bluuebunny/crossref_metadata_2025 huggingface dataset:
Constant voltage anemometer operated hot wire at subsonic speeds over wide overheats in unsteady flows
Authors: Truzzi Guido E. , Sarma Garimella R. , Chokani Ndaona
Published: December 2002

The constant voltage anemometer (CVA) was used to calibrate a hot wire over a wide overheat range. Instead of the output voltage (E) of the anemometer which is normally used, at each test point a quantity represented by “pdr” equal to the ratio of power dissipated in the hot wire (Pw) and the associated difference in the heated resistance of the hot wire (Rw) and its resistance (Ra) at the ambient fluid temperature is calculated. It is shown that the calibration curves so obtained with pdr=Pw/(Rw−Ra) as the output variable instead of E can be represented by a single calibration equation covering the wide overheat range. Overheat variation is equivalent to allowing an ambient temperature change of the fluid at a given setting. It demonstrates that this approach can be used to cover measurements using the hot wire with fluid temperature drifts without using a second hot wire for temperature corrections and without any temperature calibration. The calibration data was then applied to measure the unsteady flow in the near orifice region of synthetic jets with very good results. The measurements confirm the computational predictions that show that although there is flow reversal, over a cycle of oscillation, the synthetic jet actuator spends most of the cycle ejecting rather than ingesting fluid.
A uniaxial tensile stress apparatus for temperature-dependent magnetotransport and optical studies of thin films
Authors: Rowe A. C. H. , Fasanella K. , Hines D. R. , Zhou T. , Solin S. A.
Published: December 2002

A mechanical apparatus for the application of variable uniaxial tensile stress to thin films grown on bulk material has been designed for use in measuring the electrical and/or optical properties of a thin layer over a temperature range 4.2 K<T<300 K, and in magnetic fields up to 7 T. The induced strain is measured with a resolution of 0.0015% by monitoring the position of a laser beam reflected off the surface of the strained sample. The use of the apparatus is demonstrated on n-type InSb layers grown on GaAs where the uniaxial tensile stress is applied in the [001] direction. At 300 K and strains of up to 0.05%, an increase in the conductivity of approximately 3.5% is observed, most of which (∼2.5%) is the result of an increase in the carrier concentration. The remaining 1% is due to an increase in the carrier mobility. Using band-structure k⋅p theory and the deformation potential parameters obtained from optical spectroscopy measurements under uniaxial compression, these observations are shown to be well described by a reduction in both the fundamental band gap and the carrier effective mass with increasing tensile strain.
Probe measurements of ion energy distribution in magnetized plasmas
Authors: Ratynskaia S. V. , Demidov V. I. , Rypdal K.
Published: December 2002

A probe method for measurement of the ion energy distribution in magnetized plasmas is proposed. A cylindrical probe with end plugs is oriented parallel to the magnetic field and used to reduce the electron contribution to the total probe current. The contribution of ions to the second derivative of the total current is identified by experiments with different probe orientation and strength of the magnetic field. The ion density obtained from the measured ion energy distribution is in good agreement with the electron density obtained from measured electron energy distribution.
Characterization of multiterawatt laser-solid interactions for proton acceleration
Authors: McKenna P. , Ledingham K. W. D. , Spencer I. , McCany T. , Singhal R. P. , Ziener C. , Foster P. S. , Divall E. J. , Hooker C. J. , Neely D. , Langley A. J. , Clarke R. J. , Norreys P. A. , Krushelnick K. , Clark E. L.
Published: December 2002

A comprehensive characterization of experimental parameters in a study of proton acceleration by short-pulse laser–solid interactions at intensities up to 1019 W cm−2 is reported. Laser pulse and prepulse conditions were measured, with a contrast ratio of the order of 10−6 obtained. The focused laser intensity was experimentally calibrated using a time-of-flight spectrometer to resolve the stages of ionization of a target gas. By comparing the measured ion yields with predictions of an atomic tunneling ionization model a factor of 1.5 uncertainty in the focused intensity was determined. Drive mechanisms for mounting solid targets with thickness in the range of 0.2 to 125 μm have been developed for use with high-repetition rate lasers. A retro-focus imaging system has also been implemented to position the target relative to the laser focus. The techniques have been applied to study proton acceleration as a function of various laser and target parameters. Measurements of the energy distribution of protons as a function of laser intensity are presented for both mylar and Al targets. A maximum proton energy of 1.5 MeV was observed. A compilation of recent results from a number of laser systems on the conversion efficiency of laser energy to protons is discussed. By comparison, an efficiency of about 0.7% for the present study is encouraging for future tabletop-laser-based ion acceleration.
Micromachined droplet ejector arrays
Authors: Perçin Gökhan , Yaralioglu Göksenin G. , Khuri-Yakub Butrus T.
Published: December 2002

