bluuebunny/crossref_metadata_2025 huggingface dataset:
A survey of isotropic and anisotropic modifications of the refractive index
Authors: HOFFMANN H. J.
Published: July 2007

The effects causing differential modifications of the refractive index [MATH] are discussed. Isotropic changes are induced by the variation of the temperature and of the annealing schedule. Anisotropic changes can be caused by mechanical stresses and electric or magnetic fields. Simple dispersion formulae have been derived successfully on the basis of a one-term Sellmeier equation. It is shown that the experimental data can be fitted by these formulae. Further possibilities to vary [MATH] are mentioned.
Dynamic structure factor near the glass transition : specific features
Authors: MEZEI F.
Published: July 2007

The dynamic structure factor in the supercooled liquid state just above the glass transition temperature reveals a number of characteristic universal features in the so-called "fragile" class of glassy systems. A theory independent, unbiased analysis shows that, compared to the behaviour expected in the ordinary liquid state, one can observe : (a) a two-step pattern in the time relaxation of the liquid structure factor, the slower of these steps scaling with the bulk viscosity, (b) a relatively small, oscillatory wavenumber dependence of the relaxation dynamics, and (c) approximate master functions of unconventional shapes for both relaxation steps. These qualitative features are in agreement with predictions of recent mode coupling theories and, in addition, the quantitative behaviour is also compatible with the theoretically predicted scaling properties.
Relaxation effects in the liquid-glass transition range
Authors: ETIENNE S.
Published: July 2007

The relaxation effects and the kinetics of the liquid-glass transition are controlled by the molecular mobility with a characteristic time [MATH]. In order to calculate [MATH], we present a molecular theory based on microscopic concepts of i) quasi-point defects and ii) atomic correlated motions in disordered matter. This model is compared to other theories presented in the literature. The same physical parameters are used to describe the dynamic mechanical behavior and the thermal properties. Some applications are presented, and the spatial microfluctuations of cohesion and enthalpy are shown to be a key point in the dynamic response of disordered matter.
Nuclear spin relaxation in glassy ionic conductors
Authors: ELLIOTT S. R.
Published: July 2007

The nuclear spin relaxation behaviour in disordered ionically conducting materials is ascribed to a mechanism based on diffusion-controlled relaxation. In this target-diffusion model, relaxation is assumed to occur at interstitialcy sites, triggered by the arrival of diffusing ions. The dependence of the spin-lattice relaxation rate on the temperature, Larmor frequency and ionic concentration can all be understood with this model. The results of a nuclear spin-lattice relaxation study of the mixed-alkali effect in glasses can, on this basis, be interpreted in terms of changes in ionic mobility.
Thermodynamique des porteurs de charges dans les verres à conduction ionique
Authors: SOUQUET J. L. , COPPO D.
Published: July 2007

Ionic conductivity in glasses is mainly cationic, singly charged cation leading to the highest conductivities. Despite a large variety of chemical compositions (oxide or chalcogenide glasses) and a very large spread for room temperature conductivity (over 6 orders of magnitude) all glasses present important similarities. Below their vitreous transition temperature Tg, ionic conductivity is an activated process [MATH] and isothermal conductivity variations with composition are mainly the result of the activation energy variations ([MATH]) since dl the extrapolated values of conductivity when T tends to infinity ([MATH]) are within only one or two orders of magnitude. This behaviour is explained assuming an ionic displacement due to interstitial pairs in very low concentration. Interstitial pairs formation and migration are activated mechanisms and both contribute to the measured activation energy for ionic conduction. The usual approach developed by physicists for defect formation in ionic crystals explains very simply the similar values obtained for [MATH] but not the [MATH] variations with composition. The dissociation theory, more familiar to chemists, completes this approach and links [MATH] and [MATH] to partial entropy and enthalpy of the alkali salts dissolved in the glassy matrix. From these relationships conductivity variations with composition or quenching rate are interpreted.
Infrared reflectance studies of ionic conductive glasses. Network structure and cation dynamics
Authors: KAMITSOS E. I.
Published: July 2007

The results of our infrared reflectance studies of single and mixed alkali borate glasses are reviewed and discussed in the context of the network structure and the interactions between alkali cations and their sites in the glass. In particular, we focus on the strong influence of the alkali cation type, content, and alkali subsitution on the structure of the network building units. The dominant role of alkali ions also emerges from the analysis of their motion bands active in the far-infrared. Thus, the presence of two different distributions of anionic site environments was found to characterise the single alkali borate glasses. The corresponding analysis of the mixed alkali glass spectra revealed the existence of interactions between the dissimilar alkalis, which strongly affect their bonding characteristics. The new bonding requirements drive the glass network to perform suitable rearrangements, leading eventually to non-linear variations of the network structure with alkali substitution.
Structural studies of ionically conductive glasses by EXAFS
Authors: ROCCA F.
Published: July 2007

A good knowledge of the local structure is necessary for a better understanding of the high conductivity shown by some ions in oxide glasses. The ability of X-Ray Absorption Spectroscopy (XAS) to select a single atomic species makes this technique very useful in this field : XANES and EXAFS can give information not only on radial distribution function of atoms around the absorbing species but also on their geometrical arrangement. The experience of the last years shows that peculiar information can be extracted by studying multiple-scattering contributions and evaluating the influence of static and thermal disorder. The results of XAS studies on AgI-containing silver borate glasses are presented and discussed, with particular emphasis on the role of temperature dependent measurements in EXAFS. The comparison with results concerning other similar systems offers the possibility of discussing strengths and limits of XAS.
Echange ionique entre une couche mince obtenue par procédé sol-gel et un bain de CuCl fondu
Authors: RAADA A. , ZARZYCKI J. , PARENT C. , LE FLEM G.
Published: July 2007

