bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermoelectric properties of CoSb3 with dispersed FeSb2 particles
Authors: Katsuyama S. , Kanayama Y. , Ito M. , Majima K. , Nagai H.
Published: July 2002

We have prepared a sintered CoSb3–FeSb2 composite where the FeSb2 particles are dispersed in the CoSb3 matrix by mechanical grinding (MG) and hot pressing, and investigated the Seebeck coefficient, electrical resistivity, and thermal conductivity in order to estimate the corresponding figure of merit. The thermal conductivity of the composite is lower than that of CoSb3. The electrical resistivity of the composite increases with increasing MG time, but it is lower than that of CoSb3 at high temperature. The decrease in the thermal conductivity and the lesser increase in the electrical resistivity are ascribed to the enhancement of phonon scattering caused by the dispersion of FeSb2 particles in the CoSb3 matrix and the low electrical resistivity of FeSb2, respectively. As a result, the composite whose molar ratio of CoSb3 to FeSb2 is 0.7:0.3 and the MG time is 25 h has a maximum figure of merit value of 6.1×10−4 K−1 at 756 K. This value is much larger than the maximum value of CoSb3 at 482 K of 3.2×10−4 K−1.
Efficiency of poly(vinylidene fluoride) thin films for excitation of surface acoustic waves
Authors: Strashilov V. L.
Published: July 2002

The applicability of poly(vinylidene fluoride) (PVDF) thin stretched films to the excitation of surface acoustic waves (SAWs) is discussed. Previous results with grating transducers reveal high losses, which are attributed to large attenuation in the polymer. An experimental study was carried out on a LiNbO3 YZ delay line with the input transducer in the form of an interdigital PVDF pair. Untuned insertion losses of 80 dB were obtained at room temperature and frequency of 43 MHz. We demonstrate that these losses are largely caused by the high shear compliance of the film, which leads to degradation of the transducer electromechanical coupling. Employing the piezoelectrically strong thickness-longitudinal mode of the film, a 40 dB loss reduction was achieved. To take full advantage of this mode, a split-electrode grating transducer model with alternating polarity of electrodes is discussed. This is expected to allow efficient SAW excitation at frequencies up to tens of MHz.
Spatially resolved pore-size distribution of drying concrete with magnetic resonance imaging
Authors: Choi C. , Balcom B. J. , Beyea S. D. , Bremner T. W. , Grattan-Bellew P. E. , Armstrong R. L.
Published: July 2002

A study of the spatially resolved water-occupied pore size distribution in a drying concrete cylinder is reported. Pore sizes are obtained from freezing point depression of pore water for a temperature range of 0 to −40 °C, assuming that the freezing point is inversely proportional to pore diameter. Single-point magnetic resonance imaging techniques were used to monitor unfrozen water content as functions of position and temperature. It was observed that freezing began at −10 °C in the cylinder center, which had the highest moisture content, and with a further temperature decrease, the freezing region gradually spread to the exposed end surfaces. The central region had a broad water-occupied pore size distribution, with pore diameters as large as 10 nm. The occupied pore sizes became progressively smaller as the moisture content decreased in proximity to the exposed surfaces.
Nonequilibrium hard-disk packings with controlled orientational order
Authors: Kansal Anuraag R. , Truskett Thomas M. , Torquato Salvatore
Published: July 2002

This paper addresses one of the fundamental questions in the theory of hard-disk packings—how order within a system relates to packing density. The algorithm presented is a seed-based, growth protocol in which new disks are added sequentially to the surface of a growing cluster. The angular position of the new disk is chosen based on the minimization of an objective function designed to control order, as measured by the global bond-orientational order parameter ψ6, which varies between 0 and 1 (with 1 indicating perfect hexagonal close-packed order). Modifying the objective function allows the final packing fraction to be biased while maintaining tight control over ψ6. Inside of the range 0⩽ψ6⩽0.70, the targeted order parameter ψ6 is achieved to within two decimal places of accuracy. Furthermore, it is found that random structures (ψ6∼0.01) can be generated with packing fractions in the range 0.40⩽η⩽0.77. Interestingly, the algorithm can produce nonequilibrium hard-disk configurations that are considerably more disordered than those typical of the equilibrium fluid.
The two-plasmon decay of an extraordinary electromagnetic wave in a magnetized homogeneous plasma
Authors: Laham N. M. , Al-khateeb A. M. , Al Nasser A. S. , Odeh I. M.
Published: July 2002

The effects of the presence of a uniform magnetic field on the growth rate and the threshold intensity of the two-plasmon decay in a homogeneous plasma have been investigated. Analytical expressions for the growth rate and threshold intensity have been obtained. It has been shown that the presence of a magnetic field affects significantly the growth rate and the threshold intensity for the two-plasmon decay.
On the contrast between the Lee–Low–Pines and the generalized Fulton–Gouterman transformation
Authors: Rapp Matthias , Wagner Max
Published: July 2002

