bluuebunny/crossref_metadata_2025 huggingface dataset:
Classification of the equilibrium states of magnets with vector and quadrupole order parameters
Authors: Dem’yanenko D. A. , Kovalevskiĭ M. Yu.
Published: December 2007

A classification of the equilibrium states of magnetic condensed media whose symmetry is broken spontaneously relative to rotations in spin space and translations in configuration space is constructed. The cases of vector and tensor order parameters, which possess different transformation properties under time reversal, are examined. The conditions for unbroken symmetry and spatial symmetry of the equilibrium states for such media are formulated. In the case of a vector order parameter, the connection between these symmetry conditions and para-, ferro-, antiferro-, ferri-, and spiral magnetic states is determined. The relation of the indicated symmetry conditions with quadrupole magnetic states is determined—one- and two-axis magnetic nonmagnetics, magnetic cholesterics, and a double magnetic spiral.
Low-temperature EPR spectrum of a CENI cluster magnet
Authors: Kravchina O. V. , Kaplienko A. I. , Anders A. G. , Chervinskii D. A. , Pashkevich Yu. G. , Orendachova A. , Kajnakova M.
Published: December 2007

The energy spectrum and resonance transitions in the EPR spectrum for a four-site cluster with an exchange interaction between nearest neighbors and diagonal coupling are calculated. The cases of isotropic and anisotropic exchange couplings are analyzed. The Dzyaloshinskii–Moriya interaction is taken into account in the second case. Using the computational results, it is possible to describe the shape of the EPR absorption band of a powdered sample of the metal organic compound CENI, which undergoes appreciable changes with a change in temperature. It is shown that these changes occur because of the appearance of additional interlevel resonance transitions, whose intensity increases as temperature decreases.
Structure of mixed clusters formed in supersonic jets of Ar–N2 gas mixtures
Authors: Danylchenko O. G. , Kovalenko S. I. , Samovarov V. N.
Published: December 2007

An electron diffraction study is done on substrate-free clusters formed from nanodrops in expanding supersonic jets of Ar–N2 gas mixtures. The mean characteristic linear dimension of the clusters varies from 20to200Å, and the temperature of the clusters is 40K. The content of the gas mixtures spanned the entire concentration interval. These studies revealed that the mixed Ar–N2 clusters are enriched with Ar, and a method for determining the absolute concentrations of the cluster constituents is proposed. The structure of the clusters in relation to their size and the component concentrations is established for the first time. The diagram of the structural states is compared with published data for bulk samples grown from the liquid. A transformation from the fcc to the hcp structure on increasing cluster size is revealed, and also a transformation from the hcp to an hcp+fcc structure with increasing argon concentration; this is not observed in the bulk samples. In clusters the mutual transformations of these structures occur without a change of the interatomic distances.
Orientational isotopic effects in the thermal conductivity of CH4∕CD4 solid solutions
Authors: Krivchikov A. I. , Stachowiak P. , Pisarska E. , Jezowski A.
Published: December 2007

The thermal conductivity of (CH4)1−c(CD4)c solid solutions with c=0, 0.03, 0.065, 0.13, 0.22, 0.4, 0.78, and 1.0 is measured in the region of existence of three orientational phases: disordered (phase I), partially ordered (phase II), and completely ordered (phase III). The temperature range is 1.3–30K. It is shown that the thermal conductivity has different temperature dependences κ(T) in these phases. Its value increases with the degree of the orientational order in the phase. In phase I the thermal conductivity is independent of c and weakly dependent on T. The impurity effect in κ(T) is much stronger in the low-temperature part of phase II than in phase III. As the concentration c grows, the κ(T) curve of phase II approaches the dependence κ(T) typical of phase I. There is a hysteresis in the vicinity of the II↔III phase transition. In phase III the impurity effect in κ(T) can be considered as phonon scattering at rotational defects developing due to the difference between the moments of inertia of the CH4 and CD4 molecules. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH4–CD4 system.
Density fingering in spatially modulated Hele-Shaw cells
Authors: Tóth Tamara , Horváth Dezső , Tóth Ágota
Published: December 2007

Density fingering of the chlorite-tetrathionate reaction has been studied experimentally in a periodically heterogeneous Hele-Shaw cell where the heterogeneity is introduced in the form of spatial modulation of gap width along the front. Depending on the spatial wavelength, gap width, and chemical composition, three types of cellular structures have been observed. The initial evolution is characterized by dispersion curves, while the long time behavior is described by the change in the autocorrelation function of the front profile and in the mixing length of the patterns.
Homogeneous nucleation and growth in supersaturated zinc vapor investigated by molecular dynamics simulation
Authors: Römer F. , Kraska T.
Published: December 2007

