Authors: Naaman A. , Bhushan K. G. , Pedersen H. B. , Altstein N. , Heber O. , Rappaport M. L. , Moalem R. , Zajfman D.
Published: July 2002

Time-dependent photodetachment spectra for small electronically and vibrationally excited negatively charged carbon clusters Cn− (n=2–6) are measured using an electrostatic ion trap. The time dependence demonstrates the presence of metastable electronic states with lifetimes in the range of 10 to 200 ms. Comparison is made with available data and theoretical calculations.
Dielectric relaxation in poly(styrene-b-butadiene) copolymers with perfluorinated side chains
Authors: Floudas G. , Antonietti M. , Förster S.
Published: July 2002

The dynamics of several poly(styrene-b-butadiene) (PS-PB) copolymers with fluorocarbon side chains have been studied with dielectric spectroscopy, covering two different mesophases with long spacings between 10 and 30 nm, and two different tail lengths. It is found that fluorination results in comparably rigid side chains in the PB domains with highly anisotropic properties. In the glassy state an Arrhenius relaxation was found with an activation energy of 7.4 kcal/mol reflecting some local side chain relaxation. Above the PB glass transition a bimodal relaxation exists, comprising two cooperative processes reflecting solely the anisotropic dynamics of the side chain. The dual relaxation resembles the anisotropic dielectric properties of side chain liquid crystalline polymers in the absence of macroscopic orientation.
Enhanced transmission of laser light through thin slabs of overdense plasmas
Authors: Cairns R. A. , Rau B. , Airila M.
Published: July 2002

Enhanced transmission of electromagnetic radiation through overdense, hot, unmagnetized plasmas has been observed via particle in cell (PIC) simulations. This effect is explained in terms of a fraction of the thermal electrons oscillating in the electrostatic potential of the plasma slab ions at a frequency close to the laser frequency. These electrons are capable of resonantly transporting transverse electric currents across the plasma. As these electrons reach the far end of the slab, the currents cause electromagnetic radiation at the laser frequency to be given off.
Photopyroelectric thermal wave setup for the absolute measurement of the thermal conductivity of low density gases
Authors: Caerels J. , Glorieux C. , Thoen J.
Published: July 2002

A photopyroelectric setup for the accurate absolute measurement of the thermal conductivity of dilute gases is presented. The technique results from a modification of our previously described [Rev. Sci. Instrum. 69, 2452 (1998)] technique for the simultaneous measurement of absolute values of specific heat capacity and thermal conductivity of liquids. Measurement results are presented for different gases at room temperature and at one atmosphere as well as for the dilute Knudsen gas regime at reduced pressure. It is also demonstrated that the setup can be operated as a sensitive pressure gauge with adaptable sensitivity range.
Hosted particle positions and dipole moments of protein-filled reverse micelles
Authors: Chen Peilong , Tsao Heng-Kwong , Lu C.-Y. David
Published: July 2002

The nonlinear Poisson–Boltzmann equation is solved numerically in reverse water-in-oil droplet microemulsions with globular charged particles located at arbitrary positions inside the droplets. System free energies are calculated for both mono and dipole charged particles which are usually protein molecules. We find that average particle locations inside the droplets are always off center. Droplet dipole moments induced by the off center particles are also computed. Inter-micelle dipole–dipole interaction is estimated to be at the right magnitude to account for some of the percolation threshold decrease observed experimentally. © 2000 American Institute of Physics.
Feynman diagrams of generalized matrix models and the associated manifolds in dimension four
Authors: De Pietri Roberto , Petronio Carlo
Published: July 2002

The problem of constructing a quantum theory of gravity has been tackled with very different strategies, most of which rely on the interplay between ideas from physics and from advanced mathematics. On the mathematical side, a central role is played by combinatorial topology, often used to recover the space–time manifold from the other structures involved. An extremely attractive possibility is that of encoding all possible space–times as specific Feynman diagrams of a suitable field theory. In this work we analyze how exactly one can associate combinatorial four-manifolds with the Feynman diagrams of certain tensor theories.
A new Fourier path integral method, a more general scheme for extrapolation, and comparison of eight path integral methods for the quantum mechanical calculation of free energies
Authors: Mielke Steven L. , Truhlar Donald G.
Published: July 2002

