bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of x-ray reflectivity techniques to determine structural parameters of some silicide structures for microelectronics applications
Authors: Santucci S. , la Cecilia A. V. , Phani A. R. , Alfonsetti R. , Moccia G. , De Biase M.
Published: July 2002

X-ray reflectivity measurements have been carried out on TiN/Ti/Si- and WSix/Si-deposited and -annealed structures. The results show that for the as-deposited samples there are periodic oscillations due to a well-defined interface roughness, while for the annealed samples the disappearance of the oscillations due to increase in surface and interface roughness of the formed silicide layers is observed. We demonstrate that the analysis of x-ray reflectivity measurements collected both in the specular and in the off-specular way, even for the samples with very high surface and interface roughness, allows the determination of structural parameters of the films and interfaces.
Double-dot charge transport in Si single-electron/hole transistors
Authors: Rokhinson L. P. , Guo L. J. , Chou S. Y. , Tsui D. C.
Published: July 2002

We studied transport through ultrasmall Si quantum-dot transistors fabricated from siliconon-insulator wafers. At high temperatures, 4<T<100 K, the devices show single-electron or single-hole transport through the lithographically defined dot. At T<4 K, current through the devices is characterized by multidot transport. From the analysis of the transport in samples with double-dot characteristics, we conclude that extra dots are formed inside the thermally grown gate oxide which surrounds the lithographically defined dot.
High rate etching of 4H–SiC using a SF6/O2 helicon plasma
Authors: Chabert P. , Proust N. , Perrin J. , Boswell R. W.
Published: July 2002

The etch rate of 4H–SiC in a SF6 helicon plasma has been investigated as a function of pressure, power injected in the source, substrate bias voltage, and distance between the substrate holder and the helicon source. The highest etch rate yet reported of 1.35 μm/min along with good uniformity on 2 in. SiC substrates was achieved when this distance was minimum. Smooth etched surfaces free of micromasking have been obtained when using a nickel mask and the selectivity SiC/Ni was found to be about 50 under high etch rate conditions. Via holes have been etched to a depth of 330 μm in 4H–SiC substrates.
Repeated crystallization in undercooled Zr41Ti14Cu12Ni10Be23 liquids
Authors: Schroers Jan , Johnson William L. , Busch Ralf
Published: July 2002

Isothermal crystallization studies are performed on Zr41Ti14Cu12Ni10Be23 melts. Undercooling experiments are carried out repeatedly at 907, 860, and 750 K. The scattering of the time to reach the onset of crystallization is investigated. Results from experiments performed at 907 K show a large scatter of the onset time of crystallization. For the experiments carried out at 860 and 750 K, scattering of the onset time is two orders of magnitude smaller. These results indicate that, at high temperatures, the crystallization is governed by the time scale of the statistical nucleation events. At low temperatures, the crystallization is controlled by diffusion, resulting in a well-defined onset time for crystallization.
Nonstandard comodules for quantum matrix bialgebras
Authors: Montani H. , Trinchero R.
Published: July 2002

We carry out a construction of nonstandard matrix quantum groups comodules through a generalization of the coaction on tensor powers of a comodule. This generalization amounts to consider the situation where quantum space coordinates and its endomorphism matrix entries belong to a noncommutative quadratic algebra.
Temperature programmed desorption using time-of-flight mass spectrometry
Authors: Kim C. , Yan X.-M. , White J. M.
Published: July 2002

A new method of thermal programmed desorption (TPD) using time-of-flight mass spectrometry (TOFMS) is developed for simultaneous detection, in wide mass ranges, of large molecules and their surface reaction products. It provides a three-dimensional (3D) TPD spectrum that contains a complete set of conventional two-dimensional (2D) TPD spectra of all the masses in the mass range of interest. As an example, we present a 3D TPD spectrum of (methylcyclopentadienyl)Ir(1,5-cyclooctadiene), molecular weight 379.53 AMU, on a rhodium surface. The 3D TPD spectrum comprises TOF mass spectra, from 1 to 385 AMU, accumulating 10 000 scans every 0.6 s. By covering the whole mass range of interest in one TPD experiment, the new TOFMS-TPD eliminates guesswork required in TPD using quadrupole mass spectrometry that limits the number of masses to follow.
Iron-cored coil system for the measurement of angular distributions of charged particles
Authors: Cubric D. , Ward R. , King G. C. , Read F. H.
Published: July 2002

A novel coil system is described that enables the magnetic angle-changing technique to be used at higher energies. The system consists of coils of wire that are combined with specially shaped iron cores in such a way as to preserve localization of the magnetic field. The system extends the operational energy range of previous systems consisting of solenoids alone by a factor of 25. Measurements of the electron impact excitation of the n=2 states of helium are presented as an example of the use of the device.
Simulating a Maxwellian plasma using an electron beam ion trap
Authors: Savin D. W. , Beiersdorfer P. , Kahn S. M. , Beck B. R. , Brown G. V. , Gu M. F. , Liedahl D. A. , Scofield J. H.
Published: July 2002

