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Calculating scattering amplitudes efficiently

Description: We review techniques for more efficient computation of perturbative scattering amplitudes in gauge theory, in particular tree and one- loop multi-parton amplitudes in QCD. We emphasize the advantages of (1) using color and helicity information to decompose amplitudes into smaller gauge-invariant pieces, and (2) exploiting the analytic properties of these pieces, namely their cuts and poles. Other useful tools include recursion relations, special gauges and supersymmetric rearrangements. 46 refs., 11 figs.
Date: January 1996
Creator: Dixon, L.
Partner: UNT Libraries Government Documents Department

A new Eulerian-Lagrangian finite element simulator for solute transport in discrete fracture-matrix systems

Description: Fracture network simulators have extensively been used in the past for obtaining a better understanding of flow and transport processes in fractured rock. However, most of these models do not account for fluid or solute exchange between the fractures and the porous matrix, although diffusion into the matrix pores can have a major impact on the spreading of contaminants. In the present paper a new finite element code TRIPOLY is introduced which combines a powerful fracture network simulator with an efficient method to account for the diffusive interaction between the fractures and the adjacent matrix blocks. The fracture network simulator used in TRIPOLY features a mixed Lagrangian-Eulerian solution scheme for the transport in fractures, combined with an adaptive gridding technique to account for sharp concentration fronts. The fracture-matrix interaction is calculated with an efficient method which has been successfully used in the past for dual-porosity models. Discrete fractures and matrix blocks are treated as two different systems, and the interaction is modeled by introducing sink/source terms in both systems. It is assumed that diffusive transport in the matrix can be approximated as a one-dimensional process, perpendicular to the adjacent fracture surfaces. A direct solution scheme is employed to solve the coupled fracture and matrix equations. The newly developed combination of the fracture network simulator and the fracture-matrix interaction module allows for detailed studies of spreading processes in fractured porous rock. The authors present a sample application which demonstrate the codes ability of handling large-scale fracture-matrix systems comprising individual fractures and matrix blocks of arbitrary size and shape.
Date: July 1, 1996
Creator: Birkholzer, J. & Karasaki, K.
Partner: UNT Libraries Government Documents Department

FMGN, RENUMN, POLY, TRIPOLY: Suite of Programs for calculating and analyzing flow and transport in fracture networks embedded in porous matrix blocks

Description: This report describes a suite of programs developed at the Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab) for simulating flow and solute transport in fracture networks embedded in porous matrix blocks. The codes FMGN, RENUMN and TRIPOLY are extensions of the older codes FMG, RENUM and TRINET developed at the Berkeley Lab, and references are made to previous Berkeley Lab reports which describe those codes. The first section of this report describes the general background of TRIPOLY and the theory of treating the fluid and solute exchange between fractures and rock. The second section is a user`s manual for the programs FMGN, RENUMN, POLY and TRIPOLY. Note that the description of FMGN and RENUMN is very short in this section. FMGN and RENUMN are relatively unchanged from the old codes FMG and RENUM, and only the differences between the old and new versions are listed in this report. For work with FMGN and RENUMN we refer to the detailed description of theory and design in BILLAUX et al. and the user`s manual in BILLAUX et al. respectively. For the other codes POLY (newly developed) and TRIPOLY (features major changes compared to the old program version TRINET) a detailed user`s manual is enclosed in this report. It provides the user with sufficient information to run the programs. The third section of this report comprises some sample problems as a tutorial.
Date: September 1, 1996
Creator: Birkhoelzer, J. & Karasaki, K.
Partner: UNT Libraries Government Documents Department

Continuum-continuum Autler-Townes splitting in calcium

Description: Strong-field ionization of two-electron atoms can result in scenarios in which the electron-electron correlation plays an important role. It was recently suggested that a splitting similar to the Autler-Townes effect would occur when two ionization continua are resonantly coupled in two-electron atoms. This is obviously at variance with the case of one electron atoms where coupling between continua does not induce oscillations but instead leads to exponential decay of one continuum into another. The special case considered by Grobe and Eberly is that of a strong radiation field resonantly coupling two ionic states. Formally, the states which are coupled are continuum states (two-electron states in which one electron is in a continuum state), but nevertheless the corresponding photoelectron peak is split. Physically, the reason for this is that the electron-electron interaction transfers the energy shift of the core electron to the outgoing electron and has been dubbed {open_quotes}coherence transfer{close_quotes} by Ref.
Date: October 1, 1995
Creator: Walker, B.; Sheehy, B. & Kaluza, M.
Partner: UNT Libraries Government Documents Department

