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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

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


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

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

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

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

The electronic structure of heavy fermions: Narrow temperature independent bands

Description: The electronic structure of both Ce and U heavy fermions appears to consist of extremely narrow temperature independent bands. There is no evidence from photoemission for a collective phenomenon normally referred to as the Kondo resonance. In uranium compounds a small dispersion of the bands is easily measurable.
Date: August 1, 1996
Creator: Arko, A.J.; Joyce, J.J.; Smith, J.L. & Andrews, A.B.
Partner: UNT Libraries Government Documents Department

Baryon-baryon mixing in hypernuclei

Description: Implications of few-body hypernuclei for the understanding of the baryon-baryon interaction are examined. Octet-octet coupling effects not present in conventional, non strange nuclei are the focus. The need to identify strangeness {minus}2 hypernuclei to test model predictions is emphasized.
Date: May 1998
Creator: Gibson, B. F.
Partner: UNT Libraries Government Documents Department

S = {minus}1 and S = {minus}2 few-body hypernuclei

Description: The S = {minus}1 and S = {minus}2 few-body bound states are the focus of this discussion. Implications for the understanding of the baryon-baryon interaction are examined. Octet-octet coupling effects not found in conventional, non-strange nuclei are highlighted. TJNAF wave function tests for the S = {minus}1 sector are noted. The need to identify S = {minus}2 hypernuclei to explore the S = {minus}2 strong interaction and to test model predictions is emphasized.
Date: November 1997
Creator: Gibson, B. F.
Partner: UNT Libraries Government Documents Department

Photoelectron Spectroscopy of YbInCu{sub 4}: Direct Testing of Correlated Electron Models

Description: The electronic properties of single crystal YbInCu{sub 4} have been investigated by means of high resolution photoelectron spectroscopy. A first order, isostructural phase transition for YbInCu{sub 4} at T{sub v}=42 K leads to changes in the Kondo temperature of more than an order of magnitude (27 K vs. 400 K). This phase transition and accompanying Kondo temperature change provide the most direct test of the single impurity model (SIM) to date. Particle hole symmetry allows the SIM to be used for Yb compounds as well as Ce heavy fermions with the great advantage that the predicted Kondo resonance is found on the occupied side of the spectral weight function for Yb materials and is thus directly observable in photoemission. The photoemission results are incongruous with the single impurity model predictions for temperature dependence, binding energy and 4f occupancy, encouraging a reevaluation of the single impurity model. The experiments were conducted using the PGM undulator and 4 meter NIM beamlines at SRC. The spectra were taken at photon energies of 40 eV and 90 eV and the combined energy resolution of the analyzer and monochromator was 45- 85 meV.
Date: December 31, 1997
Creator: Joyce, J.J.; Arko, A.J.; Sarrao, J.L. & Fisk, Z.
Partner: UNT Libraries Government Documents Department

Crossing behavior of the singlet and triplet State of the negatively charged magneto-exciton in a GaAs/AlGaAs quantum well

Description: Polarized magneto-photoluminescence (MPL) measurements on a high mobility {delta}-doped GaAs/AlGaAs single quantum well from 0--60 T at temperatures between 0.37--2.1 K are reported. In addition to the neutral heavy hole magneto-exciton (X{sup 0}), the singlet (X {sub s}{sup {minus}}) and triplet (X {sub t}{sup {minus}}) states of the negatively charged magneto-exciton are observed in both polarizations. The energy dispersive and time-resolved MPL data suggest that their development is fundamentally related to the formation of the neutral magneto-exciton. At a magnetic field of 40 T the singlet and the triplet states cross as a result of the role played by the higher Landau levels and higher energy subbands in their energetic evolution, confirming theoretical predictions. The authors also observed the formation of two higher energy peaks. One of them is completely right circularly polarized and its appearance can be considered a result of the electron-hole exchange interaction enhancement with an associated electron g-factor of 3.7 times the bulk value. The other peak completely dominates the MPL spectrum at fields around 30 T. Its behavior with magnetic field and temperature indicates that it may be related to previous anomalies observed in the integer and fractional quantum Hall regimes.
Date: January 27, 2000
Partner: UNT Libraries Government Documents Department

