bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and vibrational properties of molecular beam epitaxy grown cubic (Al, Ga)N/GaN heterostructures
Authors: Frey T. , As D. J. , Bartels M. , Pawlis A. , Lischka K. , Tabata A. , Fernandez J. R. L. , Silva M. T. O. , Leite J. R. , Haug C. , Brenn R.
Published: July 2002

The radio-frequency plasma-assisted molecular beam epitaxy of cubic AlyGa1−yN/GaN heterostructures on GaAs(001) substrates is reported. Rutherford backscattering spectroscopy, high resolution x-ray diffraction, and first-order micro-Raman spectroscopy measurements were used to characterize the structural and vibrational properties of the alloy epilayers. The Al content of the alloy is in the range from 0.07<x<0.20. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic (Al, Ga)N/GaN films. The measured Raman shift of the TO phonon mode of the AlyGa1−yN alloy is in good agreement with theoretical calculations.
Two-dimensional heterodyned and stimulated infrared photon echoes of N-methylacetamide-D
Authors: Zanni Martin T. , Asplund Matthew C. , Hochstrasser Robin M.
Published: July 2002

The stimulated infrared photon echo of N-methylacetamide-D [NMAD; CH3(CO)ND(CH3)] was measured and used to determine the vibrational frequency correlation function. The correlation function was modeled as a single exponential plus a constant, and it was found that most of the NMAD vibrational frequency distribution is motionally narrowed with a pure dephasing time of 1.12 ps. The two-dimensional infrared (2D IR) spectrum of NMAD was also obtained by heterodyning the echo field with a weak local oscillator pulse. The real and imaginary portions of the 2D IR spectrum exhibit multiple peaks due to υ=0–1 and 1–2 coherences that are excited, which are not resolved in the absolute magnitude of the 2D IR spectrum. Using the correlation function determined from the stimulated photon echo, the 2D IR spectrum was accurately simulated. Resolution enhancement of the 2D IR spectrum was performed by manipulating the photon echo field with window functions. The enhanced experimental and simulated 2D IR spectra are dramatically narrowed.
Elastic strain relief in nitridated Ga metal buffer layers for epitaxial GaN growth
Authors: Kim Yihwan , Shapiro Noad A. , Feick Henning , Armitage Robert , Weber Eicke R. , Yang Yi , Cerrina Franco
Published: July 2002

Gallium nitride epitaxial layers were grown on sapphire by molecular-beam epitaxy using nitridated gallium metal films as buffer layers. The mechanical properties of the buffer layers were investigated and correlated with their chemical composition as determined by synchrotron radiation photoelectron spectroscopy. Biaxial tension experiments were performed by bending the substrates in a pressure cell designed for simultaneous photoluminescence measurements. The shift of the excitonic luminescence peak was used to determine the stress induced in the main GaN epilayer. The fraction of stress transferred from substrate to main layer was as low as 27% for samples grown on nitridated metal buffer layers, compared to nearly 100% for samples on conventional low-temperature GaN buffer layers. The efficiency of stress relief increased in proportion to the fraction of metallic Ga in the nitridated metal buffer layers. These findings suggest GaN films containing residual metallic Ga may serve as compliant buffer layers for heteroepitaxy.
Dissimilarity between the velocity and temperature fields in a perturbed turbulent thermal boundary layer
Authors: Kong Hojin , Choi Haecheon , Lee Joon Sik
Published: July 2002

Dissimilarity between the streamwise velocity and temperature fields is investigated in a perturbed turbulent thermal boundary layer using direct numerical simulation. Perturbations are provided into a turbulent boundary layer by uniform blowing or suction from a spanwise slot. The skin-friction coefficient and Stanton number are significantly changed due to blowing and suction. The streamwise evolution of the Stanton number is very different from that of the skin-friction coefficient because the pressure gradient induced by blowing or suction plays an important role in the momentum transport. Before the slot, the mean streamwise velocity changes smoothly, as compared to the mean temperature, due to the pressure gradient, and a similar behavior is also observed in the variations of the streamwise velocity and temperature fluctuations. Above the slot, the change in the mean streamwise velocity is smaller than that in the mean temperature, and after the slot the mean streamwise velocity recovers slowly to the unperturbed flow as compared to the mean temperature. By investigating each term in the mean streamwise velocity and temperature budgets, it is concluded that the mean pressure gradient is the main source of dissimilarity between the mean streamwise velocity and temperature fields. The budgets for the variances of the streamwise velocity and temperature fluctuations show that the difference between the production terms in both budget equations, which results from the existence of the mean pressure gradient, is the primary source of dissimilarity between the streamwise velocity and temperature fields above the slot, whereas downstream of the slot, the velocity pressure-gradient term (i.e., pressure gradient fluctuation) is another important source of dissimilarity. Dissimilarity between the streamwise velocity and temperature fluctuations caused by blowing persists longer in the streamwise direction than that by suction.
Chemisorption of benzene on metal dimer anions: A study by photoelectron detachment spectroscopy
Authors: Lüttgens G. , Pontius N. , Friedrich C. , Klingeler R. , Bechthold P. S. , Neeb M. , Eberhardt W.
Published: July 2002

