bluuebunny/crossref_metadata_2025 huggingface dataset:
Symmetry principles in the nuclear magnetic resonance of spinning solids: Heteronuclear recoupling by generalized Hartmann–Hahn sequences
Authors: Brinkmann Andreas , Levitt Malcolm H.
Published: July 2002

General symmetry principles for rotor-synchronized pulse sequences in magic-angle spinning solid-state nuclear magnetic resonance are presented. The theory of symmetry-based pulse sequences using π pulse elements is presented for the first time. The symmetry theory is extended to the case of generalized Hartmann–Hahn sequences, in which rotor-synchronized rf irradiation is applied simultaneously to two isotopic spin species. The symmetry principles lead to heteronuclear selection rules. The symmetry theory is used to design pulse sequences which implement heteronuclear dipolar recoupling at the same time as decoupling homonuclear spin–spin interactions, and which also suppress chemical shift anisotropies. A number of specific pulse sequences based on these principles are listed. Experimental demonstrations are given of heteronuclear two-dimensional correlation spectroscopy, heteronuclear multiple-quantum spectroscopy, and the estimation of internuclear dipolar couplings.
Local structure of amorphous Zr70Pd30 alloy studied by electron diffraction
Authors: Takagi T. , Ohkubo T. , Hirotsu Y. , Murty B. S. , Hono K. , Shindo D.
Published: July 2002

The structures of melt-spun, and subsequently annealed, Zr70Pd30 alloys, which form a nanosized icosahedral quasicrystalline phase during the primary crystallization of the amorphous phase, have been investigated by means of electron-diffraction pair-distribution-function (PDF) analysis. The PDF analysis for this alloy was performed by precise measurements of elastic halo-electron-diffraction intensities. Possible structure models for as-quenched and annealed amorphous structures were constructed with the help of reverse Monte Carlo simulations. In order to obtain the local atomic structures, the Voronoi-polyhedra analysis was performed. A considerable number of icosahedral clusters with Zr atoms in the center are present in the simulated structure of the as-quenched state, and the number of the clusters increases by annealing.
Higher excitations in coupled-cluster theory
Authors: Kállay Mihály , Surján Péter R.
Published: July 2002

The viability of treating higher excitations in coupled-cluster theory is discussed. An algorithm is presented for solving coupled-cluster (CC) equations which can handle any excitation. Our method combines the formalism of diagrammatic many-body perturbation theory and string-based configuration interaction (CI). CC equations are explicitly put down in terms of antisymmetrized diagrams and a general method is proposed for the factorization of the corresponding algebraic expressions. Contractions between cluster amplitudes and intermediates are evaluated by a string-based algorithm. In contrast to our previous developments [J. Chem. Phys. 113, 1359 (2000)] the operation count of this new method scales roughly as the (2n+2)nd power of the basis set size where n is the highest excitation in the cluster operator. As a by-product we get a completely new CI formalism which is effective for solving both truncated and full CI problems. Generalization for approximate CC models as well as multireference cases is also discussed.
Interaction between hydrophobic surfaces with metastable intervening liquid
Authors: Bratko D. , Curtis R. A. , Blanch H. W. , Prausnitz J. M.
Published: July 2002

Molecular simulation is used to elucidate hydrophobic interaction at atmospheric pressure where liquid water between apolar walls is metastable with respect to capillary evaporation. The steep increase of the estimated activation barrier of evaporation with surface–surface separation explains the apparent stability of the liquid at distances more than an order of magnitude below the thermodynamic threshold of evaporation. Solvation by metastable liquid results in a short-ranged oscillatory repulsion which gives rise to an irreversible potential barrier between approaching walls. The barrier increases with external pressure in accord with measured pressure-induced slowing of conformational transitions of biopolymers with strong hydrophobic interactions. At a sufficiently small separation, the force abruptly turns attractive signaling nucleation of the vapor phase. This behavior is consistent with the cavitation-induced hysteresis observed in a number of surface–force measurements for strongly hydrophobic surfaces at ambient conditions.
Dielectric and piezoelectric properties of (x)Pb(Mg1/3Nb2/3)O3–(1−x)Pb(Zr1/2Ti1/2)O3 thin films prepared by the sol–gel method
Authors: Yoon Ki Hyun , Lee Byoung Duk , Park Jihoon , Park Jeong Hwan
Published: July 2002

The dielectric and piezoelectric properties of sol–gel derived (x)Pb(Mg1/3Nb2/3)O3–(1−x)Pb(Zr1/2Ti1/2)O3[(x)PMN–(1−x)PZT] thin films were investigated as a function of PMN content (x=0–1). For all compositions in the (x)PMN–(1−x)PZT thin films, a well-developed perovskite phase with (111) preferred orientation was obtained at the annealing temperature range of 700–800 °C. With increasing PMN content, the dielectric constant increased, while the remnant polarization and coercive field decreased. The enhanced dielectric properties were shown in the region of x=0.2. At this composition, the dielectric constant and remnant polarization were 1750 and 12.75 μC/cm2, respectively, which indicate that the morphotropic phase boundary (MPB) exists near the composition of x=0.2. The transverse piezoelectric coefficient (d31) showed a maximum value of −84 pC/N at x=0.2. These results confirmed that enhancement of the dielectric constant and remnant polarization improved the piezoelectric properties at the MPB.
Spectral and spatial characterization of a radio frequency power absorption in high pressure helicon plasmas
Authors: Shamrai Konstantin P. , Shinohara Shunjiro
Published: July 2002

