bluuebunny/crossref_metadata_2025 huggingface dataset:
Two- and N-step correlated models for the analysis of molecular dynamics trajectories of linear molecules in silicalite
Authors: Demontis Pierfranco , Suffritti Giuseppe B. , Tilocca Antonio
Published: July 2002

Recent molecular dynamics data on the diffusion of linear diatomic and triatomic molecules in the zeolite silicalite are analyzed in terms of a new correlated model [F. Jousse, S. M. Auerbach, and D. P. Vercauteren, J. Chem. Phys. 112, 1531 (2000)] capable to account for both first- and higher-order correlation effects. This “N-step” model reproduces very well our calculated mean square displacements and diffusion coefficients of the molecules considered. The improvements with respect to the results obtained with our previous “two-step” model [P. Demontis, J. Kärger, G. B. Suffritti, and A. Tilocca, Phys. Chem. Chem. Phys. 2, 1455 (2000)] are remarkable for all molecules except chlorine, showing that only in this case the effect of (negative) correlations spanning more than two jumps between channel intersections (∼20 Å) can be neglected. The basic trajectory analysis in terms of single- and two-step models, besides being an useful reference, provides all the input data needed for the application of the N-step model. Indeed, in its silicalite formulation, the N-step model is strongly linked to the two-step one because it calculates the probability of a sequence of jumps in the same channel by means of the correlations between any two consecutive jumps. Finally, the possibility to obtain qualitative insight into the diffusive mechanism through various kind of correlation coefficients is discussed.
On the non-Arrhenius temperature dependence of the interwell electron tunneling rate in quasi two dimensional organic quantum wells
Authors: Jeong I. S. , Scott K. , Donovan K. J. , Wilson E. G.
Published: July 2002

The tunneling rate of photocreated charge carriers between layers in Langmuir–Blodgett multilayer structures is measured indirectly using the novel technique of bimolecular recombination quenching. The tunneling rate is measured as a function of the applied electrostatic potential difference between the layers as the temperature is varied between 300 and 4 K. This dependence is examined in light of the Marcus theory of charge transfer where the electrostatic potential replaces the chemical potential as the driving potential. The expectations of the Marcus theory are not met and the rate is effectively temperature independent, contrary to expectation. Other mechanisms are explored that may explain the lack of temperature dependence including the role of high frequency vibrations and the role of the zero point energy of those vibrations. The temperature dependence of the exciton dissociation probability is also examined.
Environment-induced corrections to the spin Hamiltonian as dynamic frequency shifts in nuclear magnetic resonance
Authors: Karthik G. , Kumar Anil
Published: July 2002

We derive an expression for the correction to the spin-system Hamiltonian that arises due to the system–bath interaction, starting both from the standard master equation for the spin density matrix and a perturbative diagonalization of the system–bath Hamiltonian to the second order in the interaction. We show that the dynamic frequency shifts observed in the evolution of the nuclear spin coherences are a result of these Hamiltonian corrections. We present a systematic decomposition of the relaxation superoperator into Hermitian and anti-Hermitian parts as opposed to the usual practice of partitioning it into real and imaginary parts. We point out that the relaxation-induced corrections to the coherent motion arise exclusively from the anti-Hermitian part and the dissipative effects, from the Hermitian part, both, in general, being complex. However, the secular terms of this correction are found to depend only on the imaginary and the real parts, respectively.
Quantum yields and energy partitioning in the ultraviolet photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402)
Authors: Zou Peng , McGivern W. Sean , Sorkhabi Osman , Suits Arthur G. , North Simon W.
Published: July 2002

The photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402) has been investigated at 193 nm using photofragment translational spectroscopy with vacuum ultraviolet ionization and at 193, 233, and 266 nm using state-selected translational spectroscopy with resonance-enhanced multiphoton ionization. The product branching ratios, angular distributions, and translational energy distributions were measured at these wavelengths, providing insight into the ultraviolet photodissociation dynamics of CF2BrCF2Br. The total bromine atom quantum yields were found to be 1.9±0.1 at both 193 and 233 nm and 1.4±0.1 at 266 nm. The first C–Br bond dissociation energy was determined to be 69.3 kcal/mol from ab initio calculations. The second C–Br bond dissociation energy was determined to be 16±2 kcal/mol by modeling of the bromine quantum yield. In addition, variational Rice–Ramsperger–Kassel–Marcus theory was used to calculate the secondary dissociation rates for a range of dissociation energies above threshold. These results suggest that CF2CF2Br photofragments with sufficient internal energies will undergo secondary dissociation prior to collisional stabilization under atmospheric conditions. Based on the measured translational energy distributions and product branching ratios, a model is proposed to describe the wavelength-dependent bromine quantum yield and the implications of these results to atmospheric chemistry are discussed.
Probing the effect of the H2 rotational state in O(1D)+H2→OH+H: Theoretical dynamics including nonadiabatic effects and a crossed molecular beam study
Authors: Gray Stephen K. , Balint-Kurti Gabriel G. , Schatz George C. , Lin Jim J. , Liu Xianghong , Harich Steve , Yang Xueming
Published: July 2002

