bluuebunny/crossref_metadata_2025 huggingface dataset:
Interfacial hoop stress and instability of viscoelastic free surface flows
Authors: Graham Michael D.
Published: May 2003

Viscoelasticity can dramatically exacerbate free surface flow instabilities. It is shown here that this destabilization arises, at least in part, from the combination of large tangential tension and curvature along a concave free surface. Tension and curvature generically lead to an unstable stress gradient at the free surface; a perturbation that locally thickens a fluid layer leads to a local accumulation of hoop stress, which drives further thickening of the layer. A simple theory based in this observation captures several features of experimental observations of coating flows, specifically the correlation between destabilization and extensional viscosity and the increase in wavenumber with viscoelasticity. In a particular model situation, instability of flow with a concave free surface can be reduced to a viscoelastic Rayleigh-Taylor problem, which is amenable to analytical treatment. In this situation the growth rate can be very large, because the stored elastic energy cannot balance the surface work associated with interface deformation. Application of the theory to the instability of filament stretching flow yields a prediction of the Weissenberg number below which instability does not occur that agrees well with experimental observations.
Nitrogen-related electron traps in Ga(As,N) layers (⩽3% N)
Authors: Krispin P. , Gambin V. , Harris J. S. , Ploog K. H.
Published: May 2003

Capacitance spectroscopy is used to examine the compositional dependence of deep levels in Si-doped Ga(As,N) layers grown on GaAs. We find two predominant electron traps at about 0.80 and 1.1 eV above the valence band edge EV, which do not depend on composition. For N contents above 0.1% N, the concentration of the acceptor-like gap level at EV+1.1 eV strongly increases and leads to a distinct reduction of the donor doping efficiency in Ga(As,N) layers. Based on theoretical prediction, this electron trap is tentatively associated with a split interstitial defect containing a nitrogen and an arsenic atom on the same As lattice site [(AsN)As]. The trap at EV+0.80 eV likely corresponds to nitrogen dimers, i.e., two N atoms on a single As site [(NN)As]. When approaching the critical layer thickness, this electron trap is increasingly generated during growth. The dimer defect can be removed by rapid thermal annealing at 720 °C after growth, in contrast to the stable bulk level at EV+1.1 eV. By the formation of both N-related defects, the tensile strain in Ga(As,N) is reduced.
Investigations on the nature of observed ferromagnetism and possible spin polarization in Co-doped anatase TiO2 thin films
Authors: Kim D. H. , Yang J. S. , Lee K. W. , Bu S. D. , Kim D.-W. , Noh T. W. , Oh S.-J. , Kim Y.-W. , Chung J.-S. , Tanaka H. , Lee H. Y. , Kawai T. , Won J. Y. , Park S. H. , Lee J. C.
Published: May 2003

High-quality epitaxial thin films of Co-doped anatase TiO2 (Co:TiO2) were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior altered from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like pattern. Electrical conductivity and saturation magnetization increased, seemingly consistent with the picture of carrier-induced ferromagnetism. However, we also found that the spatial distribution of Co ions became highly nonuniform and the chemical state of Co ions changed from ionic to metallic. All of these PO2 dependences, even including the transport and the magnetic properties, can be explained in terms of the formation of cobalt clusters, whose existence was clearly demonstrated by transmission–electron–microscope studies. Our work clearly indicates that the cobalt clustering will result in the room-temperature ferromagnetism observed in our Co:TiO2 films. To check the possible spin polarization of carriers in Co:TiO2 films, we also fabricated a heterojunction composed of a ferromagnetic Co:TiO2, an insulating SrTiO3, and a ferromagnetic half-metallic (La,Ba)MnO3 layer. When the magnetic field was varied, we could not observe any changes in its I–V characteristic curves, which suggests that there might be little spin-polarization effect in the anatase Co:TiO2 layer.
Electrical conductivity in metal/3,4,9,10-perylenetetracarboxylic dianhydride/metal structures
Authors: Hudej Robert , Bratina Gvido
Published: May 2003

Samples comprising 1-μm-thick layers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) sandwiched between top-most In contact and bottom-most Ag contact were synthesized on n-Si(100) substrates. Current–voltage (I–V) characteristics were measured on the structures with all the layers evaporated in sequence, and on the structures with air exposed PTCDA/In and Ag/PTCDA interfaces. The current transport in the structures fabricated without interruption of vacuum is controlled by the space charge and by the traps. Air exposure of the In/PTCDA interface introduces additional trap sites in the PTCDA layer, yielding the space-charge-limited current in the presence of traps, exponentially distributed in energy, and with a hopping transport mobility. Air exposure of the Ag/PTCDA interface introduces a discrete level of traps in addition to exponentially distributed traps.
Increased mobility from regioregular poly(3-hexylthiophene) field-effect transistors
Authors: Wang Guangming , Swensen James , Moses Daniel , Heeger Alan J.
Published: May 2003

