bluuebunny/crossref_metadata_2025 huggingface dataset:
Boundary values as Hamiltonian variables. III. Ideal fluid with a free surface
Authors: Soloviev Vladimir O.
Published: July 2002

An application of the approach to Hamiltonian treatment of boundary terms proposed in previous articles of this series is considered. Here the Hamiltonian formalism is constructed and the role of standard boundary conditions is revealed for a inviscid compressible fluid with surface tension which moves in a field of the Newtonian gravitational potential. It is shown that these boundary conditions guarantee absence of singular contributions to the equations of motion, i.e., to the Hamiltonian vector field. From the other side the Hamiltonian variation contains a nonzero boundary term. Such Hamiltonians are usually treated as “nondifferentiable” or “inadmissible.” We conclude that nondifferentiable functionals can be admissible Hamiltonians for non-ultralocal Poisson brackets. We give a four-sided picture of free surface dynamics: both in Lagrangian and in Eulerian variables and also both in variational and in Hamiltonian approaches.
Interplay between GaN and AlN sublattices in wurtzite AlxGa1−xN alloys revealed by Raman spectroscopy
Authors: Alvarez A. L. , Calle F. , Monroy E. , Pau J. L. , Sanchez-Garcia M. A. , Calleja E. , Muñoz E. , Omnes F. , Gibart P. , Hageman P. R.
Published: July 2002

The large frequency shift displayed by the longitudinal optical (LO) phonons A1(LO) and E1(LO) when going from GaN to AlN promises an accurate determination of the composition in AlxGa1−xN bulk layers by Raman spectroscopy. However, this determination is affected by a large uncertainty for low Al mole fractions (x<0.20), due to the broadened spectral line shape exhibited by these modes. A detailed study of Raman spectra recorded on layers with x<0.27, grown either on sapphire or on silicon substrates, has been performed in order to elucidate the origin of that broadening. The influence on the A1(LO) line shape of the sapphire substrate modes, compositional inhomogeneities, residual strain, and those effects inherent to the lattice dynamics of ternary alloys, is analyzed. We conclude that the broadening is caused by intrinsic inhomogeneities of the microscopic polarization fields resulting from alloying. This effect is usually obscured in other III–V compounds, such as arsenides or phosphides, due to their considerably lower LO–transverse optical splitting.
Strain field and chemical composition determination of InGaN/GaN and AlGaN/GaN multiple quantum wells grown on SiC substrates
Authors: Tagliente M. A. , De Caro L. , Tapfer L. , Waltereit P. , Brandt O. , Ploog K.-H.
Published: July 2002

We analyzed by high-resolution x-ray diffraction experiments the strain status and the chemical composition in wurtzite InxGa1−xN/GaN and AlxGa1−xN/GaN multiple quantum wells (MQW) grown on (0001) SiC substrates by plasma-assisted molecular beam epitaxy. In order to evaluate the lattice deformation in the wurtzite heterostructures, we derived a very general expression of the x-ray incidence parameter which relates the unit cell size to the experimental diffraction peaks separation. All the strain and rotation tensors components of the heterostructures could be determined, which is essential to determine the correct chemical composition. We found a coherent interface between the GaN buffer and the AlxGa1−xN/GaN superlattice, while a partial relaxation of the InxGa1−xN/GaN superlattice with respect to the GaN buffer layer was revealed. The Al and In mole fraction of AlxGa1−xN and InxGa1−xN SL layers were determined by using Vegard’s rule and simulation of the experimental x-ray patterns. We found a pronounced In segregation if the MQWs were grown under metal-stable flux condition; in contrast no appreciable segregation effect was observed under the nitrogen-stable flux condition.
Closed-form electric-field profile model for AlGaAs/GaAs heterostructures
Authors: Castro Francisco , Nabet Bahram , Chen Xiying
Published: July 2002

We propose a closed-form analytical model to describe the electric-field profile inside the GaAs layer of a modulation-doped AlGaAs/GaAs heterostructure. Derived from an accurate two-dimensional electron gas density expression, this model serves as a tool in the simulation of field effects on the spectral response of AlGaAs/GaAs metal–semiconductor–metal photodetectors. It is also suitable for analyzing surface field effects on lateral confinement mechanisms in mobility calculations for enhancement-mode heterostructure-based transistors. Results from this analytical expression are shown to agree well with numerical simulations performed on a physical device simulator and a modified self-consistent solution of Schrödinger and Poisson equations.
Influence of microstructures and crystalline defects on the superconductivity of MgB2
Authors: Serquis A. , Liao X. Z. , Zhu Y. T. , Coulter J. Y. , Huang J. Y. , Willis J. O. , Peterson D. E. , Mueller F. M. , Moreno N. O. , Thompson J. D. , Nesterenko V. F. , Indrakanti S. S.
Published: July 2002

This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB2, with the objective to improve its flux pinning. A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (Jc) at higher fields than its un-HIPed counterpart. The HIPed sample had a Jc of 10 000 A/cm2 in 50 000 Oe (5 T) at 5 K. This was 20 times higher than that of the un-HIPed sample, and the same as the best Jc reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep Jc drop with magnetic field H that occurred in the un-HIPed MgB2 pellet at H>45 000 Oe(4.5 T) and T=5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB2 pellet and to generate more dislocations in the pellets. These dispersed MgO particles and dislocations improved flux pinning also at H<45 000 Oe. The HIPing process was also found to lower the resistivity at room temperature.
Self-reciprocating radioisotope-powered cantilever
Authors: Li Hui , Lal Amit , Blanchard James , Henderson Douglass
Published: July 2002