Using an isomorphism of Coalson, we transform five different discretized path integral (DPI) methods into Fourier path integral (FPI) schemes. This allows an even-handed comparison of these methods to the conventional and partially averaged FPI methods as well as a new FPI method. It also allows us to apply to DPI methods a simple and highly effective perturbative correction scheme (previously presented for FPI methods) to account for the error due to retaining only a finite number of terms in the numerical evaluation of the propagator. We find that in all cases the perturbative corrections can be extrapolated to the convergence limit with high accuracy by using a correlated sequence of affordable calculations. The Monte Carlo sampling variances of all eight methods studied are very similar, but the variance of the perturbative corrections varies markedly with method. The efficiencies of the new FPI method (called rescaled fluctuation FPI) and one of Fourier analog methods compare favorably with that of the original FPI method. The rescaled fluctuation method not only proves practically successful, but it also gives insight into the origin of the dominant error in the conventional FPI scheme.
Vibrationally autoionizing Rydberg clusters: Spectroscopy and dynamics of pyrazine–Ar and –Xe clusters
Authors: Fujii Asuka , Kitamura Yutaka , Mikami Naohiko
Published: July 2002

Vibrational autoionization spectra of high Rydberg states of pyrazine–Ar and –Xe van der Waals clusters were observed by two-color double resonance spectroscopy. Two Rydberg series converging to the same ionization threshold appeared in the spectra of both the clusters, while only one Rydberg series was seen in bare pyrazine. One of the series of the clusters was assigned to be of “gerade,” which is the same Rydberg series as that found in bare pyrazine. The other series of the clusters was assigned to an “ungerade” series, suggesting that the symmetry breakdown of the ion core is induced by the cluster formation. For both the Rydberg series, apparently very small quantum defects were involved, and the “gerade” and “ungerade” series were tentatively assigned to the d (or s) and f Rydberg series, respectively. In comparison with the bare molecule, the quantum defects of the clusters exhibited slight shifts to the negative direction, indicating the decrease of the binding energy of the Rydberg electron. The vibrational autoionization efficiency does not change upon the cluster formation, even above the dissociation threshold of the van der Waals bond. This fact indicates that the vibrational autoionization rate is much faster than the vibrational predissociation rate.
Accurate atomic line wavelengths from astronomical sky spectra
Authors: Slanger T. G. , Huestis D. L. , Cosby P. C. , Osterbrock D. E.
Published: July 2002

Numerous atomic lines appearing in the terrestrial nightglow can be measured by HIRES, the echelle spectrometer on the 10 meter Keck I telescope on Mauna Kea. The observable nightglow lines include emissions from Na, K, Hg, Ne, N, O, and H. Agreement between the line positions and those from National Institute of Science and Technology (NIST) compilations is excellent for well-known lines, typically 2 mÅ or better. For lines which are not well-known, or cannot be measured directly in the laboratory, deviations are significant. In particular, for the optically forbidden N(2D–4S) transition the differences are substantial, 20 and 24 mÅ for the two components. Apart from improving the line positions for this transition, we also correct an error that has been perpetuated in the aeronomic literature for the last 30 years concerning the transition wavelengths. The potassium D1 line, recently discovered in the HIRES sky spectra, exhibits a position difference of 11 mÅ between the NIST and HIRES values, significant for astronomical applications. The HIRES value is shown to be in close agreement with the latest laboratory study. For the Balmer series of H lines, we report the first ground-based observation of Hγ in the geocorona, and we confirm that the positions of the H(α,β,γ) lines agree with the expectation that the principal source is direct solar resonance excitation, with a 5%–6% cascading contribution in the case of Hα. The absolute average intensities of the three lines are 3600, 900, and ∼210 millirayleighs (mR), respectively.
The origin of Λ-doubling effect for the B 1∏ and D 1∏ states of NaK
Authors: Adamson S. O. , Zaitsevskii A. , Pazyuk E. A. , Stolyarov A. V. , Tamanis M. , Ferber R. , Cimiraglia R.
Published: July 2002

