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Resonantly excited high-density exciton gas studiedvia broadbandTHz spectroscopy

Description: We report the density-dependent crossover of a resonantly photoexcited exciton gas from insulating to conducting phases. Broadband terahertz spectroscopy gives direct access to the exciton binding energy via intra-excitonic 1s-2p transitions. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with decreasing inter-particle distance. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite free-carrier and exciton terahertz conductivity. Comparison with near-infrared absorption changes illustrates a significantly enhanced energy shift and broadening of the intra-excitonic resonance.
Date: June 25, 2005
Creator: Huber, Rupert; Kaindl, Robert A.; Schmid, Ben A. & Chemla, Daniel S.
Partner: UNT Libraries Government Documents Department

Surface core-level binding energy shifts for MgO(100)

Description: This article presents theoretical and experimental results for the surface core-level binding energy, BE, shifts, SCLS, for MgO(100) and the anomalous O(1s) SCLS is interpreted in terms of the surface electronic structure.
Date: July 29, 2014
Creator: Nelin, Connie J.; Uhl, Felix; Staemmler, Volker; Bagus, Paul S.; Fujimori, Yuichi; Sterrer, Martin et al.
Partner: UNT College of Arts and Sciences


Description: The activity coefficients of HCl and NaCl in HCl--NaCl mixtures were computed from literature data. The calculations are based on the observation that at constant ionic strength and temperature the logarithm of the activity coefficient of HCi in HCl--NaCl mixtures varies linearly with NaCl concentration. (auth)
Date: July 29, 1963
Creator: Lietzke, M.H. & Stoughton, R.W.
Partner: UNT Libraries Government Documents Department


Description: Normal-coordinate calculations for the S/sub 8/ molecule were revised by use of new far-infrared spectral data and a more elaborate potential function. Representation of the observed frequencies required that the basic Urey--Bradley field be supplemented with an extra quadratic cross-term between adjacent S--S bonds. (auth)
Date: January 1, 1962
Creator: Kruse, F. H.; Scott, D. W. & McCullough, J. P.
Partner: UNT Libraries Government Documents Department

Tight-binding molecular dynamics simulations on point defects diffusion and interactions in crystalline silicon

Description: Tight-binding molecular dynamics (TBMD) simulations are performed (i) to evaluate the formation and binding energies of point defects and defect clusters, (ii) to compute the diffusivity of self-interstitial and vacancy in crystalline silicon, and (iii) to characterize the diffusion path and mechanism at the atomistic level. In addition, the interaction between individual defects and their clustering is investigated.
Date: January 23, 1995
Creator: Tang, M.; Diaz de la Rubia, T. & Colombo, L.
Partner: UNT Libraries Government Documents Department

Binding energies of hypernuclei and hypernuclear interactions

Description: In part 1 the effect of nuclear core dynamics on the binding energies of {Lambda} hypernuclei is discussed in the framework of variational correlated wave functions. In particular, the authors discuss a new rearrangement energy contribution and its effect on the core polarization. In part 2 they consider the interpretation of the {Lambda} single-particle energy in terms of basic {Lambda}-nuclear interactions using a local density approximation based on a Fermi hypernetted chain calculation of the A binding to nuclear matter. To account for the data strongly repulsive 3-body {Lambda}NN forces are required. Also in this framework they discuss core polarization for medium and heavier hypernuclei.
Date: May 1, 1996
Creator: Bodmer, A.R.; Murali, S. & Usmani, Q.N.
Partner: UNT Libraries Government Documents Department

