bluuebunny/crossref_metadata_2025_split huggingface dataset:
Does ion Bombardment Induce a Degradation of the Electronic Properties of a-Si:H Films?
Authors: Roca i Cabarrocas P. , Morin P. , Conde J. , Chu V. , Liu J.Z. , Park H.R. , Wagner S. 
Published: March 2011

ABSTRACTWe present a detailed study of the effects of a negative bias applied to the substrate on the electronic properties of a-Si:H films deposited by r.f. glow discharge. Two series of samples deposited at 30 and 100 mTorr respectively have been studied. For each series the negative d.c. bias applied to the substrate was decreased from 0 to −100 V in steps of 25 V. We observe for both series of samples an improvement of the electronic properties of the films as we decrease the substrate bias (increase the ion energy) down to − 50 V. We have found a clear correlation between the negative bias applied to the substrate and the subgap absorption, the valence band tail slope and the electron and hole μτ products.
Saturation behavior of the Light-Induced Defect Density in Hydrogenated Amorphous Silicon
Authors: Park H. R. , Liu J. Z. , i Cabarrocas P. Roca , Maruyama A. , Isomura M. , Wagner S. , Abelson J. R. , Finger F. 
Published: March 2011

ABSTRACTUsing a Kr ion laser (λ = 647.1 nm) to produce a carrier generation rate G of 3 × 1020 cm−3s−1, we have saturated the light-induced defect generation in hydrogenated (and fluorinated) amorphous silicon (a-Si:H(F)), within a few hours near room temperature. While the defect generation rate scales roughly with 1/G2, the saturation defect densities Ns,sat are essentially independent of G. The saturation is not due to thermal annealing. We have further measured Ns,sat m 37 a-Si:H(F) films grown in six different reactors under different conditions. The results show that Ns,sat lies between 5 × 1016 and 2 × 1017 cm−3, that Ns,sat drops with decreasing optical gap and hydrogen content, and that Ns,sat is not correlated with the initial defect density or with the Urbach energy.
Study of Temperature and Light Bias Effects on the Occupied Near Surface Defect Density in Hydrogenated Amorphous Silicon
Authors: Aljishi Samer , David Cohen J. , Ley Lothar 
Published: March 2011

ABSTRACTThe occupied density of states distributions in doped and undoped a-Si:H are investigated. Results reveal the presence of a distinct defect sub-band at 0.6 eV above the valence band edge Ey in n-type and in undoped films. In p-type films, this band is absent. Instead, a new band centered at 0.8 eV above Ev develops. This band is normally unoccupied and is therefore only seen under non-equilibrium conditions. A sub-band at this energy is also discovered in undoped a-Si:H films. The effects of temperature and light bias on these defect sub-bands are discussed.
Light-Induced Metastable Effects in a-Ge:H
Authors: Santos P. V. , de O. Graeff C. F. , Marcano G. , Chambouleyron I. 
Published: March 2011

ABSTRACTWe report on light induced metastable conductivity changes in a-Ge:H thin films deposited by rf-sputtering. The dark- and the photoconductivity of the samples decrease after light soaking. The changes are metastable and both the defect formation and the defect annealing processes are temperature activated. As in the case of a-Si:H films, the time evolution of the conductivity changes is well represented by a stretched exponential type of decay, with activated time constants.
Static Dipole Polarizabilities of C, N, O and F: The Importance of Spin Projection
Authors: Sosa C. , Ferris K. F. 
Published: March 2011

ABSTRACTThe static dipole polarizability for the ground state of C(3Π ), N(4∑+), O(3Π ) and F(2∑+ ) have been calculated using ab initio molecular orbital techniques. The polarizabilities are obtained from unrestricted Hartree-Fock and Many-Body Perturbation Theory wave functions with and without spin projection, which are computed by finite-field differentiation. The effect of spin contamination is shown to be important on the calculation of static dipole polarizabilities for the ground state of C, N, O and F.
Electronic Structure Calculations of Σ5 Grain Boundaries in Cu
Authors: Sowa Erik C. , Gonis A. , Zhang X.-G. 
Published: March 2011

