bluuebunny/crossref_metadata_2025 huggingface dataset:
Accurate relativistic many-body calculations of van der Waals coefficients C8 and C10 for alkali-metal dimers
Authors: Porsev Sergey G. , Derevianko Andrei
Published: June 2003

We consider long-range interactions between two alkali-metal atoms in their respective ground states. We extend the previous relativistic many-body calculations of C6 dispersion coefficients [Phys. Rev. Lett. 82, 3589 (1999)] to higher-multipole coefficients C8 and C10. Special attention is paid to usually omitted contribution of core-excited states. We calculate this contribution within relativistic random-phase approximation and demonstrate that for heavy atoms core excitations contribute as much as 10% to the dispersion coefficients. We tabulate results for both homonuclear and heteronuclear dimers and estimate theoretical uncertainties. The estimated uncertainties for C8 coefficients range from 0.5% for Li2 to 4% for Cs2.
Fiber optic reflectometer for velocity and fraction ratio measurements in multiphase flows
Authors: Chang Kuang-An , Lim Ho-Joon , Su Chin B.
Published: June 2003

A technique based on the coherent mixing of scattered signal with Fresnel reflection signal from the tip of an optical fiber is used to demonstrate the feasibility of measuring the velocity and fraction ratio of solid particles and gas bubbles or liquid droplets in a liquid or gas flow. If the liquid or gas flow is seeded with small neutrally buoyant particles, the technique is then capable of measuring the velocity as well as the fraction ratio of all three phases of the flow at a given point. The method is briefly described as follows. An optical signal derived from a diode laser driven by a constant current is launched into a single-mode optical fiber and transmitted, through a fiber coupler, to the signal fiber inserted into the test fluid. The diode laser used is a multilongitudinal mode device that has a low coherence length of about 200 μm. The coherently mixed signal propagates back to the signal fiber, through the fiber coupler, and detected by a detector. By analyzing the signal, the velocity and fraction ratio of each phase can be obtained. Using water seeded with small solid particles and air bubbles, it is demonstrated that the technique is capable of measuring the velocity in the direction parallel to the fiber. Since the only intrusion to the fluid is the tiny fiber probe (a dimension of 125 μm in diameter), the disturbance to most fluid flows is negligible, therefore, the technique is nearly nonintrusive.
Thin tape target driver for laser ion accelerator
Authors: Nayuki Takuya , Oishi Yuji , Fujii Takashi , Nemoto Koshichi , Kayoiji Tsutomu , Okano Yasuaki , Hironaka Yoichiro , Nakamura Kazutaka G. , Kondo Ken-ichi , Ueda Ken-ichi
Published: June 2003

A thin tape target driver for laser ion acceleration was developed. The driver can move a copper tape of 5 μm thickness with a positioning reproducibility of less than 30 μm (peak to valley), which is sufficient for a laser irradiation target. Using this tape target and laser pulses of energy 350 mJ and duration 60 fs, protons of energies of over 1 MeV were accelerated in the forward direction.
Feasibility of a synchrotron storage ring for neutral polar molecules
Authors: Nishimura Hiroshi , Lambertson Glen , Kalnins Juris G. , Gould Harvey
Published: June 2003

Using calculations and mathematical modeling, we demonstrate the feasibility of constructing a synchrotron storage ring for neutral polar molecules. The lattice is a racetrack type 3.6 m in circumference consisting of two 180° arcs, six bunchers, and two long straight sections. Each straight section contains two triplet focusing lenses and space for beam injection and experiments. The design also includes a matched injector and a linear decelerator. Up to 60 bunches can be loaded and simultaneously stored in the ring. The molecules are injected at 90 m/s but the velocity of the circulating beam can be decelerated to 60 m/s after injection. The modeling uses deuterated ammonia (14N2H3) molecules in a weak-field seeking state. A beam that survives 400 turns (15 s), has horizontal and vertical acceptances of 35 and 70 mm mrad, respectively, and an energy acceptance of ±2%.
In situ fluorine-modified organosilicate glass prepared by plasma enhanced chemical vapor deposition
Authors: JangJean Shiuh-Ko , Wang Ying-Lang , Liu Chuan-Pu , Hwang Weng-Sing , Tseng Wei-Tsu , Liu Chi-Wen
Published: June 2003

In situ fluorine-modified organosilicate glass (FOSG) thin films and OSG thin films were deposited at various temperatures (250–400 °C) by the plasma enhanced chemical vapor deposition method. The fluorine content in the FOSG films was adjusted by varying the SiF4/trimethylsilane gas flow ratio from 0.5 to 2. Film characteristics were investigated by examining the bonding configuration, index of refraction, surface composition, hardness, leakage current density, and breakdown field strength. The absorbance spectrum of Fourier transform infrared spectroscopy shows that the frequency of the Si–O stretching vibration mode in the FOSG films is shifted to a higher wave number (blueshift) with the increase of fluorine incorporation, while that at higher temperatures is shifted to a lower wave number (redshift). Meanwhile, the refractive index increases and decreases slightly with increasing fluorine content and deposition temperature, respectively. The dielectric constant of the FOSG is slightly lower than that of the OSG films. The FOSG films with higher fluorine content also exhibit higher mechanical hardness and higher dielectric breakdown voltage. The higher mechanical strength of the FOSG films relative to that of the OSG films is achieved as a consequence of the structural change accompanied by the incorporation of fluorine.
Nanolithography using molecular optics
Authors: Gordon Robert J. , Zhu Langchi , Schroeder W. Andreas , Seideman Tamar
Published: June 2003