The congruence energy: A contribution to nuclear masses and deformation energies

Description: The difference between measured binding energies and those calculated using a shell- and pairing-corrected Thomas-Fermi model can be described approximately by C(I) = {minus}10exp({minus}4.2{vert_bar}I{vert_bar}) MeV. The authors` interpretation of this extra binding is in terms of the granularity of quantal nucleonic density distributions, which leads to a stronger interaction for a neutron and proton with congruent nodal structures of their wave functions. The predicted doubling of this congruence energy in fission is supported by an analysis of measured fission barriers and by a study of wave functions in a dividing Hill-Wheeler box potential. A semi-empirical formula for the shape-dependent congruence energy is described.
Date: June 1, 1995
Creator: Myers, W.D. & Swiatecki, W.J.
Partner: UNT Libraries Government Documents Department

Some Comments on the Fission to Capture Cross Section Ratio

Description: Abstract: An attempt is made to correlate the thermal neutron fission cross sections of various nuclides. An empirical relationship is observed when the logs of the ratios [thermal neutron fission cross section of (Z,A)/thermal neutron capture cross section of (Z,A)] for several nuclides are plotted against their neutron binding energies [binding energy of a neutron to (Z,A)]. The log of the σf/σc ratio decreases markedly with decreasing neutron binding energy."
Date: June 1952
Creator: Huizenga, John R.
Partner: UNT Libraries Government Documents Department

THE ADIABATIC ELASTIC MODULI OF SINGLE-CRYSTAL ALPHA URANIUM AT 25 C. Work completed: January 1958. Partial Report-Metallurgy Program 4.1.16

Description: The 9 single-crystal elastic moduli pertaining to principal crystallographic axes of alpha U at 25 deg C were determined from measurements of high-frequency wave velocities for 21 modes in seven single-crystal specimens, using the phase-comparison method of McSkimin. From the results the elastic compliances, compressibilities, and Poisson"s ratios were computed for the principal axes. Th variations with crystal direction of the stiffness moduli, Young's moduli, and rigidity moduli were plotted. The nature of the anisotropy for the different moduli indicated that the nearest neighbor interatomic bonds are considerably stiffer than the next nearest bonds, which are only slightly larger in interatomic distance. (auth)
Date: June 1, 1960
Creator: Fisher, E. S.
Partner: UNT Libraries Government Documents Department

Effects of d-electrons in pseudopotential screened-exchange density functional calculations

Description: We report a theoretical study on the role of shallow d states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems. We found that the inaccurate pseudo-wavefunctions can lead to (1) an overestimation of the screened-exchange interaction between the localized d states and the delocalized higher energy s and p states and (2) an underestimation of the screened-exchange interaction between the d states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wavefunctions of d states by including the semicore s and p states of the same shell in the valence states. The correction of pseudo-wavefunctions yields band gaps and the d state binding energy with good agreements with experiments and the full potential linearized augmented planewave (FLAPW) calculations. Compared with the quasi-particle GW method, our sX-LDA results shows not only similar quality on the band gaps but also much better d state binding energy. Combined with its capability of ground state structure calculation, the sX-LDA is expected to be a valuable theoretical tool for the II-VI and III-V (especially the III-N) bulk semiconductors and nanostructure studies.
Date: August 11, 2008
Creator: Wang, Lin-Wang; Lee, Byounghak; Canning, Andrew & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Water adsorption on O(2x2)/Ru(0001) from STM experiments andfirst-principles calculations

Description: We present a combined theoretical and experimental study of water adsorption on Ru(0001) pre-covered with 0.25 monolayers (ML) of oxygen forming a (2 x 2) structure. Several structures were analyzed by means of Density Functional Theory calculations for which STM simulations were performed and compared with experimental data. Up to 0.25 monolayers the molecules bind to the exposed Ru atoms of the 2 x 2 unit cell via the lone pair orbitals. The molecular plane is almost parallel to the surface with its H atoms pointing towards the chemisorbed O atoms of the 2 x 2 unit cell forming hydrogen bonds. The existence of these additional hydrogen bonds increases the adsorption energy of the water molecule to approximately 616 meV, which is {approx}220 meV more stable than on the clean Ru(0001) surface with a similar configuration. The binding energy shows only a weak dependence on water coverage, with a shallow minimum for a row structure at 0.125 ML. This is consistent with the STM experiments that show a tendency of the molecules to form linear rows at intermediate coverage. Our calculations also suggest the possible formation of water dimers near 0.25 ML.
Date: October 15, 2007
Creator: Cabrera-Sanfelix, P.; Sanchez-Portal, D.; Mugarza, A.; Shimizu,T.K.; Salmeron, M. & Arnau, A.
Partner: UNT Libraries Government Documents Department