A reciprocating cantilever utilizing emitted charges from a millicurie radioisotope thin film is presented. The actuator realizes a direct collected-charge-to-motion conversion. The reciprocation is obtained by self-timed contact between the cantilever and the radioisotope source. A static model balancing the electrostatic and mechanical forces from an equivalent circuit leads to an analytical solution useful for device characterization. Measured reciprocating periods agree with predicted values from the analytical model. A scaling analysis shows that microscale arrays of such cantilevers provide an integrated sensor and actuator platform.
Ergodic theory and experimental visualization of invariant sets in chaotically advected flows
Authors: Mezić Igor , Sotiropoulos Fotis
Published: July 2002

We analyze a recently proposed experimental technique for constructing Poincaré maps in flows exhibiting chaotic advection and develop the theoretical framework that explains the reasons for the success of this approach. The technique is nonintrusive and, thus, simple to implement. Planar laser-induced fluorescence is employed to collect a sufficiently long sequence of instantaneous light intensity fields on the plane of section of the Poincaré map (defined by the laser sheet). The invariant sets of the flow are visualized by time-averaging the instantaneous images and plotting iso-contours of the so resulting mean light intensity field. By linking the Eulerian time averages of light intensity at fixed points in space with the Lagrangian time averages along particle paths passing through these points, we show that ergodic theory concepts can be used to show that this procedure will indeed visualize invariant sets of the Poincaré map. As the technique is based on time-averaging, we discuss the rates of convergence and show that inside regular islands the convergence is fast. An example is presented from the application of this technique to visualize the intricate web of regular islands within a steady, three-dimensional vortex breakdown bubble.
Photodissociation of CSCl2 at 235 nm: Kinetic energy distributions and branching ratios of Cl atoms and CSCl radicals
Authors: Einfeld Tina S. , Maul Christof , Gericke Karl-Heinz , Chichinin Alexei
Published: July 2002

The photodissociation dynamics of thiophosgene (CSCl2) and the respective branching ratios of both dissociation products Cl and CSCl have been studied by 3D imaging of the photodissociation product chlorine in its ground state P3/22[Cl] and excited spin–orbit state P1/22[Cl*] employing the resonance enhanced multiphoton ionization and time-of-flight technique at a dissociation wavelength of about 235 nm. A novel technique is applied where the complete three-dimensional (3D) momentum vector of a reaction product is directly determined. The kinetic energy distribution (KED) for Cl* is observed for the first time. The obtained KEDs of Cl and Cl* are different in the low kinetic energy range due to the correlating state of the partner fragment CSCl. In the case of ground state Cl the CSCl partner radical is produced in the ground X̃, Ã, and B̃ states with a contribution of 4±0.5%, 60±5%, and 36±3%, respectively. In the case of Cl* the corresponding CSCl is produced with a contribution of 7.5±0.5% in the ground X̃, 71.5±5.5% in state Ã, and 21±1.5% in state B̃. The yield of Cl*, φ(Cl*)=P(Cl*)/[P(Cl)+P(Cl*)], was found to be 0.47. No significant velocity dependence of the anisotropy parameter β could be observed. The mean value +0.03 suggests a decay on the B̃ (A1) surface.
On Keller theorem for anisotropic media
Authors: Fel Leonid G.
Published: July 2002

The Keller theorem in the problem of effective conductivity in anisotropic two-dimensional (2D) many-component composites makes it possible to establish a simple inequality σise(σi−1)⋅σise(σk)>1 for the isotropic part σise(σk) of the second rank symmetric tensor σ̂i,je of effective conductivity.
Detection of magnetic data using a magnetooptic indicator
Authors: Helseth L. E. , Il’yashenko E. I. , Baziljevich M. , Johansen T. H.
Published: July 2002

We investigate the detection of magnetic data using magnetooptical indicators with in-plane magnetization. A simple model for the magnetooptical detection system is presented. We find that the signal to noise ratio changes strongly with the bit size, the polarization noise, and the distance between the magnetic carrier and the indicator. In particular, it is found that our model a signal to noise ratio of 30 is possible for a bit size of 200 nm. We also estimate the resolution of the system, and find that a spot size of ∼200 nm can be obtained using a suitably designed solid immersion lens. Finally, we discuss detection of several magnetic tracks simultaneously.
Effect of plasma and thermal annealing on optical and electronic properties of SnO2 substrates used for a-Si solar cells
Authors: Hegedus Steven S.
Published: July 2002