We describe a technique for producing a Maxwell–Boltzmann electron energy distribution using an electron beam ion trap (EBIT). The technique was implemented on the Lawrence Livermore EBIT to simulate Maxwellian plasmas. We discuss technical and experimental issues related to these simulations. To verify the fidelity of the quasi-Maxwellian, we have measured line emission due to dielectronic recombination (DR) and electron impact excitation (EIE) of heliumlike neon, magnesium, and argon for a range of simulated electron temperatures. The ratio of DR to EIE lines in heliumlike ions is a well understood electron temperature diagnostic. The spectroscopically inferred quasi-Maxwellian temperatures are in excellent agreement with the simulated temperatures.
Diffusion-assisted reaction through a fluctuating bottleneck
Authors: Seki Kazuhiko , Tachiya M.
Published: July 2002

When a ligand escapes from a heme-pocket of a protein, the migration is geometrically controlled. A model is proposed by Zwanzig for a rate process that is controlled by passage through a fluctuating bottleneck. The model predicts that the long-time rate constant is inversely proportional to the square-root of the solvent viscosity, which is qualitatively consistent with experimentally observed rate constants. For a reverse process, namely, ligand rebinding to the heme from the solvent phase, diffusion motion of ligands in the solvent should be taken into account in addition to bottleneck fluctuations. In this article, we generalize the Zwanzig model in such a way to include the translational diffusion motion of ligands. The bimolecular rebinding rate is expressed in terms of a continued fraction which converges rapidly. It is shown that in this case the fractional power dependence does not hold for any values of the translational diffusion constant.
Pt Schottky contacts to n-GaN formed by electrodeposition and physical vapor deposition
Authors: DeLucca J. M. , Mohney S. E. , Auret F. D. , Goodman S. A.
Published: July 2002

Electrodeposited, dc magnetron sputtered, and electron beam evaporated Pt contacts to n-GaN (n=1.5×1017 cm−3) are reported. All contacts were rectifying in the as-deposited condition, and values of the barrier height were determined by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The influence of deposition conditions on the electrical characteristics of the sputtered and electrodeposited Pt contacts was further studied. Additionally, a dependence of the barrier height with time following deposition is shown. Taking into consideration all parameters of this study, the barrier height could differ by as much as 0.65 eV by I–V measurements and 0.64 eV by C–V measurements, with I–V and C–V barriers as high as 1.43 and 1.57 eV, respectively. Reverse current densities are reported for a −5 V bias with the highest and lowest median values differing by a factor of 104 as a result of the different deposition conditions. The electrical properties are believed to be strongly influenced by the presence of electrically active defects introduced during metal deposition. Deep level transient spectroscopy data support this hypothesis.
Spectroscopy and energy relaxation processes of Hg-doped solid neon, argon, and xenon
Authors: Helbing J. , Chergui M. , Haydar A.
Published: July 2002

Emission, absorption, and excitation spectroscopy has been used for a detailed analysis of the optical transitions of Hg2 trapped in cryogenic matrices. Upon excitation of electronic states correlating to the 3P1 or the 1P1 asymptote, fast nonradiative relaxation leads to emission from the lowest excited A0g+ state in all matrices, which decays monoexponentially in 1 ms in Ne, 280 μs in Ar, and 12 μs in Xe. In addition, electronically unrelaxed emission of Hg2 is reported in neon and in xenon matrices and attributed to the B1g state in neon and to the B1g state and the C0u− or A0g− states in xenon. The results are rationalized by assuming: (a) that population of the excited states occurs mainly close to the asymptotic limit, where branching is determined by nonadiabatic coupling and energetics, that are strongly environment dependent, and (b) that in Xe matrices the Hg2 states correlating to the 3P1 and 3P0 asymptotic limits are stabilized in different configurations, as a result of the very different solvation properties of the atomic 3P1 and the 3P0 state. Further emission bands are found in the vicinity of the dimer transitions, which we attribute to Hg3 and to site effects on Hg2. In particular, electronically unrelaxed emission from excited states of Hg3 is reported.
Triode electron bombardment evaporation source for ultrahigh vacuum thin film deposition
Authors: Verucchi R. , Nannarone S.
Published: July 2002

A new electron bombardment evaporation source for ultrahigh vacuum (UHV) thin film deposition is presented. It is based on an original electrodes configuration (modeled on that of a vacuum triode) featuring, besides anode and filament, a biased grid acting as a power modulator. The presence of a polarized grid helps to minimize space charge problems, to improve electron focusing and to fine control the dissipation on the evaporating material, governing the sublimation rate. The electrodes geometry was optimized by studying electron trajectories and electrical field distribution in the electrodes zone with ray-tracing analysis. Mechanical solutions and electrodes geometry were designed to ensure low outgassing and UHV compatibility, as well as to facilitate maintenance and source cleaning. The evaporator is water cooled and a mechanically operated shutter is present. The working principles and the technical details are presented together with operating data and evaporation performances.
Electric field induced second harmonic generation with and without fringes
Authors: Meshulam G. , Berkovic G. , Kotler Z. , Sa’ar A.
Published: July 2002

Electric field induced second harmonic generation (EFISH) is a well-known technique to measure the first hyperpolarizability (β) of organic molecules in solution. The characteristic experimental output is observation of oscillatory fringes of second harmonic radiation as the solution path length is changed and evaluation of β from the fringe amplitude. We present two different cases where even in the absence of these characteristic fringes β may still be evaluated: first, when using absorbing materials, and second, when using broadband laser sources. The ability to determine β by EFISH under these conditions greatly enhances the ability of this technique to measure β values over a wide range of laser frequencies. Measurements of the same molecule’s β values at different frequencies are reported, verifying the two-level model for the dispersion of β.
The metastable phase diagram of the Blume–Emery–Griffiths model in addition to the equilibrium phase diagram
Authors: Keskin Mustafa , Ekiz Cesur
Published: July 2002