FT-ICR mass spectrometry of very highly charged atomic ions

Description: The authors report FT-ICR measurements of samples trapped in the electron beam ion trap (EBIT) at Livermore. They made use of the access provided by the radial x-ray slots. They constructed two probes that could be inserted into the 2-mm wide slots. The probes each subtend only 25{degree} and face each other 180{degree} apart. They show a typical transformed spectrum of highly charged {sup 84}Kr ions obtained with this system. The spectrum was obtained by exciting with a 300-{mu}s sweep from 17.5 to 19.5 MHz. The resonance peaks of {sup 84}Kr{sup 34+}, {sup 84}Kr{sup 35+}, and {sup 84}Kr{sup 36+} are clearly visible.
Date: May 15, 1995
Creator: Schweikhard, L.; Beck, B. & Beiersdorfer, P.
Partner: UNT Libraries Government Documents Department

Fluctuations during melting

Description: Repetitive melting and recrystallization, of crystals in a solid-liquid mixture in quasi-equilibrium, is reported. The experiments were performed at constant volume and ambient temperature inside a diamond anvil cell. The results, which the authors believed are the first to show the importance of fluctuations during a first order phase transition, can be partially understood on the basis of a recent generalized theory of Ostwald ripening.
Date: October 1, 1995
Creator: Grimsditch, M. & Karpov, V.
Partner: UNT Libraries Government Documents Department

Crystal field and exchange interactions in DyT{sub 4}Al{sub 8} (T = Fe and Mn)

Description: The authors investigated the magnetic excitations in polycrystalline samples of hard magnet related compounds DyFe{sub 4}Al{sub 8} and DyMn{sub 4}Al{sub 8} by neutron spectroscopy. For both compounds the magnetic spectra at energies below 40 meV are dominated by the response of the Dy{sup 3+} ions. In DyMn{sub 4}Al{sub 8} they observed pure crystal-field transitions within the Dy{sup 3+} J = 15/2 ground multiplet ID which is split into 8 doublets under the low point-group symmetry of 4/mmm. In DyFe{sub 4}Al{sub 8} the Dy crystal-field-split states are perturbed by the molecular fields of the ordered Fe sublattice.
Date: December 31, 1994
Creator: Loewenhaupt, M.; Tils, P.; Hahn, W. & Loong, C.K.
Partner: UNT Libraries Government Documents Department

Plastic flow in fcc metals induced by single-ion impacts

Description: Irradiation of Au and Pb foils with Xe ions at temperatures between 30 and 450 K has been monitored using in-situ transmission electron microscopy. Single ion impacts give rise to surface craters on the irradiated surface with sizes as large as 12 nm. Approximately 2--5% of impinging ions produce craters on Au while only about 0.6% produce craters on Pb. Larger craters on Au frequently have expelled material associated with them. Temporal details of crater formation and annihilation has been recorded on video with a time resolution of 33 milliseconds. Craters annihilate in discrete steps due to subsequent ion impacts or anneal in a continuous manner due to surface diffusion. Craters production (those persisting for one or more video frames) as a function of temperature indicates that the surface diffusion process responsible for thermal annealing of craters has an activation energy of 0.76 eV in Au. Crater creation results from plastic flow associated with near surface cascades. Crater annihilation in discrete steps results from plastic flow induced by subsequent ion impacts, including those that do not themselves produce a crater.
Date: September 1997
Creator: Birtcher, R. C. & Donnelly, S. E.
Partner: UNT Libraries Government Documents Department

Lattice dynamics of LuPO{sub 4}

Description: Lutetium orthophosphate is an important nonmagnetic host material for rare-earth-activated luminescence applications. We have measured the LuPO{sub 4} phonon density of states and dispersion curves along the [{xi}00],[{xi}{xi}0], and [00{xi}] symmetry directions by neutron spectroscopy using polycrystalline and single-crystal samples. A quantitative analysis of the neutron results was carried out using a lattice-dynamical shell model.
Date: June 1996
Creator: Nipko, J. C.; Loong, C.-K.; Loewenhaupt, M.; Reichardt, W.; Braden, M. & Boatner, L. A.
Partner: UNT Libraries Government Documents Department

