bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic field distribution around superconducting monofilamentary Bi-2223/Ag tape
Authors: Drulis H. , Folcik L. , Drulis M.
Published: July 2002

Profiles of the magnetic field distribution around monofilamentary Bi-2223/Ag tape have been studied by one-dimensional electron paramagnetic resonance (EPR). The profiles have been taken for zero field cooling and field cooling procedures scanning the EPR probe over the tape surface at liquid helium temperature. The measurements have been performed for an original Bi-2223/Ag tape placed in an external magnetic field perpendicular to the flat surface of the sample. The obtained results indicate that magnetic flux profiles resemble those observed in thin films or slabs. The profile taken for an artificial structure made by drilling a hole in the tape exhibits a field distribution somewhat different from the one predicted by theory.
Numerical technique for integration of the Landau–Lifshitz equation
Authors: Serpico C. , Mayergoyz I. D. , Bertotti G.
Published: July 2002

In the article, a finite difference scheme for the numerical integration of the Landau–Lifshitz equation is presented. The scheme is based on the application of the midpoint rule coupled with a suitable extrapolation formula. The important properties of the scheme are the conservation of magnetization magnitude, its linearity, its second order truncation error accuracy, and the absence of spatial coupling. The accuracy of the scheme has been extensively tested by comparing numerical solutions with exact analytical solutions and by applying the scheme to the analysis of magnetization dynamics in conducting thin films.
Simulations and analysis of the coupling process of compressible vortex pairs: Free evolution and shock induced coupling
Authors: Grasso F. , Pirozzoli S.
Published: July 2002

In the present paper the dynamics of the coupling process of compressible vortex pairs is analyzed by means of extensive numerical simulations. The objective of the study is to determine the effects of the initial vortex spatial structure and of the compressibility. Different vortex structures have been considered and their influence on the vorticity dynamics has been assessed. In the case of free evolution, the numerical simulations show that two vortices in close contact evolve toward a vortex dipole; however, the structure of the resulting dipole depends upon the initial inner core structure and on the vortex intensity. In the presence of shock wave–vortex pair interaction the coupling process depends upon the strength of the shock and the type of pair. In the case of a colliding pair, the process obeys either a two- or a three-stage mechanism depending upon the shock strength, and nonlinear couples form regardless of the initial vortex Mach number. If the interacting shock is strong, the vortex pair experiences a three-stage evolution, whereby the intermediate stage is characterized by the occurrence of a transient nonsymmetric couple. In the case of passing pair, the coupling mechanism proceeds in four stages. In particular, we observe the formation of satellite vortices, whose interaction with the primary vortex during the transient gives rise to the formation of a nonsymmetric dipole and a subsequent temporary tripole due to the feeding of shear produced vorticity. The final stage of the interaction is characterized by the formation of a nonlinear dipole.
Dusty plasma expansion with a variable charge in a spherical configuration
Authors: Djebli M. , Annou R. , Zerguini T. H.
Published: July 2002

The expansion of a dusty plasma into a vacuum with a variable charge is considered self-consistently. Due to the plasma spatial finitude, loss terms are introduced in fluid equations. Particles attachment also imposes that the Boltzmann distribution be relaxed. A nonlinear differential equations system is obtained and solved numerically. A breakdown of the quasineutrality on the front along with a buildup of scale-invariant waves are reported as well.
Dual-electrode biasing for controlling ion-to-adatom flux ratio during ion-assisted deposition of diamond
Authors: Teii Kungen , Hori Masaru , Goto Toshio
Published: July 2002

A dual-bias method using a grid mesh inserted into the front of a substrate has been employed to control the ion-to-adatom flux ratio in an inductively coupled plasma for depositing crystalline materials preferring low-energy ion bombardment. The Langmuir probe measurements revealed that the ion flux toward the substrate decreased with increasing a positive substrate bias with the grid grounded, while it increased with increasing a positive grid bias with the substrate grounded. Ion energy analyses along the diffusing plasma stream by using a probe and a mass spectrometer revealed the contribution of a high-energy tail in the ion-energy distribution into the bombarding ion flux. The ion-assisted deposition of diamond at a pressure of 10 mTorr was performed at a bombarding ion energy as low as the drifting energy (∼several eV). The results indicate the need for optimizing the ion-to-adatom flux ratio for efficient migration and clustering of precursor adatoms yielding a high nucleation density over 109 cm−2.
The dispersion relation for internal acoustic-gravity waves in a baroclinic fluid
Authors: Jones R. Michael
Published: July 2002

The dispersion relation for internal waves in a fluid is generalized from the barotropic approximation to the baroclinic case to allow for the inclination of surfaces of constant density to surfaces of constant pressure. This generalization allows the barotropic approximation to be tested in a variety of situations. The dispersion relation applies to both acoustic waves and internal gravity waves propagating in either the ocean or the atmosphere. Imaginary terms in the dispersion relation proportional to the baroclinic vector indicate energy exchange between the wave and the mean flow, a result of buoyancy being a nonconservative force in a baroclinic fluid. A buoyancy calculation shows that the baroclinic generalization of the Brunt–Väisälä frequency N is given by N2=∇ρθ⋅∇p/ρ2, where ρ is density, ρθ is potential density, and p is pressure. The baroclinicity in a weather front or cyclonic ring can sometimes have as large an effect as the Earth’s rotation on the propagation of internal gravity waves.
A general nuclear magnetic resonance analysis of hetero-association of aromatic molecules in aqueous solution
Authors: Veselkov Alexei N. , Evstigneev Maxim P. , Veselkov Dennis A. , Davies David B.
Published: July 2002