Relatively high mobilities, μ=0.2 cm2 V−1 s−1 in the accumulation mode and μ=0.17 cm2 V−1 s−1 in the depletion mode, are reported for regioregular poly(3-hexylthiophene) (RR-P3HT) in field-effect transistors (FETs). Significantly higher mobility is obtained from FETs in which the RR-P3HT film is applied by dip-coating to a thickness of only 20−40 Å. These observations suggest that structural order of the semiconducting polymer at the interface between the semiconducting polymer and the SiO2 gate insulator is of paramount importance for achieving high carrier mobility. Heat treatment under nitrogen at 160 °C for 3 min increases the on/off ratio of the FET.
Mg
(
B,O
)
2
precipitation in MgB2
Authors: Liao X. Z. , Serquis A. , Zhu Y. T. , Huang J. Y. , Civale L. , Peterson D. E. , Mueller F. M. , Xu H. F.
Published: May 2003

MgB 2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), x-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long exposure to residual oxygen in flowing Ar gas at high temperatures. The effect of the precipitates in different size ranges on flux pinning is discussed.
Asymptotic analysis of dispersion characteristics in two-dimensional metallic photonic band gap structures
Authors: Smirnova Evgenya I. , Chen Chiping
Published: May 2003

We present a self-consistent technique for the asymptotic analysis of dispersion curves in two-dimensional metallic photonic band gap structures representing square and triangular arrays of metal rods. The technique is applicable for the structures with rod radii (a), which are small compared to the distance between the rods (b) and to the wavelength (λ). The induced current and charge distributions on the rods are expressed self-consistently in terms of the electromagnetic wave field. The dispersion characteristics are calculated for the TE and TM modes. The results are in agreement with those obtained previously using the Photonic Band Gap Structure Simulator code.
Structure and magnetic property changes of epitaxially grown L1-FePd isolated nanoparticles on annealing
Authors: Sato Kazuhisa , Hirotsu Yoshihiko
Published: May 2003

Isolated 10-nm-sized FePd nanoparticles with the L10-type ordered structure have been fabricated by electron-beam evaporation and postannealing above 773 K, and the structural details have been investigated by transmission electron microscopy. FePd particles were epitaxially grown on a cleaved NaCl(001) substrate and were two-dimensionally dispersed on the substrate. In FePd particles formation, Pd nanoparticles were first deposited as “seed” particles epitaxially on NaCl followed by a successive deposition of Fe particles. All the Fe particles were captured by Pd particles forming Fe/Pd nanocomplex particles with a mutual fixed orientation. Coalescence and growth of the particles were not prominent during annealing, indicating that the alloying and atomic ordering reactions proceeded mostly within each nanoparticle. The negligible coalescence can be attributed to an “anchoring effect” of the seed Pd to the coalescence growth. Moreover, both of these reactions are thought to proceed almost simultaneously during annealing at temperatures between 723 and 823 K. Most of the annealed particles were single crystal particles with c axes oriented both normal and parallel to the film plane. Large coercivities above 3 kOe were obtained after annealing at 873 K, though they were smaller than those expected from the theoretical model. The small coercivity value can be attributed to the low magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant of the present FePd nanoparticles estimated was less than half of that of the bulk materials.
Epitaxially grown GaAsN random laser
Authors: Sun B. Q. , Gal M. , Gao Q. , Tan H. H. , Jagadish C. , Puzzer T. , Ouyang L. , Zou J.
Published: May 2003

We have studied the photoluminescence properties of as-grown GaAs1−xNx epitaxial layers grown on GaAs containing 0.6%, 1.77%, and 2.8% nitrogen. We found laser emission from thick (d>200 nm) GaAs0.972N0.028 layers exhibiting the characteristic lasing properties of random lasers. This is unusual because random lasers have so far only been associated with highly disordered or random media. We believe that high gain in combination with structural inhomogeneities that are evident in these GaAs0.972N0.028 layers, can explain the random lasing in such epitaxial layers.
Fabrication of photonic band gap crystal using microtransfer molded templates
Authors: Leung W. Y. , Kang H. , Constant K. , Cann D. , Kim C.-H. , Biswas R. , Sigalas M. M. , Ho K.-M.
Published: May 2003

We have proposed and demonstrated an economical technique to fabricate a three-dimensional layer-by-layer photonic band gap structure in the infrared wavelengths. An organic polymer template structure, an inverse layer-by-layer photonic crystal structure, is assembled using the microtransfer molding technique. This template is infiltrated with sol-gel or nanoparticle titanium oxide slurry, then later removed by heat treatment at a temperature range of 550–800 °C. This method can be extended to fabricate photonic crystals operating at optical and ultraviolet frequencies.
Studies on the photodissociation and symmetry of SO2+(D̃)
Authors: Zhang Limin , Wang Zhong , Li Jiang , Wang Feng , Liu Shilin , Yu Shuqin , Ma Xingxiao
Published: May 2003