In this article we present a micromachined flextensional droplet ejector array used to eject liquids. By placing a fluid behind one face of a vibrating circular plate that has an orifice at its center, we achieve continuous ejection of the fluid. We present results of ejection of water and isopropanol. The ejector is harmless to sensitive fluids and can be used to eject fuels, organic polymers, photoresists, low-k dielectrics, adhesives, and chemical and biological samples. Micromachined two-dimensional array flextensional droplet ejectors were realized using planar silicon micromachining techniques. Typical resonant frequency of the micromachined device ranges from 400 kHz to 4.5 MHz. The ejections of water through a 4 μm diameter orifice at 3.45 MHz and a 10 μm diameter orifice at 2.15 MHz were demonstrated by using the developed micromachined two-dimensional array ejectors.
Analytical expressions for some multiplicity-free isoscalar factors of Sf⊃Sf−1
Authors: Dai Lianrong , Pan Feng , Draayer J. P.
Published: December 2002

An algebraic routine for the evaluation of analytical expressions for isoscalar factors (ISFs) of Sf⊃Sf−1 is formulated based on the linear equation method (LEM) and the analytical continuation of the rank f. As examples, ISFs of Sf⊃Sf−1 for the coupling [f−1,1]⋅[f−1,1], [f−1,1]⋅[f−2,2], [f−2,1,1]⋅[f−2,1,1] are tabulated. The results demonstrate that the number of ISF tables can be greatly reduced compared with corresponding numerical results produced using other methods.
A video tracking system for measuring the position and body deformation of a swimming fish
Authors: Wang Hao , Zeng Lijiang , Yin Chunyong
Published: December 2002

A video tracking system based on fringe tracking and camera tracking is developed to simultaneously measure the spatial position and the body deformation of a swimming fish. The control module of the system consists of a fringe pattern tracking system including one tracking mirror and a fringe pattern projector, and a video tracking system including another tracking mirror and a high-speed camera. In the control module, a target trajectory prediction algorithm is used to predict the position of a swimming fish. The two mirrors mounted on two step motors are used to track a swimming fish. The rotation angles of the tracking mirrors are recorded simultaneously. Body position and deformation are calculated from the distorted fringes based on triangulation. The relative accuracy on the body deformation is below 1%. We successfully apply the system to a swimming fish.
A programmable log-linear amplifier for wide range nuclear power measuring channels
Authors: Khaleeq M. Tahir , Alam Mahmood , Ghumman Iftikhar Ahmad
Published: December 2002

A programmable log-linear amplifier has been developed for nuclear channels. The amplifier can be programmed for logarithmic, linear or log-linear mode of operation. In the log-linear mode, the amplifier operates partially in log mode and automatically switches to linear mode at any preset point. The log-linear mode is used for wide range operation of nuclear channels and, hence, the amplifier will improve the fault finding capabilities of the nuclear channels used in power range. The amplifier is tested at nuclear reactor and the results are found in very good agreement with the designed specifications. This article presents design and construction of the amplifier and field test results.
Two-dimensional imaging of neutral alkali atom samples using surface ionization
Authors: Christandl Katharina , Lafyatis Gregory P. , Modoran Andrei , Shih Tung-Hsiu
Published: December 2002

We describe the design and characterization of a high resolution, high efficiency detector for two-dimensional imaging of neutral atoms. Incident atoms are surface-ionized by a tungsten-coated hot ribbon and the resulting ions are accelerated into a microchannel plate detector with a phosphor screen output. With this design we can detect individual alkali and other low ionization potential atoms and molecules with a spatial resolution of ∼20 μm. We find backgrounds on the order of 30 Hz over an active detection region of 20 mm2, a time response of ⩽1/30 s, and a detection efficiency of ∼50%. This detector can be used to image cold as well as thermal atomic or molecular beams.
Optical absorption study of 100-MeV chlorine ion-irradiated hydroxyl-free ZnO semiconductor quantum dots
Authors: Mohanta D. , Nath S. S. , Bordoloi A. , Choudhury A. , Dolui S. K. , Mishra N. C.
Published: December 2002

We report here synthesis and optical absorption study of energetic ion-irradiated hydroxyl-free ZnO semiconductor quantum dots. Quantum dot samples were synthesized by a quenching technique and 100-MeV chlorine ion was selected for the irradiation experiment with doses 1×1011, 5×1011, 5×1012, and 1013 ions/cm2. With an increase in fluence, the optical absorption spectra of irradiated quantum dots reveal redshift of the energy-gap parameter with respect to unirradiated (virgin) quantum dots. The narrowing of the energy gap of nanoparticles indicate particle growth under ion irradiation which is confirmed from transmission electron microscope images. The possible reason for such variations was encountered using an effective-mass approximation model which fits well for small effective mass and high-dielectric constant (ε0).
A note on a series of papers on relativistic δ-sphere interactions in quantum mechanics published by M. N. Hounkonnou and G. Y. H. Avossevou in the Journal of Mathematical Physics
Authors: Shabani J. , Vyabandi A.
Published: December 2002

In this note, we show that the definitions proposed in J. Math. Phys. 41, 24 (2000); 41, 1718 (2000); 41, 1735 (2000) for the description of relativistic δ-sphere interactions and its various generalizations are not correct.
Ultraviolet phase-modulated ellipsometer
Authors: Garcia-Caurel Enric , Moncel Jean Luc , Bos Francis , Drévillon Bernard
Published: December 2002

