bluuebunny/crossref_metadata_2025 huggingface dataset:
The Cs2 ground electronic state by Fourier transform spectroscopy: Dispersion coefficients
Authors: Amiot C. , Dulieu O.
Published: August 2002

This study presents the derivation of an accurate potential energy curve for the ground electronic state of the Cs2 molecule. High resolution laser induced emission spectra data involving vibrational levels of the ground X 1Σg+ state up to v″=135 (16 900 wave numbers) have been determined. The ground state potential energy curve is constructed by combining the inverted perturbation approach for internuclear distances up to 11 Å, with an analytical expression for longer internuclear distances. This potential curve allows an improved derivation of the dissociation energy and of the Coulombic parameters governing the Cs(6s)+Cs(6s) interaction in the electronic ground state, compared with values derived either by calculations or by recent photoassociative spectroscopy measurements. The main constants are C6=6836 (±100) a.u.=32.945 (±0.49)×106 cm−1 Å6, and De=3649.88 (±0.45) cm−1.
Development of a film sensor for static and dynamic force measurement
Authors: Castellini P. , Montanini R. , Revel G. M.
Published: August 2002

In this work an innovative double-layer film sensor for the measurement of forces is presented. The sensor is a thin film (thickness below 1 mm) based on a “sandwich” structure composed of two sensing elements glued together: one layer is a capacitive film and the other is a piezoelectric film. Both the layers are sensitive to compression loads, but they are suitable for working in different frequency ranges. In fact, while the capacitive element is capable of measuring from dc up to about 400 Hz, on the contrary the piezoelectric film works in the high frequency range. The output signals of both the sensors are acquired and then filtered and processed in order to achieve a single output signal. The piezocapacitive sensor has been developed in order to synthesize, in a small and cheap device, the capability to measure compression forces in a wide range of frequencies. The sensor is very small and has many potential applications, such as in the field of modal analysis. In particular, the very small thickness allows to insert it into a composite material to measure actual loads and excitations, as well as on the surface or between different components of a more complex system in order to obtain a smart structure. This article describes the realization of the sensor and the adopted signal processing strategies. The metrological characterization procedure is discussed and results are shown for both static and dynamic calibration of the film sensor. Finally, a simple application, that highlights the benefits of the sensor, is presented.
Direct current scanning field emission microscope integrated with existing scanning electron microscope
Authors: Wang Tong , Reece Charles E. , Sundelin Ronald M.
Published: August 2002

Electron field emission (FE) from broad-area metal surfaces is known to occur at much lower electric field than predicted by Fowler–Nordheim law. Although micron or submicron particles are often observed at such enhanced field emission (EFE) sites, the strength and number of emitting sites and the causes of EFE strongly depend on surface preparation and handling, and the physical mechanism of EFE remains unknown. To systematically investigate the sources of this emission, a dc scanning field emission microscope (SFEM) has been built as an extension to an existing commercial scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer for emitter characterization. In the SFEM chamber of ultrahigh vacuum (∼10−9 Torr), a sample is moved laterally in a raster pattern (2.5 μm step resolution) under a high voltage anode microtip for field emission detection and localization. The sample is then transferred under vacuum by a hermetic retractable linear transporter to the SEM chamber for individual emitter site characterization. Artificial marks on the sample surface serve as references to convert x, y coordinates of emitters in the SFEM chamber to corresponding positions in the SEM chamber with a common accuracy of ±100–200 μm in x and y. Samples designed to self-align in sample holders are used in each chamber, allowing them to retain position registration after non-in situ processing to track interesting features. No components are installed inside the SEM except the sample holder, which does not affect the routine operation of the SEM. The apparatus is a system of low cost and maintenance and significant operational flexibility. Field emission sources from planar niobium—the material used in high-field rf superconducting cavities for particle accelerator—have been studied after different surface preparations, and significantly reduced field emitter density has been achieved by refining the preparation process based on scan results. Scans on niobium samples at ∼140 MV/m are presented to demonstrate the performance of the apparatus.
Crystal deflector for highly efficient channeling extraction of a proton beam from accelerators
Authors: Biryukov V. M. , Chesnokov Yu. A. , Guidi V. , Kotov V. I. , Malagù C. , Martinelli G. , Scandale W. , Stefancich M. , Vincenzi D.
Published: August 2002

The design and manufacturing details of a new crystal deflector for proton beams are reported. The technique allows one to manufacture a very short deflector along the beam direction (2 mm). Thanks to that, multiple encounters of circulating particles with the crystal are possible with a reduced probability of multiple scattering and nuclear interactions per encounter. Thus, drastic increase in efficiency for particle extraction out of the accelerator was attained (85%) on a 70 GeV proton beam. We show the characteristics of the crystal deflector and the technology behind it.
Triggered alternans in an ionic model of ischemic cardiac ventricular muscle
Authors: Arce Humberto , López Alejandro , Guevara Michael R.
Published: August 2002