Theoretical estimates of reactive cross sections for O(1D)+H2(X,v=0,j)→OH(X)+H(2S), with H2 rotational quantum numbers j=0 and 1, are obtained for a range of collision energies, Ecol. Crossed molecular beam measurements are also used to infer the ratio, r1,0, of the j=1 and 0 cross sections at Ecol=0.056 eV. The theory indicates that the 1 1A′ potential surface is the most important one. However, the 2 1A′ and 1 1A″ surfaces can also contribute. Adiabatic dynamics on the 1 1A″ surface, particularly at Ecol above its 0.1 eV barrier to reaction plays a role. The 2 1A′ surface, while not correlating with ground electronic state products, can still lead to products via nonadiabatic interactions with the 1 1A′ surface. Many quantum dynamics and quasiclassical classical trajectory calculations are carried out. Accurate, ab initio based potential energy surfaces are employed. Quantum cross sections are based on helicity decoupled wave packet calculations for several values of total angular momentum. Nonadiabatic wave packet and trajectory surface hopping calculations, where appropriate, are carried out. An interesting, subtle picture emerges regarding the energy dependence of r1,0. The theoretical results indicate, somewhat surprisingly, that, for Ecol<0.1 eV,r1,0 can be less than unity owing to the anisotropy of the ground state potential. Electronically excited states and nonadiabatic effects contribute to the overall cross sections for Ecol>0.1 eV, but the full r1,0 is only weakly sensitive to excited states. Our experimentally inferred r1,0 at Ecol=0.056 eV, 0.95±0.02, is in quantitative agreement with our best calculation, which suggests that the effect of potential anisotropy is correctly described by theory. The relation between these results and previous experimental findings is discussed.
Dielectric relaxation of hot water
Authors: Yang Chia-Ning , Kim Hyung J.
Published: July 2002

Dielectric relaxation of water for a wide range of temperature (373 K⩽T⩽973 K) and density (0.07 g cm−3⩽d⩽0.66 g cm−3) is studied with the molecular dynamics simulation technique. The evolving water electronic structure is explicitly incorporated via the truncated adiabatic basis-set representation. For a given density, polarization dynamics of water tends to become faster with increasing temperature. At reduced densities, it shows a marked deviation from a single exponential decay. Another interesting feature is that at fixed T, water dielectric relaxation becomes, in general, accelerated as the density decreases. This trend is at variance with recent microwave spectroscopy measurements at elevated temperatures and reduced densities. Possible sources for the discrepancy between the simulation and experimental results are discussed.
Solid-solid transitions induced by repulsive interactions
Authors: Hemmer P. C. , Velasco E. , Mederos L. , Navascués G. , Stell G.
Published: July 2002

We show that a rich variety of crystalline structures, and a corresponding diversity of the associated phase diagrams, result from the presence in the pair potential of a soft repulsion in addition to a hard core. We use different forms for the soft repulsion, and show that the results are sensitive to the details of the potentials (in particular, their convexity) even if the range of the soft repulsion is limited to a small fraction of the hard-core diameter. Our demonstration combines exact ground-state analysis with first-order perturbation theory at finite temperatures. The relevance of our work to certain features found in real systems is also discussed.
Ferroelectric properties of Bi3.25La0.75Ti3O12 thin films prepared by chemical solution deposition
Authors: Wu Di , Li Aidong , Zhu Tao , Liu Zhiguo , Ming Naiben
Published: July 2002

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films were prepared on platinum coated silicon substrate by chemical solution deposition. The layered-perovskite phase was obtained by rapid thermal annealing the spin-on films at 650 or 700 °C for 180 s. Scanning electron micrographs showed uniform surfaces composed of spherical grains. The grain size increased with increasing annealing temperature. The remanent polarization and coercive field of 650 °C annealed film were 12.3 μC/cm2 and 48.9 kV/cm, respectively, and those of 700 °C annealed films were 18.2 μC/cm2 and 51.1 kV/cm. BLT thin films showed little polarization fatigue under 250 kV/cm bipolar cycling at 50 kHz, while fatigue properties deteriorated with decreasing cycling field and frequency. At various frequencies from 1 Hz to 50 kHz, nonvolatile polarization Pnv showed nearly no degradation over an initial period of cycling, then decayed logarithmically with switching cycles. The onset of logarithmic decay of Pnv was found to increase linearly with cycling frequency. The cycling field dependence of fatigue characteristics was interpreted in terms of competition of charge trapping which blocks the domain switching and field-assisted detrapping of charged defects which set the locked domains free. Frequency dependent fatigue properties were discussed on the basis of drift and aggregation of oxygen vacancies.
Thermoelectric properties of Pt-doped β-FeSi2
Authors: Tani Jun-ichi , Kido Hiroyasu
Published: July 2002

The thermoelectric properties of Pt-doped β-FeSi2(Fe1−xPtxSi2) have been characterized by Hall effect measurements at 300 K and by measurements of electrical resistivity, Seebeck coefficient, and thermal conductivity from 300 to 1150 K. Fe1−xPtxSi2 is n type over the measured temperature range. The electron concentration of Fe1−xPtxSi2 at 300 K is about twice the Pt atom concentration at low Pt atom concentration. The solubility limit of Pt in β-FeSi2 is estimated to be about 2.0 at. %. The electrical conduction of Fe1−xPtxSi2 can be explained by the small polaron mechanism. The thermal conductivity as well as the Seebeck coefficient of Fe1−xPtxSi2 is strongly affected by the existence of small polarons. The samples for x=0.03 and 0.05 show a maximum value of ZT=0.14 at 847 K.
Response to “Comment on ‘Improved boundary layer analysis of forced magnetic reconnection due to a boundary perturbation’ ” [Phys. Plasmas 8, 374 (2001)]
Authors: Ishizawa A. , Tokuda S.
Published: July 2002