We explore the possibility of using an intense laser beam to focus a molecular beam onto a surface to create nanowires. We show that with a grazing angle of incidence between the laser and molecular beams, it is possible to use available technology to create wires <50 nm wide and >100 μm long with a 100 W continuous wave laser. Narrower and longer features could be created with higher power lasers. This technique is very general, and may be used to deposit any atom or molecule onto an arbitrary substrate, so long as the particles may be entrained in a molecular beam and have an adequate sticking probability. The effects of spherical and chromatic aberration and laser mode structure on the focusing properties of the molecular lens are examined in detail, and design criteria for building a practical device are discussed.
Radical dynamics in unstable CF4 inductive discharges
Authors: Chabert P. , Abada H. , Booth J.-P. , Lieberman M. A.
Published: June 2003

We have observed instabilities in low-pressure planar inductive discharges in CF4 gas, in many ways similar to those seen in other attaching gases. The plasma emitted light exhibits oscillations, with the intensity modulated from 15% to 100%. The instability regions were investigated as a function of pressure, CF4 flow rate and radio-frequency power. Two matching configurations were used. When the terminus of the planar spiral antenna was directly connected to ground, we observed low-frequency oscillations between 0.5 Hz and 15 Hz. When a 400 pF capacitance was added between the terminus and ground (i.e., the capacitive coupling was reduced), the oscillation frequencies ranged from 100 Hz to 1.5 kHz. Oscillations in the concentrations of unsaturated radicals (CF and CF2) were measured during the instability, using time-resolved laser induced fluorescence. The radical dynamics were significant at frequencies of about 1 kHz. Mechanisms are proposed for these variations, including gas temperature changes and chemical reactions. A global model originally developed for SF6 chemistry was adapted to this system. The appropriate rate coefficients for CF4 were incorporated and an expression for the positive ion flux in the low electronegativity case was developed. Gas heating effects were also included.
Cluster dynamics in the range N=2–20: High resolution infrared spectra of HeN–CO
Authors: Tang Jian , McKellar A. R. W.
Published: June 2003

Infrared spectra of HeN–CO complexes with N up to about 20 have been observed in the 2145 cm−1 region of the C–O stretch vibration using a tunable diode laser spectrometer to probe pulsed supersonic expansions from moderately high pressure (⩽40 atm) cooled (>−150 °C) jet sources. Cooler (⩽0.2 K) or warmer (⩽0.5 K) effective rotational temperatures were obtained using pinhole or slit jet nozzles, respectively. Two series of R(0) transitions were observed, each correlating smoothly with the known a-type (K=0←0) and b-type (K=1←0) R(0) lines of the binary complex, He–CO. Although the b-type series starts off about 7 times stronger for N=1, it was observed to lose intensity to the a series with increasing N-value. The numbering of cluster size was reliably established up to N=14 for the a-type and N=6 for the b-type series. Some warmer lines due to higher J-value transitions [e.g., R(1)] were observed and tentatively assigned, but these were not sufficient to enable rotational analysis. Thus it has not yet been possible to separate the effects of vibrational shifts and rotational dynamics on the line positions. Two critical regions were observed in the cluster size evolution around N=7 and 15, and these may be related to the theoretically calculated maximum and minimum, respectively, in the incremental binding energy per helium atom.
Stability of 180° domain in ferroelectric thin films
Authors: Wang Biao , Woo C. H.
Published: June 2003

Ferroelectric random access memory (FRAM) has attracted much attention in the last two decades due to its ideal properties such as nonvolatility, high speed, and low power consumption. There is a strong incentive to develop high-density FRAMs, in which the switched domains, developed under low voltage or short pulses, are necessarily very small, and are therefore usually unstable and suffer from significant backswitching upon removal of the external voltage. In this investigation, a general form of energy expression for a ferroelectric material containing 180° domains is derived, from which evolution equations of the domain are established. By choosing the change in internal energy as the Liapunov function, a general formulation is developed to determine the stability conditions of the switched domain. This is applied to the case of an ellipsoidal 180° domain and yields a criterion for the stability of switched domains. We note that our approach is generally applicable to many other fields, including phase transformation, nucleation, expansion of dislocation loops in thin films, etc.
Shape dependent small cluster kinetics in the two-dimensional Ising model beyond the classical approximations
Authors: Berim Gersh O. , Ruckenstein Eli
Published: June 2003

The kinetics of small clusters of “up” spins in the two-dimensional Ising model on a square lattice is examined without the usual approximations of fixed cluster shape, constant number of “down” spins and nonsplitting–noncoagulating dynamics. New kinetic equations for the number densities of clusters of various sizes and shapes are derived and solved numerically. It is shown that the kinetic behavior of small clusters depends on their shape and that the time-dependence of the total number of down spins and the splitting and coagulation of clusters significantly affect various characteristics of the system, e.g., the range of validity of the steady-state approximation, the transient time to that state, the values of mass fluxes, etc. The influence of these factors grows with increasing temperature and supersaturation.
Theory of superradiance from photon-assisted tunneling electrons and its application to terahertz device
Authors: Asada Masahiro
Published: June 2003

