bluuebunny/crossref_metadata_2025 huggingface dataset:
Probing of flowing electron plasmas
Authors: Himura H. , Nakashima C. , Saito H. , Yoshida Z.
Published: July 2002

Probing of streaming electron plasmas with finite temperature is studied. For the first time, a current-voltage characteristic of an electric probe is measured in electron plasmas. Due to the fast flow of the electron plasmas, the characteristic curve spreads out significantly and exhibits a long tail. This feature can be explained calculating the currents collected to the probe. In flowing electron plasmas, the distribution function observed in the laboratory frame is non-Maxwellian even if the plasmas come to a state of thermal equilibrium. Another significant feature of the characteristic is that it determines a floating potential where the current equals zero, despite there being very few ions in the electron plasma. A high impedance probe, which is popularly used to determine the space potential of electron plasmas, outputs the potential. The method is available only for plasmas with density much smaller than the Brillouin limit.
Heterojunction properties of electrodeposited CdTe/CdS solar cells
Authors: Rakhshani A. E.
Published: July 2002

Solar cells of p-CdTe/n-CdS structure with a conversion efficiency of 10%±1% (AM1 spectrum, 127 mW/cm2 irradiance) were prepared by electrodeposition of CdTe on CdS-coated conducting glass. CdS was coated by chemical bath deposition. The tin–oxide conducting glass was prepared by the spray pyrolysis technique. The current–voltage, capacitance–voltage, photocurrent–voltage, photocurrent spectroscopy, and photoinduced current transient spectroscopy measurements proved to be powerful tools for the characterization of junction. The concentration of donors and acceptors in both sides of the junction are comparable, in the range of 1016 cm−3. The cell built-in potential is 1.20 V from which 0.65 V drops across the depletion width of CdTe. From the analysis of experimental data to construct the energy band diagram, it becomes evident that an electric dipole layer must exist at the CdTe/CdS interface. The presence of this layer is associated with a discontinuity of electrostatic potential at the interface by 0.28 V and a conduction-band spike of 64 meV. The conduction-band spike and an interfacial recombination center are accountable for the collection losses of photogenerated carriers. The recombination center that is likely related to the interstitial cadmium defect is energetically located 0.63 eV below the conduction-band edge and has a capture cross section of 8.8×10−12 cm2. © 2001 American Institute of Physics.
Simulation of bulk, confined, and polydisperse systems. I. A unified methodological framework
Authors: Escobedo Fernando A.
Published: July 2002

A recently developed formalism that synthesizes the description of statistical mechanical ensembles and the extrapolation of simulation data thereof is extended to encompass the formulation of semigrand canonical (SGC) type of ensembles and of ensembles suitable for confined systems and reactive equilibrium. These extensions provide a convenient framework to cast several ensembles previously used and also to formulate new variants. The advocated framework and its associated notation system are used to give general relationships for performing Monte Carlo simulations (acceptance rules) and implementing extrapolation and perturbation calculations. It is also shown how extrapolation methods can be systematically used to target equilibrium states whose specification does not allow the use of a direct simulation approach.
Radiative proton-capture nuclear processes in metallic hydrogen
Authors: Ichimaru Setsuo
Published: July 2002

Protons being the lightest nuclei, metallic hydrogen may exhibit the features of quantum liquids most relevant to enormous enhancement of nuclear reactions; thermonuclear and pycnonuclear rates and associated enhancement factors of radiative proton captures of high-Z nuclei as well as of deuterons are evaluated. Atomic states of high-Z impurities are determined in a way consistent with the equations of state and screening characteristics of the metallic hydrogen. Rates of pycnonuclear p-d reactions are prodigiously high at densities ⩾20 g/cm3, pressures ⩾1 Gbar, and temperatures ⩾950 K near the conditions of solidification. It is also predicted that proton captures of nuclei such as C, N, O, and F may take place at considerable rates, owing to strong screening by K-shell electrons, if the densities ⩾60–80 g/cm3, the pressures ⩾7–12 Gbar, and the temperatures just above solidification. The possibilities and significance of pycnonuclear p-d fusion experiments are specifically remarked.
Checkerboard composites with separated phases
Authors: Craster R. V. , Obnosov Yu. V.
Published: July 2002

Recently the authors have developed a method [SIAM J. Appl. Math. 61, 1839–1556 (2001)] capable of solving, in closed form, boundary value problems for four-phase doubly periodic checkerboard composites with continuity between the different phases. The method is based upon a novel conformal mapping that preserves the doubly periodic nature of the physical problem. The aim of the current article is to explore generalizations of that approach where we now replace continuity between some phases by nonconducting or perfectly conducting strips, thereby modeling debonding or electrodes in electrochemical devices. The specific objective is to determine effective resistivities and related parameters for these four-phase objects in a concise and explicit form.
A basic experiment on isotope scaling of transport
Authors: Bose T. , Sen A. K.
Published: July 2002

