bluuebunny/crossref_metadata_2025 huggingface dataset:
UV Tunable Laser Ablation of YBa2Cu3Ox+6: Changes in the Product Population and Kinetic Energy Distributions as a Function of the Laser Wavelength and Target Bulk Temperature
Authors: Wiedeman L. , Helvajian H.
Published: March 2011

We have conducted two separate experiments in the UV laser ablation of a sintered Yba2Cu3Ox+6wafer. We have measured the photoejected population distributions using selected UV laser wavelengths (4.01, 4.17 and 4.35 eV) near the 4.1 eV optical transition in Yba2Cu3Ox+6 In addition, we have measured the change in the ejected species kinetic energy as a function of the target bulk temperature. All the experiments were conducted at laser fluences well below the plasma formation threshold, and near that for product formation. Our results show that the UV laser ablation, at threshold fluences, proceeds via a nonthermal electronic excitation mechanism.
Influence of Beam and Target Properties on the Excimer Laser Deposition of Yba2Cu3O7−x Thin Films
Authors: Foltyn S. R. , Muenchausen R. E. , Estler R. C. , Peterson E. , Hutchinson W. B. , Ott K. C. , Nogar N. S. , Hubbard K. M. , Dye R. C. , Wu X. D.
Published: March 2011

Target modification by excimer laser exposure has been investigated. Under conditions typical in the fabrication of superconducting thin films, deposition rate decreases with exposure and significant physical and chemical modifications occur on the target surface. These modifications do not inhibit congruent evaporation.
Threshold Level Laser Ablation of Yba2Cu3Ox+6 at 351 nm, 248 nm, and 193 nm: Ejected Product Population and Kinetic Energy Distributions
Authors: Wiedeman L. , Helvajian H.
Published: March 2011

Using a sintered Yba2Cu3Ox+6 wafer in high vacuum, we have measured the photo-ejected products and kinetic energies at selected UV laser wavelengths (351 nm, 248 nm, and 193 nm) where the laser fluence is maintained near product formation threshold. At these fluences, we are well below the above surface plasma formation threshold and do not detect; any evidence for surface melting. Our results show that, for a specific laser wavelength, both the ion and the neutral mass spectrum agree. The measured spectrum shows that the photablated products consist only of atomic and diatomic species. In addition, the oxygen is bound only to the yttrium and barium. Our measurements further show that, for a specific ejected species, the yield is dependent on both the laser fluence and wavelength. However, the measured KE distribution is independent of the laser fluence for fluences near threshold. The species are ejected with mean KE between 6–9 eV and no species has kinetic energy in excess of 13 eV Our results imply that for UV laser threshold fluence ablation, the excitation/desorption process is nonthermal.
In Situ Pulsed Laser Deposition of Nd1.85Ce0.15Cu04−y
Authors: Horwitz J. S. , Chrisey D. B. , Osofsky M. S. , Grabowski K. S. , Vanderah T. A.
Published: March 2011

We have deposited thin films of the electron-doped, high temperature superconductor Nd1.85Ce0.15CuO4−y by pulsed laser deposition. Films were deposited from a stoichiometric target using a KrF excimer laser (248 nm, 250 mJ/pulse, −2 J/cm2) as a function of substrate temperature, oxygen pressure and vacuum anneal conditions. The film composition and structure, as determined by RBS and XRD, were very sensitive to the deposition and subsequent anneal conditions. Stoichiometric films were deposited at low substrate temperatures (740 °C) but contained other orientations and phases. Predominantly c-axis oriented films were formed at high substrate temperatures (900 °C) and high background pressures of oxygen (200 mtorr). These films were semiconducting when quenched on oxygen or nitrogen following deposition. Slow cooling in a vacuum, yielded superconducting films with a maximum Tc-(onset) of 15 K and Tc(R=0) of 11 K. A variation of the deposition and anneal conditions indicated that loss of copper competed with the optimization of the carrier concentration.
Laser Deposited Cubic Boron Nitride Films
Authors: Doll Gary L. , Sell Jeffrey A. , Salamanca-riba Lourdes , Ballal Ashwin K.
Published: March 2011

We report the successful growth of cubic boron nitride thin films on single crystal 100 silicon by using pulsed excimer laser ablation of a hexagonal boron nitride bulk target. Optical emission spectra were obtained during the film deposition giving insight into the deposition mechanism. The deposited films were characterized by transmission electron microscopy, scanning electron microscopy, optical microscopy, x-ray diffraction, and Auger electron microscopy. Regions of the films were found to exhibit epitaxy with the substrate.
Cluster Formation in Laser-Induced Ablation and Evaporation of Solids Observed by Laser Ionization Time-Offlight Mass-Spectrometry and Scanning Tunneling Microscopy
Authors: Tench R. J. , Balooch M. , Bernardez L. , Allen Mike J. , Siekhaus W. J. , Olander D. R. , Wang W.
Published: March 2011

Laser ionization time-of-flight mass analysis (LIMA) used pulses (5ns) of a frequency- quadrupled Nd-YAG laser (266 nm) focused onto spots of 4–100 μm diameter to ablate material, and a reflectron time of flight tube to mass-analyze the plume. The observed mass spectra for Si, Pt, SiC, and UO2 varied in the distribution of ablation products among atoms, molecules and clusters, depending on laser power density and target material.