Extended AbstractIn a recent publication [1], we presented results of electronic-structure calculations on Σ5 grain boundaries in Cu. These calculations were performed using the real-space multiplescattering theory (RSMST), a first-principles method which does not rely on translational invariance and Bloch's theorem [2, 3], and hence avoids the problems associated with treating interfaces in a periodic repeating slab geometry. A brief summary of this method follows.The RSMST is based on the concept of semi-infinite periodicity (SIP), defined as the regular repetition along a given direction of a scattering unit (atom, planes of atoms, etc.), or a set of such units. Systems with SIP possess the property of removal invariance, which states that the scattering properties (scattering matrices) of any such system remain invariant when an integral number of scattering units is removed from, or added to, the free end of the system. Using this property in conjunction with multiple-scattering theory, one can determine the electronic Green function, and hence all one-particle quantities such as the density of states (DOS), directly in real space.The essence of the method consists in a prescription for the proper renormalization of the scattering properties of the boundary sites of a cluster of atoms. Unrenormalized or “bare” sites in the interior of the cluster describe the region of interest, such as a grain boundary, while the renormalized boundary sites represent the infinite medium surrounding the cluster. This “dressing up” of the cluster is done independently for each part of a system that is characterized by its own SIP, so that grain boundaries between essentially arbitrary crystal structures can be treated.The calculations described previously [1] used given atomistic configurations and electronic one-particle potentials and computed the local DOS at atoms of interest within the grain boundaries. We are currently extending our method to incorporate charge self-consistency and total-energy capabilities. In connection with this, we have implemented four different methods of expressing mathematically the invariance properties of semi-infinite systems, and are evaluating them for optimal rates of convergence and computational efficiency. Calculations on Cu (100) monolayers will be used to illustrate these methods.
Interatomic Forces and Bonding Mechanisms in MgO Clusters
Authors: Wright Nancy F. , Painter Gayle S. 
Published: March 2011

ABSTRACTWe report results from a first-principles local spin density quantum mechanical study of the energetics and elastic properties of a series of magnesium-oxygen clusters of various morphologies. The role of quantum effects, e.g. covalency, in the bonding character of diatomic MgO is determined by comparison of classical and quantum restoring force curves. The dependence of binding properties on geometry and metal to oxygen ratio is determined by comparison of binding energy curves for a series of clusters. Results show that while gross features of the binding curves may be represented by simple interatomic potentials, details require the many body corrections of a full quantum treatment.
Molecular Dynamics Studies of Surface Indentation in Two Dimensions
Authors: Belak J. , Stowers I. F. 
Published: March 2011

ABSTRACTNon-equilibrium molecular dynamics is used to study the indentation of an ideal metallic surface by a hard spherical ball. The problem is symmetric about the axis of the ball and we choose to study the deformation in two dimensions. We use the simple embedding function F(ρ) = 2eρln ρ along with the Lennard-Jones two body potential to model a material resembling copper. We present a calculation of the st'ress field (from the internal atomistic description) for both elastic and plastic deformations. The material deforms plastically through the creation of edge dislocations and the flow of material around the sides of the indenter. The creation of dislocations is reversible—they return to the surface (anneal out) when the indenter is removed. The plastic flow leads to a permanent indentation in the surface.
Multiple Scattering Theory for Space Filling Potentials
Authors: Butler W. H. , Brown R. G. , Nesbet R. K. 
Published: March 2011

ABSTRACTMultiple scattering theory (MST) provides an efficient technique for solving the wave equation for the special case of muffin-tin potentials. Here MST is extended to treat space filling non- muffin tin potentials and its validity, accuracy and efficiency are tested by application of the two dimensional empty lattice test. For this test it is found that the traditional formulation of MST does not converge as the number of partial waves is increased. A simple modification of MST, however, allows this problem to be solved exactly and efficiently.
Molecular Water in Silica
Authors: Fairbrother Peter J. , Heggie Malcolm I. , Tole Philip , Jones Bob , Oberg Sven 
Published: March 2011