A general nuclear magnetic resonance analysis of a statistical-thermodynamical model of hetero-association of aromatic molecules in solution has been developed to take “edge effects” into consideration, i.e., the dependence of proton chemical shifts on the position of the molecule situated inside or at the edge of the aggregate. This generalized approach is compared with a previously published model, where an average contribution to proton shielding is considered irrespective of the position of the molecule in the stack. Association parameters have been determined from experimental concentration and temperature dependences of 500 MHz proton chemical shifts of the hetero-association of the acridine dye, proflavine, and the phenanthridinium dye, ethidium bromide, in aqueous solution. Differences in the parameters in the range 10%–30% calculated using the basic and generalized approaches have been found to depend substantially on the magnitude of the equilibrium hetero-association constant Khet—the larger the value of Khet, the higher the discrepancy between the two methods.
Concentration dependence of gas discharge around drops of inorganic electrolytes
Authors: Korotkov K. G. , Korotkin D. A.
Published: July 2002

This article is devoted to the study of image formation in gas discharge, initiated by a strong impulsive electromagnetic field around drops of four different nonorganic electrolytes. To describe the image mathematically we propose several parameters: the form coefficient (fractality), the entropy, and the average streamer width. We study the dependence of these parameters on concentration. The form coefficient turns out to have the best combination of stability and sensitivity in the whole range of concentrations. Statistically significant difference between the solutions and distilled water disappears at concentrations of about 2−20 N.
The influence of collision and vibrational energy on the reaction of CH3CHO+ with acetylene
Authors: Kim Ho-Tae , Liu Jianbo , Anderson Scott L.
Published: July 2002

Reaction of acetaldehyde cations with acetylene has been studied as a function of collision energy and acetaldehyde vibrational state. CH3CHO+ is state selected by resonance-enhanced multiphoton ionization, and scattered from C2H2 in a guided ion beam instrument. We have also calculated the structures and energetics of 11 different hydrogen-bonded, covalently bound, and coordination complexes, some of which are clearly intermediates in the reaction. From the product distribution, it is clear that some of most stable complexes do not participate in the reaction. The most exoergic product observed corresponds to CH3 elimination from a covalently bound complex, but this channel accounts for no more than a few percent of the total reaction signal. The dominant channel is hydrogen transfer (CH3CO++C2H3), which proceeds with >73% efficiency at low energies, dropping to a near constant 30% at energies above 1 eV. Product recoil velocity distributions indicate that this channel is complex mediated at low energies, switching to direct scattering at high energies. The hydrogen transfer reaction is weakly affected by reactant vibration, including ν3, the aldehyde CH stretch. Methyl elimination is strongly, but nonmode specifically inhibited by vibration.
Performance assessment of adiabatic quantum cellular automata
Authors: Bonci L. , Iannaccone G. , Macucci M.
Published: July 2002

We investigate the feasibility of an adiabatic logic scheme for cellular automaton systems recently proposed by Tóth and Lent [J. Appl. Phys. 85, 2977 (1999)], based on structured arrays of six-dot cells and on the use of a four-phase trapezoidal clock. With respect to the original quantum cellular automaton (QCA) concept, focused on ground state computation, a clocked scheme would have the advantage of being immune to the presence of metastable states and would allow pipelined operation, at the expense of additional complexity, due to the clock distribution circuitry. Based on realistic cell geometries and material systems, we have evaluated the obtainable switching times of QCA cells, and have determined the region in the parameter space that allows operation at a reasonable clock speed.
Quantum Monte Carlo characterization of small Cu-doped silicon clusters: CuSi4 and CuSi6
Authors: Ovcharenko I. V. , Lester W. A. , Xiao C. , Hagelberg F.
Published: July 2002

The relative energies, binding energies, and adsorption energies of three CuSi4 and two CuSi6 clusters have been computed in the fixed-node diffusion Monte Carlo (FNDMC), CASSCF, and B3LYP DFT methods. These results are compared with the earlier Hartree–Fock (HF) and B3LYP DFT investigations of these systems by two of us [C. Xiao and F. Hagelberg, J. Mol. Struct.: THEOCHEM 529, 241 (2000)]. The very close energy level spacing of the isomers under consideration confirms the previous work of Xiao and Hagelberg. The FNDMC results show some qualitative discrepancies with B3LYP DFT, and HF findings. They also confirm the appropriateness of the B3LYP DFT method for the prediction of the most stable CuSi4 isomer, while the CASSCF method compares more favorably with FNDMC for adsorption energies than B3LYP DFT.
Indium interdiffusion in annealed and implanted InAs/(AlGa)As self-assembled quantum dots
Authors: Surkova T. , Patanè A. , Eaves L. , Main P. C. , Henini M. , Polimeni A. , Knights A. P. , Jeynes C.
Published: July 2002