As a continuation of our previously published works, the metastable phase diagram of the Blume–Emery–Griffiths model is presented in addition to the equilibrium phase diagram by using the lowest approximation of the cluster variation method. We also calculate the phase transitions for the unstable branches of order parameters. The calculated first- and second-order phase boundaries of the unstable branches of the order parameters are superimposed on the equilibrium phase diagram and metastable phase diagram. It is found that the metastable phase diagram and the first- and second-order phase boundaries for the unstable branches of order parameters always exist at the low temperatures, which are consistent with the experimental and theoretical works.
Variable-range hopping conductivity and magnetoresistance in n-CuGaSe2
Authors: Lisunov K. G. , Arushanov E. , Thomas G. A. , Bucher E. , Schön J. H.
Published: July 2002

The low-temperature charge transport in n-CuGaSe2 was investigated in zero and nonzero magnetic field. Both the Mott as well as the Shklovskii–Efros regimes of the variable-range hopping are observed in different temperature intervals. The complete set of the parameters describing the properties of the localized electrons (the localization radius, the dielectric permeability, the width of the Coulomb gap, and the values of density of states at the Fermi level) are obtained by analysis of the conductivity in zero field, on one hand, and the positive magnetoresistance in a small field, on the other hand. The negative magnetoresistance in low fields is observed in all specimens in both hopping regimes. Moreover, it is interpreted as a result of quantum interference between different paths of the tunneling electrons in conditions of scattering by intermediate centers.
Electroluminescence from semitransparent Au film/nanometer SiO2/nanometer Si/nanometer SiO2/n+–Si structure under reverse bias
Authors: Heng C. L. , Sun Y. K. , Wang S. T. , Chen Y. , Qiao Y. P. , Zhang B. R. , Ma Z. C. , Zong W. H. , Qin G. G.
Published: July 2002

Nanometer SiO2/nanometer Si/nanometer SiO2 double-barrier (DB) structures, with Si layers having eleven different thicknesses from 2 to 4 nm, were deposited on n+–Si substrates using the magnetron sputtering technique. Strong electroluminescence (EL) from semitransparent Au film/DB/n+–Si structure was observed under reverse bias in a range of about 5–7 V. It is found that every EL spectrum of the structure can be decomposed into two Gaussian bands with peaks at around 1.85 and 2.25 eV, and their intensities and current swing synchronously with increasing nanometer Si layer thickness; the periodic length of swing is consistent with half of the de Broglie wavelength of the carriers. A comparison was carried out between EL from the Au/DB/n+–Si structure under reverse bias and that from the Au/DB/p–Si structure under forward bias reported previously.
Miniature rotatable vacuum cell for low-temperature thermal measurements in high magnetic field
Authors: Bondarenko V. A. , Tanatar M. A. , Kovalev A. E. , Ishiguro T. , Kagoshima S. , Uji S.
Published: July 2002

We describe a miniature vacuum cell allowing thermal measurements in liquid He medium. The small size of the cell allows rotation in the limited space of high field magnets. A useful accessory for cell evacuation is also described.
Adhesion force measurement system for micro-objects in a scanning electron microscope
Authors: Miyazaki Hideki T. , Tomizawa Yasushi , Koyano Koichi , Sato Tomomasa , Shinya Norio
Published: July 2002

An in situ measurement system for the adhesion forces acting on micrometer-sized objects in a scanning electron microscope has been developed. This system is used to measure the adhesion forces at the object–probe (needle) or the object–substrate interface, while a micro-object which is deposited on the substrate is manipulated with a single probe. The measurement of forces ranging from 50 to 3000 nN with a resolution of about 1 nN has become possible by the combination of a V-shaped double cantilever system with a stiffness of 0.1–0.3 N/m and a heterodyne laser interferometer with a resolution of 10 nm. A numerical analysis showed that the V-shaped double cantilever system is compliant only for normal displacements and is rigid toward other movements. Therefore, the reliable measurement of separation forces without applying unwanted stresses to the contact interfaces has become possible. The value of the stiffness was experimentally determined from the change in the resonant frequency when known masses were added to the tip of the cantilever system. The measured values of controlled electrostatic adhesion forces showed fairly good agreement with the calculated results. It was found that the adhesion forces between a 25-μm solder sphere and a metal probe tip under SEM observation were on the order of 100 nN.
Temporal evolution of spin-polarization in ferromagnetic tunnel junctions
Authors: Monsma D. J. , Parkin S. S. P.
Published: July 2002

The spin polarization of the simple transition-metal ferromagnets Fe, Co, and Ni can be measured using ferromagnet/insulator/superconductor tunneling structures. Measurements, carried out over the past 20 years using superconducting aluminum electrodes and alumina tunneling barriers, have given a wide range of values, especially for Ni. In this letter, we show that high-spin-polarization values are found for Ni using superconducting films formed from Cu-doped aluminum deposited in a high-vacuum sputter-deposition system. However, we find that the magnitude of the spin polarization slowly decreases over time, which we believe to result from a reaction between Ni and alumina. We conclude that the spin-polarization values measured in such studies are strongly influenced by the detailed structure of the ferromagnet–superconductor interface.
Low frequency Alfvén waves induced by toroidal flows
Authors: van der Holst B. , Beliën A. J. C. , Goedbloed J. P.
Published: July 2002