An Internal Report: Experimental Proof of Dynamical Spin Shielding in Ce from Spin-Resolved Photoelectron Spectroscopy

Description: Using Fano Effect measurements upon polycrystalline Ce, we have observed a phase reversal between the spectral structure at the Fermi Edge and the other 4f derived feature near a binding energy of 2 eV. The Fano Effect is the observation of spin polarized photoelectron emission from NONMAGNETIC materials, under chirally selective excitation, such as circularly polarized photons. Within various models, the peak at the Fermi Energy (f{sup 1} peak, quasiparticle peak, Kondo peak) is predicted to be the manifestation of the electrons which shield the otherwise unpaired spin associated with the peak at 2 eV (f{sup 0} peak or Lower Hubbard Band). Utilizing high-energy photoelectron spectroscopy, on and off resonance, the bulk nature and f-character of both features have been confirmed. Thus, observation of phase reversal between the f{sup 0} and f{sup 1} peak is a direct experimental proof of spin shielding in Ce, confirming the original model of Gunnarsson and Shoenhammer, albeit within a Hubbard picture.
Date: March 31, 2006
Creator: Tobin, J G; Yu, S W; Komesu, T; Chung, B W; Morton, S A & Waddill, G D
Partner: UNT Libraries Government Documents Department

Structure of A = 10 - 13 Nuclei with Two- Plus Three-Nucleon Interactions from Chiral Effective Field Theory

Description: Properties of finite nuclei are evaluated with two-nucleon (NN) and three-nucleon (NNN) interactions derived within chiral effective field theory (EFT). The nuclear Hamiltonian is fixed by properties of the A = 2 system, except for two low-energy constants (LECs) that parameterize the short range NNN interaction. We constrain those two LECs by a fit to the A = 3 system binding energy and investigate sensitivity of {sup 4}He, {sup 6}Li, {sup 10,11}B and {sup 12,13}C properties to the variation of the constrained LECs. We identify a preferred choice that gives globally the best description. We demonstrate that the NNN interaction terms significantly improve the binding energies and spectra of mid-p-shell nuclei not just with the preferred choice of the LECs but even within a wide range of the constrained LECs. At the same time, we find that a very high quality description of these nuclei requires further improvements to the chiral Hamiltonian.
Date: January 10, 2007
Creator: Navratil, P; Gueorguiev, V; Vary, J P; Ormand, W E & Nogga, A
Partner: UNT Libraries Government Documents Department

Spectra and Binding Energy Predictions of Chiral Interactions for 7Li

Description: Using the no-core shell model approach, we report on the first results for {sup 7}Li based on the next-to-next-to-leading order chiral nuclear interaction. Both, two-nucleon and three-nucleon interactions are taken into account. We show that the p-shell nuclei are sensitive to the subleading parts of the chiral interactions including three-nucleon forces. Though chiral interactions are soft, we do not observe overbinding for this p-shell nucleus and find a realistic description for the binding energy, excitation spectrum and radius.
Date: November 30, 2005
Creator: Nogga, A; Navratil, P; Barrett, B R & Vary, J P
Partner: UNT Libraries Government Documents Department