We confirm that our analysis, based on the conventional linear boundary layer theory, does not include the errors claimed by the Comment by Bhattacharjee, Fitzpatrick, and Wang (hereafter BFW) [Phys. Plasmas 8, 374 (2001)] and demonstrates the new reconnection process described by two reconnected fluxes forced to differ by the non-constant-ψ matching. The difference between this conventional theory and BFW’s ideal magnetohydrodynamic similarity solution related to the Sweet–Parker time scale is discussed.
Golden mean renormalization for the Harper equation: The strong coupling fixed point
Authors: Mestel B. D. , Osbaldestin A. H. , Winn B.
Published: July 2002

We construct a renormalization fixed point corresponding to the strong coupling limit of the golden mean Harper equation. We give an analytic expression for this fixed point, establish its existence and uniqueness, and verify properties previously seen only in numerical calculations. The spectrum of the linearization of the renormalization operator at this fixed point is also explicitly determined. This strong coupling fixed point also helps describe the onset of a strange nonchaotic attractor in quasiperiodically forced systems.
An extended Rayleigh model of bubble evolution
Authors: Glinsky Michael E. , Bailey David S. , London Richard A. , Amendt Peter A. , Rubenchik Alexander M. , Strauss Moshe
Published: July 2002

An extended Rayleigh model for laser generated bubbles in water and soft tissue is presented. This model includes surface tension, viscosity, a realistic equation of state, material strength and failure, stress wave emission, and linear growth of interface instabilities. The model is validated by comparison to detailed compressible hydrodynamic simulations using the LATIS computer program. The purpose of this study is to investigate the use of the extended Rayleigh model as a much faster and simpler substitute for the detailed hydrodynamic simulations when only limited information is needed. It is also meant to benchmark the hydrosimulations and highlight the relevant physics. The extended Rayleigh model and the hydrosimulations are compared using both a 1D spherical geometry with a bubble in the center and a 2D cylindrical geometry of a laser fiber immersed in water with a bubble formed at the end of the fiber. Studies are done to test the validity of the material strength and failure, stress wave emission, and the interface instability terms in the extended Rayleigh model. The resulting bubble radii, material damage radii, the emitted stress wave energies, and the size of the interface distortions are compared. Many of the trends found in the hydrosimulations are illuminated by the extended Rayleigh model owing to its relative simplicity. The extended Rayleigh model is very useful since it is accurate over a large range of parameters and it is computationally much faster than the hydrosimulations.
Deposition of microcrystalline silicon: Direct evidence for hydrogen-induced surface mobility of Si adspecies
Authors: Gerbi J. E. , Abelson J. R.
Published: July 2002

Hydrogenated microcrystalline silicon thin films can be deposited at low substrate temperatures using plasma enhanced–or hot wire–chemical vapor deposition using silane, reactive magnetron sputtering of silicon, or related techniques. Microcrystalline silicon is deposited when a large quantity of molecular hydrogen is added to the process gas such that a large flux of atomic hydrogen impinges on the film growth surface; otherwise, the films are amorphous. Three different microscopic mechanisms have been hypothesized to explain the formation of the microcrystalline phase. In essence, the hypotheses are that atomic hydrogen: (i) enhances the surface diffusion of Si adspecies, which in turn raises the probability of crystalline phase formation, (ii) promotes a subsurface transformation of amorphous into microcrystalline Si, or (iii) preferentially etches amorphous regions such that only microcrystalline Si survives to produce film growth. In this work, we critically test mechanism (i) as follows. We deposit films using dc reactive magnetron sputtering of a Si target in an argon–hydrogen plasma, which yields very poor adspecies mobility at low rates of hydrogen injection. We then increase the hydrogen injection and measure the increase in adspecies motion via the enhanced rate at which the surface smoothens for film growth on substrates with a calibrated roughness of ∼80 Å. The dynamic surface roughness and the structural phase are determined by real-time spectroscopic ellipsometry. The combination of high atomic hydrogen flux and high surface hydrogen coverage uniquely correlates with microcrystalline phase formation. Higher substrate temperatures do not increase adspecies mobility, and actually decrease it when the rate of thermal desorption becomes sufficient to decrease the surface hydrogen coverage. These results also suggest that the original identity of the Si-bearing growth species is relatively unimportant, because the atomic hydrogen flux appears to produce mobile adspecies via surface reactions. We have previously shown that subsurface transformations, mechanism (ii), can also occur. However, we find no evidence for competitive etching, mechanism (iii), under our experimental conditions.
Synthesis of tailored composite nanoparticles in the gas phase
Authors: Maisels Arkadi , Kruis F. Einar , Fissan Heinz , Rellinghaus Bernd , Zähres Horst
Published: July 2002