It has been known for several decades that electrical alternans occurs during myocardial ischemia in both clinical and experimental work. There are a few reports showing that this alternans can be triggered into existence by a premature ventricular contraction. Detriggering of alternans by a premature ventricular contraction, as well as pause-induced triggering and detriggering, have also been reported. We conduct a search for triggered alternans in an ionic model of ischemic ventricular muscle in which alternans has been described recently: a one-dimensional cable of length 3 cm, containing a central ischemic zone 1 cm long, with 1 cm segments of normal (i.e., nonischemic) tissue at each end. We use a modified form of the Luo–Rudy [Circ. Res. 68, 1501–1526 (1991)] ionic model to represent the ventricular tissue, modeling the effect of ischemia by raising the external potassium ion concentration ([K+]o) in the central ischemic zone. As [K+]o is increased at a fixed pacing cycle length of 400 ms, there is first a transition from 1:1 rhythm to alternans or 2:2 rhythm, and then a transition from 2:2 rhythm to 2:1 block. There is a range of [K+]o over which there is coexistence of 1:1 and 2:2 rhythms, so that dropping a stimulus from the periodic drive train during 1:1 rhythm can result in the conversion of 1:1 to 2:2 rhythm. Within the bistable range, the reverse transition from 2:2 to 1:1 rhythm can be produced by injection of a well-timed extrastimulus. Using a stimulation protocol involving delivery of pre- and post-mature stimuli, we derive a one-dimensional map that captures the salient features of the results of the cable simulations, i.e., the {1:1→2:2→2:1} transitions with {1:1↔2:2} bistability. This map uses a new index of the global activity in the cable, the normalized voltage integral. Finally, we put forth a simple piecewise linear map that replicates the {1:1↔2:2} bistability observed in the cable simulations and in the normalized voltage integral map.
Ab initio dipole moment functions of H232S and intensity anomalies in rovibrational spectra
Authors: Cours Thibaud , Rosmus Pavel , Tyuterev Vladimir G.
Published: August 2002

Intensities of fundamental and of two-quanta overtone and combination rovibrational bands of H2S have been evaluated from electric dipole moment functions calculated using ab initio coupled-clusters methods. Particular features and derivatives of the dipole moment are discussed in relation with observed intensity anomalies in infrared spectra of this molecule. Rovibrational energy levels and corresponding eigenfunctions have been obtained variationally using an empirical potential energy function. Calculated absolute line intensities are compared with available experimental data. The generated electric dipole moment functions have allowed to considerably improve upon previous ab initio intensity calculations for fundamental bands, and to describe for the first time major observed anomalies in intensity distributions of H2S rovibrational bands. Discrepancies between ab initio and empirical values of the integrated band intensities are 12%, 8%, and 10% for the first triad bands ν1, ν3, 2ν2 and 4% and 6% for the strongest bands ν2+ν3 and ν1+ν2.
Turbulence simulations of transport barriers with toroidal velocity
Authors: Garbet X. , Sarazin Y. , Ghendrih P. , Benkadda S. , Beyer P. , Figarella C. , Voitsekhovitch I.
Published: August 2002

The effect of a sheared toroidal velocity on a transport barrier is studied. This analysis is done by using three-dimensional global fluid simulations of electrostatic ion temperature gradient driven turbulence in tokamaks. The barrier is produced with a reversed magnetic shear. For a flat density profile, and at low collisionality, co-rotation leads to an outward motion of the barrier, whereas counter rotation leads to an inward displacement. However, the barrier displacement saturates when increasing the torque at fixed heat source. This saturation is attributed to the onset of Kelvin–Helmholtz modes. Also the central temperature is larger without external torque because the width of the transport barrier is wider. The consequence is that better confinement is obtained in absence of external torque.
Deformational injection rate measuring method
Authors: Marčič Milan
Published: August 2002

After completing the diesel engine endurance testing, we detected various traces of thermal load on the walls of combustion chambers located in the engine pistons. The engines were fitted with ω combustion chambers. The thermal load of different intensity levels occurred where the spray of fuel, fuel vapor, and air interacted with the combustion chamber wall. The uneven thermal load distribution of the combustion chamber wall results from varying injection rates in each injection nozzle hole. The most widely applied controlling methods so far for injection rate measurement, such as the Zeuch and Bosch concepts, allow measurement of only the total injection rate in multihole nozzles, without providing any indication whatsoever of the injection rate differences in individual injection nozzle holes. The new deformational measuring method described in the article allows the injection rate to be measured in each hole of the multihole nozzle. The results of the measurements using this method showed that the differences occurred in injection rates of individual injection nozzle holes. These differences may be the cause of various thermal loads on the combustion chamber walls. The criterion for injection rate is the deformation of the membrane due to an increase in the fuel quantity in the measuring space and due to the pressure waves resulting from the fuel being injected into the measuring space. The membrane deformation is measured using strain gauges, glued to the membrane and forming the Wheatstone’s bridge. We devoted special attention to the temperature compensation of the Wheatstone’s bridge and the membrane, heated up during the measurements.
Novel approach to produce polymerized hydrocarbon coatings using dielectric barrier controlled atmospheric pressure glow discharge plasma
Authors: Mishra K. K. , Khardekar R. K. , Singh Rashmi , Pant H. C.
Published: August 2002