We investigate indium interdiffusion in InAs/(AlGa)As self-assembled quantum dots by studying the changes in the optical properties of the system induced by ion implantation and/or thermal annealing. Interdiffusion of In–Ga and In–Al atoms at the interface between the dot and the (AlGa)As barrier takes place in as-grown samples and is enhanced by the postgrowth treatments. In contrast to the proposed interdiffusion as the way for suppressing the optical emission from the wetting layer, we show that it drives the system towards a predominantly two-dimensional morphology.
Planar dielectric microcavity light-emitting diodes: Analytical analysis of the extraction efficiency
Authors: Royo P. , Stanley R. P. , Ilegems M.
Published: July 2002

This article presents a simple analytical expression for the extraction efficiency of planar dielectric microcavity light-emitting diodes (MCLEDs) based on two distributed Bragg reflectors (DBRs). The calculations take into account the optical properties of the resonator (reflectivities and index contrast of the DBRs, cavity length, refractive index of the outside medium), as well as the spectral properties of the active material. The impact of these factors on the extraction efficiency is investigated and compared to the results of exact numerical calculations. Simple design rules for optimizing the extraction efficiency of MCLEDs are derived and discussed for display and fiber coupling applications.
Photoluminescence of poly(p-phenylenevinylene)–silica nanocomposites: Evidence for dual emission by Franck–Condon analysis
Authors: Ho Peter K. H. , Kim Ji-Seon , Tessler Nir , Friend Richard H.
Published: July 2002

The vibronic mode intensity pattern of the photoluminescence (PL) spectra of poly(p-phenylenevinylene) (PPV) nanocomposites dispersed with 5-nm-diam silica particles shows an apparent redistribution toward the nominal 0–0 mode with increasing silica volume fraction. Franck–Condon analysis of this variation, corrected for refractive index dispersion, reveals the presence of overlapping emission from two excited electronic states separated by 180 meV. The principal emission arises from the molecular exciton while the lower-lying one is assigned to a dipole–dipole coupled two-chain aggregate exciton. The quantum yield of the aggregate emission decreases monotonically with silica loading up to 50 vol %, whereas that of the molecular state exhibits a maximum at 15 vol %. When the samples are photoexcited below the π-π* localization edge, both of these emissions jointly redshift without a change in their relative intensities. When cooled below a transition temperature centered at 120 K, the yield of the aggregate exciton decreases sharply relative to the molecular exciton and the overall PL quantum yield (ηpl) rises. The aggregate exciton therefore appears to be formed from the molecular exciton through a phonon-assisted mechanism. At room temperature, this directly competes with de-excitation of the molecular exciton. This behavior differs from the dialkoxy-PPVs which show site-selective excitation and thus direct population of the aggregate domains. Using classical dielectric medium theories to correct for the effects of refractive index, the radiative lifetime (τr) of the molecular exciton in the various PPV compositions can be estimated. Together with the experimentally determined ηpl, this gives the ηplτr product of the molecular exciton as a function of composition. This function exhibits a maximum at 15 vol % silica, indicative of a crossover behavior that shows the competing influence of morphological disorder on the population and radiative de-excitation of this state.
Radiative and predissociative lifetimes of the A 2Σ+ state (v′=0,1) of SH and SD: A highly correlated theoretical investigation
Authors: Resende Stella M. , Ornellas Fernando R.
Published: July 2002

Doublet and quartet states of the HS radical correlating with H(2S)+S(3P,1D,1S) were investigated by ab initio calculations, at the CASSCF-MRCI/aug-cc-pV5Z level of theory. Molecular parameters and spectroscopic constants obtained for both the ground (X 2Π) and the first excited (A 2Σ+) states represent the best overall theoretical description of this system to date. Transition moments, transition probabilities, and radiative and predissociative lifetimes were also determined for the X 2Π–A 2Σ+ system. The values calculated for the radiative lifetimes of the A state show that previous results were too large. Theoretical predissociative lifetimes, although quite sensitive to the region of crossing of the potential energy curves, reproduce the experimental trends.
Mode-coupling approach for spin-facilitated kinetic Ising models
Authors: Einax Mario , Schulz Michael
Published: July 2002

The d-dimensional 2-spin facilitated kinetic Ising model is studied analytically starting from usual master equations and their transformation into a Fock-space representation. The evolution of relevant operators is rewritten in terms of a projection formalism. The obtained frequency matrices and memory terms are analyzed. Especially, the structure of the memory terms is approached by using the well-known factorization approximation as the standard decomposition technique of N-point correlation functions. The temperature dependence of the relaxation times related to the 2-spin facilitated kinetic Ising model shows a non-Arrhenius behavior. Furthermore, a characteristic stretched decay of the correlation function is obtained.
Influence of the wetting-layer growth kinetics on the size and shape of Ge self-assembled quantum dots on Si(001)
Authors: Kim H. J. , Xie Y. H.
Published: July 2002

The growth temperature of the wetting layer is used as the key variable in the study of a series of Ge self-assembled quantum dots on Si(001) substrates. A relaxed SiGe buffer layer is used for most of the samples as a means of maintaining the density of Ge dots. Ge dots are in the shapes of pyramids, domes, and superdomes, similar to those reported in the literature. A significant difference in the fraction of pyramids is observed between samples with wetting layers grown at low (280 °C) and high (650 °C) temperatures. This difference is in turn dependent on the total amount of Ge deposited or equivalently, the average size of dots. These observations point to the presence of Si during the initial stage of Ge dot formation and that the fraction of pyramids as well as the critical size for the transition from pyramids to domes are influenced by the presence of Si.
Improving reduced dimensionality quantum reaction dynamics with a generalized transition state. Application to CH4+O(3P)
Authors: Palma Juliana , Clary David C.
Published: July 2002