Homogeneous nucleation and growth of zinc from supersaturated vapor are investigated by nonequilibrium molecular dynamics simulations in the temperature range from 400to800K and for a supersaturation ranging from logS=2 to 11. Argon is added to the vapor phase as carrier gas to remove the latent heat from the forming zinc clusters. A new parametrization of the embedded atom method for zinc is employed for the interaction potential model. The simulation data are analyzed with respect to the nucleation rates and the critical cluster sizes by two different methods, namely, the threshold method of Yasuoka and Matsumoto [J. Chem. Phys. 109, 8451 (1998)] and the mean first passage time method for nucleation by Wedekind et al. [J. Chem. Phys. 126, 134103 (2007)]. The nucleation rates obtained by these methods differ approximately by one order of magnitude. Classical nucleation theory fails to describe the simulation data as well as the experimental data. The size of the critical cluster obtained by the mean first passage time method is significantly larger than that obtained from the nucleation theorem.
Patchy sticky hard spheres: Analytical study and Monte Carlo simulations
Authors: Fantoni Riccardo , Gazzillo Domenico , Giacometti Achille , Miller Mark A. , Pastore Giorgio
Published: December 2007

We consider a fluid of hard spheres bearing one or two uniform circular adhesive patches, distributed so as not to overlap. Two spheres interact via a “sticky” Baxter potential if the line joining the centers of the two spheres intersects a patch on each sphere, and via a hard sphere potential otherwise. We analyze the location of the fluid-fluid transition and of the percolation line as a function of the size of the patch (the fractional coverage of the sphere’s surface) and of the number of patches within a virial expansion up to third order and within the first two terms (C0 and C1) of a class of closures Cn hinging on a density expansion of the direct correlation function. We find that the locations of the two lines depend sensitively on both the total adhesive coverage and its distribution. The treatment is almost fully analytical within the chosen approximate theory. We test our findings by means of specialized Monte Carlo simulations and find the main qualitative features of the critical behavior to be well captured in spite of the low density perturbative nature of the closure. The introduction of anisotropic attractions into a model suspension of spherical particles is a first step toward a more realistic description of globular proteins in solution.
Theoretical predictions of nuclear magnetic resonance parameters in a novel organo-xenon species: Chemical shifts and nuclear quadrupole couplings in HXeCCH
Authors: Straka Michal , Lantto Perttu , Räsänen Markku , Vaara Juha
Published: December 2007

We calibrate the methodology for the calculation of nuclear magnetic resonance (NMR) properties in novel organo-xenon compounds. The available state-of-the-art quantum-chemical approaches are combined and applied to the HXeCCH molecule as the model system. The studied properties are Xe129, H1, and C13 chemical shifts and shielding anisotropies, as well as Xe131 and H2 nuclear quadrupole coupling constants. The aim is to obtain, as accurately as currently possible, converged results with respect to the basis set, electron correlation, and relativistic effects, including the coupling of relativity and correlation. This is done, on one hand, by nonrelativistic correlated ab initio calculations up to the CCSD(T) level and, on the other hand, for chemical shifts and shielding anisotropies by the leading-order relativistic Breit-Pauli perturbation theory (BPPT) with correlated ab initio and density-functional theory (DFT) reference states. BPPT at the uncorrelated Hartree-Fock level as well as the corresponding fully relativistic Dirac-Hartree-Fock method are found to be inapplicable due to a dramatic overestimation of relativistic effects, implying the influence of triplet instability in this multiply bonded system. In contrast, the fully relativistic second-order Møller-Plesset perturbation theory method can be applied for the quadrupole coupling, which is a ground-state electric property. The performance of DFT with various exchange-correlation functionals is found to be inadequate for the nonrelativistic shifts and shielding anisotropies as compared to the CCSD(T) results. The relativistic BPPT corrections to these quantities can, however, be reasonably predicted by DFT, due to the improved triplet excitation spectrum as compared to the Hartree-Fock method, as well as error cancellation within the five main BPPT contributions. We establish three computationally feasible models with characteristic error margins for future calculations of larger organo-xenon compounds to guide forthcoming experimental NMR efforts. The predicted Xe129 chemical shift in HXeCCH is in a novel range for this nucleus, between weakly bonded or solvated atomic xenon and xenon in the hitherto characterized molecules.
Brownian dynamics simulations of polyelectrolyte adsorption in shear flow with hydrodynamic interaction
Authors: Hoda Nazish , Kumar Satish
Published: December 2007

The adsorption of single polyelectrolyte molecules in shear flow is studied using Brownian dynamics simulations with hydrodynamic interaction (HI). Simulations are performed with bead-rod and bead-spring chains, and electrostatic interactions are incorporated through a screened Coulombic potential with excluded volume accounted for by the repulsive part of a Lennard-Jones potential. A correction to the Rotne-Prager-Yamakawa tensor is derived that accounts for the presence of a planar wall. The simulations show that migration away from an uncharged wall, which is due to bead-wall HI, is enhanced by increases in the strength of flow and intrachain electrostatic repulsion, consistent with kinetic theory predictions. When the wall and polyelectrolyte are oppositely charged, chain behavior depends on the strength of electrostatic screening. For strong screening, chains get depleted from a region close to the wall and the thickness of this depletion layer scales as N1∕3Wi2∕3 at high Wi, where N is the chain length and Wi is the Weissenberg number. At intermediate screening, bead-wall electrostatic attraction competes with bead-wall HI, and it is found that there is a critical Weissenberg number for desorption which scales as N−1∕2κ−3(lB∣σq∣)3∕2, where κ is the inverse screening length, lB is the Bjerrum length, σ is the surface charge density, and q is the bead charge. When the screening is weak, adsorbed chains are observed to align in the vorticity direction at low shear rates due to the effects of repulsive intramolecular interactions. At higher shear rates, the chains align in the flow direction. The simulation method and results of this work are expected to be useful for a number of applications in biophysics and materials science in which polyelectrolyte adsorption plays a key role.
Magnetic field induced increase of the longitudinal Kerr effect in multilayer Co∕Cu(111) films
Authors: Lukienko I. N. , Kharchenko N. F. , Zorchenko V. V. , Stetsenko A. N.
Published: December 2007

