bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface impedance of a thin superconducting film in a parallel magnetic field
Authors: Luzhbin D. A. , Kasatkin A. L. , Pan V. M.
Published: July 2002

The problem of the surface microwave impedance of a superconducting film in a static magnetic field H parallel to the surface is considered. In films of thickness d<λ (λ is the London penetration depth) the mixed state for a parallel orientation of the external field is a set of vortex rows, the number of which varies in a discrete manner at certain characteristic values Hc1(N)(d) of the magnetic field (Hc1(N)(d) is the insertion field for the Nth vortex row). The surface impedance of the film in the field of a normally incident microwave field is calculated with allowance for the contribution of oscillating vortex rows. The results obtained in the known theoretical models (Coffey–Clem and Brandt) for the surface impedance of superconductors in the mixed state are generalized to the case of thin films, for which size effects play an important role in the structure and dynamics of the vortex lattice.
The metric and strong coupling limit of the M5-brane
Authors: Gibbons G. W. , West P. C.
Published: July 2002

We find the analog of the Boillat metric of Born–Infeld theory for the M5-brane. We show that it provides the propagation cone of all 5-brane degrees. In an arbitrary background field, this cone never lies outside the Einstein cone. An energy momentum tensor for the three-form is defined and shown to satisfy the Dominant Energy Condition. The theory is shown to be well defined for all values of the magnetic field but there is a limiting electric field strength. We consider the strong coupling limit of the M5-brane and show that the corresponding theory is conformally invariant and admits infinitely many conservation laws. On reduction to the Born–Infeld case this agrees with the work of Białynicki-Birula.
Plasma enhanced metalorganic chemical vapor deposition of amorphous aluminum nitride
Authors: Harris H. , Biswas N. , Temkin H. , Gangopadhyay S. , Strathman M.
Published: July 2002

Plasma enhanced deposition of amorphous aluminum nitride (AlN) using trimethylaluminum, hydrogen, and nitrogen was performed in a capacitively coupled plasma system. Temperature was varied from 350 to 550 °C, and pressure dependence of the film structure was investigated. Films were characterized by Fourier transform infrared, Rutherford backscattering (RBS), ellipsometry, and x-ray diffraction (XRD). The films are amorphous in nature, as indicated by XRD. Variations in the refractive index were observed in ellipsometric measurements, which is explained by the incorporation of carbon in the films, and confirmed by RBS. Capacitance–voltage, conductance–voltage (G–V), and current–voltage measurements were performed to reveal bulk and interface electrical properties. The electrical properties showed marked dependence on processing conditions of the AlN films. Clear peaks as observed in the G–V characteristics indicated that the losses are predominantly due to interface states. The interface state density ranged between 1010 and 1011 eV−1 cm−2. Annealing in hydrogen resulted in lowering of interface state density values.
Ionic polarization in bending-electrostrictive response of polyurethane films
Authors: Watanabe Masashi , Shirai Hirofusa , Hirai Toshihiro
Published: July 2002

Effects of ionic polarization on hysteresis in the bending-electrostrictive response of polyurethane films are reported. The relation between the ionic polarization and charge injection from the anode was investigated using a model system. As a result, taking into account an internal electric field inside the film, the effects could be reasonably explained as follows. When the ionic polarization weakened the internal field near the anode, the charge injection was suppressed, resulting in a slow response. On the other hand, when the polarization strengthened it, the charge injection was promoted, which led to a quick response.
Helium nanodroplets and trapped Bose–Einstein condensates as prototypes of finite quantum fluids
Authors: Dalfovo Franco , Stringari Sandro
Published: July 2002

Helium nanodroplets and trapped Bose–Einstein condensates in dilute atomic gases offer complementary views of fundamental aspects of quantum many-body systems. We discuss analogies and differences, stressing their common theoretical background and peculiar features. We briefly review some relevant concepts, such as the meaning of superfluidity in finite systems, the behavior of elementary excitations and collective modes, as well as rotational properties and quantized vorticity.
Simon short circuit effect in ECRIS
Authors: Drentje A. G. , Wolters U. , Nadzeyka A. , Meyer D. , Wiesemann K.
Published: July 2002

The plasma confinement within an electron cyclotron resonance ion source significantly influences the charge state distribution and hence the performance of the source. The different axial and radial diffusion processes govern the confinement time. In many experiments it has been shown that negatively biasing the end plate in the injection region improves the charge state distribution. In a few x-ray and vacuum ultraviolet spectroscopy experiments to clarify the mechanism it is observed that the biasing improves the confinement of the plasma. It is estimated that the effect cannot be explained solely by secondary electron emission from the plate into the plasma. We propose that by biasing, the overall balance between radial ion losses and axial electron losses will change, resulting in a different diffusional mode of the entire plasma. Hence, the plasma potential and the average charge state of ions in the plasma are significantly influenced. Usually, the ion flux is dominating radial diffusion while the electron flux is dominating axial losses. This is possible due to compensating wall currents in the electrical conducting plasma chamber (“Simon short circuit”). Thus the usual approach of ambipolar diffusion does not hold in this situation. A similar effect takes place if the plasma chamber is coated with electrically insulating materials. The condition of overall flux balance to the walls is no longer fulfilled and has to be replaced by the local ambipolar particle movement. Again the entire diffusion profile of the plasma changes and the confinement improves. We examine the short circuit current as a measure for the diffusion mode in more detail and try to develop an approximate calculation on the influence of plasma potential and average charge ion state Z in the plasma. First results are presented and discussed.
The BIE100 ECR ion source
Authors: Xie Dan Z.
Published: July 2002