There is considerable divergence between theoretical predictions and tokamak experimental results for the isotopic mass effects on transport. This motivated a basic physics experiment in the Columbia Linear Machine [G. A. Navratil, J. Slough, and A. K. Sen, Phys. Plasmas 24, 184 (1982)]. Experiments were done using two different gases: hydrogen and deuterium. Measurements of particle transport driven by an E×B rotationally driven mode were made via the cross correlation of potential and density fluctuations. Unlike in tokamaks, the plasma parameter profiles and fluctuation spectra were maintained nearly invariant for both gases, thus enabling a meaningful comparison and estimation of scaling. The first series of data reveal smaller transport values for the deuterium plasma and a scaling that ranges as D⊥∝Ai−0.6 to D⊥∝Ai−0.8.
Photodissociation spectroscopy of Zn+(C2H4)
Authors: Lu W.-Y. , Kleiber P. D. , Young M. A. , Yang K.-H.
Published: July 2002

We have investigated the spectroscopy and photochemistry of Zn+(C2H4) in an angular reflectron time-of-flight mass spectrometer. We identify four absorption bands in the spectral range 220–550 nm. These bands are assigned to radiative transitions in the bimolecular complex correlating with Zn-centered and ethylene-centered absorptions, and with Zn–ethylene photo-induced charge transfer processes. The lowest energy band, assigned as 1 2B2←1 2A1, is a weak continuum consistent with a large geometry change and fast predissociation. The higher energy 1 2B1←1 2A1 band shows a long progression in the intermolecular stretch with a mode frequency of ωe=333.7 cm−1. The spectroscopic results, including partially resolved rotational structure, are consistent with a weakly bound, π-bonded complex in C2v symmetry. A Birge–Sponer analysis gave an estimate for the dissociation energies of the excited 1 2B1 state as De′=2.76 eV and the ground 1 2A1 state as De″=0.86 eV. A second structured band at still higher energies is tentatively assigned as 2 2B2←1 2A1 and shows activation of higher frequency intramolecular ethylene modes. Zn+ and C2H4+ fragment ions are observed over most of the spectral range. At higher energies (λ<250 nm) we also see a significant branching to reactive products C2H2+ and C2H3+ that result from charge transfer accompanied by C–H bond cleavage. We propose a reaction mechanism that involves coupling through an excited charge-transfer state followed by C–H bond insertion.
Linear stability and mode structure of drift tearing modes
Authors: Grasso D. , Ottaviani M. , Porcelli F.
Published: July 2002

Ion viscosity and particle diffusivity effects on the linear stability and mode structure of drift-tearing modes are investigated. If only resistivity and diamagnetic effects are taken into account, the drift-tearing mode can exist only as a propagating wave packet, which is resolved numerically as an initial value problem. Ion viscosity, particle diffusivity, as well as finite electron compressibility, also considered in this paper, lead to radially localized modes. The mode growth rate is reduced by ion viscosity, but is increased by particle diffusivity. The instability threshold is not modified. The analysis is based on the analytic and numerical solutions of a reduced three-field fluid model.
Calculation of product state distributions from resonance decay via Lanczos subspace filter diagonalization: Application to HO2
Authors: Zhang Hong , Smith Sean C.
Published: July 2002

Resonance phenomena associated with the unimolecular dissociation of HO2 have been investigated quantum-mechanically by the Lanczos homogeneous filter diagonalization (LHFD) method. The calculated resonance energies, rates (widths), and product state distributions are compared to results from an autocorrelation function-based filter diagonalization (ACFFD) method. For calculating resonance wave functions via ACFFD, an analytical expression for the expansion coefficients of the modified Chebyshev polynomials is introduced. Both dissociation rates and product state distributions of O2 show strong fluctuations, indicating the dissociation of HO2 is essentially irregular.
Momentum confinement in DIII-D shots with impurities
Authors: Stacey W. M. , Murakami M.
Published: July 2002

A neoclassical momentum transport model, consisting of gyroviscous and convective components, is applied to the analysis of momentum confinement in DIII-D [Luxon, Anderson, Batty et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 159] experiments with significant impurity content. Good agreement between predicted and measured central rotation speeds and momentum confinement times is obtained, for L-mode (low-mode) discharges with and without neon injection and for an ELMing (edge-localized modes) H-mode (high-mode) discharge. The observed improvement in momentum confinement time with increasing neon impurity content in the L-mode shots can be accounted for by a neoclassical inward convective momentum flux that increases with impurity content.
Positron annihilation study of vacancy-like defects related to oxygen precipitates in Czochralski-type Si
Authors: Brusa R. S. , Deng W. , Karwasz G. P. , Zecca A. , Pliszka D.
Published: July 2002

We report the direct measurement of vacancy-like defects related to oxygen in the oxygen precipitation process in Czochralski Si. The vacancy-like defects were detected by measuring the positron lifetime and narrowing of the positron–electron annihilation momentum distribution. Oxygen atoms surrounding the vacancy-like defects were detected by analyzing the high-momentum part of the positron–electron momentum distribution measured by a Doppler broadening coincidence technique. It was found that the majority of the defects associated with oxygen have an effective open volume smaller than that of a silicon monovacancy.
Three designs for a magnetic trap that will simultaneously confine neutral atoms and a non-neutral plasma
Authors: Dubin Daniel H. E.
Published: July 2002