The magnetohydrodynamic (MHD) spectra of toroidally rotating, axisymmetric tokamaks are analyzed. The continuous spectrum equations for the coupled localized Alfvén and slow modes are derived. The effect of flow on this spectrum is threefold: Doppler shift, centrifugal, and Coriolis effects enter. By exploiting a low-β, large aspect ratio expansion, the newly found low frequency Δm=0 Alfvén gap [van der Holst et al., Phys. Rev. Lett. 84, 2865 (2000)] located on the rational mode surfaces is analyzed. This gap is created by the geodesic curvature of the field lines, together with finite pressure, and by centrifugal and Coriolis effects. It is determined by a three mode interaction involving a central Alfvén mode and two sideband slow modes. From the same scheme, another new purely flow-induced gap inside the Δm=0 gap is found. Also, toroidal flow-induced Alfvén eigenmodes (TFAE) are found in the Δm=0 gap. All of these waves may be useful for MHD spectroscopy. Since the gap structures as well as the global wave are in the low frequency regime, they will have important implications for stability.
Block copolymer thin films on corrugated substrates
Authors: Podariu Iulia , Chakrabarti Amitabha
Published: July 2002

We numerically study morphology of symmetric diblock copolymer films with a thickness smaller than the bulk equilibrium period. These films are cast on either flat or corrugated substrates. Our results show that the formation of uniformly sized lateral domains that appear randomly on a flat surface can be controlled by using topographically patterned substrates. This is in excellent agreement with recent experimental studies. Control of lateral pattern fails if (i) the distance between the steps of the substrate is smaller than the bulk lamellar wavelength, or (ii) the lateral size of the corrugation is larger than the bulk lamellar wavelength.
Electrokinetic properties of colloids of variable charge. III. Observation of a Maxwell–Wagner relaxation mechanism by high-frequency electric-birefringence spectroscopy
Authors: Mantegazza F. , Bellini T. , Buscaglia M. , Degiorgio V. , Saville D. A.
Published: July 2002

Frequency resolved electric birefringence measurements were made on dilute aqueous suspensions of charged, rodlike latex particles. Ionic amphiphile adsorption was used to vary the particle charge as described in the first paper of this series [J. Chem. Phys. 103, 8228 (1995)]. Using the high frequency part of the birefringence spectra, the electric polarizability is computed from the Kerr constant following the approach set out in the second paper of the series [J. Chem. Phys. 109, 6905 (1998)]. Then, the polarizability spectra were interpreted with our extended Maxwell–Wagner model [J. Chem. Phys. 113, 6974 (2000), preceding paper]. Theory and experiment are in good agreement. The procedure provides a new and simple way to extract the particle charge from birefringence data.
Classical theory of molecule-surface scattering: Application to C2H2 scattering from LiF
Authors: Zhang Hongwei , Manson J. R.
Published: July 2002

A classical mechanical theory for the scattering of small, rigid molecules is developed which includes energy and momentum transfers between the surface and projectile for both translational and rotational motion. The results of calculations with this model are compared with recent high-precision measurements of the scattering of C2H2 from a clean, ordered LiF surface. The comparisons between calculations and measurements indicate that the observed angular distributions are dominated by single collisions with the surface and their shapes are due largely to translational energy (multiphonon) transfers. The calculated results for rotational energy transfer to the scattered molecules are also in reasonable agreement with measurements.
Hamiltonians and zero-curvature equations for integrable partial differential equations
Authors: Adams Malcolm R. , Krešić-Jurić Saša
Published: July 2002

We discuss the relationship between two approaches to integrable partial differential equations, one using formal affine Lie algebras and the other Banach–Lie groups. In the first approach integrability of the equations follows from commutativity of Hamiltonian flows on the Lie algebra, while in the second it follows from commutativity of certain flows induced by an action on the Banach–Lie group. We show that these two methods are essentially equivalent since one can calculate one type of the flows from the other. The classes of solutions encompassed by the two methods, however, vary significantly. We demonstrate this relationship specifically with the nonlinear Schrödinger equation.
Vapor transport within the thermal diffusion cloud chamber
Authors: Ferguson Frank T. , Heist Richard H. , Nuth Joseph A.
Published: July 2002

A review of two different, one-dimensional models of the vapor transport within the thermal diffusion cloud chamber (TDCC) is presented. In one case the assumption is made that there are no convective fluxes within the chamber and that heat and mass transport occur by diffusion only. Although in this model there are no restrictions on the transport of the two components within the chamber, the assumption of no velocities within the chamber results in an incorrect flux boundary condition for the background, carrier gas. The second model is based on the typical, stagnant background gas assumption and the equations of this model closely follow those of the classical Stefan tube problem in which there is transport of a volatile species through a noncondensible, carrier gas. Unfortunately, this model of the TDCC also suffers from the same inconsistencies as noted by several researchers for the Stefan tube. When the convective contributions to the flux are low in the stagnant background gas model, the two models give reasonably close results. For more convective situations, the supersaturation results can differ by more than 50%. One interesting feature of the zero velocity model is that it predicts a change in the supersaturation profile with pressure, whereas no pressure dependence is predicted with the stagnant background gas model. Unfortunately, the direction of this pressure change is opposite to that seen in experimental observations.
Observation of the high-resolution infrared absorption spectrum of CO2 molecules isolated in solid parahydrogen
Authors: Tam S. , Fajardo M. E.
Published: July 2002

