bluuebunny/crossref_metadata_2025 huggingface dataset:
Intraoperative blood salvage: a mathematical perspective
Authors: Hay Shauna N. , Monk Terri G. , Brecher Mark E.
Published: July 2002

BACKGROUND: Intraoperative blood salvage (IBS) is one of several blood‐conservation strategies employed to limit the use of allogeneic blood. IBS with mathematical modeling was studied to gain a better understanding of its benefits and limitations.

STUDY DESIGN AND METHODS: Equations, based on a convergent geometric series, allowed the assessment of maximum allowable blood losses to avoid allogeneic RBC exposure in relationship to the fractional recovery of IBS. Such equations allowed us to calculate the potential “savings” in allogeneic RBC volume and to define better the efficiency of IBS.

RESULTS: This model predicts that substantial amounts of RBCs (>2 units) can be recovered if there is large blood loss (e.g., >3000 mL) and if the minimal allowable Hct is low (e.g., 21%). As with other blood‐ conservation methods, this article shows that there are limits to the potential benefit of IBS for the avoidance of allogeneic RBC transfusion.

CONCLUSION: IBS can be modeled with a convergent geometric series, and the amount of blood loss is dependent on the difference between the patient's starting Hct, the minimum allowable Hct, and the fractional recovery of RBCs. Such modeling illustrates the limited utility of IBS as a sole blood‐conservation strategy.
Calculating electron current in a tight-binding model of a field-driven molecular wire: Application to xylyl-dithiol
Authors: Tikhonov Alexander , Coalson Rob D. , Dahnovsky Yuri
Published: July 2002

A recently developed Floquet theory-based formalism for computing electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic ac radiation field is applied to an experimentally relevant system, namely a xylyl–dithiol molecule in contact at either end with gold electrodes. In this treatment, a nondissipative tight-binding model is assumed to describe the conduction of electric current. Net current through the wire is calculated for two configurations of the electrode–wire–electrode system. In one, symmetric, configuration, the electrodes are close (∼2 Å) and equidistant from the bridge molecule. In the other, asymmetric configuration, one electrode is farther away (∼5 Å), representing the tip of a scanning tunneling microscope located at this distance from the bridge molecule (the other end being chemisorbed to a gold substrate). For both configurations, electron current is calculated for a range of experimental inputs, including dc bias and the intensity and frequency of the laser. Via absorption/emission of photons, resonant conditions may be achieved under which electron transport is significantly enhanced compared to the unilluminated analog. Calculations show that this can be accomplished with experimentally accessible laser field strengths.
Effects of small-scale turbulent motions on the filtered velocity gradient tensor as deduced from holographic particle image velocimetry measurements
Authors: van der Bos Fedderik , Tao Bo , Meneveau Charles , Katz Joseph
Published: July 2002

The effects of small-scale motions on the inertial range structure of turbulence are investigated by considering the dynamics of the velocity gradient tensor (Ãij) filtered at scale Δ. In addition to self-interactions and the filtered pressure Hessian, the evolution of Ãij is determined by the subgrid-scale stress tensor. As in the so-called restricted Euler dynamics, the evolution equations can be simplified by considering the invariants RΔ and QΔ of Ãij. The effects of the subgrid-scale stress tensor on RΔ and QΔ can be quantified unambiguously by evaluating conditional averages that appear in the evolution equation for the joint probability distribution function of these invariants. The required conditional averages are computed from three-dimensional measurements of fully developed turbulence in a square duct, at Reτ≈2360. The measurements are performed using holographic particle image velocimetry [Tao et al., Phys. Fluids 12, 941 (2000); Tao et al., J. Fluid Mech. 457, 35 (2002)]. The velocity distributions are spatially filtered in the inertial range using a box filter at about 30 Kolmogorov scales to separate large from small scales. The results show that the subgrid scale (SGS) stresses have significant effect on the evolution of filtered velocity gradients. In particular, along the so-called Vieillefosse tail at RΔ>0 and QΔ<0, they oppose the formation of a finite-time singularity that occurs in restricted Euler dynamics. Various other trends are quantified in different parts of the (RΔ,QΔ) plane. Included are the SGS dissipation rate of kinetic energy, and the effect of the SGS stress in modifying the so-called “discriminant,” which is a conserved quantity in restricted Euler dynamics. A priori tests of the Smagorinsky, nonlinear, and mixed models show that all reproduce the real SGS stress effect along the Vieillefosse tail, but that they fail in several other regions. An attempt is made to optimize the mixed model by letting the two coefficients be functions of RΔ and QΔ.
Chiroptical properties from time-dependent density functional theory. II. Optical rotations of small to medium sized organic molecules
Authors: Autschbach Jochen , Patchkovskii Serguei , Ziegler Tom , van Gisbergen Stan J. A. , Jan Baerends Evert
Published: July 2002

We report an implementation for the computation of optical rotations within the Amsterdam Density Functional program package. The code is based on time-dependent density functional response theory. Optical rotations have been calculated for a test set of 36 organic molecules with various density functionals, and employing basis sets of different quality. The results obtained in this work with nonhybrid functionals are comparable in quality to those recently reported by other authors for the B3LYP hybrid functional, but show a somewhat larger tendency to produce outlyers. The median error is approximately 20°/(dm g/cm3) for specific rotations [α]D as compared to experimental data (approximately 30% median deviation from experimental values). Thereby it is demonstrated that density functional computations can be employed to assist with the solution of stereochemical problems in case the specific rotations of the species involved are not small and their structures are rigid. Recent newly developed functionals are investigated with respect to their applicability in computations of optical rotations.
On the higher order mixed structure functions in laboratory shear flow
Authors: Warhaft Z. , Shen X.
Published: July 2002