Relativistic Bound States

Description: The Hamiltonian for Dirac's second-order equation depends nonlinearly on the potential V and the energy E. For this reason the magnetic contribution to the Hamiltonian for s-waves, which has a short range, is attractive for a repulsive Coulomb potential (V > 0) and repulsive for an attractive Coulomb potential (V < 0). Previous studies are confined to the latter case, where strong net attraction near a high-Z nucleus accelerates electrons to velocities close to the speed of light. The Hamiltonian is linear in the product EV/mc{sup 2}. Usually solutions are found in the regime E = mc{sup 2} + {var_epsilon}, where except for high Z, |{var_epsilon}| << mc{sup 2}. Here they show that for V > 0 the attractive magnetic term and the repulsive linear term combine to support a bound state at E = 0.5 mc{sup 2} corresponding to a binding energy E{sub b} = -{var_epsilon} = 0.5 mc{sup 2}.
Date: December 12, 2005
Creator: Ritchie, A B
Partner: UNT Libraries Government Documents Department

LDRD final report on light-powered nanovehicles.

Description: We have investigated the possibility of constructing nanoscale metallic vehicles powered by biological motors or flagella that are activated and powered by visible light. The vehicle's body is to be composed of the surfactant bilayer of a liposome coated with metallic nanoparticles or nanosheets grown together into a porous single crystal. The diameter of the rigid metal vesicles is from about 50 nm to microns. Illumination with visible light activates a photosynthetic system in the bilayer that can generate a pH gradient across the liposomal membrane. The proton gradient can fuel a molecular motor that is incorporated into the membrane. Some molecular motors require ATP to fuel active transport. The protein ATP synthase, when embedded in the membrane, will use the pH gradient across the membrane to produce ATP from ADP and inorganic phosphate. The nanoscale vehicle is thus composed of both natural biological components (ATPase, flagellum; actin-myosin, kinesin-microtubules) and biomimetic components (metal vehicle casing, photosynthetic membrane) as functional units. Only light and storable ADP, phosphate, water, and weak electron donor are required fuel components. These nano-vehicles are being constructed by self-assembly and photocatalytic and autocatalytic reactions. The nano-vehicles can potentially respond to chemical gradients and other factors such as light intensity and field gradients, in a manner similar to the way that magnetic bacteria navigate. The delivery package might include decision-making and guidance components, drugs or other biological and chemical agents, explosives, catalytic reactors, and structural materials. We expected in one year to be able only to assess the problems and major issues at each stage of construction of the vehicle and the likely success of fabricating viable nanovehicles with our biomimetic photocatalytic approach. Surprisingly, we have been able to demonstrate that metallized photosynthetic liposomes can indeed be made. We have completed the synthesis of metallized liposomes with photosynthetic ...
Date: November 1, 2003
Creator: Shelnutt, John Allen; van Swol, Frank B.; Miller, James Edward; Pereira, Eulalia; Qiu, Yan; Jiang, Ying-Bing et al.
Partner: UNT Libraries Government Documents Department

On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. [Part] 5. Comparison and Properties of Electrochemical and Chemical Rate Constants

Description: Using a theory of electron transfers which takes cognizance of reorganization of the medium outside the inner coordination shell and of changes of bond lengths inside it, relations between electrochemical and related chemical rate constants are deduced and compared with the experimental data. A correlation is found, without the use of arbitrary parameters. Effects of weak complexes with added electrolytes are included under specified conditions. The deductions offer a way of coordinating a variety of data in the two fields, internally as well as with each those in another. For example, the rate of oxidation or reduction of a series of related reactants by one reagent is correlated with that of another and with that of the corresponding electrochemical oxidation-reduction reaction, under certain specified conditions. These correlations may also provide a test for distinguishing an electron from an atom transfer mechanism. (auth)
Date: April 1, 1963
Creator: Marcus, R. A.
Partner: UNT Libraries Government Documents Department