A phase modulated ellipsometer working in the spectral range from the visible (1.5 eV) to the far ultraviolet (9.5 eV) is described and some preliminary examples of applications are shown in order to illustrate its sensitivity and accuracy. The optical configuration of the ellipsometer consists of a light source, a monochromator, a polarizer, a photoelastic modulator, a sample holder, an analyzer, and two detectors. This system has been adapted into air sealed chambers and functions under vacuum conditions to prevent absorption of the far ultraviolet radiation by atmospheric gases. The described apparatus makes it possible to perform both ex situ and in situ real time measurements. Working in the far ultraviolet (FUV) (6–10 eV), opens the interesting possibility of determining the optical properties of materials at photon energies of 6.5 and 9.5 eV, which correspond to the laser sources used in the photolithographic process. It can be anticipated that FUV phase modulated ellipsometry will appear as a useful tool for material research and characterization in the semiconductor industry.
Generation and control of field-aligned flow velocity shear in a fully ionized collisionless plasma
Authors: Kaneko T. , Odaka Y. , Tada E. , Hatakeyama R.
Published: December 2002

A modified plasma-synthesis method is developed in order to actively control an ion flow energy along magnetic-field lines, where ion and electron emitters of the same diameter are oppositely set at cylindrical machine ends and the ion emitter is concentrically segmented into three sections. The field-aligned ion flows with radially different energies, or ion flow velocity shears, are generated in a radially uniform plasma potential when each section of the segmented ion emitter is individually biased at a positive value above the plasma potential.
Spectroscopic studies of random telegraph noise in InAs quantum dots in GaAs
Authors: Panev N. , Pistol M.-E. , Jeppesen S. , Evtikhiev V. P. , Katznelson A. A. , Kotelnikov E. Yu.
Published: December 2002

We have observed and investigated random telegraph noise in the photoluminescence from InAs quantum dots in GaAs. The luminescence from a single quantum dot, exhibiting switching between two levels, has been spectrally resolved. The random telegraph noise is only observed in the presence of band filling. We find no spectral shift of the emission in the different states. It is only the intensity, mainly for higher energy peaks, that changes. The InAs quantum dots behave similarly to InP/GaInP and InGaAs/GaAs quantum dots, with respect to random telegraph noise.
Photoluminescence properties of ZnO epitaxial layers grown on lattice-matched ScAlMgO4 substrates
Authors: Makino T. , Tamura K. , Chia C. H. , Segawa Y. , Kawasaki M. , Ohtomo A. , Koinuma H.
Published: December 2002

Photoluminescence (PL) properties of ZnO films grown epitaxially on lattice-matched ScAlMgO4 substrates are investigated. PL spectra of ZnO layers measured at 5 K are dominated by neutral-acceptor bound exciton emission at 3.359 eV (I6) with a linewidth of 0.8 meV. Free exciton emission develops as temperature is raised and eventually dominates at temperatures higher than 120 K. Temperature dependence of intensity of the free exciton emission is discussed.
Texture formation in Ti–Ta alloy disilicide thin films
Authors: Özcan A. S. , Ludwig K. F. , Cabral C. , Lavoie C. , Harper J. M. E.
Published: December 2002

Texture development in (Ti,Ta)Si2, formed from Ti–Ta alloy (0–6 at. % Ta) thin films sputtered on Si(001) and poly-Si substrates, was studied using both in situ and ex situ x-ray diffraction. The addition of Ta fundamentally changes the C54 texture. At 6 at. % Ta, an effective composition for lowering the C49–C54 transformation temperature, films on both substrates develop a strong (010) texture normal to the film plane. Alloy films on Si(001) also exhibit an in-plane orientational relationship between the final C54 texture and the underlying silicon lattice. A pseudoepitaxial film relationship with the substrate may, therefore, be playing an important role in the C54 transformation process. For 32 nm films on single-crystal substrates, the in situ measurements show that significant evolution of the C54 texture occurs during the growth process, suggesting that those grains well oriented with respect to the Si substrate lattice have higher growth velocities. The possibility of a template effect by the C40 or metal-rich Ti5Si3 phase is also discussed on the basis of texture considerations.
Excited singlet (S1) state interactions of calixarenes with chloroalkanes: A combination of concerted and stepwise dissociative electron transfer mechanism
Authors: Mohanty J. , Pal H. , Nayak S. K. , Chattopadhyay S. , Sapre A. V.
Published: December 2002

Both steady-state and time-resolved studies in acetonitrile (ACN) solutions show that the excited singlet (S1) states of calixarenes (CX) undergo quenching by chloroalkanes (CA). It has been revealed by characterizing the Cl ions in the photolyzed CX–CA systems in ACN solutions that the quenching occurs due to dissociative electron transfer (DET) mechanism, whereby a C–Cl bond of the CAs undergoes dissociation on acceptance of an electron from excited CX. The bimolecular quenching constants (kq) in the present systems were correlated with the free energy changes for the concerted DET reactions based on a suitable DET theory. Such a correlation results in the recovery of an intramolecular reorganization energy, which is substantially lower to account for the C–Cl bond dissociation energy of the CAs. Comparing present results with those of an another donor–acceptor system (e.g., biphenyldiol–CA systems) where a concerted DET mechanism is applicable, it is inferred that in CX–CA systems both concerted and stepwise DET mechanisms operate simultaneously. It is proposed that the interaction of excited CXs with encaged CAs follows the stepwise mechanism whereas that with the out of cage CAs follows the concerted mechanism.
Scalar-tensor gravity and conformal continuations
Authors: Bronnikov K. A.
Published: December 2002