With the preparing of SO2+(X̃ 2A1(000)) by [3+1] multiphoton ionization of the neutral SO2 molecules at 380.85 nm, the photodissociation process and the symmetry of the excited states D̃ of SO2+ molecular ions has been investigated by measuring the photofragment SO+ excitation (PHOFEX) spectrum in ultraviolet (282–332 nm) and in visible (562–664 nm) wavelength ranges, respectively. The D̃(v100), D̃(0v20)←X̃ 2A1(000) transitions of SO2+ were assigned in the PHOFEX spectrum in the UV range. By comparing the discernible PHOFEX spectrum in UV range with the continuous PHOFEX spectrum in visible range, it is deduced that (i) around SO2+(D̃) there exists a repulsive state α 2A2 converging to the dissociation limit of SO+(X 2Π)+O(3Pg), (ii) the coupling between SO2+(D̃) and SO2+(α 2A2) leads to the dissociation to SO+(X 2Π)+O(3Pg), (iii) the symmetry of D̃ state is D̃ 2B1.
Linear scaling computation of the Fock matrix. VI. Data parallel computation of the exchange-correlation matrix
Authors: Gan Chee Kwan , Challacombe Matt
Published: May 2003

Recently, early onset linear scaling computation of the exchange-correlation matrix has been achieved using hierarchical cubature [J. Chem. Phys. 113, 10037 (2000)]. Hierarchical cubature differs from other methods in that the integration grid is adaptive and purely Cartesian, which allows for a straightforward domain decomposition in parallel computations; the volume enclosing the entire grid may be simply divided into a number of nonoverlapping boxes. In our data parallel approach, each box requires only a fraction of the total density to perform the necessary numerical integrations due to the finite extent of Gaussian-orbital basis sets. This inherent data locality may be exploited to reduce communications between processors as well as to avoid memory and copy overheads associated with data replication. Although the hierarchical cubature grid is Cartesian, naive boxing leads to irregular work loads due to strong spatial variations of the grid and the electron density. In this paper we describe equal time partitioning, which employs time measurement of the smallest sub-volumes (corresponding to the primitive cubature rule) to load balance grid-work for the next self-consistent-field iteration. After start-up from a heuristic center of mass partitioning, equal time partitioning exploits smooth variation of the density and grid between iterations to achieve load balance. With the 3-21G basis set and a medium quality grid, equal time partitioning applied to taxol (62 heavy atoms) attained a speedup of 61 out of 64 processors, while for a 110 molecule water cluster at standard density it achieved a speedup of 113 out of 128. The efficiency of equal time partitioning applied to hierarchical cubature improves as the grid work per processor increases. With a fine grid and the 6-311G(df,p) basis set, calculations on the 26 atom molecule α-pinene achieved a parallel efficiency better than 99% with 64 processors. For more coarse grained calculations, superlinear speedups are found to result from reduced computational complexity associated with data parallelism.
Ag
3
Born–Oppenheimer potential hypersurfaces
Authors: Shen Yinghau , BelBruno Joseph J.
Published: May 2003

The full Born–Oppenheimer potential energy hypersurfaces of the Ag3 system have been explored by the local spin-density scheme using an analytic potential. Our calculated physical properties, such as dissociation energies and barriers to isomerization, compare well with the available high-level configuration interaction calculations and experiments at the equilibrium geometry. Despite its simplicity, the analytic potential provides an excellent description of the neutral trimer system for silver and the other transition metals.
Liquids in confined geometry: How to connect changes in the structure factor to modifications of local order
Authors: Morineau Denis , Alba-Simionesco Christiane
Published: May 2003

The recent advances in the syntheses of mesostructured porous silicates (MCM-41 and SBA-15) allow us to study liquids confined in highly regular geometry. Hence, one might get to a better understanding of the structure and the dynamics of confined fluids. In this paper, we address the problem of the interpretation of the structure factor of a confined phase. Distortions due to geometric effects—so-called “excluded volume effects” and “cross-correlation terms”—may dominate the observed features and cannot be ignored. We present a generalization of the formalism introduced by Soper et al. It is applied in the case of a honeycomb-type lattice of parallel cylindrical pores, which corresponds to the topology of these novel porous materials. It shows that the large variations of the experimental structure factor of confined liquid benzene at room temperature are essentially attributed to an “excluded volume effect” that does not reflect different local ordering of the confined phase.
Lattice-gas study of the kinetics of the catalytic NO–CO reaction on rhodium surfaces. II. The effect of nitrogen surface islands
Authors: Bustos V. , Uñac R. , Zaera F. , Zgrablich G.
Published: May 2003

The kinetics of the catalytic reduction of NO by CO on Rh(111) was simulated by using a Monte Carlo algorithm based on a lattice-gas model. The results reported here complement those reported before by incorporating new experimental results which reveal the formation of an N–NO intermediate as necessary for molecular nitrogen production as well as the formation of N-islands on the surface. The behavior of the steady-state phase diagram for the reaction is analyzed in terms of several parameters representing different reaction schemes. It is shown how the range of conditions under which the reaction can occur is influenced by the mechanism for the formation of the N–NO intermediate, by the growth of N surface islands, and by side steps such as NO surface diffusion and NO and CO desorption.
Effect of hygroscopic nature on the electrical characteristics of lanthanide oxides (Pr2O3, Sm2O3, Gd2O3, and Dy2O3)
Authors: Jeon Sanghun , Hwang Hyunsang
Published: May 2003