Three trap designs are proposed for the simultaneous confinement of neutral atoms and a non-neutral plasma in close proximity. One design uses axially symmetric static magnetic fields with a magnetic minimum in a ring around the trap axis. Axial symmetry is required for confinement of the rotating non-neutral plasma, and the magnetic minimum traps the neutral atoms. The second design uses a rotating axially asymmetric magnetic field superimposed on a cusp field to create a time-averaged magnetic minimum (a “TOP” trap). The rotating asymmetry acts as a magnetic “rotating wall” to help confine the non-neutral plasma. In the third design, a cylindrically symmetric high-order multipole field traps the neutral atoms, which are made to rotate about the trap axis in order to avoid the magnetic null at the trap center. These designs may be useful for the production and confinement of cold antihydrogen.
Quadratic algebra associated with rational Calogero-Moser models
Authors: Caseiro R. , Françoise J.-P. , Sasaki R.
Published: July 2002

Classical Calogero–Moser models with rational potential are known to be superintegrable. That is, on top of the r involutive conserved quantities necessary for the integrability of a system with r degrees of freedom, they possess an additional set of r−1 algebraically and functionally independent globally defined conserved quantities. At the quantum level, Kuznetsov uncovered the existence of a quadratic algebra structure as an underlying key for superintegrability for the models based on A type root systems. Here we demonstrate in a universal way the quadratic algebra structure for quantum rational Calogero–Moser models based on any root systems.
Collision-induced absorption in the ν2 fundamental band of CH4. I. Determination of the quadrupole transition moment
Authors: Tipping R. H. , Brown Alex , Ma Q. , Hartmann J. M. , Boulet C. , Liévin J.
Published: July 2002

An experimental value for the quadrupole transition moment of the ν2 fundamental band of CH4 has been determined by fitting the collision-induced enhancement spectrum of CH4 with Ar as the perturber. The observed quadrupole-induced absorption increases linearly with the Ar density, ρAr, and is comparable to the allowed dipole intensity due to Coriolis interaction with the ν4 band at approximately 125 amagats. Ignoring vibration-rotation interaction and Coriolis interaction,, we equate the measured slope of the integrated intensity versus ρAr to the theoretical expression for the quadrupole-induced absorption, and obtain the value |〈0|Q|ν2〉|=0.445 ea02 for the quadrupole transition matrix element. A theoretical value 〈0|Q|ν2〉=0.478 ea02 has been determined by large-scale ab initio calculations and, considering both the theoretical approximations and experimental uncertainties, we regard the agreement as good, thus confirming our interpretation of the enhancement as due to the quadrupole collision-induced mechanism.
Dynamic mechanical behavior of filled polymers. II. Applications
Authors: Mas E. M. , Clements B. E.
Published: July 2002

A filled-polymer composite theory, based on effective medium theory, has been developed that allows the dynamic mechanical behavior to be modeled. The stress–strain behavior of the composite is derived using Schapery’s nonequilibrium thermodynamic basis for generalizing Boltzmann’s superposition principle, in conjunction with a Prony series representation for the stress relaxation moduli. Schapery’s theory allows nonlinear viscoelastic effects to be included into the mechanical response. The coefficients of the Prony series are functions of the filler’s bulk modulus, concentration, and shape. The theory is applicable to rubbery polymers and is shown to be accurate for filler concentrations up to approximately 30%. Higher concentrations can be modeled by introducing a phenomenological filler–filler correlation parameter. Effects due to heating and varying strain rate are easily accounted for in this approach. To demonstrate the utility and validity of the theory we compare its predictions to several dynamic experiments on filled-polymer composites. A discussion is provided on implementing the theory in three-dimensional finite element simulations.
Dynamics of hierarchical folding on energy landscapes of hexapeptides
Authors: Levy Yaakov , Jortner Joshua , Becker Oren M.
Published: July 2002

In this paper we apply the master equation approach to study the effects of the energy landscape topology and topography on the kinetics of folding, and on kinetic transitions of three alanine-hexapeptides analogs which involve polypeptides with neutral and charged groups and a cyclized polypeptide. We rely on the potential-energy landscapes of these molecular systems, which have been constructed using both a topological mapping analysis and a principal component analysis. It was found that the different topology and topography of the energy landscapes result in different “folding” time scales and that the systems with geometrical constraints (cyclization and opposite charges at the termini) “fold” more slowly than the unconstrained peptide. In addition, for each of the three polypeptide systems, the kinetics is nonexponential at the temperature range 400–600 K. The relaxation kinetics is characterized by logarithmic oscillations, which indicate hierarchical dynamics characterized by multiple time scales of fast (few ps) and slow (few μs) events. At higher temperatures, successive relaxation channels with similar characteristic time scales collapse into a single relaxation channel. While the kinetics of the unconstrained peptide at 600 K can be reasonably well described by a single exponential time scale, the kinetics of the constrained hexapeptides are inherently hierarchical and featured by multiple time scales even at high temperatures.
A density functional model for tuning the charge transfer between a transition metal electrode and a chemisorbed molecule via the electrode potential
Authors: Crispin Xavier , Geskin V. M. , Bureau C. , Lazzaroni R. , Schmickler W. , Brédas J. L.
Published: July 2002