A power absorption in helicon plasma excited by double m=0 antenna is considered at high argon pressures, when the electron collision frequency exceeds the driving frequency. The influence of various factors is examined, including the plasma density and nonuniformity, magnetic field, gas pressure, and antenna spectrum. The wave dispersion curves and absorbed power spectra are comparatively examined to find out the scaling laws. The spatial distribution of the deposited power is characterized both qualitatively and quantitatively by considering the absorption profiles and fractions of the total power absorbed in the under antenna and edge regions of the plasma column. Dependencies of the plasma load resistance on various parameters and magnetic field profiles are examined and compared with experimental results. Computations are performed using two alternative models, either taking into account or neglecting the excitation of quasi-electrostatic Trivelpiece–Gould (TG) waves. Results on the plasma resistance are found to be close in both models provided collisional damping of helicons is not so low as to give rise to the cavity resonances. Inclusion of the TG waves yields magnetic field profiles which are substantially localized near the antenna, and absorption profiles strongly peaking in the peripheral plasma, underneath the antenna. With neglect of the TG waves, the magnetic fields and absorption profiles are found to be much more extended in both axial and radial directions from the antenna. Theory accounting for the TG waves gives a better fit over a broad range of parameters to the measured data, especially regarding the magnetic field profiles.
Brownian ratchets and Parrondo’s games
Authors: Harmer Gregory P. , Abbott Derek , Taylor Peter G. , Parrondo Juan M. R.
Published: July 2002

Parrondo’s games present an apparently paradoxical situation where individually losing games can be combined to win. In this article we analyze the case of two coin tossing games. Game B is played with two biased coins and has state-dependent rules based on the player’s current capital. Game B can exhibit detailed balance or even negative drift (i.e., loss), depending on the chosen parameters. Game A is played with a single biased coin that produces a loss or negative drift in capital. However, a winning expectation is achieved by randomly mixing A and B. One possible interpretation pictures game A as a source of “noise” that is rectified by game B to produce overall positive drift—as in a Brownian ratchet. Game B has a state-dependent rule that favors a losing coin, but when this state dependence is broken up by the noise introduced by game A, a winning coin is favored. In this article we find the parameter space in which the paradoxical effect occurs and carry out a winning rate analysis. The significance of Parrondo’s games is that they are physically motivated and were originally derived by considering a Brownian ratchet—the combination of the games can be therefore considered as a discrete-time Brownian ratchet. We postulate the use of games of this type as a toy model for a number of physical and biological processes and raise a number of open questions for future research.
Dissipative particle dynamics for interacting systems
Authors: Pagonabarraga I. , Frenkel D.
Published: July 2002

We introduce a dissipative particle dynamics scheme for the dynamics of nonideal fluids. Given a free-energy density that determines the thermodynamics of the system, we derive consistent conservative forces. The use of these effective, density dependent forces reduces the local structure as compared to previously proposed models. This is an important feature in mesoscopic modeling, since it ensures a realistic length and time scale separation in coarse-grained models. We consider in detail the behavior of a van der Waals fluid and a binary mixture with a miscibility gap. We discuss the physical implications of having a single length scale characterizing the interaction range, in particular for the interfacial properties.
The effect of surface roughness on the adhesion of elastic solids
Authors: Persson B. N. J. , Tosatti E.
Published: July 2002

We study the influence of surface roughness on the adhesion of elastic solids. Most real surfaces have roughness on many different length scales, and this fact is taken into account in our analysis. We consider in detail the case when the surface roughness can be described as a self-affine fractal, and show that when the fractal dimension Df>2.5, the adhesion force may vanish, or be at least strongly reduced. We consider the block-substrate pull-off force as a function of roughness, and find a partial detachment transition preceding a full detachment one. The theory is in good qualitative agreement with experimental data.
Dielectric relaxation in liquid crystalline dimers
Authors: Dunmur D. A. , Luckhurst G. R. , de la Fuente M. R. , Diez S. , Peréz Jubindo M. A.
Published: July 2002