A many-fold increase of the longitudinal Kerr effect in a magnetic field has been found in multilayer Co∕Cu(111) films for definite thicknesses of the copper layers, which give extrema of the exchange coupling between the cobalt layers. It is conjectured that this phenomenon is due to the presence of a large contribution of the Co/Cu interfaces to the magneto-optic effect and is due to the paramagnetism of the conduction electrons in the interface layers, increased by hybridization of the electronic sp and d bands of copper and cobalt. It is conjectured that the periodic increase of the magneto-optic susceptibility is due to the sensitivity of the hybridization to the quantization of the transverse motion of the electrons in the copper layers.
The vibrational spectra of protonated water clusters: A benchmark for self-consistent-charge density-functional tight binding
Authors: Yu Haibo , Cui Qiang
Published: December 2007

Proton transfers are involved in many chemical processes in solution and in biological systems. Although water molecules have been known to transiently facilitate proton transfers, the possibility that water molecules may serve as the “storage site” for proton in biological systems has only been raised in recent years. To characterize the structural and possibly the dynamic nature of these protonated water clusters, it is important to use effective computational techniques to properly interpret experimental spectroscopic measurements of condensed phase systems. Bearing this goal in mind, we systematically benchmark the self-consistent-charge density-functional tight-binding (SCC-DFTB) method for the description of vibrational spectra of protonated water clusters in the gas phase, which became available only recently with infrared multiphoton photodissociation and infrared predissociation spectroscopic experiments. It is found that SCC-DFTB qualitatively reproduces the important features in the vibrational spectra of protonated water clusters, especially concerning the characteristic signatures of clusters of various sizes. In agreement with recent ab initio molecular dynamics studies, it is found that dynamical effects play an important role in determining the vibrational properties of these water clusters. Considering computational efficiency, these benchmark calculations suggest that the SCC-DFTB/molecular mechanical approach can be an effective tool for probing the structural and dynamic features of protonated water molecules in biomolecular systems.
Dynamical features of bound states of topological solitons in highly dispersive low-dimensional systems
Authors: Bogdan M. M. , Charkina O. V.
Published: December 2007

The nonstationary dynamics and interaction of topological solitons (dislocations, domain walls, fluxons) in one-dimensional systems with high dispersion are investigated. Processes of soliton complex formation are studied analytically and numerically in relation to the strength of the dispersion, soliton velocity, and distance between solitons. It is demonstrated that stable bound soliton states with complex internal structure can propagate in a dissipative medium owing to their stabilization by external forces.
Low-temperature excimer phosphorescence of vitreous ortho-bromobenzophenone
Authors: Pyshkin O. S.
Published: December 2007

Integrated and time-resolved phosphorescence spectra of vitreous ortho-bromobenzophenone are investigated at a temperature of 4.2K. An excimer emission is observed that is completely absent in the crystal. The rapid damping of the excimer emission in comparison with monomer emission is observed. It is conjectured that the excimer emission of the glass is the result of the formation of pre-dimer pairs in the preparation of the vitreous sample.
A transition state wave packet study of the H+CH4 reaction
Authors: Zhang Liling , Lu Yunpeng , Lee Soo-Y. , Zhang Dong H.
Published: December 2007

Transition state wave packet calculations have been carried out to obtain the thermal rate constants for the H+CH4 reaction on the Jordan-Gilbert potential energy surface. The eight-dimensional model for the X+YCZ3 type of reaction was employed by restricting the nonreacting CZ3 group under a C3V symmetry. We calculated the cumulative reaction probability for the total angular momentum J=0, from which the thermal rate constants were obtained for the temperature range between 250 and 500K by employing the J-K shifting approximation. It is found that the eight-dimensional rate constants agree very well with the seven dimensional ones, in which the CH bond length in the nonreacting CH3 group is fixed, suggesting that the additional mode for the symmetry stretching in CH3 group does not have any important effect on the reaction within the temperature range considered here. The present transition state wave packet results agree well with rate constants obtained from the previous seven dimensional initial state selected wave packet study, indicating the consistence of the treatments involved in these two studies. On the other hand, it is found that the energy threshold for the cumulative reaction probability for J=0 from the present study is higher than that from the full dimensional multiconfiguration time-dependent Hartree study by about 0.06eV, resulting in severe discrepancy between the present rate constants and the multiconfiguration time-dependent Hartree results.
Density functional theory for the photoionization dynamics of uracil
Authors: Toffoli D. , Decleva P. , Gianturco F. A. , Lucchese R. R.
Published: December 2007