L'échange ionique entre une couche mince de verre obtenue par procédé sol-gel (déposée sur une lame de verre à vitre par la technique de dip-coating), et un bain de CuCl fondu à 525°C et sous atmosphère d'azote a été étudié pour trois séries de verres aluminosilicates SiO2-Al2O3-X2O (X = Na, K, Li). L'expérience montre l'existence des ions Cu + monovalents introduits dans le verre.
Gel to glass transformation in base catalysed silica
Authors: DAHMOUCHE K. , BOVIER C. , BOUKENTER A. , DUMAS J. , DUVAL E. , MAI C. , SERUGHETTI J.
Published: July 2007

The effects of heat-treatments on silica xerogels prepared from base catalyzed hydrolysis and condensation of TMOS are studied by low frequency Raman scattering, scanning electron microscopy, small angle X-ray scattering and BET measurements. Raman scattering in the range 3-700 cm-1 reveals a low-frequency band that arises from the discrete particulate character of base-catalysed gels at very small scale. A continuous evolution of the structure inside particles is observed during the thermal treatment and the Raman spectra is similar to the fused silica one after treatment at 1050°C. Electron microscopy observations of the nanostructure of the xerogels correlate with the particle Raman band maximum. Complementary SAXS and BET measurements allows us to propose a model for the gel-to-glass transformation.
Structural evolution of an organic/inorganic oxide glass studied by differential scanning calorimetry and mechanical spectroscopy
Authors: CORNU J. F. , ETIENNE S. , MAI C. , PEREZ J.
Published: July 2007

Organic/inorganic 3-(trimethoxysilyl)propyl methacrylate/SiO2 based glass was synthesized by sol-gel process. One objective of the present work was to study the evolution of microstructure and mechanical properties during transformation from hybrid alcogel to hybrid glass. Results of measurements about physical properties have been obtained by mechanical tests. Microstructural investigation methods such as DSC and Mechanical Spectroscopy were used to get more informations about molecular network behaviour with thermal treatments. This paper shows that physical properties depend on time because of relaxation effects. These ones have to be measured with a well controlled synthesis chemistry to reach a defined structural state.
From colloidal-silica sols to aerogels and xerogels
Authors: FORET M. , CHOUGRANI A. , VACHER R. , PELOUS J.
Published: July 2007

By destabilization of quasi-monodisperse silica sols, fractal clusters can be obtained. Solvent evacuation gives a porous solid with fractal structure. Structural investigations of these colloidal aerogels by small-angle neutron scattering are presented, and compared to similar observations in concentrated sols and xerogels. An investigation of the vibrational modes of these materials by inelastic neutron scattering gives clear evidence of a strong non-Debye contribution in the low-frequency regime.
Nucleation and crystallisation in xerogel induced and probed by Cr
Authors: NIE W. , BOULON G.
Published: July 2007

Nucleation and crystallization from xerogels of the system SiO2-ZnO-Al2O3 are induced and probed by Cr. Besides the traditional techniques, such as X-ray diffraction, SAXS and TEM, ESR and optical techniques especially laser spectroscopy are employed for the investigation of the structural evolution during thermal treatments of these xerogels. Cr is served not only as a structure probe but also a nucleating agent. The devitrification of the system is found to be essentially initiated by the presence of traces of chromium.
Model of polarization conductivity in ionically conducting glasses
Authors: GIUNTINI J. C. , BUET F. , HENN F. , ZANCHETTA J. V. , VANDERSCHUEREN J.
Published: July 2007

We have used a hopping conduction model to calculate the frequency-dependent polarization conductivity, [MATH] in ionically conducting glasses, and we have assumed that a short-range ionic hopping mechanism occurs based on a fluctuation process. We have underlined the fundamental differences between the frequency-dependent polarization conductivity due to "bound-charge" and the dc conductivity related to "free-charge" diffusion. The law [MATH] is observed as for electronic charge-carriers in amorphous substances. The expression of parameter s is modified by the introduction of a term connected with the fluctuation probability. A plot of the function T/1-s = f(T) shows a striking difference in behaviour (slopes) between ionic glasses (Na2O-3SiO2, (Ag2S)x (GeS21-x) and electronic amorphous solids.
FIC glasses based upon AgI-TlI eutectic
Authors: TANGUY B. , PORTIER J. , ROSSIGNOL S. , REAU J. M. , VIDEAU J. J.
Published: July 2007

New vitreous conductors, shown in the TeO2-Tl2O-AgI system, are described. The ionic conductivity in these glasses increases with the AgI content. Structure consists of a covalent TeO2-Tl2O network and a AgI sublattice with only a weak interaction between them. The AgI-TlI system offers a deep eutectic for the (AgI)0.75(TlI)0.25 composition with a melting point at 220°C. This favourable medium has been selected to prepare new silver FIC glasses. So, glasses belonging to the pseudo quaternary TeO2-Tl2O-AgI-TlI system have been elaborated : the study of transport properties of [TeO2-TlO0.5]1-x [(AgI)0.75 (TlI)0.25]x glasses has shown a very high ionic conductivity for [MATH]. The influence of the Tl+ ions presence on the Ag+ ions mobility is discussed.
Nuclear magnetic resonance and ionic motion in fluorozirconate glasses
Authors: ESTALJI S. , KÜCHLER R. , KANERT O. , BÖLTER R. , JAIN H. , NGAI K. L.
Published: July 2007