Dielectric measurements are reported for liquid crystalline dimers consisting of two mesogenic groups joined through ether links by a flexible alkyl chain. Results are presented for the odd-symmetric dimer, α,ω-bis[(4-cyanobiphenyl)-4′-yloxy]undecane (BCB.O11), and the odd-asymmetric dimer, α-[(4-cyanobiphenyl)-4′-yloxy]-ω-(4-decylanilinebenzylidene-4′-oxy) nonane (CB.O9O.10). The real and imaginary parts of the electric permittivities of aligned samples were measured as functions of frequency over the range 102–109 Hz at temperatures throughout the nematic phase. Measurements were fitted as a function of frequency to the Havriliak–Negami function, and yielded relaxation times and dielectric strengths for the relaxations at each of the temperatures studied. The static dielectric anisotropy for both materials was positive. For BCB.O11, the parallel and perpendicular components of the permittivity exhibited one low frequency and one high frequency relaxation, while the parallel component of the permittivity for CB.O9O.10 showed three relaxations, which were well-separated in frequency. A generic model of liquid crystalline dimers based on mixtures of linear and bent conformers with the tetrahedral angle has been proposed [A. Ferrarini et al., Chem. Phys. Lett. 214, 409 (1993)]. This model is used to provide a qualitative interpretation of the dielectric measurements reported. To obtain a satisfactory explanation of the results, it is necessary to include additional conformers having either a hairpin shape or a bent shape with an angle of 180° minus the tetrahedral angle. The temperature dependence measured for the strengths of the dielectric relaxations is explained in terms of changes of the order parameter and changes in the relative probabilities of different shaped conformers.
Electric field tuning of a stop band in a reflection spectrum of synthetic opal infiltrated with nematic liquid crystal
Authors: Shimoda Y. , Ozaki M. , Yoshino K.
Published: July 2002

Electric field tuning of an optical stop band in a reflection spectrum is demonstrated in a synthetic opal thin film infiltrated with liquid crystal. The reflection peak of the opal shifts upon the application of the electric field, and this is based on the refractive index change caused by molecular reorientation upon application of a field. The dynamic response of the stop band shift was also studied, and fast response of the order of microsecond was observed.
Nucleated deliquescence of salt
Authors: Cantrell Will , McCrory Charles , Ewing George E.
Published: July 2002

We have studied deliquescence on the (001) face of single crystals of NaCl and find that it is a nucleated phenomenon. The phase transition occurs only after the relative humidity exceeds that found over the saturated solution by at least 5%. The contrast between our observations and previous measurements using salt powders or crystallites leads us to the conclusion that deliquescence is controlled by the differences in energy required to solvate ions from the smooth (001) face and from the defect-rich surfaces of particulate samples.
Structural and energetic heterogeneity in protein folding. I. Theory
Authors: Plotkin Steven S. , Onuchic José N.
Published: July 2002

A general theoretical framework is developed using free-energy functional methods to understand the effects of heterogeneity in the folding of a well-designed protein. Native energetic heterogeneity arising from nonuniformity in native stabilizing interactions, as well as entropic heterogeneity intrinsic to the topology of the native structure, are both investigated as to their impact on the folding free-energy landscape and resulting folding mechanism. Given a minimally frustrated protein, both structural and energetic heterogeneity lower the thermodynamic barrier to folding. When energy functions consist of pair interactions, designing in sufficient heterogeneity can eliminate the barrier at the folding transition temperature. Sequences with different distributions of native stabilizing interactions and correspondingly different folding mechanisms may still be good folders to the same structure. This theoretical framework allows for a systematic study of the coupled effects of energetics and topology in protein folding, and provides interpretations and predictions for future experiments which may investigate these effects.
Multicanonical parallel tempering
Authors: Faller Roland , Yan Qiliang , de Pablo Juan J.
Published: July 2002

We present a novel implementation of the parallel tempering Monte Carlo method in a multicanonical ensemble. Multicanonical weights are derived by a self-consistent iterative process using a Boltzmann inversion of global energy histograms. This procedure gives rise to a much broader overlap of thermodynamic-property histograms; fewer replicas are necessary in parallel tempering simulations, and the acceptance of trial swap moves can be made arbitrarily high. We demonstrate the usefulness of the method in the context of a grand-multicanonical ensemble, where we use multicanonical simulations in energy space with the addition of an unmodified chemical potential term in particle-number space. Several possible implementations are discussed, and the best choice is presented in the context of the liquid–gas phase transition of the Lennard-Jones fluid. A substantial decrease in the necessary number of replicas can be achieved through the proposed method, thereby providing a higher efficiency and the possibility of parallelization.
Complementary approaches to the ab initio calculation of melting properties
Authors: Alfè D. , Gillan M. J. , Price G. D.
Published: July 2002

Several research groups have recently reported ab initio calculations of the melting properties of metals based on density functional theory, but there have been unexpectedly large disagreements between results obtained by different approaches. We analyze the relations between the two main approaches, based on calculation of the free energies of solid and liquid and on direct simulation of the two coexisting phases. Although both approaches rely on the use of classical reference systems consisting of parametrized empirical interaction models, we point out that in the free energy approach the final results are independent of the reference system, whereas in the current form of the coexistence approach they depend on it. We present a scheme for correcting the predictions of the coexistence approach for differences between the reference and ab initio systems. To illustrate the practical operation of the scheme, we present calculations of the high-pressure melting properties of iron using the corrected coexistence approach, which agree closely with earlier results from the free energy approach. A quantitative assessment is also given of finite-size errors, which we show can be reduced to a negligible size.
Influence of the noble gas mixture composition on the performance of a plasma display panel
Authors: Gillies M. F. , Oversluizen G.
Published: July 2002