We present a new procedure to calculate rate constants from reduced dimensionality reaction probabilities. The method combines an energy-shifting correction, as used in reduced dimensionality calculations of three and four-atom reactions, with a generalized transition state. The procedure, in combination with a recently developed reduced dimensionality model, is used to calculate rate constants for CH4+O(3P)→CH3+OH and its fully deuterated counterpart. These rate constants are compared with the ones obtained using the standard technique and with experimental values. Also, we study the effect on reactivity of exciting selected modes of methane. Similarities and differences between the deuterated and undeuterated reactions are discussed.
Liquid-gas phase behavior of an argon-like fluid modelled by the hard-core two-Yukawa potential
Authors: Pini D. , Stell G. , Wilding N. B.
Published: July 2002

We study a model for an argon-like fluid parameterized in terms of a hard-core repulsion and a two-Yukawa potential. The liquid-gas phase behavior of the model is obtained from the thermodynamically Self-Consistent Ornstein–Zernike Approximation (SCOZA) of Høye and Stell, the solution of which lends itself particularly well to a pair potential of this form. The predictions for the critical point and the coexistence curve are compared to new high resolution simulation data and to other liquid-state theories, including the hierarchical reference theory (HRT) of Parola and Reatto. Both SCOZA and HRT deliver results that are considerably more accurate than standard integral-equation approaches. Among the versions of SCOZA considered, the one yielding the best agreement with simulation successfully predicts the critical point parameters to within 1%.
A method to incorporate the radial distribution function of bulk fluid into the density functional approximation
Authors: Zhou Shiqi
Published: July 2002

A systematic methodology is proposed to deal with the weighted density approximation version of classical density functional theory by employing the knowledge of the radial distribution function of bulk fluid. The present methodology results from the concept of the universality of the free energy density functional combined with test particle method. It is shown that the new method is very accurate for the predictions of the density distribution of a hard sphere fluid at different confining geometry. The universality of the present methodology is discussed.
Electron–electron counterbalance density for molecules: Exchange and correlation effects
Authors: Valderrama Elmer , Fradera Xavier , Ugalde Jesus M.
Published: July 2002

Numerical results for the electron–electron counterbalance density in a series of molecules are reported. A compact density-dependent expression for the electron–electron counterbalance density in symmetric systems is used in the discussion of the importance of the exchange and correlation effects in electron-pair distributions. In addition, the behavior of the electron–electron counterbalance density in the dissociation process of the H2 molecule is analyzed in the light of the occurrence of strong nondynamical correlation effects. Finally, the advanced density-dependent expressions allow a direct comparison with the charge concentration index and the electron–electron coalescence density, thus revealing the decisive role of exchange and correlation effects in describing the chemical bonding in molecules.
Vibrational self-consistent field approach to anharmonic spectroscopy of molecules in solids: Application to iodine in argon matrix
Authors: Bihary Z. , Gerber R. B. , Apkarian V. A.
Published: July 2002

An extension of the vibrational self-consistent field (VSCF) method is developed for quantitative calculations of molecular vibrational spectroscopy in a crystalline solid environment. The approach is applicable to fields such as matrix-isolation spectroscopy and spectroscopy of molecular crystals. Advantages of the method are that extended solid vibrations and their coupling to intramolecular modes are incorporated, and that the treatment includes anharmonic effects, both due to the intrinsic property of individual modes and due to coupling between modes. Suitable boundary conditions are adopted in treating the solid environment. In applications, e.g., molecules in rare-gas crystals, hundreds of coupled molecular and matrix modes can be handled computationally. The method is applied to the vibrational matrix-shift of iodine in an argon matrix, and the calculated overtone frequencies are compared to experimental values obtained from both time-domain coherent Raman and frequency-domain Resonance Raman measurements. The physical origin of the shifts is interpreted in detail, and the properties of the iodine–argon interactions essential to obtain the correct sign and magnitude of the shift are elucidated. An I2–Ar potential, based on anisotropic atom–atom interactions and fitted to ab initio calculations, gives the best agreement with experiment. The results show that the VSCF solid-state approach is a powerful tool for matrix spectroscopy.
Fully ab initio atomization energy of benzene via Weizmann-2 theory
Authors: Parthiban Srinivasan , Martin Jan M. L.
Published: July 2002

The total atomization energy at absolute zero, (TAE0) of benzene, C6H6, was computed fully ab initio by means of W2h theory as 1306.6 kcal/mol, to be compared with the experimentally derived value 1305.7±0.7 kcal/mol. The computed result includes contributions from inner-shell correlation (7.1 kcal/mol), scalar relativistic effects (−1.0 kcal/mol), atomic spin–orbit splitting (−0.5 kcal/mol), and the anharmonic zero-point vibrational energy (62.1 kcal/mol). The largest-scale calculations involved are CCSD/cc-pV5Z and CCSD(T)/cc-pVQZ; basis set extrapolations account for 6.3 kcal/mol of the final result. Performance of more approximate methods has been analyzed. Our results suggest that, even for systems the size of benzene, chemically accurate molecular atomization energies can be obtained from fully first-principles calculations, without resorting to corrections or parameters derived from experiment.
Ab initio multireference configuration interaction study of the electronic spectra of carbon chain anions C2n+1− (n=2–5)
Authors: Cao Zexing , Peyerimhoff S. D. , Grein Fritz , Zhang Qianer
Published: July 2002