Photoionization dynamics of the RNA base uracil is studied in the framework of density functional theory. The photoionization calculations take advantage of a newly developed parallel version of a multicentric approach to the calculation of the electronic continuum spectrum which uses a set of B-spline radial basis functions and a Kohn-Sham density functional Hamiltonian. Both valence and core ionizations are considered. Scattering resonances in selected single-particle ionization channels are classified by the symmetry of the resonant state and the peak energy position in the photoelectron kinetic energy scale; the present results highlight once more the site specificity of core ionization processes. We further suggest that the resonant structures previously characterized in low-energy electron collision experiments are partly shifted below threshold by the photoionization processes. A critical evaluation of the theoretical results provides a guide for future experimental work on similar biosystems.
Determining lifetime in silicon blocks and wafers with accurate expressions for carrier density
Authors: Bowden Stuart , Sinton Ronald A.
Published: December 2007

In recent years, many studies of silicon minority-carrier lifetime have been performed with quasi-steady-state photoconductance measurements. The method has been used to characterize the defect levels, surface passivation, and trapping effects in wafers by using absolutely calibrated data for minority-carrier lifetime versus minority-carrier injection level. This paper generalizes the quasi-steady-state photoconductance technique for use in thick wafers or blocks of silicon where the diffusion length or light absorption depth is much less than the sample thickness. The photogeneration and carrier diffusion profiles are calculated for special cases of interest. The measured effective lifetimes can then be used to estimate the bulk lifetime in the material, and report this lifetime at an appropriate average minority-carrier density for the measurement conditions. In this way, the results and measurement methodologies previously developed for use on wafers can be applied to single- or multi-crystalline silicon ingots or thick wafers. In thick silicon samples, long-wavelength weakly absorbed light can be used to reduce the effects of surface recombination on the measurement giving important advantages compared to the case of measuring unpassivated wafers. The generalization presented here offers advantages for accurately determining the bulk lifetime of silicon material prior to sawing into wafers and without requiring surface passivation.
Three-dimensional failure analysis of high power semiconductor laser diodes operated in vacuum
Authors: Yeoh Terence S. , Chaney John A. , Leung Martin S. , Ives Neil A. , Feinberg Z. D. , Ho James G. , Wen Jianguo
Published: December 2007

The damaged region of a semiconductor laser diode that failed in a vacuum environment was analyzed using focused ion beam (FIB) serial sectioning, time-of-flight secondary ion mass spectrometry (ToF-SIMS), high resolution transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), energy dispersive x-ray spectroscopy (EDS), and nanodiffraction. The FIB nanotomography models and the TEM cross sections show a damage structure extending deep into the core and originating at the diode/antireflective (AR) coating interface. Nanocrystalline gold was detected at this interface using both TEM diffraction and EDS, and the localization of gold along the core at the diode/AR interface was corroborated using 3D ToF-SIMS. A thinning of the AR coating above the failure site was observed by TEM with a corresponding increase in carbon content on the AR surface detected with EELS. It is suggested that failure proceeded by pyrolysis of adsorbed hydrocarbons on the AR coating, which, in the presence of a high optical flux, contributed to carbothermal reduction of the AR coating. As the optical flux increased, thermal gradients facilitate metal migration, leading to larger gold clusters. These clusters are sites for deep level traps and may promote catalytic reactions.
Front dynamics and macroscopic diffusion in buoyant mixing in a tilted tube
Authors: Séon T. , Znaien J. , Perrin B. , Hinch E. J. , Salin D. , Hulin J. P.
Published: December 2007

The buoyancy driven interpenetration of two fluids of different densities has been studied in a long tilted tube in the strong mixing regime for which the mean concentration profile along the tube length satisfies a macroscopic diffusion equation. Variations of the corresponding macroscopic diffusion coefficient D and of the front velocity Vf are studied as a function of the Atwood number At, the viscosity ν, the tube diameter d, and the tilt angle θ. Introducing the characteristic inertial velocity Vt and the Reynolds number Ret, the normalized front velocity Vf∕Vt and dispersion coefficient D∕(Vtd) are observed to scale, respectively, as Ret−3∕4 and Ret−3∕2 for Ret≲1000. Also, Vf increases linearly with tanθ and the ratio (D∕Vf2) remains of the order of (35±10)d∕Vt in a wide range of values of the tilt angle and of the other control parameters. This close relation observed between the variations of D and Vf2 is discussed in terms of the characteristic time for transverse mixing across the flow channel.
Turbulent mixing of two-layer stratified fluid
Authors: Whitehead J. A. , Stevenson Ian
Published: December 2007

A two-layer salt-stratified tank of water was mixed by turbulence generated by many excursions of a horizontally moving vertical rod. The objective is to observe the time-dependent response of the mean density field for ranges of Richardson number Ri>0.9 and Reynolds Number Re>600. As the density profile of the fluid gradually evolves from a single step to a mixed state over a wide range of time, there is an almost perfect collapse of all the profiles to one universal profile as a function of a similarity variable. Although the turbulent diffusion is not constant, the value in the limit of small stratification has similar magnitude to values found by others.
Semiempirical pressure-volume-temperature equation of state: MgSiO3 perovskite is an example
Authors: Garai József
Published: December 2007