The sensitivity to various types of annealing treatments of three commercially available textured SnO2 substrate materials was investigated using optical transmission and Hall effect measurements. The treatments included H2 plasma and annealing in H2/Ar, Ar, or air from 100 to 400 °C. With both types of H2 treatments, the mobility of the SnO2 having the lowest carrier density (<2×1020 cm−3) doubles from ∼30 to ∼60 cm2/V s while the mobilities of SnO2 materials having a higher carrier density were unaffected. There was no significant change in carrier density with either treatment for any SnO2. The visible transmission degrades significantly with H2 plasma at 200 °C but it is unaffected by H2/Ar annealing up to 400 °C. A 20 nm layer of sputtered ZnO is sufficient to protect the SnO2 from plasma-induced damage while still allowing the factor of 2 improvement in SnO2 mobility. The resistivity of bare SnO2 can be decreased by a factor of 2 without any loss of transmission using H2/Ar or Ar annealing at 400 °C without a protective ZnO layer. The changes were stable over 2 years of storage in air. Results are consistent with thermal desorption of oxygen from grain boundaries in a reducing ambient. An optimum SnO2 product for superstrate a-Si solar module fabrication should be deposited with a low carrier density, <2×1020 cm−3, then annealed in H2/Ar or Ar at 300–400 °C to increase the mobility without increasing the absorption losses.
Theory of free-electron maser with two-dimensional distributed feedback driven by an annular electron beam
Authors: Ginzburg N. S. , Peskov N. Yu. , Sergeev A. S. , Konoplev I. V. , Cross A. W. , Phelps A. D. R. , Robb G. R. M. , Ronald K. , He W. , Whyte C. G.
Published: July 2002

The use of two-dimensional (2D) distributed feedback is considered as a method of providing spatially coherent radiation from an oversized annular electron beam. To realize the feedback mechanism, 2D Bragg structures formed from doubly-corrugated waveguide sections of coaxial geometry are suggested. The properties of two types of coaxial cavities formed using such structures are compared: a single-section 2D Bragg cavity and a two-mirror cavity. The eigenmodes of both cavities are found and their high selectivity over both azimuthal and longitudinal indices was demonstrated. Time-domain analyses of the excitation of the cavities by an annular electron beam were carried out. The influence of the cavity parameters on the oscillation regime is analyzed and discussed. It was shown that for a specific set of 2D Bragg cavity parameters it is possible to obtain a regime of steady-state oscillations when the transverse size of the beam exceeds the wavelength by a few orders of magnitude, while outside this parameter space multimode oscillation takes place. The design of a 2D Bragg free-electron maser oscillator based on a high-current accelerator at the University of Strathclyde is discussed.
Microscopic mapping of specific contact resistances and long-term reliability tests on 4H-silicon carbide using sputtered titanium tungsten contacts for high temperature device applications
Authors: Lee S.-K. , Zetterling C.-M. , Östling M.
Published: July 2002

We report on the microscopic mapping of specific contact resistances (ρc) and long-term reliability tests using sputtered titanium tungsten (TiW) ohmic contacts to highly doped n-type epilayers of 4H-silicon carbide. The TiW ohmic contacts showed good uniformity with low contact resistivity of 3.3×10−5 Ω cm2. Microscopic mapping of the ρc showed that the ρc had a distribution that decreased from the center to the edge of the wafer. This distribution of the ρc is caused by variation of the doping concentration of the wafer. Sacrificial oxidation at high temperature partially recovered inductively coupled plasma etch damage. TiW contacts with platinum and gold capping layers have stable specific contact resistance at 500 and 600 °C in a vacuum chamber for 308 h.
Tin whiskers studied by focused ion beam imaging and transmission electron microscopy
Authors: Sheng George T. T. , Hu C. F. , Choi W. J. , Tu K. N. , Bong Y. Y. , Nguyen Luu
Published: July 2002

In surface mount technology of electronic packaging, the lead frames are finished with a layer of Pb-free solder. A large number of Sn whiskers are found on the surface of the finish, especially that of eutectic SnCu or pure Sn. The whiskers have a faster growth rate on an eutectic SnCu finish than on a pure Sn finish. Some of the whiskers on SnCu are long enough to short neighboring legs of the lead frame. We report here the study of spontaneous growth of Sn whiskers on these finishes using focused ion beam imaging and transmission electron microscopy. Cross-sectional samples, both normal and parallel to the growth direction of a whisker, were prepared. Precipitates of Cu6Sn5 in the grain boundaries of the finishes have been found. The growth of these grain boundary precipitates, due to the chemical reaction between Cu and Sn at room temperature, provides the driving force for spontaneous Sn whisker growth. Many more of these grain boundary precipitates exist in the SnCu finish than in the pure Sn finish. This is the main reason why Sn whiskers grow faster on the SnCu finish.
Impact of the defects on the electrical and optical properties of AlGaN ultraviolet photodetectors
Authors: Hanzaz M. , Bouhdada A. , Gibart P. , Omnès F.
Published: July 2002

Studies carried out on gallium nitride showed that they have a high defect density in their band gap. These defects have direct influence on the electrical characteristics of the devices, by contributing leakage currents and the trap assisted tunnel currents. In this article, we carried out numerical calculations based on the experimental data, in order to study the contribution of each component of the current. A comparison has been made between the devices based on GaN and on AlGaN. We study also the capacitance–frequency characteristics, which enable us to locate defects in the band gap. We discuss the effect of the series resistance on the current–voltage characteristics, as well as the influence of defects on the photodetector response time and on the rejection ratio.
Mechanism of low-energy electron stimulated desorption of O− from hydrogenated and hydrogen-free diamond surfaces exposed to activated oxygen
Authors: Laikhtman A. , Le Coat Y. , Hamou M. Hadj , Azria R. , Hoffman A.
Published: July 2002