Structure functions to order 8 are determined from the longitudinal and transverse velocity fluctuations measured at a single point in high Reynolds number laboratory shear flow. We examine the relationship between mixed structure functions (which are explicitly zero in isotropic flow), and those of the longitudinal and transverse structure functions. Our results confirm recent findings using SO(3) symmetry decomposition [S. Kurien and K. R. Sreenivasan, Phys. Rev. E 62, 2206 (2000)] that show that the mixed structure functions have larger scaling exponents than those of the single component structure functions, i.e., the j=2 anisotropic sector structure function scaling exponent is greater than that of the j=0 isotropic sector. However, we also show that the ratio of the j=2 to j=0 sector scaling exponents systematically decreases with increasing order, suggesting that at higher orders there is persistence of anisotropy at the smallest scales. Difficulties in the analysis due to the difference in scaling exponents of the transverse and longitudinal structure functions are addressed.
Ablative generation of surface acoustic waves in aluminum using ultraviolet laser pulses
Authors: Gospodyn J. P. , Sardarli A. , Brodnikovski A. M. , Fedosejevs R.
Published: July 2002

Surface acoustic wave generation using ablative excitation with ultraviolet laser pulses is studied experimentally in order to quantify the magnitude of response. The acoustic waves are interferometrically measured on the surface of polished aluminum samples at distances of 5–40 mm from the excitation spot. Amplitudes of the surface acoustic waves are measured as a function of laser energy (1–60 mJ) and distance from the excitation spot yielding peak-to-peak displacements of 1–40 nm. The results are compared to theoretical calculations using the Green’s function for a step impulse response. The impulse required to match the measured results is compared to the expected impulse based on plasma pressure scaling in the ablative regime and the experimentally measured response is found to agree within a factor of 2 with the theoretical predicted impulse.
Donor impurity on-center and off-center in multilayered quantum wires in the presence of magnetic field
Authors: Hsieh Cheng-Ying
Published: July 2002

The binding energies of a hydrogenic impurity located at the center and off-center of a multilayered quantum wire (MLQW) in the presence of magnetic field are studied within the framework of the effective-mass approximation. The MLQW consists of a GaAs core wire coated by a AlxGa1−xAs cylindrical shell and embedded in the bulk AlyGa1−yAs. A variational trial wave function is proposed. It is found for a small wire radius that the ground state binding energy of a hydrogenic impurity located at the center of a MLQW behaves very differently from that of a single-layered quantum wire (SLQW). The calculation shows that the binding energy depends on the potential profiles, potential barrier height, impurity position, shell thickness, magnetic field, and the difference between the Al concentration contained in the shell and bulk regions. Our trial function is also able to reproduce the binding energies of a hydrogenic impurity located at the center of a SLQW, good agreement with the previous results is obtained.
Self-organization of quasi-two-dimensional turbulence in stratified fluids in square and circular containers
Authors: Maassen S. R. , Clercx H. J. H. , van Heijst G. J. F.
Published: July 2002

Laboratory experiments on decaying quasi-2D (two-dimensional) turbulence have been performed in stratified fluids in both square and circular containers. The turbulence was generated by towing an array of vertical cylinders through the container, which was filled with either a two-layer or a linearly stratified fluid. By varying the grid configuration a different amount of angular momentum could be added to the initial flow. The evolution of the flow was visualized by 2D particle tracking velocimetry. The observed decay scenario has been investigated with emphasis on the evolution of the kinetic energy and the enstrophy of the horizontal flow, vortex statistics and the angular momentum of the flow. In particular it is shown that the experiments in both the square and the circular container support the observations from numerical simulations of decaying 2D turbulence in bounded domains with no-slip walls. Two striking examples are the experimental observation of the spontaneous spin-up phenomenon (in the square-container experiments) and the confirmation that the angular momentum of the flow in the circular-container experiment is better conserved than the total kinetic energy of the flow. The role of the initial nonzero net angular momentum on the decay of quasi-2D turbulence is investigated for both geometries and indications for an acceleration of the self-organization process are presented.
Thermomagnetic convection of magnetic fluids in a cylindrical geometry
Authors: Lange Adrian
Published: July 2002

The thermomagnetic convection of magnetic fluids in a cylindrical geometry subjected to a homogeneous magnetic field is studied. The study is motivated by a novel thermal instability [W. Luo et al., Phys. Rev. Lett. 82, 4134 (1999)]. As model system a composite cylinder with inner heating is considered which reflects the symmetry of the experimentally setup. The general condition for the existence of a potentially unstable stratification in the magnetic fluid is derived. Within a linear stability analysis the critical external induction for the onset of thermomagnetic convection is determined for dilute and nondilute magnetic fluids. The difference between both thresholds allows to test experimentally whether a test sample is a dilute fluid or not.
Transverse shock wave demagnetization of Nd2Fe14B high-energy hard ferromagnetics
Authors: Shkuratov Sergey I. , Talantsev Evgueni F. , Dickens James C. , Kristiansen Magne
Published: July 2002

