bluuebunny/crossref_metadata_2025 huggingface dataset:
Giant enhancement of orbital moments and perpendicular anisotropy in epitaxial Fe/GaAs(100)
Authors: Xu Y. B. , Tselepi M. , Guertler C. M. , Vaz C. A. F. , Wastlbauer G. , Bland J. A. C. , Dudzik E. , van der Laan G.
Published: July 2002

The spin and orbital magnetic moments and the perpendicular magnetic anisotropy of 8 and 33 monolayer epitaxial bcc Fe films grown on GaAs(100)-4×6 have been measured using x-ray magnetic circular dichronism and polar magneto-optical Kerr effect. Both the films have approximately the same spin moments of about 2.0μB close to that of the bulk value. The ultrathin film shows a giant orbital moment enhancement of about 300% with respect to the bulk value and a perpendicular interface anisotropy field HsFe–GaAs of the order of −5×104 Oe. This may be partially due to an increased degree of localization of electronic states at the Fe/GaAs interface associated with the atomic scale interface structure.
Vibrational de-excitation of v=1 H2 during collisions with a Cu(100) surface
Authors: Mowrey R. C. , McCormack D. A. , Kroes G. J. , Baerends E. J.
Published: July 2002

The dynamics of vibrational de-excitation of v=1 H2 on a Cu(100) surface is studied using a six-dimensional quantum wave packet method. The de-excitation probability increases with increasing collision energy and initial molecular rotational quantum number, j. A strong dependence on molecular orientation is found with molecules rotating with helicoptering motion (mj=j) exhibiting larger de-excitation probabilities, in general, than those with cartwheeling motion (mj=0). The final j-state distribution and quadrupole alignment are computed as functions of collision energy. The competition between vibrational de-excitation and other dynamic processes during the collision is analyzed. The total de-excitation probability is in good agreement with vibrational inelasticities from experiment but the calculations overestimate the population of scattered H2 in (v=0, j) for large j.
Random sequential adsorption and diffusion of dimers and k-mers on a square lattice
Authors: Fusco C. , Gallo P. , Petri A. , Rovere M.
Published: July 2002

We have performed extensive simulations of random sequential adsorption and diffusion of k-mers, up to k=5 in two dimensions with particular attention to the case k=2. We focus on the behavior of the coverage and of vacancy dynamics as a function of time. We observe that for k=2,3 a complete coverage of the lattice is never reached, because of the existence of frozen configurations that prevent isolated vacancies in the lattice to join. From this result we argue that complete coverage is never attained for any value of k. The long time behavior of the coverage is not mean field and nonanalytic, with t−1/2 as leading term. Long time coverage regimes are independent of the initial conditions while strongly depend on the diffusion probability and deposition rate and, in particular, different values of these parameters lead to different final values of the coverage. The geometrical complexity of these systems is also highlighted through an investigation of the vacancy population dynamics.
Intermittency and interrelationships between turbulence scaling exponents: Phase-randomization tests
Authors: Nikora V. , Goring D. , Camussi R.
Published: July 2002

A unified framework for velocity structure functions, sign-singular cancellation measures, multifractal measures of the energy dissipation and absolute values of the velocity derivative (“vorticity”), and the two-scale velocity structure functions is suggested. The scaling behavior of these measures within the inertial subrange is considered and tested using one-dimensional (1-D) velocity measurements from a high Re jet flow. In these tests we used the phase-randomization procedure that appears to be an effective tool in identifying the effects of turbulence intermittency and spectral energy flux. A number of relationships between turbulence scaling exponents are derived and tested.
Leakage through filtercake into a fluid sampling probe
Authors: Sherwood J. D. , Stone H. A.
Published: July 2002

Pore fluid can be withdrawn from reservoir rock by means of a probe lowered down a well and clamped against the rock surface. The rest of the rock surface is covered by a drilling fluid filtercake which impedes, but does not totally prevent, flow of filtrate from the wellbore into the rock and thence into the probe. The magnitude of this filtrate flow is investigated in an idealized geometry in which the porous rock, with permeability k, occupies the half-space z>0. The probe covers the circular region r<a of the plane z=0, and the rest of the plane is covered by a thin filtercake of permeability kc and thickness h. The fluid is assumed incompressible and obeys Darcy’s law, so that the fluid pressure p in the porous rock satisfies the Laplace equation. The pressure in the probe is p0<0, and p=0 in the wellbore and in the pore fluid at infinity. This mixed boundary value problem depends only on K=kca/kh. If K=0 the problem is equivalent to that of an electrified disc at constant potential p0 in unbounded space, and pore fluid is drawn from the rock at infinity. If K>0, fluid leaks from the wellbore into the reservoir, and the volume of fluid withdrawn by the probe is equal to the volume of fluid which passes from the wellbore into the rock. When 0<K≪1 fluid streamlines within the rock are similar to those for K=0 close to the probe, but emanate from the filtercake on z=0 on a length scale r∼a/K. Estimates of the hydraulic resistance of filtercakes usually encountered when drilling for petroleum indicate that this leakage flux is sufficiently small to be neglected over typical time scales for fluid sampling.
The A 3Σ−–X 3Σ− electronic transition of HC6N
Authors: Vaizert Olga , Motylewski Tomasz , Wyss Muriel , Riaplov Evgueni , Linnartz Harold , Maier John P.
Published: July 2002