In this work we report on a study of the mechanism of O− electron stimulated desorption (ESD) from hydrogenated and hydrogen-free polycrystalline diamond films exposed to thermally activated oxygen for incident electron energies in the 4–22 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes: (i) total O− and H− yields as a function of incident electron energy and (ii) kinetic-energy distribution (KED) of O− desorbed from the hydrogen-free diamond surface. The discussed ESD mechanism is referred to the information obtained from x-ray photoelectron spectroscopy, near-edge x-ray absorption fine structure, and core level H+ photodesorption measurements which reveal formation of C=O and C–O–C bonds on the hydrogen-free diamond surface, and C=O and C–O–H bonds on the hydrogenated one. Based on the maximum kinetic-energy value of O− and the ESD threshold measured for hydrogen-free surface, all low-energy (5–10 eV) O− ions are attributed to desorption by the dissociative electron attachment (DEA) to C–O–C, while DEA to C=O occurs at the incident electron energy higher than ∼10 eV. O− ESD from the hydrogenated diamond surface exposed to thermally activated oxygen is a more complicated process. Its threshold is substantially higher than for hydrogen-free diamond, and the line shape of the ESD yield curve is very similar to that of chemisorbed CO molecules. Several reaction pathways leading to production of O− by DEA are discussed for this sample. At incident electron energies higher than ∼15 eV, O− ESD proceeds also via dipolar dissociation processes for both hydrogenated and hydrogen-free diamond surfaces.
Variational density matrix functional theory calculations with the lowest-order Yasuda functional
Authors: Cioslowski Jerzy , Pernal Katarzyna
Published: July 2002

Fully variational density matrix functional theory calculations reveal a critical flaw in the Yasuda functional derived from the contracted Schrödinger equation and the lowest-order cumulant expansions of the reduced density matrices. Although it yields finite energies in conjunction with finite basis sets, it appears to be unbound from below even for one of the simplest two-electron systems, namely, the helium atom at the s limit, once a complete basis set is employed. This observation casts serious doubts upon its practical usefulness in electronic structure calculations.
Time-dependent quantum wave packet study of H+HCN→H2+CN reaction
Authors: Ma Wan-Yong , Han Ke-Li , Wang Ming L. , Zhang John Z. H.
Published: July 2002

Time-dependent quantum wavepacket calculations for the H+HCN reaction are carried out on the ab initio potential energy surface of ter Horst et al. [J. Chem. Phys. 105, 558 (1996)]. The dynamics calculations are performed using both the semirigid vibrating rotor target (SVRT) model [J. Chem. Phys. 111, 3929 (1999)] as well as the pseudo atom–diatom model. Total reaction probabilities from the initial ground state of the reagent are calculated for various values of the total angular momentum quantum number J. Reaction cross sections and rate constants are also calculated. The dynamical result from the SVRT calculation is compared with that from a pseudo atom–diatom calculation in which the HCN is treated as a pseudo diatom. Both the SVRT and pseudo atom–diatom calculations involve three degrees of freedom for the H+HCN reaction due to linearity of the HCN molecule at both reactant and transition states. The results from these two calculations are generally close to each other with some difference at high collision energies. The two models for the current system are essentially the same except that the rotational constant used is different. In particular, the SVRT model uses the correct rotational constant for the linear HCN molecule while the pseudo atom–diatom model produces a rotational constant which is much larger than the correct one.
Perturbation-facilitated optical-optical double resonance spectroscopy of the h0u−(3P1) and H1u(3P1) ion-pair states of I2
Authors: Motohiro Satoshi , Nakajima Shinsuke , Ishiwata Takashi
Published: July 2002

Perturbation-facilitated optical-optical double resonance spectroscopy has been applied to study the h0u−(3P1) and H1u(3P1) ion-pair states of I2 correlating with I−(1S)+I+(3P1). The intermediate states used were the g/u parity mixed states found in the B 3∏(0u+)−X 1∑g+ absorption spectrum near the dissociation limit. We identified several B 3∏(0u+)∼3∏(0g−) coupled states by hyperfine interactions for the first time. These states had the 0u+ and 0g− double-faced character and facilitated to combine the X 1∑g+ ground state with the h0u−(3P1) state in the (1+1) photon excitation scheme following the optical selection rules for one-photon transition. We also accessed the H1u(3P1) state through the B 3∏(0u+)∼c1g mixed state, and elucidated the heterogeneous coupling between the h0u−(3P1) and H1u(3P1) states. Our analyses covered the region of about 48 200 to 50 300 cm−1, and gave the molecular parameters of the h0u−(3P1) state valid for v=0–17 and those of the H1u(3P1) state for v=0–20. The Π3(0g−) state used as an intermediate state was analyzed through the dispersed h0u−(3P1)−3∏(0g−) emission in conjunction with the Franck–Condon factor calculations, and the principle spectroscopic constants of the Π3(0g−) state were reported.
The role of long-lived dark states in the photoluminescence dynamics of phenylene vinylene conjugated polymers
Authors: Lim Sang-Hyun , Bjorklund Thomas G. , Gaab Kevin M. , Bardeen Christopher J.
Published: July 2002