Cleaved surfaces of highly oriented pyrolytic graphite (HOPG) positioned at room temperature either 10 cm away from materials ablated at 10−5 Torr by 1–3 excimer laser (308 nm) pulses of 20 ns duration or 1 m away from materials vaporized at 10−8 Torr by 10 Nd-Glass laser pulses of 1 ms duration were analyzed by Scanning Tunneling Microscopy (STM) in air with Ångstrom resolution. Clusters up to 30 Å in diameter were observed.
Time Resolved Electroluminescence in a-Si:H p-i-n Junctions
Authors: Carius Reinhard
Published: March 2011

Time resolved and steady state electrololuminescence (EL) measurements have been performed in a-Si:H p-i-n junctions at 10K < T < 300K in order to study the recombination in these devices. At low temperatures the rise of the EL following a rectangular forward current pulse consists of a fast component with a time constant of a few μs and a slow component with a time constant of about a ms. The data suggest that the time response of the EL is determined not only by the recombination but also by the time to build up the carrier density for bimolecular radiative recombination. The fast component of the EL is attributed to the field enhanced recombination of carriers which survived from the preceding pulse. Steady state measurements reveal characteristics of bimolecular recombination. The EL spectrum is shifted by 50meV to lower energies with respect to the photoluminescence spectrum.
Optical Phenomena at the Absorption Edge of Crystalline and Amorphous Silicon
Authors: Cody George D.
Published: March 2011

Optical phenomena associated with the absorption edge of crystalline silicon (c-Si) and amorphous silicon hydride (a-Si:H) are presented and compared. The optical properties discussed include the energy dependence, dipole matrix element and density of states associated with the absorption edge; the temperature dependence of the relevant optical energy gaps, and finally the magnitude and temperature dependence of the slope of the Urbach edge for each material. The comparison suggests that the optical properties of the two materials are closely related and that the absorption edge of a-Si:H may be derived from the effect of site disorder on the zone center direct gap of c-Si.
Photothermal Investigation of Surfaces and Interfaces in Amorphous Silicon Films
Authors: Amato G. , Boarino L. , Benedetto Giuliana , Spagnolo R
Published: March 2011

A method for the determination of surface and interface states is suggested, based on the observation of interference fringe pattern in photo-thermal spectra. Some experimental results are presented and interpreted by means of a theoretical approach. The method allows in most cases to get an evaluation of surface and interface density of states.
Interface Roughness and X-Ray Reflectivity of Amorphous Semiconductor Multilayers
Authors: Persans P. D. , Ruppert A. F. , Pantojas V. , Liang K. , Hughes G. , Abeles B.
Published: March 2011

We report high-resolution small-angle x-ray reflectivity measurements on plasma-deposited a-Ge:H/a-Si:H periodic amorphous multilayers. We show that reflectivity spectra are quite complex, involving components due to relatively smooth and rough layer interfaces at different depths in the sample respectively. Model computations show that interface roughness increases from 3 É at the substrate-sample interface to more than 15 É at the top surface of a 0.5 μm multilayer stack.
Space Charge Profiles in Thick Amorphous Silicon Diodes
Authors: Hollingsworth Russell E. , Xi Jianping , Buitrago Roman
Published: March 2011

Amorphous silicon p-i-n diodes with intrinsic layer thickness exceeding 50 microns have been produced. As a probe of the carrier collection, reverse bias collection efficiency measurements using weakly absorbed light were performed. The results have shown for the first time that amorphous silicon diodes of such thickness can be fully depleted. By analyzing the shape of the collection efficiency curve, profiles of the light induced space charge within the intrinsic layer are obtained. The space charge density in thick devices is found to vary strongly with position in the intrinsic layer as well as with excitation intensity.
Effect of Residual Phosphorus on Amorphous Silicon Thin Film Transistors
Authors: Tsutsu Hiroshi , Kawamura Tetsuya , Miyata Yutaka
Published: March 2011

Effects of residual phosphorus in the channel region of amorphous silicon thin film transistors(a-Si TFTs) on the TFT characteristics were quantitatively investigated. Concentration and the depth profile of the residual phosphorus were measured by high resolution secondary ion mass spectroscopy(SIMS). The OFF characteristics of a-Si TFTs were also measured.

The SIMS data showed that the phosphorus exists about 100nm deep into intrinsic a-Si(i-a-Si), but the OFF characteristics showed that the activity of the residual phosphorus is 4 order of magnitude lower than that of heavily phosphorus doped a-Si(+-a-Si). The residual phosphorus is found to be inactive and stable, and has little effect on a-Si TFT characteristics.