Superradiance from photon-assisted tunneling electrons is analyzed quantum mechanically, and an amplifier device utilizing this phenomenon is discussed. In this device, electrons which are transported with the photon-assisted tunneling at the input port generate the amplified output due to the superradiance at the output port. Superradiance takes place from the two groups of the beat of electron waves, and the output power is proportional to the difference between these two beats. These two beats have almost the same amplitude and phase just after the photon-assisted tunneling, and their phase difference and the output power increase as the distance between the input and output ports increases. However, since the amplitude of the beat decays due to electron scattering, the increase of output power with the input–output distance is limited. An energy filtering structure, by which either of the two beat groups can be eliminated and high gain can be obtained, is also analyzed. It is shown from the analysis that the gain up to a few terahertz is possible by optimizing the device structure and the impedances at the input and output ports.
Effects of grain boundary on impurity gettering and oxygen precipitation in polycrystalline sheet silicon
Authors: Lu Jinggang , Wagener Magnus , Rozgonyi George , Rand James , Jonczyk Ralf
Published: June 2003

The effects of grain boundaries (GB) in polycrystalline sheet silicon on impurity gettering and oxygen precipitation were investigated by electron beam induced current (EBIC), deep level transient spectroscopy (DLTS), micro-Fourier-transform infrared spectroscopy (FTIR), and preferential etching/Normaski optical microscopy techniques. Both as-grown and thermally processed wafers were studied. A correlation between GB density and transition metal concentration was quantitatively established by combining DLTS and EBIC studies. It was found that four deep levels arising from Fe–B, Fe–Al, Cr–B, and Fei were present in the as-grown sample, and their concentrations decrease with increasing GB density. GB gettering was further verified by the presence of an EBIC image contrast halo around the GB. Preferential etching also revealed a precipitate density of 2×107 cm−2 on the GB. After processing, a clearly defined oxygen precipitate denuded zone formed around the GB with the interstitial oxygen concentration [Oi] decreased from 14.4 to 2.2×1017 cm−3. Micro-FTIR showed that, for both processed and as-grown samples, more silicon oxynitride appears in the GB than in the intragrain region. Since nitrogen enhances oxygen precipitation, it is likely that nitrogen preferentially precipitated on the GB during the wafer formation process and resulted in a nitrogen depletion zone, where oxygen precipitation was further suppressed and a denuded zone formed.
Interferometric analysis of ultrashort pulse laser-induced pressure waves in water
Authors: Kim Beop-Min , Komashko Aleksey M. , Rubenchik Alexander M. , Feit Michael D. , Reidt Steffen , Da Silva Luiz B. , Eichler Jürgen
Published: June 2003

Pressure waves generated by the surface ablation of water with ultrashort laser pulses in the 140 fs–10 ps duration range were studied with a Mach–Zehnder interferometry. Formation and propagation of spherical and, in ablation with longer pulses, combination of spherical and cylindrical pressure waves were observed. Measurements of the amplitude and dynamics of the pressure waves show that the conversion efficiency of laser energy to mechanical energy of the pressure waves is less than 1%, thus justifying the claim that ultrashort laser pulse ablation induces low-collateral damage and has a large potential in biomedical applications.
Ultrathin silicon oxynitride formed by low-energy electron impact plasma nitridation and chemical oxidation methods
Authors: Takahashi Masao , Tamura Masafumi , Asuha  , Kobayashi Takuya , Kobayashi Hikaru
Published: June 2003

A formation method of ultrathin silicon oxynitride layers with high-nitrogen concentrations and good electrical characteristics has been developed. This method consists of nitridation by the use of nitrogen plasma generated by low-energy electron impact and chemical oxidation. Without annealing after the nitridation or oxidation, the metal-nitrided oxide-semiconductor diodes possess poor electrical characteristics with a high-density leakage current, a large flat band shift in the negative bias direction, and a large hysteresis in the capacitance-voltage curves. With annealing in nitrogen, on the other hand, the leakage current density is markedly decreased, the flat-band voltage shifts to nearly zero gate bias, and the hysteresis is eliminated. X-ray photoelectron spectroscopy measurements show that the electrical characteristics of the oxynitride layers containing only nitrogen atoms bound to one oxygen atom and two Si atoms each, O–N(–Si)2, as nitrogen species are excellent, while the electrical characteristics become poor when the oxynitride layers contain charged species, N–(Si)4+ or high concentrations of nitrogen atoms bound to three Si atoms, N–(Si)3 or N–[Si(–O3)]3. Oxynitride layers in which nitridation is initially performed followed by chemical oxidation by immersion in nitric acid possess almost uniform nitrogen atomic concentration, N/(O+N), of 10–12% throughout the layer and high-relative dielectric constant, εr of 5.2. This εr is higher than that expected from the nitrogen concentration, indicating that nitrogen atoms have long-range interaction. When chemical oxide layers are initially formed and then nitridation is performed, the nitrogen concentration is lower (i.e., ∼8% for the surface region) and decreases with the depth, resulting in a lower εr of 4.1.
A simple, precise, and low jitter delay/gate generator
Authors: Kalisz J. , Poniecki A. , Różyc K.
Published: June 2003