Global properties of vacuum static, spherically symmetric configurations are studied in a general class of scalar-tensor theories (STTs) of gravity in various dimensions. The conformal mapping between the Jordan and Einstein frames is used as a tool. Necessary and sufficient conditions are found for the existence of solutions admitting a conformal continuation (CC). The latter means that a singularity in the Einstein-frame manifold maps to a regular surface Strans in the Jordan frame, and the solution is then continued beyond this surface. Strans can be an ordinary regular sphere or a horizon. In the second case, Strans connect two epochs of a Kantowski-Sachs type cosmology. It is shown that the list of possible types of global causal structure of vacuum space–times in any STT, with any potential function U(φ), is the same as in general relativity with a cosmological constant. This is even true for conformally continued solutions. A traversable wormhole is shown to be one of the generic structures created as a result of CC. Two explicit examples are presented: the known solution for a conformal field in general relativity, illustrating the emergence of singularities and wormholes due to CC, and a nonsingular three-dimensional model with an infinite sequence of CCs.
Fixed points of quantum operations
Authors: Arias A. , Gheondea A. , Gudder S.
Published: December 2002

Quantum operations frequently occur in quantum measurement theory, quantum probability, quantum computation, and quantum information theory. If an operator A is invariant under a quantum operation φ, we call A a φ-fixed point. Physically, the φ-fixed points are the operators that are not disturbed by the action of φ. Our main purpose is to answer the following question. If A is a φ-fixed point, is A compatible with the operation elements of φ? We shall show in general that the answer is no and we shall give some sufficient conditions under which the answer is yes. Our results will follow from some general theorems concerning completely positive maps and injectivity of operator systems and von Neumann algebras.
Sequence dependence of polymer dynamics in quenched disordered media: Weak attraction facilitates transport
Authors: Lee Sung-Joo E. , Chakraborty Arup K.
Published: December 2002

We study the dynamics of polymers in a random disordered medium of fixed obstacles using kinetic Monte Carlo methods. The polymers can have monomers which have attractive (A-type), repulsive (R-type) or neutral (H-type) interactions with the fixed obstacles that comprise the disordered medium. Several classes of homopolymers and heteropolymers with diverse sequences have been studied. Our most noteworthy result is that, above a threshold temperature, polymer bearing monomers that are attracted to the disordered medium translocate faster through the medium than those bearing neutral or repulsive monomers. We discuss how a delicate balance between energetic and entropic factors leads to this counterintuitive outcome. By examining heteropolymers with different sequences, we also find that the dependence of mobility on average composition is stronger than that on higher order correlations characterizing the sequence distribution. Connections between our results and experiments with synthetic and biological systems are noted.
Development of first-principles interaction model potentials. An application to the study of the bromide hydration
Authors: Ayala Regla , Martı́nez José M. , Pappalardo Rafael R. , Saint-Martı́n Humberto , Ortega-Blake Ivan , Sánchez Marcos Enrique
Published: December 2002

This work presents the development of first-principles bromide ion–water interaction potentials using the mobile charge density in harmonic oscillators-type model. This model allows for a flexible and polarizable character of the interacting molecules and has already been parametrized for water–water interactions. The prospected potential energy surfaces of the bromide ion-water system were computed quantum-mechanically at Hartree–Fock and Møller–Plesset second-order perturbation levels. In addition to the ion–solvent molecule pair, structures formed by the anion and two or three water molecules were considered in order to include many body effects. Minimizations of hydrated bromide clusters in gas phase [Br(H2O)n]− (n=1–6,10,15,20) and Monte Carlo computations of bromide aqueous solutions were performed to test the new potentials. Both structural and thermodynamic properties have been studied in detail and compared to the available experimental and theoretical values. From these comparisons, it was concluded the importance of including basis set superposition error corrections for the two-body interactions, and the small role of both electron correlation on the three-body terms and the four-body terms. Monte Carlo simulation results have also been used to investigate if the presence of the anion significantly affects the intramolecular geometry of the water molecules and the degree of disruption of the water solvent structure in its vicinity.
Filling and emptying kinetics of carbon nanotubes in water
Authors: Waghe Aparna , Rasaiah Jayendran C. , Hummer Gerhard
Published: December 2002

The kinetics of water filling and emptying the interior channel of carbon nanotubes is studied by molecular dynamics simulations. Filling and emptying occur predominantly by sequential addition of water to or removal from a single-file chain inside the nanotube. Advancing and receding water chains are orientationally ordered. This precludes simultaneous filling from both tube ends, and forces chain rupturing to occur at the tube end where a water molecule donates a hydrogen bond to the bulk fluid. We use transition path concepts and a Bayesian approach to identify a transition state ensemble that we characterize by its commitment probability distribution. At the transition state, the tube is filled with all but one water molecule. Filling thermodynamics and kinetics depend sensitively on the strength of the attractive nanotube–water interactions. This sensitivity increases with the length of the tubes.
Correcting off-axis effects in an on-chip resistive-pulse analyzer
Authors: Saleh O. A. , Sohn L. L.
Published: December 2002