We have determined NMR linewidths and nuclear spin relaxation rates for 19F, 7Li, and 23Na nuclei, and electrical conductivity in ZrF4-based fluoride glasses as a function of temperature. The results indicate that both Li and F are mobile in the glass containing 20 mole% LiF, whereas only F is mobile in glasses containing NaF (up to 30 mole%) or no alkali ions. Finally we compare and discuss the activation energies for the various parameters.
Near-equivalence of the role of structural unpinning number, basicity and reciprocal average electronegativity in determining the conductivity of glasses
Authors: SHASTRY M. C. R. , RAO K. J. , LEVASSEUR A. , MENETRIER M.
Published: July 2007

The chemical approach made to investigate the origin of fast ion conduction in Agl-based fast ion conducting (FIC) glasses has been extended to various ionically conducting systems containing Na+ ion. An index known as structural unpinning number (SUN), S, has been defined for this purpose based on the unscreened nuclear charge on the cation and the average electronegativity of all the anions. Variation of the log(conductivity), at a given temperature, as a function of structural unpinning number, optical basicity, [MATH], and the reciprocal average electronegativity of all the anions, [MATH], has been examined for a number of Na+-ion conducting glasses and a nearly identical variation has been noticed in all the cases. The equivalence of these chemical parameters as determinants of the conductivity behavior of glasses has thus been established and the origin of this equivalence has been discussed.
Structural approach of Ge-X, GeX2 -Ag2X (X=S,Se) glassy systems
Authors: ARMAND P. , IBANEZ A. , DEXPERT H. , BITTENCOURT D. , RAOUX D. , PHILIPPOT E.
Published: July 2007

This paper presents a summary of our structural results obtained from X-ray Absorption Spectroscopy, Anomalous Wide Angle X-ray Scattering and Small Angle X-ray scattering for germanium chalcogenide glasses. At first, the local and the medium range order existing in the structure of GeXx ([MATH] and X=S,Se) binary glasses have been determined. Then, the results obtained for (1-y)GeX2 + yAg2X ternary glasses are presented.
Surface evolution in viscoelastic medium: Relaxation and ripening
Authors: Aurongzeb Deeder M.
Published: July 2007

Roughening process in polydimethyl siloxane (PDMS) using atomic force microscopy is being studied using scaling law. As solidified PDMS showed surface undulation that is attributed to Grinfeld type instability. When the surface is annealed at 125 °C, a negative growth exponent is obtained. The growth exponent for annealing at 175 °C case was also found to be consistent with the diffusion driven model. However, the surface treated at this temperature does not exhibit any correlation length. For this particular type of solidification process, rougher surfaces led to larger growth exponent.
Particle-particle, particle-scaling function algorithm for electrostatic problems in free boundary conditions
Authors: Neelov Alexey , Ghasemi S. Alireza , Goedecker Stefan
Published: July 2007

An algorithm for fast calculation of the Coulombic forces and energies of point particles with free boundary conditions is proposed. Its calculation time scales as NlogN for N particles. This novel method has lower crossover point with the full O(N2) direct summation than the fast multipole method. The forces obtained by our algorithm are analytical derivatives of the energy which guarantees energy conservation during a molecular dynamics simulation. Our algorithm is very simple. A version of the code parallelized with the Message Passing Interface can be downloaded under the GNU General Public License from the website of our group.
On the spin-orbit splitting of CuCl2 in its Πg2 ground state
Authors: Hodges Philip J. , Ross Amanda J. , Crozet Patrick , Salami Houssam , Brown John M.
Published: July 2007

The spin-orbit splitting of CuCl2 in its ground XΠg2 state remains an unknown or, at best, poorly defined quantity. The electronic spectrum of CuCl2 has been studied by Fourier transform resolved, laser-induced fluorescence between 602 and 587nm, in an attempt to identify transitions to the upper spin component of the ground state. In order to provide a well-defined excitation process, the sample was cooled to rotational temperatures of about 10K in a free-jet expansion. Observations from just two rotationally perturbed levels in the upper electronic state, one for Cu63Cl235 and the other for Cu65Cl35Cl37, have revealed an additional feature about 482cm−1 above the Π3∕22 state. The effective rotational constants associated with these levels (0.06620cm−1 for Cu63Cl235) are significantly larger than those for the ground Π3∕22 state (0.05813cm−1). Analysis of this feature as the Π1∕22 component leads to a value of −482.9cm−1 for the spin-orbit coupling constant A and of −0.0846cm−1 for the lambda-doubling parameter (p+2q) for Cu63Cl235. Several other previously unobserved levels are also identified within 2000cm−1 of the ground state. Many of these also have anomalously large rotational constants.
Potential energy surfaces for small alcohol dimers. II. Propanol, isopropanol, t-butanol, and sec-butanol
Authors: Rowley Richard L. , Tracy Christopher M. , Pakkanen Tapani A.
Published: July 2007