A simple approach is proposed to predict the magnitude of charge transfer between a polarized transition metal electrode and a chemisorbed molecule in an electrolytic solution. The approach is based on the equalization of the chemical potentials, as defined in density-functional theory (DFT). The system is modeled by a metal cluster, a molecule, and a set of point charges (counter ions) and the properties evaluated using methods derived from DFT; specifically, we study here the chemisorption of acrylonitrile on nickel. This approach allows us to point out the various factors determining the charge transfer. The results show a linear increase in the amount of charge transferred to the chemisorbed molecule with the polarization of the metal electrode.
Magnetic properties of P-type GaMnP grown by molecular-beam epitaxy
Authors: Overberg Mark E. , Gila Brent P. , Abernathy Cammy R. , Pearton Stephen J. , Theodoropoulou Nikoleta A. , McCarthy Kevin T. , Arnason Stephen B. , Hebard Arthur F.
Published: July 2002

Growth by molecular-beam epitaxy of the dilute magnetic alloy GaMnP:C is reported. The GaMnP:C contains 9.4% Mn as determined by Auger electron spectroscopy, and is single phase as determined by x-ray diffraction, reflection high-energy electron diffraction, and transmission electron microscopy. Both magnetization and magnetotransport data are reported. The results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis at 10 K, indicating that Mn is incorporating into the GaP:C and forming the ferromagnetic semiconductor GaMnP:C. Temperature-dependent magnetization and anomalous Hall data show that magnetic behavior persists to at least 200 K, which is a very high value for a III–V based dilute magnetic semiconductor.
A numerical investigation of unsteady bubbly cavitating nozzle flows
Authors: Preston A. T. , Colonius T. , Brennen C. E.
Published: July 2002

The effects of unsteady bubble dynamics on cavitating flow through a converging-diverging nozzle are investigated numerically. A continuum model that couples the Rayleigh–Plesset equation with the continuity and momentum equations is used to formulate unsteady, quasi-one-dimensional partial differential equations. Flow regimes studied include those where steady-state solutions exist, and those where steady-state solutions diverge at the so-called flashing instability. These latter flows consist of unsteady bubbly shock waves traveling downstream in the diverging section of the nozzle. An approximate analytical expression is developed to predict the critical backpressure for choked flow. The results agree with previous barotropic models for those flows where bubble dynamics are not important, but show that in many instances the neglect of bubble dynamics cannot be justified. Finally the computations show reasonable agreement with an experiment that measures the spatial variation of pressure, velocity and void fraction for steady shockfree flows, and good agreement with an experiment that measures the throat pressure and shock position for flows with bubbly shocks. In the model, damping of the bubble radial motion is restricted to a simple “effective” viscosity, but many features of the flow are shown to be independent of the specific damping mechanism.
Calculation of the Si–H stretching–bending overtones in SiHCl3 employing ab initio potential energy and dipole moment surfaces
Authors: He Sheng-Gui , Lin Hai , Bürger Hans , Thiel Walter , Ding Yun , Zhu Qing-Shi
Published: July 2002

The Si–H stretching–bending overtones in SiHCl3 were investigated employing theoretically calculated potential energy surfaces (PES) and dipole moment surfaces (DMS). The coupled cluster method CCSD(T) was utilized to generate both one-dimensional (1D) and three-dimensional (3D) surfaces. An empirical 3D PES was also taken into consideration. The computed energy levels and band intensities agree reasonably well with observation for most of the bands. Comparison of CCSD(T) and density functional results for the very weak 2ν1 band shows that it is essential to calculate the DMS at a high level of quantum-chemical theory when cancellation of linear and quadratic contributions to the DMS is significant. The 3D ab initio PES yields more accurate band intensities than the empirical PES and therefore appears to be more realistic.
Stiction-controlled locking system for three-dimensional self-assembled microstructures: Theory and experimental validation
Authors: Agache Vincent , Quévy Emmanuel , Collard Dominique , Buchaillot Lionel
Published: July 2002

The premise of our study lies in the controlled use of the phenomenon of stiction to lock three-dimensional self-assembled polycrystalline silicon (polysilicon) microstructures. The stiction refers to the permanent adhesion of the microstructures to adjacent surfaces. It can occur either during the final stage of the micromachining process, that is to say the releasing of the microstructural material, or after the packaging of the device, due to overrange input signals or electromechanical instability. As a result, we often regard stiction as a major failure issue in the microelectromechanical systems fabrication. This letter reports both the theory of our stiction-controlled locking system operation mode and the validation of our original concept through the stiction-locking of a three-dimensional self-assembled device.
Path integral Monte Carlo applications to quantum fluids in confined geometries
Authors: Ceperley David M. , Manousakis Efstratios
Published: July 2002

Path integral Monte Carlo is an exact simulation method for calculating thermodynamic properties of bosonic systems. Properties such as superfluidity and bose condensation are directly related to multiparticle exchange cycles of individual particle paths. Such calculations of bosonic systems in confined geometries, such as helium and hydrogen on surfaces and in droplets are reviewed.
Nanosecond and femtosecond probing of the dynamics of the UV-photodissociation of perfluoroethyliodide C2F5I
Authors: Baklanov Alexey V. , Bogdanchikov Georgii A. , Aldener Mattias , Sassenberg Ulf , Persson Anders
Published: July 2002