Simple general formula describing the pressure-volume-temperature relationships (p-V-T) of elastic solids is constructed from theoretical considerations. The semiempirical equation of state (EOS) was tested to experiments of perovskite 0–109GPa and 293–2000K. The parameters providing the best fit are B0=267.5GPa, V0=24.284cm3, α0=2.079×10−5K−1, ∂B0∕∂p=1.556, and ∂α0∕∂p=−1.098×10−7K−1GPa−1. The root-mean-square deviations (RMSDs) of the residuals are 0.043cm3, 0.79GPa, and 125K for the molar volume, pressure, and temperature, respectively. These RMSD values are in the range of the uncertainty of the experiments, indicating that the five-parameter semiempirical EOS correctly describes the p-V-T relationships of perovskite. Separating the experiments into 200K ranges the semiempirical EOS was compared to the most widely used finite strain, interatomic potential, and empirical isothermal EOSs such as the Birch-Murnaghan, the Vinet, and the Roy-Roy, respectively. Correlation coefficients, RMSDs of the residuals, and Akaike Information Criteria were used for evaluating the fitting. Based on these fitting parameters under pure isothermal conditions the semiempirical p-V EOS is slightly weaker than the Birch-Murnaghan and Vinet EOSs; however, the semiempirical p-V-T EOS is superior in every temperature range to all of the investigated conventional isothermal EOSs.
Structural, magnetic, and electric properties of Dy1−xSrxCoO3−δ (0.65≤x≤0.90)
Authors: Hassen A. , Ali A. I. , Kim B. J. , Wu Y. S. , Park S. H. , Kim Bog G.
Published: December 2007

The structural, magnetic, and electric properties of Dy1−xSrxCoO3−δ perovskite have been investigated systematically over the range of doping, 0.65≤x≤0.90. The Rietveld refinements of x-ray powder diffraction patterns at room temperature indicate that the samples with 0.65≤x≤0.75 show a tetragonal structure with I4/mmm group symmetry, while the compounds with 0.80≤x≤0.90 are cubic with pm3m group symmetry. Zero field-cooled magnetization, M(T), of 0.65≤x≤0.85 samples reveals a cusp at around room temperature. For all samples, M(T) increases rapidly below 50 K due to the paramagnetism of Dy sublattice. The inverse magnetic susceptibility, χ−1(T), was described by using Curie–Weiss law. The resistivity (ρ) data can be explained according to a three-dimensional variable range hopping model in a certain temperature range. The density of states at the vicinity of Fermi level is roughly estimated.
Quantitative analysis of domain texture in polycrystalline barium titanate by polarized Raman microprobe spectroscopy
Authors: Sakashita Tatsuo , Chazono Hirokazu , Pezzotti Giuseppe
Published: December 2007

A quantitative determination of domain distribution in polycrystalline barium titanate (BaTiO3, henceforth BT) ceramics has been pursued with the aid of a microprobe polarized Raman spectrometer. The crystallographic texture and domain orientation distribution of BT ceramics, which switched upon applying stress according to ferroelasticity principles, were determined from the relative intensity of selected phonon modes, taking into consideration a theoretical analysis of the angular dependence of phonon mode intensity for the tetragonal BT phase. Furthermore, the angular dependence of Raman intensity measured in polycrystalline BT depended on the statistical distribution of domain angles in the laser microprobe, which was explicitly taken into account in this work for obtaining a quantitative analysis of domain orientation for in-plane textured BT polycrystalline materials.
Parametric-time coherent states for Smorodinsky-Winternitz potentials
Authors: Ünal Nuri
Published: December 2007

In this study, we construct the coherent states for a particle in the Smorodinsky-Winternitz potentials, which are the generalizations of the two-dimensional Kepler problem. In the third case, the system is transformed into four ocillators and the parametric-time coherent states are constructed in two coordinate frames. In the fourth case, the system is transformed into two ocillators with the reflection symmetry and the parametric-time coherent states are constructed in two coordinate frames.
The orbifolds of permutation type as physical string systems at multiples of c=26V. Cyclic permutation orbifolds
Authors: Halpern M. B.
Published: December 2007

I consider the Zλ,λ prime free-bosonic permutation orbifolds as interacting physical string systems at ĉ=26λ. As a first step, I introduce twisted tree diagrams which confirm at the interacting level that the physical spectrum of each twisted sector is equivalent to that of an ordinary c=26 closed string. The untwisted sectors are surprisingly more difficult to understand, and there are subtleties in the sewing of the loops, but I am able to propose provisional forms for the full modular-invariant cosmological constants and one-loop diagrams with insertions.
A reduced-order model of diffusive effects on the dynamics of bubbles
Authors: Preston A. T. , Colonius T. , Brennen C. E.
Published: December 2007

We propose a new reduced-order model for spherical bubble dynamics that accurately captures the effects of heat and mass diffusion. The objective is to reduce the full system of partial differential equations to a set of coupled ordinary differential equations that are efficient enough to implement into complex bubbly flow computations. Comparisons to computations of the full partial differential equations and of other reduced-order models are used to validate the model and establish its range of validity.
Plasma induced by resonance enhanced multiphoton ionization in inert gas
Authors: Shneider Mikhail N. , Zhang Zhili , Miles Richard B.
Published: December 2007