Potential energy landscapes for homogeneous dimers of propanol, isopropanol, tert-butanol, and sec-butanol were obtained using 735 counterpoise-corrected energies at the MP2∕6-311+G(2df,2pd) level. The landscapes were sampled at 15 dimer separation distances for different relative monomer geometries, or routes, given in terms of the yaw, pitch, and roll of one monomer relative to the other and the spherical angles between the two monomer centers (taken as the C atom attached to the O). The resultant individual energy surfaces and their complex topographies were also regressed using a site-site pair potential model using a modified Morse potential that provides a mathematically simple representation of the landscapes suitable for use in molecular simulations. Generalized Morse parameters were also obtained for this model from a composite regression of these energy landscapes and those previously reported for methanol and ethanol. The quality of fit for all these energy landscapes suggests that these site parameters have transferability for possible use on other alcohols.
New spectroscopic data, spin-orbit functions, and global analysis of data on the AΣu+1 and bΠu3 states of Na2
Authors: Qi P. , Bai J. , Ahmed E. , Lyyra A. M. , Kotochigova S. , Ross A. J. , Effantin C. , Zalicki P. , Vigué J. , Chawla G. , Field R. W. , Whang T.-J. , Stwalley W. C. , Knöckel H. , Tiemann E. , Shang J. , Li L. , Bergeman T.
Published: July 2007

The lowest electronically excited states of Na2 are of interest as intermediaries in the excitation of higher states and in the development of methods for producing cold molecules. We have compiled previously obtained spectroscopic data on the AΣu+1 and bΠu3 states of Na2 from about 20 sources, both published and unpublished, together with new sub-Doppler linewidth measurements of about 15 000 A←X transitions using polarization spectroscopy. We also present new ab initio results for the diagonal and off-diagonal spin-orbit functions. The discrete variable representation is used in conjunction with Hund’s case a potentials plus spin-orbit effects to model data extending from v=0 to very close to the 3S2+3P1∕22 limit. Empirical estimates of the spin-orbit functions agree well with the ab initio functions for the accessible values of R. The potential function for the A state includes an exchange potential for S+P atoms, with a fitted coefficient somewhat larger than the predicted value. Observed and calculated term values are presented in an auxiliary (EPAPS) file as a database for future studies on Na2.
Bubbling in a co-flow at high Reynolds numbers
Authors: Gordillo J. M. , Sevilla A. , Martínez-Bazán C.
Published: July 2007

The physical mechanisms underlying bubble formation from a needle in a co-flowing liquid environment at high Reynolds numbers are studied in detail with the aid of experiments and boundary-integral numerical simulations. To determine the effect of gas inertia the experiments were carried out with air and helium. The influence of the injection system is elucidated by performing experiments using two different facilities, one where the constancy of the gas flow-rate entering the bubble is ensured, and another one where the gas is injected through a needle directly connected to a pressurized chamber. In the case of constant flow-rate injection conditions, the bubbling frequency has been shown to hardly depend on the gas density, with a bubble size given by db∕ro≃[6U(k*U+k2)∕(U−1)]1∕3 for U≳2, where U is the gas-to-liquid ratio of the mean velocities, ro is the radius of the gas injection needle, and k*=5.84 and k2=4.29, with db∕ro∼3.3U1∕3 for U⪢1. Nevertheless, in this case the effect of gas density is relevant to describe the final instants of bubble breakup, which take place at a time scale much smaller than the bubbling time, tb. This effect is evidenced by the liquid jets penetrating the gas bubbles upon their pinch-off. Our measurements indicate that the velocity of the penetrating jets is considerably larger in air bubbles than in helium bubbles due to the distinct gas inertia of both situations. However, in the case of constant pressure supply conditions, the bubble size strongly depends on the density of the gas through the pressure loss along the gas injection needle. Furthermore, under the operating conditions reported here, the equivalent diameters of the bubbles are between 10% and 20% larger than their constant flow-rate counterparts. In addition, the experiments and the numerical results show that, under constant pressure supply, helium bubbles are approximately 10% larger than air bubbles due to the gas density effect on the bubbling process.
Effect of nanoparticles in nanofluid on thermal performance in a miniature thermosyphon
Authors: Liu Zhen Hua , Yang Xue Fei , Guo Guang Liang
Published: July 2007

An experiment was performed to investigate the effect of nanoparticles in the nanofluid on the thermal performance in a miniature thermosyphon. The nanofluids consisted of de-ionized water and CuO nanoparticles having an average size of 30nm. The experimental results show that the water-CuO nanofluids can greatly enhance the boiling heat transfer performance of the evaporator in thermosyphon compared with that using water at subatmospheric pressure conditions. A much lower and more uniform wall temperature of the thermosyphon can be obtained by substituting the nanofluids for water. Boiling heat transfer coefficients and the critical heat flux (CHF) of the nanofluids in the evaporator of the thermosyphon have significant increase compared with those of de-ionized water. There was an optimal mass concentration which was estimated to be 1.0wt% to achieve the maximum heat transfer performance. Operating pressure has very remarkable influences on both the heat transfer coefficients and the CHF of nanofluids, which greatly increase with the decrease of the test pressure. The heat transfer coefficient and the CHF can increase, respectively, about 160% and 120% at the pressure of 7.45kPa compared with those of water. The experimental study confirmed that the heat transfer performance of the miniature thermosyphon can evidently be strengthened by using water-CuO nanofluids.
Propagation of terahertz pulses in photoexcited media: Analytical theory for layered systems
Authors: Kužel P. , Kadlec F. , Němec H.
Published: July 2007