ABSTRACTBoth silica (SiO2 ) and water are ubiquitous in our environment and our technologies. They come together in two important processes: the oxidation of silicon by steam, which is a process step in integrated circuit fabrication, and the hydrolytic weakening of quartz, which helps us understand plate tectonics. In the former process water molecules must diffuse through an amorphous silica layer and in the latter they must diffuse through quartz to reach dislocations and bubbles to promote plasticity.We have taken a molecular fragment of SiO2 based on an a quartz c channel, with a hydrogenated external surface and with an internal water molecule. Ab initio, local density, total energy calculations, relaxing the water molecule and the atoms lining the channel, have been applied to discover the insertion energy of water in quartz. In addition, this quantity has been examined in a c axis dislocation core, in order to evaluate the ease of pipe diffusion in the case of hydrolytic weakening and to better simulate amorphous SiO2 in the case of silicon oxidation.
A Molecular Dynamics Study of Zirconium Based on An N-Body Potential: HCP/BCC Phase Transformation and Diffusion Mechanisms in the BCC-Phase
Authors: Willaime Francois , Massobrio Carlo 
Published: March 2011

ABSTRACTWe propose a simple interatomic potential model for zirconium which successfully reproduces the phonon dispersion curves in both the α- and the β-phases as well as the temperature-induced phase transformation. Two interesting phenomena that help to understand the anomalous diffusion in the high-temperature β-phase are observed: the spontaneous formation of Frenkel-pairs and the highly correlated walk of the vacancy.
Theoretical Model for Adsorbate-Induced Surface Reconstructions: A Comparison of Potassium and Oxygen on Copper
Authors: Jacobsen K. W. 
Published: March 2011

ABSTRACTA Cu(110) surface can exhibit missing-row type reconstructions if potassium or oxygen atoms are chemisorbed on the surface. In the case of potassium every second of the closepacked rows in the top layer of the copper surface is removed. The reconstruction induced by oxygen on the same surface is of a very different kind where the missing rows are formed by picking out every second atom in the close-packed rows of the copper surface. The energetics and the driving force behind the two different reconstructions are discussed and compared on the basis of effective medium calculations.
Energetics and Structures of Pt, Pd, and Ni Adatom Clusters on the Pt (001) Surface
Authors: Wright Alan F. , Daw Murray S. , Fong C. Y. 
Published: March 2011

ABSTRACTUsing the embedded atom method (EAM), we have calculated the stable configurations of Pt, Pd, and Ni clusters, containing up to nine adatoms, on the Pt (001) surface. For Pt, we predict that the stable configurations are linear chains oriented along the <110> directions for three and five adatoms, and close-packed islands otherwise. These results, for clusters containing up to seven adatoms, were reported previously in reference 4. For Pd, the results are the same except that the stable configuration predicted for five adatoms is a close-packed island. For Ni, we predict that linear chains are the stable configurations for all numbers of adatoms. In determining these stable configurations, we allowed substrate relaxations. To assess the importance of substrate relaxations, we also performed calculations on an unrelaxed substrate. We found that substrate relaxations are important with regard to the stable configurations, and that allowing relaxation lowers the energies of chains with respect to islands.
Electronic Structure Studies of Diamond/Metal Interfaces
Authors: Erwin Steven C. , Pickett Warren C. 
Published: March 2011

ABSTRACTWe have begun to investigate theoretically the electronic properties of several ideal epitaxial interfaces of diamond with Ni and Cu, both of which enjoy a close lattice match. Of particular interest is the mechanism responsible for formation of the Schottky barrier, which is not yet fully understood at the microscopic level. We find that both the barrier height and the chemical bonding at the interface are strongly dependent on interface orientation (i.e., the relative positioning of the two surfaces). For orientations near the minimum total energy geometry, the calculated Fermi level is apparently pinned around 1.7 and 2.1 eV for the (111) and (001) interfaces, respectively, relative to the valence band maximum. For orientations not near the total energy minimum, the calculated barrier height is zero. A tentative explanation for this difference is proposed.
Magnetoresistance in Granular Metals
Authors: Mclean W.L. 
Published: March 2011

ABSTRACTWe review the origins of the magnetoresistance in granular metals with emphasis on the quantum coherence effects arising from the interference of electrons traveling by different paths. We discuss the maximum in the magnetoresistance expected from scaling arguments and observed in granular aluminum. Two theories of the magnetoresistance in the variable range hopping regime, one based on the magnetic-field induced shift of the mobility edge and the other dealing with ‘directed’ hopping, are compared with experiments.
Thermoelectric Properties of Two Component Composites
Authors: Levy Ohad , Bergman David J. 
Published: March 2011