The generator of precise delay over the range of 0–650 μs is described. The delay is selected with 10 ps resolution and its jitter is below 8 ps (rms) for delays up to 10 μs. The generator was designed as a complementary metal-oxide-semiconductor programmable logic device driven by a signal generator. Three output pulses are generated: START, STOP, and GATE, all with the amplitude of 2.3 V and the switching times below 500 ps.
Ab initio study of Rg–N2 and Rg–C2 van der Waals complexes (Rg=He, Ne, Ar)
Authors: Patel Ketan , Butler Peter R. , Ellis Andrew M. , Wheeler Martyn D.
Published: June 2003

The ground-state potential energy surfaces of Rg–N2 and Rg–C2 (where Rg=He, Ne, or Ar) have been investigated at the coupled cluster singles, doubles, noniterative triples [CCSD(T)] level of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets. A basis set extrapolation procedure was employed to estimate the complete basis set limit, and the extrapolated potential energy surface was then utilized to calculate the bound intermolecular states and microwave transition frequencies of each complex. The Rg–N2 complexes were chosen to demonstrate the reliability of the extrapolation scheme, since there are abundant theoretical and experimental data already available for these complexes. The calculated binding energies and equilibrium structures of the Rg–N2 complexes compare favorably with previous semiempirical and ab initio calculations. The calculated microwave transition frequencies for Ar–N2 are in excellent agreement with experimental values (deviation <0.1% rms) whereas the equivalent Ne–N2 transitions show a greater deviation (1.3% rms). There are currently no experimental data with which to compare the binding energies and rovibrational energy levels of the Rg–C2 complexes. However, the rovibrational energy level predictions should serve as a useful guide to any future spectroscopic studies of Rg–C2 complexes.
Accurate adiabatic connection curve beyond the physical interaction strength
Authors: Magyar R. J. , Terilla W. , Burke K.
Published: June 2003

In order to better approximate and understand the exchange-correlation functional in density functional theory, the adiabatic connection curve is accurately calculated beyond the physical interaction strength using a simulated scaling method. This is done for Hooke’s atom, two interacting electrons in a harmonic well potential. Extrapolation of the accurate curve to the infinite-coupling limit agrees well with the strictly correlated electron hypothesis. The interaction strength interpolation is shown to be a good, but not perfect, fit to the adiabatic curve. Arguments about the locality of functionals and convexity of the adiabatic connection curve are examined in this regime.
Photodissociation dynamics of propene at 157.6 nm: Kinetic energy distributions and branching ratios
Authors: Lee Shih-Huang , Lee Yin-Yu , Lee Yuan T. , Yang Xueming
Published: June 2003

Photodissociation dynamics of propene at 157.6 nm has been investigated in a molecular beam apparatus using the photofragment translational spectroscopic technique combined with the vacuum ultraviolet ionization method. Eleven photofragments have been successfully detected and ascribed to eight (five binary and three triple) dissociation channels: namely, C3H5+H, C3H4+H+H, C3H4+H2, C3H3+H2+H, C2H4+CH2, C2H3+CH3, C2H2+CH4, and C2H2+CH3+H. Their branching ratios have been determined to be 1%, 7%, <0.2%, 17%, 6%, 4%, 5%, and 60%, respectively. The complicated multichannel dissociation process has a propensity towards triple dissociations, notably the C2H2+CH3+H channel. In addition, the averaged kinetic energy releases and the fractions in translational energy have also been determined from the measured kinetic energy distributions. For the binary dissociation channels, the fractions in translational energy are less than 18% except the C3H5+H channel, whereas they are more than 42% for the triple dissociation channels. An intriguing finding indicates that the C2H4+CH2 channel has a nearly identical kinetic energy distribution and a similar branching ratio to the C2H3+CH3 channel, although the former undergoes a three-center elimination process different from the C–C bond rupture occurring in the latter.
Velocity-map imaging study of the O(3P)+N2 product channel following 193 nm photolysis of N2O
Authors: Brouard M. , Clark A. P. , Vallance C. , Vasyutinskii O. S.
Published: June 2003

Velocity-map imaging has been used to characterize the velocity and angular momentum alignment distributions of the O(3PJ) products of N2O photolysis at 193 nm. The measured velocity and spatial anisotropy distributions indicate that around 60% of the available energy appears in product translation, with the remainder being released into internal excitation of the N2 cofragment. The measured O(3PJ) alignment parameters have been interpreted in terms of an instantaneous dissociation model, which suggests preferential population of ML=0 states and an electron density distribution peaking perpendicular to the direction of the breaking bond. This is consistent with dissociation on a surface of Σ− symmetry, the most likely candidate correlating with ground state products being the asymptotic Σ1 −Σ−(3A″) surface. There are several possible routes to this surface, and these are considered in light of the measured velocity distributions and velocity-dependent spatial anisotropy parameters of the O(3P) products relative to the dominant O(1D) dissociation channel.
Dissection of rovibronic band structure by polarization-resolved degenerate four-wave mixing spectroscopy
Authors: Bracamonte Alfredo E. , Vaccaro Patrick H.
Published: June 2003