The action of transverse shock waves (the shock wave propagates across the magnetization vector M) on the magnetic phase state of a Nd2Fe14B high-energy hard ferromagnetic was investigated experimentally. The design of the ferromagnetic sample, which was made as a hollow cylinder, has made it possible to dramatically reduce the amount of the explosive that initiates a transverse shock wave in Nd2Fe14B to 1.0 g (for Nd2Fe14B samples weighing 67.5 g). The results of the experiment have shown that the transverse shock wave propagating through Nd2Fe14B causes “hard ferromagnetic-to-paramagnetic” phase transformation terminating by practically complete demagnetization of Nd2Fe14B. Pulse generators based on the transverse shock wave demagnetization of hollow cylindrical Nd2Fe14B samples with diameter of 25.4 mm and length of 19.1 mm are capable of producing high-voltage pulses [peak voltage of 11.3 kV, full width at half maximum (FWHM) of 4.5 μs] and high-current pulses (peak current of 1.93 kA, FWHM of 100 μs, peak power of 27.0 kW). The effect of transverse shock wave demagnetization of high-energy hard ferromagnetic, Nd2Fe14B, was detected.
Spontaneous dissociation of a conjugated molecule on the Si(100) surface
Authors: Lin R. , Galili M. , Quaade U. J. , Brandbyge M. , Bjørnholm T. , Esposti A. Degli , Biscarini F. , Stokbro K.
Published: July 2002

The adsorption mechanism of α-sexithiophene (α-6T) on the clean Si(100)-(2×1) surface has been investigated using scanning tunneling microscopy (STM) and first principles electronic structure calculations. We find that at submonolayer coverage, the α-6T molecules are not stable and dissociate into monomers. We observe two different configurations of the monomers and have discussed the corresponding adsorption geometries based on theoretical calculations. The calculations elucidate how the fragments are absorbed on the surface, giving rise to the observed STM images. With increasing coverage, the STM images show the existence of complete α-6T molecules. In addition, results of the adsorption behavior of α-6T molecules on the H-passivated Si(100)-(2×1) surface are reported. On this surface the molecules are highly mobile at room temperature due to the weak molecule-substrate interaction. The STM results also indicate that they can easily be anchored at the defect sites.
Canonical Monte Carlo simulations of the fluctuating-charge molecular water between charged surfaces
Authors: Yang Kun-Lin , Yiacoumi Sotira , Tsouris Costas
Published: July 2002

Monte Carlo simulations have been performed to study the arrangement of water molecules between charged surfaces. The molecular water used—the four- point transferable intermolecular potential (TIP4P), together with fluctuating charge (FQ)—constitutes the water model TIP4P-FQ. The TIP4P-FQ features the same molecular frame as the TIP4P model; however, with the former, charges on the molecular sites of the same molecule are allowed to fluctuate in response to their environment. Therefore, the induced dipole moment under an external field is inherently considered in the TIP4P-FQ water model. This feature makes it more useful than multipoint fixed-charge models, such as the TIP4P, in studying water arrangement between charged surfaces. The formation of a few ordering layers of water molecules adjacent to a charged surface is confirmed by (N,V,T) Monte Carlo simulations. The preferable orientation of water molecules in this study is found to be similar to previous findings in the literature, with the oxygen atoms pointing to the positively charged surface (oxygen-down) and the hydrogen atoms pointing to the negatively charged surface (oxygen-up). Modeling results also demonstrate that the dominant orientation at the negatively charged surface is one O–H bond pointing to the surface while another O–H bond is aligned parallel to the charged surface. In addition, simulation results using the TIP4P-FQ water model show that the water density in the interfacial region increases ∼8%–14%, which is lower than the density increase observed in experimental data discussed in the literature.
Confined water in the low hydration regime
Authors: Gallo P. , Rapinesi M. , Rovere M.
Published: July 2002

Molecular dynamics results on water confined in a silica pore in the low hydration regime are presented. Strong layering effects are found due to the hydrophilic character of the substrate. The local properties of water are studied as function of both temperature and hydration level. The interaction of the thin films of water with the silica atoms induces a strong distortion of the hydrogen bond network. The residence time of the water molecules is dependent on the distance from the surface. Its behavior shows a transition from a Brownian to a non-Brownian regime approaching the substrate in agreement with results found in studies of water at contact with globular proteins.
A second-order, quadratically convergent multiconfigurational self-consistent field polarizable continuum model for equilibrium and nonequilibrium solvation
Authors: Cammi Roberto , Frediani Luca , Mennucci Benedetta , Tomasi Jacopo , Ruud Kenneth , Mikkelsen Kurt V.
Published: July 2002

We present a new implementation of the polarizable continuum model (PCM) at the multiconfigurational self-consistent field (MCSCF) level. This new MCSCF-PCM implementation is based on a second-order algorithm for the optimization of the wave function of the solvated molecule. The robust convergence properties of this approach allows for fast convergence of the PCM-MCSCF wave function for large MCSCF expansions as well as for excited states of solvated molecules. Our implementation also enables us to treat nonequilibrium solvation—that is, to treat excited molecular states generated in a fast excitation process such that not all degrees of freedom of the solvent have relaxed. To demonstrate the applicability of the approach we present calculations of solvation effects on 1,3-dipolar systems and on the ground and excited states of methylenecyclopropene.
Evanescent microwave probe measurement of low-k dielectric films
Authors: Wang Zhengyu , Kelly Michael A. , Shen Zhi-Xun , Wang Gang , Xiang Xiao-Dong , Wetzel Jeffery T.
Published: July 2002