The hygroscopic nature of lanthanide oxides such as Pr2O3, Sm2O3, Gd2O3, and Dy2O3 was characterized by means of x-ray photoelectron spectroscopy and its effect on the electrical characteristics of the compounds was investigated. Among the four samples, Pr2O3 was found to be the most reactive with water which can be attributed to the relatively large ionic radius and lower electronegativity of Pr. In contrast, Dy2O3 was the least reactive with water. A direct correlation between the hygroscopicity and electronegativity of lanthanide elements was found. With increasing hygroscopicity, a significant growth of interfacial oxide with annealing temperature was observed. A clear understanding of the nature of hygroscopic effects and the optimization of process flow will be needed for future high-k gate dielectric application.
Improved numerical technique to calculate statistical Coulomb blurring
Authors: van Veenendaal Michel
Published: May 2003

This paper studies the effects of Coulomb blurring in charged-particle optics using a method that combines Monte Carlo techniques with analytical calculations. A correction factor for the analytical theory of Jansen and Kruit is obtained. This approach strongly enhances the accuracy of the method, while maintaining the speed of calculation. The method gives good agreement with Monte Carlo simulations. The effects of aperturing are also included.
Microstructure of precipitated Au nanoclusters in MgO
Authors: Wang C. M. , Thevuthasan S. , Shutthanandan V. , Cavanagh A. , Jiang W. , Thomas L. E. , Weber W. J.
Published: May 2003

Gold nanoclusters dispersed in single crystal MgO have been prepared by ion implantation at 975 K and subsequent annealing at 1275 K for 10 h. The morphological features, size, and crystallographic orientation of the Au nanoclusters with respect to the MgO matrix, as well as the interface structure between the Au nanoclusters and MgO, have been investigated using transmission electron microscopy. During annealing, the Au clusters nucleate coherently in the MgO lattice, leading to an epitaxial orientation relationship of [010]MgO//[010]Au and (200)MgO//(200)Au that is maintained for all the Au clusters. Above a critical size of ∼5 to 8 nm, a coherent-semicoherent interface transition is observed for the Au clusters in MgO. This critical cluster size is larger than the critical size ∼3 nm based on energetic considerations. This discrepancy is discussed with respect to the point and extended defect structures at the interface between the Au clusters and the MgO matrix. The Au clusters larger than this critical size exhibit faceting on the {001} planes and internal dislocations. It is further suggested that the density of quantum antidots should depend on the size of the Au clusters.
Mechanisms of photoluminescence from silicon nanocrystals formed by pulsed-laser deposition in argon and oxygen ambient
Authors: Chen X. Y. , Lu Y. F. , Wu Y. H. , Cho B. J. , Liu M. H. , Dai D. Y. , Song W. D.
Published: May 2003

We have investigated the different mechanisms of photoluminescence (PL) of silicon nanocrystals due to the quantum confinement effect (QCE) and interface states. Si nanocrystals were formed by pulsed-laser deposition in inert argon and reactive oxygen gas. The collisions between the ejected species greatly influence the morphology of the Si nanocrystals and cause a transition from a film structure to a porous cauliflowerlike structure, as the ambient gas pressure increases from 1 mTorr to 1 Torr. The oxygen content of the Si nanocrystals increases with increasing O2 ambient pressure, and nearly SiO2 stoichiometry is obtained when the O2 pressure is higher than 100 mTorr. Broad PL spectra are observed from Si nanocrystals. The peak position and intensity of the PL band at 1.8–2.1 eV vary with ambient gas pressure, while intensity changes and blueshifts are observed after oxidation and annealing. The PL band at 2.55 eV shows vibronic structures with periodic spacing of 97±9 meV, while no peak shift is found before and after oxidation and annealing. Raman and transmission electron microscope measurements show consistent results in crystal size while more accurate atomic force microscope measurements reveal a smaller crystal size. X-ray diffraction reveals a polycrystal structure in the Si nanocrystals and the crystallinity improves after annealing. Combined with the PL spectra of Si nanocrystals obtained by crumbling electrochemically etched porous Si layer, the results clearly demonstrate that the PL band at 1.8–2.1 eV is due to the QCE in the Si nanocrystal core, while the PL band at 2.55 eV is related to localized surface states at the SiOx/Si interface.
Passivation of active recombination centers in ZnO by hydrogen doping
Authors: Ohashi Naoki , Ishigaki Takamasa , Okada Nobuhiro , Taguchi Hiroyuki , Sakaguchi Isao , Hishita Shunichi , Sekiguchi Takashi , Haneda Hajime
Published: May 2003

The effect of hydrogen doping on luminescence properties of ZnO was investigated. Hydrogen was incorporated in the ZnO crystal by irradiation with an inductively coupled plasma (ICP), in particular, the pulse modulated mode operation of ICP, and the luminescence spectra and hydrogen concentration of the resultant samples were analyzed. A hydrogenated region of 20–100 nm was formed at the sample surface by the irradiation and the concentration of hydrogen was 1017–1018 cm−3. Hydrogen doping improved the ultraviolet emission efficiency of all the samples, and the degree of improvement depended on the initial state (impurity concentration) of the original samples. The most significant improvements were recorded for the sample lightly contaminated with Cu, Al, and Li. The correlation between impurity concentration and hydrogen doping effects is discussed from the viewpoint of charge transfer between hydrogen and the other impurities.
Single complex line shapes of the B850 band of LH2
Authors: Jang Seogjoo , Silbey Robert J.
Published: May 2003

