bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of a white noise at channel inlet on the parallel and wavy convective instabilities of Poiseuille-Rayleigh-Bénard flows
Authors: Nicolas Xavier , Zoueidi Noussaiba , Xin Shihe
Published: August 2012

The present paper concerns Poiseuille-Rayleigh-Bénard mixed convection flows in horizontal rectangular air-filled channels of large spanwise aspect ratio (W/H ≥ 10) and it focuses on the primary and secondary thermoconvective instabilities made of steady longitudinal and unsteady wavy rolls for 100 ≤ Re ≤ 200, 3000 < Ra < 15 000, Pr = 0.7, and W/H = 10. Time linear stability analysis of longitudinal rolls and 3D nonlinear numerical simulations using a specially tailored finite difference code is performed for this purpose. A bibliographical review, linear stability analysis and 3D numerical simulations allow establishing the full stability diagram for Re ≤ 300 and Ra ≤ 20 000. The linear stability analysis indicates that the critical Rayleigh number Ra≈*(Re) of the neutral curve between longitudinal and wavy rolls for W/H = 10 is increased at least by a factor of 1.5 in comparison with infinite W/H. The numerical study shows that the usual definitions of growth lengths for longitudinal rolls are inappropriate and it explains the discrepancies observed on wall Nusselt numbers in the literature between experimental and numerical results for the fully developed longitudinal rolls: Nusselt number decreasing at Ra > 8000 is due to spanwise oscillations of thermoconvective rolls that favor a bulk temperature homogenization. Because they are a convective instability, wavy rolls and their space and time development are studied numerically by maintaining at channel inlet, a permanent random excitation: it is designed to cover all the modes and allows detecting the wavy roll modes that are naturally amplified by the flow and those that are damped. Wavy roll patterns are characterized with respect to its three control parameters: Re, the relative distance ɛ to the critical Rayleigh number Ra≈*, and the excitation magnitude Aexc. The growth length of the wavy rolls is shown to correlate with ɛ−0.72 and Log(Aexc). The frequency, wave number, and phase velocity of the most amplified mode, the wall averaged Nusselt number and the spanwise displacements of the wavy rolls are independent of Aexc in the fully developed zone, but depend a lot on ɛ for ɛ < 2 and nearly stabilize for ɛ > 2 (i.e., Ra > 3Ra≈*). Correlation laws as a function of Re, ɛ, and Aexc are proposed for most of the exploited quantities. Numerical simulations performed are in a good agreement with experimental results on the wavy rolls obtained by Pabiou et al. [“Wavy secondary instability of longitudinal rolls in Rayleigh-Bénard-Poiseuille flows,” J. Fluid Mech. 542, 175 (2005)10.1017/S0022112005006154]. Finally, wavy roll characteristics are shown to be potentially interesting to better homogenize the vapor depositions in the horizontal rectangular chemical vapor deposition reactors used to make thin coatings on heated substrates from gaseous components.
Contributions of the electronic spin and orbital current to the ${\rm CoCl}_4^{2-}$ CoCl 42− magnetic field probed in polarised neutron diffraction experiments
Authors: Cassam-Chenaï Patrick , Jayatilaka Dylan
Published: August 2012

Polarised neutron diffraction experiments conducted at 4.2 K on Cs3CoCl5 crystals have been analysed by using a four-dimensional model Hilbert space made of ab initio n-electron wave functions of the \documentclass[12pt]{minimal}\begin{document}${\rm CoCl}_4^{2-}$\end{document} CoCl 42− molecular ion. Two spin-orbit mixing coefficients and several configuration interaction coefficients have been optimized by fitting calculated magnetic structure factors to experimental ones, to obtain the best ensemble density operator that is representable in the model space. A goodness of fit, χ2, less then 1 has been obtained for the first time for the two experimental data sets available. In the present article, the optimized density operators are used to calculate the magnetic field densities that are the genuine observables probed in neutron diffraction experiments. Density maps of such observables are presented for the first time and numerical details are provided. The respective contributions of spin density and orbital current to the magnetic field density are analyzed.
Time-dependent partitioning theory of the control of radiationless transitions in 24-mode pyrazine
Authors: Thanopulos Ioannis , Li Xuan , Brumer Paul , Shapiro Moshe
Published: August 2012

We consider the control of internal conversion between the S2(1B2u) excited electronic state of pyrazine and the S1(1B3u) state. The study is performed both during and after the femtosecond excitation of the ground electronic state S0(1Ag) to form the S2 state. The dynamics is examined using the newly developed “effective modes” technique which enables the full computation of quantum dynamics in multi-dimensional spaces. Using this technique, we also investigate the coherent control of population transfer from S0 to the S2 and S1 electronic states. We find that the use of shaped laser pulses enables a significant delay of the internal conversion. For example, after 60 fs, the S2 population amounts to ∼60% of the initial S0 population, and remains at ∼20% after 100 fs, in contrast to the S0 electronic state which is completely depopulated within 75 fs.
Glycine phases formed from frozen aqueous solutions: Revisited
Authors: Surovtsev N. V. , Adichtchev S. V. , Malinovsky V. K. , Ogienko A. G. , Drebushchak V. A. , Manakov A. Yu. , Ancharov A. I. , Yunoshev A. S. , Boldyreva E. V.
Published: August 2012