Photoelectron detachment spectra of M2(C6H6)− (M=Pt, Pd, Pb) have been measured in the gas phase using photon energies of a Nd:YAG laser. The vibrationally resolved ground state transition from the anion to the neutral reveals an adiabatic electron affinity of (2.01±0.05) eV and (0.88±0.05) eV for Pt2(C6H6) and Pd2(C6H6), respectively. A ground state vibrational energy of (24.2±1) meV has been resolved for Pt2(C6H6). The corresponding vibrational energy of Pt2(C6H6)− amounts to (19.0±1.0) meV. The ground state vibrational energies of Pd2(C6H6) and Pd2(C6H6)− are (20.3±1.0) meV and (18.0±2.0) meV, respectively. The small vibrational frequencies suggest a perpendicular coordination (C6v-symmetry) of the benzene-adsorbed transition metal dimers. Pb2, on the other hand, is bound parallel to the benzene plane (C2v-symmetry). A closed shell ground state electron configuration is postulated for Pb2(C6H6) in contrast to the triplet ground state of unreacted Pb2. The vertical electron affinity of Pb2(C6H6) is (1.95±0.05) eV.
Structure and stability of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30
Authors: Blake Nick P. , Bryan Dan , Latturner Susan , Møllnitz Lone , Stucky Galen D. , Metiu Horia
Published: July 2002

We use density functional calculations and single-crystal x-ray diffraction measurements to study structure and bonding in the solid state clathrates Ba8Ga16Ge30, Ba8Ga16Si30, Sr8Ga16Ge30, and Ba8In16Sn30. The structures calculated by minimizing the energy provided by the density functional theory agree well with those determined by x-ray scattering. The preferred stoichiometry is found to always have 8 group II, 16 group III, and 30 group IV elements. The resultant structures are shown to be substantially more stable than the constituent elements in their standard states at room temperature and pressure. Calculations show that the group III elements prefer to be located in the six rings of the structure and are distributed to avoid bonding to one another. Motion of the group II atom (the guest) within the cages is facile, with estimated frequencies for vibration ranging from 40 to 100 cm−1. While these results may suggest a weak guest-frame bond, we find that the binding energy is over 4 eV per guest. We demonstrate that the formation of A8B16C30 from A8 and B16C30 takes place through the donation of 16 electrons (per unit cell) from the bands of A8 into the empty bands of B16C30. The guest atoms are thus charge donors. However, the spatial charge distribution of the eight donor orbitals of A8 is found to be very similar to that of the eight acceptor orbitals of B16C30. Thus while the guest is an electron donor, it is not ionic in these materials.
Hot electron generation via vacuum heating process in femtosecond laser–solid interactions
Authors: Chen L. M. , Zhang J. , Dong Q. L. , Teng H. , Liang T. J. , Zhao L. Z. , Wei Z. Y.
Published: July 2002

Hot electron generation by the vacuum heating process has been studied in the interaction of 150 fs, 5 mJ, 800 nm P-polarized laser pulses with solid targets. The measurements have suggested that the “vacuum heating” is the main heating process for the hot electrons with high energies. The energy of the vacuum-heated hot electrons has been found to be higher than the prediction from the scaling law of resonance absorption. Particle-in-cell simulations have confirmed that the hot electrons are mainly generated by the vacuum heating process under certain experimental conditions.
Observation of heterogeneity in the nanosecond dynamics of a liquid
Authors: Yang Min , Richert Ranko
Published: July 2002

We have measured the time-integrated fluorescence of coumarin 153 in a glass-forming solvent in order to assess the inhomogeneous linewidth as a function of temperature. A maximum of this width is observed at a temperature where the structural relaxation time of the solvent matches the excited state lifetime of the dye of a few nanoseconds. This feature can be explained only on the basis of a relaxation time dispersion which originates from a spatial distribution of solvent response times. Therefore, liquid dynamics in the regime of nanosecond time scales are heterogeneous with respect to the structural relaxation time.
A new separable potential operator for representing a chemical bond and other applications
Authors: Abarenkov I. V. , Tupitsyn I. I.
Published: July 2002

A new type of potential operator has several kinds of applications in electronic structure calculations. Three uses are envisaged here. First when some special region of a covalently bonded solid or very large molecule is modeled by a modest sized cluster, each dangling bond at the cluster surface can be saturated in a way that exactly reproduces the bond in the complete system. Second a similar approach can be used at the matching surface in an embedding scheme for calculations on the same type of systems. The third application is to atomic pseudopotentials where the new potential operator avoids the possibility of “ghost” states that sometimes plague the widely used Kleinman–Bylander form of the pseudopotential. The theory of the new separable potential and its application to the dangling bond problem are the main subjects of the present paper. Starting from a given potential or pseudopotential, the new separable operator modifies some of the required eigenfunctions and eigenvalues in a controlled way while conserving all other eigenvalues. The method has been tested on the molecules X–SiH3 where X=H, F, Cl, Br, or I.
Defect characterization in ZnO layers grown by plasma-enhanced molecular-beam epitaxy on (0001) sapphire substrates
Authors: Vigué F. , Vennéguès P. , Vézian S. , Laügt M. , Faurie J.-P.
Published: July 2002