For a model of the B850 band in the light harvesting complex 2 of purple bacteria, the main quantum mechanical characteristics of the single complex line shapes are studied based on the theory of the preceding paper. The model consists of single exciton states coupled to harmonic oscillator bath, with the inclusion of both static and quasistatic disorder within the exciton Hamiltonian. A closed form line shape expression is derived that can account for the non-Markovian nature of the bath for a general spectral density. The calculation of the line shape only requires the inversion of a small matrix with dimension equal to the number of exciton levels, at each frequency. For two examples of site energy modulation with definite symmetries, we examine the dependencies of the line shape on the polarization of the radiation, on the type of exciton–bath coupling, and on temperature. For a plausible example of disorder, we simulate the line shapes of the ensemble and single complex spectroscopies. Simulation of single complex line shapes suggests that the quasistatic disorder is responsible for large spectral jumps (spectral diffusion) of the lowest exciton state and make the widths of the two brightest peaks comparable.
Improved second-order Møller–Plesset perturbation theory by separate scaling of parallel- and antiparallel-spin pair correlation energies
Authors: Grimme Stefan
Published: May 2003

A simple modification of second-order Møller–Plesset perturbation theory (MP2) to improve the description of molecular ground state energies is proposed. The total MP2 correlation energy is partitioned into parallel- and antiparallel-spin components which are separately scaled. The two parameters (scaling factors), whose values can be justified by basic theoretical arguments, have been optimized on a benchmark set of 51 reaction energies composed of 74 first-row molecules. It is found, that the new method performs significantly better than standard MP2: the rms [mean absolute error (MAE)] deviation drops from 4.6 (3.3) to 2.3 (1.8) kcal/mol. The maximum error is reduced from 13.3 to 5.1 kcal/mol. Significant improvements are especially observed for cases which are usually known as MP2 pitfalls while cases already described well with MP2 remain almost unchanged. Even for 11 atomization energies not considered in the fit, uniform improvements [MAE: 8.1 kcal/mol (MP2) versus 3.2 kcal/mol (new)] are found. The results are furthermore compared with those from density functional theory (DFT/B3LYP) and quadratic configuration interaction [QCISD/QCISD(T)] calculations. Also for difficult systems including strong (nondynamical) correlation effects, the improved MP2 method clearly outperforms DFT/B3LYP and yields results of QCISD or sometimes QCISD(T) quality. Preliminary calculations of the equilibrium bond lengths and harmonic vibrational frequencies for ten diatomic molecules also show consistent enhancements. The uniformity with which the new method improves upon MP2, thereby rectifying many of its problems, indicates significant robustness and suggests it as a valuable quantum chemical method of general use.
Mechanical stresses upon hydrogen induced optical switching in thin films
Authors: Leervad Pedersen T. P. , Salinga C. , Weis H. , Wuttig M.
Published: May 2003

Films that can change their optical properties upon exposure to hydrogen offer a wide range of interesting applications. Frequently, the change of optical properties is related to a volume change, which can lead to considerable mechanical stresses causing property deterioration. In an effort to compare the behavior of different material classes that enable hydrogen induced switching, we have studied Mg, Gd, and WOx films. The measurements were carried out using a wafer curvature setup, which allows simultaneous measurements of the mechanical stress and the optical transmission and reflection of the coatings. The magnitude of the stress changes involved in the switching of these materials is found to vary significantly. Stress changes of several GPa are observed in the switching of Gd, while the switching of Mg involves stress changes of several hundreds of MPa. In the case of WOx stress changes of only the order of 100 MPa are found. Reversible switching has been found in the case of WOx. While Gd shows irreversible stress changes upon first switching, it shows reversible behavior upon subsequent switching processes. The Mg films show irreversible switching, and additionally, their durability is found to be hampered by oxidation, which is accelerated by stress induced defect formation in the films.
Thermoelectric properties of electrodeposited CuNi alloys on Si
Authors: Delatorre R. G. , Sartorelli M. L. , Schervenski A. Q. , Pasa A. A. , Güths S.
Published: May 2003

Thin films with the composition of the constantan alloy (a solid solution with 35 to 50 wt. % of Ni in Cu) have a high-thermoelectric power, which allows the fabrication of very sensitive heat-flux sensors based on planar technology. In this article, the thermoelectric properties of CuxNi100−x thin films electrodeposited on silicon were studied as a function of the composition, temperature, and thickness. The electrodeposition of thin layers on silicon is an important step for the integration of thermal sensors with semiconductor technology. The CuxNi100−x alloys were electrodeposited potentiostatically at room temperature, from a citrate electrolyte containing both copper and nickel sulfates. The layer composition was controlled by the applied potential in the range from pure copper (at −0.4 V/SCE) up to a solid solution of about 25 wt. % Cu in Ni (at −1.2 V/SCE). Extremely high values of thermoelectric power were measured for very thin layers of Cu40Ni60 on Si, showing a strong influence of the substrate. By considering the system as a thermoelectric bilayer and extracting the contribution of the semiconductor, thermopower values for the Cu40Ni60 alloys comparable to the expected ones for constantan wires were obtained.
Influence of cross-linking density on volume phase transition of liquid crystalline gels in a nematogenic solvent
Authors: Okuno Yuko , Urayama Kenji , Kohjiya Shinzo
Published: May 2003