The origin of Λ-doubling effect (q factors) for the regularly perturbed NaK B 1∏ and D 1∏ states has been investigated by means of ab initio many-body multipartitioning perturbation theory calculation of the electronic L-uncoupling matrix elements between the examined ∏1 and five lowest ∑+1 states for both Na3923K and Na4123K isotopomers. The hypothesis of pure precession was found to be valid for the B 1∏–A 1∑+ and D 1∏–C 1∑+ pairs of the interacted states, while the unique perturber approximation works properly for the D 1∏ state and completely breaks down for the B 1∏ state. The theoretical rovibronic q-factor values estimated for both states by means of the approximate sum rule agree well with their experimental counterparts and demonstrate high sensitivity to vibrational and rotational quantum numbers.
Dislocation energetics in epitaxial strained islands
Authors: Spencer B. J. , Tersoff J.
Published: July 2002

We calculate (to first order in the Burgers vector) the energy of introducing dislocations into Stranski-Krastanow islands having equilibrium shape. These results suggest that misfit dislocations of different Burgers vector tend to segregate spatially within the islands, which may be a source of mosaic structure in films formed by island coalescence.
Random walk and diffusion of hard spherical particles in quenched systems: Reaching the continuum limit on a lattice
Authors: Mercier Jean-François , Slater Gary W.
Published: July 2002

Lattice Monte Carlo methods are widely used to study diffusion problems such as the random walk of a probe particle among fixed obstacles. However, the diffusion coefficient D found with such methods generally depends on the type of lattice used. In order to obtain experimentally relevant results, one often needs to consider the continuum limit, i.e., the limit where the size of the lattice parameter is infinitely small compared to the size of both the probe particle and the obstacles. A numerical procedure to reach this limit for a single particle diffusing between quenched impenetrable obstacles is presented. As an example, the case of a system of periodic spherical obstacles is treated and a general relation between the diffusion coefficient D, the total obstructed volume f, and the dimensionality d of the problem is proposed.
Smallest diameter carbon nanotubes
Authors: Peng H. Y. , Wang N. , Zheng Y. F. , Lifshitz Y. , Kulik J. , Zhang R. Q. , Lee C. S. , Lee S. T.
Published: July 2002

Mass-selected carbon ion beam deposition (MSIBD) was used to demonstrate that the diameter of a carbon nanotube could be as small as 0.4 nm, the theoretical limit predicted but never experimentally reached so far. The deposition was performed at an elevated temperature much lower than the high temperatures (800–1000 °C) needed for deposition of carbon nanotubes by conventional methods. High-resolution transmission electron microscopy showed that the combination of the stress induced by the ion impact and the C migration at the temperature applied formed graphitic sheets with their normal (c axis) parallel to the surface of the silicon substrate. Some sheets closed to form multiwall nanotubes. The smallest diameter of the innermost tube was found to be 0.4 nm. The novel use of MSIBD (a pure method, catalyst free, low deposition temperature, easily applied to large surfaces without surface pretreatment capable of pattern-writing) may significantly advance the carbon nanostructure technology.
An ac method for the precise measurement of Q-factor and resonance frequency of a microwave cavity
Authors: Nebendahl B. , Peligrad D.-N. , Požek M. , Dulčić A. , Mehring M.
Published: July 2002

We have developed a new and fast method for the determination of the complex frequency shift of a microwave resonant cavity. The method is based on frequency modulation of the microwave source around the cavity resonance and detection of the 2nd and 4th harmonic of the modulation frequency. With this procedure the static measurement of the response amplitude is not necessary and all the data are obtained through a single ac channel. The optimal frequency deviation is shown to be comparable to the cavity resonance width.
Single-electron tunneling dynamics in a turnstile designed with asymmetric tunnel barriers
Authors: Nakashima Hiroshi , Uozumi Kiyohiko
Published: July 2002