BIE100, an all-permanent-magnet-based electron cyclotron resonance (ECR) ion source, has been under development at Berkeley Ion Equipment (BIE). With a relatively small but optimized source magnet volume, the maximum peak magnetic field strengths of the BIE100 reach 13 kG on axis and 11 kG at the plasma chamber walls, respectively. Microwaves of frequencies of 12.75 and 14 GHz are to simultaneously heat the plasma electrons. The development goal of this ECR ion source is to further explore the capabilities of ECR ion source in the efficient production of the intermediate and high charge state rare ion beams needed for the generation of rare ion beams and industrial applications. This article will present the design features of the BIE100 ion source and its current status.
Surface-hopping dynamics of a spin-boson system
Authors: Mac Kernan Donal , Ciccotti Giovanni , Kapral Raymond
Published: July 2002

The spin-boson model is solved within the framework of quantum-classical dynamics using our recently-developed surface-hopping scheme. The quantum-classical equation of motion is expressed in an adiabatic basis and its solution is constructed from an ensemble of trajectories which undergo nonadiabatic transitions and evolve coherently on the adiabatic surfaces. Details of the algorithm for the simulation of the dynamics are presented and the method of simple Monte Carlo sampling used to evaluate the expectation values of observables is discussed. The simulation method is applied to a spin-boson system with a harmonic bath composed of ten oscillators with an Ohmic spectral density. For the spin-boson model the present implementation of quantum-classical dynamics is exact and the results of our surface-hopping simulations are in accord with previous numerically exact results for this model.
Highly oriented indium tin oxide films for high efficiency organic light-emitting diodes
Authors: Kim H. , Horwitz J. S. , Kim W. H. , Kafafi Z. H. , Chrisey D. B.
Published: July 2002

Highly oriented indium tin oxide (ITO) thin films were grown by pulsed-laser deposition (PLD) on glass and single-crystal yttria-stabilized zirconia (YSZ) substrates. The structural, electrical, and optical properties of these films were investigated as a function of oxygen partial pressure. Films were deposited at substrate temperature of 300 °C in mixed gases (12 mTorr of argon and 1–50 mTorr of oxygen) using a KrF excimer laser (248 nm and 30 ns full width at half maximum) at a fluence of 1.2 J/cm2. ITO films (300 nm thick), deposited by PLD on YSZ at 300 °C in a gas mixture of 12 mTorr of argon and 6 mTorr of oxygen, exhibit a low electrical resistivity (1.6×10−4 Ω cm) with a high transparency (∼74%) at 550 nm. ITO films deposited on both glass and YSZ substrates have been used as an anode contact in organic light-emitting diodes. A comparison of the device performance for the two substrates shows that the device fabricated on the ITO/YSZ has a higher external quantum efficiency than that of the device fabricated on the ITO glass.
Effects of tensile, compressive, and zero strain on localized states in AlInGaN/InGaN quantum-well structures
Authors: Aumer M. E. , LeBoeuf S. F. , Moody B. F. , Bedair S. M. , Nam K. , Lin J. Y. , Jiang H. X.
Published: July 2002

The recombination dynamics of optical transitions as well as strain effects in AlInGaN/In0.08Ga0.92N quantum wells (QWs) were studied. QW emission energy, photoluminescence decay behavior, photoluminescence emission line shape, and nonradiative recombination behavior were found to be strong functions of strain as well as localization. The degree of carrier localization was inferred by modeling several aspects of optical behavior obtained from variable temperature time-resolved photoluminescence experiments. According to the modeling results, the degree of localization was found to be a minimum for unstrained QWs and increased as either tensile or compressive strain increased, indicating that InGaN QW microstructure is a function of the lattice-mismatch-induced strain experienced during deposition.
The effect of convection on a propagating front with a liquid product: Comparison of theory and experiments
Authors: McCaughey B. , Pojman J. A. , Simmons C. , Volpert V. A.
Published: July 2002

This work is devoted to the investigation of propagating polymerization fronts converting a liquid monomer into a liquid polymer. We consider a simplified mathematical model which consists of the heat equation and equation for the depth of conversion for one-step chemical reaction and of the Navier-Stokes equations under the Boussinesq approximation. We fulfill the linear stability analysis of the stationary propagating front and find conditions of convective and thermal instabilities. We show that convection can occur not only for ascending fronts but also for descending fronts. Though in the latter case the exothermic chemical reaction heats the cold monomer from above, the instability appears and can be explained by the interaction of chemical reaction with hydrodynamics. Hydrodynamics changes also conditions of the thermal instability. The front propagating upwards becomes less stable than without convection, the front propagating downwards more stable. The theoretical results are compared with experiments. The experimentally measured stability boundary for polymerization of benzyl acrylate in dimethyl formamide is well approximated by the theoretical stability boundary.
Laser generation of acoustic waves in liquids and gases
Authors: Sigrist Markus W.
Published: July 2002