A resistive-pulse analyzer is a device that utilizes measurements of the electrical resistance of a solution-filled pore to determine the size of particles that pass through the pore. The relation between particle size and changes in the pore’s resistance is complicated by particles that travel off the central axis of the pore. Here, we present data taken using a microfabricated pore and latex colloids that illustrates the effects of off-axis particles, and propose an algorithm for removing those effects from the data. We show that the ability to remove off-axis effects increases the precision of devices that transport particles through the pore with a pressure-driven flow relative to those that use electrophoretic flow.
Near-field enhanced Raman spectroscopy using side illumination optics
Authors: Hayazawa Norihiko , Tarun Alvarado , Inouye Yasushi , Kawata Satoshi
Published: December 2002

We demonstrate near-field enhanced Raman spectroscopy with the use of a metallized cantilever tip and highly p-polarized light directed onto the tip with side illumination optics using a long working distance objective lens. The highly p-polarized light field excites surface plasmon polaritons localized at the tip apex, which results in the enhanced near-field Raman scattering. In this article, we achieved an enhancement factor of 4000 for Rhodamine 6G molecules adsorbed on a silver island film. The side illumination is also applicable to an opaque sample and to near-field photolithography.
Stimulation of electrical conductivity in a π-conjugated polymeric conductor with infrared light
Authors: Meskers S. C. J. , van Duren J. K. J. , Janssen R. A. J.
Published: December 2002

Irradiation with infrared light is found to stimulate the electrical conductivity of a film of an organic polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in conductivity is found to be linear in the intensity of the irradiation (4–400 mW/cm2). Both frequency and time domain measurements reveal that the change in resistance induced by irradiation, relaxes according to ΔR(t)∝(1/t)0.6, with t as the time after excitation. As a possible mechanism for this relaxation, we model the diffusion of heat from the polymer film to the supporting glass substrate. Assuming that the change in resistance is linear with the raise in temperature caused by the infrared irradiation, one predicts a ΔR(t)∝(1/t)0.5 dependence. The similarity between the model and experimental behavior is taken as an indication that the relaxation is limited by heat transport from the polymer film and that the thermalization of the charge carriers occurs on a shorter time scale. Electrical characterization is complemented with optical measurements. These show infrared-induced transient absorption of the polymer film with practically the same relaxation behavior as the change in resistance. This suggests that the optical transients are also due to thermal excitations. In the sub-ps time domain, measurements of the change in optical transmission (ΔT/T) induced by the infrared pulse show a very short-lived component with a lifetime close to the instrumental resolution (∼500 fs). The rapid response is followed by a slow component that decays according to (ΔT/T)(t)∝(1/t)0.65. This is interpreted in terms of cooling of the excited charge carriers limited by heat transport, indicating that the thermalization of the carriers occurs on the sub-ps time scale.
Optical absorption spectroscopy of Fe2+ and Fe3+ ions in LiNbO3
Authors: Basun S. A. , Evans D. R. , Bunning T. J. , Guha S. , Barnes J. O. , Cook G. , Meltzer R. S.
Published: December 2002

We provide a method to measure concentrations of Fe2+ and Fe3+ ions in LiNbO3. The method, based only on optical absorption spectroscopy, provides a convenient means to determine the absorption cross sections for Fe2+ and Fe3+ ions, along with an independent measurement of the Fe2+ and Fe3+ concentrations in a given LiNbO3 crystal. The absorption cross section of the Fe2+ ions found by this method correlated well with a previously measured value. The cross section of the Fe3+ ions was obtained. It was also found that in some samples a significant fraction of Fe ions did not contribute to the absorption in the visible.
Structure and positron annihilation spectra of tin incorporated in mesoporous molecular sieves
Authors: Zhang H. Y. , He Y. J. , Chen Y. B. , Wang H. Y.
Published: December 2002

Mesoporous molecular sieves (MCM-41) consist of an ordered array of silica tubules comprised of pores with uniform controllable diameters in the nanometer range. Tin was successfully incorporated into MCM-41 using wet chemical techniques. Detailed structural analysis via x-ray diffraction and high resolution transmission electron microscopy confirm this, and indicate that, after sintering samples in air, SnO2 crystal nanoclusters formed in the channels. These conclusions are further supported by a study of the positron annihilation spectrum. In particular, the insensitivity, after incorporation of tin, of the long-lived component of the positron annihilation spectrum to whether an air or a vacuum annealing atmosphere is used indicates that tin in the MCM-41 channels hinders the entry of quenching oxygen from the air. Furthermore, after sintering, the complete loss of this long-lived component indicates that SnO2 nanoclusters fill the channels.
Transport in a slowly perturbed convective cell flow
Authors: Itin A. P. , de la Llave R. , Neishtadt A. I. , Vasiliev A. A.
Published: December 2002

We study transport properties in a simple model of two-dimensional roll convection under a slow periodic (period of order 1/ε≫1) perturbation. The problem is considered in terms of conservation of the adiabatic invariant. It is shown that the adiabatic invariant is well conserved in the system. It results in almost regular dynamics on large time scales (of order ∼ε−3 ln ε) and hence, fast transport. We study both generic systems and an example having some symmetry.
Pseudo Jahn–Teller origin of instability of molecular high-symmetry configurations: Novel numerical method and results
Authors: Bersuker Isaac B. , Balabanov Nikolai B. , Pekker David , Boggs James E.
Published: December 2002