We report the observation of high-resolution (0.008 cm−1) infrared absorption spectra of CO2 molecules isolated in solid parahydrogen (pH2) matrices at T=2.4 to 4.8 K. Several extremely sharp (0.01 to 0.04 cm−1 full-width-at-half-maximum) absorption features appear in the 2343.5 to 2345 cm−1 region. We assign the three strongest peaks to the v3 mode of isolated CO2 molecules. The spectra are consistent with trapping of the CO2 molecules in three distinct double-substitutional sites in hcp and fcc regions of the pH2 solid. We offer several hypotheses as to the origins of the numerous weaker absorption features.
Estimation of the Kolmogorov constant C0 by direct numerical simulation of a continuous scalar
Authors: Weinman K. A. , Klimenko A. Y.
Published: July 2002

The coefficient C0, which determines the effective turbulent diffusion in velocity space, is fundamental in Lagrangian modeling. Others, for example, Yeung and Pope [J. Fluid Mech. 207, 531 (1989)] have investigated this coefficient numerically with direct numerical simulation (DNS) by analyzing the dispersion of discrete particles. In our paper we estimate the coefficient C0 by using DNS to study the initial evolution of continuous passive scalar fields. Using an equivalent probability density function (pdf) and conditional moment analyses we examine the initial transient behavior for passive scalar mixing, with both linear and Gaussian-type initial distributions in the velocity space. Our estimates of C0 are found to be consistent with the data found in the literature.
Triaxial magnetic field gradient system for microcoil magnetic resonance imaging
Authors: Seeber D. A. , Hoftiezer J. H. , Daniel W. B. , Rutgers M. A. , Pennington C. H.
Published: July 2002

There is a great advantage in signal to noise ratio (S/N) that can be obtained in nuclear magnetic resonance (NMR) experiments on very small samples (having spatial dimensions ∼100 μm or less) if one employs NMR “micro” receiver coils, “microcoils,” which are of similarly small dimensions. The gains in S/N could enable magnetic resonance imaging (MRI) microscopy with spatial resolution of ∼1–2 μm, much better than currently available. Such MRI microscopy however requires very strong (>10 T/m), rapidly switchable triaxial magnetic field gradients. Here, we report the design and construction of such a triaxial gradient system, producing gradients substantially greater than 15 T/m in all three directions, x, y, and z (and as high as 50 T/m for the x direction). The gradients are switchable within time ∼10 μs and adequately uniform (within 5% over a volume of [600μm3] for microcoil MRI of small samples.
Critical analysis of the apoferritin adsorption at solid–liquid interfaces in the framework of a particular adsorption model
Authors: Lavalle Ph. , Gergely C. , Lustig A. , Ball V.
Published: July 2002

The adsorption mechanism of horse spleen apoferritin on smooth Si(Ti)O2 surfaces was investigated by means of optical wave guide lightmode spectroscopy (OWLS) as well as with atomic force microscopy (AFM), for which images of high resolution were obtained on muscovite mica surfaces. By the use of both experimental methods, the adsorption process could be studied from a kinetic as well as from a statistical thermodynamics point of view. This approach allowed to test the hypothesis of the occurrence of a particular type of deposition mechanism, namely the random sequential adsorption (RSA), by evaluating all the requirements that should be fulfilled in such a process. Only the requirement relative to the kinetics of the adsorption process, and subsequently, the estimation of the surface coverage at saturation is fulfilled by our experiments. From the fit of the theoretical kinetic equations corresponding to the RSA model to the experimental adsorption kinetics we find that the apoferritin molecules occupy an area of 140±30 nm2, in agreement with the values found by counting the number of particles per unit area in the AFM experiments and also with the saturation level of the adsorption isotherm. From our experiments we found that the evolution of the surface coverage close to saturation did not follow the expected power law evolution with time in the framework of the RSA model. Moreover, the dependence of the density fluctuations on the sub-surface area in the AFM image is not consistent with the expected evolution obtained by computer simulations based on the RSA model. These results emphasize the difficulty to study the adsorption mechanism of proteins at solid—liquid interfaces in the framework of any given adsorption model.
Symmetry and designability for lattice protein models
Authors: Wang Tairan , Miller Jonathan , Wingreen Ned S. , Tang Chao , Dill Ken A.
Published: July 2002

Native protein folds often have a high degree of symmetry. We study the relationship between the symmetries of native proteins, and their designabilities—how many different sequences encode a given native structure. Using a two-dimensional lattice protein model based on hydrophobicity, we find that those native structures that are encoded by the largest number of different sequences have high symmetry. However only certain symmetries are enhanced, e.g., x/y-mirror symmetry and 180° rotation, while others are suppressed. If there are many possible mutations which leave the native state of a particular protein stable, then, by definition, the state is highly designable. Hence, our findings imply that insensitivity to mutation implies high symmetry. It appears that the relationship between designability and symmetry results because protein substructures are also designable. Native protein folds may therefore be symmetric because they are composed of repeated designable substructures.
Off-resonant vibrational excitation: Orientational dependence and spatial control of photofragments
Authors: Machholm Mette , Henriksen Niels E.
Published: July 2002