The effect of cross-linking density (Cx) on volume phase transition of two liquid crystalline (LC) networks comprising dissimilar mesogens in a low molar mass LC has been investigated. Cross-linking density alters the modulus and nematicity of LC network each of which directly influence one of the three governing forces (elastic force, nematic interaction, isotropic mixing interaction) for the equilibrium swelling: An increment in Cx increases the network modulus, while it reduces the nematic-isotropic transition temperature of the pure (dry) LC network (TNIN). The LC networks in the nematogen are discontinuously transformed from the swollen state into the shrunken state, as a result of a single nematic phase formation inside the gel at TNIG. It has been found that as Cx increases, the magnitude of discontinuous volume change at TNIG decreases; TNIG shifts to higher temperatures, which is opposite to the Cx dependence of TNIN. As an extreme case of high cross-linking density, the LC networks with Cx=10 mol % in the nematic solvent exhibits the nematic–isotropic transition but do not undergo appreciable volume transition. The experimental swelling–temperature curves are compared with the prediction of a mean field theory for nematic gel. The theory successfully describes the effect of cross-linking density on the swelling and phase behavior observed.
Fluid–fluid coexistence in colloidal systems with short-ranged strongly directional attraction
Authors: Kern Norbert , Frenkel Daan
Published: May 2003

We present a systematic numerical study of the phase behavior of square-well fluids with a “patchy” short-ranged attraction. In particular, we study the effect of the size and number of attractive patches on the fluid–fluid coexistence. The model that we use is a generalization of the hard sphere square well model. The systems that we study have a stronger tendency to form gels than the isotropic square-well system. For this reason, we had to combine Gibbs ensemble simulations of the fluid–fluid coexistence with a parallel tempering scheme. For moderate directionality, changes of the critical density and the width of coexistence curves are small. For strong directionality, however, we find clear deviations from the extended law of corresponding states: in contrast to isotropic attractions, the critical point is not characterized by a universal value of the reduced second virial coefficient. Furthermore, as the directionality increases, multiparticle bonding affects the critical temperature. We discuss implications for the phase behavior, and possibly crystallization, of globular proteins.
Quantum dynamics study of isotope effect for H+CH4 reaction using the SVRT model
Authors: Zhang Xin , Yang Guang-Hui , Han Ke-Li , Wang M. L. , Zhang John Z. H.
Published: May 2003

The semirigid vibrating rotor target model is applied to study the isotope effect in reaction H+CH4→H2+CH3 using time-dependent wave-packet method. The reaction probabilities for producing H2 and HD product channels are calculated. The energy dependence of the reaction probabilities shows oscillating structures for both reaction channels. At low temperature or collision energies, the H atom abstraction is favored due to tunnelling effect. In partially deuterated CHxDy (x+y=4), the breaking of the C–H bond is favored over that of the C–D bond in the entire energy range studied. In H+CHD3 reaction at high energies, the HD product dominates simply due to statistical factor.
Structure and electronic spectroscopy of naphthalene–acenaphthene van der Waals dimer: Hole-burning, dispersed fluorescence, and quantum chemistry calculations
Authors: Das Aloke , Nandi Chayan K. , Chakraborty Tapas
Published: May 2003

Electronic spectroscopy of 1:1 van der Waals dimer of naphthalene and acenaphthene has been studied in a supersonic free jet expansion by measuring the laser-induced fluorescence excitation, dispersed fluorescence, and two-color hole-burning spectra. In fluorescence excitation spectrum the dimer exhibits a long progression of an intermolecular vibration, and similar vibronic structures are observed also in emission spectra from the origin region of the S1 surface. The excimer formation from the locally excited state appears as a barrier crossing process and excess vibronic energy required to cross the barrier is about 420 cm−1. The equilibrium structure and binding energy of the dimer are computed by the ab initio quantum chemistry method at the MP2/6-31G and MP2/6-31+G*//MP2/6-31G levels. A parallel-displaced structure, in which two molecules are displaced from a fully overlapping geometry by 1.16 and 0.45 Å, respectively, along the long and short molecular axes, and maintains a vertical separation of 3.48 Å between two molecular planes, is found to be the most stable in the ground state. The BSSE corrected MP2/6-31+G*//MP2/6-31G binding energy of the dimer is 9.2 kcal/mol. The observed spectral and dynamical characteristics of the mixed dimer are compared to those reported for the naphthalene homodimer, and the differences are interpreted in terms of geometry and exciton resonance interactions.
Pair dispersion and preferential concentration of particles in isotropic turbulence
Authors: Zaichik Leonid I. , Alipchenkov Vladimir M.
Published: May 2003

The objective of the paper is to present a statistical model for predicting pair dispersion and preferential concentration of particles suspended in an isotropic homogeneous turbulent flow field. This model is based on a kinetic equation for the probability density function of the relative velocities of two particles. The model developed is applied to predict the pair relative velocity statistics and the accumulation effect of heavy particles in a steady-state suspension. The effect of particle inertia on the predicted Eulerian two-point particle velocity correlations is demonstrated and compared with known results of numerical simulation.
Extending the 3ω method for thin-film analysis to high frequencies
Authors: Raudzis C. E. , Schatz F. , Wharam D.
Published: May 2003