The structural properties of high-quality (0001)ZnO/Al2O3 films grown by plasma-enhanced molecular-beam epitaxy are investigated by x-ray diffraction and transmission electron microscopy. The only defects encountered are threading dislocations with a density of 1010–4×1010 cm−2. Most numerous dislocations are pure-edge dislocations (Burgers vector of 1/3〈112̄0〉), which accommodate slight in-plane misorientations between subgrains. The oxygen polarity of these films is also established.
Chemical reaction dynamics with stochastic potentials below the high-friction limit
Authors: Shepherd Tricia D. , Hernandez Rigoberto
Published: July 2002

Thermally activated escape over a fluctuating barrier is studied by numerical simulations of the Langevin equation. A class of merged harmonic oscillator potentials are used to model barrier fluctuations continuously over the entire domain of the dynamics. Though the friction constant in the Langevin equation is Ohmic (local), the correlation in the stochastic potentials is taken to be exponential (nonlocal) with a specified decay constant. These models have previously been restricted to the overdamped regime. In this limit, the high friction constants ensure the system’s thermalization despite the additional forces acting on the system effected by the stochastic potential of mean force. To insure that equipartition is obeyed below the overdamped regime, the friction constant is modified so as to dissipate this excess energy. This phenomenological choice is exact if the fluctuations in the stochastic potential are induced by a Gaussian random force and are either uniform across the configuration space or the dynamics are determined only at the transition state. Otherwise it provides agreement with the exponent of the equilibrium distribution to second order in the inverse temperature. With this scheme, the stochastic dynamics are shown to exhibit both the Kramers turnover and resonant activation over a wide range of friction and decay constants. The zero and infinite limits of the latter are also obtained using both simulations and analytic rate formulas.
Atomic scale simulation of defect production in irradiated 3C-SiC
Authors: Devanathan R. , Weber W. J. , Gao F.
Published: July 2002

Molecular dynamics simulations using a modified Tersoff potential have been used to study the primary damage state and statistics of defect production in displacement cascades in 3C-SiC. Recoils with energies from 0.25 to 50 keV have been simulated at 300 K. The results indicate that: (1) the displacement threshold energy surface is highly anisotropic; (2) the dominant surviving defects are C interstitials and vacancies; (3) the defect production efficiency decreases with increasing recoil energy; (4) defect clusters are much smaller and more sparse compared to those reported in metals; and (5) a small fraction of the surviving defects are antisite defects.
Efficient and high-power AlGaN-based ultraviolet light-emitting diode grown on bulk GaN
Authors: Nishida Toshio , Saito Hisao , Kobayashi Naoki
Published: July 2002

By introducing thick bulk GaN as a substrate, we improved the performance of an AlGaN-based ultraviolet (UV) light-emitting diode (LED). The output power exceeds 3 mW at the injection current of 100 mA under a bare-chip geometry. Internal quantum efficiency is estimated as more than 80%, and the peak wavelength is 352 nm. The maximum power exceeds 10 mW at a large current injection of 400 mA, with an operation voltage of less than 6 V. These results indicate that an efficient UV LED is intrinsically possible by the combination of appropriate device design and the nitride substrate. By introducing packaging technology to enhance extraction efficiency, we will have a compact and efficient UV light source in the wide wavelength range of 200–360 nm, similar to conventional longer-wavelength LEDs.
Direct perturbation theory of magnetic properties and relativistic corrections for the point nuclear and Gaussian nuclear models
Authors: Hennum Alf C. , Klopper Wim , Helgaker Trygve
Published: July 2002

Starting from the Lévy-Leblond equation, which is the four-component nonrelativistic limit of the Dirac equation, a direct perturbation theory of magnetic properties and relativistic corrections is developed and implemented for point-charge and finite nuclei. The perturbed small components are regularized by projecting them onto an auxiliary small-component basis of Gaussian functions. The relevant operators and matrix elements are derived for the point-nuclear and Gaussian nuclear models. It is demonstrated how the usual paramagnetic spin-orbit, Fermi-contact, and spin-dipole integrals of Ramsey’s theory can be evaluated in the same manner as field and field-gradient integrals—that is, as derivatives of potential-energy integrals. A few illustrative calculations are performed.
Exact results for parallel-chain kinetic models of biological transport
Authors: Kolomeisky Anatoly B.
Published: July 2002

In order to describe the observed behavior of single motor proteins moving along linear molecular tracks, a class of stochastic models is studied which recognizes the possibility of parallel biochemical pathways. Extending the theoretical analysis of Derrida [J. Stat. Phys. 31, 433 (1983)], exact results are derived for the velocity and dispersion of a discrete one-dimensional kinetic model which consists of two parallel chains of N states and M states, respectively, with arbitrary forward and backward rates. Generalizations of this approach for g>2 parallel chains models are briefly sketched. These results and other properties of parallel-chain kinetic models are illustrated by various examples.
Equilibrium and nonequilibrium molecular-dynamics simulations of the central force model of water
Authors: Bresme Fernando
Published: July 2002