ABSTRACTAn explicit expression for the bulk effective thermoelectric coefficient αe of a two component composite is derived. This coefficient is found to depend only on the bulk effective electric conductivity σe and thermal conductivity γe, as well as on the moduli of the pure components. Using this expression and making standard scaling assumptions about the forms of σe and γe, the scaling behavior of αe, and of the effective figure of merit Ze are investigated. This behavior depends strongly on the thermal and electric conductivities ratios in the pure components. Upper and lower bounds for αe are calculated from its analytical properties.
Tunneling and Percolation Behavior in Granular Metals
Authors: Balberg I. , Wagner N. , Goldstein Y. , Weisz S.Z. 
Published: March 2011

ABSTRACTThe nature of the percolation process in granular metals is examined for the first time by a computer simulation of a system of metallic grains embedded in an insulating matrix. Assuming that the intergrain conduction is due to quantum mechanical tunneling it is found that a percolation-like critical behavior of the conductivity is obtained, but that a percolation universal behavior will be found only in a very special case. In contrast, the behavior of the electrical noise does not deviate substantially from the universal one. Comparison of these results with recent experimental observations suggests that in the metallic range, both transport properties are controlled by the continuous metallic network rather than by intergrain tunnelin.. We propose that the metallic network resembles the previously studied system of ‘inverted random voids’.
Inverse Transport Problems for Composite Media
Authors: Mcphedran Ross C. , Milton Graeme W. 
Published: March 2011

ABSTRACTFor two-component ceramic metal composites we address the inverse problem of estimating the volume fraction occupied by each component from measurements of the transmission and absorption coefficients at various frequencies of the applied radiation. We devise the algorithms, each based on representation formulas and bounds for the complex dielectric constant that were developed by Bergman and Milton. One inversion algorithm works remarkably well when the measurements are exact but fails completely when small errors are present. The second random throw algorithm works well when the measurement errors are large but is computationally intensive and is limited to small data sets. The third method based on single and two-point inversion, while not so accurate, is simple and efficient. The last two algorithms are implemented with experimental data and good agreement is obtained with the experimentally measured true volume fraction.
Infrared Absorption by Granular Metals
Authors: Tanner D.B. , Kim Y.H. , Carr C.L. 
Published: March 2011

ABSTRACTThe infrared properties of granular metals and superconductors are qualitatively in accord with effective medium ideas, with insulating behavior below a percolation transition and metallic response above. An exception is the far-infrared absorption at low metallic concentrations, which is much stronger than theoretical predictions. Measurements of superconductors and of normal metals in different hosts suggest that this absorption is predominately electric dipole rather than the magnetic dipole (eddy current) absorption which is expected to be the dominant low-frequency loss in highly conducting particles. Measurements of clustered and non-clustered samples suggest that the strong far-infrared absorption does not arise from the clustering together of the individual metallic particles, although clustering does lead to about a tenfold increase in absorption.
Superconducting and Mechanical Properties of YBa2Cu3O7−x / Silver Granular Solids
Authors: Niculescu H. K. , Gielisse P. J. , Hascicek Y. S. , Testardi L. R. 
Published: March 2011

ABSTRACTIn this paper an experimental study of YBa2Cu3O7−x/silver composites is presented and the relationship between microstructure, superconducting and mechanical properties is discussed. Silver contents ranging from 0 to 17 vol% fell all below the percolation treshold. X-ray and Wavelength Dispersive Spectrometry analyses indicate that Ag remains in a free state, and is located mainly at the grain boundaries. A tenfold increase in Jc. and a more than twofold increase in strength are the benefits of the improved microstructure of silver composites. The Tc remains unaffected.
Indium-Impregnated Porous Glass: Magnetotransport and Superconducting Transition
Authors: Graf M. J. , Huber C. A. , Huber T. E. , Salzberg A. P. 
Published: March 2011