An evanescent microwave probe (EMP) has been used to characterize low-k dielectric films grown on Si wafers at 2 GHz. Several families of low-k dielectric films with varying film thicknesses have been examined. The EMP signal (shift in resonant frequency) was found to scale with the “reduced electrical length,” defined as the film thickness/dielectric constant. The universal functional dependence derived from the experimental results is quantitatively consistent with the results of both analytical modeling and finite element simulations. These results demonstrate the unique capability of EMP to accurately characterize the application relevant parameter of low-k thin films.
Axially dependent dielectric relaxation response of natural hydroxyapatite single crystals
Authors: Kalogeras Ioannis M. , Vassilikou-Dova Aglaia , Katerinopoulou Anna
Published: July 2002

A thermally stimulated depolarization current (TSDC) study of single crystals of natural fluorine-rich hydroxyapatite, polarized with the electric field (Ep) applied parallel and perpendicular to the c axis, has revealed relaxation modes with axially dependent dielectric behavior. The (re)orientation of the OH− ions in the columns and the hydroxyls of the structural H2O molecules give rise to two prominent thermocurrent bands in the temperature range of ∼200–340 K. Several weak current bands, below ∼200 K, are related to the relaxation of impurity–Ca2+ vacancy (I–V) dipoles. The I–V mechanisms are particularly effective in the case where Ep⊥c axis and are considered to take place via cation jumps on planes perpendicular to the [0001] crystal axis, at regions near “distorted” Ca(II) triangles (i.e., triangles which include impurity ions and Ca2+ vacancies). The band attributed to rotational relaxation of the OH− ions in the columns shows particular strength when Ep⊥c axis, and an extensive distribution of relaxation times. The distribution is due to the multiplicity of physicochemical interactions between the OH− ions and their ionic and molecular neighbors (hydrogen bonds with adjacent F−, Cl−, and O2− ions, varying steric hindrances, etc.). The changes induced in the TSDC spectra after annealing processes at selected high temperatures are interpreted based on the gradual loss of structural H2O molecules located in oxygen or calcium vacancies and the resulting local changes in the crystal lattice.
Analysis of polarization propagation along a semiconductor-based quantum cellular automaton chain
Authors: Girlanda M. , Macucci M.
Published: July 2002

We present a simulation of a chain of three quantum cellular automaton (QCA) cells defined in a GaAs/AlGaAs heterostructure by means of depletion gates, focusing on the evaluation of the voltage unbalance that must be applied to the gates to enforce the correct polarization of the driver cell and propagate it through the other two cells. We use the configuration interaction method in each cell, including the electrostatic coupling between cells with an iterative self-consistent procedure. In particular, we investigate the issue, addressed in the recent literature, of chain malfunction due to the adverse effect on the driven cells of the electric field from the gates defining the driver cell. Our conclusion is that, as long as the gate voltage unbalance polarizing the driver cell is smaller than a threshold depending on geometric and material parameters, correct operation of the QCA wire can be obtained.
Femtosecond laser pulse control of electron transfer processes
Authors: Mančal Tomáš , Kleinekathöfer Ulrich , May Volkhard
Published: July 2002

Laser-pulse guided ultrafast electron transfer (ET) is studied theoretically for different types of donor–acceptor systems. The pulse initiates an optical transition from the electronic ground state into an excited state and controls the ET. The computations concentrate on systems where (a) the excited state (donor) is coupled to an acceptor level and where (b) the ET proceeds as an internal conversion from the excited state to the ground state. For both examples the manifold of vibrational coordinates is mapped on a single reaction coordinate coupled to a dissipative reservoir of further coordinates. Utilizing the methods of dissipative quantum dynamics combined with the optimal control (OC) scheme, it is demonstrated that control fields really exist which drive the ET in the required manner. Various properties of the OC algorithm are discussed when applied to dissipative dynamics and a scheme is proposed to avoid pinning in a local extremum.
Cumulative reaction probability by constrained dynamics: H transfer in HCN, H2CO, and H3CO
Authors: Baloı̈tcha E. , Lasorne B. , Lauvergnat D. , Dive G. , Justum Y. , Desouter-Lecomte M.
Published: July 2002

A strategy to obtain quantum corrections to the cumulative reaction probability from a subspace of active coordinates is analyzed. The kinetic energy operator exactly takes into account the constraints due to inactive coordinates. The geometry of the inactive skeleton is adiabatically adjusted to the dynamical variables or simply frozen according to the coupling to the active space. Dynamics is carried out using the curvilinear coordinates of the Z-matrix so that computation of the potential energy surface and dynamics are coupled. The cumulative reaction probability N(E) is obtained directly in a large range of energy by a time independent formulation of the Zhang and Light transition state wave packet method. NnD(E) is first computed in the active n-dimensional space and then convoluted with a bath. The efficiency of the Chebyshev expansion of the microcanonical projection operator δ(E−ĤnD) appearing in the quantum expression of NnD(E) is checked. The method is implemented for the study of tunneling effect in H transfer. The coordinates are three spherical coordinates referred to the frozen or adiabatic skeleton. We compare the quantum corrections brought about by different 2D groups of internal coordinates.
Effects of NH3 and N2 additions to hot filament activated CH4/H2 gas mixtures
Authors: Smith James A. , Wills Jonathan B. , Moores Helen S. , Orr-Ewing Andrew J. , Ashfold Michael N. R. , Mankelevich Yuri A. , Suetin Nikolay V.
Published: July 2002