Glycine phases formed when aqueous solutions were frozen and subsequently heated under different conditions were studied by Raman scattering, x-ray diffraction, and differential scanning calorimetry (DSC) techniques. Crystallization of ice Ih was observed in all the cases. On cooling at the rates of 0.5 K/min and 5 K/min, glassy glycine was formed as an intermediate phase which lived about 1 min or less only, and then transformed into β-polymorph of glycine. Quench cooling of glycine solutions (15% w/w) in liquid nitrogen resulted in the formation of a mixture of crystalline water ice Ih and a glassy glycine, which could be preserved at cryogenic temperatures (80 K) for an indefinitely long time. This mixture remained also quite stable for some time after heating above the cryogenic temperature. Subsequent heating under various conditions resulted in the transformation of the glycine glass into an unknown crystalline phase (glycine “X-phase”) at 209–216 K, which at 218–226 K transformed into β-polymorph of glycine. The “X-phase” was characterized by Raman spectroscopy; it could be obtained in noticeable amounts using a special preparation technique and tentatively characterized by x-ray powder diffraction (P2, a = 6.648 Å, b = 25.867 Å, c = 5.610 Å, β = 113.12º); the formation of “X-phase” from the glycine glassy phase and its transformation into β-polymorph were followed by DSC. Raman scattering technique with its power for unambiguous identification of the crystalline and glassy polymorphs without limitation on the crystallite size helped us to follow the phase transformations during quenching, heating, and annealing. The experimental findings are considered in relation to the problem of control of glycine polymorphism on crystallization.
Temporal evolution, morphology, and settling of the sediment plume in a model estuary
Authors: Henniger R. , Kleiser L.
Published: August 2012

We examine the transport and settling of particles in a laboratory-scale model estuary using direct numerical simulation. The configuration is a shallow saltwater-filled basin with large horizontal dimensions. The particle-laden freshwater enters the basin over a relatively small inlet. Turbulence is generated by the collapse of Kelvin–Helmholtz vortices triggered in the freshwater/saltwater stratified mixing layer. The flow is computed until a statistically stationary solution is attained. The results demonstrate a significant increase of the particle settling speed compared to Stokes particle settling. For the onset of the simulation, this increase is attributed to sheet and finger convection. At later stages, we find that the increased settling speed is a result of the turbulent mixing of the particle suspension with clear ambient fluid, the density differences between the different phases, and the Stokes settling. The increase of the settling velocity is most pronounced for large Reynolds numbers, Richardson numbers of the particle suspension, and Stokes settling speeds. Apart from rather large Richardson numbers of the particle suspensions, variations of these parameters do not alter the flow qualitatively. For sufficiently small Richardson numbers, the shape of the particle plume is characterized by a distinct near-surface plume, a zone of fast particle settling beneath this plume, and a nepheloid layer at the bottom of the basin. These features are a result of the buoyant freshwater-particle mixture, the entrainment of ambient saltwater and the settling enhancement of the particulate phase.
Electronic conduction and microstructure in polymer composites filled with carbonaceous particles
Authors: Mdarhri A. , Brosseau C. , Zaghrioui M. , El Aboudi I.
Published: August 2012

Physical and physico-chemical properties of polymer filled with carbon black (CB) particles, namely, the microstructure dependence of these properties, are not only interesting on their own but are particularly important for electronic applications as they can impose limits on the sensitivity of a device. With this purpose, we report on an experimental study of the structural and electrical properties of semi-crystalline ethylene-co-butyl acrylate polymer filled with conductive CB nano-particles. We found that the value of the direct current conductivity exhibits a jump of 12 orders of magnitude over a small change in CB concentration and is due to a percolation-like behavior. To assess the temperature evolution of supercolative samples, we present measurements of the conductivity as function of temperature. Above the glass transition temperature of the polymer, the CB network restricts the motions of the polymer chains. This behavior was ascribed to the change in CB mesostructure in the polymer matrix as probed by scanning electron microscopy and atomic force microscopy as well as to the difference in the thermal expansion between the two phases. In addition to the observed conductivity increase, the effect of adding CB particles in the polymer matrix is to increase the thermal stability as is probed by thermogravimetric analysis tests. The room temperature alternating current conductivity, studied over the frequency range from 100 Hz to 15 MHz, is interpreted as arising mainly from inter-aggregate polarization effects. By considering carefully the CB content of the alternating current conductivity, we found that our experimental data agree well with the Sheng’s model of fluctuation-induced tunnelling of charge carriers over nanometric gaps between adjacent CB aggregates. For studying the filler content dependence of the effective permittivity, several mixing laws and effective medium theories have been used. The observed discrepancies between our experimental data and these theoretical predictions may be occur partly because these analysis contain an inaccurate knowledge of the physicochemical properties of the carbonaceous phase, give a poor description of the interfaces in these complex heterostructures, or both. As part of the present investigation, present results are compared to transport properties of polystyrene-cobutyl acrylate latex and epoxy resin matrices filled with different loadings of multiwalled carbon nanotubes (MWCNT) and over wide temperature and frequency ranges. It is remarkable that the MWCNT’s anisotropy (length-to-diameter ratio close to 100) manifests itself in percolation-like behavior with lower threshold volume fraction and different mesostructure than that evidenced for CB filled samples.
Crystal lattice properties fully determine short-range interaction parameters for alkali and halide ions
Authors: Mao Albert H. , Pappu Rohit V.
Published: August 2012