The role of intermediate nonluminescent states in the relaxation of singlet excitons in the conjugated polymer poly(phenylene vinylene) (PPV) and its soluble derivative poly[(2-methoxy-5-hexyloxy-p-phenylene) vinylene] (MH-PPV) is investigated. Time-resolved luminescence and absorption measurements provide evidence for a long-lived, weakly emissive species in PPV at 17 K, in addition to the luminescent singlet state. Ground state recovery times at this temperature provide evidence that up to 40% of the initially excited chromophores end up in a state that does not relax back to the ground state on the 5 ns time scale of the experiment. As the temperature is raised from 17 K to 290 K, the redshifted emission disappears, the fluorescence decay becomes more rapid, and the magnitude of the long-time bleach decreases. These results can be understood in terms of a three-level model where the initially excited singlet state decays nonradiatively via two separate channels: thermally-activated direct relaxation to the ground state, and nanosecond relaxation into a third, long-lived dark state. As the temperature increases, the thermally-activated process increases at the expense of both the fluorescence and the intermediate dark state population. Using this model, a temperature-independent dark state formation time of 1.8 ns was found for PPV, and 1.1 ns for MH-PPV. Our data and modeling provide no evidence for a subpicosecond relaxation channel in the decay of the luminescent excitons in these phenylene vinylene polymers.
Microwave mixing in microbridges made from YBa2Cu3O7−x thin films
Authors: Il’in K. S. , Siegel M.
Published: July 2002

We present a systematic study of the response of a YBa2Cu3O7−x thin-film microbridge to millimeter wave radiation. The dependencies of the microwave response spectrum on the bias voltage and operating temperature have been measured on samples made from YBa2Cu3O7−x films with different thicknesses. Once the bias voltage exceeds the value corresponding to the first maximum of differential resistance, the value of the −3 dB roll-off frequency of the microbridge response is drastically reduced. On the other hand, at temperatures close to Tc the roll-off frequency rises to about 20 GHz. For high resistivity YBa2Cu3O7−x films with relatively low upper critical magnetic fields the −3 dB roll-off frequency is about 7 GHz even at temperatures much smaller than Tc. To explain the results obtained we consider the presence of two additive mechanisms that contribute to the YBa2Cu3O7−x thin-film microbridge response to microwave radiation: vibration of magnetic vortices and heating of electrons and phonons. The competition between these two mechanisms defines the bias voltage and temperature dependencies of the −3 dB roll-off frequency as well as the recently reported dependence of the intermediate frequency bandwidth of YBa2Cu3O7−x hot-electron bolometer mixers on the local oscillator frequency. Numerical calculations based on this assumption are in a good qualitative agreement with the experimental results.
Effect of contact resistance in solid-state thermionic refrigeration
Authors: Ulrich Marc D. , Barnes Peter A. , Vining Cronin B.
Published: July 2002

An analytical model of thermionic emission cooling that includes contact resistance is presented. The electrical current density necessary for peak operation of thermionic emission coolers is such that even the slightest resistance in the contacts to the devices will significantly reduce the cooling and coefficient of the performance. The effect of contact resistance is analyzed numerically using a model of thermionic emission cooling based on Fermi–Dirac statistics. The cooling and coefficient of performance are shown to be reduced dramatically by even the slightest contact resistance.
Guided ion beam studies of the reaction of Nin+ (n=2–16) with D2: Nickel cluster-deuteride bond energies
Authors: Liu Fuyi , Liyanage Rohana , Armentrout P. B.
Published: July 2002

The kinetic-energy dependences of the reactions of Nin+ (n=2–16) with D2 are studied in a guided ion beam tandem mass spectrometer. The products observed are NinD+ for all clusters and NinD2+ for n=5–16. Reactions for formation of NinD+ are observed to exhibit thresholds, whereas cross sections for formation of NinD2+ (n=5–16) exhibit no obvious barriers to reaction. Rate constants of D2 chemisorption on the cationic clusters are compared with results from previous work on neutral nickel clusters. Nin+–D bond energies as a function of cluster size are derived from threshold analysis of the kinetic-energy dependence of the endothermic reactions, and are compared to previously determined metal–metal bond energies, D0(Nin+–Ni). The bond energies of Nin+–D generally increase as the cluster size increases, and parallel those for Nin+–Ni for many clusters. These trends are explained in terms of electronic and geometric structures for the Nin+ clusters. The bond energies of Nin+–D for larger clusters (n⩾11) are found to be close to the value for chemsorption of atomic hydrogen on bulk phase nickel.
Electrostatics on particles: Phenomenological and orientational density functional theory approach
Authors: HaDuong Tap , Phan Sébastien , Marchi Massimo , Borgis Daniel
Published: July 2002

In order to describe efficiently the solvation of complex solutes in computer simulations, we introduce several simple particle-based models with the requirement that they yield, on average, either an exact or approximate representation of the macroscopic laws of electrostatics. First, in a phenomenological approach, electrostatics of continuous media is formulated in terms of a polarization density free energy functional, which is projected on randomly distributed discrete Lennard-Jones pseudoparticles. The resulting model is that a polarizable fluid, in which the induced dipoles describe both orientational and electronic polarization. The problem of the connection between the macroscopic dielectric constant and the pseudoparticles polarizability is examined and important deviations with respect to the commonly accepted Clausius–Mossotti relation are found. Dipolar saturation effects can also be added to the model to yield a “nonlocal Langevin solvent model” and an approximate, numerically very efficient, “local Langevin solvent model.” The two models are implemented in molecular dynamics simulations and their solvation properties are compared to continuous electrostatics for simple solutes such as spherical ions or ion pairs. Their computational efficiency is also discussed and compared to explicit microscopic solvent models. Then a statistical mechanics approach based on orientational density functional theory ideas is presented. Starting from a microscopic Hamiltonian describing a polar solvent, and for a given position of all the solvent molecules, a preliminary thermodynamic average over all the possible orientations of the molecules is performed. This can done by defining an orientational free-energy functional which, at a formal stage, is perfectly well-defined and exact. Minimization of the functional with respect the angular degrees of freedom yields an effective Hamiltonian acting on the translational degrees of freedom only which can be explored via molecular dynamics simulations. The simplest approximation for the orientational functional yields a version of the nonlocal Langevin solvent model mentioned above. More general approximations are suggested.
Nondegenerate monopole mode of single defect two-dimensional triangular photonic band-gap cavity
Authors: Huh Joon , Hwang Jeong-Ki , Ryu Han-Youl , Lee Yong-Hee
Published: July 2002