Resonance enhanced multiphoton ionization and cavity ring down spectroscopies have been used to provide spatially resolved measurements of relative H atom and CH3 radical number densities, and NH column densities, in a hot filament (HF) reactor designed for diamond chemical vapor deposition and here operating with a 1% CH4/n/H2 gas mixture—where n represents defined additions of N2 or NH3. Three-dimensional modeling of the H/C/N chemistry prevailing in such HF activated gas mixtures allows the relative number density measurements to be placed on an absolute scale. Experiment and theory both indicate that N2 is largely unreactive under the prevailing experimental conditions, but NH3 additions are shown to have a major effect on the gas phase chemistry and composition. Specifically, NH3 additions introduce an additional series of “H-shift” reactions of the form NHx+H⇌NHx−1+H2 which result in the formation of N atoms with calculated steady state number densities >1013 cm−3 in the case of 1% NH3 additions in the hotter regions of the reactor. These react, irreversibly, with C1 hydrocarbon species forming HCN products, thereby reducing the concentration of free hydrocarbon species (notably CH3) available to participate in diamond growth. The deduced reduction in CH3 number density due to competing gas phase chemistry is shown to be compounded by NH3 induced modifications to the hot filament surface, which reduce its efficiency as a catalyst for H2 dissociation, thus lowering the steady state gas phase H atom concentrations and the extent and efficiency of all subsequent gas phase transformations.
Structure effects on electron-optical phonon interaction in GaAs/AlxGa1−xAs quantum wells
Authors: Lee H. C. , Sun K. W. , Lee C. P.
Published: July 2002

Based on the dielectric continuum model, we have studied the electron-optical phonon scattering rates in GaAs/AlxGa1−xAs quantum wells with different structure parameters. It was found that the scattering rate of the symmetric interface phonon mode has a stronger dependence on the Al composition in the barriers than that of the confined mode. The effective phonon energy emitted by hot electrons in GaAs/AlxGa1−xAs quantum wells with various Al compositions was estimated and the calculated value agrees with the experimental results qualitatively. For the dependence on the well width, scattering rates of the S+ mode drop considerably as the well width is increased. The dependence of the electron-optical phonon interaction on structure parameters can be clearly explained by the H and G factors defined in the article.
Strain in cracked AlGaN layers
Authors: Einfeldt S. , Dießelberg M. , Heinke H. , Hommel D. , Rudloff D. , Christen J. , Davis R. F.
Published: July 2002

The strain relaxation due to cracks of different depths in AlGaN layers grown on GaN template layers has been investigated using spatially resolved cathodoluminescence spectroscopy, high-resolution x-ray diffraction and two-dimensional finite element simulations. The experimental data consistently show that the relief of tensile stress increases with decreasing crack spacing. The measured strain profiles between the cracks are well described by the theoretical calculations for small crack spacings; whereas, deviations for larger crack spacings have been found. The latter is discussed in terms of inelastic strain relaxation mechanisms, the reliability of the deformation potential for AlGaN employed in this article, and the spatial variations in the properties of the AlGaN, e.g., its composition.
High-frequency near-field microscopy
Authors: Rosner Björn T. , van der Weide Daniel W.
Published: July 2002

Conventional optics in the radio frequency (rf) through far-infrared (FIR) regime cannot resolve microscopic features since resolution in the far field is limited by wavelength. With the advent of near-field microscopy, rf and FIR microscopy have gained more attention because of their many applications including material characterization and integrated circuit testing. We provide a brief historical review of how near-field microscopy has developed, including a review of visible and infrared near-field microscopy in the context of our main theme, the principles and applications of near-field microscopy using millimeter to micrometer electromagnetic waves. We discuss and compare aspects of the remarkably wide range of different near-field techniques, which range from scattering type to aperture to waveguide structures.
Interfacial reactions of Ni on Si1−xGex (x=0.2, 0.3) at low temperature by rapid thermal annealing
Authors: Zhao H. B. , Pey K. L. , Choi W. K. , Chattopadhyay S. , Fitzgerald E. A. , Antoniadis D. A. , Lee P. S.
Published: July 2002

The interfacial reaction of Ni with relaxed Si1−xGex (x=0.2,0.3) films in the low temperature range, viz., 300–500 °C, has been investigated and compared with that of Ni with Si (i.e., x=0). Ni2(Si1−xGex) and Ni3(Si1−xGex)2 were observed at 300 °C whereas a uniform film of Ni(Si1−xGex) was formed at 400 °C for both Si0.8Ge0.2 and Si0.7Ge0.3 substrates. At 500 °C, a mixed layer consisting of Ni(Si1−yGey) and Si1−zGez was formed with a relation of z>x>y. Sheet resistance measurement results show that the silicided film attains its lowest value at an annealing temperature of 400 °C. The approximate values of the resistivity of the corresponding uniform Ni(Si1−xGex) (x=0.2, 0.3) derived from the transmission electron microscope and sheet resistance results are 19 and 23 μΩ cm, respectively.
Vibrational modes of thin oblate clouds of charge
Authors: Jenkins Thomas G. , Spencer Ross L.
Published: July 2002