Accurate models of alkali and halide ions in aqueous solution are necessary for computer simulations of a broad variety of systems. Previous efforts to develop ion force fields have generally focused on reproducing experimental measurements of aqueous solution properties such as hydration free energies and ion-water distribution functions. This dependency limits transferability of the resulting parameters because of the variety and known limitations of water models. We present a solvent-independent approach to calibrating ion parameters based exclusively on crystal lattice properties. Our procedure relies on minimization of lattice sums to calculate lattice energies and interionic distances instead of equilibrium ensemble simulations of dense fluids. The gain in computational efficiency enables simultaneous optimization of all parameters for Li+, Na+, K+, Rb+, Cs+, F−, Cl−, Br−, and I− subject to constraints that enforce consistency with periodic table trends. We demonstrate the method by presenting lattice-derived parameters for the primitive model and the Lennard-Jones model with Lorentz-Berthelot mixing rules. The resulting parameters successfully reproduce the lattice properties used to derive them and are free from the influence of any water model. To assess the transferability of the Lennard-Jones parameters to aqueous systems, we used them to estimate hydration free energies and found that the results were in quantitative agreement with experimentally measured values. These lattice-derived parameters are applicable in simulations where coupling of ion parameters to a particular solvent model is undesirable. The simplicity and low computational demands of the calibration procedure make it suitable for parametrization of crystallizable ions in a variety of force fields.
Screenings and vertex operators of quantum superalgebra $U_q(\widehat{{sl}}(N|1))$Uq(sl̂(N|1))
Authors: Kojima Takeo
Published: August 2012

We construct the screening currents of the quantum superalgebra \documentclass[12pt]{minimal}\begin{document}$U_q(\widehat{sl}(N|1))$\end{document}Uq(sl̂(N|1)) for an arbitrary level k ≠ −N + 1. We show that these screening currents commute with the superalgebra modulo total difference. We propose bosonizations of the vertex operators by using the screening currents. We check that these vertex operators are the intertwiners among the Fock-Wakimoto representation and the typical representation for rank N ⩽ 4.
Molecular dynamics and neutron scattering study of the dependence of polyelectrolyte dendrimer conformation on counterion behavior
Authors: Wu Bin , Chen Wei-Ren , Egami Takeshi , Li Xin , Liu Yun , Wang Yongmei , Do Changwoo , Porcar Lionel , Hong Kunlun , Liu Li , Smith Gregory S. , Smith Sean C.
Published: August 2012

Atomistic molecular dynamics (MD) simulations and contrast variation small angle neutron scattering (SANS) have been combined to investigate the Generation-5 polyelectrolyte polyamidoamine starburst dendrimer. This work reveals the dendrimer conformational dependence on counterion association at different levels of molecular charge. The accuracy of the simulations is verified through satisfactory comparison between modeled results, such as excess intra-dendrimer scattering length density distribution and hydration level, and their experimental counterparts. While the counterion distributions are not directly measureable with SANS, the spatial distribution of the counterions and their dendrimer association are extracted from the validated MD equilibrium trajectories. It is found that the conformation of the charged dendrimer is strongly dependent on the counterion association. Sensitivity of the distribution of counterions around charged amines to the counterion valency is qualitatively explained by adopting Langmuir adsorption theory. Moreover, via extending the concept of electrical double layer for compact charged colloids, we define an effective radius of a charged dendrimer including the spatial distribution of counterions in its vicinity. Within the same framework, the correlation between the strength of intra-dendrimer electrostatic repulsion and the counterion valency and dynamics is also addressed.
Krylov subspace methods for computing hydrodynamic interactions in Brownian dynamics simulations
Authors: Ando Tadashi , Chow Edmond , Saad Yousef , Skolnick Jeffrey
Published: August 2012

Hydrodynamic interactions play an important role in the dynamics of macromolecules. The most common way to take into account hydrodynamic effects in molecular simulations is in the context of a Brownian dynamics simulation. However, the calculation of correlated Brownian noise vectors in these simulations is computationally very demanding and alternative methods are desirable. This paper studies methods based on Krylov subspaces for computing Brownian noise vectors. These methods are related to Chebyshev polynomial approximations, but do not require eigenvalue estimates. We show that only low accuracy is required in the Brownian noise vectors to accurately compute values of dynamic and static properties of polymer and monodisperse suspension models. With this level of accuracy, the computational time of Krylov subspace methods scales very nearly as O(N2) for the number of particles N up to 10 000, which was the limit tested. The performance of the Krylov subspace methods, especially the “block” version, is slightly better than that of the Chebyshev method, even without taking into account the additional cost of eigenvalue estimates required by the latter. Furthermore, at N = 10 000, the Krylov subspace method is 13 times faster than the exact Cholesky method. Thus, Krylov subspace methods are recommended for performing large-scale Brownian dynamics simulations with hydrodynamic interactions.
Absolute cross sections for electron scattering from furan
Authors: Maljković J. B. , Blanco F. , Čurík R. , García G. , Marinković B. P. , Milosavljević A. R.
Published: August 2012