The laser generation of sound in liquids and gases is reviewed. The sound-generating mechanisms of laser interaction with matter are discussed with emphasis on the thermoelastic process. The studies on strongly absorbing liquids include detailed theoretical considerations of the thermoelastic sound generation with pulsed lasers. Acoustic waveforms for H2O and D2O are calculated analytically on the basis of a model laser-pulse shape. Both free and rigid boundaries on the surface of the liquid are considered. Good agreement between theory and experiments with respect to waveforms and amplitudes is obtained. The experiments are performed with a hybrid CO2 laser and piezoelectric or optical detection of the acoustic transients. In view of a present controversy, special emphasis is put on the temperature dependence of the acoustic amplitudes in H2O, D2O, and in aqueous MgSO4 solutions. Good agreement is found between experimental data and a new, pure thermal model which takes heat conduction into account. The distortion of the acoustic waveform during the propagation through the liquid is treated in terms of sound absorption, diffraction, and nonlinear acoustics. A simple experimental method for the determination of Beyer’s nonlinearity parameter B/A is presented. In the last section some characteristics of photoacoustic spectroscopy (PAS) in gaseous media are reviewed. This method has been demonstrated to be highly sensitive. The measurement of absorption coefficients as low as 10−8 cm−1 is possible. PA studies on H2O vapor are discussed with new results on line and continuum absorption in the 9–11-μm wavelength range. Finally, the impact of PAS on trace gas analysis is demonstrated. With PAS the detection of gas concentrations in the ppb range is feasible. The operational characteristics of a stationary CO laser and a mobile CO2 laser-PAS system are presented, including first results on continuous in situ air pollution monitoring.
Sheath thickness and potential profiles of ion-matrix sheaths for cylindrical and spherical electrodes
Authors: Conrad J. R.
Published: July 2002

Analytic expressions have been obtained for the potential profile and sheath thickness of the transient ion-matrix sheath which forms when a large negative step potential is applied to planar, cylindrical, and spherical electrodes immersed in a plasma.
Scanning electron-acoustic microscopy of MgO crystals
Authors: Urchulutegui M. , Piqueras J. , Llopis J.
Published: July 2002

The capability of scanning electron-acoustic microscopy in the characterization of MgO crystals has been studied. The conditions for the observation of different surface and subsurface features in as-grown and deformed crystals are described and the results are discussed on the basis of thermal and nonthermal mechanisms of acoustic signal generation.
Dependence of H− production upon the work function of a Mo surface in a cesiated hydrogen discharge
Authors: Wada M. , Pyle R. V. , Stearns J. W.
Published: July 2002

The photoelectric work function of a cesiated molybdenum converter surface in a cesiated hydrogen discharge and the negative-hydrogen-ion (H−) current produced at the surface were simultaneously measured. With the negative bias potential of the converter constant, the H− yield increased exponentially as the work function was decreased by introducing Cs into the discharge. The yield of H− current was always higher for a higher bias potential on the surface, provided the measured surface work function was nearly the same. When the concentration of Cs in a discharge was nearly constant, the bias potential of the surface at which the H− production became maximum was observed. At this bias potential, the surface work function was close to the work-function minimum.
Rate constants for the etching of intrinsic and doped polycrystalline silicon by bromine atoms
Authors: Walker Zane H. , Ogryzlo Elmer A.
Published: July 2002

The rate constants for the reaction of bromine atoms with intrinsic and heavily doped n+-type polycrystalline silicon, as well as their temperature dependencies, are reported for the first time. The rate constant for intrinsic silicon is given as k=(4±2)×107 nm min−1 Torr−1e−(62±2kJ/mol)/RT and for n+-type silicon with a phosphorus number density of 5×1018 cm−3, k=(1±1)×1011 nm min−1 Torr−1e−(62±2kJ/mol)/RT. Although the etch rates for the n+-type silicon are 2–3 orders of magnitude larger than for intrinsic silicon at the same temperature and atom concentration, the difference in the two rate constants is due to a change in the pre-exponential Arrhenius factor and not in the activation energy.
Early stages of melting in Si under nanosecond laser pulse irradiation: A time-resolved study
Authors: Solis J. , Afonso C. N.
Published: July 2002

Time-resolved reflectivity (TRR) measurements are performed in crystalline Si under UV and visible wavelength irradiation. The former are carried out with ArF excimer laser pulses whereas the latter are performed in micron-sized areas irradiated with Ar+ laser pulses by means of a novel experimental setup. It is the first time that TRR measurements in the nanosecond regime are performed in micron-sized irradiated areas although they are very suitable to characterize processes in phase change optical storage and microelectronics applications. The energy density melting thresholds at both Ar+ and ArF laser wavelengths are determined. The reflectivity values obtained for pulse fluences just above the melting threshold show that melting proceeds inhomogeneously being the near-surface region formed by a mixture of solid and liquid phases without a well-defined interface. The comparison of the results obtained with uv and visible irradiation indicates that inhomogeneous melting is a general phenomenon which does not depend on the irradiation wavelength. It is present in the early stages of the melting process and its origin is related to the phase nucleation process itself. As the laser fluence is increased, the evolution of the melt duration exhibits a ‘‘different’’ behavior which is related to the formation of a homogeneous molten layer on top of the surface.
Properties of the TiSi2/p+n structures formed by ion implantation through silicide and rapid thermal annealing
Authors: Erzgräber H. B. , Zaumseil P. , Bugiel E. , Sorge R. , Tittelbach-Helmrich K. , Richter F. , Panknin D. , Trapp M.
Published: July 2002