The influence of the noble gas mixture composition on the plasma display panel performance is investigated in test panels with a design which resembles the one used in commercial panels. Single gases and binary and ternary mixtures of He, Ne, Ar, Kr, and Xe are applied, where the Xe concentration is varied from 0% to 100%. The performance is characterized in terms of the panel luminance, efficacy, and discharge voltages. It is found that while an increase in efficacy and luminance can be achieved in several multicomponent mixtures it is necessary to examine the associated increase in the firing voltage, Vf. If one considers the luminance versus Vf dependence, then binary NexXe1−x mixtures are optimal to achieve the highest efficacy values at the lowest Vf. The maximum efficacy gain factor in high Xe partial pressure mixtures is about a factor of 3 with respect to the mixture applied in default commercial panels.
Volumetric hydrogen storage in single-walled carbon nanotubes
Authors: Liu C. , Yang Q. H. , Tong Y. , Cong H. T. , Cheng H. M.
Published: July 2002

Macroscopically long ropes of aligned single-walled carbon nanotubes (SWNTs), synthesized by a hydrogen and argon arc discharge method, were cold pressed into tablets without any binder for measurements of their volumetric hydrogen storage capacity. The typical apparent density of the tablets was measured to be around 1.7 g/cm3 with respect to a molding pressure of 0.75 Gpa. A volumetric and mass hydrogen storage capacity of 68 kg H2/m3 and 4.0 wt %, respectively, was achieved at room temperature under a pressure of 11 MPa for suitably pretreated SWNT tablets, and more than 70% of the hydrogen adsorbed can be released under ambient pressure at room temperature. Pore structure analysis indicated that the molding process diminished the mesopore volume of the SWNT ropes, but exerts little influence on their intrinsic pore textures.
Carbon nanotube composites for thermal management
Authors: Biercuk M. J. , Llaguno M. C. , Radosavljevic M. , Hyun J. K. , Johnson A. T. , Fischer J. E.
Published: July 2002

Single-wall carbon nanotubes (SWNTs) were used to augment the thermal transport properties of industrial epoxy. Samples loaded with 1 wt % unpurified SWNT material showed a 70% increase in thermal conductivity at 40 K, rising to 125% at room temperature; the enhancement due to 1 wt % loading of vapor grown carbon fibers was three times smaller. Electrical conductivity data showed a percolation threshold between 0.1 and 0.2 wt % SWNT loading. The Vickers hardness rose monotonically with SWNT loading up to a factor of 3.5 at 2 wt %. These results suggest that the thermal and mechanical properties of SWNT-epoxy composites are improved, without the need to chemically functionalize the nanotubes.
Biochemical clocks and molecular noise: Theoretical study of robustness factors
Authors: Gonze D. , Halloy J. , Gaspard P.
Published: July 2002

We report a study of the influence of molecular fluctuations on a limit-cycle model of circadian rhythms based on the regulatory network of a gene involved in a biochemical clock. The molecular fluctuations may become important because of the low number of molecules involved in such genetic regulatory networks at the subcellular level. The molecular fluctuations are described by a birth-and-death stochastic process ruled by the chemical master equation of Nicolis and co-workers and simulated by Gillespie’s algorithm. The robustness of the oscillations is characterized, in particular, by the probability distribution of the first-return times and the autocorrelation functions of the noisy oscillations. The half-life of the autocorrelation functions is studied as a function of the size of the system which controls the magnitude of the molecular fluctuations and of the degree of cooperativity of some reaction steps of the biochemical clock. The role of the attractivity of the limit cycle is also discussed.
Influence of structural transition on transport and optical properties of Ni2MnGa alloy
Authors: Zhou Y. , Jin Xuesong , Xu Huibin , Kudryavtsev Y. V. , Lee Y. P. , Rhee J. Y.
Published: July 2002

The transport and optical properties of Ni2MnGa alloy in the martensitic and austenitic states were investigated. The dependence of the temperature coefficient of resistivity on temperature shows kinks at the structural and ferroparamagnetic transitions. Electron-magnon and electron-phonon scattering are analyzed to be the dominant scattering mechanisms of the Ni2MnGa alloy in the martensitic and austenitic states, respectively. It was observed that the martensite reveals a higher Debye temperature compared to the austenite, which was explained by a difference in the restoring coefficient between atoms. A Lorentz model that consists of two oscillators was employed to fit the interband contribution to optical conductivity (OC). It is found that the structural transition plays a more important role for the changes in OC spectrum than the thermal-expansion effect.
Concentration dependent nitrogen diffusion coefficient in expanded austenite formed by ion implantation
Authors: Mändl S. , Rauschenbach B.
Published: July 2002