We report results of measurements and calculations of absolute cross sections for electron scattering from furan molecules (C4H4O). The experimental absolute differential cross sections (DCSs) for elastic electron scattering were obtained for the incident energies from 50 eV to 300 eV and for scattering angles from 20º to 110º, by using a crossed electron-target beam setup and the relative flow technique for calibration to the absolute scale. The calculations of the electron interaction cross sections are based on a corrected form of the independent-atom method, known as the screening corrected additivity rule (SCAR) procedure and using an improved quasifree absorption model. The latter calculations also account for rotational excitations in the approximation of a free electric dipole and were used to obtain elastic DCSs as well as total and integral elastic cross sections which are tabulated in the energy range from 10 to 10 000 eV. All SCAR calculated cross sections agree very well with both the present and previously published experimental results. Additionally, calculations based on the first Born approximation were performed to calculate both elastic and vibrationally inelastic DCSs for all the modes of furane, in the energy range from 50 eV to 300 eV. The ratios of the summed vibrational to elastic DCSs are presented and discussed. Finally, the present results for furan are compared with previously published elastic DCSs for the tetrahydrofuran molecule and discussed.
Excitation transfer induced spectral diffusion and the influence of structural spectral diffusion
Authors: Rosenfeld Daniel E. , Fayer Michael D.
Published: August 2012

The theory of vibrational excitation transfer, which causes spectral diffusion and is also influenced by structural spectral diffusion, is developed and applied to systems consisting of vibrational chromophores. Excitation transfer induced spectral diffusion is the time-dependent change in vibrational frequency induced by an excitation on an initially excited molecule jumping to other molecules that have different vibrational frequencies within the inhomogeneously broadened vibrational absorption line. The excitation transfer process is modeled as Förster resonant transfer, which depends on the overlap of the homogeneous spectra of the donating and accepting vibrational chromophores. Because the absorption line is inhomogeneously broadened, two molecules in close proximity can have overlaps of their homogeneous lines that range from substantial to very little. In the absence of structural dynamics, the overlap of the homogeneous lines of the donating and accepting vibrational chromophores would be fixed. However, dynamics of the medium that contains the vibrational chromophores, e.g., a liquid solvent or a surrounding protein, produce spectral diffusion. Spectral diffusion causes the position of a molecule's homogeneous line within the inhomogeneous spectrum to change with time. Therefore, the overlap of donating and accepting molecules’ homogeneous lines is time dependent, which must be taken into account in the excitation transfer theory. The excitation transfer problem is solved for inhomogeneous lines with fluctuating homogeneous line frequencies. The method allows the simultaneous treatment of both excitation transfer induced spectral diffusion and structural fluctuation induced spectral diffusion. It is found that the excitation transfer process is enhanced by the stochastic fluctuations in frequencies. It is shown how a measurement of spectral diffusion can be separated into the two types of spectral diffusion, which permits the structural spectral diffusion to be determined in the presence of excitation transfer spectral diffusion. Various approximations and computational methodologies are explored.
Explicitly correlated Gaussian calculations of the 2D Rydberg states of the boron atom
Authors: Sharkey Keeper L. , Bubin Sergiy , Adamowicz Ludwik
Published: August 2012

Accurate non-relativistic variational calculations are performed for the seven lowest members of the 2D Rydberg series (1s22s2p2, and 1s22s2nd, n = 3, …, 8) of the boron atom. The wave functions of the states are expanded in terms of all-electron explicitly correlated Gaussian basis functions and the effect of the finite nuclear mass is directly included in the calculations allowing for determining the isotopic shifts of the energy levels. The Gaussian basis is optimized independently for each state with the aid of the analytic energy gradient with respect to the Gaussian parameters. The calculations represent the highest accuracy level currently achievable for the considered states. The computed energies are compared with the available experimental data.
Solvated calcium ions in charged silica nanopores
Authors: Bonnaud Patrick A. , Coasne Benoît , Pellenq Roland J.-M.
Published: August 2012

Hydroxyl surface density in porous silica drops down to nearly zero when the pH of the confined aqueous solution is greater than 10.5. To study such extreme conditions, we developed a model of slit silica nanopores where all the hydrogen atoms of the hydroxylated surface are removed and the negative charge of the resulting oxygen dangling bonds is compensated by Ca2+ counterions. We employed grand canonical Monte Carlo and molecular dynamics simulations to address how the Ca2+ counterions affect the thermodynamics, structure, and dynamics of confined water. While most of the Ca2+ counterions arrange themselves according to the so-called “Stern layer,” no diffuse layer is observed. The presence of Ca2+ counterions affects the pore filling for strong confinement where the surface effects are large. At full loading, no significant changes are observed in the layering of the first two adsorbed water layers compared to nanopores with fully hydroxylated surfaces. However, the water structure and water orientational ordering with respect to the surface is much more disturbed. Due to the super hydrophilicity of the Ca2+-silica nanopores, water dynamics is slowed down and vicinal water molecules stick to the pore surface over longer times than in the case of hydroxylated silica surfaces. These findings, which suggest the breakdown of the linear Poisson–Boltzmann theory, provide important information about the properties of nanoconfined electrolytes upon extreme conditions where the surface charge and ion concentration are large.
Vibrational deexcitation and rotational excitation of H2 and D2 scattered from Cu(111): Adiabatic versus non-adiabatic dynamics
Authors: Muzas A. S. , Juaristi J. I. , Alducin M. , Muiño R. Díez , Kroes G. J. , Díaz C.
Published: August 2012