Multireference configuration interaction calculations on the vertical transition energies of the low-lying excited states in carbon chain anions C2n+1− (n=2–5) are carried out. Calculated vertical term energies confirm the previously suggested assignments to the first and second Π2←X2Π band systems in matrix isolation spectroscopy. The lowest 1 2Π←X 2Π electronic transitions of C5−, C7−, C9−, and C11−, are calculated at 2.66, 2.27, 1.90, and 1.54 eV, respectively, with large oscillator strengths. Relevant theoretical evidences for a more detailed assignment of the observed transitions are presented. Generally, corresponding excitation energies in the C2n+1− carbon chain anions are found descending with an increase of the chain. The equilibrium geometries and harmonic vibrational frequencies of these linear carbon chains determined by the density functional theory approach also show reasonable agreement with available experimental data.
Electroluminescence thermal quenching in alternating-current thin-film electroluminescent devices
Authors: Baukol B. A. , Hitt J. C. , Wager J. F. , Sun S.-S.
Published: July 2002

Electroluminescence (EL) thermal quenching in alternating-current thin-film electroluminescent (ACTFEL) devices refers to a reduction in the luminance with increasing temperature, which is in excess to that of normal thermal quenching and is concomitant with a reduction in the transferred charge. A comparison of thermal quenching trends for ZnS:Mn, SrS:Ce, SrS:Cu,Ag, and multilayer SrS:Cu,Ag/SrS:Ce ACTFEL devices is presented. Respectively, ZnS:Mn; SrS:Cu and SrS:Cu,Ag; and SrS:Ce and multilayer SrS:Cu,Ag/SrS:Ce ACTFEL devices exhibit very little (normal), a large amount (of primarily EL), or a small amount (of primarily normal) thermal quenching. Insertion of one or more interface layers of SrS:Ce significantly reduces the extent of EL thermal quenching in a SrS:Cu,Ag ACTFEL device. Simulation suggests that SrS:Ce interface layers decrease the rate and displace the location of hole creation by band-to-band impact ionization away from the SrS:Cu,Ag layer, where hole trapping at Cu or Ag acceptor sites leads to EL thermal quenching via thermally activated annihilation of positive space charge due to thermionic emission of holes from Cu or Ag acceptor traps.
Optical characterization of excess carrier lifetime and surface recombination in 4H/6H–SiC
Authors: Galeckas A. , Linnros J. , Frischholz M. , Grivickas V.
Published: July 2002

The high-injection lifetime and surface recombination parameters have been investigated in as-grown 4H and 6H–SiC epilayers subjected to various process treatments. A depth-resolved optical transient absorption technique was utilized to evaluate the influence of film thickness and surface treatment on carrier lifetime. We demonstrate that besides polishing and ion implantation, both natural and thermal oxidation may also result in lifetime reduction due to enhanced surface losses. Moreover, a long-term stability test has revealed a substantial degradation of lifetime characteristics, consistent with a spontaneous surface oxidation and slow relaxation of SiO2/SiC interface states. We show that for common film thickness <100 μm, the effective lifetime is dominated by surface leakage, which is found, generally, to be higher in 4H compared to 6H–SiC.
The dielectric relaxation time of ice V, its partial anti-ferroelectric ordering and the role of Bjerrum defects
Authors: Johari G. P. , Whalley E.
Published: July 2002

The dielectric relaxation spectra of ice V has been studied over the 133–270 K range, at frequencies in the 0.05 Hz–300 kHz range. Its dielectric relaxation time, τ, is 1.5 s at 133 K. The Arrhenius plot of its τ is found to bend sharply at ∼200 K, and the Arrhenius energy decreases from 49.9 kJ/mol at 260 K to 23.3 kJ/mol at 160 K. This confirms the inference from the orientational ordering studies that τ of ice V at 125 K is much less than 100 s [J. Chem. Phys. 112, 7169 (2000)]. The increase in the equilibrium permittivity on cooling ice V from 266 to 133 K is 77% of that anticipated from a decrease in its volume and thermal energy. The discrepancy is attributed to a gradual increase in orientational ordering of water molecules in its structure, leading to a partially antiferroelectric state at low temperatures. A comparison made with the earlier studies of uncontaminated (H2O and D2O) ice Ih and N2- and CO2-contaminated (H2O) ice Ih (impurities in the latter used to generate extrinsic Bjerrum defects), showed that the decrease in the Arrhenius energy on cooling ice V may be explained without invoking a change in mechanism from Bjerrum defects diffusion to ionic defects diffusion with zero activation energy.
An edge pedestal model based on transport and atomic physics
Authors: Stacey Weston M.
Published: July 2002