Conventionally, low-pressure (<1 Torr) electrical discharges are used for material processing and thin-film deposition. These schemes suffer mainly due to the high cost of equipment and the complexity of operations. The atmospheric pressure glow discharge plasma is developed using a threaded styled electrode in different configurations, and these reactors are used to produce plasma polymerized coatings, required on plane substrates as self-supporting films to obtain membranes for blocking holes in cavities, and on microballoon targets, which are used as fuel containers for inertial confinement fusion, to avoid DT gas permeation. Helium gas is used as the supporting gas for formation and stabilization of atmospheric pressure glow discharge plasma reactors. Ethylene and acetylene gases are used as monomers to produce plasma polymerized hydrocarbon films. These films are characterized using scanning electron microscopy. Plasma polymerized coatings of thickness 100 nm–10 μm with a smooth surface finish (rms<100 nm) are deposited successfully. The surface finish is further improved using a postdischarge configuration. Preliminary results are very encouraging but further progress is to be made in this area. We are also planning to extend this technique for C:H coating of microballoons, which are used as fuel containers in inertial confinement fusion.
Monolithic fused silica suspension for the Virgo gravitational waves detector
Authors: Amico P. , Bosi L. , Carbone L. , Gammaitoni L. , Marchesoni F. , Punturo M. , Travasso F. , Vocca H.
Published: August 2002

Thermal noise in mirror suspension wires is the most severe limit to the low frequency sensitivity of interferometric gravitational wave detectors presently under construction. The pendulum mode mechanical quality factor of the suspension last stage significantly affects the noise level of the detector output. The monolithic design for this suspension stage, using a low dissipation material, is proposed for Virgo. High mechanical Qs and high breaking strengths have been obtained for monolithic fused silica fibers. A low dissipation and high strength bonding technique using potassium silicate bonding is proposed.
Design and performance of a frequency chirped external cavity diode laser
Authors: Repasky Kevin S. , Switzer Gregg W. , Carlsten John L.
Published: August 2002

The design of a frequency chirped external cavity diode laser (CECDL) in the Littrow configuration with an intra-cavity electro-optic crystal is presented and its tuning and noise performance is studied. The CECDL has a center wavelength of 793 nm with a 20 nm tuning range by mechanical rotation of the feedback grating about a pivot point. Rapid tuning of the CECDL is achieved electronically by voltage control of the electro-optic crystal. The electro-optic tuning response of this laser is 2.01 MHz/V and linear frequency chirps of 800 MHz ranging in duration from 3 to 337 μs are demonstrated. The maximum electro-optic tuning is set by the external cavity mode spacing of 2.4 G Hz for this laser.
Bichromator for electron density measurements
Authors: McNeill Donald H.
Published: August 2002

A bichromator system consisting of two interference filters with different bandwidths centered on the Hα line and photomultiplier detectors for measuring the time evolution of the electron density in hydrogen-containing plasmas is described. This method is based on the time variation in the width of the Stark broadened line, as a result of which the ratio of the signals in the two detector channels is a function of the plasma electron density. Various aspects of this system are discussed briefly, including background light (line and continuum), self-absorption, choice of filter bandwidths, and detection limits. The apparatus is inexpensive, easy to calibrate, and comparatively insensitive to minor changes in optical setup. It has been used for measuring the electron density in laser spark plasmas in room air, hydrogen, and dry air plus hydrogen.
Reversible maps in two-degrees of freedom Hamiltonian systems
Authors: Zare K. , Tanikawa K.
Published: August 2002

It has been shown that a sub-class of two-degrees of freedom Hamiltonian systems possesses a reversing symmetry discovered by Birkhoff in the restricted problem of three bodies. This mixed space–time reversing symmetry, which is different from the classical time reversal symmetry, can be shared by time-reversible as well as time-irreversible systems. Examples of time-irreversible systems which possess this reversing symmetry are the restricted problem of three bodies as shown by Birkhoff in 1915, and a special case of the motion of a rigid body with a fixed point discussed in this paper. If a Hamiltonian system possesses this Birkhoff reversing symmetry, then there exists a surface of section for which the corresponding Poincaré map is Birkhoff-reversible. The Birkhoff-reversibility of this map may be used to study its global dynamics such as the locations and the distribution of the stable and unstable periodic points, the distribution of stable and chaotic regions, and the identification of the scattering regions.
Polymer depletion interaction between a colloid particle and a wall: A Monte Carlo study
Authors: Milchev Andrey , Bhattacharya Aniket
Published: August 2002

An off-lattice bead–spring model of a polymer solution in a container with impenetrable walls is used to study the depletion interaction of a colloid particle with the planar wall by means of a Monte Carlo simulation. As expected, this interaction is found to depend essentially on the ratio ρ=R/Rg of the particle radius R to the mean radius of gyration Rg of the polymer chains in the case of dilute and semidilute solutions. For large particle to polymer size ratio ρ>1 this effective force is attractive and decreases steadily with growing distance D of the colloid from the wall. It is found to scale linearly with ρ in agreement with recent theoretical predictions. In the opposite case of ρ<1 the depletion force is found to change nonmonotonically with D and go through a maximum at a particular distance Dmax⩽Rg. In both cases, however, local variations of the polymer density profile, which we detect at higher polymer concentrations, are found to influence the depletion force and even to change it locally from attraction to repulsion. The monomer density distribution far away from/or around the colloid in the vicinity of the wall is also investigated and related to the observed behavior of the depletion force.
Electronic structure of poly(tri-heterocycle) based on furan and thiophene: The role of the syn links
Authors: Doretto R. L. , Laks B.
Published: August 2002