We have studied survival and rotational excitation probabilities of H2(vi = 1, Ji = 1) and D2(vi = 1, Ji = 2) upon scattering from Cu(111) using six-dimensional (6D) adiabatic (quantum and quasi-classical) and non-adiabatic (quasi-classical) dynamics. Non-adiabatic dynamics, based on a friction model, has been used to analyze the role of electron-hole pair excitations. Comparison between adiabatic and non-adiabatic calculations reveals a smaller influence of non-adiabatic effects on the energy dependence of the vibrational deexcitation mechanism than previously suggested by low-dimensional dynamics calculations. Specifically, we show that 6D adiabatic dynamics can account for the increase of vibrational deexcitation as a function of the incidence energy, as well as for the isotope effect observed experimentally in the energy dependence for H2(D2)/Cu(100). Furthermore, a detailed analysis, based on classical trajectories, reveals that in trajectories leading to vibrational deexcitation, the minimum classical turning point is close to the top site, reflecting the multidimensionally of this mechanism. On this site, the reaction path curvature favors vibrational inelastic scattering. Finally, we show that the probability for a molecule to get close to the top site is higher for H2 than for D2, which explains the isotope effect found experimentally.
Influence of charging process and size distribution of dust grain on the electric conductivity of dusty plasma
Authors: Duan Ji-Zheng , Wang Cang-Long , Zhang Jian-Rong , Ma Sheng-Qian , Hong Xue-Ren , Sun Jian-An , Duan Wen-Shan , Yang Lei
Published: August 2012

The effects of dust size distribution and charging process of dust grains on the complex electric conductivity of dusty plasmas have been investigated in the present paper. Comparisons are made between real dusty plasma in which there are many different dust grain species and the mono-sized dusty plasma (MDP) in which there is only one kind of dust grain whose size is the average dust size. In some cases the complex electric conductivity of real dusty plasma is larger than that of MDP, while in other cases it is smaller than that of MDP, it depends on the dust size distribution function.
Mechanical relaxation in a Zr-based bulk metallic glass: Analysis based on physical models
Authors: Qiao J. C. , Pelletier J. M.
Published: August 2012

The mechanical relaxation behavior in a Zr55Cu30Ni5Al10 bulk metallic glass is investigated by dynamic mechanical analysis in both temperature and frequency domains. Master curves can be obtained for the storage modulus G′ and for the loss modulus G′′, confirming the validity of the time-temperature superposition principle. Different models are discussed to describe the main (α) relaxation, e.g., Debye model, Havriliak-Negami (HN) model, Kohlrausch-Williams-Watt (KWW) model, and quasi-point defects (QPDs) model. The main relaxation in bulk metallic glass cannot be described using a single relaxation time. The HN model, the KWW model, and the QPD theory can be used to fit the data of mechanical spectroscopy experiments. However, unlike the HN model and the KWW model, some physical parameters are introduced in QPD model, i.e., atomic mobility and correlation factor, giving, therefore, a new physical approach to understand the mechanical relaxation in bulk metallic glasses.
High phosphorous doped germanium: Dopant diffusion and modeling
Authors: Cai Yan , Camacho-Aguilera Rodolfo , Bessette Jonathan T. , Kimerling Lionel C. , Michel Jurgen
Published: August 2012

The in situ n-type doping of Ge thin films epitaxial grown on Si substrates is limited to 1 × 1019 cm−3 by the phosphorous out-diffusion during growth at 600 °C. By studying the phosphorous diffusion in Ge with different background doping, we find that the diffusion coefficient is extrinsic and is enhanced 100 times in Ge doped at 1 × 1019 cm−3 compared to intrinsic diffusivity. To achieve higher phosphorous concentration, delta-doped layers are used as a dopant source for phosphorous in-diffusion. We show that the doping level is a result of the competition between in-diffusion and dopant loss. The high diffusivity at high n-type carrier concentration leads to a uniform distribution of phosphorous in Ge with the concentration above 3 × 1019 cm−3.
Enhanced surface losses of organic solar cells induced by efficient polaron pair dissociation at the metal/organic interface
Authors: Yang Wenchao , Li De-Li , Yao Yao , Hou Xiaoyuan , Wu Chang-Qin
Published: August 2012

As a growing importance is placed on developing more efficient organic solar cells, understanding the behavior of free charge carriers at the metal/organic (M/O) interface is critical. One of the current challenges is understanding surface losses, essentially the loss of free charge carriers at the electrode/organic interface. In this paper, we use device model simulations to study such phenomena and we pay particular attention to the role of polaron pair (PP) M/O interfacial dissociation. The origin of surface losses is through the extraction of free charge carriers from the wrong electrodes, or direct surface recombination of PPs. Through simulation, we find that a high injection barrier leads to a large surface loss. In addition, surface loss increases with both the interfacial dissociation rate and PP diffusivity. Efficient interfacial dissociation can significantly enhance surface losses if the PP diffusivity is relatively large. Furthermore, current voltage characteristics reveal that surface losses undermine the device operating parameters and efficiency. Interlayers inserted at the M/O interface could block wrong electrode carriers, suppress the interfacial dissociation and reduce surface losses.
Voltage-induced deformation in dielectric
Authors: Liu Liwu , Liu Yanju , Leng Jinsong
Published: August 2012