Equilibrium and nonequilibrium molecular-dynamics simulations of the central force model of water (CFM) [Lemberg and Stillinger, J. Chem. Phys. 62, 1677 (1975)] are presented. We consider a model based on a functional form introduced in theoretical studies of associating systems employing integral equations [F. Bresme, J. Chem. Phys. 108, 4505 (1998)]. Results on thermodynamic, dynamic, dielectric, and coexistence properties are presented. The central force model shows satisfactory agreement with the experimental results in all these cases. In addition, nonequilibrium molecular-dynamics simulations show that the CFM predicts a decrease of the thermal conductivity with temperature, as observed in the experiment, but this dependence is reproduced qualitatively at temperatures characteristic of supercooled states. These results emphasize the need for further studies of the heat conduction and properties of water in these conditions. Overall the present potential should provide a basis for further theoretical and simulation studies of complex systems where water is present.
Photoluminescence properties of MgxZn1−xO alloy thin films fabricated by the sol-gel deposition method
Authors: Zhao Dongxu , Liu Yichun , Shen Dezhen , Lu Youming , Zhang Jiying , Fan Xiwu
Published: July 2002

The photoluminescence properties of MgxZn1−xO alloy thin films fabricated by the sol-gel deposition method were studied. The Mg2+ content in the films was up to 0.36 and they had the ZnO wurtzite structure. The band gap of the films can be controlled between 3.40 and 3.93 eV by adjusting the Mg2+ proportions. Transmittance spectroscopy was used to characterize the excitonic structure of the alloys, which the excitonic character is clearly visible at room temperature. The intense ultraviolet photoluminescence was observed at room temperature. This emission is indicative of the excitonic nature of the material.
Theory of thermal conductivity of dense simple fluids
Authors: Rah Kyunil , Chan Eu Byung
Published: July 2002

A theory of thermal conductivity of simple liquids is developed in a way parallel with the theory of shear and bulk viscosities reported in previous papers. A molecular theoretic expression for the thermal conductivity of simple liquids is presented, which consists of two distinctive parts: one that is given in terms of intermolecular forces and the structure of the liquid described by equilibrium pair correlation function and thus strongly depends on the density, and the other that is given in terms of the Chapman–Enskog thermal conductivity and thus independent of the density. The density dependent part of the thermal conductivity is given in terms of the intermolecular force, the equilibrium pair correlation function, and self-diffusion coefficient in a form similar to the shear and bulk viscosities of the liquid, or in terms of the shear viscosity in a form generalizing to liquids the Eucken relation of thermal conductivity and shear viscosity. The theoretical result obtained for thermal conductivity is tested against experimental data available on argon in the literature.
Stress control in GaN grown on silicon (111) by metalorganic vapor phase epitaxy
Authors: Feltin Eric , Beaumont B. , Laügt M. , de Mierry P. , Vennéguès P. , Lahrèche H. , Leroux M. , Gibart P.
Published: July 2002

The strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy has been investigated. The insertion of AlN/GaN superlattices was found to decrease the stress sufficiently for avoiding crack formation in an overgrown thick (2.5 μm) GaN layer. X-ray diffraction and photoluminescence measurements are used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layers. A reduction of threading dislocation density is also observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed with a full width at half maximum of the bound exciton line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) x-ray rocking curve also attests the high crystalline quality of GaN on Si (111), when using these AlN/GaN superlattices.
Thermodynamic theory of thermophotovoltaic solar energy conversion
Authors: Badescu Viorel
Published: July 2002

The thermophotovoltaic (TPV) efficiency is maximized by using three optimization parameters, namely, the absorber and PV solar cell temperatures and the voltage across the cell. Various combinations of spherical and (disk) plane absorbers and solar cells are analyzed. The best performance is associated with a combination plane absorber–plane solar cell. The thermal design has a significant influence on the optimum PV cell band gap. In the case of a normal thermal design, the cell temperature is usually high and depends strongly on the band gap. When a very good thermal design is considered, the optimum cell temperature is less than 30 degrees higher than the ambient temperature and decreases with an increase in the band gap. Increasing the ratio rA/rc between the absorber and PV cell radii leads to increased TPV efficiency. The optimum absorber temperature (practically) does not depend on rA/rc.
Integrated all-fiber variable attenuator based on hybrid microstructure fiber
Authors: Kerbage C. , Hale A. , Yablon A. , Windeler R. S. , Eggleton B. J.
Published: July 2002

We present a design for modulating light in a single-mode optical fiber by achieving efficient field interaction between the fundamental mode of an air–silica microstructure optical fiber and tunable materials incorporated in the air holes of the fiber. This method involves tapering microstructure optical fiber where light is guided by the air holes and modulated by a polymer incorporated in these capillary voids. Using this fiber design, we experimentally demonstrate a variable attenuator (loss filter) with 30 dB dynamic range, insertion loss of less than 0.8 dB, and minimal polarization dependence.
Optical constants of wurtzite ZnS thin films determined by spectroscopic ellipsometry
Authors: Ong H. C. , Chang R. P. H.
Published: July 2002

The complex dielectric functions of wurtzite ZnS thin films grown on (0001) Al2O3 have been determined by using spectroscopic ellipsometry over the spectral range of 1.33–4.7 eV. Below the band gap, the refractive index n is found to follow the first-order Sellmeir dispersion relationship n2(λ)=1+2.22λ2/(λ2−0.0382). Strong and well-defined free excitonic features located above the band edge are clearly observed at room temperature. The intrinsic optical parameters of wurtzite ZnS such as band gaps and excitonic binding energies have been determined by fitting the absorption spectrum using a modified Elliott expression together with Lorentizan broadening. Both parameters are found to be larger than their zinc blende counterparts.
Spark model of pulsed discharge in water
Authors: Lu Xinpei , Pan Yuan , Liu Kefu , Liu Minghai , Zhang Hanhong
Published: July 2002