Off-resonant and resonant vibrational excitation with short intense infrared (IR) laser pulses creates localized oscillating wave packets, but differs by the efficiency of the excitation and surprisingly by the orientational dependence. Orientational selectivity of the vibrational excitation of randomly oriented heteronuclear diatomic molecules can be obtained under simultaneous irradiation by a resonant and an off-resonant intense IR laser pulse: Molecules with one initial orientation will be vibrationally excited, while those with the opposite orientation will be at rest. The orientation-dependent response to the IR fields is due to the anharmonicity of the potential. A subsequent ultraviolet laser pulse in resonance at the outer turning point of the vibrational motion can then dissociate the oscillating molecules, all with the same orientation, leading to spatial control of the photofragment distribution.
Phase transitions of deuterated CO(NH2)2 under pressure
Authors: Góra D. , Parlinski K.
Published: July 2002

Minimizing the phenomenological potential energy of the deuterated urea we have confirmed the stability of the phase with symmetry: P4̄21m (Z=2), and found two other phases P21212 (Z=2) and P212121 (Z=4), which occur at zero temperature and under increasing pressure. The phase transitions are accompanied by a softening of Γ2(B1) zone center, and Z3 zone boundary modes, respectively.
A theoretical study of the CuO3 species
Authors: Deng Ke , Yang Jinlong , Zhu Qingshi
Published: July 2002

Density functional calculations are performed to study the CuO3 species in the neutral, cationic and anionic charge states. Three stable isomers OCuO2 (C2v), Cu(O3) (C2v) and OCuOO (Cs) are obtained. The most stable geometries of the CuO3 species are found to depend strongly on the charge states. For the neutral, the ground state is the OCuO2 quartet, while it is the Cu(O3)+ singlet for the cation. For the anion, the OCuO2− singlet and OCuOO− triplet are found to be quasi-isoenergetic and both are lower in energy than the Cu(O3)− singlet. The equilibrium bondlengths, vibrational frequencies and electronic configurations are obtained. A theoretical assignment for the features in the photoelectronic spectrum (PES) is given at the local spin density approximation level. Our results compare well with the available experimental results and show that the anionic OCuO2 (C2v) and OCuOO (Cs) isomers coexist in the PES experiment.
Transient recirculation in a slowly varying tube impulsively rotated about its axis with constant angular velocity
Authors: MacDonald D. A.
Published: July 2002

For time t̄<0, viscous fluid is in slow flow through a long straight axially symmetric tube whose radius, ā, varies slowly with axial distance, x̄. When t̄=0 the tube is impulsively rotated about its axis with angular velocity, Ω̄, at which angular speed it is thereafter maintained. During the transition from zero angular velocity, when t̄<0, to solid body rotation, when t̄→∞, the flow in the tube can briefly exhibit striking physical behavior, markedly different from the flow in the stationary tube. We present a linearization of the Navier–Stokes equations, valid when the Blasius parameter ε, which governs the magnitude of the inertial forces, tends to zero and the swirl parameter, λ, which is the ratio of a representative tube wall velocity, Ω̄ā0, to a representative axial velocity, tends to infinity, with the product ελ2≡Γ held fixed. An analytic solution suitable for computation and valid for suitably large t̄ is presented and streamlines are plotted for a typical diverging and a typical converging tube at time t̄=0.6ā02/ν when Γ=40. The relevance of the results to the phenomenon of vortex breakdown in tubes is discussed.
The effect of chain density on the frictional behavior of surfaces modified with alkylsiloxanes and immersed in n-alcohols
Authors: Clear Susannah C. , Nealey Paul F.
Published: July 2002

Concepts of viscoelastic behavior of bulk polymer systems were used to describe maxima in the friction behavior of self-assembled monolayers (SAMs) measured with lateral force microscopy (LFM) as a function of sliding velocity and applied normal load in the presence of solvents (plasticizers). The objective of this study was to investigate whether decreases in the chain density of the SAMs caused maxima in the friction force to shift to higher sliding velocities; such a shift is indicative of shorter chain relaxation times in less dense, less ordered SAMs. Complete SAMs were formed from octadecyltrichlorosilane (OTS) and partial SAMs were formed from n-octadecylmethyldichlorosilane (2Cl) and n-octadecyldimethyldichlorosilane (1Cl), all on silicon/silicon dioxide substrates. With decreasing chain density, solvent partitioning into the monolayer should increase as OTS<2Cl<1Cl. In butanol and pentanol, the maxima in the friction force shifted to higher sliding velocities with decreasing chain density, and the relaxation times calculated for the partial 2Cl films were an order of magnitude shorter than those for SAMs of OTS. For both SAMs of OTS and 2Cl films, maxima shifted to lower sliding velocities with increases in the applied normal load and with increases in the chain length n of the solvent. The higher compressibility of 2Cl films caused greater shifts in the maxima for similar increases in the applied normal load. The increase with n was consistent with both a mechanism of solvent partitioning controlled by the free volume distribution in the SAM and a mechanism of insertion. The relaxation times of the alkyl chains were related to a molecular model of energy dissipation involving the adsorption and desorption of the chain ends to and from the surface of the probe tip.
The criteria for the onset of double-diffusive instabilities at a vertical boundary
Authors: Kerr Oliver S.
Published: July 2002