The two-dimensional monopole mode is investigated as a candidate for a nondegenerate photonic band-gap (PBG) cavity mode. This monopole mode formed in a single defect triangular PBG cavity is truly nondegenerate and remains nondegenerate in the sense that the introduction of asymmetry does not result in splitting of the mode. Several methods to tune the resonant frequency of the mode are studied. The radii, positions, and shapes of the nearest-neighbor air holes are varied in this analysis using the three-dimensional finite-difference time-domain calculation. The quality factor of the monopole mode formed in a slab waveguide structure is found to be larger than that of the dipole mode when the shape of the nearest-neighbor holes is elliptical.
Thermal stability of dopants in laser annealed silicon
Authors: Takamura Y. , Jain S. H. , Griffin P. B. , Plummer J. D.
Published: July 2002

As semiconductor device dimensions continue to decrease, the main challenge in the area of junction formation involves decreasing the junction depth while simultaneously decreasing the sheet resistance. Laser annealing is being investigated as an alternative to rapid thermal annealing to repair the damage from ion implantation and to activate the dopants. With this technique, uniform, box-shaped profiles are obtained, with dopant concentrations that can exceed equilibrium solubility limits at normal processing temperatures. Unfortunately, these super-saturated dopant concentrations exist in a metastable state and deactivate upon further thermal processing. In this article, we describe a comprehensive study of the deactivation kinetics of common dopants (P, B, and Sb) across a range of concentrations and annealing conditions. For comparison, As deactivation data from the literature is also presented. P and As deactivate substantially at temperatures as low as 500 °C, while Sb at moderate concentrations and B remain fully active until 700 to 800 °C. It is proposed that As and P deactivate through the formation of small dopant-defect clusters while B deactivates through precipitation. The proximity to the surface is shown to be a second-order effect.
A new theoretical insight into the nature of intermolecular interactions in the molecular crystal of urea
Authors: Gora Robert W. , Bartkowiak Wojciech , Roszak Szczepan , Leszczynski Jerzy
Published: July 2002

The nature of interactions in the molecular crystal of urea is analyzed in terms of the interaction energy decomposition. The influence of the electron correlation effects was estimated on the basis of the calculated second order Møller–Plesset corrections and their analysis. In the crystal, the urea molecules form infinite ribbons which reveal strong cooperative effects. The hydrogen-bonded interactions of the orthogonal ribbons do stabilize the crystal, whereas interactions between parallel tapes are repulsive. The stability of the crystal structure is determined by a subtle balance between these two types of interactions. Although, the electron correlation effects are stabilizing, their contribution is rather small in comparison with the total interaction energy.
A self-calibrating, multichannel streak camera for inertial confinement fusion applications
Authors: Donaldson W. R. , Boni R. , Keck R. L. , Jaanimagi P. A.
Published: July 2002

Self-calibrating, multichannel UV streak cameras have been designed, and six units have been deployed on the OMEGA laser system. These instruments acquire 12 channels simultaneously on a low-noise, charge-coupled-device camera in single-shot operation. The instruments can discern temporal features out to a bandwidth of 11 GHz, and the peak signal-to-noise ratio in each channel is 200:1. The unique feature of this system is the self-calibration ability built into it. The geometric distortions, flat field, and sweep speed of each channel can be measured and adjusted on a routine basis. By maintaining a strick regime of weekly calibrations, accurate power-balance measurements on the OMEGA laser can be obtained. These cameras represent a cost-effective solution for power balancing the OMEGA laser system.
Observation and analysis of the infrared spectra of O2–HF near 3950 cm−1 and O2–DF near 2900 cm−1
Authors: Fawzy Wafaa M. , Lovejoy Christopher M. , Nesbitt David J. , Hougen Jon T.
Published: July 2002