A simple analytical solution to the potential energy which an electron feels when tunneling through an ultrasmall tunnel capacitor in mesoscopic circuits in the semiclassical regime is presented. An effect of induced charges (image charges) on the plates of the tunnel capacitors when an electron is transferred from one plate to the other is taken into account. The derived expression for the potential energy of a tunneling electron is used to analyze a transport property of a turnstile designed with four identical tunnel barriers whose energy profiles are asymmetric, such tunnel barriers are called asymmetric tunnel barriers (ATBs). This turnstile is expected to show the zero-bias current due to synergism of the ATB nature, i.e., the rectification effect, and the turnstile effect when a periodic signal is applied to its gate. The image force is found to have a significant rule on the single-electron traffic in mesoscopic devices/circuits especially involving ATBs.
Stable photoluminescence of zinc oxide quantum dots in silica nanoparticles matrix prepared by the combined sol–gel and spray drying method
Authors: Mikrajuddin  , Iskandar F. , Okuyama K. , Shi F. G.
Published: July 2002

A sol–gel method was employed to produce a zinc oxide (ZnO) colloid consisting of ZnO nanocrystalline particles with an average diameter of ∼3 nm, and subsequently mixed with a silica (SiO2) colloid. The mixture was finally spray dried to form a powder nanocomposite. It was found that the green photoluminescence (PL) exhibited by the composite was very stable: the intensity, position, and shape do not change even after being aged over 30 days. Thus, the ZnO/SiO2 nanocomposite has a much improved PL stability over ZnO colloids, which is often found to undergo a significant redshift even after aging over a few days. Our results are expected to have significant technological implications.
Photoelectron imaging using an ellipsoidal display analyzer
Authors: Dütemeyer T. , Quitmann C. , Kitz M. , Dörnemann K. , Johansson L. S. O. , Reihl B.
Published: July 2002

We have built an ellipsoidal display analyzer (EDA) for angle-resolved photoelectron spectroscopy and related techniques. The instrument is an improved version of a design by Eastman et al. [Nucl. Instrum. Methods 172, 327 (1980)] and measures the angle-resolved intensity distribution of photoelectrons at fixed energy I(θ,φ)|E=const.. Such two-dimensional cuts through the Brillouin zone are recorded using a position-sensitive detector. The large acceptance angle (Δθ=43° in the polar direction and Δφ=360° in the azimuthal direction) leads to a collection efficiency which exceeds that of conventional hemispherical analyzers by a factor of about 3000. Using ray-tracing calculations we analyze the electron optical properties of the various analyzer components and optimize their arrangement. This minimizes distortions and aberrations in the recorded images and greatly improves the performance compared to previous realizations of this analyzer. We present examples demonstrating the performance of the analyzer and its versatility. Using a commercial He-discharge lamp we are able to measure complete angular distribution patterns in less than 5 s. The energy and angular resolution are ΔEEDA=85 meV and Δθ=1.2°, respectively. Complete stacks of such cuts through the Brillouin zone at different kinetic energies E can be acquired automatically using custom software. The raw data are processed leading to a three-dimensional set (I(EB,k∥) of photoelectron intensity versus binding energy E and wave vector k∥. From this all relevant information, like the dispersion relations EB(k∥) along arbitrary directions of the Brillouin zone or Fermi-surface maps, can then be computed. An additional electron gun enables low-energy electron diffraction, Auger electron spectroscopy, and electron energy-loss spectroscopy. Switching between electrons and photons as the excitation source is possible without any movement of the sample or analyzer. Because of the high acquisition speed it is possible to study the electronic structure of solids as a function of an external parameter (i.e., temperature) or to make animated movies showing, for example, the evolution of electronic states in reciprocal space. After installation of this EDA at a synchrotron providing tunable photon energy, the full power of the instrument will come into play by adding techniques like constant final state or constant initial state spectroscopy, and x-ray photoelectron diffraction.
A four-dimensional quantum mechanical state-to-state study of the H2+C2H→H+C2H2 reaction
Authors: Szichman Henrik , Gilibert Miquel , González Miguel , Giménez Xavier , Aguilar Antonio
Published: July 2002