Judicious selection of polarization characteristics in degenerate four-wave mixing (DFWM) spectroscopy is shown to provide a facile and robust means for discriminating rovibronic features according to their changes in rotational angular momentum, ΔJ. Building upon a perturbative (weak-field) treatment of the resonant DFWM response, theoretical analyses are presented for a collinear arrangement of linearly polarized electromagnetic waves that interact with an isotropic ensemble of gas-phase target molecules. The polarization unit vectors for two input fields (E1 and E3) are presumed to be fixed along the Y-axis (φ1=φ3=π/2), while the remaining incident field (E2) has its orientation within the transverse X–Y plane specified by angular coordinate φ2. For φ2=π/4, complete elimination of high-J Q-branch (ΔJ=0) and P-/R-branch (ΔJ=±1) structure is found to occur when the detected direction of linear polarization is switched between the limiting values of φ4,Q=−18.43° and φ4,P/R=+26.57°, respectively. These predictions are corroborated experimentally by probing the tunneling-split origin region of the tropolone Ã 1B2–X̃ 1A1 (π*←π) absorption system under ambient, bulk-gas conditions. Other polarization-based schemes for rovibronic branch suppression are discussed as are the effects incurred by (strong-field) optical saturation phenomena. The implementations of DFWM suggested by this work should prove useful for dissecting and unraveling the congested spectra often exhibited by massive polyatomic species.
Analysis of electromagnetic oscillations in dielectric cathode with grid electrodes
Authors: Hayashi Yasushi , Okino Akitoshi , Hotta Eiki
Published: June 2003

We have performed analytical studies on the radio frequency (rf) electromagnetic (EM) waves in a ferroelectric cathode with a grid electrode. During operation of the cathode, a trigger voltage induces a very strong field at the vacuum–metal–ferroelectric triple junction, which results in local saturation of the material’s dielectric property near the corresponding area. High frequency oscillations generated from the saturation then lead to the formation of standing waves. With the practical geometry and the material property used in the experiments, the oscillation frequency of the lowest mode (TM01) is found to overlap that of the domain oscillation of the ferroelectric material. This provides domains with the possibility of gaining energy from the EM wave through resonance to sustain their oscillation, which is considered to be related with electron emission. The rf pass band was studied by varying the geometry and the material properties of the cathode, and this may lead to optimization of the cathode shape.
Magnetic force images of nanomagnetic domains taken with platinum-coated tips
Authors: Teschke O. , Kleinke M. U. , Dotto M. E. R. , Soares D. M. , Knobel M. , de Souza E. F.
Published: June 2003

This article deals with magnetic force microscope images of nanosized domains in Co-coated films made by Pt-coated tips as well as micromagnetic images of data tracks written in recording media. Pt-coated tips have improved image delineation of the magnetic field distribution compared to images obtained by Co-coated hard magnetic tips. The force acting on Pt-coated tips in the magnetic field of the substrate was modeled assuming a paramagnetic tip. Due to the ferromagnetic nature of the interaction between the tip and substrate the spatial resolution of hard magnetic tips was shown to be inadequate to measure details of the features of nanosized domains. A comparison of the magnetic images made by Pt-coated tips with topographic images shows that magnetic domains resist thermal erasure at ambient temperature when they are formed of eight metallic grains.
Prediction of thermodynamic properties of krypton by Monte Carlo simulation using ab initio interaction potentials
Authors: Nasrabad Afshin Eskandari , Deiters Ulrich K.
Published: June 2003

The vapor–liquid equilibria of pure krypton were calculated by Gibbs ensemble Monte Carlo simulation using two different ab initio pair potentials. One pair potential was obtained from coupled-cluster calculations, using the CCSD(T) level of theory and two successive correlation consistent basis sets, aug-cc-pVTZ and -pVQZ. The resulting pair potentials were extrapolated to obtain the basis set limit of the interaction energies. The second ab initio potential was taken from literature. It is shown that the coupled-cluster potential leads to a quantitative prediction of second virial coefficients, vapor pressures, and orthobaric densities, if Axilrod–Teller triple-dipole potentials are included in the simulations.
Electronic and vibrational coherence effects in broadband transient absorption spectroscopy with chirped supercontinuum probing
Authors: Dobryakov Alexander L. , Kovalenko Sergey A. , Ernsting Nikolaus P.
Published: June 2003

The theory of transient absorption with chirped supercontinuum probing is developed for electronic excitation of a vibronic four-level scheme. Dephasing is modeled as homogeneous in the Bloch approximation together with inhomogeneous broadening. The time correction routine for transient spectra is shown to be identical to that of the nonresonant case which was treated previously. Contributions from sequential and coherent terms are calculated and compared to each other. The simulated data reveal how electronic and vibrational dephasing, detuning, and inhomogeneous broadening manifest themselves in the coherent component of femtosecond transient absorption spectra.
The use of molecular dynamics for the study of solution properties of guar gum
Authors: Laguna M. Teresa R. , Tarazona M. Pilar , Saiz Enrique
Published: June 2003