A combined matrix and gas phase study is presented to identify the A 3Σ−–X 3Σ− electronic transition of the linear triplet isomer of HC6N and isotopic derivative DC6N. Absorption spectra have been observed in a 6 K neon matrix after mass selective deposition and in the gas phase by cavity ring down spectroscopy through a supersonic planar plasma. The band origin of the 000 A 3Σ−–X 3Σ− electronic transition of HC6N is determined to be at 21 208.60(5) cm−1, shifted ∼30 cm−1 to the blue of the neon matrix value. Rotational analysis indicates that the chain is slightly stretched on electronic excitation, yielding B0′=0.027 92(5) cm−1. Transitions to vibrationally excited levels in the upper A 3Σ− state are observed as well. The results are compared with a rotationally resolved spectrum of the 000 A 3Σu−–X 3Σg− electronic transition of the isoelectronic HC7H species.
The electronic structure of CuCl2 and CuBr2 from anion photoelectron spectroscopy and ab initio calculations
Authors: Wang Xue-Bin , Wang Lai-Sheng , Brown Reuben , Schwerdtfeger Peter , Schröder Detlef , Schwarz Helmut
Published: July 2002

The electronic structures of CuX2 (X=Cl and Br) have been investigated in the gas phase by means of anion photodetachment photoelectron spectroscopy and ab initio theory. The photoelectron spectra of CuX2− were recorded at two photon energies, 193 and 157 nm. Well-resolved and rich photodetachment features in the spectra provide unprecedented details for the low-lying electronic states of CuCl2 and CuBr2. The excitation energies for five low-lying electronic states of CuX2 were determined, and they explain well the two previously observed optical absorption bands. The electron affinities for CuCl2 and CuBr2 were determined to be identical, 4.35±0.05 eV within the experimental uncertainty. Both the anions and neutral CuX2 species were calculated to be linear with only a slight bond length variation between the charged and neutral species. The calculated electron affinities and vertical excitation energies for the excited states agree well with the experimental values, yielding a definite assignment for the electronic states of CuCl2 and CuBr2.
Domain wall resistance in ultrathin manganite films
Authors: Li Qi , Hu Y. F. , Wang H. S.
Published: July 2002

Resistance related to magnetic domain walls in compressive-strained epitaxial manganite ultrathin films has been studied. The samples were demagnetized in different ways to induce either multidomain or single domain states. Very large difference in resistance was observed between the two states, which was attributed to the domain wall resistance. The magnitude of the domain wall resistance was found to be different in different manganite compounds. We have shown that large domain wall resistance can be obtained in strained ultrathin manganite films and the result cannot be simply explained by the existing models.
Dependence on film thickness of grain boundary low-field magnetoresistance in thin films of La0.7Ca0.3MnO3
Authors: Todd N. K. , Mathur N. D. , Blamire M. G.
Published: July 2002

The magnetoresistance of grain boundaries in the perovskite manganites is being studied, both in polycrystalline materials, and thin films grown on bicrystal substrates, because of interest in low-field applications. In this article we show that epitaxial films grown on SrTiO3 bicrystal substrates of 45° misorientation show magnetoresistance behavior which is strongly dependent on the thickness of the film. Thin films, e.g., 40 nm, can show a large low-field magnetoresistance at low temperatures, with very sharp switching between distinct high and low resistance states for fields applied in plane and parallel to the boundary. Thicker films show a more complex behavior of resistance as a function of field, and the dependence on the angle between the applied field and the grain boundary is altered. These changes in magnetoresistance behavior are linked to the variation in morphology of the films. Thin films are coherently strained, due to the mismatch with the substrate, and very smooth. Thicker films relax, with the formation of defects, and hence different micromagnetic behavior.
Spin glass phase in spin-density-wave Cr–Co alloys
Authors: Galkin V. Yu. , Ali N. , Ortiz W. A. , Fawcett E.
Published: July 2002

A spin glass (SG) phase was observed in the spin-density wave (SDW) alloys Cr1−xCox at high Co concentrations (x⩾13%). After zero-field cooling, the temperature dependence of the magnetization M(T) of Cr0.87Co0.13 and Cr0.85Co0.15, measured in an applied field H=100 Oe with a SQUID magnetometer, exhibits a low T maximum, characteristic of a SG. Cooling in the measuring field, however, gives quite a different behavior. The field dependence of the magnetization M(H) is nonlinear, with a pronounced hysteresis. A strong time relaxation M(t) is also an indication of a SG phase. SG in Cr1−xCox is formed at high impurity concentrations, possibly due to clustering of the magnetic impurities.
Electrical and magnetoresistivity studies in chemical solution deposited La(1−x)CaxMnO3 thin films
Authors: Angappane S. , Murugaraj P. , Sethupathi K. , Rangarajan G. , Sastry V. S. , Chakkaravarthi A. Arul , Ramasamy P.
Published: July 2002