The electronic structures of poly(OSO) oligomers were theoretically analyzed. Models based on neutral and charged oligomers whose all rings (3<n<15) are linked to have syn or anti chains were considered. Geometrical optimizations were carried out at the semiempirical level using the parametric method 3 (PM3). The results indicated that the chain of the syn oligomer have a small curvature. For singly oxidized oligomers, the positive charge is concentrated around furan rings with a quinoid bond alternation pattern that extends at most six rings. For doubly oxidized oligomers, we also found a quinoid structure which extends up to eight rings. The energy of the electronic transitions and their associated oscillator strength values were also calculated for neutral and charged oligomers, so the UV–vis absorption spectra is presented. The calculations were done using the intermediate neglect of differential overlap Hamiltonian in combination with the single configuration–interaction technique in order to include correlations effects. The results indicated that the difference between the lowest unoccupied molecular orbital and highest occupied molecular orbital energies of the neutral oligomers are weakly correlated to the kind of their chains, although the different intensities of the lowest energy absorption peaks can be related to rings linked at the syn form.
Effective procedure for energy optimizing antisymmetrized geminal power states
Authors: Weiner B. , Ortiz J. V.
Published: August 2002

A procedure for energy optimizing an antisymmetrized geminal power state, which contains a description of correlation effects in an unbiased fashion, is presented. This procedure overcomes difficulties and shortcomings of past optimization procedures for antisymmetrized geminal power states. It is shown that the computational cost scales as r5, where r is the size of the spin orbital atomic basis set, which is superior to the scaling costs of multiexcited complete active space self-consistent field calculations, but unlike such calculations their is no preselection of configurations or excitation level. The variational parameters are the geminal coefficients with respect to a fixed atomic orbital basis, which allows one to classify these variables into chemically significant and non significant ones according to their magnitudes and thus reduce size of calculations.
Subsurface segregation of yttria in yttria stabilized zirconia
Authors: de Ridder M. , van Welzenis R. G. , van der Gon A. W. Denier , Brongersma H. H. , Wulff S. , Chu W.-F. , Weppner W.
Published: August 2002

The segregation behavior in 3 and 10 mol % polycrystalline yttria stabilized zirconia (YSZ), calcined at temperatures ranging from 300 to 1600 °C, is characterized using low-energy ion scattering (LEIS). In order to be able to separate the Y and Zr LEIS signals, YSZ samples have been prepared using isotopically enriched Z94rO0 instead of natural zirconia. The samples are made via a special precipitation method at a low temperature. The segregation to the outermost surface layer is dominated by impurities. The increased impurity levels are restricted to this first layer, which underlines the importance of the use of LEIS for this study. For temperatures of 1000 °C and higher, the oxides of the impurities Na, Si, and Ca even cover the surface completely. The performance of a device like the solid oxide fuel cell which has an YSZ electrolyte and a working temperature around 1000 °C, will, therefore, be strongly hampered by these impurities. The reduction of impurities, to prevent accumulation at the surface, will only be effective if the total impurity bulk concentration can be reduced below the 10 ppm level. Due to the presence of the impurities, yttria cannot accumulate in the outermost layer. It does so, in contrast to the general belief, in the subsurface layer and to much higher concentrations than the values reported previously. The difference in the interfacial free energies of Y2O3 and ZrO2 is determined to be −21±3 kJ/mol.
Magnetized foils as π flippers in neutron spin-echo spectrometry
Authors: Rekveldt M. Theo , Bouwman Wim G. , Kraan Wicher H.
Published: August 2002

In neutron spin-echo spectrometry Larmor precession regions are used, in which the homogeneity of the field line integrals along the neutron paths play an essential role. In the application of this technique for small-angle scattering, the transmission angle encoding happens by strong precession gradients in a certain direction in a magnetic field. Here the use of magnetized foils as π flippers will be discussed. They appear to compensate very efficiently for field line inhomogeneities and moreover they create an effectively strong gradient in Larmor precession, needed for the small-angle scattering application. The π flippers consist of magnetized foils of the proper thickness, magnetized by the field of the precession devices itself. Application of such π flippers in a spin-echo small angle neutron scattering setup with fields perpendicular and parallel to the beam direction is discussed. It appears that such flippers avoid corrections for the inhomogeneity of the field line integrals for the main part in both options.
Wave dissipation by electron Landau damping in low aspect ratio tokamaks with elliptic magnetic surfaces
Authors: Grishanov N. I. , Ludwig G. O. , de Azevedo C. A. , Neto J. P.
Published: August 2002