In this work we compare the generalized Fulton–Gouterman transformation (GFGT) and the Lee–Low–Pines transformation (LLPT). We apply both to a general electron-phonon Hamiltonian including the Fröhlich Hamiltonian, an electronic periodic potential, phononic anharmonicities, and Umklapp processes. We respectively discuss the ability of the two unitary transformations to achieve approximate electronic diagonalization in general and also for a particular electronic Bloch base. We further consider their possible application to multiparticle systems. It turns out that in general the GFGT is a more successful technique than the LLPT.
The theory of chemically induced dynamic electron polarization for radical pair recombination in low dimensional space
Authors: Shushin A. I. , Batchelor S. N.
Published: July 2002

The kinetics of multiplet chemically induced dynamic electron polarization (CIDEP) generation for radical pair (RP) recombination in low dimensional media (the dimensionality n⩽3) is theoretically analyzed in detail. Simple analytical expressions for the time dependent amplitude Pe(t) of the multiplet CIDEP are derived. These expressions show that Pe(t) is sensitive to the dimensionality only in the limit of weak spin dependent interaction Q and large diffusion coefficient D, when ξ=Qd2/D≪1, where d is the distance of closest approach. In the opposite limit ξ>1, the dependence Pe(Q) is universal Pe(Q)∼Q/(1+ΛQ/D), where Λ∼d.
A magnetohydrodynamic model of rotating magnetic field current drive in a field-reversed configuration
Authors: Milroy Richard D.
Published: July 2002

A numerical model has been developed to study the use of a Rotating Magnetic Field (RMF) as an electron current drive mechanism for the formation and sustainment of a field-reversed configuration (FRC). Previous models assumed a fixed ion model, but here a full two-dimensional (r-θ) magnetohydrodynamic model has been developed. The model has been applied to two classes of problems: (1) For the sustainment problem, a RMF is applied to a preexisting FRC. (2) For the formation problem, a RMF is applied to a plasma column with an initially uniform axial magnetic field and background plasma density. The RMF-induced current reverses this bias field, forming a FRC. The code employs an option to include some three-dimensional effects to satisfy the average β condition and equalize pressure and density between inner and outer field lines, when it is applied to sustainment simulations.
Rotational energy relaxation of individual rotational states in liquids
Authors: Jang Joonkyung , Stratt Richard M.
Published: July 2002

The manner in which most molecules reorient in liquids bears little resemblance to the process in the gas phase. For small-moment-of-inertia species such as the hydrides, however, the observation of discrete spectroscopic lines corresponding to individual isolated-molecule quantum transitions suggests that one is actually seeing single-molecule dynamics perturbed only weakly by the environment—just as one sees with solution-phase vibrational behavior. We examine here the degree to which such individual rotational quantum states remain well defined in liquids by considering the rates of discrete energy-level-to-energy-level transitions in solution. For rotational quantum states that do preserve their free-rotor character in a liquid, we find that the transition rate between angular momentum states obeys a rotational Landau–Teller relation strikingly similar to the analogous expression for vibration: the rate is proportional to the liquid’s rotational friction evaluated at the transition frequency. Subsequent evaluation of this friction by classical linearized instantaneous-normal-mode theory suggests that we can understand this relationship by regarding the relaxation as a kind of resonant energy transfer between the solute and the solution modes. On specializing to the particular cases of H2 and D2 in Ar(l), we find that the most critical modes are those that move the light solute’s center of mass with respect to a single nearby solvent. This observation, in turn, suggests a generalization of instantaneous-normal-mode ideas that transcends both linear coupling and harmonic dynamics: an instantaneous-pair theory for the relaxation of higher-lying levels. By employing a linearized instantaneous-normal-mode theory of relaxation within the liquid band and an instantaneous-pair theory for higher-frequency relaxation, we find that the resonant-transfer paradigm is reasonably successful in reproducing molecular dynamics results spanning a wide range of different rotational states.
In-plane magnetization of garnet films imaged by proximal probe nonlinear magneto-optical microscopy
Authors: Wegner D. , Conrad U. , Güdde J. , Meyer G. , Crecelius T. , Bauer A.
Published: July 2002

We show that small in-plane magnetization components in magnetic garnet films with perpendicular anisotropy can be imaged using nonlinear magneto-optical proximal probe microscopy, i.e., magnetization-induced second harmonic generation together with a scanning near-field optical microscope. A relationship exists between the in-plane magnetization domains we observed and the typical maze-like out-of-plane magnetization domains that are simultaneously imaged recording the linear Faraday effect.
Comment on “Optical properties of CdTe1−xSx (0⩽x⩽1): Experiment and modeling” [J. Appl. Phys. 85, 7418 (1999)]
Authors: Djurišić Aleksandra B. , Li E. Herbert
Published: July 2002

Wei et al. [J. Appl. Phys. 85, 7418 (1999)] performed the room temperature spectroscopic ellipsometry to determine the dielectric function of CdTe1−xSx films. They have fit the obtained dielectric function using the Holden’s model dielectric function [Phys. Rev. B 56, 4037 (1997)], and derived conclusions about the line shape at the band gap E0. However, their description of the fitting procedure is ambiguous, and some model parameters in Table I [J. Appl. Phys. 85, 7418 (1999)] are missing which makes it impossible to reproduce their calculations. Furthermore, the results of Wei et al. [J. Appl. Phys. 85, 7418 (1999)] do not represent conclusive proof of the advantages of their approach over other models available in the literature.
Stability of Si–C films prepared by a pulsed arc discharge method: Thermal treatment and heavy-ion irradiation
Authors: Khriachtchev L. , Vainonen-Ahlgren E. , Sajavaara T. , Ahlgren T. , Keinonen J.
Published: July 2002