These results enabled us to fabricate inverted staggered a-Si TFTs by the simplest process using only 2 photo-mask steps and 1 self-aligned exposure.
The Programmability of Amorphous Silicon Analogue Memory Elements
Authors: Hajto J. , Rose M. J. , Snell A. J. , LeComber P. G. , Owen A. E.
Published: March 2011

This paper outlines recent results from work on non-volatile, analogue switching effects in amorphous silicon devices. The implication for the switching mechanism of these results and also the technological applications of such analogue devices are discussed. Particular reference is made to their use as re-programmable, non-volatile weighting elements in neural network synapses.
Compensating for Light Soaking Effects in Optically Addressed Spatial Light Modulators Incorporating A-Si:H Photodiodes
Authors: Walker C. M. , Landreth B. , Moddel G.
Published: March 2011

We characterize the effects of light soaking on optically addressed spatial light modulators (OASLMs) incorporating a-Si:H photodiodes and relate these effects to the light-induced degradation observed in the photodiodes. After illuminating the OASLMs with white light for up to 100 hours, the rise time of the OASLM optical response decreases, the fall time increases, and the undesirable modulation in the absence of write light (electrical modulation) also increases. We were able to compensate for these effects and approximately restore the original optical transfer characteristics by applying a dc offset voltage across the device. The results can be interpreted by considering series resistance changes in the a-Si:H photodiode.
Growth of Amorphous, Microcrystalline, and Epitaxial Silicon in Low Temperature Plasma Deposition
Authors: Tsai C. C. , Anderson G.B. , Anderson R.
Published: March 2011

This paper examines near-equilibrium and non-equilibrium film formation processes in the plasma deposition of Si and their effects on network propagation, defect generation and interface quality based on the concept of film formation as a balance between deposition and ‘etching.’ Using non-F reactant gases and conventional PECVD, epitaxial Si growth was achieved at 250°C. By adjusting the extent of ‘etching’, one can selectively obtain amorphous, microcrystalline, polycrystalline, and epitaxial Si using similar process gases and reactors.
Hydrogenated Silicon Films Prepared by Remote Plasma CVD
Authors: Kim Sung Chul , Lee Seung Kyu , Soe Sung Mo , Koh Sung Ok , Ihm Sung Shil , Jun Jung Mok , Kim Tae Gon , Chung Moon Hyun , Lee Kyung Ha , Song Hyon Kynn , Jang Jin , Hwang Jung Tae , Chung Kwan Soo
Published: March 2011

We have studied the improvement of the quality of undoped a-Si:H deposited by remote-plasma chemical vapour deposition. The effects of reactant gas concentration, rf power, substrate bias voltage on the electrical and optical properties have been investigated. Some hydrogen dilution of si lane improves the photoeletric property and a high rf power gives rise to the defect creation due to the ion bombardment on the growing surface. The positive substrate bias improves the quality of undoped a-Si:H.
The Low-Temperature Metal-Organic Chemical Vapor Deposition (Ltmocvd) Route to Amorphous Silicon Semiconductors
Authors: Kaloyeros Aain E. , Corbett James W. , Tobcano Paul J. , Rizk Richard B.
Published: March 2011

Preliminary results are presented for a new approach proposed by the present investigators to solve the problem of light-induced degradation in amorphous silicon semiconductors. The approach uses low-temperature metal-organic chemical vapor deposition (LTMOCVD) of tailored organometallic precursors. The precursors employed are non-toxic, non-hazardous and easy to handle. In the present paper, a-Si:H films were grown, using argon with various hydrogen concentrations as carrier gas, in a cold-wall CVD reactor at a reactor pressure of 1-10 torr and substrate temperature in the range 300–450°C. Characterization studies were performed using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and extended electron-energy-loss fine structure spectroscopy (EXELFS). The results of these studies showed that the films were uniform, continuous, adherent and highly pure--contaminant levels were below the detection limits of XPS. In addition, EXELFS results showed that short-range order (SRO), consisting of the same tetrahedral coordinated units found in crystalline silicon, does exist in all the amorphous samples, regardless of hydrogen concentration. However, the degree of stuctural disorder in the silicon local tetrahedral units decreased as hydrogen was added.
Measurements of the Transient Photoconductivity During the Growth of A-Si:H Multilayers
Authors: Neitzert H. C. , Werner A. , Kunst W. , Kunst M.
Published: March 2011

The deposition process of multiple layer structures of intrinsic and p-type hydrogenated amorphous silicon was followed by measuring the microwave detected transient photoconductivity (TRMC) during the film growth. In an i-p-i structure we can show that after deposition of an upper layer of about 500 nm, former deposited layers do not influence the TRMC-signal any more. In an i-p+-i-p structure we can clearly distinguish between p-layers of different doping concentration.
Optical Studies of Microstructure in a-Si:H
Authors: Vaněček M. , Červinka D. , Favre M. , Curtins H. , Shah A. V.
Published: March 2011

A set of samples grown under different substrate temperature has been characterised by complex optical, photoelectrical and transport measurements. A very sensitive and simple method for detection of optical scattering in a device quality amorphous silicon has been demonstrated. The spectral dependence of the optical scattering coefficient αs(E) can give us information on the nature and concentration of the scattering centers in a-Si:H thin films.
Hydrogen Stability in Hydrogenated Amorphous Germanium
Authors: Beyer W. , Herion J. , Wagner H. , Zastrow U.
Published: March 2011

The thermal stability of hydrogen in hydrogenated amorphous germanium films was studied by hydrogen evolution and by deuterium and hydrogen inter-diffusion experiments. Similar to a-Si:H, the hydrogen stability in a-Ge:H is found to depend strongly on the film structure and on the position of the Fermi level.
Thermally-Induced Change of Conductivity and Defect Density in Amorphous Silicon-Germanium Alloys
Authors: Shimizu Tatsuo , Xu Xixiang , Sasaki Hiroyuki , Yan Hui , Morimoto Akiharu , Kumeda Minoru
Published: March 2011

Thermally-induced metastable phenomena in amorphous silicon-germanium alloys were studied by conductivity and ESR measurements. Fast cooling from 250 °C reduced both dark- and photo-conductivities by a factor of 3–4 while the neutral defect density remained unchanged. Thermally-induced change in conductivity relaxed towards equilibrium with a stretched exponential form. The thermal equilibrium temperature was found to be roughly proportional to the optical gap for a–Si:H, a–Sii−xCx:H, a–Si1−xNx:H and a–Si1−xGex:H:F.
Hydrogen Diffusion in a-Sic:H
Authors: Demichelis F. , Pirri C. F. , Tresso E. , Amato G. , Della Mea G.
Published: March 2011

In order to improve our understanding of the motion of hydrogen and the mechanism by which structural changes occurr, two sets of a-SiC:H samples obtained by glow-discharge at different substrate temperatures and power densities, respectively without and with hydrogen flow, were analyzed.