Silicided p+n junctions and contact resistivity test structures were formed by implantation of BF2 through TiSi2 in crystalline as well as in Si+ or Ge+ preamorphized silicon. A subsequent rapid thermal annealing at 950 °C in nitrogen atmosphere was performed to activate the dopant, to remove the ion implantation damage, to increase the silicide conductivity, and to improve the electrical characteristics of the junction. Very low leakage currents and low contact resistivities were measured on samples without preamorphization. With Si+ or Ge+ implantation the channeling of boron was suppressed but residual defects below the original amorphous-crystalline (a-c) interface gave rise to an increased leakage current. A Ti-related defect level was found by deep level transient spectroscopy in the silicon substrate up to a depth of some micrometers.
Magnetic characterization of amorphous intermetallic materials MTe2 (M=Fe and Co)
Authors: Zhang Jian H. , Wu Biao , O’Connor Charles J. , Simmons William B.
Published: July 2002

The amorphous intermetallic materials of FeTe2 and CoTe2 have been synthesized by the reaction of the Zintl anion, Te2−2, with Fe2+ and Co2+ in aqueous solutions. The magnetic properties of these materials have been examined as a function of temperature between 2.5 and 300 K. Both FeTe2 and CoTe2 exhibit strong antiferromagnetic coupling between localized moments but without long-range magnetic ordering. FeTe2 and CoTe2 undergo a transition to a spin-glass state at 5 and 6 K, respectively. Thermal treatment of the freshly prepared sample of FeTe2 at a moderate temperature changes it to the crystalline phase which shows 3D antiferromagnetic ordering at ∼100 K.
Time-resolved measurements of highly polymerized negative ions in radio frequency silane plasma deposition experiments
Authors: Howling A. A. , Sansonnens L. , Dorier J.-L. , Hollenstein Ch.
Published: July 2002

The time-resolved fluxes of negative polysilicon hydride ions from a power-modulated rf silane plasma have been measured by quadrupole mass spectrometry and modeled using a simple polymerization scheme. Experiments were performed with plasma parameters suitable for high-quality amorphous silicon deposition. Polysilicon hydride anions diffuse from the plasma with low energy (approximately 0.5 eV) during the afterglow after the electron density has decayed and the sheath fields have collapsed. The mass dependence of the temporal behavior of the anion loss flux demonstrates that the plasma composition is influenced by the modulation frequency. The negative species attain much higher masses than the positive or neutral species and anions containing as many as sixteen silicon atoms have been observed, corresponding to the 500 amu limit of the mass spectrometer. This suggests that negative ions could be the precursors to particle formation. Ion–molecule and ion–ion reactions are discussed and a simple negative ion polymerization scheme is proposed which qualitatively reproduces the experimental results. The model shows that the densities of high mass negative ions in the plasma are strongly reduced by modulation frequencies near 1 kHz. Each plasma period is then too short for the polymerization chain to propagate to high masses before the elementary anions are lost in each subsequent afterglow period. This explains why modulation of the rf power can reduce particle contamination. We conclude that for the case of silane rf plasmas, the initiation steps which ultimately lead to particle contamination proceed by negative ion polymerization.
Eddy-current interaction with an ideal crack. II. The inverse problem
Authors: Bowler John R. , Norton Stephen J. , Harrison David J.
Published: July 2002

Eddy-current inversion is the process whereby the geometry of a flaw in a metal is derived from electromagnetic probe measurements. An inversion scheme is described for finding the shape and size of cracks from eddy-current probe impedance measurements. The approach is based on an optimization scheme that seeks to minimize a global error function quantifying the difference between predicted and observed probe impedances. The error minimum is sought using a standard descent algorithm that requires a knowledge of the gradient of the error with respect to a variation of the flaw geometry. Computation of the gradient is based on a provisional flaw estimate, then the flaw estimate is updated in a ‘‘direction’’ that reduces the error. The process continues iteratively until a convergence criterion has been satisfied. Then the final flaw estimate should match the shape of the real defect. An equation for the gradient has been derived using an integral formulation of the ideal crack problem. Numerical estimates of the error gradient and the probe impedances have been calculated using approximations based on the moment method. Tests of the inversion scheme using single frequency probe impedance measurements have been carried out by calculating the shapes of narrow slots in aluminum alloy plates. Good agreement is found between the optimum profiles and the measured slot shapes.
Distribution of incident ions and retained dose analysis for a wedge-shaped target in plasma source ion implantation
Authors: Malik Shamim M. , Muller D. E. , Sridharan K. , Fetherston R. P. , Tran Ngoc , Conrad J. R.
Published: July 2002

A wedge-shaped target was implanted with nitrogen ions using the plasma source ion implantation process, in order to understand the effects of the target edges on the energy and fluence distribution of incident ions. Experimental measurements and analysis of retained dose on silicon samples affixed on the surface of the target, showed results consistent with those predicted by theoretical models. Higher retained dose and greater implantation depths were observed in the vicinity of the edge contained by the normal angle as compared to the edges contained by the acute angles. The target face with smaller area accumulated, on the average, higher dose compared to the face with the larger area.
Anisotropic networks with stable dipole orientation obtained by photopolymerization in the ferroelectric state
Authors: Hikmet R. A. M. , Lub J.
Published: July 2002