A model is presented for the calculation of the characteristic scale lengths from transport considerations in the edge pedestal region of high confinement (H-mode) plasmas. The model is based on the requirements of heat and particle removal through the edge. Atomic physics effects on edge density and temperature gradient scale lengths are taken into account. An empirical fit for the width of the edge pedestal transport barrier is employed. Model problem calculations indicate that the model predicts the magnitudes and some trends of characteristic gradient scale lengths observed in current experiments.
Dot patterns from second-harmonic and sum-frequency generation in polycrystalline ZnSe
Authors: Chinh Tran Duc , Seibt Wolfgang , Siegbahn Kai
Published: July 2002

During a comparative study of second-harmonic generation (SHG) and sum-frequency generation (SFG) in single-crystal and polycrystalline ZnSe, the polycrystalline material showed a dot pattern in the SHG as well as in the SFG output. No such pattern was found in the output from the single-crystal ZnSe sample. The second-harmonic and also the sum-frequency dot pattern, resembling a diffraction pattern, could be observed over a wide tuning range of the fundamental beam, between 1.1 and 1.6 μm, generated by a femtosecond optical parametric amplifier. The size of the observed pattern is dependent on wavelength and the dots show a characteristic polarization.
Effect of dc bias on dielectric properties of Cd2Nb2O7 ceramics
Authors: Ang Chen , Bhalla A. S. , Guo Ruyan , Cross L. E.
Published: July 2002

The effect of dc bias on dielectric properties of cadmium pyroniobate Cd2Nb2O7 ceramics is studied in this article. Without dc bias, two obvious dielectric peaks around 80 and 180 K are observed; by applying dc bias, the two peaks are greatly suppressed and finally eliminated at 15 kV/cm, however, a peak with no frequency dispersion shows up at 192 K. The results show high electric-field tunability of the dielectric constant with a low dielectric loss at radio frequencies. The electric-field dependence of the dielectric constant around 80 K can be well described by the modified Devonshire relation including the Langevin-type cluster term, i.e., ε(E)=ε1−ε2E2+ε3E4+(Pjxj/ε0)[cosh(Exj)]−2. The fit parameters indicate that the polar cluster carries polarization P=∼0.28–0.65 mC/m2 with the cluster size of L=∼24–30 nm.
Refractive index change caused by electron irradiation in amorphous As–S and As–Se thin films coated with different metals
Authors: Nordman Nina , Nordman Olli
Published: July 2002

The refractive index change caused by electron irradiation was measured in amorphous As–S and As–Se thin films coated with different metals. Metal atoms/ions diffused into the films during irradiation. The diffusion was dependent on the metal and influenced the refractive index. The influence was smallest in As40S60 films although these films possessed the highest overall refractive index changes. Au atoms/ions were almost immobile in all films while Ag atoms/ions had the highest mobility. Their high mobility allowed them to diffuse laterally within the film.
The effect of thermal annealing on the properties of Al–AlOx–Al single electron tunneling transistors
Authors: Scherer H. , Weimann Th. , Zorin A. B. , Niemeyer J.
Published: July 2002

The effect of thermal annealing on the properties of Al–AlOx–Al single electron tunneling transistors is reported. After treatment of the devices by annealing processes in forming gas atmosphere at different temperatures and for different times, distinct and reproducible changes of their resistance and capacitance values were found. According to the temperature regime, we observed different behaviors with regard to the resistance changes, namely the tendency to decrease the resistance by annealing at T=200 °C, but to increase the resistance by annealing at T=400 °C. We attribute this behavior to changes in the aluminum oxide barriers of the tunnel junctions. The good reproducibility of these effects with respect to the changes observed allows the proper annealing treatment to be used for postprocess tuning of tunnel junction parameters. Also, the influence of the annealing treatment on the noise properties of the transistors at low frequency was investigated. In no case did the noise figures in the 1/f regime show significant changes.
Representations of Uεres(sl2) via restricted q-Fock spaces
Authors: Liu Xufeng , Sun Changpu
Published: July 2002

Two restricted C[q,q−1]-forms of the well known q-boson algebra are introduced and the corresponding restricted q-Fock spaces defined. All of the irreducible highest weight representations, including the infinite dimensional ones, of Uεres(sl2) of type 1 are constructed through the restricted q-Fock spaces.
Anomalous behavior of Sb implanted Si after mega-electron-volt carbon irradiation
Authors: Dey Soma , Varma Shikha
Published: July 2002

We have investigated the dopant behavior of 1.5 MeV implanted Sb in Si(100) both prior to and following irradiation with 8 MeV C+ ions. The irradiation stimulates the regrowth in silicon lattice and induces a high Sb substitution of 93% after a thermal anneal of 400 °C. At higher temperatures, a significant Sb diffusion towards the amorphous/crystalline interface is detected in the ion-irradiated sample. It is suggested that the vacancy supersaturation produced during ion irradiation is dominantly responsible for the significant modification of dopant substitution, redistribution, and diffusion. Point defects created during precipitation also seem to influence the dopant diffusion.
Defect generation and breakdown of ultrathin silicon dioxide induced by substrate hot-hole injection
Authors: Vogel Eric M. , Edelstein Monica D. , Suehle John S.
Published: July 2002