Results from structural, optical and electrical measurements show that annealing and hydrogen diffusion allow for a possible reorganization of the a-SiC:H amorphous network.
Material Properties of P-Type A-Sic Layers Using Either Diborane or Trimethylboron as Doping Gas
Authors: Lechner P. , Gorn M. , RÜbel H. , Scheppat B. , Kniffler N.
Published: March 2011

Hydrogenated amorphous SiC:H films prepared by rf glow discharge from a silane-methane mixture have been B-doped either from diborane or from trimethyl boron (TMB). In the latter a higher hydrogen content is found explaining a higher Tauc optical gap EG, however also a lower dark conductivity for the same boron concentration in the film. Thus the figure of merit of our p-layers doped from TMB is not improved. Further the observed narrowing of the Tauc gap could not be correlated with a decrease of the hydrogen content at high B-doping. The reduction of EG is mainly explained by a drastic broadening of the absorption edge caused by creation of boron induced defects and structural disorder but not due to a shift of the absorption edge.
The Movement of Mobility Edges in Hydrogenated Amorphous Silicon
Authors: Lee S. , Heller D. , Wronski C. R
Published: March 2011

Internal photoemission of both electrons and holes is used to investigate the movement of the mobility edges in high quality intrinsic, undoped hy-drogenated amorphous silicon (a-Si:H) with temperature and electrical field. The electron mobility edge is found to move up in energy by ∼40meV between 298K and 120K. On the other hand, the hole mobility edge remains essentially unchanged between 298K and 160K. The injection (and collection) of photoemitted holes is less efficient than that for electrons and in the films studied could not be measured below 160K.
Theoretical Calculations of the Nonlinear Dielectric Function of Inhomogeneous Thin Films
Authors: Risser Steven M. , Ferris Kim F.
Published: March 2011

The optical response of thin dielectric films can be influenced by grain morphology and the presence and distribution of defects. In the limit of random defects and small electric field amplitudes, approximate methods exist to model the real part of the dielectric constant in terms of volume fractions and bulk dielectric constants of the film components. Explicit inclusion of nonlinear polarizabilities and details of the microstructure, such as particle phase, shape, and orientation requires a more exact approach.

We have developed a method to self-consistently determine the local internal electric field and polarization in the long wavelength limit for model films with a random distributions of defects of arbitrary phase and orientation. From this we have calculated the real part of the dielectric constant as a function of nonlinear polarizability of the components, and have shown the effect of defect phase and orientation on the dielectric constant of the film.
Elastic Constants in C-plane of Single Crystal Bi 2Sr 2Ca Cu 208 between 80 K and 260 K
Authors: Wu Jin , Wang Yening , Yan Yifeng , Zhao Zhongxian
Published: March 2011

The temperature dependence of the in-plane C11 C22. C12 and C66 modes between 80 and 260 K of superconducting crystals of Bi2Sr2Ca1Cu208 have been obtained via the measurements of ultrasonic-velocities. The anisotropic elasticity in the a-b plane of single crystal Bi2 Sr2Ca1Cu2O8 is manifest. The shear modulus of sound propagation along the [110] with the polarization has been also calculated and shows an overall trend of softening over a wide temperature range above Tc. The shear modulus C6 6 shows three obvious softening minima around 240–250 K, 150 K and 100 K.
Electronic Structures at Metal/Alumina Interfaces
Authors: Kohyama M. , Kose S. , Kinoshita M. , Yamamoto R.
Published: March 2011

The electronic structure of the interface between α-Al2O3(0001) and Nb layers has been calculated using the empirical tight-binding method and the slab model. It has been shown that a direct chemical bond of both covalent and ionic characters can be established at the interface between the surface O atoms of α-Al2O3 and the Nb atoms, which is consistent with the recent photoelectron spectroscopy study. General trends of the electronic structure and chemical bond at the interfaces between α-Al2O3.(0001) and a series of 4d transition metals have been examined using the same theoretical method. In all cases, there exist similar interactions between the surface O atoms and the metal atoms, and it has been observed that the occupancy in the portion of the antibonding peaks of the local density of states at the interface increases as the atomic number of the transition metal increases.
First Principles Model for Electronic Properties of Non-Ideal Surfaces of Refractory Interstitial Alloys
Authors: Ivanovsky A. L. , Gubanov V. A.
Published: March 2011