A quantum mechanical approach to treat diatom–triatom exchange processes of the type AB+CDE→A+BCDE is presented. The initial nine degree-of-freedom problem is simplified to a reaction having active only five of such degrees of freedom, which emulates a rotating–stretching AB molecule colliding colinearly with a linear CDE molecule. This model is then applied to study the H2+C2H→H+C2H2 reaction. In the present work, the H2 rotations are treated using the infinite-order-sudden-approximation (IOSA) method, whereas the coupled states (CS or jz) approximation is employed to uncouple the total angular momentum J from internal rotations. Thus, a four-dimensional mathematical analysis is performed, which allows the computation of state-to-state reactive probabilities and cross sections. The bending vibrational levels of the acetylene C2H2 molecule are calculated on the basis of a one single degenerate bending expansion, i.e., just one H (the attacked one) is considered to bend, the remainder being frozen. Present results show that the product acetylene is formed in highly excited vibrational states, particularly if either the reacting asymmetric CH or symmetric C2 stretches are involved. Finally, rate constant results are compared with two other theoretical treatments and with experiments.
Diffusion in phase space with molecular friction: A numerical study for undercooled liquids
Authors: Schulz Beatrix M. , Trimper Steffen , Schulz Michael
Published: July 2002

We present a molecular dynamic simulation (MD) to study relaxations in a Lennard-Jones liquid with an additional molecular friction term. To avoid crystallization the atomic size is varied. The simulation is done for a fixed density. The problems of MD-simulations in undercooled systems with a Newtonian friction term and under conservation of the total momentum are overcome by our analysis. The origin of friction terms in a more microscopical approach is discussed. We calculate trajectories connecting the minima in the phase space of the total system. It is shown that both structural relaxations and vibrations of atoms around their fixed positions can be analyzed by those so-called pseudotrajectories used here. The size of cooperatively rearranging regions and the length of jumps between neighbored structures are studied.
Push–pull electro-optic polymer modulators with low half-wave voltage and low loss at both 1310 and 1550 nm
Authors: Zhang Hua , Oh Min-Cheol , Szep Attila , Steier William H. , Zhang Cheng , Dalton Larry R. , Erlig Hernan , Chang Yian , Chang Daniel H. , Fetterman Harold R.
Published: July 2002

Push–pull polymeric electro-optic Mach–Zehnder (MZ) modulators with Vπ of 1.2 and 1.8 V at 1310 and 1550 nm, respectively, with an interaction length of 2 cm are demonstrated. These devices were made from second-order nonlinear optic guest–host polymers that consisted of a phenyltetraene bridged high μβ chromophore guest and an amorphous polycarbonate host. Poling was done in N2 atmosphere to avoid chromophore bleaching by oxidation. A MZ-like two-arm microstrip line was used as the driving electrode in these devices. The optical response dropped 3 dB electrical from 2 to 20 GHz. These 3 cm long devices have 5 dB total chip loss at both wavelengths and good thermal stability.
Comparison of thermostatting mechanisms in NVT and NPT simulations of decane under shear
Authors: Delhommelle Jerome , Evans Denis J.
Published: July 2002

Nonequilibrium molecular dynamics (NEMD) simulations play a major role in characterizing the rheological properties of fluids undergoing shear flow. However, all previous studies of flows in molecular fluids either use an “atomic” thermostat which makes incorrect assumptions concerning the streaming velocity of atoms within their constituent molecules, or they employ a center of mass kinetic (COM) thermostat which only controls the temperature of relatively few degrees of freedom (3) in complex high molecular weight compounds. In the present paper we show how recently developed configurational expressions for the thermodynamic temperature can be used to develop thermostatting mechanisms which avoid both of these problems. We propose a thermostat based on a configurational expression for the temperature and apply it to NEMD simulations of decane undergoing Couette flow at constant volume and at constant pressure. The results so obtained are compared with those obtained using a COM kinetic thermostat. At equilibrium the properties of systems thermostatted in the two different ways are of course equivalent. However, we show that the two responses differ far from equilibrium. In particular, we show that the increase in the potential energy of the internal modes with increasing shear is only observed with a Gaussian isokinetic COM thermostat in both NVT and NPT simulations. There is no such increase with the configurational thermostat, which, unlike the Gaussian isokinetic COM thermostat, correctly accounts for the internal degrees of freedom of the molecular fluid.
Unsteady flow of thin liquid film on a disk under nonuniform rotation
Authors: Dandapat B. S.
Published: July 2002