Size exclusion chromatography with dual detection, i.e., employing a refractive index, concentration sensitive, detector together with a multiangle light scattering detector which is sensitive to molecular size, has been applied to study the solution properties of guar gum in water with different concentrations of K2SO4 at 25 °C. The analysis of a single highly polydisperse sample is enough for obtaining calibration curves for molecular weight and radius of gyration and the scaling law coefficients. The influence of the ionic strength on the conformational properties of the polymer can also be analyzed. Moreover, unperturbed dimensions can be obtained by extrapolation of the values measured in a good solvent. The value of the characteristic ratio of the unperturbed dimensions thus obtained is Cn=〈r2〉0/nl2≈19±1. A theoretical analysis is also included. Thus, molecular dynamics procedures were employed to analyze the conformational properties of an oligomer of guar gum under different conditions; namely, standing alone in vacuo, in bulk solid state and in water solution, both with and without salt. These conformational properties were then employed to compute molecular dimensions of Monte Carlo generated chains with different lengths according to standard procedures of the matrix multiplication scheme, thus allowing the evaluation of both perturbed and unperturbed dimensions which are in very good agreement with the experimental values. Moreover our result permits the explanation of the discrepancies among experimental and theoretical values reported in the literature.
Methodology for the evaluation of yield strength and hardening behavior of metallic materials by indentation with spherical tip
Authors: Ma Dejun , Ong Chung Wo , Lu Jian , He Jiawen
Published: June 2003

This article presents a methodology for evaluating the yield strength and hardening behavior of metallic materials by spherical indentation. Two types of assumed material behaviors with a pure elastic-Hollomon’s power law hardening and a pure elastic-linear hardening were considered separately in the models of spherical indentation. The numerical relationships between the material properties and indentation responses were established on the basis of dimensional and finite element analysis. As the first approximation to the real plastic flow properties, the yield strengths and hardening behaviors determined from the spherical indentation loading curve and the numerical relationships were used to derive the intersecting points between Hollomon’s power law hardening curve and linear hardening line. Through proceeding the three parameter’s regression analysis with Swift’s power law function for the intersecting points determined at different maximum indentation depths, the final yield strength and hardening behavior of tested material can be obtained. The validation of this method was examined by investigating three groups of materials with near linear hardening behavior, near Hollomon’s power law hardening behavior, and initial yield plateau. It is concluded that the proposed method is applicable to a wide variety of materials which exhibit separate hardening behaviors.
Mean-field approach for a statistical mechanical model of proteins
Authors: Bruscolini Pierpaolo , Cecconi Fabio
Published: June 2003

We study the thermodynamical properties of a topology-based model proposed by Galzitskaya and Finkelstein for the description of protein folding. We devise and test three different mean-field approaches for the model, that simplify the treatment without spoiling the description. The validity of the model and its mean-field approximations is checked by applying them to the β-hairpin fragment of the immunoglobulin-binding protein (GB1) and making a comparison with available experimental data and simulation results. Our results indicate that this model is a rather simple and reasonably good tool for interpreting folding experimental data, provided the parameters of the model are carefully chosen. The mean-field approaches substantially recover all the relevant exact results and represent reliable alternatives to the Monte Carlo simulations.
Effects of confined longitudinal optical phonons on the exciton binding energy in a cubic quantum dot
Authors: El Amrani B. , Fliyou M. , Bensaid L. , Lamcharfi T. , Rahmani K. , Bouayad M.
Published: June 2003

The binding energy of an exciton in a cubic quantum dot has been calculated with and without including the electron (hole)-confined longitudinal optical phonon interactions, using a variational approach and within the effective mass approximation. The quantum confinement effect is described by an infinitely deep well in the envelope-function approximation. The charge carrier phonon coupling is treated within the adiabatic approximation. As the dot size increases, the results show that the contribution of the confined longitudinal optical phonon on the binding energy decreases and the correction to the excitonic state increases.
Laue transmission diffraction optics for thin film stress calculation
Authors: French B. L. , Bilello J. C.
Published: June 2003

White beam Laue transmission diffraction topography has been used to determine the curvature of a crystalline substrate, from which the stress in the overlaying coating is determined. A detailed analytical method has been developed which correlates the introduction of curvature to the substrate in two orthogonal directions with attendant changes in the dimensions of a given topographic reflection. The size of a given reflection is found to depend on the dimensions of the incident beam, the horizontal and vertical incident beam divergences, the orthogonal curvatures of the crystalline substrate, the camera length, and the indices of the selected reflection. The thermal tests of sputtered polycrystalline Ta and Cr films on Si (100) substrates are used to demonstrate the applicability of this phenomenon. Simultaneous observation of film delamination and quantification of associated stresses is shown to be possible.
Dynamics of steady and unsteady operation of inductive discharges with attaching gases
Authors: Chabert P. , Lichtenberg A. J. , Lieberman M. A. , Marakhtanov A. M.
Published: June 2003