Underwater sparks have long been used in many fields, but up until now there are no widely accepted model for them. In this article we present a more practical model for them, taking into account the nature of the plasma of pulsed discharge in water, where the following corrections are included: (1) correction of the ionization potentials; (2) pressure caused by the effect of charge interaction on the bound state electrons; (3) line spectra; and (4) mass flow caused by evaporation, condensation, and bubble wall ablation. Using this model, the time-varying pressure, temperature, and density can be obtained for given circuit parameters. The simulation results are in good agreement with the experimental measurements. We also give a simulation with capacitance of 1.25 nF, initial voltage of 4×104 V, circuit inherent inductance of 20 nH, and circuit resistance of 15 mΩ; the maxima of the pressure, temperature, and electron density are about 5×104 K, 5×103 atm, and 4×1026 m−3, respectively.
Effect of an orientational phase transformation on low-temperature creep in n-H2
Authors: Alekseeva L. A. , Stetsenko Yu. E. , Butenko Yu. V. , Solodovnik A. A.
Published: July 2002

The low-temperature stationary creep of solid normal hydrogen grown in a polycrystalline form is investigated. The experimental temperature range 1.4–4.2 K includes the temperature Tc of the phase transformation of n-H2. It is found that as the temperature of n-H2 decreases to Tc the stationary creep rate of the samples abruptly decreases. This which could be due to stiffening of the n-H2 lattice as a result of orientational ordering of the orthomolecules.
Influence of average size and interface passivation on the spectral emission of Si nanocrystals embedded in SiO2
Authors: Garrido Fernandez B. , López M. , Garcı́a C. , Pérez-Rodrı́guez A. , Morante J. R. , Bonafos C. , Carrada M. , Claverie A.
Published: July 2002

The correlation between the structural (average size and density) and optoelectronic properties [band gap and photoluminescence (PL)] of Si nanocrystals embedded in SiO2 is among the essential factors in understanding their emission mechanism. This correlation has been difficult to establish in the past due to the lack of reliable methods for measuring the size distribution of nanocrystals from electron microscopy, mainly because of the insufficient contrast between Si and SiO2. With this aim, we have recently developed a successful method for imaging Si nanocrystals in SiO2 matrices. This is done by using high-resolution electron microscopy in conjunction with conventional electron microscopy in dark field conditions. Then, by varying the time of annealing in a large time scale we have been able to track the nucleation, pure growth, and ripening stages of the nanocrystal population. The nucleation and pure growth stages are almost completed after a few minutes of annealing time at 1100 °C in N2 and afterward the ensemble undergoes an asymptotic ripening process. In contrast, the PL intensity steadily increases and reaches saturation after 3–4 h of annealing at 1100 °C. Forming gas postannealing considerably enhances the PL intensity but only for samples annealed previously in less time than that needed for PL saturation. The effects of forming gas are reversible and do not modify the spectral shape of the PL emission. The PL intensity shows at all times an inverse correlation with the amount of Pb paramagnetic centers at the Si–SiO2 nanocrystal–matrix interfaces, which have been measured by electron spin resonance. Consequently, the Pb centers or other centers associated with them are interfacial nonradiative channels for recombination and the emission yield largely depends on the interface passivation. We have correlated as well the average size of the nanocrystals with their optical band gap and PL emission energy. The band gap and emission energy shift to the blue as the nanocrystal size shrinks, in agreement with models based on quantum confinement. As a main result, we have found that the Stokes shift is independent of the average size of nanocrystals and has a constant value of 0.26±0.03 eV, which is almost twice the energy of the Si–O vibration. This finding suggests that among the possible channels for radiative recombination, the dominant one for Si nanocrystals embedded in SiO2 is a fundamental transition spatially located at the Si–SiO2 interface with the assistance of a local Si–O vibration.
Near-field optical patterning on azo-hybrid sol–gel films
Authors: Landraud N. , Peretti J. , Chaput F. , Lampel G. , Boilot J.-P. , Lahlil K. , Safarov V. I.
Published: July 2002

We report on the near-field optical patterning of photochromic sol–gel films with subwavelength resolution. The sample containing functionalized azobenzene species is locally illuminated in the visible absorption band of these photochromes through the aperture of a metallized tapered optical fiber. The surface topography imaged by in situ shear-force microscopy reveals that, due to repeated photoisomerization cycles of the azobenzene molecules, photoinduced matter migration occurs under the tip leading to the formation of a surface relief. The shape of this structure is characteristic of the electromagnetic field distribution and strongly depends on the tip-to-sample distance. In near-field illumination conditions, protrusions of lateral dimension as small as 60 nm (≈λ/10) are currently produced. When repeating this process, compact arrays of nanodots are optically inscribed.
Integrally gated carbon nanotube-on-post field emitter arrays
Authors: Hsu David S. Y. , Shaw Jonathan
Published: July 2002