Longitudinal dielectric permittivity elements are derived for radio-frequency waves in an axisymmetric tokamak with elliptic magnetic surfaces, for arbitrary elongation and inverse aspect ratio. A collisionless plasma model is considered. Drift-kinetic equation is solved separately for untrapped (passing or circulating) and three groups of the trapped particles as a boundary-value problem. Bounce resonances are taken into account. A coordinate system with the “straight” magnetic field lines is used. Permittivity elements, evaluated in the paper, are suitable to estimate the wave dissipation by electron Landau damping (e.g., during the plasma heating and current drive generation) in the frequency range of Alfvén, fast magnetosonic, and lower hybrid waves, for both the large and low aspect ratio tokamaks. The dissipated wave power is expressed by the summation of terms including the imaginary parts of both the diagonal and nondiagonal elements of the longitudinal permittivity.
Absorption spectra of mixed two-dimensional cyanine aggregates on silver halide substrates
Authors: Bakalis Lisette D. , Rubtsov Igor , Knoester Jasper
Published: August 2002

Using the coherent potential approximation (CPA), we study the absorption spectra of two-dimensional molecular aggregates formed from binary random molecular mixtures. In addition to the substitutional randomness, we include Gaussian randomness in the transition frequencies within each of the two classes of molecules. The latter is motivated by the considerable disorder that is typical for two-dimensional aggregates. By comparing to exact diagonalization results for small clusters, we show that the CPA gives an excellent description of the spectra for this kind of disorder, both in the cases of amalgamation and persistence type mixing. Taking into account long-range excitation transfer interactions mediated by the extended molecular transition dipoles, we analyze experimental spectra of amalgamation-type mixed cyanine aggregates adsorbed on AgBr {100} surfaces that are also reported here. We find good agreement between theory and experiment for the position, the width, and the general shape of the absorption line as a function of the mixing ratio. The analysis also allows us to estimate the slip angle characterizing the structure of these aggregates.
Platinum contamination issues in ferroelectric memories
Authors: Boubekeur H. , Mikolajick T. , Pamler W. , Höpfner J. , Frey L. , Ryssel H.
Published: August 2002

The contamination risk of processing with platinum electrodes on device performance in ferroelectric memories is assessed in this work. Details of platinum diffusion to the active regions at annealing temperatures of 800 °C are investigated by secondary ion mass spectroscopy, deep level transient spectroscopy, and Rutherford backscattering spectrometry techniques. Cross sectional transmission electron microscopy and local elemental analysis by energy dispersive x-ray spectroscopy were used to examine the precipitation of Pt in defect free silicon as an eventual cause of gate oxide degradation. The impact of platinum contamination on device performance is evaluated under the typical ferroelectric memory processing conditions. Results from leakage current and charge to breakdown measurements of intentionally contaminated diode and metal–oxide–semiconductor (MOS) structures, respectively, are presented. The results show that the degradation depends strongly on device design and configuration. A phosphorus doped polysilicon plug, which has the function of connecting the select transistor to the capacitor module, provides effective gettering regions and prevents the diffusion of Pt atoms to the active regions. Under typical processing conditions, no evident Pt precipitates were observed and up to a concentration level of 4×1014 atoms/cm2, the leakage current of intentionally contaminated diodes does not increase, if the contamination occurs after front-end phosphorus doped poly-Si processing. Results from constant current charge to breakdown show a small number of breakdown events due to redeposition of Pt at the periphery of the MOS structure. The risk of processing with Pt electrodes in ferroelectric memories requires great care. Precautions like sealing the back surface and incorporating phosphorus doped polysilicon as the plug material are necessary to avoid the detrimental effects of Pt.
Collision processes of CHy and CHy+ hydrocarbons with plasma electrons and protons
Authors: Janev R. K. , Reiter D.
Published: August 2002

The critical fusion reactor design issue of tritium codeposition in tokamaks with carbon as wall material is closely linked with the plasma chemistry involving hydrocarbons. A complete set of cross sections for all important electron- and proton-impact processes with CHy (y=1–4) hydrocarbon impurities and their ions CHy+ is presented. The cross sections are derived on the basis of most recent experimental information and well established cross section scaling relationships. The cross sections are presented in closed analytic forms convenient for implementation in plasma simulation codes.
Optical constants and thickness determination of very thin amorphous semiconductor films
Authors: Chambouleyron I. , Ventura S. D. , Birgin E. G. , Martínez J. M.
Published: August 2002

This contribution addresses the relevant question of retrieving, from transmittance data, the optical constants, and thickness of very thin semiconductor and dielectric films. The retrieval process looks for a thickness that, subject to the physical input of the problem, minimizes the difference between the measured and the theoretical spectra. This is a highly underdetermined problem but, the use of approximate—though simple—functional dependencies of the index of refraction and of the absorption coefficient on photon energy, used as an a priori information, allows surmounting the ill posedness of the problem. The method is illustrated with the analysis of transmittance data of very thin amorphous silicon films. The method enables retrieval of physically meaningful solutions for films as thin as 300 Å. The estimated parameters agree well with known data or with optical parameters measured by independent methods. The limitations of the adopted model and the shortcomings of the optimization algorithm are presented and discussed.
Analysis of negative differential conductance in a two-island Coulomb blockade system by a polytope approximation in phase space
Authors: Evans Gareth J. , Mizuta H.
Published: August 2002