High quality magnetoresistive La(1−x)CaxMnO3 thin films have been prepared by the chemical solution deposition technique. A solution of propionate precursors of lanthanum, calcium, and manganese in propionic acid was used for this purpose. Films of varying compositions (x varying from 0.1 to 0.4) were spin coated on to LaAlO3(100) and SrTiO3(100) substrates at room temperature and pyrolyzed in the temperature range 600–850 °C. For fixed compositions, annealing at higher temperatures shifts the insulator–metal transition temperature (TI–M) to higher values accompanied by a reduction in the resistivity values. The TI–M variation for different x values was found to be less pronounced in the compositions x=0.2, 0.3, and 0.4. Typical TI–M values of 283 K and 290 K were obtained for La0.7Ca0.3MnO3 coated on LaAlO3 and SrTiO3 substrates, respectively, when annealed at 850 °C. The substrate effect was found to be more pronounced for the x value 0.1 which showed two peaks (one at 271 K and another at 122 K) in the ρ-T curve. The roles of substrate mismatch, composition variation, and annealing temperatures are discussed.
Femtosecond vibrational-electronic four-wave-mixing spectroscopy
Authors: Cho Minhaeng
Published: July 2002

Two novel two-dimensional vibrational-electronic spectroscopies are theoretically proposed. The first method uses electronically off-resonant fields to create a vibrational coherence state in the ground electronic state and then thus created transient grating is probed by using a time-delayed electronically resonant field. The second method is to use an infrared pulse to create a vibrational coherence state and an electronically two-photon resonant field is used to probe the IR-induced vibrational grating in time. These two methods are found to be useful in selectively studying vibronic coupling of the solvent intermolecular vibrational modes to the electronic transition of a chromophore dissolved in an isotropic medium.
Entropy and entropy production of finite chemical reaction systems influenced by Gaussian noise
Authors: Zhao Nanrong , Luo Jiuli
Published: July 2002

In this paper we present a stochastic formalism to reveal the meso-statistical significance of the entropy and entropy production of finite chemical reaction systems influenced by the Gaussian noise. As an extension of Shanon entropy a generalized stochastic entropy is introduced without missing any information from both internal and external fluctuations coexisting in this kind of system. An explicit expression of the entropy production valid for the steady state affected by the Gaussian white noise has been obtained. It turns out that the contributions of noise to entropy production of the fluctuations are due to the effect that it intensifies the deviations of the effective probability behavior from the Poisson distribution and the central limit theorem. Meanwhile, a nontrivial shift of the Gibbs’ entropy production is inevitable if the noise is multiplicative. Finally, as an illustration of the Gaussian colored noise, the influence of the Ornstein–Uhlenbeck noise to entropy production is also discussed. In the very small correlation time limit, it shows similar regularities but in a more complicated and implicit form. An approach to transform the stochastic formalism into a thermodynamic formulation is also mentioned briefly.
Structural and dielectric investigation of doped Sr0.61Ba0.39Nb2O6 crystals
Authors: Li Ming-Hua , Chong Tow-Chong , Xu Xue-Wu , Kumagai Hirohiko
Published: July 2002

Undoped, Ce, and Co-doped Sr0.61Ba0.39Nb2O6 crystals were grown by the Czochralski method. The temperature dependence of dielectric constants was measured experimentally. The peak temperatures in dielectric constants of the doped crystals were detected to be lower by around 7 °C than that of the undoped crystal. Doping also results in the dispersion of ferroelectric phase transition due to the more disordered structure. Nonisoelectronic substitution of doping ions and lattice deformation are discussed by taking some properties of doping ion into account, such as radius, charge status, polarization, and concentration. The influences of the doping ions on the dielectric behaviors are also discussed. In Ce and Co double-doped crystals, the interaction between the two doping ions results in the compensation of the extra charges and counteraction of the structure deformation.
Phase separation of binary homopolymer and ternary homopolymer–copolymer mixtures through Gibbs ensemble simulations
Authors: Poncela Antonio , Rubio Ana M. , Freire Juan J.
Published: July 2002

A Gibbs ensemble algorithm is implemented for mixtures of homopolymers and copolymers in a cubic lattice with coordination number z=26. We consider symmetric binary mixtures of homopolymer chains and also study the influence of the presence of symmetric diblock copolymers on these mixtures. All the chain lengths in a given mixture are identical (chains up to 128 units are included). A certain proportion of vacancies is introduced in each simulation. The phase separation curves are characterized by fitting the coexistence data to theoretical expressions. The dependence of the critical temperature on chain length, fraction of vacancies and also on the volume fraction of the copolymer in the mixture has been analyzed. Despite differences between models, the conclusions with this algorithm are similar to those obtained from previous simulations performed with techniques in which finite size interfacial effects may be important. Since the computational cost of our algorithm is reasonable, the present method can be considered an efficient way to study phase separation in different types of polymer mixtures.
Generalized tight-binding molecular dynamics scheme for heteroatomic systems: Application to SimCn clusters
Authors: Menon Madhu
Published: July 2002

Generalized tight-binding molecular dynamics scheme is extended to treat heteroatomic interactions in covalent clusters. The simplicity of the method derives from the minimum number of adjustable parameters needed. Application to small and intermediate size SimCn clusters including SiC heterofullerenes show good agreement with experiment.
Oxidation and phase transitions of epitaxial tin oxide thin films on (1̄012) sapphire
Authors: Pan X. Q. , Fu L.
Published: July 2002