Multiwalled carbon nanotubes were grown using chemical vapor deposition on the tops of blunt vertical silicon posts in cells having a horizontal gate aperture of conventional field emitter design. We obtained over 1 mA total emission current from a single array, or 0.3 μA per cell at 40 V. In addition to the low voltage operation, the most distinctive differences from conventional field emitter arrays include their stability and the lack of catastrophic arcing without any special sample preparation.
Experimental investigation of the collapse of a turbulent wake in a stably stratified fluid
Authors: Bonnier Marion , Eiff Olivier
Published: July 2002

This paper is devoted to an experimental characterization of the collapse under buoyancy forces of the turbulent motions in the wake of a sphere in a linearly stable saline stratification. This phenomenon is part of the transition between two commonly recognized regions of the wake, the near wake and the far wake. It is shown here that the wake evolves in four distinct stages: the three-dimensional near wake, the collapse, a transition region, and finally the far wake. Hot-film measurements in the near wake and the region beyond show that initially the mean defect velocity decreases as in a nonstratified fluid, but then increases briefly before continuing to decrease again. The short period of mean defect-velocity increase or mean-flow acceleration characterizes the collapse and results in the higher levels of kinetic energy found in stratified wakes as compared to nonstratified wakes. The far-wake region of the flow was studied by a digital particle image velocimetry technique yielding decay laws for the defect velocity as well as the vertical component of vorticity for both laminar and turbulent wake regimes. The decay of these far-wake quantities was found not to depend on the regime considered, implying that the decay rates are independent of the collapse. However, it was found that the characteristic longitudinal scales do depend on the Froude number, implying a selective memory of the initial conditions in spite of collapse.
Molecular simulation of crystal growth in n-eicosane
Authors: Waheed N. , Lavine M. S. , Rutledge G. C.
Published: July 2002

Molecular dynamics is used to obtain crystal growth rates for a model n-alkane. A united atom model of bulk n-eicosane exhibits an observable phase change from an amorphous phase to a close-packed hexagonal phase, in the presence of a crystal surface. Rates are calculated from the translation of the order–disorder transition in the simulation cell as a function of time. The temperature dependence of crystallization is analyzed in terms of Ziabicki’s rate model, and behavior is considered in light of more coarse-grained models.
Calculation of the electromagnetic field radiated by an elastic wave in a type-II superconductor
Authors: Fil V. D.
Published: July 2002

Formulas describing the conversion of an elastic wave into an electromagnetic field at the boundary between a type-II superconductor and the vacuum are obtained which admit passing to both the limiting cases of the normal and Meissner states. A strategy that would permit experimental measurement of the dynamical parameters of the vortex lattice is discussed. It is pointed out that the study of the inertial (Stewart–Tolman) component of the radiated field is a promising area of research.
Behind the weak excitonic emission of ZnO quantum dots: ZnO/Zn(OH)2 core-shell structure
Authors: Zhou H. , Alves H. , Hofmann D. M. , Kriegseis W. , Meyer B. K. , Kaczmarczyk G. , Hoffmann A.
Published: July 2002

The structure of ZnO quantum dots prepared via the wet chemical method was studied. By introducing an annealing treatment (150 °C–500 °C), we also investigated the effect of the change in the structure of the dots on their luminescence properties. Our studies revealed that the surface of the as-prepared dots is passivated by a thin layer of Zn(OH)2, thus, the dots consist of a ZnO/Zn(OH)2 core-shell structure. We present evidence that the weak excitonic transition of ZnO quantum dots is strongly correlated with the presence of the surface shell of Zn(OH)2. When Zn(OH)2 is present, the excitonic transition is quenched.
Bright visible photoluminescence from silica nanotube flakes prepared by the sol–gel template method
Authors: Zhang Ming , Ciocan Eugenia , Bando Y. , Wada K. , Cheng L. L. , Pirouz P.
Published: July 2002

We report macroscopic synthesis of silica nanotubes by the sol–gel template method. A large number of silica nanotubes with small diameters (30–50 nm) were produced and were shaped into flakes successfully. Strong photoluminescence (PL) was observed in both as-grown and annealed nanotube flakes. The PL spectra have maxima at 2.55 and 2.30 eV for the as-grown and annealed samples, respectively; the PL intensity of annealed nanotubes is much higher than that of as-grown nanotubes. The strong emission may be due to the Si–OH complex located on both the inner and outer surfaces of the nanotubes. The nanotube flakes we prepared may have potential applications in future integrated optical devices.
Electronic defect studies of ladder-type polymers
Authors: Alagiriswamy A. A. , Narayan K. S.
Published: July 2002

Electrically active defects have a profound impact on the semiconducting properties of conjugated polymer systems. Defect-induced thermally stimulated current (TSC) in poly (benzimadazobenzophenanthroline), a ladder-type, high-temperature conjugated polymer, is studied in detail. The TSC results identify the nature and the energetics of the trap levels in the polymer. Variations in the results as a function of the initial trap-filling parameters and the thermal history were also observed. The long-lived component in the photoinduced current decay, an indicator of defect states, is also studied within this context. The barrier limiting processes of the photocurrent are correlated with the results obtained from TSC measurements and we speculate on the origin of these defects.
Perturbation theory for nearly integrable multicomponent nonlinear PDEs
Authors: Shchesnovich V. S.
Published: July 2002