In this study a ferroelectric liquid crystal (FLC) monoacrylate (S)-4-(1-ethoxycarbonyl-1-ethyloxy)phenyl 4′-(11-acryloyloxyundecyloxy)biphenyl-4-carboxylate was used. Room-temperature spontaneous polarization of the monoacrylate was measured to be 190 nC/cm2. The monoacrylate was provided with 10% LC diacrylate. After inducing macroscopic molecular and dipolar orientation in the ferroelectric phase, polymerization within the reactive system was induced photochemically. In this way an anisotropic polymer network with a spontaneous polarization was produced. Within this network the crosslinks were formed by the diacrylate molecules. The anisotropic network was transparent and highly birefringent. The system showed high thermal stability and remained anisotropic even after being heated to elevated temperatures. The presence of dipolar orientation within the anisotropic networks was characterized by piezoelectric measurements. The piezoelectric coefficient within these networks was found to be highly dependent on the direction of the applied strain. The highest value was measured in the direction perpendicular to the molecular orientation and was 29 pC/N.
Temperature dependence of the Urbach edge in GaAs
Authors: Johnson S. R. , Tiedje T.
Published: July 2002

The temperature dependence of the optical-absorption edge (Urbach edge) of GaAs is measured in semi-insulating and n-type GaAs (n=2×1018 cm−3) over the temperature range from room temperature to 700 °C. Both the optical absorption and the temperature are measured using a diffuse reflectance technique. The characteristic energy of the exponential absorption edge is found to increase linearly with temperature, from 7.5 meV at room temperature to 12.4 meV at 700 °C, for semi-insulating GaAs. The temperature dependent part of the width of the Urbach edge for semi-insulating GaAs is six times smaller than predicted by the standard theory where the edge width is proportional to the phonon population.
Electrochromism of fluorinated and electron-bombarded tungsten oxide films
Authors: Azens A. , Granqvist C. G. , Pentjuss E. , Gabrusenoks J. , Barczynska J.
Published: July 2002

Reactive dc magnetron sputtering of W was performed in a plasma of Ar+O2 with and without CF4 addition and substrate bias. Structural studies by atomic force microscopy, X-ray diffraction, infrared reflectance spectroscopy, and Raman spectroscopy indicated that the electron e bombardment associated with a positive substrate bias led to grain growth and partial crystallization while maintaining a high density of W=O double bonds presumably on internal surfaces. Electrochemical measurements showed that the durability under extended Li+ intercalation/ deintercalation was excellent for e-bombarded oxide films and poor for oxyfluoride films. Spectrophotometric studies of the electrochromism yielded that the color/bleach dynamics was slow for the e-bombarded oxide but fast for the oxyfluoride. The range of optical modulation was large. Tandem films, with a thin protective layer of e-bombarded oxide covering a thicker oxyfluoride layer, were able to combine rapid dynamics with good durability.
Li intercalation in transparent Ti–Ce oxide films: Energetics and ion dynamics
Authors: Mattsson M. Stro/mme , Azens A. , Niklasson G. A. , Granqvist C. G. , Purans J.
Published: July 2002

Films of Ti dioxide, mixed Ti–Ce oxide, and Ce dioxide were produced by reactive dc magnetron sputtering. Electrochemical lithiation was probed by chronopotentiometry, cyclic voltammetry together with optical transmittance recording, and impedance spectroscopy. Evidence was found for inserted electrons being accommodated in Ce 4f states; this contention was supported by preliminary results from x-ray absorption fine-structure spectroscopy. These electrons do not produce luminous electrochromism. The variation of the chemical diffusion coefficient of Li, with film composition and Li content, was also studied.
Magnetovolume effects and anomalous compressibility of Nd2Fe17 and Nd2(FeTi)17 compounds
Authors: Arnold Z. , Kamarád J. , Morellon L. , Ibarra M. R.
Published: July 2002

We present the results of direct measurements of the isothermal compressibility up to 13 kbar on Nd2Fe17 and Nd2(FeTi)17 compounds and the results of measurements of the linear thermal expansion on Nd2Fe17 compound under hydrostatic pressure up to 6 kbar in temperature range 200–400 K covering both ferromagnetic and paramagnetic region using a modified strain gauge method. A large magnetic contribution to the compressibility κm∼0.15 Mbar−1 was observed in the ferromagnetic state of the Nd2Fe17 compound. The pronounced lambda-shaped anomalies in the thermal expansion coefficient α (negative values of α) and the maximum of compressibility were simultaneously observed nearby the Curie temperature. The corresponding thermodynamic relations are used to analyze the pressure behavior.
New correlation procedure for the improvement of resolution of deep level transient spectroscopy of semiconductors
Authors: Istratov A. A.
Published: July 2002

The selectivity and sensitivity of deep level transient spectroscopy (DLTS) depend significantly on the choice of the correlation function. In this article, the factors limiting the resolution of correlation DLTS are discussed. It is argued that the weighting functions reported to date provide asymmetrical rate windows, being effective filters only for slow transients. To overcome this limit, a correlation procedure, based on the Gaver–Stehfest algorithm for the inverse Laplace transformation, is proposed. Using this procedure one can obtain a temperature scanned DLTS mode resolution comparable to the resolution of sophisticated methods of inversion of the Laplace integral equation.
Spatial and temporal variations of CF and CF2 radical densities in high-density CF4 plasmas studied by laser-induced fluorescence
Authors: Suzuki C. , Sasaki K. , Kadota K.
Published: July 2002