We present a detailed model for the evolution of resonance enhanced multiphoton ionization (REMPI) produced plasma during and after the ionizing laser pulse in inert gas (argon, as an example) at arbitrary pressures. Our theory includes the complete process of the REMPI plasma generation and losses, together with the changing gas thermodynamic parameters. The model shows that the plasma expansion follows a classical ambipolar diffusion and that gas heating results in a weak shock or acoustic wave. The gas becomes involved in the motion not only from the pressure gradient due to the heating, but also from the momentum transfer from the charged particles to gas atoms. The time dependence of the total number of electrons computed in theory matches closely with the results of coherent microwave scattering experiments.
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overdamped Josephson junctions above 4.2K for voltage metrology
Authors: Lacquaniti V. , De Leo N. , Fretto M. , Maggi S. , Sosso A.
Published: December 2007

The temperature dependence of the quantized voltage steps near the superconducting transition temperature of Nb∕Al–AlOx∕Nb overdamped Josephson junctions irradiated at 75GHz has been studied. These junctions can switch between the hysteretic and nonhysteretic regime either by varying a fabrication parameter or, as shown in the present paper, by changing the junction operating temperature. Due to the high values of JC and VC up to 75kA∕cm2 and 0.5mV at 4.2K, respectively, and to the moderate temperature dependence of these parameters, the height of the first rf-induced step was larger than 1mA at T=6–8K, corresponding to 0.7–0.9T∕TC on a junction of 5×5μm2. The flatness at the nanovolt level of this step demonstrates the feasibility of programable or ac voltage standards operating in a cryocooler setup.
Pulse like hydrogen sensing response in Pd nanoparticle layers
Authors: Khanuja Manika , Varandani Deepak , Mehta Bodh R.
Published: December 2007

This study reports an enhanced and unusual pulse like hydrogen sensing response in Pd nanoparticle layers. The faster H adsorption due to increased surface area and closure of conducting paths as a result of lattice expansion on hydride formation are the primary reasons for this. In comparison, Pd thin films exhibit a slow and subdued sensing response because of the overlap of the above two opposing effects and hydrogen induced lattice strain. Temperature independent conductivity in the temperature range of 20–300K confirms the presence of interparticle gaps in the case of Pd nanoparticle layers.
Oxide thickness dependence of swift heavy ion-induced surface tracks formation in silicon dioxide on silicon structures at grazing incidence
Authors: Carvalho A. M. J. F. , Touboul A. D. , Marinoni M. , Ramonda M. , Guasch C. , Saigne F. , Bonnet J. , Gasiot J.
Published: December 2007

The influence of the oxide thickness in the surface tracks formation in thin silicon dioxide layered-silicon substrate (SiO2-Si) irradiated with swift heavy ion is dealt with. In this respect, SiO2-Si samples with different oxide thicknesses have been characterized using atomic force microscopy before and after 7.51 MeV/u Xe ion irradiation at a grazing incident angle of 1° relative to the surface plane. Experimental evidence of the existence of a threshold thickness in the formation of swift heavy ion-induced surface tracks has been addressed and discussed according to the thermal spike theory. This experimental upshot can be helpful when assessing metal–oxide–semiconductor ultrathin-gate oxide reliability issues and for growth of silicon-based nanostructures.
Microcavity top-emitting organic light-emitting devices integrated with microlens arrays: Simultaneous enhancement of quantum efficiency, cd/A efficiency, color performances, and image resolution
Authors: Yang Chih-Jen , Liu Su-Hao , Hsieh Hsing-Hung , Liu Chih-Che , Cho Ting-Yi , Wu Chung-Chih
Published: December 2007

A long bothering issue in microcavity organic light-emitting devices (OLEDs) is the difficulty to simultaneously achieve enhanced cd/A efficiency, enhanced external quantum efficiency, enhanced color saturation, and stable colors with viewing angles in the same device design. In this work, we show that microcavity top-emitting OLEDs integrated with microlenses may provide a universal approach for simultaneously achieving all these desired nice characteristics. Furthermore, the pixel blurring often occurring in employment of microlenses to conventional bottom-emitting OLEDs is significantly suppressed by combination of top-emitting microcavity OLEDs and microlenses.
Density variation of nanoscale patterns in thermal nanoimprint lithography
Authors: Woo Young Seok , Kim Jae Kwan , Lee Dong Eon , Suh Kahp Yang , Lee Woo Il
Published: December 2007

Density variation of nanoscale patterns in thermal nanoimprint lithography was studied both by experiments and molecular dynamics simulations. A simple soft imprinting technique was used to fabricate various nanopatterns (70nm and 600nm lines and 150nm dots) over a large area (2×3cm2). Local density was measured by the relative magnitude of van der Waals interactions between a sharp tip and the patterned surface. In order to investigate the mechanism of density variation, molecular dynamic simulations were performed. Experimental and simulation results demonstrated that the density of the pressed region (valleys) was higher than that of the cavity region (hills) when a simple amorphous polymer is thermally imprinted with a patterned mold.
Spontaneous reorientation of bimetal multilayer nanowires
Authors: Ma Fei , Ma Sheng-Li , Xu Ke-Wei , Chu Paul K.
Published: December 2007