When subjected to a high electric field, dielectrics deform. For a hard dielectric, voltage-induced deformation is usually small, and the potential for deformation is determined by its breakdown voltage. If a dielectric is soft, the voltage-induced deformation is significantly large. The potential for deformation is determined by the breakdown voltage together with electromechanical instability and snap-through instability. Based on the theoretical researches conducted by Zhao and Suo [Phys. Rev. Lett. 104, 178302 (2010)] and Li et al. [Int. J. Smart Nano Mater. 2(2), 59-67 (2011)], taking the dielectric elastomer soft material as research object, we introduce three kinds of material limits: strain-stiffening, polarization saturation, and breakdown voltage. The effect of material limits on voltage-induced deformation is analyzed. Also investigated is the influence of pre-stretch and material parameters on the deformation. For a specific soft dielectric material under certain pre-stretch, the theoretical maximum electrical actuation deformation can be determined.
Supercurrent in Nb/InAs-nanowire/Nb Josephson junctions
Authors: Günel H. Y. , Batov I. E. , Hardtdegen H. , Sladek K. , Winden A. , Weis K. , Panaitov G. , Grützmacher D. , Schäpers Th.
Published: August 2012

We report on the fabrication and measurements of planar mesoscopic Josephson junctions formed by InAs nanowires coupled to superconducting Nb terminals. The use of Si-doped InAs-nanowires with different bulk carrier concentrations allowed to tune the properties of the junctions. We have studied the junction characteristics as a function of temperature, gate voltage, and magnetic field. For junctions with high doping concentrations in the nanowire, Josephson supercurrent values up to 100 nA are found. Owing to the use of Nb as superconductor, the Josephson coupling persists at temperatures up to 4 K. In all junctions, the critical current monotonously decreased with the magnetic field, which can be explained by a recently developed theoretical model for the proximity effect in ultra-small Josephson junctions. For the low-doped Josephson junctions, a control of the critical current by varying the gate voltage has been demonstrated. We have studied conductance fluctuations in nanowires coupled to superconducting and normal metal terminals. The conductance fluctuation amplitude is found to be about 6 times larger in superconducting contacted nanowires. The enhancement of the conductance fluctuations is attributed to phase-coherent Andreev reflection as well as to the large number of phase-coherent channels due to the large superconducting gap of the Nb electrodes.
Structure and ferromagnetism in vanadium-doped LiNbO3
Authors: Sheng P. , Zeng F. , Tang G. S. , Pan F. , Yan W. S. , Hu F. C.
Published: August 2012

Doping into LiNbO3 (LN) and studying its magnetism might provide an alternative way for fabricating diluted magnetic compounds with potential application in the field of spintronics. Room-temperature ferromagnetic V-doped LN with V contents of 1–3 at. % was prepared by ion-beam implantation. The samples exhibit a maximum atomic magnetic moment of 3.82 μB/V at a V doping concentration of 2 at. %. Structural characterization and first principle calculation suggest that the magnetism most likely arises from the oxygen vacancy around the V dopant. X-ray absorption near-edge spectroscopy reveals that the V atom principally substituted for the Nb atom in the LN lattice and that the V is octahedrally coordinated but with a large distortion. It also showed that oxygen vacancies are present in the third shell of the doped V atoms. With the aid of first-principle calculations, we constructed the electronic structure of this system and demonstrated that the O vacancies play an important role in modulating the magnetism. These O vacancies can release the magnetic moment of the V dopant and enhance spin coupling over a long range. Two O vacancies are estimated to have an atomic magnetic moment of 4 μB, which is consistent with the results of magnetic measurements.
Statistical mechanical origin of hysteresis in ferroelectrics
Authors: Yang Feng , Hu Guangda , Xu Baixiang , Wu Weibing , Yang Changhong , Wu Haitao
Published: August 2012

Hysteresis in ferroelectric materials is a strong non-linear phenomenon, the origin of which has drawn considerable attention. Based on the hopping probability function of the lattice-center ion crossing the ferroelectric double-well potential, a statistical mechanical theory is proposed to model ferroelectric hysteresis behavior. The hopping probability function is obtained from the statistical distribution function of ions in energy space, which is derived from the mathematical permutation and combination of the occupy-states of lattice-center ion. The calculated hysteresis agrees well with experimental measurements. In particular, the model provides interesting explanations to the polarization current and the coercive field, which differ from traditional microscopic models and thermodynamic theories.
Effect of Ge on dislocation nucleation from surface imperfections in Si-Ge
Authors: Li Z. , Picu R. C. , Muralidhar R. , Oldiges P.
Published: August 2012