In situ groundwater bioremediation

Description: In situ groundwater bioremediation of hydrocarbons has been used for more than 40 years. Most strategies involve biostimulation; however, recently bioaugmentation have been used for dehalorespiration. Aquifer and contaminant profiles are critical to determining the feasibility and strategy for in situ groundwater bioremediation. Hydraulic conductivity and redox conditions, including concentrations of terminal electron acceptors are critical to determine the feasibility and strategy for potential bioremediation applications. Conceptual models followed by characterization and subsequent numerical models are critical for efficient and cost effective bioremediation. Critical research needs in this area include better modeling and integration of remediation strategies with natural attenuation.
Date: February 1, 2009
Creator: Hazen, Terry C.
Partner: UNT Libraries Government Documents Department

Effects of organic carbon supply rates on mobility of previously bioreduced uranium in a contaminated sediment

Description: Bioreduction-based strategies for remediating uranium (U)-contaminated sediments face the challenge of maintaining the reduced status of U for long times. Because groundwater influxes continuously bring in oxidizing terminal electron acceptors (O{sub 2}, NO{sub 3}{sup -}), it is necessary to continue supplying organic carbon (OC) to maintain the reducing environment after U bioreduction is achieved. We tested the influence of OC supply rates on mobility of previously microbial reduced uranium U(IV) in contaminated sediments. We found that high degrees of U mobilization occurred when OC supply rates were high, and when the sediment still contained abundant Fe(III). Although 900 days with low levels of OC supply minimized U mobilization, the sediment redox potential increased with time as did extractable U(VI) fractions. Molecular analyses of total microbial activity demonstrated a positive correlation with OC supply and analyses of Geobacteraceae activity (RT-qPCR of 16S rRNA) indicated continued activity even when the effluent Fe(II) became undetectable. These data support our earlier hypothesis on the mechanism responsible for re-oxidation of microbial reduced U(IV) under reducing conditions; that microbial respiration caused increased (bi)carbonate concentrations and formation of stable uranyl carbonate complexes, thereby shifted U(IV)/U(VI) equilibrium to more reducing potentials. The data also suggested that low OC concentrations could not sustain the reducing condition of the sediment for much longer time.
Date: May 15, 2008
Creator: Wan, J.; Tokunaga, T.K.; Kim, Y.; Brodie, E.; Daly, R.; Hazen, T.C. et al.
Partner: UNT Libraries Government Documents Department

Subsurface Biogeochemical Heterogeneity (Field-scale removal of U(VI) from groundwater in an alluvial aquifer by electron donor amendment)

Description: Determine if biostimulation of alluvial aquifers by electron donor amendment can effectively remove U(VI) from groundwater at the field scale. Uranium contamination in groundwater is a significant problem at several DOE sites. In this project, the possibility of accelerating bioreduction of U(VI) to U(IV) as a means of decreasing U(VI) concentrations in groundwater is directly addressed by conducting a series of field-scale experiments. Scientific goals include demonstrating the quantitative linkage between microbial activity and U loss from groundwater and relating the dominant terminal electron accepting processes to the rate of U loss. The project is currently focused on understanding the mechanisms for unexpected long-term ({approx}2 years) removal of U after stopping electron donor amendment. Results obtained in the project successfully position DOE and others to apply biostimulation broadly to U contamination in alluvial aquifers.
Date: June 1, 2006
Creator: Long, Philip E.; Lovley, Derek R.; N’Guessan, A. L.; Nevin, Kelly; Resch, C. T.; Arntzen, Evan et al.
Partner: UNT Libraries Government Documents Department

Structure, energy, and electronic properties of the {Sigma} = 13 {l_brace}510{r_brace} tilt grain boundary structure in Si

Description: The authors have examined a variety of structures for the {l_brace}510{r_brace} symmetric tilt boundary in Si, using first-principles calculations. These calculations show that the observed structure in Si is the lowest energy structure. This structure is more complicated than what is necessary to preserve four-fold coordination. They compare the results to classical and tight-binding models, in order to test these empirical approaches.
Date: December 31, 1997
Creator: Morris, J.R.; Lu, Z.Y.; Ring, D.M.; Xiang, J.B.; Ho, K.M.; Wang, C.Z. et al.
Partner: UNT Libraries Government Documents Department