We report on a method to obtain tailored nanoparticle aggregates of two components in the gas phase. The method is based on the modification of the Brownian collision rate by charging the nanoparticles. Particles of different components are charged oppositely in order to obtain composite nanoparticle aggregates via preferential coagulation. The resulting composite aggregates are uncharged, which allows for their separation from both, charged unaggregated particles and charged aggregates of only one component. The mean size and standard deviation of each particle component can be adjusted by means of differential mobility analysis. Experimental results are presented for composites of PbS and Ag.
Epitaxial growth of transparent p-type conducting CuGaO2 thin films on sapphire (001) substrates by pulsed laser deposition
Authors: Ueda K. , Hase T. , Yanagi H. , Kawazoe H. , Hosono H. , Ohta H. , Orita M. , Hirano M.
Published: July 2002

Transparent p-type conducting CuGaO2 thin films were prepared on α-Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60° with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (∼80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3×10−2 S cm−1, 1.7×1018 cm−3, and 0.23 cm2 V−1 s−1, respectively.
The conformational behavior of polyglycine as predicted by a density functional model with periodic boundary conditions
Authors: Improta Roberto , Barone Vincenzo , Kudin Konstantin N. , Scuseria Gustavo E.
Published: July 2002

Representative conformations of polyglycine are studied by means of density functional calculations, performing complete geometry optimizations under periodic boundary conditions. The calculated stability order and the equilibrium geometries are in good agreement with the available experimental results. The performance of four density functionals (LSDA, PBE, BLYP, VSXC) are compared both for the dipeptide analogue and for the infinite homopolypeptide. Our results indicate that PBE and BLYP are the models of choice for analyzing conformational equilibriums in polypeptides. While the geometry of the different conformations of polyglycine and the stability order are almost converged at the 6–31G(d) level, the relative energies are not stable until the 6–311++G(2d,2p) basis set level is reached. A comparison between the geometries of glycine dipeptide analogue and of glycine infinite homopolypeptide allows us to gain further insights on the influence of long range effects on the geometry and the stability of the different conformers. This study shows the feasibility of complete high level ab initio optimizations of infinite polypeptides, paving the route for new interesting applications of reliable quantum mechanical methods to biological systems.
Excitation efficiency of electrons and holes in forward and reverse biased epitaxially grown Er-doped Si diodes
Authors: Markmann M. , Neufeld E. , Sticht A. , Brunner K. , Abstreiter G.
Published: July 2002

In this letter, we report on the excitation efficiency of erbium ions by hot electrons and holes in Si:Er:O and Si1−yCy:Er pn diodes at 10 K. In forward bias, a higher electroluminescence efficiency at 1.54 μm is observed for incorporating the erbium ions in the p region of the diode, where enough holes are present to form bound excitons for erbium excitation. In reverse bias, electrons turn out to be 5000 times more efficient in impact exciting of Er3+ than holes at equal space-charge region widths. A dark region of 45 nm for electrons and about 70 nm for holes is present where no erbium excitation is possible. Impact excitation of Er codoped by C is much less efficient than for O codoping.
Harmonic functions and angular momentum for anyon systems
Authors: Milewski Jan , Lulek Tadeusz
Published: July 2002

A method of construction of harmonic functions for the system of anyons on a plane is given. Spectra of angular momentum are derived. Anyonic statistics implies a restriction of the range of angular momentum which is stronger in comparison to the bosonic and fermion case.
Shock induced mechanical response of a γ-TiAl alloy
Authors: Millett J. C. F. , Bourne N. K. , Jones I. P.
Published: July 2002

Plate impact experiments have been performed on samples of the γ-titanium alloy, Ti-48Al-2Cr-2Nb-1B to determine its mechanical response during shock loading. This alloy displays pronounced precursor decay. Comparison with the findings of a previous investigation on a similar material shows that the variation in the Hugoniot Elastic Limit can be explained in terms of the microstructure. The Hugoniot has also been determined, and stress, shock, and particle velocity have been measured as independent variables. The difference of these data from those of a similar intermetallic alloy have been explained in terms of the microstructure. It has been observed that the measured Hugoniot (in stress-particle velocity space) lies above that calculated from the measured shock velocities. We believe that this is due to the high degree of work hardening that this type of intermetallic alloy displays.
Crystallization in block copolymer melts: Small soft structures that template larger hard structures
Authors: Fairclough J. Patrick A. , Mai Shao-Min , Matsen Mark W. , Bras Wim , Messe Loic , Turner Simon C. , Gleeson Anthony J. , Booth Colin , Hamley Ian W. , Ryan Anthony J.
Published: July 2002

The crystallization of shear oriented oxyethylene/oxybutylene (E/B) diblock copolymers has been studied by simultaneous small and wide angle x-ray scattering. Crystallization of ordered melts can be accompanied by a change in length scale and retention of the melt orientation. Lamellar melts crystallize with an increase in length scale with multiply folded E blocks and the B blocks slightly stretched from their melt conformation. Crystallization from oriented gyroid melts leads to an increase in length scale with preferred melt directions being selected. The retention of layer planes on crystallization from an ordered melt is caused by the local stretching of chains and the locally one-dimensional structure, despite the relative strength of the structural process. We demonstrate that an interfacial preordering effect can cause crystallographic register to jump length scales in a soft matter system showing epitaxial crystallization.
On the simulation of the entropy of macromolecules with different flexibilities
Authors: Meirovitch Hagai
Published: July 2002