We studied the structural behavior and electrical transport properties of epitaxial α-SnO thin films grown on the (1̄012) α-Al2O3 (sapphire) substrate. Hall effect measurements revealed that the epitaxial as-deposited SnO film is a p-type semiconductor. In situ x-ray diffraction studies show that the α-SnO phase is metastable and will transform into SnO2 with the rutile type structure when annealed at high temperatures in air. The onset of this phase transformation was observed to begin approximately at 300 °C during heating. Shortly thereafter, rutile SnO2 was observed to coexist with α-SnO and intermediate products such as Sn and Sn3O4. After being annealed at temperatures above 600 °C, the film then fully transformed into the rutile SnO2 phase. Our results show that the α-SnO to SnO2 structural transformation proceeds initially by the localized disproportionate redistribution of internal oxygen at low temperature, followed by the transformation of the remaining SnO phase and intermediate phases into SnO2 via the inward diffusion of external oxygen at higher temperatures. Most of the SnO2 crystallites nucleate epitaxially on α-SnO with the orientation relationship of (101)SnO2//(001)SnO and their growth processes are controlled by the (101)SnO2//(001)SnO interfaces, leading to a (101) texture and a laminar grain shape for SnO2. The relationship between the electrical transport properties and the structural evolution of the film has also been investigated.
Nonadiabatic instanton calculation of multistate electron transfer reaction rate: Interference effects in three and four states systems
Authors: Jang Seogjoo , Cao Jianshu
Published: July 2002

For multistate electron transfer reactions with quantum reaction coordinates, nonadiabatic instanton theory can provide a powerful and direct means of calculating the reaction rate without any limitation to the magnitudes of electronic coupling constants. In order to examine its performance in detail, the theory is applied to simple model systems with three and four electronic states which have one and two bridge states respectively. Calculations for three states systems, varying the through–bond coupling constant, show that the nonadiabatic instanton theory reproduces the results of perturbation and adiabatic instanton theories in the limits of small and large coupling constants, respectively. In the absence of through–space coupling, the crossover between the two limits is smooth and monotonic. However, in the presence of through–space coupling, the crossover pattern becomes sensitive to the relative phase of the two electronic channels and demonstrates substantial interference effects. For a four states system that has two interfering through–bond coupling paths, similar interference effect was observed. These results show that the nonadiabatic instanton method can serve as a favorable means of understanding the general kinetics and exploring the interference effects in the low-temperature bridge mediated and/or proton coupled electron transfer systems.
Optical properties of self-ion-implanted Si(100) studied by spectroscopic ellipsometry
Authors: Mori Hirofumi , Adachi Sadao , Takahashi Mitsutoshi
Published: July 2002

Microstructural and optical properties of self-ion-implanted Si(100) have been studied using atomic force microscopy (AFM) and spectroscopic ellipsometry. The Si+ ions are implanted at 150 keV with fluences ranging from 2×1014 to 2×1015 cm−2. The AFM image indicates that the Si(100) surface is shiny flat before and after Si+-ion implantation (rms roughness of ∼0.3 nm). An effective-medium approximation and a linear regression analysis suggest that the ion-implanted surface can be explained by a physical mixture of microcrystalline (μc−) and amorphous silicon (a-Si). It has been shown that the complex dielectric function ε(E) of μc-Si differs appreciably from that of c-Si, especially in the vicinity of the sharp critical-point features. This difference in ε(E) can be successfully explained by increasing the broadening value at each critical point. The volume fraction of a-Si is found to be simply expressed as fa=([Si+]/A)α, where [Si+] is the ion fluence in cm−2, A (=1.4×1015 cm−2) is an amorphization-threshold fluence, and α(=1.42) is an amorphization-rate factor of the Si+ ions.
Formation and dissociation of triply charged fragment ions of benzene
Authors: Scheuermann F. , Salzborn E. , Hagelberg F. , Scheier P.
Published: July 2002

Electron impact ionization of small singly charged hydrocarbon ions was applied to produce and investigate multiply charged molecular ions. The stability of triply charged hydrocarbon ions depends strongly on the number of hydrogen atoms of the molecule. C4Hj3+ were observed only for molecules that contain more than 2 hydrogen atoms. Parallel, highly correlated ab initio calculations were performed for these molecular ions. The binding energies obtained by these computations agree well with the experimental findings. Moreover, there is quantitative agreement between the experiment and the calculations on the kinetic energy of the fragment ions upon Coulomb explosion.
The surface-induced static director distribution in thin nematic liquid crystal films: A deuterium nuclear magnetic resonance spectroscopy study
Authors: Luckhurst Geoffrey R. , Miyamoto Tetsuo , Sugimura Akihiko , Takashiro Tsukasa , Timimi Bakir A.
Published: July 2002

We have studied the static director distribution in thin nematic liquid crystal cells with different film thicknesses and different surface anchoring strengths using a combination of deuterium nuclear magnetic resonance (NMR) spectroscopy and continuum theory. A nematic liquid crystal, 4-pentyl-d2-4′-cyanobiphenyl (5CB-d2) deuteriated in the α position of the pentyl chain, was confined between two glass plates with both weak and strong anchoring conditions; the anchoring strengths were measured by using a saturation voltage method. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of the applied electric field, which can be used to explore the director deformation. The deuterium NMR spectra predicted by continuum theory involving the magnetic, electric, elastic, and unified surface anchoring energies are found to be in good agreement with experiment. The investigation also reveals that profiles of the director distribution for the case with a magnetic field have a top-hat shape different to the bowler-hat shape, which is well known as the director profile for the case with no magnetic field.
Response to “Comment on ‘Finding finite-time invariant manifolds in two-dimensional velocity fields’ ” [Chaos 11, 427 (2001)]
Authors: Haller G.
Published: July 2002