We study Si–C films (Si content from 0 to 33 at. %) prepared with a pulsed arc discharge method. The structural modifications introduced by annealing up to 1100 °C, irradiation with 53 MeV I10+127 ions, and deposition onto heated substrates are characterized by Raman spectroscopy. For all the treatments, the structural modifications decrease when the Si concentration increases. Moreover, for high Si content (33 at. %), the energetic iodine ions are found to recover efficiently the structure degraded in preliminary high-temperature annealing. The experiments demonstrate Si-induced stabilization of the three-dimensional Si–C network, which is interpreted as deepening of the corresponding potential energy well. It is highly possible that Si–C films can possess a superior thermodynamic stability for an optimal Si concentration.
Theoretical consideration for carrier transport noise in nonequilibrium steady-state operation of metal–oxide–semiconductor field-effect transistor
Authors: Sumino Daijiro , Omura Yasuhisa
Published: July 2002

This article describes a theoretical approach to carrier-density-fluctuation-induced high-frequency transport noise in n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) under the condition of nonequilibrium steady-state operation. A partial differential equation describing the charge density fluctuation is formulated assuming the use of the current continuity condition. The model presented herein is applied to the analysis of the high-frequency transport noise in MOSFETs in a higher drain-to-source electric field range than is possible with the state-of-the-art model because the current continuity condition is taken into account. In contrast to the past result, it is shown that the normalized fluctuation power decreases or increases as the drain voltage increases, and it saturates at a certain value. When the drain voltage increases, the ratio of the carrier transit time to the effective relaxation time of fluctuation increases, which promotes the averaging of fluctuation. As a result, the increase in fluctuation is suppressed comparing with past theoretical results. In addition, the dependence of the carrier-density-fluctuation-induced transport noise on gate voltage is considered because both the local channel potential and the local carrier density inside the channel are taken into account in formulating the normalized fluctuation power. It is demonstrated that the normalized fluctuation power increases with gate voltage in the low gate voltage range and saturates at the certain value in the high gate voltage range. When the gate voltage increases, the ratio of the carrier transit time to the effective relaxation time of fluctuation decreases, which suppresses the averaging of fluctuation.
Epitaxial Pb(Zr0.52Ti0.48)O3/La0.35Nd0.35Sr0.3MnO3 heterostructures for fabrication of ferroelectric field-effect transistor
Authors: Wu Wenbin , Wong K. H. , Mak C. L. , Choy C. L. , Zhang Y. H.
Published: July 2002

Epitaxial La0.35Nd0.35Sr0.3MnO3 (LNSMO) thin films and Pb(Zr0.52Ti0.48)O3(PZT)/LNSMO heterostructures have been grown on LaAlO3 (001) substrates by the pulsed laser deposition method. The oxygen concentration in the LNSMO films is quite sensitive to the deposition oxygen pressure and can be controlled during the fabrication process. It is, however, stable against in situ postdeposition thermal treatments. Consequently, the resistivity and the metal-semiconductor transition temperature of the LNSMO films can be tuned and fixed during film growth. Electrical measurements on the Pt/PZT/LNSMO ferroelectric capacitor show a remnant polarization of ∼35 μC/cm2 and a coercive field of 30–40 kV/cm at low driving voltages. Switching endurance tests suggest no polarization loss up to about 1010 bipolar switching cycles. The advantages of using epitaxial LNSMO films as the semiconducting channel in an all-perovskite ferroelectric field-effect transistor are discussed.
Effects of multi-energetic grain-boundary trapping states on the electrical characteristics of poly-CdSe thin film transistors
Authors: Lee Shih-Chung , Lee M. J.
Published: July 2002

A physically based two-dimensional simulation for a polycrystalline CdSe thin-film transistor with multi-energetic trapping states located 0.11, 0.33, and 0.67 eV below the conduction band in the semiconductor and localized at the grain boundaries was presented. It was shown that the experimentally observed gradual transition from the exponential (or subthreshold) to linear (or post-threshold) regime is due to the effect of the trapping–detrapping process at the shallower trapping level of 0.11 eV, whereas the subthreshold slope has a strong dependence on the density distribution of the deeper trap at 0.33 eV, and the traps at 0.67 eV have no experimentally observable effect on the shape of the transfer characteristics of TFTs. Furthermore, each trap energy level only affects the shape of the transfer characteristics in a range of gate voltages when |EF −ET|<3kT. The fixed oxide charge density at the SiO2/CdSe interface was extracted from C–V measurements on metal-oxide-semiconductor capacitors and shown to be 3×1011/cm2.
Suppression of microwave magnetic envelope solitons by continuous wave magnetostatic wave signals
Authors: Scott Mark M. , Fetisov Yuri K. , Synogach Valeri T. , Patton Carl E.
Published: July 2002