The local states (LS) and hypothetical scanning (HS) methods of Meirovitch enable one to calculate approximately the absolute entropy and free energy from a sample generated with any simulation procedure. These are general methods which have been applied to a wide range of systems, magnets, lattice-gas and fluid models, as well as polymers and biological macromolecules. While LS and HS are based on the same theoretical grounds, calculation of their transition probabilities (TPs) is significantly different. TPs (LS) are obtained from pure “geometrical” considerations, by counting the number of occurrence of the so-called “local states,” whereas TPs (HS) depend on the interaction energy and thus become more complicated to calculate for realistic models of proteins, for example. LS is suitable for calculating the entropy of fluctuations around a well-defined structure such as the α helix of a peptide, where HS fails; however, HS performs very efficiently at the other extreme, for random coiled polymers with strong excluded volume interactions. In this paper we test for the first time the efficiency of LS in the flexible regime by applying it to models of self-avoiding walks (SAWs) on a square lattice. LS is found to be significantly less efficient than HS. This suggests that to efficiently treat macromolecules with variable regions of flexibility, such as crystalline polymers or protein loops (with relatively flexible side chains), methods that are hybrid of LS and HS will be more efficient than LS and HS individually. Potential hybrid methods are discussed and two methods, denoted LS1 and LS2, are applied to the SAWs models and their efficiency is studied. The present calculations shed new light on various theoretical aspects of our approach. In particular, a recently suggested best-fit procedure for increasing the accuracy of the free energy, based on the correlation between an approximate free energy and its fluctuation, is found to perform well already for relatively bad approximations.
Recursion operators of some equations of hydrodynamic type
Authors: Gürses M. , Zheltukhin K.
Published: July 2002

We give a general method for constructing recursion operators for some equations of hydrodynamic type, admitting a nonstandard Lax representation. We give several examples for N=2 and N=3 containing the equations of shallow water waves and its generalizations with their first two general symmetries and their recursion operators. We also discuss a reduction of N+1 systems to N systems of some new equations of hydrodynamic type.
Far-ranged transient motion of “hot” oxygen atoms upon dissociation
Authors: Schintke Silvia , Messerli Stéphane , Morgenstern Karina , Nieminen Jouko , Schneider Wolf-Dieter
Published: July 2002

Scanning tunneling microscopy at low temperature reveals that upon dissociative adsorption of oxygen on Ag(001) “hot” adatoms have separated in a far-ranged transient motion to two different intrapair distances around 2 and 4 nm, corresponding to 7 and 14 surface lattice constants, respectively. Manipulation experiments on oxygen atoms displaying different contrasts in the images and model calculations suggest that the transient motion ends up not only in the stable fourfold hollow site but also in two metastable sites.
Chemical equilibria in slitlike pores
Authors: Borówko M. , Zagórski R.
Published: July 2002

The reaction ensemble Monte Carlo method is used to study chemical equilibrium in Lennard-Jones fluids confined in slitlike pores. An impact of an external field on the chemical equilibrium is being discussed. The results are compared with the bulk system behavior. The composition of equilibrium mixtures is determined for various values of parameters characterizing the system. The scaling particle–Monte Carlo (SP–MC) method is applied to evaluate the excess chemical potential in equilibrium mixture. These values are used to calculate the reaction constant.
Active matrix of amorphous silicon multijunction color sensors for document imaging
Authors: Lemmi F. , Mulato M. , Ho J. , Lau R. , Lu J. P. , Street R. A. , Palma F.
Published: July 2002

An integrated color image sensor, made entirely with amorphous silicon (a-Si:H) large-area technology, is presented. The a-Si:H based sensor is a double-junction p-i-n-i-p photodiode that discriminates two spectral bands according to the bias voltage. The active-matrix addressed array has 512×512 pixels with 75 μm pixel pitch and uses thin-film transistors as pixel switches. The array structure and the spectral response are discussed, and color images taken by the system using two bias voltages demonstrate the compatibility of color sensors with large-area active-matrix addressing techniques.
Evolution of barrier asymmetry in magnetic tunnel junctions
Authors: Brückl H. , Schmalhorst J. , Reiss G. , Gieres G. , Wecker J.
Published: July 2002

Magnetic tunnel junctions usually consist of different layer stacks at the two sides of the tunneling barrier. The exposure of these junctions to high temperatures thus can induce large asymmetries in the electronic potential with respect to the barrier. Using the example of Co/Cu/Co/Al2O3/Co tunnel junctions, we show that the measured current/voltage characteristics develop a large asymmetry upon annealing at a temperature larger than 230 °C. This is accompanied by a Cu enrichment at one side of the barrier and cannot be explained by simply using the work function differences between the electrode materials. Thus, a Cu–Al2O3 intermixing zone at the barrier interface must be taken into account, which leads to an asymmetric step-like barrier shape. The interpretation is supported by numerical evaluation of model barriers which reproduce the experimental asymmetries if an intermixing zone of only 0.2 nm thickness is assumed.
Generalized gyrotron theory with inclusion of adiabatic electron trapping in the presence of a depressed collector
Authors: Airila M. I. , Dumbrajs O.
Published: July 2002