Lapeyre, Hua, and Legras have recently suggested that the detection of finite-time invariant manifolds in two-dimensional fluid flows, as described by Haller and Haller and Yuan, can be substantially improved. In particular, they suggested (a) a change of coordinates to strain basis before the application of Theorem 1 of Haller and (b) the use of a nondimensionalized time computed from Theorem 1. Here we discuss why these proposed steps will not result in a significant overall improvement. We verify our arguments in a more detailed computation of the example analyzed in Lapeyre, Hau, and Legras (the Kida ellipse), as well as in a two-dimensional barotropic turbulence simulation. While in both of these examples the techniques suggested by Lapeyre, Hau, and Legras reveal additional thin regions of hyperbolicity near vortex cores, they also lead to an overall loss of detail in the global computation of finite-time invariant manifolds.
Unusual substitutional properties of Cu in bulk polycrystalline samples of La0.7Ca0.3Mn1−xCuxO3−δ
Authors: Tovstolytkin A. I. , Pogorilyi A. N. , Belous A. G. , Yanchevski O. Z.
Published: July 2002

The transport and magnetoresistive properties of bulk polycrystalline samples of the La0.7Ca0.3Mn1−xCuxO3−δ (x⩽0.15) system are studied in the temperature range 77 K–300 K. All the samples investigated exhibit the giant magnetoresistance effect associated with the transition from the ferromagnetic metallic to the paramagnetic insulating state. As a function of copper concentration, the temperature Tp of the resistivity peak first decreases from 193 K (x=0) to 108 K (x=0.10) and then gradually grows, reaching 120 K at x=0.15. Significant temperature broadening of the resistive transition as well as anomalous behavior of the peak value of the resistivity are observed near x=0.10. The unusual properties of the La0.7Ca0.3Mn1−xCuxO3−δ system are well explained in terms of a mixed valence of the Cu ions.
Adapting optimal control theory and using learning loops to provide experimentally feasible shaping mask patterns
Authors: Hornung Thomas , Motzkus Marcus , de Vivie-Riedle Regina
Published: July 2002

In this article we present two algorithms that provide optimal control pulses directly applicable in experiment. These are, namely, the optimal control algorithm, in which we incorporated spectral pressure during optimization, and complementarily a theoretical variant of the learning loop based on an evolutionary algorithm. With these tools a productive dialog between theory and experiments on optimal control is imaginable. The implementation of these pulses is assured by calculating the optimal mask pattern, that serves as an interface between theory and experiment.
Level-crossing statistics of the horizontal wind speed in the planetary surface boundary layer
Authors: Edwards Paul J. , Hurst Robert B.
Published: July 2002

The probability density of the times for which the horizontal wind remains above or below a given threshold speed is of some interest in the fields of renewable energy generation and pollutant dispersal. However there appear to be no analytic or conceptual models which account for the observed power law form of the distribution of these episode lengths over a range of over three decades, from a few tens of seconds to a day or more. We reanalyze high resolution wind data and demonstrate the fractal character of the point process generated by the wind speed level crossings. We simulate the fluctuating wind speed by a Markov process which approximates the characteristics of the real (non-Markovian) wind and successfully generates a power law distribution of episode lengths. However, fundamental questions concerning the physical basis for this behavior and the connection between the properties of a continuous-time stochastic process and the fractal statistics of the point process generated by its level crossings remain unanswered.
Performance evaluation of the multi-stage Mott polarimeter using the Monte Carlo simulation
Authors: Tamura Keiji , Yasuda Masaaki , Kotera Masatoshi , Murata Kenji
Published: July 2002

The Mott polarimeter is the most popular detection system of the spin polarization of electron beams. However, the Mott polarimeter has a significant disadvantage, low sensitivity. We propose a multi-stage Mott polarimeter that provides higher sensitivity than the conventional one. Three-dimensional electron trajectories in target films are calculated by a Monte Carlo simulation of electron scattering based on the single scattering model. The Mott scattering cross section for polarized electrons is used in the simulation. We calculate the figure of merit of the multi-stage Mott polarimeter to evaluate the performance. Moreover, we evaluate the performance of the multi-stage Mott polarimeter taking the supporting films and the apertures into consideration.
Comparative analysis of time-resolved and time-integrated x-ray data from long pulse Z-pinch implosions on Saturn
Authors: Apruzese J. P. , Thornhill J. W. , Whitney K. G. , Davis J. , Deeney C. , Coverdale C. A.
Published: July 2002