Spectra were recorded in the H–F stretching fundamental region for O2–HF and in the D–F region for O2–DF, using a laser difference-frequency spectrometer coupled to a slit-nozzle expansion. By varying the ratio of oxygen to carrier gas, beam temperatures ranging from 5 to 16 K were obtained. One standard uncertainty for the relative frequency position of unblended lines is 0.0001 cm−1. Each spectrum was visually subdivided into a stronger (cold) spectrum and a weaker (hot) spectrum. Lines in the cold spectrum were fit to nearly experimental error, using a rotational Hamiltonian for open-shell complexes taken from the literature. For O2–DF, 21 rotational and spin–rotational parameters (10 each for the upper and lower state plus the band origin) were used to fit 86 transitions to a standard deviation of 0.0002 cm−1. For O2–HF, 23 rotational and spin–rotational parameters were used to fit 83 transitions to a standard deviation of 0.0003 cm−1. The slightly poorer quality of the fit for O2–HF than for O2–DF is probably related to the somewhat larger vibrational amplitudes expected for the van der Waals motions in the protonated species. In spite of strenuous efforts, a simultaneous global fit to measurement error of the eight cold-spectrum branches and fourteen hot-spectrum branches could not be achieved, suggesting some improvement in the model used to derive the fitting Hamiltonian may be necessary.
On a debate over the simulation and mapping of physical clusters in small cells
Authors: Reiss Howard , Djikaev Yuri , Bowles Richard K.
Published: July 2002

This paper attempts to resolve some issues in a published debate concerning the types of approximations involved, either implicitly or explicitly, in an innovative method for the simulation of small physical clusters. The method consists of first simulating the probability that a cluster will be found in a small subvolume of a macrovolume, followed by the use of this probability in the evaluation of the equilibrium number of clusters in the macrosystem. Under certain conditions the quantitative accuracy of the method remains high, regardless of the approximations. However, some of the approximations are subtle and impact the fundamental basis of statistical mechanics. Therefore, it is important to understand them as thoroughly as possible.
“Direct” measurement of the growth rate during the C49 to C54 transformation in TiSi2: Activation energy
Authors: Privitera S. , La Via F. , Quilici S. , Meinardi F. , Grimaldi M. G. , Rimini E.
Published: July 2002

The growth rate of the C54 phase of TiSi2 inside a C49 matrix has been measured by micro-Raman imaging by following the time evolution of the C54 grain radius. The measurement is the most direct that has been achieved up to now, being completely independent of the nucleation process. From the Arrhenius plot, an activation energy of 3.8±0.6 eV the growth process alone has been determined.
Polarized infrared spectroscopy at oblique incidence of optical phonons in In1−xGaxAs epilayers on InP
Authors: Rowell N. L. , Shin H. K. , Lockwood D. J. , Poole P. J.
Published: July 2002

The concentration dependence of the zone-center optical phonons in strained In1−xGaxAs epilayers grown by chemical beam epitaxy on InP(100) has been measured with oblique angle, polarized far infrared reflectivity measurements. The spectra, containing distinct peaks at the phonon frequencies, were curved resolved for the individual phonon modes which included GaAs-like longitudinal optic (LO) and transverse optic (TO), disorder induced, and InAs-like LO and TO phonons. The concentration dependence of the phonon frequencies, examined for 0.31<x<0.56, showed a nearly linear dependence on gallium fraction.
Pseudolinearity in the inverse dielectric susceptibility of poled Pb(Mg1/3Nb2/3)O3–PbTiO3 crystals
Authors: Viehland D. , Li J. F.
Published: July 2002

For poled oriented 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystals, dielectric measurements have revealed pseudolinear (Curie–Weiss) behavior for temperatures below a phase transition; however, significant deviations are observed in the paraelectric phase. The value of the Curie constant (C) in the poled polydomain condition was found to be significantly larger than that in a poled monodomain condition. It is believed that polarization heterogeneity exists within the poled polydomain condition.
Growth mechanisms of carbon nanotubes using controlled production in ultrahigh vacuum
Authors: Hövel H. , Bödecker M. , Grimm B. , Rettig C.
Published: July 2002

We present a method for the preparation of single walled carbon nanotubes (SWNTs) on a highly oriented pyrolytic graphite (HOPG) surface in ultrahigh vacuum (UHV), for which the preparation parameters for the production of metal clusters, fixed to nanometer sized pits on the surface, and the subsequent deposition of carbon can be controlled separately. Using cobalt as the cluster metal we carried out a comprehensive study concerning the influence of the substrate temperature (up to 900 °C) and the effective film thickness for the carbon evaporation. With scanning tunneling microscopy in UHV at room temperature and at T=77 K we observed single, separated SWNTs of about 50 nm length, which frequently were angled or branched and included junctions between sections of different tube diameters. With a statistical evaluation of tube diameters, tube lengths, and cluster heights, we obtained new insights into the growth mechanisms. An increase of tube diameters with increasing substrate temperature and a strong catalytic activity of cobalt clusters with sizes below 4 nm is in agreement with experimental results for the gas phase growth and recent calculations for several growth mechanisms. At T=77 K the atomic structures of the SWNT were imaged together with atomic resolution on the HOPG substrate. The presence of branched SWNTs and the observed alignment of the lattice structure of the SWNT and the HOPG both indicate that the tube growth in our case probably takes place at the moving end of the SWNT and not at the fixed clusters, different from recent experiments using chemical vapor deposition for nanotube growth on substrates.
Modeling of thermal stresses in passivated interconnects
Authors: Hsueh Chun-Hway
Published: July 2002