The influence of continuous wave (cw) magnetostatic wave signals on microwave magnetic envelope soliton pulse formation and propagation in magnetic films has been examined. Pulsed and cw microwave signals were applied to the input of a single crystal yttrium–iron–garnet film magnetostatic wave delay line. The nominal operating frequency was 4.8 GHz. The pulse signals served to form solitons with no cw power present. Under suitable conditions, the cw signal served to inhibit or eliminate the soliton formation and propagation. The suppression effect was measured as a function of the cw signal frequency and power. The suppression is maximized when the cw signal frequency coincides with the pulse carrier frequency. At this frequency, an input cw power of 80 mW is sufficient to suppress completely a soliton pulse formed from a 10 ns wide, 500 mW peak power input pulse.
Discotic amphiphiles
Authors: Henderson J. R.
Published: July 2002

Solutions of disk-shaped molecules that self-assemble into linear aggregates pose an interesting question as to whether or not it might sometimes be appropriate to regard the solute as an amphiphile and, if so, how could the strength of this amphiphilic behavior be quantified or measured. This paper proposes an answer by mapping linear self-assembly onto the isodesmic chemical equilibria of an exactly solvable one-dimensional model. In particular, the concentration dependence of the aggregation number is seen to be dominated by one of two different regimes, depending on the presence and strength of a solvophobic solute core. The amphiphilic regime is associated with a large value of the Henry Law constant for solvating the two ends of a linear aggregate. Here, the aggregation is driven by solvent “pressure” and the aggregation number rises rapidly with increasing concentration to quickly saturate at its high concentration value. In the opposite regime, the linear self-assembly is driven by solute–solute attraction and the aggregation number displays the well-known square-root-concentration form. This analysis defines protocols for identifying and classifying discotic amphiphiles, from experimental data on any aspect of a linear aggregation distribution.
Exact solution of the quantum Calogero–Gaudin system and of its q deformation
Authors: Musso Fabio , Ragnisco Orlando
Published: July 2002

A complete set of commuting observables for the Calogero–Gaudin system is diagonalized, and the explicit form of the corresponding eigenvalues and eigenfunctions is derived. We use a purely algebraic procedure exploiting the coalgebra invariance of the model; with the proper technical modifications this procedure can be applied to the q-deformed version of the model, which is then also exactly solved.
Selective modification of the Er3+ 4I11/2 branching ratio by energy transfer to Eu3+
Authors: Strohhöfer C. , Kik P. G. , Polman A.
Published: July 2002

We present an investigation of Er3+ photoluminescence in Y2O3 waveguides codoped with Eu3+. As a function of europium concentration we observe an increase in decay rate of the erbium 4I11/2 energy level and an increase of the ratio of photoluminescence emission from the 4I13/2 and 4I11/2 states. Using a rate equation model, we show that this is due to an energy transfer from the 4I11/2→4I13/2 transition in erbium to europium. This increases the branching ratio of the 4I11/2 state towards the 4I13/2 state and results in a higher steady state population of the first excited state of erbium. Absolute intensity enhancement of the 4I13/2 emission is obtained for europium concentrations between 0.1 and 0.3 at. %. In addition, the photoluminescence due to upconversion processes originating from the 4I11/2 state is reduced. Using such state-selective energy transfer the efficiency of erbium doped waveguide amplifiers can be increased.
Homogeneous substrate heating using a CO2 laser with feedback, rastering, and temperature monitoring
Authors: Barrington S. J. , Eason R. W.
Published: July 2002

We report results from a highly versatile and scalable raster-scanned continuous wave CO2 laser heater incorporating a dual wavelength photodiode-based temperature monitoring system with feedback control. The system is able to uniformly heat substrates inside a vacuum chamber, reduce the occurrence of substrate fracture and allow almost negligible heat loss to the deposition chamber and surroundings. The minimum achievable temperature difference across the substrate is, in practice, limited by the fastest achievable raster scan speed.
Linear and nonlinear Rayleigh–Bénard–Marangoni instability with surface deformations
Authors: Regnier V. C. , Dauby P. C. , Lebon G.
Published: July 2002

Thermoconvective instabilities in a bilayer liquid–gas system with a deformed interface are investigated. In the first part of the work which is devoted to a linear approach, emphasis is put on the role of the upper gas layer on the instability phenomenon. The condition to be satisfied by the gas to remain purely conductive is established. The so-called Oberbeck–Boussinesq approximation is discussed and its range of validity is carefully defined. Instead of the classical Rayleigh, Marangoni, crispation, and Galileo numbers, new dimensionless groups are introduced. A critical comparison with several previous works is made. The nonlinear analysis consists in studying the different convective patterns which can appear above the threshold. Particular attention is devoted to the shape of the interface and the so-called “hybrid” relief. The amplitude of the deformation is also determined and comparison with experimental data is discussed.
Surface charge induced modifications of the structure and dynamics of mixed dipolar liquids at solid–liquid interfaces: A molecular dynamics simulation study
Authors: Senapati Sanjib , Chandra Amalendu
Published: July 2002