A unique set of physico-chemical properties of the implantation phases (transition metal carbides and nitrides) and their wide-scale application in production processes motivated an intensive research in the electronic properties of the phases [1, 2]. A great attention attaches recently to the study of specific features of the electronic surface states of refractory compounds, the role of the electronic states being decisive in the formation of adsorption, catalytic, emission and other characteristics of the compounds. Available calculations of the energy spectrum of the high-symmetry faces of some 3d-5d-metal carbides and nitrides performed with the use of the band theory methods [4, 5] made it possible to establish principal features peculiar to the formation of surface states for ideal systems.
Calculated Properties of Carbon Defects in Silicon
Authors: Tersoff J.
Published: March 2011

The energies of carbon defects in silicon have been calculated, using an empirical classical potential, and used to infer defect properties and reactions. The results accord well with a variety of experimental data, including defect structures, activation energies for defect motion, and coupling to strain.
Dynamical Simulation of the Motion of Curved Grain Boundaries Composed of Pyramidal-Shaped Ledges
Authors: Jhan Re-Jhen , Bristowe P. D.
Published: March 2011

A dynamical simulation of curved grain boundaries composed of pyramidal-shaped ledges has shown that the boundaries can move by local conservative shuffles of atoms or groups of atoms such that one adjoining crystal grows at the expense of the other. In the model system studied, the shuffles often take the form of correlated rotational displacements about the axis normal to the boundary. The simulations provide support for the atomic mechanism proposed by Babcock and Balluffi to explain their observation of grain boundary migration without the participation of SGBDs.
Molecular Dynamics Simulations of Cluster Impacts on Metallic Targets
Authors: Shapiro Mark H. , Tombrello T. A.
Published: March 2011

A modified version of the multiple-interaction code SPUT2 has been used to simulate impacts of 63-atom Cu clusters on six–layer Cu targets. Simulations were carried out with cutoff times of 100 and 500 fs for an incident cluster energy of 63 keV (1 keV/atom). Significant enhancements were observed in the maximum potential and kinetic energies achieved in the early phase of the collision cascade. Some hard collisions yielded atoms with potential energies as high as 925 eV (in the CM frame). This is almost twice the energy allowed in an isolated two-body collision. The number of hard collisions per time-step vs potential energy is well-fitted with a decaying exponential, allowing extrapolation to higher energies. These results together with similar results for Al clusters impacting Au targets suggest that non-linear collisional effects cannot explain the high D–D fusion rates seen in Beuhler, Friedlander, and Friedman's recent experiment [l].
Favored Boundaries in the Structural Unit Model of Grain Boundaries
Authors: Khantha M. , Vitek V. , Goldman M.
Published: March 2011

A systematic geometrical procedure for predicting favored boundaries in the structural unit model is presented. The method is applicable to both symmetric and asymmetric tilt boundaries. The predictions are confirmed by modeling the structures of tilt boundaries belonging to low symmetry ( and [221]) axes in f.c.c. and b.c.c. structures. The results confirm the applicability of the structural unit model for relatively high-index tilt axes.
New Surface Atomic Structures for III-V(110)-p(1x1)-Sb(1ML):Chemical Bonding and Electronic Structure
Authors: LaFemina John P. , Duke C. B. , Maflhiot C.
Published: March 2011

Tight-binding total energy computations are used to examine the chemical bonding and electronic structure for two new minimum-energy surface atomic structures for p(lxl) overlayers of Sb on III-V(110) surfaces. The bonding in each of these structures is unique, having no analog in either the bulk or small molecule coordination chemistry of these materials, and is a phenomenon uniquely associated with the constrained epitaxical growth of the Sb overlayer.
Oxygen Dissociation and Adsorption on Si(100) Reconstructed Surfaces
Authors: Miyamoto Yoshiyuki , Oshiyama Atsushi
Published: March 2011

A detailed picture of dissociation and adsorption of oxygen on the Si(100) reconstructed surface is presented on the basis of the total-energy band structure and force calculations within the local density approximation with use of the normconserving nonlocal pseudopotentials. Dissociation of an oxygen molecule occurs at any site on the Si(100) surface. The resulting oxygen atom is adsorbed on several (meta)stable sites depending on which site the preceding molecule dissociates. Peculiar relaxation of the top-layer Si atoms is found upon oxygen adsorption. Calculated vibrational energy and valence density of states in the most stable geometry are reasonably consistent with the experimental data available, i.e., HREELS and UPS. Finally, we have found that an oxygen molecule penetrates through the oxygen-covered Si(100) surface, on which the Si dangling bonds are terminated, and then dissociates in the vicinity of the Si bond center site.
Classical Potentials for Covalent Solids and Clusters: Application to Silicon and Silicon Dioxide
Authors: Chelikowsky James R. , Glassford Keith M.
Published: March 2011

Pairwise interactions lead to close-packed structures which can be easily described by classical potentials. However, covalent solids such as silicon or silica form open structures which cannot be described by such interactions. We have developed new interatomic force fields which describe the phase stability of silicon and small silicon clusters. We consider three body forces which are adjusted to describe “covalent” →, “metallic” phase transitions instead of small amplitude atomic vibrations. These forces result in a highly accurate phase diagram for crystalline polytypes. Our force filed can be easily modified to describe energies and structures of Sin vapor phase clusters. A key aspect of the cluster problem is the transfer of bond strength from dangling bonds to back bonds. We show as a function of this back bond strength that a covalent →. metallic “first order” structural phase transition occurs in the cluster structures. We also examine the role of three body forces in silica in light of recent pressure measurements. We find that the recently developed pairwise forces are not sufficient to predict accurately the pressure dependence of the structural parameters.
Application of the Embedded Atom Method to Pb and Be
Authors: Karimi M. , Yang Z. , Tibbits P. , Ila D. , Dalins I. , Vidali G.
Published: March 2011