Relaxation oscillations in charged particle densities are seen in low-pressure inductive discharges, if attaching gases such as SF6 and Ar/SF6 mixtures are used, such that the plasma contains negative ions. These oscillations occur in the neighborhood of the transition between lower power capacitive operation and higher power inductive operation. An instability, which occurs around this transition, can be described by a global model, that consists of time-dependent equations for the electrons, ions, and electron temperature. The model qualitatively agrees with experimental observations, but leaves significant quantitative differences to be explained. To understand the reasons for the model’s inability to make quantitative predictions, the local conditions for instability are determined for a model somewhat simplified from the one that approximates the experiment, but retains most of the important physics. The simplified model allows explicit determination for the local transition to instability and identifies two parameters that govern the transition. The local criteria are related to the global criterion that all fixed points are unstable. The importance for the instability of the physical parameters of the ion wall loss and of the ratio of capacitive to inductive power transfer has led to a reexamination of the size of these factors, and gives better quantitative agreement of the model to experiments.
Thermally stimulated luminescence from vapor-transport-equilibrated LiTaO3 crystals
Authors: Chirila M. M. , Garces N. Y. , Halliburton L. E. , Evans D. R. , Route R. K. , Fejer M. M.
Published: June 2003

Thermally stimulated luminescence (TSL), optical absorption, and electron paramagnetic resonance (EPR) have been used to characterize the emission of ultraviolet light from undoped LiTaO3. The crystals in this study were grown from a congruent melt and then subjected to a vapor-transport-equilibration (VTE) treatment. Two overlapping TSL peaks occur at 94 and 98 K, with each showing a 350 nm maximum in its spectral emission. These peaks are observed after a 77 K exposure of the crystals to x rays or lasers (266, 325, or 355 nm). Congruent crystals from the same boule (but not VTE treated) produced no measurable emission in similar experiments. During excitation of the VTE-treated crystals at 77 K, holes are localized on oxygen ions, either self-trapped or with a nearby stabilizer, and electrons are trapped at regular Ta5+ ions and at Fe3+ impurities (present at trace levels), where they form Ta4+ and Fe2+ ions. The hole centers and the Fe3+ ions have characteristic EPR spectra, and a broad optical absorption band peaking near 1600 nm is attributed to the Ta4+ electron center. Upon warming, the trapped hole centers formed during the initial excitation become unstable below 100 K and move to Ta4+ ions, thus causing radiative recombination in the TaO6 units.
Electrical and structural characteristics of yttrium oxide films deposited by rf-magnetron sputtering on n-Si
Authors: Evangelou E. K. , Wiemer C. , Fanciulli M. , Sethu M. , Cranton W.
Published: June 2003

Yttrium oxide dielectric films were grown by rf-magnetron sputtering on n-Si(100) substrates and annealed in vacuum at temperatures ranging from 400 to 600 °C. The main aim of this work was the investigation of the interface between the dielectric film and silicon. Both structural (x-ray diffraction and transmission electron microscopy) and electrical characterization were used for this purpose. No structural change was observed on the interfacial native oxide layer after annealing at 600 °C for 1 h. Metal–oxide–semiconductor structures defined by the evaporation of Al electrodes show low leakage currents, moderate dielectric constant (around 14), and high densities of positive charges trapped in the oxide. Hysteresis effects in capacitance–voltage (C–V) curves reduce with the annealing temperature. Another interesting observation is the parallel shift of the C–V curves along the voltage axis with frequency. An insulator trap model is proposed to explain this behavior.
Determination of crystallite size in a magnetic nanocomposite using extended x-ray absorption fine structure
Authors: Calvin S. , Miller M. M. , Goswami R. , Cheng S.-F. , Mulvaney S. P. , Whitman L. J. , Harris V. G.
Published: June 2003

An extended x-ray absorption fine structure was collected for a soft magnetic material comprising very fine nanoscale crystallites of nickel within coarser iron matrix grains. Using a simple spherical model and the spectra of bulk standards, the nickel crystallite size was estimated. Comparison with transmission electron microscopy images confirms that this technique yields a size weighted toward smaller crystallites, whereas Scherrer analysis yields sizes weighted toward larger crystallites. The iron crystallite size was also estimated by this technique in order to ascertain the effect of a nonspherical morphology. This technique shows promise for in situ analyses of materials containing nanoscale crystallites and as a complement to Scherrer analyses.
Influence of quantum effects on giant magnetoresistance for multilayer structures
Authors: Zeng X. H. , Chen H. X. , Xu X. L. , Ruda H. E. , Qiao Bi
Published: June 2003

A description of the nonlinear conductivity of mesoscopic systems is formulated and used to describe quantum effects in the giant magnetoresistance of multilayer structures, based on a subdynamics approach. Consideration of the nonlinear response makes it possible to discuss the influence of bias and temperature on giant magnetoresistance. Calculations show that for sandwich structures, the giant magnetoresistance increases when the bias increases, and decreases for negative bias. Compared with the influence of bias, the variation of giant magnetoresistance with temperature is small.
Development and applications of time-bunched and velocity-selected positron beams
Authors: Merrison J. P. , Charlton M. , Aggerholm P. , Knudsen H. , van der Werf D. P. , Clarke J. , Poulsen M. R.
Published: June 2003