A numerical method is presented for finding the eigenfunctions (normal modes) and mode frequencies of azimuthally symmetric non-neutral plasmas confined in a Penning trap whose axial thickness is much smaller than their radial size. The plasma may be approximated as a charged disk in this limit; the normal modes and frequencies can be found if the surface charge density profile σ(r) of the disk and the trap bounce frequency profile ωz(r) are known. The dependence of the eigenfunctions and equilibrium plasma shapes on nonideal components of the confining Penning trap fields is discussed. The results of the calculation are compared with the experimental data of Weimer et al. [Phys. Rev. A 49, 3842 (1994)] and it is shown that the plasma in this experiment was probably hollow and had mode displacement functions that were concentrated near the center of the plasma.
Photoinduced band-bending effect of low temperature GaAs on AlGaAs/InGaAs/GaAs modulation-doped transistors
Authors: Folkes Patrick A. , Gumbs Godfrey , Xu Wen
Published: July 2002

Low temperature photoluminescence (PL) measurements on pseudomorphic modulation-doped transistors (PHEMTs) with a low-temperature (LT) GaAs layer in the GaAs buffer layer clearly show a decrease in the quantum well PL transition energies compared to a PHEMT with no LT GaAs. Self-consistent calculations of the quantum well PL transition energies and oscillator strengths show that the observed decrease in PL energies can be attributed to a larger photoinduced band bending in PHEMTs with an undoped GaAs/LT GaAs interface compared to the photoinduced band bending in PHEMTs with an undoped GaAs/semi-insulating GaAs interface.
Amide I vibrational modes in glycine dipeptide analog: Ab initio calculation studies
Authors: Cha Sangyob , Ham Sihyun , Cho Minhaeng
Published: July 2002

Coupling between two local amide I vibrational motions of peptides has been quantitatively estimated by assuming that the two peptides interact with each other via dipole–dipole interaction, the so-called transition dipole coupling (TDC) model. The TDC theory has been applied to describing amide I IR and Raman band envelopes and further used to interpret the two-dimensional IR pump–probe and photon echo spectra of polypeptides recently. In order to quantitatively test the validity of the TDC model or in general dipole–dipole interaction model for a dipeptide, we carry out systematic investigations, by using both the ab initio calculation methods and extended TDC theory, on the potential energy surface, vibrational frequencies of symmetric and antisymmetric amide I vibrational normal modes, transition dipole and transition polarizability, IR and Raman intensities of the two modes, IR-Raman noncoincidence phenomena in the full Ramachandran space for a model dipeptide, glycine dipeptide analog. It is found that the spectroscopic properties of dipeptide can be quantitatively well described by the TDC model, but the quantities related to the potential energy surface such as absolute magnitudes of vibrational frequencies and frequency splitting between the two normal modes cannot be accounted for by using the TDC model. A further investigation of dimeric system with two formamide molecules is presented and the applicability of the TDC model to through space vibrational interaction as a function of intermolecular distance between the two peptides is examined.
Lattice strain and lattice expansion of the SrRuO3 layers in SrRuO3/PbZr0.52Ti0.48O3/SrRuO3 multilayer thin films
Authors: Jia C. L. , Contreras J. Rodrı́guez , Poppe U. , Kohlstedt H. , Waser R. , Urban K.
Published: July 2002

In SrRuO3/PbZr0.52Ti0.48O3/SrRuO3 multilayer thin films on SrTiO3 substrates the different lattice distortion behavior of the top and the bottom SrRuO3 film layer is found and characterized by means of transmission electron microscopy. The bottom SrRuO3 layer is compressively strained in the film plane by a constraint of the SrTiO3 substrate. In contrast, in the interface area of the top SrRuO3 layer, a lattice dilatation is measured not only in the film plane but also parallel to the film normal. The misfit strain, the lead interdiffusion and the oxygen concentration in this area are investigated and discussed as possible reasons for the unexpected lattice dilatation along the film normal direction.
Strain field and scattered intensity profiling with energy dispersive x-ray scattering
Authors: Croft M. , Zakharchenko I. , Zhong Z. , Gurlak Y. , Hastings J. , Hu J. , Holtz R. , DaSilva M. , Tsakalakos T.
Published: July 2002

Two powerful synchrotron x-ray scattering techniques for residual strain depth-profiling and tomography-like scatter-intensity profiling of materials are presented. The techniques utilize energy dispersive x-ray scattering, from a fixed microvolume, with microscanning of the specimen being used to profile its interior. The tomography-like profiles exploit scattering-cross-section variations, and can be contrast enhanced by separately monitoring scattering from different crystal structures. The strain profiling technique is shown to finely chronicle the internal strain variation over several mm of steel. Detailed strain profiling for a cantilever spring demonstrates the interplay of residual and external stresses in elastic/plastic deformation. Since surface compression, by shot peening, is a classic method to fortify against fatigue failure, the strain profile for a shot-peened, surface-toughened material is determined and discussed in terms of a simple elastic–plastic stress/strain model. Finally the lattice strains in a WC/Co coated steel composite material are discussed.
Single-band envelope-function model in the full Brillouin zone for electronic structure calculation in semiconductor nanostructures
Authors: Mirčetić A. , Ikonić Z. , Milanović V. , Indjin D.
Published: July 2002