Multilayered structures are proposed to extend the size range at which metal nanowires can transform spontaneously from one orientation to another due to the interfacial stress. The principle is demonstrated on gold-palladium (Au–Pd) multilayer nanowires by using the molecular dynamics (MD) method and two distinctly different mechanisms are identified for the reorientation which take place mainly via slipping and twinning at low temperature and by amorphization and recrystallization at high temperature. Our results show that the time response of devices fabricated with the nanostructures is consequently affected.
Silver halide single-mode fibers with improved properties in the middle infrared
Authors: Lewi T. , Shalem S. , Tsun A. , Katzir A.
Published: December 2007

In this letter, we report the design and development of improved step-index silver halide (AgClBr) single mode fibers. These fibers exhibited round and symmetrical field mode distribution while efficiently suppressing the higher order modes. The losses of the fundamental mode were 10–20dB∕m at λ=10.6μm, and the minimum fiber length needed for single mode operation was ∼50cm. These fibers will be useful as spatial filters for middle infrared interferometry.
Background gas effects on the generation of nanopatterns on a pure silicon wafer with multiple femtosecond near field laser ablation
Authors: Wen Sy-Bor , Greif Ralph , Russo Richard E.
Published: December 2007

Significant laser energy was transferred from the near field scanning optical microscope tip to a silicon wafer producing nanopatterns on the sample surface. The patterns changed from nanoprotrusions to nanocraters when the background gas was changed from air to argon. Two physical mechanisms were attributed to this dramatic change, namely, oxidation and laser ablation. When air was present, oxidation dominated over ablation in the formation of the nanoprotrusions obtained after multiple laser pulses. When oxygen was absent, e.g., pure argon was the background gas, laser ablation was dominant, and nanocraters resulted after multiple laser pulses.
Determination of the specific resistance of individual freestanding ZnO nanowires with the low energy electron point source microscope
Authors: Weber D. H. , Beyer A. , Völkel B. , Gölzhäuser A. , Schlenker E. , Bakin A. , Waag A.
Published: December 2007

A low energy electron point source microscope is used to determine the electrical conductivity of freestanding ZnO nanowires. The nanowires were contacted with a manipulation tip and I-V curves were taken at different wire lengths. From those, the specific resistance was calculated and separated from the contact resistance. By comparing the specific resistances of ZnO nanowires with diameters between 1100 and 48nm, a large surface contribution for the thin nanowires was found. A geometric model for separation between surface and bulk contributions is given.
Effects of K∕Na ratio on the phase structure and electrical properties of (KxNa0.96−xLi0.04)(Nb0.91Ta0.05Sb0.04)O3 lead-free ceramics
Authors: Wu Jiagang , Xiao Dingquan , Wang Yuanyu , Zhu Jianguo , Wu Lang , Jiang Yihang
Published: December 2007

( K x Na 0.96 − x Li 0.04 ) ( Nb 0.91 Ta 0.05 Sb 0.04 ) O 3  (x=0.32–0.56) lead-free piezoelectric ceramics were prepared by conventional sintering. The effects of K∕Na ratio on the phase structure and electrical properties of the ceramics were studied. These results indicate that the phase structure undergoes a transition from orthorhombic to tetragonal phase with increase of x. The ceramics with x=0.38 exhibit enhanced electrical properties (d33∼306pC∕N; kp∼48%; kt∼49%; Tc∼337°C; ϵr∼1327; tanδ∼2.5%; Pr∼34.9μC∕cm2; Ec∼11.3kV∕cm). Enhanced electrical properties of the ceramics should be attributed to the polymorphic phase transition near room temperature. These results show that the ceramic with x=0.38 is a promising lead-free piezoelectric material.
Percolation in random networks of heterogeneous nanotubes
Authors: Li Jiantong , Zhang Zhi-Bin , Zhang Shi-Li
Published: December 2007

The electrical performance of random carbon nanotube network transistors is found by Monte Carlo simulation to strongly depend on the nature of the conduction path percolating the network. When the network is percolated only by semiconducting nanotube pathways (OSPs), the transistors can directly achieve both high on current and large on/off current ratio. Based on percolation theory, the present work predicts that there exist specific nanotube coverage domains within which OSP has the highest probability and becomes predominant. Simulation results show that the coverage domains depend on the network dimension, nanotube length, and the fraction of metallic nanotubes.
C‐reactive protein and the mode of onset of labor in term pregnancies
Authors: WISER AMIR , SIVAN EYAL , DULITZKI MOTI , CHAYEN BENJEMIN , SCHIFF EYAL , BAR‐CHAIM ADINA , SIMCHEN MICHAL J.
Published: December 2007

Objective. To evaluate whether C‐reactive protein (CRP) levels in maternal serum at post‐date follow‐up can predict the mode of onset of spontaneous delivery in term pregnancies and the interval to delivery. Study design. Women at 40 completed weeks of gestation were recruited. The interval from CRP sampling to delivery and mode of onset of delivery were noted. Results. Sixty women were divided into 3 groups according to the mode of onset of labor. Group A presented with spontaneous onset of contractions, Group B presented with pre‐labor ruptured membranes (PROM), and Group C completed 42 weeks without spontaneous initiation of labor. There was no association between CRP values and latency from time of sampling to delivery. Mean serum CRP in Group B (12.7 mg/l) was significantly higher than in either Group A (7.2 mg/l) or Group C (10.2 mg/l) (p = 0.01). In addition, CRP values in the upper quartile (>12.2 mg/l) had a 37.5% positive predictive value for PROM, with a negative predictive value of 93%. Conclusion. Women whose labor will start with PROM have higher CRP values at post‐date follow‐up than women whose labor starts otherwise.
Which factors play a role in clinical decision‐making in external cephalic version?
Authors: KOK MARJOLEIN , VAN DER STEEG JAN WILLEM , MOL BEN W.J. , OPMEER BRENT , VAN DER POST JORIS A.
Published: December 2007