Optical pump–terahertz probe spectroscopy has become a widely used experimental tool for the investigation of the ultrafast far-infrared response of polar systems. In this paper the authors present an analytical method of calculating the propagation of ultrashort terahertz pulses in photoexcited media. The transient terahertz wave form transmitted through the sample is equal to a product of the incident terahertz field (at a mixed frequency), transient susceptibility, and a so called transfer function which depends on the properties of the sample in equilibrium. The form of the transfer function is derived for general layered systems and for specific cases including one-dimensional photonic crystals, thin films, and bulk samples. Simplified expressions directly applicable to the analysis of the experimental results related to the most common sample geometries are shown and discussed.
Hydronium ion motion in nanometer 3-methyl-pentane films
Authors: Bell Richard C. , Wu Kai , Iedema Martin J. , Cowin James P.
Published: July 2007

An ion soft-landing approach was applied to study the motion of hydronium (D3O+) and cesium (Cs+) ions from 84to104K in glassy 3-methyl-pentane (3MP) films vapor deposited on Pt(111). Both ions were found to have very similar mobilities in 3MP. The span of ion mobilities probed is from ∼10−18to∼10−13m2V−1s−1. Ion transport in these films was studied as a function of film thickness and electric field strength. The drift velocity was found to be linear with applied field below about 2×108V∕m and deviated from linearity above this. To a large extent, D3O+ and Cs+ motion in 3MP was well predicted by a simple continuum-based ion mobility model in films from 25 to 20 000 ML thick (including pronounced perturbations 7 ML from both the vacuum and Pt interfaces). The mobility varied with temperature more slowly than predicted by Stokes’ law, which may be due to extended inhomogeneous structures in the 3MP near its glass transition at 77K.
Effect of symmetric and asymmetric In0.2Ga0.8As wells on the structural and optical properties of InAs quantum dots grown by migration enhanced molecular beam epitaxy for the application to a 1.3 μm laser diode
Authors: Ryu S. P. , Lee Y. T. , Cho N. K. , Choi W. J. , Song J. D. , Lee J. I. , Kwack H. S. , Cho Y. H.
Published: July 2007

In this article, we present an in-depth study of the effects of the structural and optical properties of InAs “dots in an In0.2Ga0.8As well” (DWELL) and InAs “dots in an asymmetric In0.2Ga0.8As well” (asym. DWELL) grown by migration-enhanced molecular beam epitaxy. The energy spacing (ΔE1) between the ground-state and the first-excited-state transitions increases from 66 meV for the DWELL to 73 meV for the asym. DWELL. These results are consistent with ΔE1 measured by photoluminescence excitation and the values of activation energy fitting. The photoluminescence linewidth of the asym. DWELL (40 meV) is narrower than that of the DWELL (70 meV), which shows superior uniformity in the former over the latter. The trends of the properties of the DWELL and the asym. DWELL deduced from the structural properties are in good agreement with those from the optical properties. From the results, it is strongly supported that the asym. DWELL is more suitable for application to long wavelength optical communication than the DWELL counterpart.
Corridor of existence of thermodynamically consistent solution of the Ornstein-Zernike equation
Authors: Vorob’ev V. S. , Martynov G. A.
Published: July 2007

We obtain the exact equation for a correction to the Ornstein-Zernike (OZ) equation based on the assumption of the uniqueness of thermodynamical functions. We show that this equation is reduced to a differential equation with one arbitrary parameter for the hard sphere model. The compressibility factor within narrow limits of this parameter variation can either coincide with one of the formulas obtained on the basis of analytical solutions of the OZ equation or assume all intermediate values lying in a corridor between these solutions. In particular, we find the value of this parameter when the thermodynamically consistent compressibility factor corresponds to the Carnahan-Stirling formula.
Structural, magnetic, and transport properties of Zr-substituted La0.7Sr0.3MnO3
Authors: Kim M. S. , Yang J. B. , Cai Q. , James W. J. , Yelon W. B. , Parris P. E. , Malik S. K.
Published: July 2007

Zr-substituted perovskites La0.7Sr0.3Mn1−xZrxO3 with 0⩽x⩽0.20 were investigated by neutron diffraction (ND), magnetization, electric resistivity, and magnetoresistance measurements. ND refinements reveal that substituted Zr4+ goes only to the Mn site. Because of its large size, this leads to a Zr-solubility limit at x⩽0.10. The x⩽0.10 samples exhibit a rhombohedral structure (space group R3¯c) from 10K to room temperature. For the x⩽0.10 samples, the cell parameters a and c, and volume increase continuously with increasing Zr content. In addition, the structural distortion of the MnO6 octahedra increases with increasing Zr content. Zr-substituted La0.7Sr0.3MnO3 exhibits metallic behavior at low temperature. The field dependent resistivity suggests that electron-electron scattering is dominant and a two-magnon scattering process emerges with increasing temperature. The contribution from the two-magnon scattering in resistivity becomes larger with increasing Zr content. A maximum magnetoresistance (MRmax) of 35% is obtained for the x=0.03 sample at H=5T. The MRmax shifts to a higher temperature region upon application of an external magnetic field.
Large scale SiC∕SiOx nanocables: Synthesis, photoluminescence, and field emission properties
Authors: Wang X. J. , Tian J. F. , Bao L. H. , Hui C. , Yang T. Z. , Shen C. M. , Gao H.-J. , Liu F. , Xu N. S.
Published: July 2007