Nucleation of dislocation loops from sharp corners playing the role of stress concentrators located on the surface of Si1−xGex strained layers is studied. The surface is of {100} type and the concentrator is oriented such as to increase the applied resolved shear stress in one of the {111} glide planes. The mean stress in the structure is controlled through the boundary conditions, independent of the Ge concentration. Shuffle dislocations are considered throughout, as appropriate for low temperature-high stress conditions. The effect of Ge atoms located in the glide plane, in the vicinity of the glide plane and at larger distances is studied separately. It is observed that Ge located in the glide plane leads to the reduction of the activation energy for dislocation nucleation. The activation volume in presence of Ge is identical to that in pure Si. Ge located in {111} planes three interplanar distances away from the active glide plane has little effect on nucleation parameters. The far-field Ge contributes through the compressive normal stress it produces and leads to a slight reduction of the activation energy for shuffle dislocation nucleation.
Effects of Gd substitution on microstructures and low temperature dielectric relaxation behaviors of SrTiO3 ceramics
Authors: Fang Liang , Dong Wen , Zheng Fengang , Shen Mingrong
Published: August 2012

In this paper, Sr1 − 3x/2GdxTiO3 ceramics (0.00 ≤ x ≤ 0.05) were prepared from powders obtained by a sol–gel method. X-ray diffraction results showed that the Gd ion substituted Sr site, and the unit cell volume of doped samples decreased with increasing Gd doping concentrations, which was due to the difference between Sr/Gd ionic radii. Moreover, the structural change also led to an increase in the antiferrodistortive transition temperature. Dielectric properties of all samples were investigated over broad temperature and frequency ranges, and four dielectric relaxation behaviors were identified in different samples. It was found that two intrinsic dielectric relaxation behaviors for the pure SrTiO3 ceramics could be gradually disappeared with increasing Gd doping concentrations, which was ascribed to the change of domain state induced by Sr vacancies. On the other hand, the other two dielectric relaxation behaviors were induced by Gd doping. The relaxation behavior with activation energy in the range of 100–130 meV was originated from oxygen vacancies, while another relaxation behavior with activation energy in the range of 200–230 meV corresponded to the thermal motions of Ti4+ ions.
Multiferroicity and phase transitions in Tm-substituted GdMnO3
Authors: Li L. , Lin L. , Yan Z. B. , He Q. Y. , Liu J.-M.
Published: August 2012

The multiferroic behaviors of polycrystalline Gd1−xTmxMnO3 are investigated by measuring the structural, magnetic, dielectric, and ferroelectric properties. Remarkable polarization is observed by partial Tm-substitution of Gd ions, and can be proposed to arise from the contribution of both the symmetric exchange striction (S·S term) and asymmetric exchange striction (S × S term) in the ab-plane cycloidal structure at low substitution level. With further substitution, the polarization is gradually reduced, and it is suggested that the ferroelectric phase evolves from the ab-plane cycloidal structure to the bc-plane cycloidal structure where only the antisymmetric exchange striction (S × S term) contributes to the polarization.
Josephson current through a quantum-dot ring: π-junction transition and persistent current magnification
Authors: Yi Guangyu , An Limin , Gong Wei-Jiang , Wu Haina , Wei Guozhu
Published: August 2012

By means of the exact diagonalization approach, the Josephson and persistent currents in a superconductor/quantum-dot ring/superconductor (S/QDR/S) structure are theoretically investigated. The ground state is obtained within zero bandwidth approximation in which the superconductors are replaced by effective local pairing potentials. It is found that Josephson current can flow through this structure in the presence of various electron correlations. Furthermore, in the half-filled case, a novel 0-π transition behavior is observed, which arises from the interplay of interdot antiferromagnetic coupling and electron correlations. When the symmetry of the two arms in the QDR is broken down, the quantum interference efficiently causes the persistent current magnification, even in the case of equilibrium and zero magnetic flux.
Observation of parallel-antiparallel magnetic coupling in ultrathin CoFeB-MgO based structures with perpendicular magnetic anisotropy
Authors: Cheng Chih-Wei , Shiue C. H. , Cheng Tsung-I , Chern G.
Published: August 2012

A series of MgO/CoFeB/Ta(x)/CoFeB/MgO multilayered structures is fabricated by sputtering. Magnetic parallel-antiparallel oscillatory behavior is observed as a function of Ta thickness, while perpendicular magnetic anisotropy (PMA) also exists due to the MgO stabilization. The oscillatory period is ∼1.3 nm with a maximum interlayer exchange coupling (IEC) of ∼0.02 erg/cm2. The Ta spacer can be replaced by a layer of other metals to form a general perpendicular synthetic antiferromagnetic structure. The tuning of IEC and PMA by insertion of Ru is discussed.
Influence of intergranular exchange coupling on the magnetization dynamics of CoCrPt:SiO2 granular media
Authors: Brandt R. , Tibus S. , Springer F. , Fassbender J. , Rohrmann H. , Albrecht M. , Schmidt H.
Published: August 2012

We investigate the effect of Co+ irradiation on the magnetization dynamics of CoCrPt:SiO2 granular media. Increasing irradiation levels reduce the saturation magnetization and effective anisotropy, which decrease the intrinsic magnetization precession frequency. Furthermore, increasing intergranular exchange coupling results in a qualitative change in the behavior of the magnetic material from a collection of individual grains to a homogeneous thin film, as evidenced in both the switching behavior and dynamics. The frequency change cannot be explained by single crystal macrospin modeling, and can only be reproduced by the inclusion of the dipolar effects and anisotropy distribution inherent in a granular medium.
The electronic and optical properties of carbon-doped SrTiO3: Density functional characterization
Authors: Li N. , Yao K. L.
Published: August 2012