Expanded austenite, formed after nitrogen plasma immersion ion implantation or low energy nitriding of austenitic stainless, is characterized by a high nitrogen content CN of up to 20 at. % and an unusual fast diffusion, which in general cannot be described using a single diffusion coefficient. Here, the concentration dependent diffusivity is calculated for several experimental parameters and steel alloys. Two mathematical simplifications of the general diffusion theory, well justified for physical reasons, helped in solving the equations. First, a constant surface concentration was assumed, despite a constant nitrogen flux into the surface, and, second, only mobile nitrogen atoms in a stationary steel matrix were considered. Thus, it was possible to solve the Boltzmann–Matano equation and obtain the concentration dependent diffusion coefficient D(CN). In all cases, a step-like behavior, with a high value for high nitrogen contents and a low value for low ones, is found, with the transition point between a nitrogen concentration of 5 and 17 at. %, depending on the sample.
Age-related changes in the ‘‘complexity’’ of cardiovascular dynamics: A potential marker of vulnerability to disease
Authors: Lipsitz Lewis A.
Published: July 2002

Healthy physiologic control of cardiovascular function is a result of complex interactions between multiple regulatory processes that operate over different time scales. These include the sympathetic and parasympathetic nervous systems which regulate beat-to-beat heart rate (HR) and blood pressure (BP), as well as extravascular volume, body temperature, and sleep which influence HR and BP over the longer term. Interactions between these control systems generate highly variable fluctuations in continuous HR and BP signals. Techniques derived from nonlinear dynamics and chaos theory are now being adapted to quantify the dynamic behavior of physiologic time series and study their changes with age or disease. We have shown significant age-related changes in the 1/fx relationship between the log amplitude and log frequency of the heart rate power spectrum, as well as declines in approximate dimension and approximate entropy of both heart rate and blood pressure time series. These changes in the ‘‘complexity’’ of cardiovascular dynamics reflect the breakdown and decoupling of integrated physiologic regulatory systems with aging, and may signal an impairment in cardiovascular ability to adapt to external and internal perturbations. Studies are currently underway to determine whether the complexity of HR or BP time series can distinguish patients with fainting spells due to benign vasovagal reactions from those due to life-threatening cardiac arrhythmias. Thus, measures of the complexity of physiologic variability may provide novel methods to monitor cardiovascular aging and test the efficacy of specific interventions to improve adaptive capacity in old age.
Two forms of spiral-wave reentry in an ionic model of ischemic ventricular myocardium
Authors: Xu Aoxiang , Guevara Michael R.
Published: July 2002

It is well known that there is considerable spatial inhomogeneity in the electrical properties of heart muscle, and that the many interventions that increase this initial degree of inhomogeneity all make it easier to induce certain cardiac arrhythmias. We consider here the specific example of myocardial ischemia, which greatly increases the electrical heterogeneity of ventricular tissue, and often triggers life-threatening cardiac arrhythmias such as ventricular tachycardia and ventricular fibrillation. There is growing evidence that spiral-wave activity underlies these reentrant arrhythmias. We thus investigate whether spiral waves might be induced in a realistic model of inhomogeneous ventricular myocardium. We first modify the Luo and Rudy [Circ. Res. 68, 1501–1526 (1991)] ionic model of cardiac ventricular muscle so as to obtain maintained spiral-wave activity in a two-dimensional homogeneous sheet of ventricular muscle. Regional ischemia is simulated by raising the external potassium concentration ([K+]o) from its nominal value of 5.4 mM in a subsection of the sheet, thus creating a localized inhomogeneity. Spiral-wave activity is induced using a pacing protocol in which the pacing frequency is gradually increased. When [K+]o is sufficiently high in the abnormal area (e.g., 20 mM), there is complete block of propagation of the action potential into that area, resulting in a free end or wave break as the activation wave front encounters the abnormal area. As pacing continues, the free end of the activation wave front traveling in the normal area increasingly separates or detaches from the border between normal and abnormal tissue, eventually resulting in the formation of a maintained spiral wave, whose core lies entirely within an area of normal tissue lying outside of the abnormal area (“type I” spiral wave). At lower [K+]o (e.g., 10.5 mM) in the abnormal area, there is no longer complete block of propagation into the abnormal area; instead, there is partial entrance block into the abnormal area, as well as exit block out of that area. In this case, a different kind of spiral wave (transient “type II” spiral wave) can be evoked, whose induction involves retrograde propagation of the action potential through the abnormal area. The number of turns made by the type II spiral wave depends on several factors, including the level of [K+]o within the abnormal area and its physical size. If the pacing protocol is changed by adding two additional stimuli, a type I spiral wave is instead produced at [K+]o=10.5 mM. When pacing is continued beyond this point, apparently aperiodic multiple spiral-wave activity is seen during pacing. We discuss the relevance of our results for arrythmogenesis in both the ischemic and nonischemic heart.
Models of defibrillation of cardiac tissue
Authors: Krinsky V. , Pumir A.
Published: July 2002

Heterogeneities, such as gap junctions, defects in periodical cellular lattices, intercellular clefts and fiber curvature allow one to understand the effect of an electric field in cardiac tissue. They induce membrane potential variations even in the bulk of the myocardium, with a characteristic sawtooth shape. The sawtooth potential, induced by heterogeneities at large scales (tissue strands) can be more easily observed, and lead to stronger effects than the one induced at the cellular level. In the generic model of propagation in cardiac tissue (FitzHugh), 4 mechanisms of defibrillation were found, two mechanisms based on excitation (EA,EM), and two—on de-excitation (DA,DM). The lowest electric field is required by an EM mechanism. In the Beeler–Reuter ionic model, mechanism DM is impossible. We critically review the experimental basis of the theory and propose new experiments.
The silicon liquid-crystal light valve
Authors: Efron U. , Grinberg J. , Braatz P. O. , Little M. J. , Reif P. G. , Schwartz R. N.
Published: July 2002