Large scale, high yield SiC∕SiOx nanocables have been synthesized by thermal evaporation of carbon powders and silicon powders in the presence of Fe3O4 nanoparticle catalysts. Transmission electron microscopy and high-resolution transmission electron microscopy show that the nanocables consist of a 50–300nm single-crystalline β-SiC core wrapped with a 10–20nm amorphous SiOx shell. The nanocables have two broad photoluminescence peaks located around 390 and 460nm when the 250nm ultraviolet fluorescent light excitation is applied at room temperature. The results of field emission measurement of SiC∕SiOx nanocables indicate the low turn-on and threshold electric fields of 3.2 and 5.3V∕μm at the vacuum gap of 200μm, respectively. When the vacuum gap was increased to 1000μm, the turn-on and threshold electric fields were decreased to 1.1 and 2.3V∕μm, respectively. The SiC∕SiOx nanocables with good photoluminescence and field emission properties are promising candidates for ultraviolet-blue emitting devices, flat panel displays, and semiconductor field emitters.
On the exchange-hole model of London dispersion forces
Authors: Ángyán János G.
Published: July 2007

First-principles derivation is given for the heuristic exchange-hole model of London dispersion forces by Becke and Johnson [J. Chem. Phys. 122, 154104 (2005)]. A one-term approximation is used for the dynamic charge density response function, and it is shown that a central nonempirical ingredient of the approximate nonexpanded dispersion energy is the charge density autocorrelation function, a two-particle property, related to the exchange-correlation hole. In the framework of a dipolar approximation of the Coulomb interaction around the molecular origin, one obtains the so-called Salem-Tang-Karplus approximation to the C6 dispersion coefficient. Alternatively, by expanding the Coulomb interaction around the center of charge (centroid) of the exchange-correlation hole associated with each point in the molecular volume, a multicenter expansion is obtained around the centroids of electron localization domains, always in terms of the exchange-correlation hole. In order to get a formula analogous to that of Becke and Johnson, which involves the exchange-hole only, further assumptions are needed, related to the difficulties of obtaining the expectation value of a two-electron operator from a single determinant. Thus a connection could be established between the conventional fluctuating charge density model of London dispersion forces and the notion of the “exchange-hole dipole moment” shedding some light on the true nature of the approximations implicit in the Becke-Johnson model.
On the breakup of fluid films of finite and infinite extent
Authors: Diez Javier A. , Kondic Lou
Published: July 2007

We study the dewetting process of thin fluid films that partially wet a solid surface. Using a long-wave (lubrication) approximation, we formulate a nonlinear partial differential equation governing the evolution of the film thickness, h. This equation includes the effects of capillarity, gravity, and an additional conjoining/disjoining pressure term to account for intermolecular forces. We perform standard linear stability analysis of an infinite flat film, and identify the corresponding stable, unstable, and metastable regions. Within this framework, we analyze the evolution of a semi-infinite film of length L in one direction. The numerical simulations show that for long and thin films, the dewetting fronts of the film generate a pearling process involving successive formation of ridges at the film ends and consecutive pinch-off behind these ridges. On the other hand, for shorter and thicker films, the evolution ends up by forming a single drop. The time evolution as well as the final drops pattern show a competition between the dewetting mechanisms caused by nucleation and by free surface instability. We find that precise computations, requiring quadrupole precision of computer arithmetic, are often needed to avoid spurious results.
Scaling of pressure drop for oscillatory flow through a slot
Authors: Raju Reni , Gallas Quentin , Mittal Rajat , Cattafesta Louis
Published: July 2007

Reduced-order modeling and design of zero-net mass-flux actuators require an understanding of several fundamental flow mechanisms governing the performance of these devices. One such aspect is the pressure drop due to oscillatory flow through a slot. In this Brief Communication, we use numerical simulations and experiments to examine physics-based scaling laws for the various mechanisms that determine the net pressure drop, and critically evaluate a proposed phenomenological model for predicting the nonlinear component of the pressure drop in oscillatory slot flows.
n
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-doped-layer-free microcrystalline silicon thin film transistors fabricated with the CuMg as source/drain metal
Authors: Wang M. C. , Chang T. C. , Liu Po-Tsun , Xiao R. W. , Lin L. F. , Li Y. Y. , Yeh F. S. , Chen J. R.
Published: July 2007

The feasibility of using CuMg as source/drain metal electrodes for n+-doped-layer-free microcrystalline silicon thin film transistors (μ-Si:H TFTs) has been investigated. The Ohmic-contact characteristic has been achieved by using the CuMg alloy as source/drain metal. Furthermore, a wet etching process of Cu alloy source/drain metal has been completed by using the ferric chloride base etchant. The proposed μ-Si:H TFT has shown similar electrical characteristic with the μ-Si:H TFT with n+-doped layer. The experimental result also showed that the CuMg alloy was suitable for the replacement of n+-doped layer in thin film transistor liquid-crystal displays.
Dimension results of multifractional Brownian sheets
Authors: Wu Dongsheng
Published: July 2007

Let {B̂H(t)(t)}={B̂H(t)(t),t∊R+N} be an (N,d)-multifractional Brownian sheet with N Hurst functionals Hℓ(tℓ)∊CKℓ(R+,(0,1)) for some Kℓ∊(0,1)(ℓ=1,…,N) defined by a harmonizable representation. Under some regularity conditions on Hℓ(tℓ)’s, we establish the result on the modulus of continuity of the sample paths and determine the Hausdorff and packing dimensions of the range B̂H(t)([a,b]) and the graph GrB̂H(t)([a,b]) for [a,b]⊂[ε,T]N, where 0<ε<T<∞, for the multifractional Brownian sheet.
Dispersion of silicon nonlinearities in the near infrared region
Authors: Lin Q. , Zhang J. , Piredda G. , Boyd R. W. , Fauchet P. M. , Agrawal G. P.
Published: July 2007