Objective. To assess the clinical factors that influence the estimates of clinicians of the success of an external cephalic version (ECV), and the subsequent management decisions made by clinicians. Design. We constructed 16 fictional vignettes of women with a term fetus in breech position eligible for ECV. Setting. Secondary and tertiary clinics in The Netherlands. Population. Thirty‐seven gynaecologists, residents and midwifes. Methods. Sixteen case summaries concerning a hypothetical patient eligible for ECV. Potential prognostic factors that varied between the cases were parity, maternal body mass index, engagement of the fetus, amniotic fluid, fetal growth, fetal presentation and placental localisation. For each case presentation, the clinicians were asked for their inclination to perform an ECV, and whether or not they would use tocolysis. Results. The estimated probabilities of success varied between 20 and 60%. The number of clinicians that would attempt an ECV varied per case between 32 and 97%. Amniotic fluid and engagement contributed 80% of the variation in the decision to perform ECV. In the case of oligohydramnios or an engaged breech, the clinicians tended not to perform an ECV. Conclusion. Amniotic fluid and engagement seem to be the main factors in the clinical decision‐making of clinicians in ECV. This decision‐making is probably experience based. Systematic knowledge of clinical prognosticators and subsequent assessment of their prognostic capacity is needed.
Seasonal influence on the admittance of pre‐eclampsia patients in Tygerberg Hospital
Authors: IMMINK ANNELIES , SCHERJON SICCO , WOLTERBEEK RON , STEYN D. WILHELM
Published: December 2007

Background. Previous studies suggest that the occurrence of pre‐eclampsia is seasonally distributed. This retrospective study aims to determine whether there is a seasonal variation in the number of admissions and the prevalence of women with pre‐eclampsia in Tygerberg Hospital, South Africa. Methods. The number of women admitted with a diagnosis of pre‐eclampsia, eclampsia or haemolysis elevated liver enzymes low platelets (HELLP) syndrome (together called pre‐eclampsia) was obtained from hospital records from 2002 to 2003 for each season and month. Associations were analysed with Odds Ratios (OR). Furthermore, these data was compared with the Cape Town temperatures recorded on each day over the period, as well as the total rainfall for each month. Bivariate logistic regression of the probability of pre‐eclampsia on temperature and rainfall was performed. Results. Pre‐eclampsia occurred in 11.5% of all admissions (1,329/11,585). The prevalence was highest in winter (13.6% pre‐eclampsia patients from all admissions). Women admitted in winter had a higher risk of developing pre‐eclampsia compared to those admitted in summer (OR =1.69, 95% CI: 1.07–1.53). The risk of developing pre‐eclampsia in June was higher than in February (summer in South Africa, reference month) (OR =2.81, 95% CI: 2.06–3.83). There was a significant correlation between the number of admissions with pre‐eclampsia and the minimum daily temperature (r = −0.620, p = 0.032). Conclusions. Pre‐eclampsia occurs more frequently in winter at Tygerberg Hospital, South Africa. The findings have implications for future research related to the aetiology of pre‐eclampsia as well as for clinical care.
New attempt using labio‐vestibular flap technique to manage circumcised women with Rokitansky syndrome
Authors: THABET SAEED MOHAMED AHMED , ALI ATHMAR HUSSEIN
Published: December 2007

Objective. To assess the efficacy of the labio‐vestibular flap technique in managing circumcised women with Rokitansky syndrome and in correcting the sexuality defects caused by the anomaly present and by female circumcision. Design. A prospective, comparative, clinical study. Setting. Kasr El Aini School of Medicine, Cairo University, Egypt. Patients. Thirty‐four circumcised patients with Rokitansky syndrome were divided into 2 groups; Group 1 consisted of 22 cases treated with the labio‐vestibular flap technique, and Group 2 consisted of 12 cases treated with McIndoe's technique. Main outcome measures. Pre and postoperative clinical assessment of the newly formed vagina and sexuality. The new flap is formed of the remaining parts of the labia minora and the adjoining parts of the vestibule, and is used to cover the posterior and lateral walls. Meanwhile, the anterior wall is covered by the epithelium of the original blind vaginal pouch. The modified Kasr El Aini sexual assessment sheet assesses sexuality. Results. Rokitansky syndrome could be classified into 4 clinical types according to the degree of development of the uterus. The labio‐vestibular technique was simple and not associated with any graft rejection, hair growth or contraction of the reformed tract. Dyspareunia and marital failure were less significantly recorded in the labio‐vestibular technique than in McIndoe's technique. But, the most significant results were the improvement in sexuality, in addition to restoration of genital continuation, menstruation and fertility in some cases. Conclusion. The labio‐vestibular flab technique is the most simple and most suitable line of treatment for circumcised cases with Rokitansky syndrome. In these cases, the technique achieved good results in correcting sexual defects after circumcision.