The Riemann–Hilbert problem associated with the integrable PDE is used as a nonlinear transformation of the nearly integrable PDE to the spectral space. The temporal evolution of the spectral data is derived with account for arbitrary perturbations and is given in the form of exact equations, which generate the sequence of approximate ordinary differential equations in successive orders with respect to the perturbation. For vector nearly integrable PDEs, embracing the vector nonlinear Schrödinger and complex modified Korteweg–de Vries equations, the main result is formulated in a theorem. For a single vector soliton the evolution equations for the soliton parameters and first-order radiation are given in explicit form.
Electrical properties and devices of large-diameter single-walled carbon nanotubes
Authors: Javey Ali , Shim Moonsub , Dai Hongjie
Published: July 2002

Individual large-diameter (∼3 to 5 nm) semiconducting single-walled carbon nanotubes (SWNTs) are found to exhibit ambipolar field-effect transistor (FET) behavior, with easily accessible n- and p-conduction channels by simple electrostatic gates. The effects of temperature and ultraviolet radiation on their electrical properties are elucidated, shedding light into the intrinsic behavior of SWNTs in this relatively large-diameter regime. The ambipolar SWNT-FETs can be readily used as building blocks for functional nanoelectronic devices such as voltage inverters that operate under ambient conditions.
Imaging of quantized magnetostatic modes using spatially resolved ferromagnetic resonance
Authors: Tamaru S. , Bain J. A. , van de Veerdonk R. J. M. , Crawford T. M. , Covington M. , Kryder M. H.
Published: July 2002

We present a measurement technique for performing spatially resolved ferromagnetic resonance and directly imaging quantized magnetostatic modes in magnetic samples that undergo high frequency magnetic drive fields (up to 8 GHz). The dynamic response of a 50×50 μm2 permalloy structure (100 nm thick) under a 7.04 GHz highly nonuniform drive field was measured as a function of the dc bias field using this technique. The magnetization variation observed indicates that quantized magnetostatic mode waves appear at certain bias fields, with the number of nodes decreasing with an increase in the bias field. We tentatively assign the indices of each mode using the Damon–Eshbach (DE) model. Similar modes have been observed for a similar sample geometry using an inductive measurement and they showed good agreement with the DE model. However, the result measured using this technique showed some discrepancy with the DE model and the spatial patterns observed are more complicated than simple one-dimensional standing waves. This complexity suggests that analysis beyond that of the DE model is required to explain the observations.
Single crystal hexaferrite phase shifter at Ka band
Authors: Zuo Xu , Shi Ping , Oliver S. A. , Vittoria C.
Published: July 2002

A phase shifter was fabricated from a scandium doped M-type (SCM) barium hexaferrite single crystal substrate, and tested over microwave frequencies from 20 to 40 GHz and bias magnetic fields from 0 to 2 kOe. At a microwave frequency of 40 GHz and bias magnetic field of 1.3 kOe, the differential phase-shift rate was 42°/kOe mm and the loss rate was 0.7 dB/mm. The phase shifter utilized the large uniaxial anisotropy field inherent in ScM single crystals, where the crystallographic c axis and the magnetic bias field were parallel to each other and in the substrate plane. The performance of the phase shifter was modeled by the conjugate method for transmission electron microscopy wave and compared with experimental results.
Intergrain tunneling and magnetotransport properties in CrO2–RuO2/TiO2
Authors: Manoharan S. Sundar , Sahu Ranjan K. , Elefant D. , Schneider C. M.
Published: July 2002

We report an enhanced negative magnetoresistance ratio of about 16% at 4.2 K at 1 T for CrO2/TiO2 composites, compared to CrO2 powder compacts and CrO2/RuO2 composite. The magnetoresistance ratio of 60% at 4.2 K is noted when TiO2 acts as tunneling barrier. The absence of substantial barrier heights with RuO2 grains does not alter the magnetoresistance compared to the CrO2 powder compacts. The insulating TiO2 grains increases the grain boundary barrier for the spin polarized tunneling, thereby enhancing the low field magnetoresistance.
Collision-induced intramolecular energy flow and C–H bond dissociation in excited toluene
Authors: Ree J. , Kim Y. H. , Shin H. K.
Published: July 2002

The collision-induced intramolecular energy flow and C–H bond dissociation in toluene have been studied using classical dynamics procedures. The molecule initially contains high amounts of vibrational excitation in the methyl C–H stretch and the nearby benzene ring C–H stretch and it is in interaction with Ar. The two excited C–H stretches are coupled to each other through two C–C stretching, two H–C–C bending and one C–C–C bending modes, all of which are initially in the ground state. At 300 K, the energy lost by the excited molecule upon collision is not large and it increases slowly with increasing total vibrational energy content between 10 000 and 40 000 cm−1. Above the energy content of 40 000 cm−1, energy loss increases rapidly. Near 65 000 cm−1 energy loss takes a maximum value of about 1000 cm−1. The temperature dependence of energy loss is weak between 200 and 400 K. When the energy content is sufficiently high, either or both C–H bonds can dissociate, producing free radicals, C6H5CH2, C6H4CH3, or C6H4CH2. The ring C–H dissociation occurs almost entirely in a direct-mode mechanism on a subpicosecond time scale. Nearly half of methyl group C–H dissociation events occur on a subpicosecond time scale and the rest through a complex-mode collision in which bond dissociation occurs several picoseconds after the initial impact. In the complex-mode collision, Ar binds to the radical forming a weakly bound benzyl⋯Ar complex. In both dissociative and nondissociative events, intramolecular energy flow is efficient, taking place upon the initial impact on a subpicosecond time scale.
Micromagnetic three-dimensional simulation of the pinning field in high temperature Sm(Co,Fe,Cu,Zr)z magnets
Authors: Scholz W. , Fidler J. , Schrefl T. , Suess D. , Matthias T.
Published: July 2002