We have derived the embedding energy functional and two body potential of the Embedded Atom Method (EAM) using decreasing exponentials for both the electron density and the two body potential. The embedding function was obtained from the equation of state given by Rose et al. Because of the form of the embedding function, the equilibrium lattice constant, cohesive energy, and bulk modulus are automatically satisfied. The two parameters Φe and γ of the two body potential were determined by fitting to shear modulus and the single vacancy formation energy. Contributions of up to the third nearest neighbors were included in the evaluation of the charge density ρ and the two body potential Φ. The stability and anisotropy of each structure were estimated and compared with the available experimental data.
Calculation of Local Fields for Clusters of Ellipsoids Within the T-Katrix Approach
Authors: Gomez Manuel , Fonseca Luis F. , Cruz Luis , Vargas William
Published: March 2011

A T-matrix formalism is used to calculate local electric fields around clusters of prolate ellipsoids in the long wavelength regime. The calculations are performed as a function of interparticle distance as well as angle of orientation. The observed red shifts in the resonant wavelengths of the characteristic peaks are shown to obey an exponential relationship as a function of interparticle separation and a sinusoidal relationship as a function of angle of rotation of the ellipsoid. The behavior of the cluster is discussed and the two effects, of separation and rotation, are compared.
Conductivity Fluctuations in Two-Component Finite Systems
Authors: Axell Jörgen , Helsing Johan
Published: March 2011

In all applications and experimental setups one has to do measurements on finite samples with some average properties. Due to statistical fluctuations in the realization of the specific sample one has, there will be a spread in the measured properties. In this paper we consider electrical conduction in a two-phase material, where the two phases have different conductivities. We study the size dependence of the fluctuations in the effective conductivity and give the dependence of the sample size and the dependence of the resistance ratio, especially when the latter is close to one. A simple model and numerical simulations are presented.
Particle Interaction Model of Permittivity Enhancement in Metal-Insulator Composites
Authors: Doyle W. T. , Jacobs I. S.
Published: March 2011

A simple effective spherical cluster model of generalized susceptibility enhancements in disordered monodisperse suspensions of conducting spheres is compared with experimental measurements of reduced permittivity and conductance. Good agreement with experiment is obtained by treating the suspension in the Clausius-Mossotti approximation as a mixture of isolated spheres and compact spherical metallized clusters.
Dielectric Properties of Percolating Nanocrystals
Authors: Marquardt P. , Nimtz G.
Published: March 2011

Microwave spectroscopy revealed a non-metallic effective dielectric function of non- supported nanocrystal networks. This new result is in agreement with the size-induced metal-insulator transition (SIMIT) observed in isolated sub-μm conductors. Separating the particle contribution ε = ε1 + iε2 from the measured effective dielectric response leads to the conclusion that percolating networks of SIMIT particles are characterized by an enormous positive value of El and a conductivity σ ∼ ε2 much smaller than expected for metallic networks. Another non-metallic feature of the networks is the weak thermal influence on ε1 and ε2. All properties change sensitively with the filling factor f of the network. Their unusual dielectric properties make low- density nanocrystal networks candidates for applications e.g. as high permittivity capacitor materials or temperature-independent resistors.
Electrical Conduction in Granular Metal Films
Authors: Morris J.E. , Mello A. , Adkins C.J.
Published: March 2011

In the early 1970's, Abeles et al developed a widely accepted model for electrical conduction in granular metal films based upon an assumed correlation between metal particle size and inter-particle tunneling gap width. The paper critically examines this assumption and other aspects of the theory. No observation of any such correlation has been reported, nor should it be expected, particularly in discontinuous films. The latter point requires separate considerations of the very different structures of discontinuous metal films, granular metals and cermets, using the “granular” term here in a more limited sense than usual.

Other aspects of the theory considered include its implied percolation topology and the field effect. Experimental data are also re-evaluated in terms of other common models. The paper concludes with a brief review of recent work in discontinuous metal films with suggestions for future theoretical and experimental work.
Numerical Simulation of Hopping Conductivity in Granular Metals
Authors: Chen L. F. , Sheng Ping , Abeles B. , Zhou M. Y.
Published: March 2011

Electrical conduction in granular metals is simulated by mapping the hopping conductance between pairs of metal grains onto a simple cubic lattice with bonds between neighbors. By considering a log-normal distribution of grain sizes and the effect of disorder potential, the numerically calculated network conductance exhibit clear deviation from simple activation. Plotting -log a vs. T-½, where σ denotes conductivity and T the temperature, gives good straight line behavior with slopes comparable to those measured experimentally. Our results are noted to differ from those of Adkins et al.
Field Effect Experiments on Discontinuous Metal Films
Authors: Adkins C J
Published: March 2011

We describe experiments in which the energetics of electrical conduction in discontinuous metal films are probed by inducing carriers electrostatically into the films. Both conductivity and thermopower show field effects that are generally of the expected form but they are very much smaller than the classic model for conduction in these systems would predict. Furthermore, in some cases at least, they can be shown to result from dielectric relaxation processes rather than from direct modification of basic transport energetics.