The ns photodissociation of perfluoroethyliodide C2F5I at 266 nm has been studied by using the resonant two-photon ionization (R2PI) technique. Recoil anisotropy parameters as well as average translational energy of the I atoms in the fine structure states P1/22 and P3/22 have been determined. The main contribution (99%) to the absorption at 266 nm was found to be caused by a parallel transition to the Q03 state which gives mainly excited-state atoms I(2P1/2). The ground-state atoms I(2P3/2) were found to appear mainly (88%) from the primarily excited Q03 state via curve-crossing Q03–Q11 and to a lesser extent (12%) from direct absorption by a perpendicular transition to the Q11 and Q13 states. The fs pump–dump technique in combination with ns R2PI probing of the fragments I(2P1/2) and I(2P3/2) and time-of-flight mass spectrometry have been applied to probe the early stage dynamics of the C2F5I molecule on the excited state Q03 potential energy surface (PES). The evolution time of the excited molecule to the point where the energy gap between the excited state Q03 and the ground-state potential energy surfaces drops to a value of about 12 440 cm−1 was found to be 52±13 fs. This time corresponds to about 0.8 Å extension of the C–I bond distance. The molecular dynamics simulation with DFT calculated ground-state PES and Q03 PES with the shape calculated for methyl iodide found in the literature gives reasonable agreement with the experimental result for the evolution time.
Experimental study of axial flow in a vortex ring
Authors: Naitoh Takashi , Fukuda Naohiko , Gotoh Toshiyuki , Yamada Hideo , Nakajima Kei
Published: July 2002

For a vortex ring at Re=1600, it was found that two types of azimuthal flow (axial flow) exist inside the vortex tube. The axial flow appears when the amplitude of a wavy deformation of the vortex ring becomes appreciable. The axial flow starts to grow before the wave-breaking and its speed increases gradually. In order to investigate the relation between the deformation of the vortex core and the axial flow, the flow field was examined experimentally using smoke visualization and the smoke wire technique simultaneously. Axial flow velocities are estimated from flow visualization images.
Molecular mechanisms of photo-induced spectral diffusion of single terrylene molecules in p terphenyl
Authors: Bordat Patrice , Brown Ross
Published: July 2002

An empirical molecular model of crystals of p-terphenyl doped with terrylene is developed and compared with recent experiments on single guest molecules. The model provides an assignment of the experimental electronic origins to the substitution sites and an interpretation of the photo-induced frequency jumps of single terrylene molecules. The Stark shifts of terrylene in the different sites are estimated by a coupled Hartree–Fock semi-empirical calculation. The topology of the many-dimensional energy surface of the X1 electronic origin is explored with simulated annealing.
Theory and simulations of squeeze-out dynamics in boundary lubrication
Authors: Zilberman S. , Persson B. N. J. , Nitzan A.
Published: July 2002

The dynamics of expulsion of the last liquidlike monolayer of molecules confined between two surfaces (measured recently for the first time [J. Chem. Phys. 114, 1831 (2001)]) has been analyzed by solving the two-dimensional Navier–Stokes equation combined with kinetic Monte Carlo simulations. Instabilities in the boundary line of the expelled film produce a rough boundary for all length scales above a critical value. The squeeze-out of liquid is shown to result from the 2D-pressure gradient in the lubrication film in the contact area. The Monte Carlo simulations agrees well with experiments, reproducing most qualitative and quantitative features. In particular it shows the formation of small islands, which (in the absence of pinning mechanism) drift slowly to the periphery of the contact area. We calculate the drift velocity analytically as a function of the distance of the island to the periphery of the contact area. Experiments indicate that some kind of pinning mechanism prevails, trapping fluid pockets for very long times. When including such pinning areas in the simulations, three distinct squeeze phases and time scales were observed: (1) initial fast squeeze of most of the fluid; (2) slower squeeze of unpinned fluid pockets; (3) long term pinning of fluid pockets. We also show that a distribution of small pinning areas may produce a synergistic effect, slowing down the second phase of the squeeze, compared to a small number of big pinning areas. The paper presents a new stochastic numerical approach to problems of moving boundaries which naturally accounts for thermal fluctuations and their effect in unstable dynamics.
On the symplectic structures for geometrical theories
Authors: Cartas-Fuentevilla R.
Published: July 2002

We present a new approach for constructing covariant symplectic structures for geometrical theories, based on the concept of adjoint operators. Such geometric structures emerge by direct exterior derivation of underlying symplectic potentials. Differences and similarities with other approaches and future applications are discussed.
Design and performance of a stable first crystal mount for a cryogenically cooled Si monochromator at the Advanced Photon Source
Authors: Dufresne E. M. , Arms D. A. , Dierker S. B. , Clarke R. , Yacoby Y. , Pitney John , MacHarrie Bob , Pindak Ron
Published: July 2002