When a body of fluid with a vertical salinity gradient is heated at a vertical boundary instabilities can sometimes appear. Several criteria for when these instabilities should be observed have been proposed, based on experiments and theory. Four different criteria will be examined which would appear to be mutually incompatible. Each of the experimentally derived criteria provides a good measure for the onset of instability in the corresponding experiments. When compared to the theoretical predictions the experimentally derived criteria are all shown to be compatible with the theory if the constraints of the experiments are included, but all would be expected to fail if the parameters of the respective experiments were extended.
Optimally defined Racah–Casimir operators for su(n) and their eigenvalues for various classes of representations
Authors: de Azcárraga J. A. , Macfarlane A. J.
Published: July 2002

This paper deals with the striking fact that there is an essentially canonical path from the ith Lie algebra cohomology cocycle, i=1,2,…,l, of a simple compact Lie algebra 𝔤 of rank l to the definition of its primitive Casimir operators C(i) of order mi. Thus one obtains a complete set of Racah–Casimir operators C(i) for each 𝔤 and nothing else. The paper then goes on to develop a general formula for the eigenvalue c(i) of each C(i) valid for any representation of 𝔤, and thereby to relate c(i) to a suitably defined generalized Dynkin index. The form of the formula for c(i) for su(n) is known sufficiently explicitly to make clear some interesting and important features. For the purposes of illustration, detailed results are displayed for some classes of representation of su(n), including all the fundamental ones and the adjoint representation.
Liquid-phase epitaxial growth of amorphous silicon during laser annealing of ultrashallow p+/n junctions
Authors: Chong Y. F. , Pey K. L. , Lu Y. F. , Wee A. T. S. , Osipowicz T. , Seng H. L. , See A. , Dai J.-Y.
Published: July 2002

We have investigated the effect of laser annealing on the recrystallization of the preamorphized layer during the formation of ultrashallow p+/n junctions. The results from channeling Rutherford backscattering spectrometry clearly indicate that the preamorphized layer has been completely annealed with a single-pulse laser irradiation at 0.5 J/cm2. These data are further verified by high-resolution cross-sectional transmission electron microscopy. It is proposed that the preamorphized layer has recrystallized to a single-crystalline structure via liquid-phase epitaxy. No observable extended defects are present in the recrystallized region after laser annealing.
Reduced magnetoresistance in magnetic tunnel junctions caused by geometrical artifacts
Authors: Matsuda K. , Watari N. , Kamijo A. , Tsuge H.
Published: July 2002

Spuriously reduced magnetoresistance (MR) ratios have been observed in magnetic tunnel junctions in which a square contact portion with dimensions smaller than the width of the lead electrodes connects both the top and bottom lead electrodes. The phenomenon becomes apparent by measuring the magnetoresistance of the junctions with various sizes systematically varied under a fixed line width of the electrodes. Observed junction size dependence of resistance (R)×area(A) products and MR ratios were analyzed through finite difference calculation, and it was found that there exist junction sizes for which R×A products and MR ratios are larger and smaller, respectively, than the intrinsic ones.
Effects of prepoling and polarization on photoemission from ferroelectric ceramics
Authors: Boscolo I. , Cialdi S. , Tazzioli F. , Catani L. , Giannessi L.
Published: July 2002

We describe the photoemission of lanthanum-doped lead zirconate titanate when illuminated with 355 nm radiation, with/without prepoling and with/without the application of a polarizing field. To explain the different behaviors observed, we propose a physical picture based on the defects moving toward the surface and the donor electrons squeezing into a very thin layer at the surface. The model and the experimental results indicate that ferroelectric ceramics could become interesting photoemitters.
Robust half metalicity in FexCo1−xS2
Authors: Mazin I. I.
Published: July 2002

The FexCo1−xS2 system is predicted, on the basis of density functional calculations, to be a half metal for a wide concentration range. Unlike most known half metals, the half metalicity in this system is expected to be very stable with respect to crystallographic disorder and other types of defects.
Expansion in the distance parameter for two vortices close together
Authors: Burzlaff J. , Kellegher E.
Published: July 2002

Static vortices close together are studied for two different models in two-dimensional Euclidean space. In a simple model for one complex field, an expansion in the parameters describing the relative position of two vortices can be given in terms of trigonometric and exponential functions. The results are then compared to those of the Ginzburg–Landau theory of a superconductor in a magnetic field at the point between type-I and type-II superconductivity. For the angular dependence a similar pattern emerges in both models. The differences for the radial functions are studied up to third order.
Dynamical quenching of laser-induced dissociations of diatomic molecules in intense infrared fields: Effects of molecular rotations and misalignments
Authors: Abou-Rachid Hakima , Tung Nguyen-Dang T. , Atabek O.
Published: July 2002