We discuss possible explanations, including that large potential disorder is present and that transport is controlled by energy structure that moves with the chemical potential, as would be the case i correlation energies determined carrier mobility. Experiments that measure the charging energies of the metal particles directly suggest that correlation is important, and we argue that progress in understanding conduction in granular metals depends on development of a model in which correlation energies play a critical role in controlling transport.
Use of Dielectric Mixture Equations for Estimating Permittivities of Solids from Data on Pulverized Samples
Authors: Nelson Stuart O. , You Tian-Su
Published: March 2011

The complex permittivities of solid and pulverized samples of two plastics, Rexolite 1422 and Kynar, were measured at frequencies of 2.45, 11.5, and 22.0 GHz at 25°C by the short-circuited waveguide technique. Several dielectric mixture equations and extrapolation of functions of the real and imaginary parts of the permittivity that are linear with bulk density were then used to estimate the permittivities at solid-material densities from measurements on the pulverized samples. For these materials, the best estimates of the permittivities were provided by extrapolations that are consistent with the Complex Refractive Index and Landau and Lifshitz, Looyenga mixture equations. The Bottcher mixture equation often gave values very close to the Landau and Lifshitz, Looyenga equation, and the Bruggeman-Hanai, Rayleigh, and Lichtenecker mixture equations gave increasingly larger permittivity estimates in that order.
Oxygen-Induced Reversible Structural Change of Supported fine Silver Particles Observed by In Situ Optical Absorption and X-ray Diffraction
Authors: Yanase Akihisa , Komiyama Hiroshi , Tanaka Kazunobu
Published: March 2011

In situ measurements of optical absorption and x-ray diffraction for small supported silver particles were performed under defined atmospheres at temperatures below 723 K. We found, for the first time, reversible changes both in optical absorption spectrum and lattice parameter of silver particles against the cyclic exchange of ambient gases between 5% H2/He and 1% 02/He. The present data indicate that the adsorption of oxygen results in a reduction in surface stress of silver particles, leading to a change in the shape of particles.
Dynamical Properties of Weakly Coupled Josephson Systems
Authors: Lee K. H. , Xia T.-K. , Stroud D.
Published: March 2011

We review recent work on the dynamical behavior of coupled resistively-shunted Josephson junctions, with emphasis on our own calculations. We present a model which allows for the inclusion of finite temperature, disorder, d.c. and a. c. applied currents, and applied magnetic fields. We discuss applications to (a) calculations of critical currents and IV characteristics; (b) harmonic generation and microwave absorption by finite clusters of Josephson junctions; (c) critical energies for vortex depinning; and (d) quantized voltage plateaus in arrays subjected to combined d.c. and a. c. currents. Possible connections to the behavior of granular high-temperature superconductors are briefly discussed.
Quantum Fluctuations in Granular Superconductors
Authors: Fishman R.S.
Published: March 2011

A 1/z expansion is used to study the coupling of phase fluctuations in an array of superconducting grains. By lowering Tc. and increasing the critical grain diameter αc,fluctuations suppress the long-range order in the material. When the temperature T is of order U = e2/C, where C is the grain capacitance, peaks appear in the fluctuation corrections to both the specific heat and the short-range order parameter. These peaks are induced by the coupling between thermal fluctuations of the charge and quantum fluctuations of the phase on neighboring grains.
Superconductivity and Tunneling Spectroscopy in Granular and Homogeneous Quench Condensed thin Films
Authors: Valles J. M. , Dynes R. C.
Published: March 2011

Quench-condensed films of simple superconductors can be deposited with a morphology which is either granular (grain sizes on the scale of tens of angstroms) or homogeneous. Transport and tunneling studies of the normal and superconducting states of these films as a function of sheet resistance have revealed profound differences between them. In uniform films Tc. and the energy gap decrease continuously with decreasing film thickness. Superconductivity is destroyed by the reduction of the amplitude of the order parameter. In granular films, the grains are sufficiently large to support superconductivity in each grain. Long range phase coherence is destroyed by intergrain phase coherence breaking.
On The Optimized Mixing of Order and Disorder in High-Tc Ceramic Superconductors
Authors: Mezzelli E.
Published: March 2011

Coherent excitations of localized states, represented by a selfsimilar Cayley tree, produce a gap in the density of states, which is deeper as the number of generations increases.

The model can suitably describe systems in which an optimized concentration of “defects” allows self similarity of the state-tree and coherent coupling between clusters of localized states.

A renormalization of the state-tree leads to a hierarchy of the energy eigenvalues.
Percolation Effects in the Mechanical Properties of Granular Ni-A12O3 thin films
Authors: Schlesinger T.E. , Gavrin A. , Cammarata R.C. , Chien C.-L.
Published: March 2011

The mechanical properties of sputtered Ni-Al2O3 granular thin films were investigated by low load microharaness testing. It was found that the microhardness of these films displayed a percolation threshold at a nickel volume fraction of about 0.6, below which the hardness is greatly enhanced. This behavior is qualitatively similar to the electrical and magnetic properties of these types of films. A percolation threshold in hardness can be understood as due to a change in the mechanism for plastic deformation.
Magnetic and Structural Properties of Granular Iron-Silicon Dioxide Thin Films
Authors: Carey M.J. , Parker F.T. , Berkowitz A.E.
Published: March 2011