We have investigated domain wall pinning in Sm(Co,Fe,Cu,Zr)z based magnets with 3D micromagnetic simulations using a hybrid finite element/boundary element method. Transmission electron micrographs of magnets with Sm2(Co,Fe)17 matrix and Sm(Co,Cu)5−7 precipitation phases were used to model the cellular microstructure. The pinning field was investigated for varying thickness of the intercellular phase.
Nonstationarity of strong collisionless quasiperpendicular shocks: Theory and full particle numerical simulations
Authors: Krasnoselskikh V. V. , Lembège B. , Savoini P. , Lobzin V. V.
Published: July 2002

Whistler waves are an intrinsic feature of the oblique quasiperpendicular collisionless shock waves. For supercritical shock waves, the ramp region, where an abrupt increase of the magnetic field occurs, can be treated as a nonlinear whistler wave of large amplitude. In addition, oblique shock waves can possess a linear whistler precursor. There exist two critical Mach numbers related to the whistler components of the shock wave, the first is known as a whistler critical Mach number and the second can be referred to as a nonlinear whistler critical Mach number. When the whistler critical Much number is exceeded, a stationary linear wave train cannot stand ahead of the ramp. Above the nonlinear whistler critical Mach number, the stationary nonlinear wave train cannot exist anymore within the shock front. This happens when the nonlinear wave steepening cannot be balanced by the effects of the dispersion and dissipation. In this case nonlinear wave train becomes unstable with respect to overturning. In the present paper it is shown that the nonlinear whistler critical Mach number corresponds to the transition between stationary and nonstationary dynamical behavior of the shock wave. The results of the computer simulations making use of the 1D full particle electromagnetic code demonstrate that the transition to the nonstationarity of the shock front structure is always accompanied by the disappearance of the whistler wave train within the shock front. Using the two-fluid MHD equations, the structure of nonlinear whistler waves in plasmas with finite beta is investigated and the nonlinear whistler critical Mach number is determined. It is suggested a new more general proof of the criteria for small amplitude linear precursor or wake wave trains to exist.
Modeling kinetic shifts in threshold collision-induced dissociation. Case study: Dichlorobenzene cation dissociation
Authors: Muntean Felician , Heumann Lars , Armentrout P. B.
Published: July 2002

A threshold collision-induced dissociation (CID) study is performed on dichlorobenzene cation dissociation of both the ortho and para isomers. Ab initio calculations are performed on the system to investigate the details of the potential energy surface with respect to Cl atom loss and to provide the molecular parameters necessary for CID cross section modeling. The effects of kinetic shifts on the CID threshold determinations are investigated using a model that incorporates statistical unimolecular decay theory. The model is tested using unimolecular dissociation rate constants as a function of energy provided by earlier photoelectron–photoion-coincidence (PEPICO) experiments. The different possible sets of parameters involved in the CID model, their effect on the dissociation rates, and their effect on the final CID threshold determination are discussed. A tight transition state is observed to reproduce the experimental dissociation rates better than a phase-space limit loose transition state, a result attributed to a potential energy surface that is much more attractive than a simple ion-induced dipole potential. The dissociation thresholds derived from CID data are in reasonable agreement with the ones derived from fitting the PEPICO rates when similar transition state assumptions are used. A final analysis of the CID data yields 0 K dissociation energies for the Cl atom loss from dichlorobenzene of 3.22±0.17 eV for the ortho isomer and 3.32±0.18 eV for the para isomer. In the present study we support a mechanism that the dissociations of the two isomers proceed through a direct bond cleavage, rather than through isomerization to a common intermediate.
Combining semiclassical time evolution and quantum Boltzmann operator to evaluate reactive flux correlation function for thermal rate constants of complex systems
Authors: Yamamoto Takeshi , Wang Haobin , Miller William H.
Published: July 2002

The semiclassical (SC) initial value representation (IVR) provides a way for including quantum effects into classical molecular dynamics simulations. Implementation of the SC-IVR to the thermal rate constant calculation, based on the reactive flux correlation function formalism, has two major obstacles: (1) the SC integrand may be highly oscillatory with respect to the initial phase space variables; and (2) matrix elements of the Boltzmannized flux operator, which are crucial in generating the initial (or final) distribution for the SC trajectories, are generally not available in analytic forms. In this paper, we present practical ways of overcoming these two barriers for the SC calculation of thermal rate constants. For the first problem, we show that use of a symmetric flux–flux correlation function, together with the generalized Filinov transformation technique, can significantly smooth the corresponding SC integrand and make the calculation practical for quite large systems. For the second problem, we propose a general method for evaluating matrix elements of the Boltzmannized flux operator “on-the-fly,” based on the combination of the imaginary-time path integral technique with the Metropolis random walk algorithm. Using these approaches, it is shown that thermal rate constants can be obtained for systems with more than 100 degrees of freedom, as well as for reactions in the deep tunneling regimes where quantum effects are significant.