Hole-induced bulk and interface defect generation and breakdown in ultrathin silicon dioxide (2.0 and 3.0 nm) are studied using substrate hot-hole injection. The results show that although these substrate hot holes are effective in creating electrically active damage in the dielectrics, these defects are very ineffective in causing breakdown as compared to those defects created by constant voltage tunneling stress. Identical to hole trapping in thicker oxides, substrate hot-hole defect generation was independent of electric field, decreased with decreasing thickness, and increased with decreasing temperature. The defect generation and breakdown of ultrathin oxides by substrate hot-hole stress is significantly different from that observed for constant voltage tunneling stress. The results suggest that the degradation and breakdown of ultrathin silicon dioxide cannot be explained by the trapping of hot holes alone.
Pressure effect on magnetic and transport properties of Zn doped La0.91Mn0.95O3 manganite
Authors: Markovich V. , Rozenberg E. , Gorodetsky G. , Mogilyansky D. , Revzin B. , Pelleg J.
Published: July 2002

The effect of hydrostatic pressure P up to 1 GPa and applied magnetic fields H up to 1.5 T on the magnetic and transport properties of La0.91Mn0.95O3 doped with 7 at. % Zn at the temperature range of 15–350 K is reported. It was found that the doping by Zn changes the low temperature conductance from a metallic-like to a nonmetallic one. Doping by Zn of the initial self-doped sample shifts the Curie temperature from 252 to 204 K. In both compounds the applied pressure enhances the ferromagnetic coupling and Curie temperature. The pressure coefficients of the Curie temperature are equal for both compounds being dTC/dP≈13 K/GPa. Above the Curie temperature the resistivity of Zn-doped manganite obeys Mott’s law rather than the Arrhenius behavior. Both parameters, A and T0 of Mott localization law decrease under pressure indicating a reduction in the localization energy. © 2001 American Institute of Physics.
Plasma expansion in the preshock region
Authors: Burm K. T. A. L. , Goedheer W. J. , Schram D. C.
Published: July 2002

The supersonic expansion of an underexpanding argon plasma from a high density arc source with small dimensions into a low-pressure vessel with large dimensions is studied by an extended one-dimensional nonlocal thermal equilibrium fluid model, called SPIRIT. In an expanding plasma the velocity increases and the pressure, the density, and the temperatures decrease severely. In this article the virtual source model is discussed first, which is a model describing the expanding plasma as originating from a virtual source. The virtual source model includes some viscosity and heat transport in simplified form, but most of the viscosity and heat transport contributions are neglected. The SPIRIT code includes the full energy and momentum balances. The inclusion of viscosity and heat sources may lead to deviations from an adiabatic and/or isentropic expansion. The SPIRIT code can analyze the deviations. When deviations are small, the isentropic expressions from gas dynamics can be used to model expanding plasma too. Model outcomes are compared with experimental data.
Specific rate constants k(E,J) for the dissociation of NO2. I. Time-resolved study of rotational dependencies
Authors: Abel B. , Kirmse B. , Troe J. , Schwarzer D.
Published: July 2002

The effect of rotational excitation on the specific rate constants k(E,J) of the unimolecular decomposion of NO2 has been investigated. Time-resolved pump- and probe experiments with sub-ps time resolution are reported in which NO2 molecules with well-defined rotational and vibrational energy distributions were optically excited near and above the dissociation threshold. The subsequent unimolecular decay of the reacting NO2 molecules was probed by time-resolved laser-induced fluorescence of the disappearing NO2 via excitation to Rydberg states. At constant photolysis wavelength, increasing rotational energy (up to 310 cm−1) was found to leave the overall decay rate nearly unaffected. This observation can be rationalized by assuming a compensation of the angular momentum and energy dependences of the specific rate constants when J and E are changed at the same time. Keeping the total energy E nearly constant and changing J independently, the effect of rotation on the decay rate can be separated and observed more clearly. From the experimental data we conclude that, for sufficiently high J and constant total energy, molecules with larger J dissociate more slowly than molecules with small J, which is in agreement with predictions from statistical unimolecular rate theory.
Backward Raman amplification of ionizing laser pulses
Authors: Malkin V. M. , Fisch N. J.
Published: July 2002

Ultraintense laser pulse compression by means of backward Raman scattering of a laser pump in plasma might be accomplished alternatively, and possibly technologically more simply, at ionization fronts.
On the stochastic mechanics of the free relativistic particle
Authors: Pavon Michele
Published: July 2002

Given a positive energy solution of the Klein–Gordon equation, the motion of the free, spinless, relativistic particle is described in a fixed Lorentz frame by a Markov diffusion process with nonconstant diffusion coefficient. Proper time is an increasing stochastic process and we derive a probabilistic generalization of the equation (dτ)2=−(1/c2)dXν dXν. A random time-change transformation provides the bridge between the t and the τ domain. In the τ domain, we obtain an M4-valued Markov process with singular and constant diffusion coefficient. The square modulus of the Klein–Gordon solution is an invariant, nonintegrable density for this Markov process. It satisfies a relativistically covariant continuity equation.
Calogero–Moser systems with periodic and doubly periodic interaction potentials and loop algebras
Authors: Fleury M.
Published: July 2002

For a finite number of particles on the line pairwise interacting with 1/r2 potential, the positions are given by the eigenvalues of some time-dependent matrix. Infinite periodic or doubly periodic replication of the particles yields Calogero–Moser systems with periodic or doubly periodic interaction potential. We are thus led to consider matrices of infinite order, which are identified with Fourier series with matrix coefficients, depending on an additional parameter. These distributional loops of matrices (tori of matrices in the doubly periodic case) are shown to obey simple (partial) differential equations, which allow us to determine them explicitly. Thus we obtain the already known solution of the Calogero–Moser system on the circle, and provide a new insight for the system on the torus.
Perturbation theory free from secular terms for the equations of motion of anharmonic oscillators
Authors: Fernández Francisco M.
Published: July 2002