One loop graviton self-energy in a locally de Sitter background

Description: The graviton tadpole has recently been computed at two loops in a locally de Sitter background. We apply intermediate results of this work to exhibit the graviton self-energy at one loop. This quantity is interesting both to check the accuracy of the first calculation and to understand the relaxation effect it reveals. In the former context we show that the self-energy obeys the appropriate Ward identity. We also show that its flat space limit agrees with the flat space result obtained by Capper in what should be the same gauge. 15 refs., 4 figs., 10 tabs.
Date: February 1996
Creator: Tsamis, N. C. & Woodard, R.P.
Partner: UNT Libraries Government Documents Department

The formation of metal/metal-matrix nano-composites by the ultrasonic dispersion of immiscible liquid metals

Description: Ultrasonic energy has been used to disperse one liquid metallic component in a second immiscible liquid metal, thereby producing a metallic emulsion. Upon lowering the temperature of this emulsion below the mp of the lowest-melting constituent, a metal/metal-matrix composite is formed. This composite consists of sub-micron-to-micron- sized particles of the minor metallic phase that are embedded in a matrix consisting of the major metallic phase. Zinc-bismuth was used as a model system, and ultrasonic dispersion of a minor Bi liquid phase was used to synthesize metal/metal-matrix composites. These materials were characterized using SEM and energy-dispersive x-ray analysis.
Date: December 1, 1996
Creator: Keppens, V.M.; Mandrus, D.; Boatner, L.A. & Rankin, J.
Partner: UNT Libraries Government Documents Department

How close are hyperdeformed states to the scission point?

Description: The HFB method with the Gogny force is used to study the effects of reflection asymmetry at I = 0{h_bar} on the barriers separating superdeformed and hyperdeformed minima from fission in the {sup 176}W and {sup 168}Yb nuclei. The fission barrier for the HD minimum is reduced by 5 MeV in {sup 176}W when reflection asymmetry is taken into account.
Date: December 1, 1996
Creator: Robledo, L.M.; Egido, J.L. & Chasman, R.R.
Partner: UNT Libraries Government Documents Department

Solid-particle erosion of an Al{sub 2}O{sub 3}-SiC-TiC composite

Description: An electrodischarge-machinable Al{sub 2}O{sub 3}-SiC-TiC composite developed by Industrial Ceramic Technology, Inc., has a high fracture toughness, 9.6{+-}0.6 MPm{sup 1/2}, as measured by indentation, and a Vickers hardness of 20.3{+-}0.6 GPa. The composite`s resistance to solid-particle erosion was measured for 143-{mu}m dia SiC particles impacting at 20-90{degree} angles and 50-100 m/s velocities. Erosion rate exhibited a maximum for normal incidence, and the erosion resistance was better than that of commercial Al{sub 2}O{sub 3}. SEM indicated that material wastage was by a combination of brittle fracture and microplasticity.
Date: November 1, 1996
Creator: Jiang, M.; Goretta, K.C.; Singh, D.; Routbort, J.L. & Schuldies, J.J.
Partner: UNT Libraries Government Documents Department

Atomic electron correlations in intense laser fields

Description: This talk examines two distinct cases in strong optical fields where electron correlation plays an important role in the dynamics. In the first example, strong coupling in a two-electron-like system is manifested as an intensity-dependent splitting in the ionized electron energy distribution. This two-electron phenomenon (dubbed continuum-continuum Autler-Townes effect) is analogous to a strongly coupled two-level, one-electron atom but raises some intriguing questions regarding the exact nature of electron-electron correlation. The second case examines the evidence for two-electron ionization in the strong-field tunneling limit. Although their ability to describe the one-electron dynamics has obtained a quantitative level of understanding, a description of the two (multiple) electron ionization remains unclear.
Date: November 1998
Creator: DiMauro, L. F.; Sheehy, B.; Walker, B.; Agostini, P. A. & Kulander, K. C.
Partner: UNT Libraries Government Documents Department