Development of thin liquid film under nonuniform rotation has been studied numerically by using finite difference technique under the assumption of planar interface. For impulsive rotation of the disk an anomalous behavior of the rate of film thinning with the variation of the Reynolds number is observed in a different time zone. It is also shown that the azimuthal velocity field develops into the entire depth faster with the smaller impulsive rotation. A physical explanation for the above observation is provided. Further it is found that faster rate of thinning can be obtained if the spinner starts impulsively and then increases its spinning rate continuously.
Variational approach to a turbulent swirling pipe flow with the aid of helicity
Authors: Yoshizawa Akira , Yokoi Nobumitsu , Nisizima Shoiti , Itoh Sanae-I. , Itoh Kimitaka
Published: July 2002

A turbulent swirling flow in a circular pipe is studied with the aid of a variational method. A prominent feature of the flow is the occurrence of a retarded or reversed mean-flow profile near the central axis. Diffusion effect is generally enhanced in a turbulent flow. A mechanism under which these two properties coexist is sought with resort to a variational approach combined with helicity. The mean-flow field is expressed by the mean vorticity proportional to its curl. On this basis, the centerline axial vorticity is shown to play a central role in the occurrence of axial-velocity retardation or reversal in a turbulent swirling pipe flow. Suggestions on modeling the Reynolds stress are obtained from the mean-vorticity equation.
Dynamics of hydrogen bonding in an elementary model of water
Authors: Evans Glenn T.
Published: July 2002

The dynamics of the hydrogen bond breaking and reforming are investigated using a model for water in which each water molecule is a hard sphere with four sticky spots located at the corners of a tetrahedron. H-bonding arises when a pair of particles attach at their sticky spots by means of the narrow, anisotropic square well potential. To escape the square well, and break the H-bond, a solvent molecule must collide with the bonded pair and communicate sufficient energy along the line of centers so as to exceed the threshold energy. The calculated time correlation function describes the fluctuation in the number of H-bonded pairs. Its correlation time, associated with the three-body direct rupture, is roughly 0.83 ps in water at 300 K and obeys an Arrhenius law. After bond rupture, the restituting solvent molecule can return (or backscatter) and in so doing, cause the initial dimer to reform its H-bond. As a result, the overall correlation time for bond breaking is roughly 14 ps. Employed here are aspects of the kinetic theory of square well fluids together with Wertheim’s theory for associating systems.
Mössbauer studies of melt-spun Pr2Fe14B ribbons
Authors: Hamdeh H. H. , Zhang X. , Ho J. C. , Chang W. C. , Chang H. W. , Liou Y. S.
Published: July 2002

Melt-spun ribbons of tetragonal Pr2Fe14B exhibit favorable hard magnet characteristics. Technically relevant materials based on this compound, however, generally contain a certain amount of soft magnetic α-Fe or Fe3−xBx for remanence enhancement through exchange coupling. The nominal off-stoichiometric compositions lead to metallurgical complications, which are not easily resolvable by standard phase identification techniques such as x-ray diffraction and thermal magnetic analysis. As a viable alternative, Fe57-Mössbauer spectroscopy can be used to delineate individual Fe sites. To provide a basis for such an approach, this report gives Mössbauer parameters including hyperfine magnetic field, isomer shift, and quadrupole splitting as obtained from a single-phase melt-spun Pr2Fe14B ribbon.
Synthesis of large area aligned carbon nanotube arrays from C2H2–H2 mixture by rf plasma-enhanced chemical vapor deposition
Authors: Wang Y. H. , Lin J. , Huan C. H. A. , Chen G. S.
Published: July 2002

Large area aligned carbon nanotube (CNT) arrays have been successfully synthesized from C2H2 and H2 mixture by rf plasma-enhanced chemical vapor deposition (without hot filament) on iron-coated silicon substrates. H2 plasma (not H2 gas) was confirmed to play the role of reducing iron oxide to metallic iron and promoting the formation of evenly separated particles, as well as being the primary factor in synthesizing aligned CNTs. The addition of H2 gas with no plasma during the growth resulted in randomly oriented CNTs. Meanwhile, without the addition of H2, the C2H2 plasma resulted in the growth of very fine worm-like carbon fibers. Using substrates with a thicker catalyst layer (>90 nm) reduced the CNT density significantly.
Functionalization of silicon step arrays II: Molecular orientation of alkanes and DNA
Authors: Crain J. N. , Kirakosian A. , Lin J.-L. , Gu Yuedong , Shah Rahul R. , Abbott Nicholas L. , Himpsel F. J.
Published: July 2002