We develop a time–independent perturbation theory in operator form for the equations of motion of classical and quantum-mechanical anharmonic oscillators. The method focuses on the frequency of the motion in the former case and in a frequency operator in the latter, producing a power series in a small coupling constant. The resulting expressions are free from secular instabilities that commonly appear in perturbation treatments of nonlinear problems.
Temperature evolution of the translational density of states of liquid water
Authors: De Santis Alberto , Ercoli Alessandro , Rocca Dario
Published: July 2002

The molecular dynamics technique is used to study the relative dynamics of tagged pairs of molecules and to derive the related translational density of states (DOS) of liquid water at 243, 273, and 373 K. The modes that compose the short-time dynamics of centers of mass are obtained. The dynamical quantities studied are characterized by a fast-time decay followed by a plateau whose height increases with the temperature and with the initial pair separation. The plateau is attributed to the nonharmonic motions and its height is related to the pair relative diffusion coefficient. An exponential relaxation is used to represent the way the system follows to reach the diffusive behavior; the derived relaxation times agree with those reported in the literature describing the fast translational dynamics. The frequencies of the other short-time modes are related to the main frequencies of the solid, while the mode damping is analyzed in terms of the damped harmonic oscillator model; it is found that the Gaussian damping gives a better fit to the DOS than the exponential one. The temperature evolution of the various modes is investigated and related to the microscopic pair dynamics. In particular, the modes at low frequencies (ω≅50 cm−1) are absent in the vibrations along the hydrogen bond (longitudinal modes); they are present in the transverselike dynamics of other pair states. The temperature increase produces the intensity decrease of the 50-cm−1 band and the pile-up of the DOS intensity towards zero frequency. The decay constants of these two effects have a different temperature dependence, which confirms the oxygen bending nature of the 50-cm−1 band and its independence on the relaxational-like dynamics.
Net current generation and beam transport efficiency of grad-B-drift transported relativistic electron beams
Authors: Rose D. V. , Welch D. R. , Ottinger P. F. , Schumer J. W.
Published: July 2002

Numerical simulations of grad-B drifting, high-current, relativistic electron beams are presented. The simulations use a hybrid fluid/particle-in-cell code to study the net-current and conductivity evolution for 200 to 900 kA, 1.3 MeV annular electron beams in a background gas of nitrogen (N2) at pressures of 1–60 Torr. Optimum guide-wire current and gas pressure for efficient beam transport are found from the simulations to be ∼40 kA and ∼5–15 Torr of N2, respectively, with energy transport efficiencies as high as 80% for transport distances up to 200 cm. For beam currents and/or gas pressures near the high end of the ranges considered, large net currents significantly alter the magnetic-field profile and result in decreased transport efficiency. Transport efficiencies up to 90% are also found for a self-pinched transport mode in the 1–15 Torr N2 range with no wire current.
Improved atomic force microscope cantilever performance by ion beam modification
Authors: Hodges Alex R. , Bussmann Konrad M. , Hoh Jan H.
Published: July 2002

The performance of atomic force microscopy cantilevers, as measured by the resonant frequency and spring constant, is directly dependent on the shape of the cantilever. Here we have improved the performance of conventional silicon nitride cantilevers by using focused ion beam milling to minimize the width of the cantilever legs. The resonant frequency in solution for any given spring constant is increased by two- to threefold, and the thermal noise in a given bandwidth is correspondingly reduced.
Effect of gas heating on the spatial structure of a traveling wave sustained Ar discharge
Authors: Henriques J. , Tatarova E. , Dias F. M. , Ferreira C. M.
Published: July 2002

In this work we report a theoretical and experimental study of the influence of gas heating on the spatial structure of a microwave Ar discharge sustained by a traveling surface wave. The theoretical analysis is based on a discharge model which couples in a self-consistent way electron and heavy particle kinetics, discharge electrodynamics, and gas thermal balance. The set of coupled equations used includes the electron Boltzmann equation, the rate balance equations for the most important excited species and charged particles, the gas thermal balance equation, and the equations describing wave propagation and power dissipation. The principal collisional and radiative processes which determine the populations in the Ar(3p54s) and Ar(3p54p) levels are accounted for. The field strength necessary for steady-state discharge operation is obtained from the balance between total rates of ionization (including direct and step-wise ionization and energy pooling reactions) and of electron loss due to the diffusion to the wall and bulk recombination. The gas thermal balance equation is solved using the experimentally obtained wall temperature as a boundary value. The model determines the axial discharge structure, i.e., the axial variation of the main discharge quantities. An experimental validation of the model predictions is achieved using probe techniques, emission spectroscopy, and radiophysics methods. In particular, spatially resolved measurements of the electron energy distribution function, gas temperature, wave electric field components, and wave attenuation have been carried out. As a result of the nonuniform wave power absorption along the wave path the gas temperature varies along the column. This variation induces axial changes in the neutral density and the reduced electric field which strongly affects the particle kinetics and the discharge electrodynamics, as demonstrated here.