Sensor-based demand controlled ventilation

Description: In most buildings, occupancy and indoor pollutant emission rates vary with time. With sensor-based demand-controlled ventilation (SBDCV), the rate of ventilation (i.e., rate of outside air supply) also varies with time to compensate for the changes in pollutant generation. In other words, SBDCV involves the application of sensing, feedback and control to modulate ventilation. Compared to ventilation without feedback, SBDCV offers two potential advantages: (1) better control of indoor pollutant concentrations; and (2) lower energy use and peak energy demand. SBDCV has the potential to improve indoor air quality by increasing the rate of ventilation when indoor pollutant generation rates are high and occupants are present. SBDCV can also save energy by decreasing the rate of ventilation when indoor pollutant generation rates are low or occupants are absent. After providing background information on indoor air quality and ventilation, this report provides a relatively comprehensive discussion of SBDCV. Topics covered in the report include basic principles of SBDCV, sensor technologies, technologies for controlling air flow rates, case studies of SBDCV, application of SBDCV to laboratory buildings, and research needs. SBDCV appears to be an increasingly attractive technology option. Based on the review of literature and theoretical considerations, the application of SBDCV has the potential to be cost-effective in applications with the following characteristics: (a) a single or small number of dominant pollutants, so that ventilation sufficient to control the concentration of the dominant pollutants provides effective control of all other pollutants; (b) large buildings or rooms with unpredictable temporally variable occupancy or pollutant emission; and (c) climates with high heating or cooling loads or locations with expensive energy.
Date: July 1, 1997
Creator: De Almeida, A.T. & Fisk, W.J.
Partner: UNT Libraries Government Documents Department

Volume-based considerations for the metal-insulator transition of CMR oxides

Description: The sensitivity of {rho} [electrical resistivity] to changes in volume which occur through: (1) applied pressure, (2) variations in temperature, and (3) phase transitions, is evaluated for some selected CMR oxides. It is argued that the changes in volume associated with phase changes are large enough to produce self pressures in the range of 0.18 to 0.45 GPa. The extreme sensitivity of the electrical resistivity to pressure indicates that these self pressures are responsible for large features in the electrical resistivity and are an important component for occurrence the metallicity below {Tc}. It is suggested that this is related to a strong volume dependence of the electron phonon coupling in the CMR oxides.
Date: March 1, 1998
Creator: Neumeier, J. J.; Hundley, M. F.; Cornelius, A. L. & Andres, K.
Partner: UNT Libraries Government Documents Department

The effect of displacement cascades on small helium bubbles in aluminum and gold

Description: The evolution of individual helium bubbles in thin foils of gold and aluminum irradiated with 400 keV Ar+ and 200 keV Xe+ has been followed with in-situ transmission electron microscopy for a comparison between the effects of dilute (Al) and dense (Au) collision cascades. Bubble shrinkage in Al has been attributed to direct displacement of the gas out of the bubbles. Effects in Au, include the disappearance and Brownian motion of bubbles under irradiation, and are consistent with thermal spike processes seen in molecular dynamics simulations.
Date: December 1, 1994
Creator: Donnelly, S.E.; Valizadeh, R.; Vishnyakov, V.; Birtcher, R.C. & Templier, C.
Partner: UNT Libraries Government Documents Department

Valence proton-neutron interaction strengths from double binding energy differences

Description: Empirical p-n interaction strengths have been extracted from experimental mass data using double-difference binding energy equations. The especially strong interactions for self-conjugate nuclei will be discussed as well as microstructure found for deformed and in doubly magic regions. Valence correlation Schemes provide a basis to comment on the stability of medium mass near-drip-line nuclei and superheavy elements.
Date: 1996-10~
Creator: Brenner, D. S.; Foy, B. D.; Warner, D. D.; Zamfir, N. V. & Casten, R. F.
Partner: UNT Libraries Government Documents Department

Lateral interaction energy derived from Frumkin isotherm for c(2 x 2) Br/Ag(100)