Detailed description is given of the structure, operation, fabrication, and performance of the novel, fast-response silicon liquid-crystal spatial light modulator. Applications for large screen displays and optical data processing are described.
Dynamical critical exponent of the 3D Ising model
Authors: Wansleben S. , Landau D. P.
Published: July 2002

Using Monte Carlo simulations we investigate the linear (equilibrium) dynamic critical behavior of L×L×L simple cubic Ising models with periodic boundary conditions. The critical exponent z is determined from the scaling behavior of the relaxation time as a function of lattice size. Our estimate of z=2.03±0.04 agrees well with the theoretical prediction obtained by ε expansion.
Blocked impurity band detectors—an analytical model: Figures of merit
Authors: Szmulowicz Frank , Madarasz Frank L.
Published: July 2002

We are presenting an analytic model for the figures of merit of a novel extrinsic infrared quantum detector—the blocked impurity band (BIB) detector. The detector consists of top and bottom contacts, a heavily doped active layer, and a nearly intrinsic layer, called the blocking layer, to stop the motion of hopping carriers in the impurity band. The responsivity, gain, excess noise factor, and detectivity of the BIB detector are calculated as functions of the device dimensions, doping concentrations, and the applied reverse bias, which controls the electric field in the depletion region, devoid of hopping carriers, of the device underneath the blocking layer. Central to our model is the inclusion of impact ionization of carriers in the calculation of the detector response and of the associated noise. For practical detector dimensions and doping concentrations, and at 2-V reverse bias, we calculate the responsivity to be on the order of 2 A/W, and detectivities, with 1012 photons/cm2 s background photon flux, on the order of 1013 cm Hz1/2/W. We provide analytic expressions for the figures of merit which should prove useful to other researchers in the field who want to optimize the detector design.
Modeling of shock-induced chemical reactions in powder mixtures
Authors: Horie Yukie , Kipp Marlin E.
Published: July 2002

Chemical reactions in inorganic powder mixtures under high-pressure shock wave loading are described by two mathematical models, one homogeneous and the other heterogeneous. The models are formulated based upon existing results of observations on post-shock samples of Al-Ni, Al-Ti, and ZnO-Fe2O3 mixtures. Two mechanisms were isolated for the development of the initial models: (1) the creation of a nonequilibrium mixture by dynamic mass mixing, and (2) ensuing chemical reactions. The homogeneous model was evaluated under shock conditions using a one-dimensional wave propagation code, and suggested requisite conditions for the thermal excursion of localized reactions: a localized initial peak temperature of 1000–2000 K and reaction time constants of 1 μs or less. Calculations indicate that reactions occur while the sample is under shock loading, consistent with observations of post-shock samples.
A magnetic position sensor
Authors: Garshelis I. J. , Fiegel W. S.
Published: July 2002

A position sensor utilizing a slender ferromagnetic element, a small permanent magnet, and interactive conductors is described. The element, first uniformly magnetized by passage of the magnet over its entire length, is divided by subsequent motion of the magnet into two regions having opposite polarizations on each side of the magnet position. The regions are differentiated by their unequal incremental permeabilities in small bias fields. The effective ac permeability of the element as a whole is shown to be a linear function of magnet position. Data from an experimental sensor in which permeability was sensed by the inductance of an encircling solenoid confirm this linear transfer function. Utility as a measurement and control element in machinery, instruments, and liquid level devices is apparent from the achievable performance. Expected applications will be in the range from a few centimeters to a few meters.
Electron transport in ZnS
Authors: Ruda H. E. , Lai B.
Published: July 2002

Transport characteristics of N-ZnS are calculated by solving the Boltzmann transport equation using a variational method (including all major scattering mechanisms and screening). The dependence of electron mobility on carrier concentration, for a range of compensation ratio and ionized impurity concentration, are given at both 300 and 77 K. This provides a rapid means for determining material quality. Mobility limits of 230 cm2/V s (n∼1019 cm−3) and over 3000 cm2/V s (n<1014 cm−3) are calculated at 300 and 77 K, respectively. The temperature dependence of the mobility is calculated and agrees favorably with experimental data.
The extrinsic nature of nonlinear behavior observed in lead zirconate titanate ferroelectric ceramic
Authors: Li Shaoping , Cao Wenwu , Cross L. E.
Published: July 2002