The effect of electrostatic adiabatic electron trapping in the presence of a depressed collector has been included in gyrotron efficiency computations. The results are presented as general contour plots in the control parameter plane. A condition for trapping is derived in terms of the retarding voltage. The overall electron efficiency as a function of the retarding voltage has been calculated for a representative set of control parameters including the effect of electron velocity spread. It is found that the onset of trapping seriously decreases the efficiency.
Dielectric studies of Cd1−x−yZnxMnyTe crystals
Authors: Lin H. M. , Chen Y. F. , Shen J. L. , Chou W. C.
Published: July 2002

We report on the dielectric properties of Cd1−x−yZnxMnyTe alloys studied by capacitance and dissipation factor measurements at temperature 5 K<T<475 K and frequency 20 Hz<f<1 MHz. A Debye-like relaxation of dielectric behavior has been observed, which is found to be a thermally activated process. The activation energies obtained from the capacitance and dissipation factor measurements are in excellent agreement. By means of our measurements, it is believed that the dielectric character of the carrier hopping among structural defects is responsible for the observed Debye relaxation. The relationship between the activation energy and Zn concentration has been established. The results are described by the four-center model, in which the number of Zn atoms appearing in the nearest-neighbor sites of a defect can have four possible configurations.
Negative resistance contribution of a domain-wall structure in a constricted geometry
Authors: Theeuwen S. J. C. H. , Caro J. , Schreurs K. I. , van Gorkom R. P. , Wellock K. P. , Gribov N. N. , Radelaar S. , Jungblut R. M. , Oepts W. , Coehoorn R. , Kozub V. I.
Published: July 2002

We study the magnetoresistance (MR) of Py/Py, Co/Py, Co/Co, Ni/Ni, and Co/Cu point contacts (Py=permalloy=Ni80Fe20). These devices are narrow constrictions or channels (diameter, length ≈30 nm) between two thin film electrodes. Due to the small size of the constriction, which is comparable to a bulk domain-wall (DW) thickness, a DW can be caught in it. For almost all material combinations studied we find that low resistance contacts show an MR minimum at zero field (H=0) of magnitude 0.4%–1.3%, for temperatures between 1.5 and 293 K. The minimum occurs for all field orientations with respect to the channel axis. When the contact resistance increases beyond the value set by a diameter-to-length ratio for the channel of about unity, the resistance minima at H=0 evolve into a maximum/minimum combination as expected for a predominant anisotropic magnetoresistance (AMR) effect. We use micromagnetic calculations based on magnetostatic and exchange interactions to obtain the magnetization in the constriction. These calculations predict that, due to the finite channel length, there are two partial DWs at either side of the channel. For high resistance contacts this agrees with the observed AMR, which results from scattering in the homogeneously magnetized material in the channel. The MR minimum for low resistance contacts arises from the DWs, which cause a resistance decrease. We attribute this decrease to a change of spin-dependent diffuse scattering at the constriction boundary due to the DWs.
Defects introduced in cadmium telluride by γ irradiation
Authors: Cavallini A. , Fraboni B. , Dusi W. , Zanarini M. , Hage-Ali M. , Siffert P.
Published: July 2002

The properties peculiar to high resistivity CdTe:Cl are of great interest because of its application as a radiation detector. The compensation process responsible for the materials semi-insulating character implies the presence in the lattice of impurities and defects which have not yet been thoroughly characterized. The use of CdTe:Cl as a detector exposes the material to high fluxes of ionizing radiation which alter the crystal stoichiometry and affect the resulting electrical and optical properties, but few and scattered experimental data are available about radiation effects on this compound. In this work we have carried out an extensive investigation of the effects of γ irradiation on CdTe:Cl by photoinduced current transient spectroscopy analyses. We have identified the deep levels with activation energies up to midgap and we have followed their evolution with increasing irradiation doses up to 50 kGy, the dose which totally degrades the material detecting properties.
Unexpected stability of [Cu⋅Ar]2+, [Ag⋅Ar]2+, [Au⋅Ar]2+, and their larger clusters
Authors: Walker N. R. , Wright R. R. , Barran P. E. , Cox H. , Stace A. J.
Published: July 2002

Experimental observations following the ionization of neutral group 11 metal/argon complexes have revealed the presence of doubly charged ions of the form [M⋅Arn]2+ for n in the range 1–6. Of particular interest are two features of the results. First, the unexpected stability of the dimer ions, [M⋅Ar]2+, since similar species involving a molecule rather than a rare gas atom are often unstable with respect to charge transfer. Ab initio calculations show the dimers owe their stability to a combination of a strong electrostatic interaction and the high ionization energy of argon. A second feature to the results is the high relative intensities of the [M⋅Ar4]2+ and [M⋅Ar6]2+ ions. Calculations show these complexes to consist of square-planar D4h structures, with the additional two atoms in [M⋅Ar6]2+ occupying axial sites, which are Jahn–Teller distorted. The calculated relative binding energies support the preferential stability of these two structures.
An ab initio study of the lattice distortions induced by nonisovalent Ge2+, Sn2+, and Pb2+ substitutional impurities in crystalline NaCl
Authors: Aguado Andrés
Published: July 2002