We explore the single-band envelope-function model in the entire Brillouin zone for electronic structure calculation of nanostructures that involve semiconductors with one or more significant valleys in the conduction band. Comparison with the numerically more demanding empirical pseudopotential calculation shows reasonably good accuracy of the method described here, which correctly predicts the quantized state energies, splittings in multivalley semiconductor nanostructures, as well as the intersubband transition derived optical properties. This makes it potentially useful for modeling nanostructures and nanodevices based on Si and other materials.
Temperature dependence of intrinsic infrared absorption in natural and chemical-vapor deposited diamond
Authors: Piccirillo Clara , Davies Gordon , Mainwood Alison , Scarle Simon , Penchina C. M. , Mollart T. P. , Lewis K. L. , Nesládek M. , Remes Z. , Pickles C. S. J.
Published: July 2002

Empirical rules are derived that describe the temperature dependence of the infrared absorption spectra of pure diamond for photons of energy hν=500–4000 cm−1. We show that with increasing temperature in the range 14<T<850 K, all the features in the infrared spectrum shift to lower frequency at very similar fractional rates. The rate for all the features is, to ±13%, Δν/ν=cn(Ee) where c=−0.027 and n(Ee) is the Bose–Einstein population factor with Ee=860 cm−1. The intensities of the optical absorption involving the creation of two phonons of energies E1 and E2 are expected to increase with T in proportion to [1+n(E1)][1+n(E2)]. This expression, combined with the fractional shift rule for the energies of each mode, allows high temperature two-phonon spectra to be simulated accurately from a low temperature spectrum. The temperature dependence of the three-phonon band between 2665 and 3900 cm−1 is precisely fitted without adjustable parameters by using the shift rule in conjunction with a modified density of three-phonon states. Absorption at 10.6 μm is shown to involve the simultaneous destruction and creation of phonons. Its strong temperature dependence in the range 300<T<800 K is accurately described, without any adjustable parameters, in terms of three main components: the destruction of one phonon of 335 cm−1 and the creation of a second of 1275 cm−1; the shift to lower energy of the phonons; and a three-phonon process involving the destruction of one and the creation of two phonons. The analysis demonstrates why diamond has to be effectively cooled when used for the windows of a high-power CO2 laser.
Diagnostics of nanosecond dynamics of the plasma produced during KrF excimer laser ablation of zirconia in vacuum
Authors: Li Peng , Lim Daniel , Mazumder Jyoti
Published: July 2002

A 248 nm KrF excimer laser was used to ablate the yttria stabilized ZrO2 target in vacuum while an intensified charge coupled device camera was used to get the time-resolved side view images of the induced plume/plasma. Two components, plume and plasma, can be clearly distinguished from the images with delay time less than 300 ns. The center of the plasma is found moving along the direction tilted ∼55°, instead of 90°, from the surface of the target while the processing laser came along ∼40°. The movement velocities and the explosion rates of the plasma during the first 2 μs after the laser strike were calculated. Time- and spatial-resolved emission spectra from excited Zr atoms in the plasma have been measured to determine the corresponding excitation temperature. The dynamic evolution of the plasma is outlined based on the experimental results.
Vibrational energy levels of methyl cation
Authors: Yu Hua-Gen , Sears Trevor J.
Published: July 2002

The potential energy surface for methyl cation is calculated using an extrapolated full coupled-cluster/complete basis set (FCC/CBS) theory. The equilibrium C–H bond length is obtained as 1.0884 Å. The vibrational energy levels of both CH3+ and CD3+ cations are computed using a variational method, based on the accurate potential energy surface. The fundamental frequencies in cm−1 are predicted to be 2942.27 (v1), 1377.82 (v2), 3108.29 (v3), and 1387.01 (v4) for CH3+, and 2082.99 (v1), 1070.31 (v2), 2339.99 (v3), and 1015.70 (v4) for CD3+, respectively.
Deep-level defects in InGaAsN grown by molecular-beam epitaxy
Authors: Kaplar R. J. , Ringel S. A. , Kurtz Steven R. , Klem J. F. , Allerman A. A.
Published: July 2002

Deep-level transient spectroscopy (DLTS) studies on both p-type unintentionally doped and n-type (Si-doped), 1.05 eV band gap InGaAsN grown by molecular-beam epitaxy are reported. Two majority-carrier hole traps were observed in p-type material, H3′ (0.38 eV) and H4′ (0.51 eV), and no evidence was found for the presence of minority-carrier electron traps. In n-type material, we observed a shallow distribution of electron levels, E1′, as well as a deep electron trap E4′ (0.56 eV) and a deep hole trap H5′ (0.71 eV). All DLTS peaks observed were broad and are thus consistent with continuous defect distributions and/or groups of closely spaced discrete energy levels in the band gap. Comparison of the spectra to previously reported spectra of metalorganic chemical vapor deposition-grown InGaAsN of the same composition revealed some similarities and some differences, suggesting that some of the observed deep levels are due to intrinsic physical sources, whereas others are specific to the growth technique used.
Electrical characterization of gas discharges using a numerical treatment. Application to dielectric barrier discharges
Authors: Petit M. , Jidenko N. , Goldman A. , Goldman M. , Borra J. P.
Published: July 2002