We present a new design for mounting a cryogenically cooled Si crystal which gives greatly improved beam stability. The design has been successfully implemented at the University of Michigan, Howard University, Bell Laboratories-Lucent Technologies Collaborative Access Team (MHATT-CAT) 7ID Beamline of the Advanced Photon Source. Before the installation of the new crystal mount, our Si (lll) cryogenically cooled monochromator was sensitive to the pressure fluctuations of the liquid nitrogen coolant, such that the angle of incidence on the first crystal varied linearly with the applied pressure in the cooling lines, causing beam motion of about 250 μm, 60 m from the source. The key element of the design is a symmetrically positioned cooling manifold which balances the forces caused by pressure fluctuations. With this new mount, the typical beam stability is now about 10 μm, comparable to the source stability.
The supersymmetric technique for random-matrix ensembles with zero eigenvalues
Authors: Ivanov D. A.
Published: July 2002

The supersymmetric technique is applied to computing the average spectral density near zero energy in the large-N limit of the random-matrix ensembles with zero eigenvalues: B, DIII-odd, and the chiral ensembles (classes AIII, BDI, and CII). The supersymmetric calculations reproduce the existing results obtained by other methods. The effect of zero eigenvalues may be interpreted as reducing the symmetry of the zero-energy supersymmetric action by breaking a certain Abelian symmetry.
Generalized form of the conserved quantity in constant-temperature molecular dynamics
Authors: Terada Tohru , Kidera Akinori
Published: July 2002

A generalized form of the conserved quantity in the constant-temperature molecular dynamics (MD) simulation is proposed as a measure of accuracy of MD simulations. This quantity is defined as the deviation of the distribution functions, or the Jacobian determinant, generated by the MD trajectory, from the ideal canonical value. For the Nosé–Hoover equations, this has the same form as the Hamiltonian of Nosé’s extended system. We calculated the conserved quantities for a series of constant-temperature simulations of a small protein, crambin, in water, and used them to evaluate the accuracy of the simulations under various conditions; i.e., with the Gaussian isokinetic or Nosé–Hoover equations, with flexible or rigid-body water, and with a single- or multiple-time-step algorithm. New integrators, based on the decomposition of the exponential Liouville operators, were developed for the simulation with rigid-body water. The comparison of the conserved quantities showed that the Gaussian isokinetic equations produced almost the same degree of accuracy as the Nosé–Hoover equations, and that the rigid-body treatment of water and the multiple-time-step algorithm greatly improved the accuracy.
Nuclear magnetic resonance scattering across interfaces via the dipolar demagnetizing field
Authors: Brown S. M. , Sen P. N. , Cory D. G.
Published: July 2002

The dipolar demagnetizing field in nuclear magnetic resonance of liquids is a convenient means of measuring the scattering function corresponding to a two-species spatial correlation function. In particular, the method applies where the distance to be measured is long compared to the diffusion length. Here we explore theoretically and experimentally the dipolar demagnetizing field scattering in the oil phase of a polydisperse oil/water emulsion. The initial spin grating is created in one phase, and the scattering from the other phase is observed. The experimental results confirm that structural properties of the system, including the average bubble size distribution, can be probed using the dipolar field.
Electrohydrodynamic flow of a dielectric liquid due to autonomous injection of charge by a needle electrode
Authors: Higuera F. J.
Published: July 2002

An analysis is presented of the creeping axisymmetric flow induced by Coulomb’s force in a dielectric liquid subject to autonomous unipolar injection of electric charge through a paraboloidal metallic electrode. A stationary flow and a I∝V3 current-voltage characteristic are found when the constant voltage applied between the injecting electrode and another, collecting electrode is well above the injection threshold but below a certain critical value. For larger voltages the flow and the electric current become time dependent, owing to the instability of a charged layer that surrounds the injecting electrode. Qualitative order of magnitude estimates are presented for both regimes.
Spin-orbit coupling and conical intersections in molecules with an odd number of electrons. III. A perturbative determination of the electronic energies, derivative couplings and a rigorous diabatic representation near a conical intersection
Authors: Matsika Spiridoula , Yarkony David R.
Published: July 2002

When the spin–orbit interaction is included, the character of a conical intersection in a molecule with an odd number of electrons differs dramatically from that of its nonrelativistic counterpart. In contrast to the two-dimensional branching space (η=2) in the nonrelativistic case, for these conical intersections the branching space is five-dimensional (η=5) in general, or three-dimensional (η=3) when Cs symmetry is present. Recently we have introduced an algorithm, based on analytic gradient techniques, to locate such conical intersections and used related techniques to efficiently construct and study the properties of the vectors defining the branching space. Here we extend this analysis. A perturbative description of the η=3 case is reported and used to determine the energy, derivative couplings, and a “rigorous” diabatic basis in the vicinity of a conical intersection. The perturbative results are compared with those of exact numerical calculations employing model Hamiltonians. The implications for the nuclear motion problem are discussed.
Efficient time-independent wave packet scattering calculations within a Lanczos subspace: H+O2 (J=0) state-to-state reaction probabilities
Authors: Zhang Hong , Smith Sean C.
Published: July 2002

An efficient Lanczos subspace method has been devised for calculating state-to-state reaction probabilities. The method recasts the time-independent wave packet Lippmann–Schwinger equation [Kouri et al., Chem. Phys. Lett. 203, 166 (1993)] inside a tridiagonal (Lanczos) representation in which action of the causal Green’s operator is affected easily with a QR algorithm. The method is designed to yield all state-to-state reaction probabilities from a given reactant-channel wave packet using a single Lanczos subspace; the spectral properties of the tridiagonal Hamiltonian allow calculations to be undertaken at arbitrary energies within the spectral range of the initial wave packet. The method is applied to a H+O2 system (J=0), and the results indicate the approach is accurate and stable.
Transition of the sheath structure in an electrostatic probe from electropositive to electronegative plasma
Authors: Fernández Palop J. I. , Ballesteros J. , Hernández M. A. , Morales Crespo R.
Published: July 2002