The authors present the detailed characterization of the wavelength dependence of two-photon absorption and the Kerr nonlinearity in silicon over a spectral range extending from 1.2to2.4μm. They show that silicon exhibits a significant increase in its nonlinear figure of merit with increasing wavelengths beyond the two telecommunication bands. They expect their results to provide guidance for extending nonlinear silicon photonics into new spectral regimes.
Extremely high oxygen sensing of individual ZnSnO3 nanowires arising from grain boundary barrier modulation
Authors: Xue X. Y. , Feng P. , Wang Y. G. , Wang T. H.
Published: July 2007

Extremely high oxygen sensing is realized from individual ZnSnO3 nanowires with abundant grain boundaries. The current across one single ZnSnO3 nanowire increases by about six orders of magnitude, from 1.20×10−7to3.78×10−1μA, as the oxygen pressure decreases from 3.7×104to1.0×10−4Pa. Such a drastic sensing is ascribed to grain boundary barrier modulation. This interpretation is confirmed by the sensing experiments under UV illumination. The results demonstrate a promising approach to realize miniaturized and highly sensitive oxygen sensors.
X-ray diffraction study of nanocrystalline tungsten nitride and tungsten to 31 GPa
Authors: Ma Yanzhang , Cui Qiliang , Shen Longhai , He Zhaoming
Published: July 2007

X-ray diffraction measurement determined that a newly synthesized nanocrystalline tungsten nitride (W2N) has a substantially larger cell parameter than its bulk material. Yet the lattice of a metal nanocrystalline tungsten (W) remained unchanged. High-pressure diffraction study to 31 GPa resolved a much lower bulk modulus of 240 GPa for nanocrystalline W2N and a relatively unchanged bulk modulus of 307 GPa for nanocrystalline W compared to its bulk material. We found that the metallic bonding of a metal is not affected by reduction of the grain size. The enlarged cell parameter and the relatively low bulk modulus of W2N reflect the size effect of nanocrystalline W2N.
Anisotropic magnetic properties in [110] oriented epitaxial La0.7Sr0.3MnO3 films on (0001) sapphire
Authors: Bhosle V. , Prater J. T. , Narayan J.
Published: July 2007

We have grown virtually strain-free [110] oriented epitaxial La0.7Sr0.3MnO3 films on (0001) sapphire and investigated orientation dependent magnetic properties to study magnetic anisotropy in detail. Measurements of angular dependence of magnetization in the plane of LSMO films reveal that the easy axis lies along [002], whereas [1¯10] acts as the hard axis. We also show that there is no out-of-plane anisotropy, when the magnetization is measured along the equivalent directions, which suggests uniaxial magnetic anisotropy. The observation of uniaxial magnetic anisotropy in the LSMO films deposited on sapphire is attributed to magnetocrystalline anisotropy and is substantiated with detailed transmission electron microscopy analysis and theoretical correlations.
Density functional theory study of CHx (x=1–3) adsorption on clean and CO precovered Rh(111) surfaces
Authors: Yang Ming-Mei , Bao Xin-He , Li Wei-Xue
Published: July 2007

C H x  (x=1–3) adsorptions on clean and CO precovered Rh(111) surfaces were studied by density functional theory calculations. It is found that CHx (x=1–3) radicals prefer threefold hollow sites on Rh(111) surfaces, and the bond strength between CHx and Rh(111) follows the order of CH3<CH2<CH. A slight attraction between adsorbed CHx and CHx, CHx and CO, CO and CO radicals/molecules at coverage of 1∕9–1∕4 ML is found, and considerable repulsion is built up at coverage higher than 1∕4 ML. It is found that CHx adsorption results in the reduction of work function due to charge transfer from the adsorbates to the substrate. For CH3 adsorption, charge accumulation between the substrate and carbon atom is identified, which results in C–H stretch mode softening. For CO precovered surfaces, the mode softening is prevented due to the weakened interaction between CHx and substrates from repulsive interaction between adsorbates. Conversely, the overall charge transfer from the CHx (x=1–3) to the substrate enhances the charge back donation from the substrate to the empty antibonding states of adsorbed carbon monoxide, which results in the softness of the C–O stretch, respectively. The C 1s surface core-level shifts for CHx with and without the presence of CO were calculated, and a negative shift with respect to the carbon in atop CO on Rh(111) is found, and the negative shift follows the order of CO<CH3<CH2<CH. The results are analyzed in details by difference of charge density and projected density of states.
Phase space representations and perturbation theory for continuous-time histories
Authors: Burch Aidan
Published: July 2007

We consider two technical developments of the formalism of continuous-time histories. First, we provide an explicit description of histories of the simple harmonic oscillator on the classical history phase space, comparing and contrasting the Q, P, and Wigner representations; we conclude that a representation based on coherent states is the most appropriate. Second, we demonstrate a generic method for implementing a perturbative approach for interacting theories in the history formalism, using the quartic anharmonic oscillator. We make use of the identification of the closed-time-path generating functional with the decoherence functional to develop a perturbative expansion for the latter up to second order in the coupling constant. We consider both configuration space and phase space histories.
Energy criterion of global existence for supercritical nonlinear Schrödinger equation with harmonic potential
Authors: Chen Guanggan , Zhang Jian , Wei Yunyun
Published: July 2007