In this article a numerical method for an electrical data treatment which is aimed to define the key electrical parameters monitoring the thermal and chemical properties of gas discharges, in particular dielectric barrier discharges is presented. Numerous electrical parameters characterize the electrical discharges: pulses onset voltage, pulse number, pulse height, pulse width, power injected in the discharge cell, etc. Our treatment using Matlab® allows us to get such data, as is shown in the present article for a point-to-plane dielectric barrier discharge. Moreover it shows how a separation between the different components of the current signals remaining after the elimination of the capacitive component can be obtained. Much attention is paid, in particular, to the impulsive components (streamers and Trichel pulses components) and on the nonimpulsive ones constituting what we call the pseudocontinuous components, generally ignored in the literature. So four different categories of charges are considered: for the positive voltage alternation, a positive-pulse charge (Qimp+) and a positive pseudocontinuous charge (Qcont+) and for the negative alternation, a negative-pulse charge (Qimp−) and a negative pseudocontinuous charge (Qcont−). The article also intends to show how the main electrical parameters (current charge and power) can be determined by different ways and how to select the best way for representative measurements in every situation.
Transient heat flux measurement using a surface junction thermocouple
Authors: Sanderson S. R. , Sturtevant B.
Published: July 2002

A new form of surface junction thermocouple sensor has been developed and tested. The novel feature of the design is the use of a tapered fit between two coaxial thermocouple elements to form a thin, robust junction. The gauge has a response time on the order of 1 μs and is suitable for measuring large transient heat fluxes in hypervelocity wind tunnels. Asymptotic analysis is used to demonstrate the operating principles and to assess the errors associated with the finite thickness of the surface junction. Spectral deconvolution methods are used to infer a mean square optimal estimate of the surface heat flux from time resolved surface temperature measurements. This improved signal processing method is applicable to transient heat flux gauges of all types. Potential reducible error sources and other systematic errors are described. Measurements of the heat flux about the forebody of a cylindrical body in a hypervelocity flow demonstrate the functioning of the gauge and are used to obtain statistical estimates of the repeatability of the technique. The measured heat fluxes are compared with established theoretical predictions.
Comparison of electronic decay of valence ionized fluorinated carbanions and their acids
Authors: Müller I. B. , Zobeley J. , Cederbaum L. S.
Published: July 2002

The ionization and double-ionization spectra of fluorinated carbanions of various chain lengths are compared with those of their corresponding acids. For the acidic systems we find a dramatic relative shift of the double-ionization spectra to higher energies due to the presence of just one additional proton. The impact of the proton on the ionization spectra is also important, but results in only half of the double-ionization spectra’s shift. A molecular electronic decay mechanism is found to be operative in the valence region of the molecules under investigation. The impact of this decay is more substantial for the anions. The threshold for electronic decay (i.e., the first double-ionization potential) is at much lower energy for the anions than for the acids. Interestingly, the localization pattern of the holes in the decay channels is, in contrast to the decay threshold, only a little affected by protonation. We also compare the impact of electron correlation effects on the ionization and double-ionization spectra of the series of fluorinated carbanions and of their acids.
The ab initio potential energy surface and vibrational-rotational energy levels of X 2Σ+ MgOH
Authors: Koput Jacek , Carter Stuart , Peterson Kirk A. , Theodorakopoulos Giannoula
Published: July 2002

The equilibrium structure and potential energy surface of magnesium monohydroxide in its ground doublet state, X 2Σ+ MgOH, have been determined from large-scale ab initio calculations using the spin-restricted coupled-cluster method, RCCSD(T), with basis sets of double-through quintuple-zeta quality. The effects of core-electron correlation on the calculated molecular parameters were investigated. The vibrational-rotational energy levels of various MgOH isotopomers were calculated using the variational method. The spectroscopic constants determined are found to be in remarkably good agreement with experimental data.
Dynamics of a vertically falling film in the presence of a first-order chemical reaction
Authors: Trevelyan Philip M. J. , Kalliadasis Serafim , Merkin John H. , Scott Stephen K.
Published: July 2002

The evolution of a vertically falling film in the presence of a simple first-order (exothermic or endothermic) chemical reaction is considered. The heat of reaction sets up surface tension gradients that induce thermocapillary stresses on the free-surface, thus affecting the evolution of the film. By using a long-wave expansion of the equations of motion and associated boundary conditions, we derive a nonlinear partial differential equation of the evolution type for the local film thickness. We demonstrate that, when the surface tension is an increasing function of temperature an exothermic reaction has a stabilizing effect on the free surface while an endothermic reaction is destabilizing. We construct bifurcation diagrams for permanent solitary waves and show that, in all cases the solution branches exhibit limit points and multiplicity with two branches, a lower branch and an upper branch. Time-dependent computations of the free-surface evolution equation show that the system always approaches a train of coherent structures that resemble the lower branch solitary waves. We also examine the absorption characteristics through the interface and we demonstrate that an endothermic reaction enhances absorption and mass transport. The opposite is true for an exothermic reaction.