A theoretical analysis of the lattice distortions induced by nonisovalent Ge2+, Sn2+, and Pb2+ substitutional impurities in crystalline NaCl, and of the off-center equilibrium position adopted by those impurities in their ground ns2 electronic configuration is presented. The calculations are based in the cluster approach, and involve large active clusters embedded in an accurate quantal representation of the crystalline environment. The charge compensation problem is dealt with by considering several allocations of a cationic vacancy in the host lattice. The obtained distortions involve in all cases the concerted movement of a large number of host crystal ions. Those distortions are presented and discussed in terms of simple packing and Madelung considerations.
Writhing geometry at finite temperature: Random walks and geometric phases for stiff polymers
Authors: Maggs A. C.
Published: July 2002

This paper studies the geometry of a semiflexible polymer at finite temperatures. The writhe correlation functions can be calculated from the properties of Gaussian random walks on the sphere. The writhe of a polymer is analogous to geometric or Berry phases studied in optics and wave mechanics. These results can be applied to confocal microscopy studies of stiff filaments and to simulations of short DNA loops.
Optical alignment of liquid crystals
Authors: Dantsker D. , Kumar J. , Tripathy S. K.
Published: July 2002

An optical method is described for the preparation of a liquid crystal cell. In one, single exposure step a surface relief grating is generated in a photopolymer layer deposited on a glass substrate. Such substrates then orient liquid crystals in contact with them and homogeneous alignment results. An analysis of the nematic liquid crystal orientation in terms of nematic liquid crystal cell parameters as encountered in applications has been made. Consideration of the different mechanisms proposed to explain the origin of liquid crystal alignment and of the conditions required for the generation of surface relief grating in a photopolymer layer has led to the conclusion that both the photoinduced chromophore theory and the surface oriented polymer chains theory are unable to explain the alignment mechanism. On the other hand, the elastic strain energy theory, which is based on geometrical factors, agrees with the observed spatial molecular orientation of the liquid crystals.
Geometric and statistical properties in the evolution of material surfaces in three-dimensional chaotic flows
Authors: Giona Massimiliano , Adrover Alessandra
Published: July 2002

In this article we analyze the invariant geometric properties of three-dimensional (3-D) chaotic flows. Attention is focused on the statistical (measure-theoretical) characterization of the asymptotic evolution of material surfaces forming the boundary between fluid elements, which can be characterized quantitatively in terms of intermaterial contact area density. The approach developed by Giona and Adrover [Phys. Rev. Lett. 81, 3864 (1998)] for diffeomorphisms (Poincaré map of two-dimensional periodically forced flows) is extended to three-dimensional autonomous systems, for which a relation is obtained between intermaterial contact area density and stretching field. The Arnold–Beltrami–Childress flow is considered as a model system. The statistical and singular properties of the intermaterial contact area measure are addressed and some as yet unsolved fundamental issues related to nonautonomous three-dimensional flows are discussed.
Elastic properties of nanocrystalline zirconium–silicon–boron thin films
Authors: Chirita M. , Xia H. , Sooryakumar R. , Tolle J. B. , Torres V. M. , Wilkens B. J. , Smith D. J. , Kouvetakis J. , Tsong I. S. T.
Published: July 2002

Thin amorphous films of ZrB3 grown on Si(111) substrates by chemical-vapor deposition are, upon annealing at 960 °C, transformed to films composed of nanocrystallites (6–10 nm extent) with a nominal composition of Zr0.9Si0.3B3. The independent elastic constants of the layers are determined from the dispersion of the surface and pseudo-Sezawa acoustic excitations and reveal large enhancements in the C11 and C44 constants accompanying the conversion of ZrB3 to the nanocrystalline phase. Since the transverse sound velocities of the binary and Zr0.9Si0.3B3 are in near resonance with the sound velocity of Si (VTSi=4.8 km/s), only the Rayleigh surface wave is localized to the film while all higher-order acoustic modes are evanescent. Despite the strong decay channels, high-lying excitations with velocities as large as 25 km/s (≫VTSi) are observed in Brillouin light scattering. Insight into these acoustic properties is provided by evaluating the elastodynamic Green’s functions and associated acoustic-mode densities.
Linear gyroviscous stability of field-reversed configurations with static equilibrium
Authors: Iwasawa Naotaka , Ishida Akio , Steinhauer Loren C.
Published: July 2002

A discrepancy persists between field-reversed configuration experiments, which are generally stable, and theoretical predictions of instability. The common consensus has been that the stability is the result of finite Larmor radius (FLR) effects. An FLR analysis is presented that finds the self-consistent displacement functions and complex frequency. This is done using the linear gyroviscous model, a fluid-based representation of FLR that allows a wide range of equilibria and modes to be examined with modest computations. The conclusion is that FLR in static FRC fails to explain the observed stability. The cause of stability must lie elsewhere.
Nuclear magnetic resonance measurements of fluctuations in air–water two-phase flow: Pipe flow with and without “disturbing” section
Authors: Le Gall F. , Pascal-Ribot S. , Leblond J.
Published: July 2002