The nonlinear electric and electromechanical responses of lead zirconate titanate Pb(ZrxTi1−x)O3 ceramics to an external ac electric field have been measured under different driving levels. The onset of measurable nonlinearity is observed to be accompanied by the appearance of hysteresis loops. This lossy nature suggests that the nonlinearities in a ferroelectric ceramic are generated by the domain-wall motion. In addition, aging experiments and the bias field dependence of the threshold field (onset of nonlinearity) all indicate the extrinsic nature of the nonlinear behavior of ferroelectric ceramics. A phenomenological theory of Arlt, Dederichs, and Herbiet [Ferroelectrics 74, 34 (1987)] has been extended to include the nonlinear contributions. With only 90° wall vibration being considered, the theory leads to some basic understanding of the experimental results.
Electron transport and energy-band structure of InSb
Authors: Jung Y. J. , Park M. K. , Tae S. I. , Lee K. H. , Lee H. J.
Published: July 2002

Various n-InSb crystals, pure and Te doped, were prepared using the horizontal zone-refining technique. The electrical conductivity and Hall coefficient of these samples were measured as functions of temperature from 90 to 470 K. By self-consistently fitting the theoretical transport parameters to the experimental data, a set of parameters was refined and/or determined. They include the optical band-gap variation E0(T) = [0.235 − 3.2× 10−4T2/(220 + T)] eV, the heavy-hole effective mass m*hh = (0.20 ± 0.02)me, and the acoustic deformation potential Ed = (13 ± 1) eV. Various scattering mechanisms and impurity phenomena are also discussed.
Calibration of the nitrogen vibron pressure scale for use at high temperatures and pressures
Authors: Schmidt S. C. , Schiferl D. , Zinn A. S. , Ragan D. D. , Moore D. S.
Published: July 2002

Coherent anti-Stokes Raman scattering (CARS) and spontaneous Raman spectroscopy have been used to obtain vibrational spectra of shock-compressed and static high-pressure fluid nitrogen, respectively. Vibrational frequencies were obtained from the CARS data using a semiclassical model for these spectra. Spontaneous Raman vibrational frequencies were determined by fitting data using a Lorentz-shape line. A functional form was found for the dependence of the vibrational frequency on pressure and temperature to 40 GPa and 5000 K, respectively. By fitting the vibrational data to this form, a pressure scale based on the fluid nitrogen vibron has been calibrated for use at very high temperature. The nitrogen vibron pressure scale was used to determine the fluid-δ nitrogen phase boundary up to 20 GPa and 900 K.
Photoacoustic frequency-domain depth profiling of continuously inhomogeneous condensed phases: Theory and simulations for the inverse problem
Authors: Mandelis Andreas , Peralta Samuel B. , Thoen Jan
Published: July 2002

An application is presented of the Hamilton–Jacobi formulation of thermal-wave physics [A. Mandelis, J. Math. Phys. 26, 2676 (1985)] to the problem of photoacoustic depth profiling of inhomogeneous solids with arbitrary, continuously varying thermal diffusivity profiles. Simple expressions for the modulation frequency dependence of the photoacoustic signal in the case of exponential thermal diffusivity profiles are obtained, and a working general method for solving the inverse problem and obtaining arbitrary diffusivity depth profiles is demonstrated through computer simulations. The method was found to possess excellent profile reconstruction fidelity.
Molecular-dynamics simulations of atomic processes at the low-temperature diamond (111) surface
Authors: Pailthorpe B. A.
Published: July 2002

The deposition of low-energy carbon atoms onto a low-temperature diamond (111) surface is studied by molecular-dynamics computer simulations. A Stillinger–Weber potential [F. H. Stillinger and T. A. Weber, Phys. Rev. B 31, 5262 (1985)], with a reparametrization derived from quantum-mechanical energy calculations for small tetrahedral carbon clusters, is used to model the interatomic interactions. The penetration of 1–100-eV neutral carbon atoms into the diamond (111) surface at 100 K and the resultant surface atom rearrangements and induced film stress are studied. For intermediate energies (20–60 eV) the incident atom penetrates beneath the exposed (111) surface and significantly increases the lateral compressive stress in the diamond film. The emerging picture is that diamond films grow from below the exposed surface in a region of locally high stress and tetrahedral coordination.
Non-Newtonian flow effects during spin coating large-area optical coatings with colloidal suspensions
Authors: Britten Jerald A. , Thomas Ian M.
Published: July 2002

Multilayer sol-gel optical high reflectors with greater than 99% reflection have been prepared on substrates up to 20 cm in diameter by spin coating silica/alumina colloidal suspensions. These coatings are radially nonuniform, owing to the extensive shear-thinning rheology of the high-index alumina suspension. To a large degree the film thickness nonuniformity can be compensated for by the reflection bandwidth. The rheological properties of the alumina suspension under steady shear have been measured. The low-shear reduced viscosity and the shear-thinning time constant are shown to vary exponentially with φ2, where φ is the solids volume fraction. At φ=0.1 the sol has effectively gelled. A model for spincoating with a non-Newtonian fluid has been developed that uses the Carreau rheology model to fit the measured viscometric data. Modeling and experimental results show that as long as these non-Newtonian effects are sufficiently large (as in this case) the radial film uniformity is determined only by these parameters and cannot be significantly influenced by spin rate, initial solids fraction, or any other parameters under the control of the operator. However, most of the film thickness variation occurs in the first 1–2 cm from the substrate center, leaving the remainder almost uniform. Therefore the degree of nonuniformity does not appreciably increase with increasing substrate size.
Titanium nitride oxidation chemistry: An x-ray photoelectron spectroscopy study
Authors: Saha Naresh C. , Tompkins Harland G.
Published: July 2002