Molecular dynamics simulations are carried out to investigate the structural and dynamical properties of binary Stockmayer liquids near charged solid surfaces at varying surface charge density. The two solvent components differ widely in their polarity. The dipolar mixtures are formed at varying composition and the properties of the interfacial molecules are calculated in terms of several equilibrium and dynamical quantities such as the number density and polarization profiles, electrostriction at surfaces, linear and angular velocity autocorrelation functions, perpendicular (z) and parallel (x,y) components of translational diffusion tensors and rotational diffusion coefficients. The extent of selective adsorption of one species against the other at the surfaces is investigated as a function of surface charge density and composition and its effects on translational and rotational diffusion of interfacial molecules are discussed. The dynamical properties of the interfaces are also compared with those of the bulk.
The role of electrode material and polarization fatigue on electron emission from ferroelectric Pb(ZrxTi1−x)O3 cathodes
Authors: Angadi M. , Auciello O. , Krauss A. R. , Gundel H. W.
Published: July 2002

Electron emission from bulk Pt/PZT/Pt and RuO2/PZT/RuO2 ferroelectric cathodes is reported. The total emission current (0.8 A) and polarization fatigue are high for ferroelectric cathodes with Pt electrodes, while emission current (0.18 A) and polarization fatigue are significantly lower for cathodes with RuO2 electrodes. The data correlate with earlier results from research on ferroelectric memories, which suggest that oxygen vacancies near the electrode/ferroelectric interface and through the bulk of the ferroelectric layer play a major role in controlling the polarization switching of PZT-based capacitors and consequently in controlling the electron emission process in PZT cathodes. The strong correlation between polarization fatigue and emission observed in the present investigation provides evidence for the role of polarization inversion on the electron emission phenomenon. In addition, the data provide evidence for the role that oxygen vacancies play in controlling electron emission from PZT cathodes.
Force measurement in a nanomachining instrument
Authors: Gao Wei , Hocken Robert J. , Patten John A. , Lovingood John
Published: July 2002

Two miniature, high sensitivity force transducers were employed to measure the thrust force along the in-feed direction and the cutting force along the cross-feed direction in a nanomachining instrument. The instrument was developed for conducting fundamental experiments of nanocutting especially on brittle materials. The force transducers of piezoelectric quartz type can measure machining forces ranging from 0.2 mN to 10 N. The submillinewton resolution makes it possible to measure the machining forces in the cutting experiments with depths of cut as small as the nanometer level. The stiffness and resonant frequency of the force transducers are 400 mN/nm and 300 kHz, respectively, which meet the specification of the instrument. A force transducer assembly is designed to provide a mechanism to adjust the preload on the force transducer and to decouple the measurement of forces. The assembly consists of three dual-axis circular flexures and a subframe. The axial stiffness of the flexures is designed to be greater than 6×107 N/m and the lateral stiffness of the flexures is designed to be 1×106 N/m to provide proper decoupling of forces.
Role of atomic charge transfer on sintering of TiO2 nanoparticles: Variable-charge molecular dynamics
Authors: Ogata Shuji , Iyetomi Hiroshi , Tsuruta Kenji , Shimojo Fuyuki , Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya
Published: July 2002

The stability of surface structure and space-charge distribution in free TiO2 nanoparticles are investigated for both rutile and anatase phases using a variable-charge interaction potential in which atomic charges vary dynamically depending on their environment. We find the dynamic charge transfer: (i) enhances atomic diffusion at surfaces of the spherical nanoparticles at high temperatures; and (ii) creates additional repulsive force between the two nanospheres through formation of a double-charge surface layer in each nanosphere. The surface diffusion due to the charge transfer clearly distinguishes the two nanospheres with different underlying crystalline structures; the surface diffusion constant of the anatase sphere is almost three times as great as that of the rutile sphere. Variable-charge molecular dynamics simulations are then applied to sintering of two TiO2 nanospheres. It turns out that the enhanced surface diffusion in the anatase nanosphere at high temperatures significantly promotes neck formation between the two spheres.
Broad spectral sensitization of organic photovoltaic heterojunction device by perylene and C60
Authors: Feng W. , Fujii A. , Lee S. , Wu H. , Yoshino K.
Published: July 2002

The characteristics of heterojunction devices consisting of indium-tin-oxide (ITO)/poly(3-alkylthiophene) (PAT)/N,N′-diphenyl glyoxaline-3, 4, 9, 10-perylene tetracarboxylic acid diacidamide (PV)/Al, ITO/PAT-PV/PV/Al, and ITO/PAT-PV-C60/PV/Al, which are typical photoinduced rectifiers, are reported. The device under dark conditions is almost insulating; however, when exposed to light marked rectification and strong photoresponse over a broad spectral range covering near-infrared to visible and UV wavelengths are observed. The effect of PV and C60 doping as well as annealing on spectral photoresponse and photoelectric conversion was investigated. The results show that the efficient photoinduced electron transfer from PAT to PV and C60 occurs at the interface between the composite and junction.
Size control of carbon-induced Ge quantum dots
Authors: Beyer A. , Müller E. , Sigg H. , Stutz S. , Grützmacher D. , Leifeld O. , Ensslin K.
Published: July 2002

Self-assembled C-induced Ge dots are islands which are not formed by the Stranski–Krastanov mode of growth. They are formed by a three-dimensional mode originating from the undulating strain fields of the C alloyed Si (100) surface. This opens additional possibilities to control the size and the shape of these dots by modifying the strain fields of the C-alloyed Si surface. Here, we show that the amount of C deposited prior to the growth of the Ge islands strongly effects the diameter and height of the dots. Increasing the C coverage to 0.3 monolayer leads to the formation of comparably compact islands. Consequently, the photoluminescence of the dots is shifted to lower energies compared to dots grown with lower C coverages.
Formation, structure and bond dissociation thresholds of gas-phase vanadium oxide cluster ions
Authors: Bell R. C. , Zemski K. A. , Justes D. R. , Castleman A. W.
Published: July 2002

