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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Local atomic and electronic structure of oxide/GaAs and SiO2/Si interfaces using high-resolution XPS

The chemical structures of thin SiO2 films, thin native oxides of GaAs (20-30 A), and the respective oxide-semiconductor interfaces, have been investigated using high-resolution X-ray photoelectron spectroscopy. Depth profiles of these structures have been obtained using argon ion bombardment and wet chemical etching techniques. The chemical destruction induced by the ion profiling method is shown by direct comparison of these methods for identical samples. Fourier transform data-reduction methods based on linear prediction with maximum entropy constraints are used to analyze the discrete structure in oxides and substrates. This discrete structure is interpreted by means of a structure-induced charge-transfer model.

Grunthaner, F. J.↗
NASA NTRS1982

Proton NMR studies of the electronic structure of ZrH/sub x/

The proton spin lattice relaxation times and Knight shifts were measured in f.c.c. (delta-phase) and f.c.t. (epsilon-phase) ZrH/sub x/ for 1.5 or = to x or = to 2.0. Both parameters indicate that N(E/sub F/) is very dependent upon hydrogen content with a maximum occurring at ZrH1 83. This behavior is ascribed to modifications in N(E/sub F/) through a fcc/fct distortion in ZrH/sub x/ associated with a Jahn-Teller effect.

Attalla, A.↗
NASA NTRS1984

Electronic structure and properties of magnetic defects in Co(1+x)Al(1-x) and Fe(1+x)Al(1-x) alloys

CoAl and FeAl compounds are developed along two directions. Magnetic susceptibility and specific heat at low temperature on (NiCo)Al and (CoFe)Al ternary alloys are in good agreement with band calculations. Results on magnetization and specific heat under field at low temperature on nonstoichiometric compounds show clearly the importance of the nearest neighbor effects. In the case of CoAl, the isolated cobalt atoms substituting aluminum are characterized by a Kondo behavior, and, for FeAl, the isolated extra iron atoms are magnetic and polarize the matrix. Moreover, for the two compounds, clusters of higher order play a considerable part in the magnetic properties for CoAl, these clusters also seem to be characterized by a Kondo behavior, for FeAl, these clusters whose moment is higher than in the case of isolated atoms, could be constituted of excess parts of iron atoms.

Abbe, D.↗
NASA NTRS1985

On 3d bonding in the transition metal trimers - The electronic structure of equilateral triangle Ca3, Sc3, Sc3(+), and Ti3(+)

It is pointed out that transition metals and transition metal (TM) compounds are currently of considerable interest because of their relevance to catalysis and to materials science problems such as hydrogen embrittlement and crack propagation in metals. The present paper is concerned with complete active space Self-Consistent Field (SCF) externally contracted configuration interaction (CASSCF/CCI) calculations for the low-lying states of Sc3 and Sc3(+). A comparison is conducted regarding the bonding in the Ca3, Sc3, and Cu3 molecules. This comparison makes it possible to predict general trends for the TM trimers. Attention is given to the qualitative features of the bonding in the TM trimers, the basis sets and other technical details of the calculations, the calculated results for Sc3 and Sc3(+), and conclusions from this work.

Walch, S. P.↗
NASA NTRS1988

Symmetry and equivalence restrictions in electronic structure calculations

A simple method for obtaining MCSCF orbitals and CI natural orbitals adapted to degenerate point groups, with full symmetry and equivalnece restrictions, is described. Among several advantages accruing from this method are the ability to perform atomic SCF calculations on states for which the SCF energy expression cannot be written in terms of Coulomb and exchange integrals over real orbitals, and the generation of symmetry-adapted atomic natural orbitals for use in a recently proposed method for basis set contraction.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1990

Electronic structure Fermi liquid theory of high T(sub c) superconductors: Comparison with experiments

For years, there has been controversy on whether the normal state of the Cu-oxide superconductors is a Fermi liquid or some other exotic ground state. However, some experimentalists are clarifying the nature of the normal state of the high T(sub c) superconductors by surmounting the experimental difficulties in producing clean, well characterized surfaces so as to obtain meaningful high resolved photoemission data, which agrees with earlier positron-annihilation experiments. The experimental work on high resolution angle resolved photoemission by Campuzano et al. and positron-annihilation studies by Smedskjaer et al. has verified the calculated Fermi surfaces in YBa2Cu3O7 superconductors and has provided evidence for the validity of the energy band approach. Similar good agreement was found for Bi2Sr2CaCu2O8 by Olson et al. As a Fermi liquid (metallic) nature of the normal state of the high T(sub c) superconductors becomes evident, these experimental observations have served to confirm the predictions of the local density functional calculations and hence the energy band approach as a valid natural starting point for further studies of their superconductivity.