ABSTRACTMeasurements of the resistive superconducting transition and magnetoresistance of indium metal in the restricted geometry of 50 Å size porous Vycor glass are reported. We have mapped out the upper critical magnetic field versus temperature phase diagram for these transitions for fields up to 20 T and temperatures down to 100 mK. The results are consistent with magnetization measurements by previous authors and can be generally interpreted in terms of the description of inhomogeneous type-II superconductors. The field-induced resistive transitions exhibit unusual behavior not probed by previous bulk-type measurements which may be related to the microstructure of the composite.
Spin Glass Magnetic Behavior of Iron/Silica Gel Nanocomposites
Authors: Shull Robert D. , Ritter Joseph J. 
Published: March 2011

ABSTRACTHomogeneous gelled composites of iron and silica containing 5-40 wt.%Fe prepared by low temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst) were heated to 380°C in the presence of hydrogen gas. X-ray diffraction and M6ssbauer effect measurements, and transmission electron microscope (TEM) observations show these materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. Magnetic susceptibility data indicate the materials became either superparamagnetic or ferromagnetic at room temperature. On cooling, the magnetization data furthermore show that the hydrogenated materials containing ll-30% Fe become magnetic spin glasses at temperatures less than 30 K. Magnetic history effects are observed in addition to displaced hysteresis loops below their spin freezing temperatures (Tf). For field-cooled materials at 10 K, the displacement of the hysteresis loops along the field axis indicates the presence of a unidirectional anisotropy which decreases with the cooling field. Both superparamagnetic-to-spin glass and ferromagnetic-to-spin glass transitions are observed in these nanocomposites. Tf varies with the Fe content from ∼30 K for the 11%Fe nanocomposite to ∼10 K for a content near 33%Fe.
Magnetic Properties of Nanostructured Fe/Ag Composite Metal Films
Authors: Liou S.H. , Zhang Y.X. , Deangelis R.J. 
Published: March 2011

ABSTRACTWe have synthesized a series of nanostructured Fe/Ag composite metal films. The much enhanced magnetic coercivity was found to scale with the particle size which is process controlled. At a low substrate temperature(< 200°C), the film consists of small clusters of Fe(< 100Å) which behave superparamagnetically with a blocking temperature above 100K.
Wave Velocities in Sediments
Authors: Marion Dominique , Nur Amos , Yin Hezhu 
Published: March 2011

ABSTRACTSystematic relations between porosity and compressional velocity Vp in the three component (sand, grains, clay and brine) systems (1) porous sandstone, (2) sands, and (3) suspensions, were obtained using experimental data and models. In Cemented Shaley Sandstones Vp was found to correlate linearly with porosity and clay content. The velocities in clean sandstones are about 7% higher than those predicted by the linear fit, indicating that a small amount of clay significantly reduces the elastic moduli of sandstones.For uncemented shaley sand, a model for the dependence of sonic velocity and porosity on clay content and compaction was developed for sand with clay dispersed in the pore space and for shale with suspened sand grains. The model closely mimics the experimentally observed minimum for porosity and the peak in velocity versus clay content. The results explain much of the scatter in velocity data in-situ. Velocity in suspensions at ϕ = 39% of grains in brine is close to values predicted by the Reuss (Isostress) average. Velocity dispersion, as suggested by Biot (1956 a,b) is calculated and observed in coarser sediments such as sand, whereas velocities in the finer clay and silt follow Biot's low frequency value.In total, our results provide the complete dependence of velocity on porosity in brine saturated sediment with clays, ranging from pure quartz to pure clay and water. Our results also highlight the crucial role of the critical porosity ϕ at about 39%, and the transition from cemented to uncemented sands.
Effect of Sodium Ions on the Dielectric Conductivity of Porous Silica in Humid Environments
Authors: Cao Wanqing , Gerhardt Rosario , Wachtman John B. 
Published: March 2011

ABSTRACTPartially sintered silica gels were immersed in different NaCI solutions to deliberately introduce a small amount of sodium ions ranging from 387 to 9900 ppm. Dielectric measurements were carried out in the frequency range 12 to 107Hz under various relative humidity conditions. Two relaxation processes were observed in the impedance and modulus planes and were assigned to two separate regions in the hydrated silica surface. The top region is expected to be rich in hydrated sodium since its modulus relaxation time depends strongly on the relative humidity as well as the sodium content. The interfacial region is believed to be composed of structured water since its dielectric conductivity is affected mainly by the amount of water adsorbed on the pore surfaces.