New approach for the determination of the nuclear level density parameters

Description: The nuclear level density parameters of the stable nuclides have been determined from the neutron resonance data on the basis of new values for the spin dispersion parameter. The latter parameter is deduced from the spin dependent average level spacings of s-wave neutron resonances of odd target nuclides. In addition, the temperature dependence of the level density parameter is studied and an analytical relation for it is presented.
Date: August 1, 1998
Creator: Mughabghab, S.F. & Dunford, C.L.
Partner: UNT Libraries Government Documents Department

Calculation of the {alpha}-particle ground state

Description: The Correlated Hyperspherical Harmonic expansion method is used to calculate alpha-particle properties with a realistic hamiltonian consisting of the Argonne V14 two nucleon and Urbana model VIII three nucleon potentials. The calculated binding energy, mass radius and wave percentages are close to the corresponding quantities obtained with Green's Function Monte Carlo and Faddeev-Yakubovsky techniques.
Date: October 1, 1994
Creator: Viviani, M.; Kievsky, A. & Rosati, S.
Partner: UNT Libraries Government Documents Department

Fast Through-Bond Diffusion of Nitrogen in Silicon

Description: The authors report first principles total energy calculations of interaction of nitrogen in silicon with silicon self-interstitials. Substitutional nitrogen captures a silicon interstitial with 3.5 eV binding energy forming a {l_angle}001{r_angle} split interstitial ground state geometry, with the nitrogen forming three bonds. The low energy migration path is through a bond bridge state having two bonds. Fast diffusion of nitrogen occurs through a pure interstitialcy mechanism; the nitrogen never has less than two bonds. Near-zero formation energy of the nitrogen interstitialcy with respect to the substitutional rationalizes the low solubility of substitutional nitrogen in silicon.
Date: July 12, 2000
Partner: UNT Libraries Government Documents Department

Ion kinetics and thermochemistry pertinent to mass spectrometric organic speciation. Progress report, November 15, 1992--14 November 1995

Description: Essentially all of the completed/in progress studies during the last contract period have involved the NIST HPMS unit. Three distinct areas of in-house research are recognizable: (i) determinations of binding energies and entropies for association and cluster ions, which is accomplished by measuring the temperature dependence of the appropriate equilibrium, (ii) measurement of the temperature dependence unimolecular and bimolecular rate constants, and (iii) evaluation of PA`s, HA`s, and IP`s via measurement of variable-temperature equilibria of the type AH{sup +} + B {leftrightarrow} BH{sup +} + A, A{sup -} + BH {leftrightarrow} AH + B{sup -}, and A{sup +} + B {leftrightarrow} B+ + A. Key results from some representative projects are summarized below.
Date: May 1, 1998
Creator: Sieck, L.W.
Partner: UNT Libraries Government Documents Department

Ab initio calculation of tight-binding parameters

Description: We calculate ab initio values of tight-binding parameters for the f- electron metal Ce and various phases of Si, from local-density functional one-electron Hamiltonian and overlap matrix elements. Our approach allows us to unambiguously test the validity of the common minimal basis and two-center approximations as well as to determine the degree of transferability of both nonorthogonal and orthogonal hopping parameters in the cases considered.
Date: December 1, 1997
Creator: McMahan, A. K. & Klepeis, J. E.
Partner: UNT Libraries Government Documents Department

Relativistic calculation of the triton binding energy and its implications

Description: First results for the triton binding energy obtained from the relativistic spectator or Gross equation are reported. The Dirac structure of the nucleons is taken into account. Numerical results are presented for a family of realistic OBE models with off-shell scalar couplings. It is shown that these off-shell couplings improve both the fits to the two-body data and the predictions for the binding energy.
Date: July 1, 1996
Creator: Stadler, Alfred & Gross, Franz
Partner: UNT Libraries Government Documents Department