In this paper we describe a new NMR sequence designed to measure the liquid mass flux and the liquid momentum flux fluctuations in a two-phase flow, in order to study the action of the flow on transverse mechanical structures. An upward air–water bubbly flow is investigated in a circular pipe under atmospheric pressure and room temperature for two geometrical configurations: with and without a “disturbing” section. Results show that the addition of the “disturbing” section on the experimental set-up led to the appearance of an instability phenomenon that greatly influences the relative intensity of both mass and momentum liquid flux fluctuations. Moreover, the spectral analysis of the former quantities demonstrates that the fluctuations of the liquid space fraction play a major role in the behavior of the liquid momentum flux fluctuations.
Relaxation behavior of nonlinear optical response in borate glasses containing gold nanoparticles
Authors: Sasai Jun , Hirao Kazuyuki
Published: July 2002

The relaxation time of nonlinear optical response has been obtained by femtosecond pump–probe measurements for borate glasses containing gold nanoparticles prepared by conventional melt-quenching and heat treatment techniques. Size effects of gold nanoparticles on the relaxation process were investigated. The relaxation time of a fast relaxation component increases with a decrease of the particle radius. The tail of the decay curve due to a slow relaxation component increases with an increase of the particle size. These results indicate that the particle size is an important factor for relaxation processes. In addition, the tail component is very small and is not affected by the pump intensity. This shows that the borate glass matrix has superior properties in the slow relaxation process.
Ion heat pinch in reversed magnetic shear tokamak plasmas
Authors: Wang A. K. , Qiu X. M.
Published: July 2002

In the well-known reversed shear discharges, it is observed that the ion thermal diffusivity (χi) falls below the standard neoclassical value (χineo), i.e., χi<χineo. In this paper, local turbulent ion thermal pinch (χit<0) is proposed as a candidate for interpreting the experimental results from χi=χineo+χit<χineo. To test the idea, the two-fluid theory, developed by Weiland and the Chalmers group [J. Weiland et al., Nucl. Fusion 29, 1810 (1989); H. Nordman et al., ibid 30, 983 (1990)], is used in the reversed magnetic shear tokamak plasma to study the drift mode and associated ion heat transport. The theory is extended here to include both the radial electrical field shear (dEr/dr) and electron fluid velocity (Ve) in the sheared coordinate system. Remarkably different from B−1dEr/dr, k⋅Ve directly includes the safety factor q as well as the E×B velocity VE itself, where k is the magnetic configuration-dependent wave vector. As a result, the synergetic effects of B−1dEr/dr and k⋅Ve, especially those of k⋅Ve, lead to the local turbulent ion heat pinch in the negative and weak magnetic shear region because of the wave-particle resonance. The impact of B−1dEr/dr and k⋅Ve on the growth rate and ion heat pinch is numerically investigated. Qualitatively, the present results are in good agreement with the experimental trends.
Dynamic behavior of thermionic dispenser cathodes under ion bombardment
Authors: Cortenraad R. , van der Gon A. W. Denier , Brongersma H. H. , Gärtner G. , Raasch D. , Manenschijn A.
Published: July 2002

We have investigated the surface coverage and electron emission of thermionic dispenser cathodes during 3 keV Ar+ ion bombardment, thereby simulating the bombardment of the cathodes by residual gases that takes place in cathode-ray tubes as used in television sets. During the ion bombardment at the operating temperature of 1030 °C, a dynamic equilibrium is established between the sputter removal and resupply mechanisms of the Ba and O atoms that form the dipole layer on the cathode substrate. We demonstrated that the performance of the cathodes under ion bombardment is governed by the O removal and resupply rates. It was found that the Ba resupply rate is almost an order of magnitude higher than the O resupply rate, but that the Ba can only be present on the surface bound to O atoms. Therefore, the Ba/O ratio is approximately equal to unity during the ion bombardment. Based on the investigations of the removal and resupply processes, we proposed a model that accurately describes the surface coverage and electron emission during the ion bombardment, including the dependence of the ion flux and cathode temperature.
Structure formation of a single polymer chain. I. Growth of trans domains
Authors: Fujiwara Susumu , Sato Tetsuya
Published: July 2002

Molecular dynamics simulations are carried out to study structure formation of a single polymer chain with 500 CH2 groups. Our simulations show that the orientationally ordered structure is formed at a low temperature both by gradual stepwise cooling and by quenching from a random configuration at a higher temperature. The growth of the global orientational order proceeds in a gradual manner in the case of gradual stepwise cooling, whereas it proceeds in a stepwise manner in the case of quenching. The latter feature endorses the previously proposed hypothetical grand view of self-organization [e.g., see T. Sato, Phys. Plasmas 3, 2135 (1996)]: when a system is driven far from equilibrium, it will evolve to a more stable state in a stepwise fashion irrespective of its fundamental interaction forces. From the microscopic analysis of the structure formation process, we find the following characteristic features: (i) In the case of gradual stepwise cooling, the global orientational order grows gradually through the incorporation of small trans domains and the surrounding trans segments into the largest trans domain. (ii) In the case of quenching, the growth of the orientational order is either due to the incorporation of small trans domains and the surrounding trans segments into the largest trans domain or to the elongation of the trans segments in the largest trans domain.