Analytical modeling is performed to obtain closed-form solutions for the thermal stresses in passivated interconnects in microelectronic devices. A periodic unit cell of a planar passivated interconnect on a substrate is considered. When the aspect ratio of the cross section of the interconnect line has a limiting value, zero or infinity, closed-form solutions for the volume-averaged thermal stresses in the interconnect can be readily derived. These analytical solutions agree with existing finite element calculations. Using the modified shear lag model, the general closed-formed solutions for the thermal stress distributions in the cross section of the unit cell normal to the interconnect line are derived. Specific results are calculated for the system of Al interconnect lines on Si substrate with SiO2 passivation. The effects of the geometrical parameters in the system (e.g., interconnect aspect ratio, distance between interconnect lines, and passivation thickness) on the thermal stress distributions are examined.
Rayleigh–Bénard flow of a rarefied gas and its attractors. I. Convection regime
Authors: Stefanov S. , Roussinov V. , Cercignani C.
Published: July 2002

In this paper we investigate the long time behavior (final state) of the Rayleigh–Bénard (RB) flow of a rarefied monatomic gas for a set of the nondimensional Knudsen and Froude numbers in the intervals Kn∈[1.0×10−3,4×10−2], Fr∈[1.0×10−1,1.5×103]. For the most part of the computations the third nondimensional parameter, the ratio of the cold and hot wall temperatures is fixed to Tc/Th=0.1, corresponding to a large temperature difference (Th serves as reference temperature), for which the RB system is believed to reach most of the possible final states (attractors). The low Knudsen numbers allow the problem to be investigated numerically by using two completely different methods: direct simulation Monte Carlo (DSMC) method (molecular approach) and finite difference (FD) method (continuum approach based on the model of compressible viscous heat conducting gas with state-dependent transport coefficients). As a result the effect of rarefaction on the onset of convection in the two-dimensional case is studied and the zone of convection is delineated. The gravitational instability of the RB system is analyzed by using the exact solution of pure-conduction state. As a result two analytical conditions for large and small Froude numbers, respectively, are suggested as exterior bounds of the zone of convection. Both DSMC and FD calculations, which are in good agreement in the considered set of the governing parameters, confirm the validity of these estimations. The observed hysteresis loops between the found out co-existing attractors within the zone of convection is another finding of the presented study. The results for the lowest Knudsen number considered (Kn=0.001), where a set of qualitatively new regimes and final states have been found, are presented separately in paper II.
Stokes-flow instability due to interfacial surfactant
Authors: Frenkel Alexander L. , Halpern David
Published: July 2002

The linear stability of a two-fluid shear flow with an insoluble surfactant on the flat interface is investigated in the Stokes approximation. Gravity is neglected in order to isolate the Marangoni effect of the surfactant. In contrast to all earlier studies of related fluid systems, we encounter (i) the destabilization (here, of a shear flow) caused solely by the introduction of an interfacial surfactant and (ii) the destabilization (here, of a system with a surfactant) caused solely by the imposition of a Stokes flow. Asymptotic long-wave expressions for the growth rates are obtained.
Wave focusing on the line
Authors: Aktosun Tuncay , Rose James H.
Published: July 2002

Focusing of waves in one dimension is analyzed for the plasma-wave equation and the wave equation with variable speed. The existence of focusing causal solutions to these equations is established, and such wave solutions are constructed explicitly by deriving an orthogonality relation for the time-independent Schrödinger equation. The connection between wave focusing and inverse scattering is studied. The potential at any point is recovered from the incident wave that leads to focusing to that point. It is shown that focusing waves satisfy certain temporal-antisymmetry and support properties. Discontinuities in the spatial and temporal derivatives of the focusing waves are examined and related to the discontinuities in the potential of the Schrödinger equation. The theory is illustrated with some explicit examples.
Growth and optical characterization of Ga2O3 nanobelts and nanosheets
Authors: Dai L. , Chen X. L. , Zhang X. N. , Jin A. Z. , Zhou T. , Hu B. Q. , Zhang Z.
Published: July 2002

Monoclinic Ga2O3 nanobelts and nanosheets are obtained by a simple chemical route involved with H2O at 950 °C in Ar atmosphere. Electron microscopy observations reveal that the as-synthesized Ga2O3 nanobelts and nanosheets are structurally uniform, single crystalline, and most of them are free of defects and dislocations. The nanobelts are growing along with [001] facets, and the nanosheets are stacked up by (011) facets. The Raman scattering spectrum of Ga2O3 nanostructures shows a 30 cm−1 redshift at high wave numbers in comparison with that of bulk Ga2O3 powder. The photoluminescence spectrum reveals that there exists a stable blue emission band centered at 460 nm, which is mainly attributed to the oxygen vacancies in the Ga2O3 nanostructures.
Determination of carrier concentration in n-ZnSe by reflectance difference spectroscopy: Experimental results and model calculation
Authors: Kumagai N. , Hanada T. , Yao T. , Yasuda T.
Published: July 2002

We have performed in situ and ex situ measurements of reflectance difference (RD) spectra for Cl-doped n-type ZnSe grown by molecular beam epitaxy. The linear electro-optic (LEO) effect (or Pockels effect) in the depletion layer induces characteristic RD features near the E1 and E1+Δ1 transition energy. The intensity of these features is correlated with the carrier concentration determined from capacitance–voltage measurements. A quantitative model is presented to explain the observed dependence of the LEO signal on the carrier concentration. This model quantitatively considers the effect of the finite surface state density. A good fit with the experimental results was obtained by assuming the surface state densities of 3.8×1013 and 2.6×1013 cm−2 eV−1, respectively, for in situ measurement of a Se-terminated surface at 300  °C and for ex situ measurement of an oxide-covered surface at room temperature.