Freeman, A. J.↗
NASA NTRS1989

On the electronic structure and spectrum of CuF2 and CuCl2

Results are reported from SDCI and CPF ab initio computations for the ground and low-lying states of CuF2 and CuCl2. A large Gaussian basis set with functions up to the g level is employed, and the results are presented in tables and characterized in detail. Particular attention is given to the bond distances and force constants for the ligand-field states and the bond distances for the charge-transfer states. Good agreement is obtained between the CPF and SDCI values and with published experimental data.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Electronic structure of light-emitting porous Si

Characterization of light-emitting porous Si films with X-ray photoelectron spectroscopy is reported. Only traces of O are detected on HF-etched samples, in contradiction to an earlier report that oxides are a significant component of porous Si. Si 2p and valence-band measurements demonstrate that the near-surface region of high porosity films which exhibit visible luminescence consists of amorphous Si.

Vasquez, R. P.↗
NASA NTRS1994

Conformational Characteristics of Poly(tetrafluoroethylene) (PTFE) Based Upon Ab Initio Electronic Structure Calculations on Model Molecules

Conformational energy contours of perfluoroalkanes, determined from ab initio calculations, confirm the well-known spitting of trans states into two minima at plus or minus 17 degrees but also show that the gauche states split as well, with minima at plus or minus 124 degrees and plus or minus 84 in order to relieve steric crowding. The directions of such split distortions from the perfectly staggered states are strongly coupled for adjacent pairs of bonds in a manner identical to the intradyad pair for poly (isobutylene) chains. These conformational characteristics are fully represented by a six-state rotational isomeric state (RIS) model for PTFE comprised of t(+), t(-), g(sup +)+, g(sup +)-, g(sup -) + and g(sup -)-states, located at the split energy minima. The resultant 6 x 6 statistical weight matrix is described by first-order interaction parameters for the g+(+) (ca. 0.6 kcal/mol) and g+- (ca. 2.0 kcal/mol) states, and second order parameters for the g(sup +)+g(sup +)+ (ca 0.6 kcal/mol) and g(sup +)+g(sup -)+ (ca. 1.0 kcal/mol) states. This six-state RIS model, without adjustment of the geometric or energy parameters as determined from the ab initio calculations, predicts the unperturbed chain dimensions and the fraction of gauche bonds as a function of temperature for PTFE in good agreement with available experimental values.

Smith, Grant D.↗
NASA NTRS1995

X-Ray Photoemission Measurements and Electronic Structure of HgBa2CaCu2O6+Delta

X-ray photoelectron spectroscopy data presented here are the first on an epitaxial thin film of a mercury cuprate superconductor, the first in which signals from the superconducting phase dominate all the core level spectra, and the first to exhibit a clear Fermi edge in the valence band region. Mercury is found to be in a single chemical state, contrary to reports based on sintered polycrystalline pellets.

x-ray photoemission photoelectron spectroscopy mer↗
NASA NTRS1990

Structural and electronic changes in the growth of mercury overlayers on Cu(001) - A helium beam scattering, LEED and ARPES study

We used ABS, LEED and angle-resolved photo-electron spectroscopy (ARPES) to study bilayer films of Hg on Cu(001). In the surface temperature range of 180 to 330 K, the first Hg layer forms two ordered phases, a c(2x2) (with coverage-0.5 of Cu(001)) and a high density (partially commensurate) c(4x4) (coverage-0.62). ARPES data show that there is little or no dispersion of the 5d band of Hg. ABS data show that this layer is not flat, with in-registry Hg atoms lying about 0.15 below the not-in-registry Hg atoms. From ABS we find that the second layer forms a completely registered c(4x4) phase. From ARPES we obtain that the second layer has an electronic structure, particularly the 5d levels, characteristic of bulk mercury. Preliminary results of calculations of the structure of the bilayer are given.

Vidali, G.↗