The formation and structure of gas-phase vanadium oxide cluster anions are examined using a guided ion beam mass spectrometer coupled with a laser vaporization source. The dominant peaks in the anion total mass distribution correspond to clusters having stoichiometries of the form (VO2)n(VO3)m(O2)q−. Collision-induced dissociation studies of the vanadium oxide species V2O4–6−, V3O6–9−, V4O8–10−, V5O11–13−, V6O13–15−, and V7O16–18− indicate that VO2, VO3, and V2O5 units are the main building blocks of these clusters. There are many similarities between the anion mass distribution and that of the cation distribution studied previously. The principal difference is a shift to higher oxygen content by one additional oxygen atom for the stoichiometric anions (VxOy−) as compared to the cations with the same number of vanadium atoms, which is attributed to the extra pair of electrons of the anionic species. The oxygen-rich clusters, VxOy(O2)−, are shown to more tightly adsorb molecular oxygen than those of the corresponding cationic clusters. In addition, the bond dissociation thresholds for the vanadium oxide clusters ΔE(V+–O)=6.09±0.28 eV, ΔE(OV+–O)=3.51±0.36 eV, and ΔE(O2V−–O)=5.43±0.31 eV are determined from the energy-dependent collision-induced dissociation cross sections with Xe as the collision partner. To the best of our knowledge, this is the first bond dissociation energy reported for the breaking of the V–O bond of a vanadium oxide anion.
Laser-based apparatus for extended ultraviolet femtosecond time-resolved photoemission spectroscopy
Authors: Siffalovic P. , Drescher M. , Spieweck M. , Wiesenthal T. , Lim Y. C. , Weidner R. , Elizarov A. , Heinzmann U.
Published: July 2002

A novel laser-based apparatus is presented utilizing high harmonic radiation for visible pump–EUV probe experiments on ultrafast processes. True femtosecond temporal resolution is achieved by a monochromator making use of dedicated narrowband multilayer mirrors rather than gratings for selection of single harmonic orders in the photon energy range between 66 and 73 eV. First applications of this new light source for electron spectroscopy on gas phase helium and xenon demonstrate the selection of a single high harmonic order with the intensity ratio between the selected and its adjacent harmonic not exceeding 10:1. A pump–probe study of hot electron production on a solid Pt(110) surface yields a cross-correlation corresponding to a temporal system resolution of 100 fs.
CF, CF2, and SiF densities in inductively driven discharges containing C2F6, C4F8, and CHF3
Authors: Hebner G. A.
Published: July 2002

Laser induced fluorescence was used to measure the spatially resolved CF, CF2, and SiF radical density in inductively driven discharges containing fluorocarbon gases. Measurements of the spatially resolved CF density were performed in C2F6, and CHF3 containing discharges as functions of inductive power, pressure and bias condition on a silicon substrate. In addition, CF rotational temperatures were calculated, assuming saturated spectra. Measurements of the spatially resolved CF2 and SiF density were performed in C4F8, C2F6, and CHF3 containing discharges as functions of inductive power, pressure and bias condition. SiF rotational temperatures were also estimated. As the induction coil power was increased, the SiF density in the center (r=0 cm) increased while the CF2 density decreased and the CF density slightly decreased. In all cases, the radical density in the center of the glow increased with pressure changes from 5 to 30 mTorr while changes in the bias power had little influence on any of the measured radical densities. The spatial distribution of the CF and SiF density peaked in the center of the discharge. The CF2 density had a local maximum in the center of the plasma with a decreasing density at the edge of the glow. However, the CF2 density outside the glow region was a factor of 2–6 higher than the density inside the glow region, depending on the gas. CF and SiF rotational temperatures were between 450 and 750 K.
Study of charge control and gate tunneling in a ferroelectric-oxide-silicon field effect transistor: Comparison with a conventional metal-oxide-silicon structure
Authors: Lin Yih-Yin , Zhang Yifei , Singh Jasprit , York Robert , Mishra Umesh
Published: July 2002

It is known that conventional metal-oxide-silicon (MOS) devices will have gate tunneling related problems at very thin oxide thicknesses. Various high-dielectric-constant materials are being examined to suppress the gate currents. In this article we present theoretical results of a charge control and gate tunneling model for a ferroelectric-oxide-silicon field effect transistor and compare them to results for a conventional MOS device. The potential of high polarization charge to induce inversion without doping and high dielectric constant to suppress tunneling current is explored. The model is based on a self-consistent solution of the quantum problem and includes the ferroelectric hysteresis response self-consistently. We show that the polarization charge associated with ferroelectrics can allow greater controllability of the inversion layer charge density. Also the high dielectric constant of ferroelectrics results in greatly suppressed gate current.
Virtual instrument for testing the hearing impaired
Authors: Norian K. H.
Published: July 2002