A two-island single-electron tunneling system is presented that exhibits negative differential conductance (NDC) based on Coulomb blockade. The NDC mechanism is explained by introducing a simple analysis method, the polytope approximation. A condition for NDC to occur is analyzed fully by using the polytope approximation.
Electrical resistivity and Hall coefficient of C49, C40, and C54 TiSi2 thin-film phases
Authors: Mammoliti F. , Grimaldi M. G. , La Via F.
Published: August 2002

The electrical resistivity and Hall coefficient of C49, C40, and C54 TiSi2 thin films were studied in the temperature range of 4–350 K. The residual resistivity was the highest (52–72 μΩ cm) for the C49 phase, lower for the C40 (22–33 μΩ cm), and the lowest for the C54 phase (2.8–3.8 μΩ cm). On the other hand, the Debye temperature decreases from the C54 phase (470–490 K), to the C49 (451 K), and finally to the C40 phase (420 K). The Hall coefficient of the C49 and C40 phases exhibits a temperature independent value. A change of sign for the Hall coefficient occurs in the case of C54 at a temperature of about 80 K indicating that a multicarrier conduction mechanism exists in C54 TiSi2. The value of the scattering length le for the C54 samples is two to three times larger than for the C49 and C40 samples. This difference in the scattering length is suggested to be due to the presence of intrinsic defects in the C49 and C40 phases and/or to the large amount of grain boundaries.
Current sheet formation in three-dimensional magnetic configurations
Authors: Bulanov S. V. , Echkina E. Yu. , Inovenkov I. N. , Pegoraro F.
Published: August 2002

The formation of electric current sheets in a high-conductivity three-dimensional plasma configuration is studied in the neighborhood of the critical points of the magnetic field. First, with the help of a Clebsch variable representation, the propagation of magnetohydrodynamic (MHD) waves in azimuthally symmetric current-free configurations is investigated. Exact solutions of the MHD equations that describe the self-similar evolution of a magnetic configuration near a critical point are presented. Then, the nonlinear plasma evolution is simulated with an MHD code. The excitation of an electric current at the boundary of the computational region results in a self-consistent plasma flow and in the change of the structure of the magnetic field. No electric current sheet is formed when both the equilibrium configuration and the perturbations are azimuthally symmetric.
The structure of frozen phases in slit nanopores: A grand canonical Monte Carlo study
Authors: Ayappa K. G. , Ghatak Chandana
Published: August 2002

Freezing of soft spheres in slit nanopores is investigated using Grand canonical Monte Carlo simulations. The pores are in equilibrium with a liquid located close to the liquid–solid coexistence region in the bulk Lennard-Jones phase diagram. In addition to layering, the confined fluid is found to possess in-plane order, leading to the formation of frozen phases which give rise to a sequence of solid–solid transformations as the pore width is varied. Transformations between n layered triangular to n+1 layered square lattices and between n layered square to triangular lattices, are observed for n=1, 2, 3, and 4. The transition from triangular to square lattices occurs via an intermediate buckled phase which is characterized by increased out-of-plane motion, while maintaining in-plane triangular order. Buckling was found to decrease with increasing number of layers. The transition between square to triangular lattices at a fixed number of layers is accompanied by a lowering of the solvation force, resulting in a doublet in the solvation force maxima. Influence of fluid–wall interactions on the nature of the frozen phases are studied by comparing the structures formed with a 10-4-3 and 10-4 fluid–wall potential. The solid structures are classified based on their closest 3D counterparts.
Monte Carlo studies of adsorption of a sequenced polyelectrolyte to patterned surfaces
Authors: McNamara J. , Kong C. Y. , Muthukumar M.
Published: August 2002

The adsorption of negatively charged polyelectrolyte and a polyampholyte to patterned surfaces has been investigated by using off-lattice Monte Carlo simulations. The surface is decorated by stripe and checkerboard patterns of mixed charges. The polymer has periodic charge segments which can potentially match the periodicity of the surface pattern. Our results show that the chain entropy of a flexible polymer disrupts and prevents full pattern recognition between the charge sequence on the polymer and the surface pattern. Quantities such as average adsorption energy and radii of gyration of the adsorbed polymer are dictated by the size of the surface pattern and the correlation of the polymer charge density
Light-induced change of charge transfer band in nanocrystalline Y2O3:Eu3+
Authors: Song Hongwei , Chen Baojiu , Peng Hongshang , Zhang Jisen
Published: August 2002

In this letter, we report the observation of ultraviolet light-induced change of excited charge-transfer band in nanocrystalline Y2O3:Eu3+, which is attributed to the rearrangements of local environments surrounding the Eu3+ ions in the near surface of nanoparticles.
Measurements of helicon wave propagation and Ar II emission
Authors: Scharer J. , Degeling A. , Borg G. , Boswell R.
Published: August 2002