Description: The structure of the bromide adlayer on Ag(100) and the adsorption isotherm have been determined by using in situ surface x-ray scattering techniques and chronocoulometry. Bromide adsorbed on Ag(100) forms a fourfold-hollow-site lattice gas and the adsorption saturates at 1/2 monolayer in a c(2 x 2) structure. The Frumkin isotherm has been employed to fit the experimentally obtained isotherm. Using the experimentally determined electrosorption valency, the lateral interaction energy of 220 meV/atom at full coverage is obtained.
Date: October 1997
Creator: Wang, J. X.; Ocko, B. M. & Wandlowski, T.
Partner: UNT Libraries Government Documents Department

On properties of boundaries and electron conductivity in mesoscopic polycrystalline silicon films for memory devices

Description: The authors present the results of MD modeling on the structural properties of grain boundaries (GB) in thin polycrystalline films. The transition from crystalline boundaries with low mismatch angle to amorphous boundaries is investigated. It is shown that the structures of the GBs satisfy a thermodynamical criterion suggested in a cited reference. The potential energy of silicon atoms is closely related with a geometrical quantity -- tetragonality of their coordination with their nearest neighbors. A crossover of the length of localization is observed to analyze the crossover of the length of localization of the single electron states and properties of conductance of the thin polycrystalline film at low temperature. They use a two-dimensional Anderson localization model, with the random one site electron charging energy for a single grain (dot), random non-diagonal matrix elements, and random number of connections between the neighboring grains. The results on the crossover behavior of localization length of the single electron states and characteristic properties of conductance are presented in the region of parameters where the transition from an insulator to a conductor regimes takes place.
Date: February 1, 1998
Creator: Berman, G.P.; Doolen, G.D.; Mainieri, R.; Rehacek, J.; Campbell, D.K.; Luchnikov, V.A. et al.
Partner: UNT Libraries Government Documents Department

Baryon number non-conservation and the topology of gauge fields

Description: An introduction to the subject of baryon number non-conservation in the electroweak theory at high temperatures of energies is followed by a summary of our discovery of an infinite surface of sphaleron- like configurations which play a key role in baryon number non- conserving transitions in a hot electroweak plasma.
Date: October 1, 1996
Creator: Axenides, M.; Johansen, A.; Nielsen, H.B. & Toernkvist, O.
Partner: UNT Libraries Government Documents Department

Unitarity-based techniques for one-loop calculations in QCD

Description: Perturbative QCD, and jet physics in particular, have matured sufficiently that rather than being merely subjects of experimental studies, they are now tools in the search for new physics. This role can be seen in the search for the top quark, as well as in recent speculations about the implications of supposed high-E{sub T} deviations of the inclusive-jet differential cross section at the Tevatron. One of the important challenges to both theorists and experimenters in coming years will be to hone jet physics as a tool in the quest for physics underlying the standard model. As such, it will be important to measurements of parameters of the theory or of non-perturbative quantities such as the parton distribution functions, as well as to searches for new physics at the LHC. Jet production, or jet production in association with identified photons or electroweak vector bosons, appears likely to provide the best information on the gluon distribution in the proton, and may also provide useful information on the strong coupling {alpha}{sub s}. In order to make use of these final states, the authors need a wider variety of higher-order calculations of matrix elements. Indeed, as the authors shall review, next-to-leading order calculations are in a certain sense the minimal useful ones. On the other hand, these calculations are quite difficult with conventional Feynman rules. In the following sections, the authors will discuss some of the techniques developed in recent years to simplify such calculations.
Date: June 1, 1996
Creator: Bern, Z.; Dixon, L. & Kosower, D.A.
Partner: UNT Libraries Government Documents Department

Efficient analytic computation of higher-order QCD amplitudes

Description: The authors review techniques simplifying the analytic calculation of one-loop QCD amplitudes with many external legs, for use in next-to-leading-order corrections to multi-jet processes. Particularly useful are the constraints imposed by perturbative unitarity, collinear singularities and a supersymmetry-inspired organization of helicity amplitudes. Certain sequences of one-loop helicity amplitudes with an arbitrary number of external gluons have been obtained using these constraints.
Date: March 1, 1995
Creator: Bern, Z.; Chalmers, G.; Dixon, L.; Dunbar, D.C. & Kosower, D.A.
Partner: UNT Libraries Government Documents Department