Although the 3ω method was originally introduced to measure the thermal conductivity of bulk material and was later developed further to include thin-film materials, many thermal and geometrical parameters influence the 3ω signal. We show that several of these parameters can be determined simultaneously from a single 3ω measurement. With the commonly used frequency range, however, some parameter combinations cannot be determined uniquely. To overcome this problem we have developed an electronic setup for measurements from 5 Hz up to 200 kHz. We show numerically and experimentally that we can determine four physical parameters from a single measurement in this frequency range. This makes the 3ω method very suitable for microelectronics and microsensor testing, where physical parameters should ideally be determined by purely electrical measurements.
Vibrational corrections to indirect nuclear spin–spin coupling constants calculated by density-functional theory
Authors: Ruden Torgeir A. , Lutnæs Ola B. , Helgaker Trygve , Ruud Kenneth
Published: May 2003

At the present level of electronic-structure theory, the differences between calculated and experimental indirect nuclear spin–spin coupling constants are typically as large as the vibrational contributions to these constants. For a meaningful comparison with experiment, it is therefore necessary to include vibrational corrections in the calculated spin–spin coupling constants. In the present paper, such corrections have been calculated for a number of small molecular systems by using hybrid density-functional theory (DFT), yielding results in good agreement with previous wave-function calculations. A set of empirical equilibrium spin–spin coupling constants has been compiled from the experimentally observed constants and the calculated vibrational corrections. A comparison of these empirical constants with calculations suggests that the restricted-active-space self-consistent field method is the best approach for calculating the indirect spin–spin coupling constants of small molecules, and that the second-order polarization propagator approach and DFT are similar in performance. To illustrate the usefulness of the presented method, the vibrational corrections to the indirect spin–spin coupling constants of the benzene molecule have been calculated.
Three-dimensional optical polarization tomography of single molecules
Authors: Prummer Michael , Sick Beate , Hecht Bert , Wild Urs P.
Published: May 2003

We apply the concept of tomography to polarization-sensitive optical microscopy of single fluorophores to determine the three-dimensional orientation of molecular absorption dipoles with isotropic sensitivity. Wide-field microscopy provides the opportunity to monitor simultaneously three-dimensional rotation and two-dimensional translation of many molecules in parallel. For orientation determination the molecules are illuminated from different directions of incidence with linearly polarized light. In each exposure the excitation along a particular projection of the absorption dipole on the electric field leads to a distinct fluorescence intensity. Five exposures are sufficient to determine the full orientation of the fluorophores. To demonstrate the potential of the method we determine the orientation and position of individual immobilized lipid membrane markers. The shot-noise-limited isotropic angular resolution is 2°. For time-resolved studies the bandwidth can be expanded up to 200 Hz.
Writhing geometry of open DNA
Authors: Rossetto V. , Maggs A. C.
Published: May 2003

Motivated by recent experiments on DNA torsion-force-extension characteristics we consider the writhing geometry of open stiff molecules. We exhibit a cyclic motion which allows arbitrarily large twisting of the end of a molecule via an activated process. This process is suppressed for forces larger than femtonewtons which allows us to show that experiments are sensitive to a generalization of the Călugăreanu–White formula for the writhe. Using numerical methods we compare this formulation of the writhe with recent analytic calculations.
Biasing a transition state search to locate multiple reaction pathways
Authors: Peters Baron , Liang WanZhen , Bell Alexis T. , Chakraborty Arup
Published: May 2003

A variety of chemical systems exhibit multiple reaction pathways that adjoin to a common reactant state. In fact, any reaction producing side products or proceeding via a stable intermediate involves a species possessing at least two reaction pathways. Despite improvements in ab initio transition-state search algorithms it remains difficult to detect multiple reaction pathways. Typically, multiple reaction pathways can only be detected by intuitively varying the initial point in the transition-state search trajectory. This reliance on intuition limits the ability to discover new and unexpected chemistry using ab initio methods. This paper proposes a systematic and intuition-free method for biasing a transition-state search to identify multiple reaction pathways originating from a common reactant state. The method allows the successive location of transition states, with each successful search contributing to a cumulative bias potential for the following search. The method is applicable to all psuedo-Newton–Raphson-type transition-state searches. The procedure was tested for a model potential energy surface and for the thermal rearrangement of trans-1,4-dimethylcyclobutene. In the latter case, four reaction pathways were discovered: two exothermic conrotatory ring openings leading to hexadienes, an endothermic methyl migration pathway leading to a carbene, and an exothermic rearrangement leading to 3-methyl-1,4-pentadiene. In accordance with experiment, the calculations predict that the conrotatory pathway leading to trans,trans-2,4-hexadiene is the kinetically dominant pathway. The methodology was also used to compute selectivities for competitive pathways producing trans and cis triflouropentadiene products in the thermal rearrangement of 3-triflouromethyl-cyclobutene. Again, results were in accord with experimental observations.
Dressed basis for highly excited molecular vibrations
Authors: Kellman Michael E. , Dow Mark W. , Tyng Vivian
Published: May 2003