Wave magnetic field, optical and Langmuir probe measurements are carried out to examine fast and thermal electron contributions to plasma ionization in a helicon plasma source. For the optical measurements, an Ar II line (443 nm) with a particularly short lifetime (7 ns) is chosen to resolve the excitation rate within a radio frequency period (73.7 ns). Information is then obtained regarding the acceleration processes and their action on the electron distribution that caused the excitation. Spatio-temporal measurements of 443 nm peak emission show that the emission is modulated at the source frequency. The peak count phase of the modulation propagates along the plasma at a comparable speed as the local helicon wave phase velocity. Computer modeling utilizing lab data is carried out to examine wave field effects on electron acceleration and ionization contributions arising from non-Maxwellian fast electrons.
Crystal structure, molecular dynamics, and polar properties of pyridinium fluorochromate
Authors: Paja̢k Z. , Małuszyńska H. , Szafrańska B. , Czarnecki P.
Published: August 2002

The crystal and molecular structure of pyridinium fluorochromate has been determined at 293, 240, and 150 K by x-ray diffraction. All three phases are orthorhombic with the following sequence of space groups and solid–solid phase transitions disclosed also by differential thermal analysis and differential scanning calorimetry, Cmcm→ lim 258 KCmc21→ lim 224 KPbca. Only the low temperature phase is well ordered while the two high temperature phases reveal molecular disorder of both pyridinium and fluorochromate ions. H1 and F19 nuclear magnetic resonance study has been performed as a function of temperature to describe molecular dynamics of both ions. The dielectric and pyroelectric measurements together with these results allow us to characterize the low temperature phase III as antiferroelectric, the mesophase II as ferroelectric, with the Curie point at 258 K, and the high temperature phase I as paraelectric.
Spatiotemporal control of cardiac alternans
Authors: Echebarria Blas , Karma Alain
Published: August 2002

Electrical alternans are believed to be linked to the onset of life-threatening ventricular arrhythmias and sudden cardiac death. Recent studies have shown that alternans can be suppressed temporally by dynamic feedback control of the pacing interval. Here we investigate theoretically whether control can suppress alternans both temporally and spatially in homogeneous tissue paced at a single site. We first carry out ionic model simulations in a one-dimensional cable geometry which show that control is only effective up to a maximum cable length that decreases sharply away from the alternans bifurcation point. We then explain this finding by a linear stability analysis of an amplitude equation that describes the spatiotemporal evolution of alternans. This analysis reveals that control failure above a critical cable length is caused by the formation of standing wave patterns of alternans that are eigenfunctions of a forced Helmholtz equation, and therefore remarkably analogous to sound harmonics in an open pipe. We discuss the implications of these results for using control to suppress alternans in the human ventricles as well as to probe fundamental aspects of alternans morphogenesis.
Direct measurement of Urbach tail and gap state absorption in CuGaSe2 thin films by photothermal deflection spectroscopy and the constant photocurrent method
Authors: Meeder A. , Marrón D. Fuertes , Rumberg A. , Lux-Steiner M. Ch. , Chu V. , Conde J. P.
Published: August 2002

The applicability of photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) to chemical vapor deposited and physical vapor deposited CuGaSe2 (CGSe) thin films is investigated. Process dependent variations in direct band-gap energies, Urbach tail widths, bulk and surface defect densities of the films are given as a function of composition. Differences in the PDS and CPM absorption spectra of more than one order of magnitude are found. Using the standard a-Si theory of PDS and CPM analysis, a two-layer system consisting of a defect-rich near-surface layer and a less defective bulk layer is found in device-quality films. Optical improvements due to the Ga+Se annealing step as a second stage of the growth process result in a reduced bulk defect density while the near-surface defect density remains unchanged.
Simulation of time evolution of clocked and nonclocked quantum cellular automaton circuits
Authors: Bonci L. , Gattobigio M. , Iannaccone G. , Macucci M.
Published: August 2002

We have studied, by means of a Monte Carlo simulation, data propagation in quantum cellular automaton circuits, comparing the performance of nonclocked and clocked implementations. Based on two choices of fabrication parameters, we have evaluated the maximum achievable clock speed for different operating temperatures, concluding that better performance can be obtained with the clocked architecture, which, however, involves significant technological difficulties. The large discrepancy existing between a simple estimate of the cell switching time, based on the RC time constant, and our results is discussed and explained by means of an intuitive argument. Two different circuits have been investigated, a binary wire and a majority voting gate, obtaining analogous results, with a slightly smaller maximum clock rate for the majority voting gate, due to the larger number of cells involved.
Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies
Authors: Peng D. L. , Hihara T. , Sumiyama K. , Morikawa H.
Published: August 2002