The pseudo Jahn–Teller (PJT) effect as the only source of instability of molecular high-symmetry configurations in nondegenerate states is given a deeper insight by means of a novel method of ab initio evaluation of the vibronic Kv and nonvibronic K0 contributions to the curvature K=K0+Kv of the adiabatic potential energy surface in the direction of distortion. The method overcomes two essential difficulties: (1) the low accuracy in calculation of K0 because of the singularities at the nuclei where existing basis sets yield inadequate results, and (2) the lack of sufficiently accurate data on excited states (including the continuum spectrum) for calculation of Kv. This is achieved by summing up the contributions of the excited states to result in expressions with only the ground state wave function and its derivatives, and excluding the singularities by canceling mutually compensating diagonal matrix elements in K0 and Kv. After these essential changes K=K0+Kv is no longer a small difference between two large numbers, and it can be calculated with reasonable accuracy, while the separation of Kv allows one to reveal the PJT origin of the instability: the direction of distortion for which the negative Kv value overcomes the positive K0 one. A further insight into the origin of the instability is reached by estimating the relative contribution of the most active excited states for different distortions (and for the same distortions in similar compounds), since the contribution of the continuum states (the largest part of Kv) does not affect the bonding and hence the differences in Kv. Illustrative numerical calculations were carried out on several series of molecular systems: planar AH4, A=B−, C, N+, O2+, Si, planar BH3, CH3−, NH3, octahedral MH6, M=Sc3−, Ti2−, V−, Cr, Mn+, and octahedral TeF62−, IF6−, and XeF6.
Accurate correlation consistent basis sets for molecular core–valence correlation effects: The second row atoms Al–Ar, and the first row atoms B–Ne revisited
Authors: Peterson Kirk A. , Dunning Thom H.
Published: December 2002

Correlation consistent basis sets for accurately describing core–core and core–valence correlation effects in atoms and molecules have been developed for the second row atoms Al–Ar. Two different optimization strategies were investigated, which led to two families of core–valence basis sets when the optimized functions were added to the standard correlation consistent basis sets (cc-pVnZ). In the first case, the exponents of the augmenting primitive Gaussian functions were optimized with respect to the difference between all-electron and valence–electron correlated calculations, i.e., for the core–core plus core–valence correlation energy. This yielded the cc-pCVnZ family of basis sets, which are analogous to the sets developed previously for the first row atoms [D. E. Woon and T. H. Dunning, Jr., J. Chem. Phys. 103, 4572 (1995)]. Although the cc-pCVnZ sets exhibit systematic convergence to the all-electron correlation energy at the complete basis set limit, the intershell (core–valence) correlation energy converges more slowly than the intrashell (core–core) correlation energy. Since the effect of including the core electrons on the calculation of molecular properties tends to be dominated by core–valence correlation effects, a second scheme for determining the augmenting functions was investigated. In this approach, the exponents of the functions to be added to the cc-pVnZ sets were optimized with respect to just the core–valence (intershell) correlation energy, except that a small amount of core–core correlation energy was included in order to ensure systematic convergence to the complete basis set limit. These new sets, denoted weighted core–valence basis sets (cc-pwCVnZ), significantly improve the convergence of many molecular properties with n. Optimum cc-pwCVnZ sets for the first-row atoms were also developed and show similar advantages. Both the cc-pCVnZ and cc-pwCVnZ basis sets were benchmarked in coupled cluster [CCSD(T)] calculations on a series of second row homonuclear diatomic molecules (Al2, Si2, P2, S2, and Cl2), as well as on selected diatomic molecules involving first row atoms (CO, SiO, PN, and BCl). For the calculation of core correlation effects on energetic and spectroscopic properties, the cc-pwCVnZ basis sets are recommended over the cc-pCVnZ ones.
Drift, diffusion, and trapping of hydrogen in p-type GaN
Authors: Seager C. H. , Myers S. M. , Wright A. F. , Koleske D. D. , Allerman A. A.
Published: December 2002

Using capacitance–voltage measurements we have measured both the diffusion and the field-induced drift of H in GaN p/n+ diodes grown by metalorganic vapor phase epitaxy. Our data are well described by a computational model which simulates all of the important electronic processes as well as the drift, diffusion, and trapping of hydrogen in the GaN lattice. The experimental data demonstrate that H exists in the positive charge state; they also suggest that hydrogen diffusivity is anisotropic in this hexagonal material. In the temperature range from ∼200 to 310 °C we have determined that the sum of the activation energies for diffusion and binding of H+ to magnesium acceptors is 2.03 eV. This is ∼0.6 eV larger than previous density functional theory estimates of this quantity. We present first-principles calculations which demonstrate the observed diffusion anisotropy and which suggest possible reasons why previous treatments underestimate the barrier for hydrogen diffusive motion.
Permalloy thin films on MgO(001): Epitaxial growth and physical properties
Authors: Michelini F. , Ressier L. , Degauque J. , Baulès P. , Fert A. R. , Peyrade J. P. , Bobo J. F.
Published: December 2002