The dynamical dissociation quenching (DDQ) effect is a new mechanism for laser-induced vibrational trapping of molecules in the infrared (IR) spectral range. Previously demonstrated for one-dimensional, prealigned diatomic molecules [see F. Châteauneuf, T. Nguyen-Dang, N. Ouellet, and O. Atabek, J. Chem. Phys. 108, 3974 (1998)], the effect was shown to result from a proper synchronization of the molecular motions with the oscillations of the laser electric field. The present paper explores the influence of rotations and misalignment of the molecular system on the DDQ effect. To this end, the two-dimensional (radial and angular) wave-packet dynamics of the H2+ and HD+ molecular ions are considered in an intense IR laser field starting from two types of initial angular distributions: The first type of distributions is appropriate for a field-free, pure angular momentum eigenstate and denotes typically an initially nonaligned, nonoriented molecule. The second type denotes a more or less well aligned and/or oriented initial condition, and is described by an angular width Δ which is considered a parameter in terms of which the efficiency of the DDQ effect are monitored. We demonstrate that the DDQ effect remains efficient whenever a proper compromise is achieved between angular localization and angular-momentum (action) minimization. From the detailed analysis of the time-resolved dynamics, a time scale is also estimated for the molecule-field synchronization process which underlies the DDQ effect. An ultrafast laser-induced rotational-electronic energy transfer is found to compete with the DDQ effect, in the case the initial rotational state denotes an almost perfect alignment and/or orientation situation.
Formation of ammonium halide particles from pure ammonia and hydrogen halide gases: A theoretical study on small molecular clusters (NH3–HX)n (n=1, 2, 4; X=F, Cl, Br)
Authors: Cherng Bob , Tao Fu-Ming
Published: July 2002

The mechanism for the gas-phase production of particulate ammonium halides is studied by density functional theory and ab initio calculations on the molecular clusters of ammonia–hydrogen halide, (NH3–HX)n, n=1, 2, 4; X=F, Cl, Br. The equilibrium structures of the clusters demonstrate an abrupt transition from hydrogen-bonding in the single unit of NH3–HX to the NH4+⋯X− ion-pair interactions in (NH3–HX)4 resulting from spontaneous proton transfer from hydrogen halide to ammonia. The onset of the ion-pair form is at n=2 for HCl and HBr and at n=4 for HF. The ion-pair structure is the stable form of the larger clusters to maximize favorable electrostatic interactions and to closely resemble the crystal structures of ammonium halides. The calculated vibrational frequencies of the clusters provide additional evidence in support of the transition from hydrogen-bonding to ion-pair interactions as cluster size increases. The study shows that it is possible to form particulate ammonium halides directly from pure NH3 and HX vapors in the homogeneous gas phase.
Multiarrangement photodissociation calculations utilizing negative imaginary potentials
Authors: Abrashkevich Dmitri G. , Brumer Paul , Shapiro Moshe
Published: July 2002

A new method for calculating total and partial cross sections for photodissociation processes which produce more than one chemical product is presented. By using negative imaginary absorbing potentials, the method reduces the multiarrangement problem to a set of single-arrangement problems. In this way, the state-to-state photodissociation transition amplitudes are calculated directly using the artificial channel method coupled to an efficient log–derivative propagator. In addition, the discrete position operator representation is used to significantly simplify the calculations of the potential matrix elements. The method is shown to provide accurate cross sections for the resonant photodissociation of a model CO2 system.
Corona discharge supersonic free-jet for III–V nitride growth via A 3Σu+ metastable nitrogen molecules
Authors: Jordan D. C. , Burns C. T. , Doak R. B.
Published: July 2002

A corona discharge supersonic free-jet is shown to be a nearly pure source of A 3Σu+ metastable molecular nitrogen, an electronically excited and chemically active form of N2 that is ideally suited to nitride semiconductor growth. Optical emission spectroscopy at various distances downstream of the supersonic nozzle reveals a cascade through the excited state manifold of N2 triplet states to populate the A 3Σu+ state. Appearance potential spectroscopy (mass spectrometer electron bombardment ionization yield, measured as a function of electron impact energy) delivers the composition of the terminal molecular beam. A 3Σu+ molecules are the dominant activated species in the beam, which otherwise contains only nonreactive X 1Σg+ ground state nitrogen molecules plus a minor amount of S04 nitrogen atoms. Up to 1.56% number fraction of the beam is A 3Σu+, providing 1.0×1017 metastables sr−1 s−1.
Theoretical study of the He–HCN, Ne–HCN, Ar–HCN, and Kr–HCN complexes
Authors: Toczyłowski Rafał R. , Doloresco Fred , Cybulski Sławomir M.
Published: July 2002

The two-dimensional potential energy surfaces for the He–HCN, Ne–HCN, Ar–HCN, and Kr–HCN complexes are presented. Calculations have been performed using single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)] and the augmented correlation-consistent polarized triple-zeta basis set (aug-cc-pVTZ) with an additional (3s3p2d2f1g) set of bond functions. The potentials have been used to find the vibration–rotation energies of the four complexes and their deuterated analogs. The frequencies of rotational or rovibrational transitions found for He–HCN and Ar–HCN are in very good agreement with the experimental results. Good agreement is also obtained with the experimental rotational transition frequencies for Kr–HCN. For Ne–HCN, on the other hand, the agreement with the experimental data is not as good, but can be improved by using larger basis sets.