LABVIEW programing was used to build a virtual instrument to assess the needs of individual hearing impaired subjects to enable them to hear speech in background noise. The instrument acquires the noisy speech, adaptively removes noise from speech, and then feeds the speech to the subject. Design changes can quickly and easily be made to the instrument to assess the specific requirements of the electronic circuit of the hearing aid for the individual subject. The instrument provides a novel way of providing custom electronics for hearing aids.
Superintegrable systems in quantum mechanics and classical Lie theory
Authors: Sheftel M. B. , Tempesta P. , Winternitz P.
Published: July 2002

The relation is established between some concepts of quantum mechanics and those of soliton theory. In particular, superintegrable systems in two-dimensional quantum mechanics are shown to be invariant under generalized Lie symmetries and to allow recursion operators.
Diagram projection rules for recoupling diagrams in the Racah–Wigner category
Authors: Joyce William P.
Published: July 2002

This paper gives three important techniques for recoupling coefficients of an arbitrary number or irreducible representations: factorization as a product of smaller degree recoupling coefficients, diagram projection projecting a recoupling diagram to obtain constraints on recoupling coefficients, and primitive expansion expressing a recoupling coefficient in terms of primitive recoupling coefficients. The first technique reduces a recoupling coefficient to its essential and simplest form. The second is many faceted allowing one to work at the recoupling level. Importantly, recoupling diagrams can be used to construct a complete set of constraints on the recoupling coefficients. The third allows large degree recoupling coefficients to be expressed in terms of primitive recoupling coefficients. An example of this is the fact that the 9j symbol is defined by the 3j and 6j symbols.
Magnetic-field-dependent plasma composition of a pulsed aluminum arc in an oxygen ambient
Authors: Schneider Jochen M. , Anders André , Yushkov George Yu.
Published: July 2002

A variety of plasma-based deposition techniques utilize magnetic fields to affect the degree of ionization as well as for focusing and guiding of plasma beams. Here we use time-of-flight charge-to-mass spectrometry to describe the effect of a magnetic field on the plasma composition of a pulsed Al plasma stream in an ambient containing intentionally introduced oxygen as well as for high vacuum conditions typical residual gas. The plasma composition evolution was found to be strongly dependent on the magnetic field strength and can be understood by invoking two electron impact ionization routes: ionization of the intentionally introduced gas as well as ionization of the residual gas. These results are characteristic of plasma-based techniques where magnetic fields are employed in a high-vacuum ambient. In effect, the impurity incorporation during reactive thin-film growth pertains to the present findings.
A novel Y-type reactor for selective excitation of atmospheric pressure glow discharge plasma
Authors: Xia Guan-Guang , Wang Jin-Yun , Huang Aimin , Suib Steven L. , Hayashi Yuji , Matsumoto Hiroshige
Published: July 2002

A novel Y-type atmospheric pressure ac glow discharge plasma reactor has been designed and tested in CO reduction with hydrogen and the reverse water–gas shift reaction. The reactor consists of a Y-type quartz tube with an angle of 120°–180° between the two long arms, two metal rod electrodes serving as high voltage terminals and two pieces of aluminum foil which were wrapped outside of the quartz tubes as a ground electrode. Different combinations of input power applied on this three- electrode system can lead to selective plasmas on one side, two sides, or can also generate a stable arc between the two high voltage terminal electrodes. The ability to selectively activate different species with this type of apparatus can help to minimize side reactions in plasmas to obtain desirable products. The Y-type reactor may provide a novel means to study fundamental problems regarding radical reactions.
Optical properties of single carrier polymer diodes under high electrical injection
Authors: Campbell I. H. , Smith D. L. , Neef C. J. , Ferraris J. P.
Published: July 2002

We present measurements of the modulation of the near absorption edge optical properties of single carrier diodes made from a soluble poly(p-phenylene vinylene) derivative. Pulsed electrical excitation was used to acquire the optical characteristics for average injected charge densities up to 1018 cm−3. For injected carrier densities below 5×1017 cm−3, the modulation of the optical properties are dominated by electroabsorption effects; at higher densities, the effects of the injected charge predominate. We find that there is no significant absorption by the injected charge below the optical absorption threshold and that there is substantial bleaching of the lowest energy optical transitions at high carrier density. These results imply that charge carrier absorption in this polymer does not limit the performance of organic light-emitting diodes and diode lasers.
Mirrorless optical bistability of an ultrathin glassy film built up of oriented J-aggregates: Effects of two-exciton states and exciton–exciton annihilation
Authors: Glaeske H. , Malyshev V. A. , Feller K.-H.
Published: July 2002

We theoretically analyze the optical response of an ultrathin film with accounting for the effects of creating two exciton states as well as their annihilation in an individual aggregate. We show that these two processes do not violate mirrorless optical bistability of the film found in the framework of the ground-to-one exciton state transitions but, contrary to that, act towards improving the conditions for bistability to occur. In particular, the killing action of inhomogeneous broadening of the one-exciton absorption spectrum may be considerably reduced due to the exciton–exciton annihilation process giving hope for creating a unit for an all-optical device based on such systems.
Temperature in the classical microcanonical ensemble
Authors: Rickayzen Gerald , Powles Jack G.
Published: July 2002