We report a study of the oxidation of TiN. In previous work, the oxidation kinetics for 350–450 °C were reported and an initiation time prior to fast oxidation was identified. In this study, x-ray photoelectron spectroscopy was used to investigate the oxidation mechanisms at 350 °C during this initiation time period. The oxide thickness increases slowly with oxidation time and the film appears to change from an amorphous TiO2 layer to a crystalline TiO2 layer. Spectral features which are intermediate between TiO2 and TiN are reported and a model involving grain boundary oxidation is proposed. One of the thicker oxides studied was annealed in vacuum to 700 °C. Following oxidation, some of the capping oxide and much of the intermediate material is no longer in the analysis volume and we suggest that the oxygen and nitrogen is being dissolved into the bulk in much the same way that nonevaporable getters are activated before use.
Surface roughness and the ultrasonic detection of subsurface scatterers
Authors: Nagy Peter B. , Rose James H.
Published: July 2002

The influence of surface roughness on the detection of subsurface scatterers using phase-coherent, ultrasonic immersion transducers is reported. Experiments were conducted with phase-coherent transducers, immersed in a liquid bath, that insonified samples with roughened surfaces. Measurements were made of: (1) the signal-to-noise ratio for a subsurface void when viewed through a rough surface; (2) the frequency-dependent loss (between 2 and 20 MHz) that was induced in the transmission and reflection coefficients of the coherent beam for flat aluminum plates with root-mean-square roughness varying between 5 and 50 μm; and (3) the effects of surface roughness on the material noise. The following results were found. First, surface roughness greatly degraded the signal-to-noise ratio (S/N) for some typical inspection geometries. Second, after appropriate normalization, the transmission and reflection coefficients for the coherent beam were found to be nearly universal functions of the angle of incidence, except near the critical angles. Large reductions (several orders of magnitude) in the phase-coherent signal S resulted for some typical inspection setups. The noise, on the other hand, remained nearly unchanged for the same setups. The degradation in S/N was attributed to the randomization of the phase of the coherently transmitted and reflected beams. Both the sensitivity of the signal and the relative insensitivity of the backscatter noise are explained in terms of the phase-screen approximation. This simple approximation is also used to explain the nearly universal form of the normalized transmission and reflection coefficients, and to predict that the backscatter noise may be either slightly enhanced or slightly decreased by surface roughness.
Broadband tunability of a far-infrared free-electron laser
Authors: Bakker R. J. , van der Geer C. A. J. , Jaroszynski D. A. , van der Meer A. F. G. , Oepts D. , van Amersfoort P. W.
Published: July 2002

A unique property of the free-electron laser (FEL) is its capability to be tuned continuously over a wide spectral range. This is a major difference with all other high-power lasers. However, the tunability of first-generation FELs used to be quite poor (typically 10% or less), due to constraints imposed by the accelerator and the undulator. The free electron laser for infrared experiments (FELIX) uses an undulator with an adjustable gap, which permits wavelength scans over an octave in typically 2 min without the need for any readjustment of the electron beam. Results obtained in operation of the long-wavelength FEL of the FELIX facility are presented. These involve measurements of the spectral range covered (16–110 μm), the output power, and the influence of the cavity desynchronism. The results are compared with numerical simulations.
Grain boundary scattering in the normal state resistivity of superconducting NbN thin films
Authors: Tyan Jyh-Haur , Lue Juh Tzeng
Published: July 2002

NbN films prepared by reactive dc sputtering at high ambient pressure exhibit anomalous normal-state resistivity, as pervasively reported in literature. Most of the prepared films exhibit negative temperature coefficient of resistivity (TCR), while a few films are metallic (TCR≳0) near room temperature and are nonmetallic (TCR<0) at temperatures between 200 and 300 K. Theoretical fittings by the model of grain-boundary scattering yield a better congruence with the experimental data, than those by exploiting the model of electron localization. This suggests that the transport mechanism of sputtered film prevails manipulated by grain boundary scattering rather than by defect scattering or by impurity scattering.
Preparation and characterization of (Bi2S3)1−x(CdS)x solid solution thin films
Authors: Misra Susama , Padhi H. C.
Published: July 2002

Solid solution thin films of (Bi2S3)1−x(CdS)x for different compositions x=0,0.35,0.60,0.81,1.0 are prepared for the first time by electroless deposition on glass and Si 〈111〉 substrates. Composition of the films has been measured using helium Rutherford backscattering method which are in good agreement with mole fraction of parent nitrate solutions used for the preparation. Scanning electron microscopy studies indicated formation of good quality films and band-gap studies of the films confirmed the formation of solid solution. The band gaps of the samples of different compositions were found to vary linearly with composition, decreasing with increase in Bi2S3 atomic concentration.