Theoretical dynamical studies of metal clusters and cluster-ligand systems

Description: In what follows we use the term cluster to designate a cohesive group of like atoms (molecules), i.e., bare ({open_quotes}neet{close_quotes} {open_quotes}naked{close_quotes}) clusters. More generally, the term is also used for organo- and inorganometallic compounds, i.e., ligated clusters. Although the approaches and techniques used by the various disciplines to study metal-ligand interactions are quite different, many of the central subjects and issues are common for them. The common subjects include possible geometric structures and isomeric forms, structural (isomerization) transitions, stability, fluxionality, structure-reactivity correlation (or lack of it), role of coordination, etc. However, the precise interpretation of these issues and the details emphasized by the different disciplines are dictated by the nature of the objects studied and may not, therefore, be identical. For example, questions regarding structures, isomerization transitions, fluxionality or even melting of metal clusters refer to the state and properties of the metal network itself. The same questions, when asked in connection with organo- and inorganometallic compounds, often refer to the arrangements and rearrangements of the ligands attached to a metal framework of a fixed structure. Of course, when required, the state of and changes in the metal framework are considered as well. The fields of metal-containing molecular compounds, surface science, and physics and chemistry of clusters furnish complementary information on a broad variety of metal-ligand systems. A comprehensive understanding of the nature and properties of these systems, as defined by the type and number of metal atoms and ligands involved, can be achieved only through a mutual awareness of and continuing progress in all of these research areas.
Date: June 1, 1995
Creator: Jellinek, J.
Partner: UNT Libraries Government Documents Department

Theoretical study of ion/macrocycle interactions using both hybrid quantum mechanical-molecular mechanical and ab initio methods

Description: Synthetic macrocycles have drawn much experimental and theoretical interest since Pederson first synthesized the crown-ether 18-crown-6 (18c6) in 1967. Crown-ethers show a remarkable range of specificity for a wide variety of cations that depends, in part, on the size of the ether, the type of donor atoms (e.g. oxygen, nitrogen, sulfur), and the polarity of the solvent. Crown-ethers and related macrocycles are of particular interest to research efforts in chemical separations applied to environmental remediation. For example, at the Hanford nuclear facility {sup 9O}Sr{sup 2+} and {sup 137}Cs{sup +} are two major generators of heat which complicate the disposal of nuclear waste. One example of the use of crown-ethers for radionuclide separation is the Strontium Extraction (SREX) process which uses di-t-butylcyclohexano-18-crown-6 for recovering {sup 9O}Sr{sup 2+} from acidic solution. A more thorough understanding of fundamental interactions of cation/crown-ether solution chemistry may provide the basis for rational design of new ligands useful in the separation of these and other radionuclides from radionuclide-containing waste streams at hazardous waste storage facilities. There is also a growing interest in the use of crown-ethers, cryptands, and other ligands for use in chemical sensors. Specifically, fluoroionophores, which consisting of a fluorophore (e.g. dye-molecule) linked to an ionophore (e.g. crown-ether), exhibit measurable changes in the photophysical properties of the fluorophore upon ion binding by the ionophore. Fluoroionophores would be useful for monitoring ground-water aquifers and industrial effluent streams for low-levels of hazardous radionuclides and other toxic metals.
Date: August 1, 1995
Creator: Thompson, M.A.; Glendening, E.D.; Feller, D. & Kendall, R.
Partner: UNT Libraries Government Documents Department

Fate of neptunium in an anaerobic, methanogenic microcosm.

Description: Neptunium is found predominantly as Np(IV) in reducing environments, but Np(V) in aerobic environments. However, currently it is not known how the interplay between biotic and abiotic processes affects Np redox speciation in the environment. In order to evaluate the effect of anaerobic microbial activity on the fate of Np in natural systems, Np(V) was added to a microcosminoculated with anaerobic sediments from a metal-contaminated fresh water lake. The consortium included metal-reducing, sulfate-reducing, and methanogenic microorganisms, and acetate was supplied as the only exogenous substrate. Addition of more than 10{sup {minus}5} M Np did not inhibit methane production. Total Np volubility in the active microcosm, as well as in sterilized control samples, decreased by nearly two orders of magnitude. A combination of analytical techniques, including VIS-NIR absorption spectroscopy and XANES, identified Np(IV) as the oxidation state associated with the sediments. The similar results from the active microcosm and the abiotic controls suggest that microbian y produced Mn(II/HI) and Fe(II) may serve as electron donors for Np reduction.
Date: December 21, 1998
Creator: Banaszak, J. E.
Partner: UNT Libraries Government Documents Department