Starting from a multiresonance spectroscopic Hamiltonian fit by Baggott to experimental levels of H2O, an approximate Hamiltonian is devised using a prediagonalized “dressed” zero-order basis, within which a residual, effective single-resonance coupling operator acts. The dressed basis incorporates many of the effects of nonintegrability, while the effective resonance furnishes much of the simplicity of integrable systems. Numerical tests are performed for two distinct dressed bases, in which different resonance operators are chosen as the residual effective coupling. Excellent agreement with the energies and eigenvectors of the exact system is obtained for each of the two dressed bases.
Photoelectron imaging of carbonyl sulfide cluster anions: Isomer coexistence and competition of excited-state decay mechanisms
Authors: Surber Eric , Sanov Andrei
Published: May 2003

We investigate the structure and decay of (OCS)n− cluster ions (n=2–4) using photoelectron imaging spectroscopy. The results indicate the coexistence of isomers with OCS− and covalently bound (OCS)2− cluster cores. A several-fold decrease in the relative abundance of the dimer-based species is observed for n=3 and 4 compared to n=2. The OCS−(OCS)n−1 cluster ions undergo direct photodetachment similar to OCS−⋅H2O, while (OCS)2−(OCS)n−2 exhibits both direct electron detachment and cluster decomposition via ionic fragmentation and autodetachment. The autodetachment originates from either the excited states of the parent cluster or internally excited anionic fragments. It is described using a statistical model of thermionic emission, which assumes rapid thermalization of the excitation energy. A decrease in the relative autodetachment yield in the trimer and tetramer cluster ions, compared to the covalent dimer, is attributed to competition with ionic fragmentation.
Transient and steady-state amplitudes of forced waves in rectangular basins
Authors: Hill D. F.
Published: May 2003

A weakly-nonlinear analysis of the transient evolution of two-dimensional, standing waves in a rectangular basin is presented. The waves are resonated by periodic oscillation along an axis aligned with the wavenumber vector. The amplitude of oscillation is assumed to be small with respect to the basin dimensions. The effects of detuning, viscous damping, and cubic nonlinearity are all simultaneously considered. Moreover, the analysis is formulated in water of general depth. Multiple-scales analysis is used in order to derive an evolution equation for the complex amplitude of the resonated wave. From this equation, the maximum transient and steady-state amplitudes of the wave are determined. It is shown that steady-state analysis will underestimate the maximum response of a basin set into motion from rest. Amplitude response diagrams demonstrate good agreement with previous experimental investigations. The analysis is invalid in the vicinity of the “critical depth” and in the shallow-water limit. A separate analysis, which incorporates weak dispersion, is presented in order to provide satisfactory results in shallow water.
Noise-source turbulence statistics and the noise from a Mach 0.9 jet
Authors: Freund J. B.
Published: May 2003

Turbulence statistics that are relevant to jet noise modeling but difficult to measure in experiments are computed using a previously validated simulation database of a Mach 0.9 cold jet. Initial focus is on fourth-order statistics that are at the core of acoustic analogy based models built on both the Lilley and Lighthill equations. Common simplifications of fourth-order correlations based on normal statistics are found to be accurate. We see that although two-point correlations are well fitted by exponential functions, as is typical of turbulence at all but the lowest Reynolds numbers, the spatially integrated fourth-order space/retarded-time covariances, which are used in the models, are instead very well fitted by Gaussian functions of different widths for different components, which is counter to conventional modeling practice. We also examine the components of Lighthill’s analogous noise source that are linear and quadratic in velocity fluctuations, as well as components that are deviations from p′=a∞2ρ′. The spectrum from the linear components is more peaked and more direction dependent than the spectral shape of the quadratic component’s noise, which is relatively independent of angle. These two components are also correlated, especially at small angles where their mutual correlation coefficient reaches as low as −0.4, which casts doubt on models that treat these so-called shear noise (linear) and self-noise (quadratic) terms as distinct. The p′−a∞2ρ′ contribution is relatively small, but not negligible as might be expected for this nearly isothermal jet. The total radiated power of the quadratic terms is nearly the same as that of all components combined. It is shown that the standard Lighthill framework does not lead to a straight forward designation of what noise comes from what region of the jet.
A subgrid-scale mixing model for large-eddy simulations of turbulent reacting flows using the filtered density function
Authors: Cha Chong M. , Trouillet Philippe
Published: May 2003

The filtered density function approach [Colucci et al., Phys. Fluids 10, 499 (1999); Jaberi et al., J. Fluid Mech. 401, 85 (1999)] for large-eddy simulations of initially nonpremixed turbulent reacting flows is extended to encompass the flamelet regime. This is done by developing a model which describes the effect of realistic, Arrhenius chemical kinetics on the subgrid mixing time scale for reactive scalars. The model is based on mapping closure and flamelet modeling and can readily be applied to many existing micro-mixing models where the time scales of the reacting scalars appear explicitly in the formulation. Testing of the model using spatially filtered direct numerical simulation data of a turbulent reacting jet [Boersma, Center for Turbulence Research Annual Research Briefs (Stanford University/NASA Ames, 1999), pp. 59–72] show a significant improvement over the generally used estimate of substituting the time scale of the reacting scalars by that of a conserved scalar. The impact of internal intermittency on the performance of the new model is discussed.