Alkanes and DNA are adsorbed via thiol groups on ultrathin gold films that have been templated on stepped Si(111)7×7 surfaces. The orientation of the adsorbed molecules is determined by polarization-dependent near edge x-ray absorption fine structure spectroscopy from the C 1s and N 1s core levels. An anisotropy in the polar distribution is found which is consistent with models in which the molecules have a preferred tilt angle with respect to normal but a random azimuthal distribution. In order to obtain maximum coverage and an oriented overlayer it is necessary to deposit more than 3 nm of Au (12 monolayers).
Quantal two-center Coulomb problem treated by means of the phase-integral method. III. Quantization conditions in the general case expressed in terms of complete elliptic integrals. Numerical illustration
Authors: Athavan N. , Lakshmanan M. , Fröman N.
Published: July 2002

In this paper we take up the quantal two-center problem where the Coulomb centers have arbitrary positive charges. In analogy with the symmetric case, treated in the second paper of this series, we use the knowledge on the quasiclassical dynamics to express the contour integrals in the first- and third-order approximations of the phase-integral quantization conditions, given in the first paper of this series of papers, in terms of complete elliptic integrals. For various values of the distance between these charges the accuracy of the formulas obtained is illustrated by comparison with available numerically exact results.
The unimolecular dissociation of the OH stretching states of HOCl: Comparison with experimental data
Authors: Weiß J. , Hauschildt J. , Schinke R. , Haan O. , Skokov S. , Bowman J. M. , Mandelshtam V. A. , Peterson K. A.
Published: July 2002

The unimolecular dissociation of the (v1,0,0) pure OH stretching states of hypochlorous acid (HOCl) in the ground electronic state is investigated for v1=6–9. The dynamics calculations are performed on an accurate potential energy surface and employ filter diagonalization in connection with an imaginary absorbing potential. The dependence of the linewidth (or dissociation rate) on the total angular momentum is emphasized. Resonance enhancements due to mixings with other vibrational states, which have substantially larger rates, are clearly observed—in qualitative agreement with recent measurements. The average width increases, in quantitative agreement with experiments, by four orders of magnitude, from 10−4 cm−1 for v1=6 to about 1 cm−1 for v1=9.
Intrinsic stress evolution in aluminum nitride thin films and the influence of multistep processing
Authors: Rajamani Ashok , Beresford Rod , Sheldon Brian W.
Published: July 2002

Curvature measurements were used to investigate intrinsic stresses in AlN thin films grown by molecular beam epitaxy. In situ stress evolution experiments show that tensile stress is generated at the time where individual islands coalesce into a continuous film, and for some time after coalescence. Compressive stress is also generated both before and after coalescence. Introducing an intermediate annealing stage appears to reduce tensile stress evolution after coalescence, ultimately increasing the final intrinsic compressive stress. The maximum reduction in the tensile intrinsic stress is obtained when the sample is annealed at roughly the time where the islands coalesce into a continuous film.
Algebraic perturbation theory for dipolar fluid mixtures in a weak external field: Application for isomerization equilibria
Authors: Kronome Gergely , Szalai István , Liszi János
Published: July 2002

We study the effect of the external electrostatic field on dipolar fluid mixtures by extending the algebraic perturbation theory of Kalikmanov [Phys. Rev. E 59, 4085 (1999), valid for one-component dipolar fluids] to the multicomponent case. Our reference system is a dipolar hard sphere mixture, while the external field is treated as a perturbation. Using the mean spherical approximation to describe the reference system in the case when the components differ only by their dipole moments (and not their size), the external field contributions to the free energy, pressure and chemical potentials are derived. As an application, we use these expressions to examine the field dependence of the isomerization equilibrium in a two-component dipolar mixture. It is found that in isomerization equilibrium, the concentration of the more dipolar component increases with the external field, as well as the dielectric constant of the mixture, while the pressure is decreased by the field.
Theoretical mechanistic study on the ion–molecule reactions of CCN+/CNC+ with H2O and HCO+/HOC+ with HCN/HNC
Authors: Tao Yu-guo , Ding Yi-hong , Liu Jian-jun , Li Ze-sheng , Huang Xu-ri , Sun Chia-Chung
Published: July 2002