Laser-induced fluorescence spectroscopy has been employed for the measurements of the ground-state CF and CF2 radical densities in low-pressure and high-density CF4 plasmas generated by helicon wave discharges. In the pulsed operation (5 Hz, 10 ms duration), the radial profiles of the CF and CF2 densities were hollow shape in the discharge phase, which indicates that both radicals were desorbed from the wall and were lost in the plasma column. In the continuous-wave operation, roughly uniform radial profiles were observed for both radicals. Therefore the desorbed radicals in the pulsed discharge seem to originate from the adsorbed species in the afterglow of the previous discharge.
Optical and electrochemical properties of Li+ intercalated Zr–Ce oxide and Hf–Ce oxide films
Authors: Veszelei M. , Kullman L. , Stro/mme Mattsson M. , Azens A. , Granqvist C. G.
Published: July 2002

Sputter deposited Zr–Ce oxide and Hf–Ce oxide films were investigated with regard to structure, optical absorption, and electrochemical properties. X-ray diffractometry and Rutherford backscattering spectrometry showed that polycrystalline Zr–Ce oxide and Hf–Ce oxide films had cubic crystal structures for 40–100 mol % CeO2 and 50–100 mol % CeO2, respectively. Cyclic voltammetry was performed in an electrolyte of propylene carbonate containing LiClO4. The charge capacity was ∼60 mC/cm2μm for a Zr–Ce oxide film with a Ce/Zr atom ratio of ∼13 as well as for a Hf–Ce oxide film with a Ce/Hf atom ratio of ∼2. A decrease of the charge capacity was noted after ∼1000 voltammetric cycles, with the mixed oxide films being far more stable than CeO2. In situ optical transmittance measurements showed that both Zr–Ce and Hf–Ce oxide films remained essentially transparent during Li+ intercalation. Chronopotentiometry measurements were used to elucidate effects of the electronic structure during Li+ intercalation.
Promotion of crystallization and magnetic property improvement enhancement of the energy product in Nd5.5Fe66B18.5Cr5Co5 by magnetic field heat treatment
Authors: Zhao T. M. , Hao Y. Y. , Xu X. R. , Yang Y. S. , Hu Z. Q.
Published: July 2002

An externally applied magnetic field during heat treatment of a melt spun Nd5.5Fe66B18.5Cr5Co5 alloy can promote its crystallization from amorphous state and induce a grain refinement. The stronger the applied magnetic field, the lower the crystallization temperature. Holding the annealing time for 5 min, the crystallization occurs at 680 °C in conventional heat treatment, however, it happens at 650 °C when annealed with a 0.15 T field strength and 630 °C with a 0.25 T field strength. The maximum energy product for Nd5.5Fe66B18.5Cr5Co5 powder under optimum heat treatment with an applied magnetic field can increase about 17% because of the enhancement of exchange coupling role between the soft and hard phase due to the full crystallization and the grain size reduction.
Phase formation and resistivity in the ternary system Ti–Nb–Si
Authors: Mouroux Aliette , Roux Marylène , Zhang Shi-Li , d’Heurle François M. , Cabral Cyril , Lavoie Christian , Harper James M. E.
Published: July 2002

We present measurements of the pseudobinary phase diagram of the TiSi2–NbSi2 system. This disilicide system has recently become important because of the enhanced formation of the low resistivity C54 phase of TiSi2 by addition of Nb. The solubility limit of Nb in C54 TiSi2 at 1000 °C is found to lie between 10% and 16% at the metal site, and the solubility limit of Ti in C40 NbSi2 at 1000 °C is between 76% and 79.5% at the metal site. Adding Nb to C54 TiSi2 increases the unit cell volume at a rate of 0.035% per at. % Nb. Adding Nb to C40 (Ti,Nb)Si2 increases the unit cell volume at a rate of 0.034% per at. % Nb. The presence of Nb enhances the formation of the C54 phase and improves its thermal stability. The desirable low resistivity of the C54 phase is increased by 1.2 μΩ cm per at. % Nb.
Effect of pre-amorphization of polycrystalline silicon on agglomeration of TiSi2 in subquarter micron Si lines
Authors: Bae Jong-Uk , Sohn Dong Kyun , Park Ji-Soo , Lee Byung Hak , Han Chang Hee , Park Jin Won
Published: July 2002

We investigated the effect of C49-to-C54 conversion behavior on the agglomeration of Ti-silicide fabricated on subquarter micron polycrystalline Si lines by comparing pre-amorphized samples with conventional ones. Pre-amorphization of polycrystalline Si enhances the C49-to-C54 transformation on subquarter micron linewidths, however, it results in the early development of macroscopic grooves during the second rapid thermal annealing. From these results, we suggest the existence of an extra thermal budget during the second rapid thermal annealing of the pre-amorphized samples, which deteriorates the thermal stability of the C54-TiSi2 featured line. It is also shown that C49-to-C54 transformation on a 0.2 μm linewidth in the conventional samples has two kinds of competing factors when postannealing is added. One is the tendency of C54 transformation and the other is agglomeration of C49-TiSi2 grains. During high temperature annealing (>850 °C), C49-TiSi2 has been agglomerated by a thinning mechanism instead of transformation into the C54 phase.
Density-matrix functional theory for the N-particle ground state
Authors: Zumbach Gil , Maschke Klaus
Published: July 2002