A simple model for the sheath structure which takes place in the surroundings of a metallic plane is used to analyze the Bohm criterion in electronegative plasmas. As it is well known, the Bohm criterion which determines the electric potential at the sheath edge can be multivalued in electronegative plasmas. In this article we analyze which value is the correct one. It is shown that when the positive ion current provided by choosing the highest potential at the sheath edge is equal to the one obtained by choosing the lowest, a transition in the sheath dynamics is produced. A new statement of the Bohm criterion which has great advantages is proposed.
Electronic energy dissipation in chloro-aluminum phthalocyanine/methanol system following nonlinear interaction with a train of picosecond pulses
Authors: Wei Tai-Huei , Huang Tzer-Hsiang , Hu Jung-Kuang
Published: July 2002

Using the Z-scan technique, we study the nonlinear interaction of a dilute methanolic solution of chloro-aluminum phthalyocyanine with a ∼70 ns long train of laser pulses, each of 18 ps duration. The observations are explained by taking into account, in addition to the photoexcitation of a ground state molecule by a single pulse, thermal lensing and photoabsorption by an excited molecule (singlet or triplet) produced by previous pulses. A microscopic picture of the heating process in the solution is provided.
Controlling the formation of luminescent Si nanocrystals in plasma-enhanced chemical vapor deposited silicon-rich silicon oxide through ion irradiation
Authors: Kim T. G. , Whang C. N. , Sun Yohan , Seo Se-Young , Shin Jung H. , Song J. H.
Published: July 2002

The effect of ion irradiation on the formation of luminescent Si nanocrystals from silicon-rich silicon oxide (SRSO) films deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (PECVD) whose Si content ranged from 33 to 50 at. % is investigated. As-deposited SRSO films contained a high density of irregular-shaped Si nanocrystals. Irradiating these films with 380 keV Si at room temperature to a dose of 5.7×1015 cm−2 prior to anneal at 1000 °C is found to increase the luminescence intensity due to Si nanocrystals over the films. Based on the x-ray photoemission spectra and the dependence of the luminescence intensity on the irradiating ion dose, anneal time, and the silicon content of the film, we propose the destruction of pre-existing Si clusters by ion irradiation to be an important factor responsible for the observed enhancement of luminescence, and suggest that preanneal irradiation may be a viable method to control the formation of luminescent Si nanocrystals in PECVD-deposited silicon-rich silicon oxide.
Brillouin light scattering study on the elastic properties of thick sputtered c-BN films
Authors: Wittkowski T. , Jorzick J. , Jung K. , Hillebrands B. , Keunecke M. , Bewilogua K.
Published: July 2002

The Brillouin light scattering technique is used to observe thermally excited acoustic phonons in backscattering geometry in a transparent, 1.8 μm thick diode sputtered c-BN film. The Rayleigh mode at the film surface and the quasilongitudinal bulk mode were detected under various angles of light incidence. The angular dependent phase velocity of the bulk wave provides evidence for an elastic anisotropy of the film material. Complete descriptions of the elastic properties are presented, assuming either isotropic or hexagonal film symmetry. The reduction of the film stiffness in comparison to the single crystal and the origin of the elastic anisotropy of c11/c33≈4/5, where c11 and c33 are the respective extensional stiffness constants parallel and perpendicular to the film, is discussed. Both effects are predominantly caused by sp2-bonded material and a structured film growth.
Residual dipolar coupling in the CP/MAS nuclear magnetic resonance spectra of spin-1/2 nuclei coupled to quadrupolar nuclei application of floquet theory
Authors: Kampermann H. , Bain Alex D. , Dumont R. S.
Published: July 2002

The CP/MAS spectra of spins-1/2 directly bonded to a quadrupolar nucleus (for example, C13 coupled to N14) often show fine structure, due to dipolar coupling. As the magnetic field is increased, splittings decrease and disappear. For the C13–N14 system, spectra appear as doublets with an intensity ration of 1:2. The mechanism for this is well-understood. Internuclear dipolar coupling, normally averaged out by spinning, appears because the N14 nucleus is not quantized exactly along the static magnetic field—the molecule-fixed quadrupolar interaction is not negligible compared to the lab-frame Zeeman interaction. Sample spinning cannot remove dipolar coupling if the quantization direction depends on molecular orientation. While the principle is clear, a fully detailed theory is lacking. Perturbation theory or average Hamiltonian theory work well if the quadrupolar interaction is small, or is very large, with respect to Zeeman term. However, there is little available that permits the calculation of the full spinning sideband spectra for all strengths of the quadrupolar interaction. In this paper, the spin system is solved exactly, by full matrix diagonalization, for a given orientation. This involves a careful analysis of the definition of quantization. Floquet theory is adopted to handle sample spinning—the time-dependent Hamiltonian is replaced by an expanded time-independent “Floquet” Hamiltonian. Due to the time-dependent quantization direction resulting from quadrupolar coupling, off-diagonal blocks must be computed by numerical Fourier transformation. Once the Floquet matrix is set up, and dephasing of coherences accounted, the result is a comprehensive means of simulating spectra, valid for all ranges of parameters.
Translational diffusion and reorientational relaxation of water analyzed by site–site generalized Langevin theory
Authors: Yamaguchi T. , Chong S.-H. , Hirata F.
Published: July 2002