A detailed [C2H2NO+] potential-energy surface in singlet, including 48 minimum isomers and 73 transition states, is built up at the B3LYP/6-311G(d,p) and CCSD(T)/6-311G(2df,p) (single-point) levels in order to explore the mechanisms of the important ion–molecule reactions between CCN+/CNC+ and H2O and between HCO+/HOC+ and HCN/HNC. For the reactions of both CCN+ and CNC+ towards H2O, product HCO++HNC may be the most abundant followed by the much less HCO++HCN and then HCNH++CO. Significant discrepancies on the product distributions are found between our calculated results and two previous experimental findings. On the other hand, for the HCO++HCN/HNC reactions, the barrierless association may lead to the stable adducts OC(H)NCH+/OC(H)CNH+, while the proton-transfer may barrierlessly lead to product HCNH++CO via the hydrogen-bound complexes OC…HNCH+/OC…HCNH+. For the HOC++HCN/HNC reactions, both the barrierless proton-transfer and association-elimination processes can lead to HCNH++CO via the complexes CO…HNCH+/CO…HCNH+ and unstable adducts HOCNCH+/HOCCNH+, respectively. The computations reported in this paper may represent the first theoretical study on the chemical reactivity of the C2N+ ion, and may thus provide a useful guide for understanding the mechanisms of the other analogous reactions such as those of C2N+ with CH4, NH3, H2S and CH3OH, etc. The present calculations may also provide useful information for future laboratory investigations on the HCO+/HOC++HCN/HCN reactions that have not been previously studied. Interstellar implications of the title reactions are discussed.
Hydrogen bonding in supercritical methanol studied by infrared spectroscopy
Authors: Bulgarevich Dmitry S. , Otake Katsuto , Sako Takeshi , Sugeta Tsutomu , Takebayashi Yoshihiro , Kamizawa Chiyoshi , Shintani Daisuke , Negishi Akira , Tsurumi Chikao
Published: July 2002

We report the first Fourier transform infrared spectroscopic studies of the hydrogen-bonding interactions in supercritical methanol and methanol/methanol-d1 mixtures. At 523 K and below ∼0.3 mol/l, methanol chiefly existed in a monomeric form. However, at 523 K and between ∼0.3 and 2 mol/l, in the regions of O–H, C–O, and C–H stretching vibrations, there were clear indications of methanol monomer–dimer equilibrium. The effect of temperature on the equilibrium constant was also investigated. As a result, the partial molar volume difference and enthalpy of the dimerization were estimated and correlated with the behavior of the methanol isothermal compressibility. It was also concluded that at 523 K and density higher than ∼15 mol/l, the hydrogen-bonding equilibrium started to shift toward larger aggregates.
Optimization of white polychromatic semiconductor lamps
Authors: Žukauskas A. , Vaicekauskas R. , Ivanauskas F. , Gaska R. , Shur M. S.
Published: July 2002

A stochastic method of optimization of a white-light source that relies on additive color mixing of the emissions from colored light-emitting diodes (LEDs) was developed. The method allows for finding the optimal wavelengths of LEDs in order to obtain the best possible trade off between luminous efficacy and the general color rendering index (CRI) of the white source for an arbitrary number of primary LEDs. Optimal solid-state lamps composed of two, three, four, and five different LEDs were analyzed. We show that a dichromatic LED lamp can only provide high efficacy with a general CRI close to zero, whereas trichromatic and quadrichromatic lamps are able to cover the entire range of reasonable general CRI values. The optimization of quintichromatic LED lamps and lamps with a higher number of primary color LEDs yields a negligible benefit in improving CRI but provides for quasicontinuous spectra that might be required for special lighting needs.