Permalloy thin films were deposited onto MgO(001) substrates by standard sputtering technique at growth temperatures ranging from 200 to 800 °C. Both reflection high-energy electron diffraction (RHEED) experiments and atomic force microscopy observations reveal that the substrate temperature for two-dimensional epitaxial growth should not exceed 350 °C. A series of permalloy films with thicknesses ranging from 3.5 to 100 nm was prepared at 300 °C. All films were grown as (001) single crystal with the same crystallographic orientations as the MgO(001) substrates. As evidenced by RHEED and x-ray diffraction, films with thicknesses larger than about 20 nm present a structural relaxation. The in-plane magnetization hysteresis loops obtained by longitudinal Kerr loop measurements exhibit a 〈100〉 fourfold cubic anisotropy and an enhancement of the magnetic softness with decreasing film thickness, both likely originating from the large interfacial strain configuration.
Characterization of silicon dioxide film by high spatial resolution cathodoluminescence spectroscopy
Authors: Yoshikawa M. , Matsuda K. , Yamaguchi Y. , Matsunobe T. , Nagasawa Y. , Fujino H. , Yamane T.
Published: December 2002

We have measured the cathodoluminescence (CL) spectra of silicon dioxide (SiO2) films prepared by a variety of methods and observed peaks at 280, 445, 480, 520, and 640 nm. A comparison of the CL and electron spin resonance spectra of Si-doped quartz glass showed that the 445 and 480 nm peaks originated from oxygen vacancy centers, which were different from E′ centers. From CL measurement of a cross section of thermally grown SiO2 film deposited on a Si substrate, we found that the relative intensities of the 445 and 480 nm peaks were stronger than that of the CL peak at 640 nm. This CL peak was attributed to the nonbridging oxidation hole centers or their precursors. Furthermore, CL measurement of a cross section of a 16 M dynamic random-access memory device revealed that the relative intensity in the gate oxide film was stronger than that in the interlayer oxide film. These data suggest that the content of oxygen vacancy centers increases in the vicinity of the interface between the SiO2 film and the Si substrate relative to that of the nonbridging oxidation hole centers. CL spectroscopy provides us with a large amount of data on defects in the SiO2 film of Si-based electronic devices at the 0.1 μm level.
Current-heating formation of small holes in thin gold or silver films
Authors: Perea-López Nestor , Rakov Nikifor , Xiao Mufei
Published: December 2002

We describe a process to form small holes in thin gold or silver films. The prepared films are useful for optical studies on surface plasmon induced transmission as well as some related phenomena.
Surface domain imaging in external magnetic fields
Authors: Steierl G. , Liu G. , Iorgov D. , Kirschner J.
Published: December 2002

We report on experimental advances in scanning electron microscopy with polarization analysis that allow the observation of ferromagnetic domains in external magnetic fields of about 0.1 T. This is achieved by using a modified electron optics that produces a magnetic field at the sample surface that is spatially confined on the length scale of 0.1 mm. During imaging, primary and secondary electrons pass through the magnetic field without significant disturbance. We demonstrate that the primary electron beam may be used to keep track of the generated magnetic field. As an exemplary application, the switching processes of rectangular Permalloy elements are analyzed.
Novel resonant-frequency sensor to detect the kinetics of protein adsorption
Authors: Clark Alison J. , Whitehead Lorne A. , Haynes Charles A. , Kotlicki Andrzej
Published: December 2002

Proteins prefer interfaces, and in aqueous solutions they rapidly adsorb to available solid–liquid interfaces. The adsorption process often involves a change in protein conformation at the surface that can result in functional inactivation of the protein. These changes in protein conformation, which are thought to lead to the formation of an entangled gel-like layer of denatured protein, are responsible for a number of deliterious processes, including biofouling on contact lenses and medical implants. The adsorption process is generally irreversible; dilution of protein in the solution phase does not result in protein desorption from the solid. Presumably, this is due to the effects of the protein denaturation and entanglement process on the rate constant for desorption. Nonspecific protein adsorption to solid–liquid interfaces is, therefore, a kinetically controlled process. Hence, measuring and understanding the kinetics of protein adsorption to solid surfaces, including the kinetics of protein conformational changes, is of considerable interest. We have developed a sensor that responds to protein adsorption kinetics and also, we believe, is sensitive to protein conformational changes during adsorption. The device is operated by monitoring the change in resonant frequency of an elastomeric film (25 μm thick), as an aqueous protein solution is exposed to the surface. Since the mass of a monolayer of the protein or other adsorbent is an extremely small fraction of the mass of the film, the observed change in resonant frequency is due almost entirely to changes in the surface tension of the film. Upon exposing the elastomeric film to a protein solution, we observe a continuing change in resonant frequency for more than 24 h, which is well beyond the time it would take for the population of proteins on the surface to equilibrate under diffusion-limited kinetics. This prolonged response is likely due to the surface energy changes of the sensor as the adsorbed protein molecules change their conformation. We present a description of the basic operation of this sensor as well as some examples of its response to bulk protein and surfactant concentrations.
Ab initio study of the resonant electron attachment to the F2 molecule
Authors: Brems V. , Beyer T. , Nestmann B. M. , Meyer H.-D. , Cederbaum L. S.
Published: December 2002