We discuss the one-particle density-matrix functional theory for the N-particle ground-state problem. Using the variational principle we obtain a set of self-consistent equations which could be useful for practical calculations. The derived relation between the eigenvalues of the density matrix and the chemical potential agrees with that of Gilbert. The present theory is compared with the Kohn–Sham and the Hartree–Fock approach to the ground state problem. It is found that both approaches can be obtained from the present theory under certain assumptions.
Measurement of the wave-vector dependent diffusion coefficient in concentrated particle dispersions
Authors: van Megen W. , Ottewill R.H. , Owens S.M. , Pusey P. N.
Published: July 2002

Measurements are reported of the wave-vector dependent diffusion coefficient D(q) in the region of the first two peaks in the static structure factor for concentrated nonaqueous dispersions of polymethylmethacrylate spheres (diam≂0.4 μm). The particle volume fractions are taken up to the point of spontaneous crystallization. In order to minimize the effects of multiple scattering in these dynamic light scattering experiments, the particles are dispersed in solvent mixtures having refractive indexes close to those of the particles. The measured data for D(q) are compared with the results of two recent theoretical predictions, viz. the approach of Beenakker and Mazur based on a partial resummation of many-particle interactions, and the empirical effective two-particle mobility method of Snook et al.
Influence of polydispersity on dynamic light scattering measurements on concentrated suspensions
Authors: van Veluwen A. , Lekkerkerker H. N. W. , de Kruif C. G. , Vrij A.
Published: July 2002

The dynamic structure factor for concentrated dispersions of small polydisperse hard spherical particles has been measured and compared with theoretical predictions. Experimental evidence for the occurrence of a slowly decaying mode has been found. The dynamic structure factor is analyzed in terms of two independent modes, associated with collective diffusion and exchange diffusion. Calculations of the mode amplitudes are extended to the case of combined optical and size polydispersity. The correspondence between measurements and theory is quite satisfactory.
Monte Carlo path integration for the real time propagator
Authors: Makri Nancy , Miller William H.
Published: July 2002

Monte Carlo methods are described for evaluating the Feynman path integral representation of the (real time) propagator (time evolution operator), exp(−iHt/ℏ). The approach is based on the modified Filinov algorithm presented earlier by Makri and Miller [Chem. Phys. Lett. 139, 10 (1987)]. Numerical calculations are presented for time evolution in a symmetric double well potential, as well as in a Morse potential.
The external heavy atom effect: Theory of spin–orbit coupling of alkali and noble metals in rare gas matrices
Authors: Pellow Robert , Vala Martin
Published: July 2002

Recent data from magnetic circular dichroism (MCD) and optical absorption studies of alkali and noble metal atoms in rare gas matrices have shown an interesting and significant influence of the rare gas on the spin–orbit (SO) splitting of the metal excited 2P state. The excited state SO coupling constants extracted from these data can be classified into three major categories: (I) positive and greater than gas phase values, (II) positive and less than gas phase values, and (III) negative and less than gas phase values. A theory for the spin–orbit coupling of a noble or alkali metal in a rare gas matrix is presented. A supermolecule model in which the metal is surrounded by 12 tetradecahedrally situated rare gas atoms is used to calculate the SO splitting. The splitting is found to depend on three quantities: the metal–rare gas σ- and π-type overlap integrals, the ratio of rare gas-to-metal SO coupling constants, and the metal–rare gas molecular orbital mixing coefficients. Using computed overlap integrals and empirical SO coupling constants it is shown that the model is capable of predicting qualitatively the three major categories of metal–rare gas spin–orbit constants. Finally, a semiempirical approach is utilized to estimate the mixing coefficients and quantitative SO ‘‘reduction’’ factors are determined. This parameterless theory is shown to predict all the important trends well.
Vibrational studies of reactive intermediates of aromatic amines. IV. Radical cation time-resolved resonance Raman investigation of N, N-dimethylaniline and N, N-diethylaniline derivatives
Authors: Poizat O. , Guichard V. , Buntinx G.
Published: July 2002

The radical cation time-resolved resonance Raman spectra of various isotopic derivatives of N, N-dimethylaniline (DMA), N, N-diethylaniline (DEA), N, N-dimethyl-p-toluidine (4MDMA) and 3, 5, N, N-tetramethylaniline (3,5DMDMA) are reported in the 300–1800 cm−1 range. Excitation was in the weak radical cation absorption around 480 nm. Complete vibrational assignments are proposed. The band activity and the changes in frequency with respect to the neutral molecules are consistent with a quinoidal-type conformation of the framework close to planarity. Stabilization of this conformation is observed when the phenyl ring contains methyl substituents. The analysis of the Raman enhancements suggests that the quinoidal character of the radical structure is significantly lowered in the resonant excited state. An obvious analogy is found between the spectra of DMA+ ⋅ and of the biphenyl radical cation, which clearly indicates that (i) a nearly common chromophore structure characterizes these two radical cations and (ii) the distortion of this chromophore structure in the resonant excited state is comparable in both compounds, i.e., the biphenyl+ ⋅* ←biphenyl+ ⋅ and DMA+ ⋅* ←DMA+ ⋅ transitions are of similar nature. These results are consistent with structural previsions from simple molecular orbital considerations and a comprehensive interpretation of the Raman spectra is given in terms of HOMO population.
Comment on: Is n=4 a magic number for He+n?
Authors: Balasubramanian K.
Published: July 2002