We systematically studied structural and magnetic characteristics of size- monodispersed Fe and oxide-coated Fe cluster assemblies with the mean cluster sizes of 7–16 nm. Transmission electron microscopy and scanning electron microscopy (SEM) observations show that the Fe clusters in the assemblies maintain their original size at room temperature. In the SEM images, a random stacking of the Fe clusters and a porous structure with a low cluster packing fraction of about 25% are observed. For the Fe cluster assemblies, magnetic coercivity (Hc) at room temperature increases from 4×101 to 4×102 Oe by increasing the mean cluster size from 7.3 to 16.3 nm. Using the experimental values of the coercivity at T⩾100 K and the fitting values of blocking temperature TB from Hc=Hc0[1−(T/TB)1/2], we estimated the values of magnetic anisotropy constant K of the order of 106 erg/cm3 from TB=KV/25kB, which is larger by an order of magnitude than the bulk Fe value (5×105 erg/cm3). Such a large effective anisotropy at T⩾100 K is ascribed to the large surface anisotropy effects of the small clusters and the low cluster-packing fraction of the Fe cluster assemblies. For the oxide-coated Fe cluster samples, the coercivity strongly depends on the oxygen gas flow rate during deposition, cluster size, and temperature. In the case of a high oxygen gas flow rate (namely high surface-oxidized clusters), the ferrimagnetic oxide shell crystallites also affect the coercivity at T>50 K: The hysteresis loop shift disappears, leading to a complex change in the coercivity and an enhancement of the effective anisotropy constant.
Influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN quantum wells
Authors: Kuroda Takamasa , Tackeuchi Atsushi
Published: August 2002

We studied the influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN multiple quantum wells (MQWs) mainly in relation to a quantum-confined Stark effect. We performed a systematic time-resolved photoluminescence measurement of MQWs for various carrier densities and three different well widths (2.5, 4.0, and 5.5 nm). We show that the energy shift and the change in carrier lifetime are explained well by the free carrier screening effect which compensates for the internal electric field.
Propagation through heterogeneous substrates in simple excitable media models
Authors: Bub Gil , Shrier Alvin
Published: August 2002

The interaction of waves and obstacles is simulated by adding heterogeneities to a FitzHugh–Nagumo model and a cellular automata model. The cellular automata model is formulated to account for heterogeneities by modelling the interaction between current sources and current sinks. In both models, wave fronts propagate if the size of the heterogeneities is small, and block if the size of the heterogeneities is large. For intermediate values, wave fronts break up into numerous spiral waves. The theoretical models give insights concerning spiral wave formation in heterogeneous excitable media.
Instabilities and spatio-temporal chaos of long-wave hexagon patterns in rotating Marangoni convection
Authors: Mancho Ana Marı́a , Riecke Hermann , Sain Fil
Published: August 2002

We consider surface-tension driven convection in a rotating fluid layer. For nearly insulating boundary conditions we derive a long-wave equation for the convection planform. Using a Galerkin method and direct numerical simulations we study the stability of the steady hexagonal patterns with respect to general side band instabilities. In the presence of rotation, steady and oscillatory instabilities are identified. One of them leads to stable, homogeneously oscillating hexagons. For sufficiently large rotation rates the stability balloon closes, rendering all steady hexagons unstable and leading to spatio-temporal chaos.
Double mode condensates of a flowing plasma as possible relaxed states
Authors: Steinhauer Loren C.
Published: August 2002

Stationary energy (SE) states are found for a flowing two-fluid plasma. These are of interest because quiescent relaxed plasma equilibria, if they exist, should be a subset of the SE states. The platform for the analysis is a two-fluid flowing plasma, which is more realistic than the single-fluid model used in most treatments of relaxed plasmas. The two-fluid model is characterized by two helicities, one for each species. Including flow allows for the possibility that flow may be an important ingredient in relaxed states. The analysis expands the flow and field vectors in a complete basis set of divergence-free vectors. This reduces the problem to algebra. It leads at once to the prediction of SE states that are a two-point spectrum of the basis set, i.e., double-mode condensates. The properties of these SE states are shown to depend on their location in helicity space, a two-dimensional parameter space of the ion and electron helicities. The migration of a SE state as a result of resistive and viscous dissipation is also shown.
Time-varying response of molecular electron devices: A fundamental requirement for organic nanoelectronics
Authors: Woo Raymond , Pati Ranjit , Karna Shashi P.
Published: August 2002

A simple formalism within the framework of the time-dependent Green’s function theory is developed to calculate the time-varying response of molecular devices under small-signal conditions. Results are obtained for a model molecular wire consisting of benzene-1,4-dithiol sandwiched between two Au electrodes. From the analysis of the results, it is shown that, within the conceivable frequency range that would be used in a molecular electronic system, none of the existing molecular electronic device architectures is capable of exhibiting a measurable dynamic response. An energy condition is established that must be met for a molecule to exhibit a measurable dynamic response.
Electron emission from deep level defects EL2 and EL6 in semi-insulating GaAs observed by positron drift velocity transient measurements
Authors: Tsia J. M. , Ling C. C. , Beling C. D. , Fung S.
Published: August 2002

A ±100 V square wave applied to a Au/semi-insulating SI–GaAs interface was used to bring about electron emission from and capture into deep level defects in the region adjacent to the interface. The electric field transient resulting from deep level emission was studied by monitoring the positron drift velocity in the region. A deep level transient spectrum was obtained by computing the trap emission rate as a function of temperature and two peaks corresponding to EL2 (Ea=0.81±0.15 eV) and EL6 (Ea=0.30±0.12 eV) have been identified.