Dissociative attachment to and vibrational excitation of diatomic molecules by electron impact is discussed within the projection operator approach. The present method lifts the assumption of separability of the discrete-continuum coupling term Vdε(R), i.e., it is no longer required to write it as a product of a function depending on coordinate R and energy ε separately. The method is applied to the Σu2 resonant dissociative electron attachment to and vibrational electron excitation of the F2 molecule. The required ab initio data have been computed using a recently developed algorithm. This algorithm is based on the Feshbach–Fano partitioning technique and the R-matrix method (FFR). The FFR method is discussed in the context of this particular application.
Structures of metallic clusters: Mono- and polyvalent metals
Authors: Lai S. K. , Hsu P. J. , Wu K. L. , Liu W. K. , Iwamatsu M.
Published: December 2002

We present detailed numerical results on the ground state structures of metallic clusters. The Gupta-type many-body potential is used to account for the interactions between atoms in the cluster. Both the genetic algorithm technique and the basin hopping method have been applied to search for the global energy minima of clusters. The excellent agreement found in both schemes for the global energy minima gives credence to the optimized energy values obtained. For four monovalent and one polyvalent metals studied in this work and within the accuracy of the energies presented here, we find that the global energy minima predicted by the basin hopping method are the same as those values obtained by the genetic algorithm. Our calculations for the ground state energies of alkali metallic clusters show regularities in the energy differences, and the cluster growth pattern manifested by this same group of clusters is generally icosahedral, which is quite different from the close-packed and decahedral preferentially exhibited by the tetravalent lead clusters. Considering the inherent disparities in the electronic properties and the bulk structures in these metals (body-centered cubic for alkali metals and face-centered cubic for the lead metal), it is not unreasonable to conjecture that the valence electrons do play a subtle role in the conformation of metallic clusters.
Classical field descriptions for ultrashort tightly-focused laser pulses
Authors: Wang P. X. , Wang J. X.
Published: December 2002

The wave equations for an ultrashort tightly-focused laser pulse in Hermite–Gaussian (0,0) mode are solved approximately. We obtain the analytical field expressions, which are exact up to the second order of the parameters 1/(k0L) and 1/(k0w0) (k0 is the laser wave number, w0 the laser beam waist, and L the laser pulse length). Our solutions can be reduced to usual paraxial ones naturally and more precise compared with the usual paraxial ones.
Effect of lattice expansion on the magnetotransport properties in layered manganites La1.4Sr1.6−yBayMn2O7
Authors: Zhu Hong , Zhu DeLiang , Zhang YuHeng
Published: December 2002

The layered manganite La1.4Sr1.6−yBayMn2O7 was prepared to investigate the relation between magnetotransport properties and lattice expansion in manganites with Sr3Ti2O7-type structure. The lattice expansion caused by Ba2+ substitution enhances the ferromagnetic coupling within the perovskite MnO2 bilayers. Lightly Ba2+ doped samples display the colossal magnetoresistance effect common in the mixed-valent manganese oxides. In the heavily Ba2+ doped samples, the three-dimensional ferromagnetic transition disappears, whereas the metal–insulator transition and the marked magnetoresistance effect remain. By increasing Ba+ content, the metal–insulator transition unexpectedly shifts to a higher temperature. This abnormal behavior may be due to the correlation between spin clusters and percolative transport process in these materials.
Auger recombination in narrow-gap semiconductor superlattices incorporating antimony
Authors: Grein C. H. , Flatté M. E. , Olesberg J. T. , Anson S. A. , Zhang L. , Boggess T. F.
Published: December 2002

A comparison is performed between measured and calculated Auger recombination rates for four different narrow-gap superlattices based on the InAs/GaSb/AlSb material system. The structures are designed for optical or electrical injection for mid-infrared laser applications, with wavelengths ranging from 3.4 to 4.1 μm. The electronic band structures are computed employing an accurate 14-band restricted basis set (superlattice K⋅p) methodology that utilizes experimental information about the low-energy electronic structure of the bulk constituents. The superlattice band structures and their associated matrix elements are directly employed to compute Auger recombination rates. Varying amounts of Auger recombination suppression are displayed by the various superlattices as compared to bulk mid-infrared systems. The greatest disagreement between theory and experiment is shown for the structure predicted to have the most Auger suppression, suggesting the suppression is sensitive either to theoretical or growth uncertainties.
Annealing behavior of magnesium and aluminum implanted with iron ions
Authors: Reuther H.
Published: December 2002

Magnesium and aluminum were alloyed by implantation with iron ions. The implantation energy was 200 keV while the ion doses ranged over several decades from 1×1015 up to 9×1017 cm−2. In this way highly disordered alloyed layers with up to 90 at.% iron were obtained. To study possible ordering, stabilization, and redistribution processes samples were annealed in vacuum at subsequently increasing temperatures up to 600 °C for aluminum and up to 400 °C for magnesium. The annealing process was observed by conversion electron Mössbauer spectroscopy, Auger electron depth profiling, and x-ray diffraction. In the case of the Fe–Al system different iron aluminides are formed. In the case of the Fe–Mg system (insoluble with each other), α-iron is precipitated in small clusters in the final state.