We describe the development of an instrument for the production of low energy positron beams that are bunched in time, and the use of a velocity selection device. The bunching unit was constructed from forty seven separate elements, coupled in series in a capacitor chain to reduce the delay time for propagation of the applied voltage pulse along the electrode system and to facilitate operation at frequencies up to 100 kHz. A parabolic potential distribution for time focusing was used. Tests with a dc positron beam produced from a radioactive source are described, together with measurements in which the buncher was used to compress positron pulses produced from an electron accelerator-based beam. Computer simulations of particle trajectories in the buncher have been performed resulting in a detailed evaluation of the factors that govern and limit the time resolution of the instrument. A sector magnet used to velocity-select intermediate energy positrons is described and its performance discussed.
Magnetization reversal in antiferromagnetically coupled ternary layers
Authors: Wang Y. J. , Chui S. T.
Published: June 2003

The magnetization reversal in selected antiferromagnetically coupled multilayers has been studied by solving the Landau–Lifshitz–Gilbert equation. The multilayers are composed of 10 nmt1/3 nmt2/10 nmt3, where t1, t2, and t3 are soft magnetic material and are separated each other by thin nonmagnetic spacers. Our results suggest that the coupled ternary layers can possess good permanent magnet properties: A coercivity of 5–6 KOe and a hysteresis loop which can become approximately rectangular with an appropriate choice of parameters. The coercivity in such coupled layers originates from the interlayer coupling and the demagnetization field between layers.
Optimization of a Josephson voltage array based on frequency dependently damped superconductor–insulator–superconductor junctions
Authors: Hassel Juha , Seppä Heikki , Grönberg Leif , Suni Ilkka
Published: June 2003

We have developed a programmable Josephson voltage standard based on an array of superconductor–insulator–superconductor junctions. The junctions are damped by an external frequency-dependent shunt circuit, which allows operation at Shapiro steps with n>1. We derive optimization criteria for the design parameters for a fast and stable array with low power consumption. An array able to generate 1.0 VRMS ac voltage signal with sub-ppm accuracy is experimentally demonstrated. Theoretically it is also shown that a fast programmable 10 V array can be realized.
A fiber–optic interferometer for in situ measurements of plasma number density in pulsed-power applications
Authors: Smith L. M. , Keefer D. R. , Wright N. W.
Published: June 2003

A fiber–optic-based interferometer has been designed, constructed, and applied to plasma measurements in pulsed-power systems. The beam from a 1310 nm solid-state diode laser is coupled into a single-mode fiber and split into two beams, one of which was passed through an acousto-optic modulator to frequency shift the light. Both beams travel through approximately 30 m of fiber, with the probe volume consisting of a short air or vacuum gap in the probe beam fiber where the light is collimated and collected by lenses. The beams are then recombined on a photodiode, producing a time-varying sinusoidal intensity signal that is phase modulated with the presence of a plasma in the probe volume. This configuration allows for remote measurements of plasma electron number density, and is robust with respect to vibration in the plasma source and electromagnetic interference. Tests indicate that phase measurement accuracies of ±0.045 rad corresponding to number density accuracies of ±1.2×1019 m−2 at 1310 nm are achievable with this device. A description of this interferometer, including refinements needed to achieve these accuracies, is presented along with the results of tests performed on a coaxial plasma gun.
Field-ionization electron detector at low temperature of 10 mK range
Authors: Shibata M. , Tada M. , Kishimoto Y. , Kominato K. , Haseyama T. , Ogawa I. , Matsuki S. , Yamada S. , Funahashi H. , Yamamoto K.
Published: June 2003

Selective field-ionization electron detector at low temperature of 10 mK range was developed with a channel electron multiplier. The field-ionization electrode system is attached to the bottom plate of the mixing chamber of a dilution refrigerator and ionized electrons are transported to a channel electron multiplier at the 1 K temperature pumping stage through a series of ring focusing electrodes. The channel electron multiplier is heated up to more than 20 K with a heating coil to maintain its long-term operational conditions, while keeping its environment temperature to 1 K. Rydberg states in Rb with 80≲n≲150 were successfully field ionized and detected with this system at the long-term operating temperature of 12–15 mK.
Trap density in conducting organic semiconductors determined from temperature dependence of J−V characteristics
Authors: Kumar Vikram , Jain S. C. , Kapoor A. K. , Poortmans J. , Mertens R.
Published: June 2003

Space charge limited currents in organic semiconductors are frequently observed to obey the power law J−Vm and are attributed to an exponential distribution of traps having two parameters, namely the characteristic distribution energy Et and the trap concentration Ht. We determine these parameters from the J(V) characteristics at two or more temperatures reported in literature.
Activation energy analysis as a tool for extraction and investigation of p–n junction leakage current components
Authors: Czerwinski A. , Simoen E. , Poyai A. , Claeys C.
Published: June 2003

The origin of p–n junction reverse current is investigated by a method based on the analysis of the leakage current activation energy. Its main advantages lie in the possibility to distinguish multiple reverse-bias dependent leakage components and determine their mechanisms, which can be different than the Shockley–Read–Hall or field-enhanced generation mechanisms. This is illustrated for state-of-the-art silicided shallow junctions, exhibiting a local Schottky effect, due to small-area silicide penetrations. An estimate of the area of the Schottky (or Shannon) contacts follows from the analysis. The method may be used for various semiconductor materials and leakage current origins.