We show that a formula for the temperature of a classical system, originally given by Rugh, can be generalized. The result is that the inverse of the temperature is proportional to the average of the function ∇.(B/B.∇H), where H is the Hamiltonian of the system and B is an arbitrary vector function of the coordinates in phase space. Special cases of the new formula include a number of familiar results including the virial theorem.
Low-temperature kinetics of exciton–exciton annihilation of weakly localized one-dimensional Frenkel excitons
Authors: Ryzhov I. V. , Kozlov G. G. , Malyshev V. A. , Knoester J.
Published: July 2002

We present results of numerical simulations of the kinetics of exciton–exciton annihilation of weakly localized one-dimensional Frenkel excitons at low temperatures. We find that the kinetics is represented by two well-distinguished components: a fast short-time decay and a very slow long-time tail. The former arises from excitons that initially reside in states belonging to the same localization segment of the chain, while the slow component is caused by excitons created on different localization segments. We show that the usual bimolecular theory fails in the description of the behavior found. We also present a qualitative analytical explanation of the nonexponential behavior observed in both the short- and the long-time decay components. Finally, it is shown that our theoretical estimate for the annihilation time of the fast component is in good agreement with data obtained from transient absorption experiments on J-aggregates of pseudoisocyanine.
Migration of a van der Waals bubble: Lattice Boltzmann formulation
Authors: Holdych D. J. , Georgiadis J. G. , Buckius R. O.
Published: July 2002

A numerical study of the isothermal migration of a two-dimensional bubble in Poiseuille flow is reported here for vapor–liquid density and dynamic viscosity ratios of 1/8, Red=1, and Ca=2. A lattice Boltzmann model with a van der Waals equation of state is employed to simulate the diffuse interface for three interface thickness to bubble diameter ratios, 1/5, 1/10, and 1/20. Point-by-point comparisons with the sharp-interface incompressible counterpart (reported in the literature) reveal velocity discrepancies which are more evident on the vapor side. These differences are a manifestation of a finite mass flux through the interface, associated with driven finite–thickness interfaces. An analytical study of the one-dimensional analog of the traveling diffuse interface problem explains this phenomenon and shows that this flux vanishes as a result of viscous dissipation as the interface thickness tends to zero. This trend is corroborated by the two-dimensional lattice Boltzmann results.
Domain wall structure in Permalloy films with decreasing thickness at the Bloch to Néel transition
Authors: Trunk T. , Redjdal M. , Kákay A. , Ruane M. F. , Humphrey F. B.
Published: July 2002

The Bloch to Néel wall transition is investigated in Permalloy films between 160 and 10 nm thickness using direct integration of the Landau–Lifshitz–Gilbert equation in a three-dimensional Cartesian lattice. At 80 nm, the wall is a symmetric Bloch wall characterized by two adjoining vortices with the magnetization at the wall center pointing perpendicular to the plane of the material throughout the thickness. The Bloch to Néel transition takes place between 35 and 30 nm, below which the wall becomes a symmetric Néel wall. For the Bloch walls, our wall energy per unit area calculations match reasonably well the results of A. Hubert’s Ritz method calculations [Magnetic Domains (Springer, New York, 1998), p. 251] and A. E. Labonte’s numerical calculations [J. Appl. Phys. 40, 2450 (1969)]. For the Néel walls, however, our results indicate an approximately 70% higher energy for thicknesses of 30 nm and below, since the Néel wall tails are included. For thicknesses below 160 nm, the anisotropy energy component is low, and both C-shaped and symmetric Bloch walls are dominated by exchange interaction. As the wall transforms from Bloch to Néel below 35 nm, the energy contribution changes from 76% exchange and 24% demagnetization to 70% demagnetization and 30% exchange, respectively. Wall widths are computed for thicknesses between 10 and 640 nm along with the out-of-plane magnetization due to the presence of the vortex.
Far-wing scattering studies on the reaction Li*(2p,3p)+H2→LiH(v″=1,2,J″)+H
Authors: Bililign Solomon , Hattaway Brian C. , Robinson Tito L. , Jeung Gwang-Hi
Published: July 2002

We did laser pump–probe far-wing scattering experiments to study the photochemical reactions Li*(2p,3p)+H2→LiH(v″=1,2,J″)+H. We show in this work that the Li(2p)+H2⇒LiH+H reaction takes place when the kinetic energy of the reactants are high enough. We think that the Li(2p)+H2 collisions take place preferentially in bent near-C2v geometry and that the LiH2 2A′ intermediate plays a major role where wide-amplitude internal vibrations eventually result in losing a hydrogen atom from the LiH2 complex. We also have observed the Li(3p)+H2⇒LiH+H reaction. The rotational state distribution of the LiH product for excitation in the red wing is identical to that found in the blue wing region, it does not show any preference to high or low rotational states, and no asymmetry is observed in the reactive to nonreactive branching ratio. We show different possibilities for the reactive and nonreactive collisions and provide a tentative explanation of the reaction mechanism using the highly accurate ab initio potential energy surfaces. We also show that the harpooning model cannot be used to explain the reaction mechanism in the Li*+H2 collision for all the atomic states from Li(2s) to Li(3d), and that no long range electron transfer occurs from the metal atom to the hydrogen molecule.