This paper is concerned with the supercritical nonlinear Schrödinger equation with a harmonic potential which describes the attractive Bose-Einstein condensate under a magnetic trap. We establish two types of new invariant evolution flows. Then, in terms of the Hamiltonian invariants, we derive a new sharp energy criterion for global existence and blowing up of solutions of the equation, which can be precisely computed in the Bose-Einstein condensate.
Electrical and magnetic properties of chemically derived nanocrystalline cobalt ferrite
Authors: Sivakumar N. , Narayanasamy A. , Shinoda K. , Chinnasamy C. N. , Jeyadevan B. , Greneche J.-M.
Published: July 2007

Nanocrystalline cobalt ferrite particles of 8nm grain size were synthesized by coprecipitation technique and subsequently suitably heat treated to obtain higher grain sizes. The experimentally observed changes in the dc electrical conductivity and Curie temperature with heat treatment have been attributed to the changes in the cation distributions as obtained from the Mössbauer and extended x-ray absorption fine structure (EXAFS) measurements and to the grain size. The activation energies for conduction as determined from the Arrhenius plots suggest that the conductivity is due to hopping of both electrons and holes. The observed decrease in conductivity when the grain size is increased from 8to92nm is clearly due to the predominant effect of migration of some of the Fe3+ ions from octahedral to tetrahedral sites, as is evident from in-field Mössbauer and EXAFS measurements. But the higher conductivity of the 102 and 123nm particles compared to that of the 92nm particles is attributed to the higher grain size, since the cation distribution is found to be the same for all these three samples. The Néel temperature increases from 709K for the as-prepared particles (8nm)to809K for the 92nm particles because of the change in the cation distribution and it remains almost the same for the higher grain sizes as there is no further change in the cation distribution.
Pinning effect and thermal stability study in L1 FePt-pinned spin valves
Authors: Zhao Hui , Zhang Zongzhi , Ma Bin , Jin Q. Y.
Published: July 2007

L 1 0 -ordered FePt alloy is introduced into the synthetic antiferromagnetically coupled spin valves as an exchange pinning layer. Large switching field of 1824Oe for the reference layer and a high giant magnetoresistance (GMR) ratio of 7.0% are observed in such FePt-pinned spin valves, which are comparable to those in MnIr-pinned samples with similar structures. However, the GMR ratio in FePt-pinned spin valve exhibits a poor thermal stability. The rough surface of the FePt layer causes the atoms to interdiffuse easily at lower temperature, resulting in some FePt formation in the CoFe reference layer. This magnetic isotropic FePt alloy with (111) texture could degrade the uniaxial anisotropy of the reference layer, leading to the reduction of GMR ratio.
Shear mode electromechanical coupling coefficient k15 and crystallites alignment of (112¯) textured ZnO films
Authors: Yanagitani Takahiko , Kiuchi Masato , Matsukawa Mami , Watanabe Yoshiaki
Published: July 2007

ZnO film, in which the crystallite c axis lies in the substrate plane [(112¯0) textured ZnO], is a good candidate for application in shear mode piezoelectric devices. The relationships between the degree of crystallites alignment and the shear mode electromechanical coupling coefficient k15 in (112¯0) textured ZnO films have been investigated. Forty pure-shear mode high overtone bulk acoustic resonators consisting of the (112¯0) textured ZnO film were prepared. The film was varied in crystallites alignment and film thickness. The degrees of crystallites alignment of the films were determined by x-ray pole figure analysis. The k15 values of the films were estimated from the conversion loss characteristics of the resonators. A significant correlation was observed between dispersion of the x-ray poles and the k15 values. However, the k15 values in the thinner films were clearly reduced as compared with those in the thicker films despite their similarities in crystallites alignment. In addition, unexpected second harmonic mode resonance was detected in the thinner films. We concluded that the piezoelectrically inactive layer in the initial film growth deteriorated the k15 value in thinner films.
Internal orientations in externally deformed nematic polymer liquid crystals
Authors: Walasek Janusz
Published: July 2007

The system of linear polymer liquid crystal (PLC) macromolecules, each modeled by semiflexible chain of alternate connected flexible and stiff rodlike sequences, is externally deformed. As a result, two orientation phases for hard rods are generated. One of them is nematic N+ with orientation parameter s>0; this has the place in monomer liquid crystal (MLC) systems and in PLCs. The nematic N− phase with s>0 appears in deformed PLCs only. This causes the fact that orientation of PLC chains, as a whole, is generated also by the system deformation. A discussion of that effect is the main goal of this article. The change of orientation is also discussed dependent on changes of the system temperature and parameters of the chain structure, such as the liquid crystal component concentration, contour lengths of stiff and flexible parts, and internal interactions parameters. Average shape of PLC chain and the shape anisotropy are calculated and discussed.
Surface enhanced Raman with anodized aluminum oxide films
Authors: Zhang C. , Abdijalilov K. , Grebel H.
Published: July 2007

Aluminum is not a platform of choice for surface enhanced Raman scattering (SERS) experiments despite its large negative permittivity value (larger than gold or silver at optical wavelengths). It is also widely believed that an oxide layer on top of any platform substantially impedes SERS signals. Yet, anodized aluminum oxide may be perforated in an organized fashion and we have used it to examine SERS of single walled carbon nanotubes (SWCNTs) at micron length and fullerene (C60) at the nanoscale. The signal-to-noise ratio of the corresponding Raman signals exhibited a large signal enhancement for SWCNTs but not for C60. We attributed the SERS to the formation of standing surface charge waves in this subwavelength environment.