Two series of Al:Mg wire array Z-pinch implosions were performed on the Saturn pulsed-power generator [R. B. Spielman et al., in Proceedings of the 2nd International Conference on Dense Z-Pinches, Laguna Beach, CA, 1989, edited by N. R. Pereira, J. Davis, and N. Rostoker (American Institute of Physics, New York, 1989), p. 3]. In one series, the wire number was varied, in the other, the load mass was varied. In all cases an initial array diameter of 40 mm was maintained. Sufficient x-ray data were obtained in these experiments to allow the plasma conditions to be inferred by matching x-ray data to that calculated by a collisional-radiative equilibrium (CRE) model. In fitting the data, the plasma has been assumed to be either uniform [zero dimension (0D)] or to vary one-dimensionally (1D) with radius. On one shot, a comparison of these two methods of data analysis performed with both time-resolved and time-integrated data has been carried out. While the same trends in plasma conditions as a function of load mass and wire number are seen, the uniform plasma assumption yields higher electron temperatures, lower core densities, but higher K-shell radiating mass fractions than those obtained from the 1D assumption. When comparing time-resolved and time-integrated spectroscopic data, a pronounced difference is seen in a density-sensitive line ratio whose time-resolved values corroborate experimental evidence that much of the K-shell radiation is emitted from a high density core plasma of spatial extent less than about 300 μm.
Direct-product formalism for calculating magnetic resonance signals in many-body systems of interacting spins
Authors: Nevzorov Alexander A. , Freed Jack H.
Published: July 2002

Modern applications of nuclear magnetic resonance and electron spin resonance, and especially quantum computing problems call for more effective formalisms to describe relaxation and evolution of various orders of coherence in the presence of motions of the interacting spins. Here we suggest a formulation for the description of multiquantum spin states based on direct-product structures that take into account the inherent permutation symmetries and quantum coherences of a multispin system. Convenient recursion relations are obtained for the matrix representations of the N-body Hamiltonian superoperator and pulse propagators. This allows one to obtain compact expressions for the evolution of magnetic resonance signals when the dipolar interactions amongst spin-bearing molecules are modulated by their motions. These expressions include the free-induction decay and solid echoes, as well as the decay of higher-order coherences under the assumption of statistical independence of the motions of the spin-bearing molecules. Exact results, not requiring this assumption, are obtained for the case of the truncated dipolar Hamiltonian. Important phenomena that arise in multispin systems, such as instantaneous diffusion and spectral diffusion arising from motions, are studied more rigorously by solving the equation for the time evolution of the spin-density states. The many-body magnetic-resonance signals in the presence of motions are obtained by solving the appropriate stochastic Liouville equations. These solutions may be compared to solid-echo experiments to extract translational diffusion coefficients even in the slow motional regime.
Valence one-electron and shake-up ionization bands of polycyclic aromatic hydrocarbons. I. Benzene, naphthalene, anthracene, naphthacene, and pentacene
Authors: Deleuze Michael S. , Trofimov Alexander B. , Cederbaum Lorenz S.
Published: July 2002

The valence ionization bands of benzene and of polyacenes ranging from naphthalene to pentacene have been entirely assigned by means of one-particle Green’s function calculations, performed using the third-order algebraic-diagrammatic construction [ADC(3)] scheme and series of basis sets of improving quality. For the sake of consistency, the computations are based on correlated (DFT/B3LYP) rather than uncorrelated geometries. Ionization bands pertaining to π-orbitals are subject to a severe shake-up contamination at already quite low binding energies (e.g., down to 8.0 eV in the case of pentacene). In sharp contrast, the orbital picture of ionization holds to a much greater extent within the σ-band system (e.g., for pentacene, up to binding energies of 14.6 eV). Despite the intricacy of ionization bands, and, possibly, vibrational complications, ADC(3) spectra consistently match photoionization measurements up to the inner-valence region, where the orbital picture completely breaks down.
A structural study of the hexafluorobenzene from liquid to supercritical conditions using neutron diffraction and molecular dynamics
Authors: Danten Yann , Cabaço M. Isabel , Tassaing Thierry , Besnard Marcel
Published: July 2002

This paper is devoted to an investigation of the local order in hexafluorobenzene by neutron diffraction and molecular-dynamics simulations. Experimentally, the fluid has been studied under isobaric (P∼16 MPa) and isothermal (T∼573 K) conditions in a broad density domain ranging from the liquid value (ρ∼1600 kg m−3) down to typical densities on the supercritical domain (ρ∼60 kg m−3). It is found that the translational ordering, which is characterized by two well defined shells of neighboring molecules at ambient conditions, is gradually weakened and extends only to the first shell as the density decreases in the supercritical domain. This behavior is in full agreement with the general trend reported for the positional ordering in our previous investigations for some cyclic molecules. In marked contrast, the short range orientational ordering existing in the first shell of molecule is almost preserved. It is found that parallel and perpendicular configurations of a pair of neighboring molecules coexist, although parallel configurations are found predominant at the shortest intermolecular distances. Finally, the structural evolution in the three fluids, hexafluorobenzene, benzene and 1,3,5-trifluorobenzene are comparatively discussed. It is argued that the orientational ordering in the hexafluorobenzene is closer to that existing in 1,3,5-trifluorobenzene than in liquid benzene. However, no presence of stacked dimers (C6F6)2 have been revealed from both experimental and simulated structural analyze.
Ultrafast dichroism spectroscopy of anthracene in solution. I. Inertial versus diffusive rotation in benzyl alcohol
Authors: Zhang Yunhan , Sluch Mikhail I. , Somoza Mark M. , Berg Mark A.
Published: July 2002