The translational and rotational diffusion coefficients and the dielectric spectrum of water at the ambient condition are calculated using the exponential memory model previously proposed by us. The translational diffusion coefficient is in good agreement with experiments and computer simulations. However, the rotational diffusion and dielectric relaxation of the present theory are about ten times as fast as those of experiments. In order to clarify the origin of the disagreement, the memory kernel is directly obtained from the molecular-dynamics simulation and compared with the exponential model. It is found that the long-time part of the memory kernel, which is not considered in the exponential model, is dominant in the rotational diffusion and the dielectric relaxation of water.
Collapse of Lennard-Jones homopolymers: Size effects and energy landscapes
Authors: Calvo F. , Doye J. P. K. , Wales D. J.
Published: July 2002

The collapse of Lennard-Jones homopolymers is investigated by means of Monte Carlo simulations and the inherent structure/superposition approach, with special emphasis on finite size effects. At thermal equilibrium, the polymers undergo a series of phase changes from the zero temperature folded state to a coexistence state, a molten globule state, the coil state, and finally to a high-temperature “soft” state where the bond lengths vary significantly from their equilibrium value. The correlation between the thermodynamic characteristics of the polymers and those of clusters is interpreted in terms of the energy landscapes of the two systems.
Simulation of characteristics of a molecular single-electron tunneling transistor with a discrete energy spectrum of the central electrode
Authors: Shorokhov V. V. , Johansson P. , Soldatov E. S.
Published: July 2002

Current–voltage curves of molecular single-electron tunneling transistors are simulated based on a modified theory of single electronics that accounts for the discreteness of the energy spectrum of the molecule. The simulation was performed including effects of energy relaxation of the electrons in the molecule for two limiting cases of fast and slow relaxation, and for both equidistant and randomly spaced energy levels of the molecule. An efficient recursion method allowing a fast calculation of the Gibbs canonical distribution for electrons in the molecule is suggested and realized. A comparison of the simulated I–V curves with the experimental ones shows that the experimental conditions correspond to the slow relaxation case.
Characterization of strained Si/Si1−xGex/Si heterostructures annealed in oxygen or argon
Authors: Lindgren A.-C. , Chen C. , Zhang S.-L. , Östling M. , Zhang Y. , Zhu D.
Published: July 2002

The strained Si/Si1−xGex/Si layer heterostructure heat treated from 700 °C to 950 °C in Ar (annealing) or O2–C2H2Cl2 (oxidation) was characterized using high-resolution x-ray diffraction in combination with Rutherford backscattering. Only small changes to the structure are observed up to 800 °C, within the resolution limits of diffraction and backscattering. Severe strain relaxation occurs at 950 °C and the heterostructure tends to relax more during annealing in Ar than during oxidation in O2–C2H2Cl2. The strain relaxation is mainly caused by interdiffusion of Si and Ge rather than formation of misfit dislocations. Diffusion of Si interstitials generated during oxidation into the heterostructure is suggested as the cause responsible for the less pronounced interdiffusion of Si and Ge in the oxidized samples.
Diffusion influenced end-to-end reaction of a flexible polymer chain: The memory effect
Authors: Bandyopadhyay Tusar , Ghosh Swapan K.
Published: July 2002

The end-to-end diffusion dynamics of an ideal Rouse chain in solution, as estimated by fluorescence resonance energy transfer between the chain ends via Förster mechanism, is investigated theoretically through a non-Markovian reaction-diffusion equation. The resulting equation, with as well as without the memory term included, is solved numerically and the calculated survival probability, especially in the presence of memory term, compares extremely well with the recently reported Brownian dynamics simulation data for a variety of choice of Förster transfer parameters.
A combined model for {311} defect and dislocation loop evolution: Analytical formulation of kinetic precipitation model
Authors: Gencer Alp H. , Dunham Scott T.
Published: July 2002

Accurate modeling of extended defect kinetics is of primary importance for prediction of transient enhanced diffusion (TED) following ion implantation of silicon. Our previously developed moment-based model [Gencer and Dunham, J. Appl. Phys. 81, 631 (1997)] accurately accounts for formation and evolution of {311} defects and can be used to predict TED under subamorphizing conditions. Using experimental knowledge about the distribution of the {311} defect population, and making approximations on the sums that are encountered in the model, we are able to simplify this model. We demonstrate that these simplifications don’t affect the predictive capabilities of the model for {311} defect kinetics and TED. Furthermore, we are able to extend the model, under the same simplifying assumptions, to account for dislocation loop formation from {311} defect unfaulting and dislocation loop evolution, giving a unified model for interstitial aggregation in silicon. The resulting analytical model does not impose any computational speed penalty when the loop extension is turned on, making it applicable to a wide range of problems.