Possible explanations for the unusual abundance of the n=4 peak in the recent experimental mass spectra of helium clusters generated by Buchenau et al. are considered since theoretical calculations reveal that the He atom is bound by only 0.02–0.03 eV in He+4 relative to He+3.
Equilibrium and nonequilibrium solvation and solute electronic structure. I. Formulation
Authors: Kim Hyung J. , Hynes James T.
Published: July 2002

A theoretical formulation is developed to describe the electronic structure of an immersed solute, electrostatically coupled to a polar and polarizable solvent. The solvent is characterized, in the dielectric continuum approximation, by electronic and orientational polarizations. Starting from a general free-energy expression for the quantum solute–solvent system, a time-independent nonlinear Schrödinger equation is derived. The nonlinearity arises from the assumed equilibration of the solvent electronic polarization Peqe, to the solute electronic wave function Ψ and the solvent orientational polarization Por. When Por is arbitrary, there is nonequilibrium solvation. When Por is equilibrated to Peqe and Ψ, equilibrium solvation obtains. The theory is illustrated for a model symmetric electron donor–acceptor solute system in a two state basis set description. Solution of the nonlinear Schrödinger equation in the presence of arbitrary Por yields nonequilibrium solvation stationary states (NSS) for the solute–solvent system, including the solvent-dependent solute electronic structure, and the associated free energies. When Por=Peqor, the corresponding equilibrium solvation states (ESS) and their characteristics are obtained. The NSS are classified into three distinct regimes, according to the relative strengths of the electronic coupling, which tends to delocalize the solute electronic distribution, and the solvent polarization, which tends to localize it. The ESS stability characteristics are also important in this classification. Two of the regimes correspond to activated electron transfer processes, and differ according to whether there is or is not a continuous free-energy path leading from localized reactants to localized products. The third regime, in which the electronic coupling dominates the solvent polarization, corresponds to stable delocalized states between which spectroscopic transitions are of interest. Finally, the inclusion of electronic exchange in the theory leads to the necessity of more than one solvent coordinate in order to describe the free-energy surface for the solute–solvent system.
Relative stabilities of fullerene, cumulene, and polyacetylene structures for Cn : n=18–60
Authors: Feyereisen Martin , Gutowski Maciej , Simons Jack , Almlöf Jan
Published: July 2002

The relative stabilities of closed fullerene, cumulene, and polyacetylene carbon structures, as well as the cohesive energies for clusters of size n=18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 50, and 60 have been examined using ab initio self-consistent-field and second-order Mo/ller–Plesset perturbation theory and analytical derivative geometry optimization methods. These geometries and relative stabilities constitute the primary findings of this work. All calculations were carried out using the disco program with atomic basis sets derived from van Duijneveldt’s carbon (6s,3p) primitive orbital basis set, contracted to [3s2p]. For n≥32, the fullerenes are predicted to be the most stable, and their cohesive energies are predicted to increase monotonically as n varies from 24 to 60. The optimized geometries obtained here are very near those obtained earlier by others for the few species where such data exist. Based on earlier work employing larger atomic orbital bases, the relative energies of the fullerene structures are expected to be lowered even further as larger basis sets are employed. Hence, it is likely that the smallest cluster for which the fullerene structure is the most stable has n<32. Comparing our computed electron binding energy for C−24 to the experimental vertical detachment energy of this species supports the claim that n=24 may be the smallest energetically favored fullerene.
Femtosecond vibrational transition-state dynamics in a chemical reaction
Authors: Pedersen S. , Bañares L. , Zewail A. H.
Published: July 2002

We report the femtosecond dynamics of the vibrational motion in the transition-state evolution of an isomerization reaction. The observed nonstatistical and bound behavior reflects the localization in selective torsional and bending modes (of the 72 normal modes of the system). The multidimensionality of the potential energy surface is examined by comparing experiments with theoretical calculations.
Structural features of the NO/Ru(001) adsorption complexes: A linear combination of Gaussian-type orbitals local density functional model cluster analysis of high-resolution electron energy loss spectroscopy data
Authors: Neyman Konstantin M. , Rösch Notker , Kostov Krassimir L. , Jakob Peter , Menzel Dietrich
Published: July 2002

High-resolution electron energy loss spectra (HREELS) of NO adsorbed at low temperature on the Ru(001) surface are reported with particular emphasis on the low coverage regime. The improved resolution compared to earlier studies allowed one to clearly separate the various vibrational bands and to establish correlations among them. The experimental data are analyzed with the help of linear combination of Gaussian-type orbitals local density functional model cluster calculations. We conclude that the loss peak of the low coverage samples at 1130 cm−1 can be attributed to the stretching vibrations of upright oriented μ3-bridge nitrosyl species which are bound in an unusual configuration, i.e., via the oxygen atom. The dominating feature at small coverages around 1400 cm−1, corresponding to the intramolecular vibration of NO moieties at the same site but bound via nitrogen, is calculated in agreement with experiment. Bent structures of the adsorbate are energetically less favored both for the μ3-ON and μ3-NO adsorption complexes. In contrast to the adsorption on the threefold hollow sites, NO molecules in the on-top position at Ru(001) bear a positive charge and vibrate at much higher frequencies. The importance of Pauli repulsion for the vibrational frequencies of adsorbed species is emphasized.