The electronic properties and optical activities of C-doped cubic SrTiO3 in perovskite structure are studied by first-principles calculation. The calculated formation energies show that the substitutional C impurity is preferentially occupied at the Ti site. For C@O, the C impurity introduces spin-polarized gap states, and the associated electron excitations among the valence band, the conduction band and the gap states produce various visible-light absorption thresholds. For C@Ti, some C gap states of s-character appear near the bottom of the conduction band, which result in the lowered optical transition energy and thus the visible light absorption as observed in the experiment.
Spurious indications of energetic consequences of decoherence at short times for scattering from open quantum systems
Authors: Mayers J. , Reiter G.
Published: August 2012

Dreismann, Gray and Blach (DGB) have claimed that neutron scattering from molecular hydrogen at energy transfers E sufficiently large to break the H-H bond, gives E ∼3% larger than predicted by conventional quantum theory. DGB presented this claim as the first experimental evidence for energetic consequences of decoherence of quantum entangled particles due to interactions with the environment. It is shown here this claim is entirely spurious. DGB obtained disagreement with conventional theory by changing the geometrical description of the Vesuvio instrument at ISIS, they used to collect their data. Instead of using the default scattering angles obtained from neutron diffraction, DGB used scattering angles obtained using a steel rule and protractor. DGB then manufactured apparent but in fact completely spurious evidence for quantum decoherence effects from the large measurement errors in the scattering angles they used. These give shifts to both higher and lower E than predicted by conventional quantum theory. DGB simply ignored detectors giving shifts to lower values of E. DGB also ignored previously published H2 data that clearly contradict their claim.
Schottky barriers measurements through Arrhenius plots in gas sensors based on semiconductor films
Authors: Schipani F. , Aldao C. M. , Ponce M. A.
Published: August 2012

The oxygen adsorption effects on the Schottky barriers height measurements for thick films gas sensors prepared with undoped nanometric SnO2 particles were studied. From electrical measurements, the characteristics of the intergranular potential barriers developed at intergrains were deduced. It is shown that the determination of effective activation energies from conduction vs. 1/temperature curves is not generally a correct manner to estimate barrier heights. This is due to gas adsorption/desorption during the heating and cooling processes, the assumption of emission over the barrier as the dominant conduction mechanism, and the possible oxygen diffusion into or out of the grains.
Vitrification in human and domestic animal embryology: work in progress
Authors: Vajta Gábor
Published: August 2012

According to the analysis of papers published in major international journals, rapidly increasing application of vitrification is one of the greatest achievements in domestic animal and especially human embryology during the first decade of our century. This review highlights factors supporting or hampering this progress, summarises results achieved with vitrification and outlines future tasks to fully exploit the benefits of this amazing approach that has changed or will change many aspects of laboratory (and also clinical) embryology. Supporting factors include the simplicity, cost efficiency and convincing success of vitrification compared with other approaches in all species and developmental stages in mammalian embryology, while causes that slow down the progress are mostly of human origin: inadequate tools and solutions, superficial teaching, improper application and unjustified concerns resulting in legal restrictions. Elimination of these hindrances seems to be a slower process and more demanding task than meeting the biological challenge. A key element of future progress will be to pass the pioneer age, establish a consensus regarding biosafety requirements, outline the indispensable features of a standard approach and design fully-automated vitrification machines executing all phases of the procedure, including equilibration, cooling, warming and dilution steps.
University students’ perceptions of environmental risks to infertility
Authors: Remes Olivia , Whitten Amanda N. , Sabarre Kelley-Anne , Phillips Karen P.
Published: August 2012

Background Canadian young adults may be at risk of future infertility due to the high incidence of sexually transmissible infections (STIs) in this population. Young adults’ perceptions of environmental risks, including contaminants, STIs and lifestyle habits on infertility, have not been examined. We have therefore designed a qualitative study to explore risk perceptions, awareness and knowledge of common environmental risk factors for infertility in a multiethnic sample of young adults. Methods: Semistructured interviews were carried out with 40 university undergraduate students (16 men and 24 women) in Ottawa, Canada, followed by qualitative analysis of interview transcripts to identify major themes. Results: The following broad themes described participants’ risk perceptions about infertility and (1) environmental contaminants: knowledge gaps, media reports and negative perception of chemicals; (2) STIs: superficial understanding of their role in infertility, general awareness, associations with sexual behaviours and knowledge gaps; and (3) lifestyle: protective benefits of healthy lifestyle, dose or exposure effects for smoking and alcohol, and knowledge gaps. Students demonstrated a superficial understanding of environmental risks, at times relying on media reports and anecdotal information to support their beliefs. Conclusions: This next generation of potential infertility patients exhibits a general understanding of environmental risks to infertility; however, young adults are overly optimistic that healthy lifestyle behaviours will safeguard future fertility. STIs represent the most significant modifiable risk factors for this age group; a message that can be supported by sexual and reproductive health education and promotion with greater emphasis on the long-term outcomes of STIs, including infertility.