Dichroism experiments with 150 fs time resolution on anthracene in benzyl alcohol are presented as a function of viscosity from 14.4 cP (274 K) to 2.7 cP (329 K). These measurements test a qualitative prediction of the viscoelastic picture of liquid dynamics, specifically that earlier “inertial” dynamics have a viscosity independent rate, whereas later “diffusive” dynamics have a rate directly proportional to viscosity. This paper focuses on two components of the dichroism decay that are assigned to rotational motion. A third component is assigned to electronic-state solvation and is analyzed in a companion paper [J. Chem. Phys. 115, 4231 (2001)]. The longest component is due to rotational diffusion and is very well described by a hydrodynamic model with slip boundary conditions. A fast decay component in the subpicosecond region is found and shown to have a viscosity-independent rate. It is assigned to inertial rotation by comparison to the computer simulations of Jas et al. [J. Chem. Phys. 107, 8800 (1997)]. Inertial rotation extends out to at least 1 ps, longer than the range commonly assumed for inertial dynamics. Over much of this range, the inertial rotation is not free-rotor-like, but is strongly modified by interaction with the solvent. The inertial rotation also accounts for the “missing” anisotropy found when the rotational diffusion fits are extrapolated to zero time.
Photoluminescence of δ-doped ZnSe:(Te,N) grown by molecular beam epitaxy
Authors: Kuskovsky Igor L. , Tian C. , Sudbrack C. , Neumark G. F. , Lin W.-C. , Guo S. P. , Tamargo M. C.
Published: July 2002

We have studied the low temperature photoluminescence (PL) of a δ-doped ZnSe:(Te,N) system using two different types of samples, one with single δ layers separated by undoped spacers and the other with three adjacent δ layers in each doping cycle. We have concluded that both Te and N participate in radiative recombination. We observe a relatively low PL efficiency (compared to samples without N) for these samples, and we suggest that Auger recombination is a likely mechanism, although a role of slow donor–acceptor pair PL and consequent nonradiative processes cannot be ruled out.
Time response of protection in event of vacuum failure based on Nude ionization gauge controller
Authors: Gao Hui , Wang Qiuping , Wang Weibin , Wu Qinglin , Chen Wentong , Sheng Liusi , Zhang Yunwu
Published: July 2002

This article describes the design and application of fast-response vacuum protection sensor module, based on the Nude ionization gauge and a homemade controller named GH07X. A simulative test indicated that the controller’s response time was less than 200 μs when 1 atm air rushed into the vacuum system through a pulsed valve with 0.8 mm orifice nozzle and the emitting current of the Nude gauge was 4 mA. The experiment result showed that the response time mainly depended on the gas density as well as the electron emitting current of the Nude gauge filament. Compared with the vacuum protection sensors based on sputter ion pump and cold-cathode gauge, GH07X is faster and reliable besides, GH07X can be used as an ultrahigh-vacuum slow valve interlock controller with response time of 100 ms, which is faster than other gauge controllers. The widely used field–bus interface CAN and common serial interface RS232/RS485 are embedded in GH07X controller system.
Gradients of vibrational coordinates from the variation of coordinates along the path of a particle
Authors: Pesonen Janne
Published: July 2002

The gradients of vibrational coordinates are needed in order to form the exact vibrational kinetic energy operator of a polyatomic molecule. In my previous work [J. Chem. Phys. 112, 3121 (2000)], it was demonstrated that they can be easily obtained for any geometrically defined shape coordinates by the direct vectorial differentiation. However, there is in some cases a more practical way to obtain these gradients from the variation of coordinates along the path of a particle. This approach can be used effectively to find the gradients of the shape coordinates, which are given as implicit functions of the nuclear positions. As a new application, I use this method to obtain the gradients of the eigenvalues of the moment tensor.
Comment on “Mechanisms for the anomalous dependence of carrier lifetime on injection level and photoconductance on light intensity” [J. Appl. Phys. 89, 332 (2001)]
Authors: Macdonald Daniel , Cuevas Andrés
Published: July 2002

In a recent article [S. Z. Karazhanov, J. Appl. Phys. 89, 332 (2001)], Karazhanov proposed a single-level recombination model as an explanation for the anomalous dependence of the carrier lifetime on injection level observed in cast multicrystalline silicon. This approach contrasts with previous models which involved the use of two distinct levels, one causing recombination and the other only trapping. The purpose of this Comment is to outline some critical considerations which suggest that only a two-level (or indeed a multilevel) model can satisfactorily explain the experimental observations.
Physical significance of second electron-pair moments in position and momentum spaces
Authors: Koga Toshikatsu , Matsuyama Hisashi
Published: July 2002

For many-electron atoms, the generalized electron-pair density function g(q;a,b) represents the probability density function for the magnitude |ari+brj| of two-electron vector ari+brj to be q, where a and b are real-valued parameters. It is pointed out that the second moments 〈q2〉(a,b), associated with g(q;a,b), are related with several physical properties such as the diamagnetic susceptibility χd, the form factor F(μ), the incoherent scattering function S(μ), the minus first moment M−1 of the oscillator strength density, and the dipole polarizability α. The corresponding moments 〈t2〉(a,b) in momentum space are connected with the mass polarization correction εM, the first moment M1 of the oscillator strength density, and the nuclear momentum squared 〈p02〉 as well as the electronic kinetic energy 12〈∑ipi2〉. The average excitation energy ΔE is also estimated from 〈q2〉(a,b) and 〈t2〉(a,b).
Effect of carrier trapping time on performance of alternating-current thin-film electroluminescent devices
Authors: Krasnov Alex N. , Hofstra Peter G.
Published: July 2002