Fe-SiO2 granular films were produced by co-sputtering from separate Fe and SiO2 targets. Films deposited on silicon nitride membranes were thin enough to observe directly by transmission electron microscopy (TEM). The coercive force (Hc) was strongly dependent on temperature and volume fraction of Fe as observed in previous investigations, but with lower Hc. Two features of the films with low Fe concentrations were remarkable. The hysteresis loops show high remanence, and the room temperature TEM images show stable domain structure and ripple patterns. This behavior is inconsistent with a model of non-percolating, non-interacting superparamagnetic Fe grains.
Mechanical Properties of Colloidal Gels Subject to Particle Rearrangement
Authors: Shih Wan Y. , Shih Wei-Heng , Aksay Ilhan A.
Published: March 2011

In a colloidal suspension with attractive interaction, particles form aggregates that settle to the bottom of the container. As the concentration of particles is increased, the overlapping of the aggregates (flocs) produces a continuous network throughout the suspension before settling occurs and a colloidal gel is formed. Colloidal gels may be divided into hard gels and soft gels.1 Hard gels are those formed by fine particles such as silica or boehmite of about 0.01 µm in size with high interparticle attraction energies. Under small shear rates, there is little restructuring in the gel network. The flocs that pack to form the gel network still retain their fractal structure, which gives rise to the scaling behavior of various mechanical properties of a gel with respect to particle concentration. For example, the storage modulus G' of a hard gel remains constant at small strain and increases in a power-law fashion with particle concentration, (φ, as G' - φn. There can be two regimes, a strong-link regime and a weak-link regime. In both regimes, the exponent n can be expressed in terms of the fractal dimension of the flocs, D, and the fractal dimension of the backbone of the flocs, x, while the form of n depends on what regime the system is in. Furthermore, a gel may crossover from the strong-link regime to the weak-link regime as the particle volume fraction is increased2.
Transport of Toluene in Microporous Vycor Class
Authors: Abeles Ben , Chen L. F. , Johnson J. W. , Drake J. M.
Published: March 2011

A quasi-equilibrium model is developed for capillary condensation flow, in which surfacediffusion is included. The model is in excellent agreement with the experimental data for the flow of toluene in untreated Vycor glass and in Vycor glass whose pores were derivatized with C18H38. The observed increase in viscosity with decreasing pore size is ascribed to a stagnant monolayer adsorbed to the pore walls. The surface diffusion coefficient at 22ºC is 5 x 10-5 cm2/secfor both the derivatized and untreated samples.
Healing of Pores in Metals by Magnetic Pressure Pulses
Authors: Betedhtin V.I. , Pakhomov A.B. , Peregood B.P. , Petrov A.I. , Razuvaeva M. V.
Published: March 2011

The problem of healing of microscopic pores and cracks in the temperature field and understatic and dynamic pressures attracts considerable attention. Indeed, regeneration of continuity leads to a significant improvement of physical and mechanical properties of the materials where micropores were formed as a result of long—term loading (high—temperature creep, fatigue), surface deformation (friction, wear, grinding), specialconditions of processing (casting, powder metallurgy, condensed films) [1– 10]. For instance, the elimination of porosity allows longer lifetimes [1– 7] and higher ultimate strength [8], elastioonstants [91], and also electric and magnetic properties [10]to be achieved.
Characterization of the Electric Field in Electrophotographic Granular Systems
Authors: Schein L. B.
Published: March 2011

Within all electrophotographic (or xerographic) copiers and laser printers are development systems where the image is reproduced. Usually these development systems contain 200 μm diameter metallic, magnetic balls called carrier particles. Attached electrostatically to these balls are 10μm diameter polymer particles called toner which eventually end up on the paper forming the black images given to customers. The process bywhich the toner leaves the carrier and ends up on the paper is driven by electric fieldsin the development system. These fields are determined by the applied voltage and the dielectric properties of the granular mixture of metal carrier balls, polymeric toner, andair. The average electric field is determined by the dielectric constant of this granular mixture. But on a microscopic scale that a toner would experience, i.e. 10 μm, the electric field has significant structure. This structure has been determined both experimentally and theoretically. For example, the electric field peaks underneath a chainof carrier particles (which are chained up by a magnetic field). Models of the development process clearly indicate that toner particles transfer from the carrier particles only when the toner particles are directly under a carrier chain, where one would expect the field to be a maximum. However, recently we have succeeded in showing experimentally that the value of the electric field that the toner particles experience is the average field, described by the dielectric constant. Theoretical questions concerning the calculation of the electric field in a granular system raised by this unexpected result will bediscussed.
Oxide Nonlinear Conductors: A Review
Authors: Levinson Lionel M. , Philipp Herbert R.
Published: March 2011

Oxide nonlinear conductors are granular electronic ceramic materials whose electrical behavior is dominated by grain boundary interfaces states. The material is typically comprised of relatively conducting oxide grains with electrical barriers at the grain boundaries induced by segregated impurity ions. Oxide nonlinear conductors can exhibit a highlynonlinear current-voltage relationship with a 5% change in voltage serving to increase the current flow an order of magnitude or more.

This review outlines our understanding of the microstructure, grain boundary properties and electrical behavior of these variable resistor or “varistor” granular materials. ZnO varistors currently have widespread application in protecting power and signal level electrical circuits against dangerous voltage surges and a brief overview will be given of the applications of this material.