In spite of a significant number of studies on memory effect in alternating-current thin-film electroluminescent devices, the phenomenon is still unclear. In the present work we investigate some peculiarities of memory behavior of the devices from the viewpoint of the nonequilibrium carrier trapping time. Applying a combination of unique voltage staircase waveforms, we demonstrate that the degree of memory of the device is determined by the trapping time of the carriers at the insulator–semiconductor interface and the sulfur vacancy (Vs) concentration in the bulk of the active layer. We also show that separating electrons and holes before the pulse arrival can increase the effective field across the phosphor layer.
Superfluid solvation structure of OCS in helium clusters
Authors: Kwon Yongkyung , Whaley K. Birgitta
Published: July 2002

We make a detailed study of the local solvation structure and energetics of an OCS molecule in clusters of He4 at finite temperatures. Calculations are made with the path integral Monte Carlo method, incorporating the exchange permutation symmetry of the bosonic He4 atoms. Analysis of the local extent of superfluidity is made with an approximate exchange path estimator developed previously. The sensitivity of the helium solvation structure to the interaction potential is examined with calculations for two recently published He–OCS potentials, and the vibrational shift of the antisymmetric OCS vibration is estimated from a set of vibrationally adiabatic potentials. We comment on possible effects of molecular rotation on the local solvation structure, and discuss the microscopic two-fluid analysis of the rotational spectroscopy of OCS in HeN4.
Generally polarized acoustic waves trapped by high aspect ratio electrode gratings at the surface of a piezoelectric material
Authors: Laude V. , Khelif A. , Pastureaud Th. , Ballandras S.
Published: July 2002

It has been shown theoretically and demonstrated experimentally that shear horizontal (SH) surface waves can exist when the surface of an isotropic substrate is perturbed by a strong corrugation, for instance consisting of deep grooves etched in the substrate, whereas these waves cannot exist without this perturbation. It is shown in this article that a periodic array of metallic electrodes (wires) exhibiting large aspect ratios deposited over a piezoelectric substrate give rise to surface acoustic waves with general polarization. The admittance of an interdigitated transducer, which is a basic tool for predicting the waves parameters, is calculated by a combination of finite element analysis and a boundary integral method. This approach has been extended to obtain the polarization of the acoustic waves. For different piezoelectric substrates, we predict various surface acoustic modes and their polarization. Along with mostly SH modes, we also find modes mostly polarized in the sagittal plane. We discuss the frequency behavior of the surface modes as a function of the electrode height compared to the period.
Bifurcation and scaling of drift wave turbulence intensity with collisional zonal flow damping
Authors: Malkov M. A. , Diamond P. H.
Published: July 2002

Interacting drift wave–zonal flow turbulence is examined at the spectral level of description using an extended “predator–prey” model. Analytic solutions that describe both the linear scaling of transport with ion–ion collisionality as well as the saturation regime are obtained for a simple model of drift wave turbulence. A theory of self-regulation in this system is presented. The possibility of bifurcation to a state with higher turbulence level and transport is demonstrated. This bifurcation is associated with the appearance of a condensate solution at the largest scales. The possible relevance of this phenomenon to the bursting events of turbulence and transport recently observed in gyrokinetic simulations of ITG instability is discussed.
Morphing the He–OCS intermolecular potential
Authors: Howson Joanna M. M. , Hutson Jeremy M.
Published: July 2002

A potential energy surface for He–OCS that agrees with experimental rotational spectra to within 1 MHz is presented. The potential was first calculated at a grid defined in prolate spheroidal coordinates, which give stabler interpolations than Jacobi coordinates. Coupled cluster calculations at the CCSD(T) level were carried out with an aug-cc-pVTZ basis set. The potential was then morphed, a procedure that scales the energy and the intermolecular distance in a coordinate-dependent way. The parameters of the function used for morphing were determined by a least-squares fit to the experimental data. The global minimum of the recommended potential, at −50.2 cm−1, is 4.8 cm−1 deeper than the unscaled potential of Higgins and Klemperer [J. Chem. Phys. 110, 1383 (1999)]. The morphing procedure increases the well depth by more at the sulfur end than at the oxygen end.
Uniaxial stress system for electron paramagnetic resonance studies
Authors: Krupski M. , Kaszyński R. , Krupska A. , Gierszal H.
Published: July 2002

A standard X-band electron paramagnetic resonance cavity has been adapted to measurements under uniaxial stress parallel to the external magnetic field direction. The system is applied to study single crystals under stress up to 100 MPa in the temperature range 100–400 K.
Dynamic measurement access, a new technique for fast thermal conductivity measurement
Authors: Zawilski B. M. , Tritt Terry M.
Published: July 2002

A new technique called dynamic measurement access (DMA) is presented in this article which was developed for relatively easy, fast, and accurate thermal conductivity measurements. Utilizing a commercial cryocooler adapted for this experiment, the DMA technique allows the measurement of typically 400 data points in a temperature range between 300 and 10 K, during an elapsed time of approximately 90 min. This technique provides very good precision and requires a very simple experimental setup, which is described in this article.