Semiempirical Tight-binding Calculations Research Articles

Influence of Oxygen Vacancies on the Electronic Structure of Yttrium Oxide

The influence of oxygen vacancies on the electronic structure of yttrium oxide was investigated both experimentally and theoretically. By means of X‐ray absorption spectroscopy, at the yttrium K and L11 edges and at the oxygen K edge, information on the local densities of states of various orbital symmetries, around each type of atom, was obained. The total density of states in the valence band was studied using X‐ray photoelectron spectroscopy. Experimental results were compared with those obtained on stoichiometric yttria. The evolution of the densities of states is well reproduced by self‐consistent, semiempirical tight‐binding calculations applied to clusters of increasing size, in which oxygen vacancies are introduced. The charge transfer between oxygen and yttrium is modified, as experimentally observed from the study of the O KLL Auger line shape. The decrease of the local atomic charge on oxygen is theoretically confirmed by the tight‐binding calculations, showing a tendency toward a more covalent bond in nonstoichiometric yttria.

Electronic structure of yttrium oxide.

The electronic structure of stoichiometric Y{sub 2}O{sub 3} is studied both theoretically and experimentally. X-ray absorption, performed through total-yield measurements at the {ital K} and {ital L}{sub II} edges of yttrium atoms and the {ital K} edge of oxygen atoms, through total-yield measurements has given access to empty states of various orbital characters in the conduction band. The shape of the valence band was obtained by means of photoemission spectroscopy. These results are compared with self-consistent, semiempirical tight-binding calculations applied to clusters of increasing sizes, which lead to local densities of states. We show that the quasi-octahedral local environment of the yttrium atoms is responsible for the splitting of their {ital d} levels. However, this ligand-field approach is not sufficient to account for the yttrium and oxygen {ital K} thresholds, nor for the shape of the valence band for which band effects are important. The two-peak structure of the oxygen {ital K} edge results from the hybridization of the O {ital p} orbitals with the {ital e}{sub {ital g}} and {ital t}{sub 1{ital u}} states on the yttrium atoms. Three peaks are detected in the valence band: We show that they suggest the existence of direct-hopping terms between neighboring oxygenmore » atoms. Finally, the various calculations have given an estimate of the degree of covalency of the Y-O bond.« less

Atomic substitutions in YBa2Cu

We have performed semiempirical tight-binding calculations of the electronic structure of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, with d and s orbitals included for all the metal atoms and p and s orbitals for the oxygen. Here we report studies of the following atomic substitutions: Al, Fe, Co, Ni, and Zn replacing Cu; Sr and La replacing Ba; Tl, Pb, and Bi replacing Y; and F and N replacing O. In each case, the modification of the local densities of states, the atomic valences, and the Fermi energy were calculated.

Theoretical Studies of Oxygen Vacancies in YBa2Cu3O7-y

ABSTRACTWe have performed semiempirical tight-binding calculations of the electronic structure of YBa2Cu3O7, with dand s orbitals included for all the metal atoms and p and sorbitals for oxygen. Here we report studies of oxygen vacancies on the O(1) chain sites in YBa2Cu3O7-y. The modification of the density of states ρ(E) and the shift of the Fermi energy Ep were calculated for 0 < y ≤ 1.0. The Fermi energy is found to increase monotonically with y, confirming the expectation that oxygen vacancies act as donors. Also, ρ(EF)is found to decrease with y.

Photoemission from ordered physisorbed adsorbate phases: N2 on graphite and CO on Ag(111).

The measured two-dimensional band structure of physisorbed molecular monolayers has been used to determine the structure of the ordered phase. For the previously reported ordered herringbone structure of ${\mathrm{N}}_{2}$ on graphite we observe a $k$-dependent splitting in the $3{\ensuremath{\sigma}}_{g}$ level which is a consequence of the intermolecular interaction of the two molecules per unit cell. Our measured dispersion is consistent with the structure proposed by recent LEED calculations and with a semi-empirical tight-binding calculation. For CO on Ag(111), which has not been studied by LEED, we find an orientationally ordered phase where there are two molecules per unit cell with the CO lying parallel to the surface.

Site inequivalence inFe1+xNb3−xSe10

${\mathrm{Fe}}_{1+x}{\mathrm{Nb}}_{3\ensuremath{-}x}{\mathrm{Se}}_{10}$ has a novel structure made up of trigonal prismatic and octahedral coordinated metal atoms. The Se atoms are of two major types, those bound in Se pairs and single Se atoms, and while only Nb cations are found on the trigonal sites and the Fe (along with the remaining Nb) is found on the octahedral sites. To permit the study of the effects of disorder and site inequivalence on the electronic structure of this compound, the x-ray photoelectron spectra of the core and valence electrons were examined. The spectra show two distinct types of Se. The relative intensities of the two Se spectra and comparisons with the spectra of related compounds identify the Se at greater binding energy with the closely spaced Se pairs in the trigonal chains and those at smaller binding energy with the rest of the Se atoms. The spectra of the Nb atoms in the two different sites were not resolved and differ by not more than 0.2 eV in binding energy. However, the Nb $3d$ spectra showed more broadening than in similar compounds, probably due to the Nb site inequivalence and disorder intrinsic to the octahedral chains. All Nb atoms are consequently chemically similar. Chemical shifts indicate an oxidation state for Nb in ${\mathrm{Fe}}_{1+x}{\mathrm{Nb}}_{3\ensuremath{-}x}{\mathrm{Se}}_{10}$ between these of Nb${\mathrm{Se}}_{2}$ and Nb${\mathrm{Se}}_{3}$. The spectra of the Fe atoms were relatively broad, and showed no resolved structure. These results are consistent with a recent semiempirical tight-binding band calculation which suggests that the conduction-electron density is low and resides mainly on the trigonal prismatic chain.

Periodic perturbations of a polymer

The reduction of the short range translational symmetry of a polymer chain (caused by, for example, the loss of an isoconformational structure) is accommodated as a periodic perturbation in a semi-empirical tight-binding LCAO calculation. Using the results of a calculation on an unperturbed chain in a perturbational mixing procedure an energy matrix can be calculated whose order is equal to (or even less than) that of the unperturbed structure. The method is applied to the generation of various chain conformations from all-trans polyacetylene and is shown to work successfully.

A model pseudopotential calculation of the electronic structure of Si (111)-1 × 1 surface

We report a model pseudopotential calculation of the electronic structure of Si (111)-1 × 1 unrelaxed, relaxed and expanded surfaces. It is shown that our results for the surface states on these faces are in good agreement with results from other self-consistent pseudopotential and semi-empirical tight-binding calculations.

Realistic Tight-Binding Calculations of Surface States of Si and Ge (111)

We give semiempirical tight-binding calculations of dangling- and back-bond surface bands and resonances for unreconstructed relaxed Si and Ge (111) surfaces. For Si at high-symmetry points these are in very good agreement with those of Appelbaum and Hamann. Comparison of the results with the energy-loss-spectroscopy data of Rowe and Ibach suggests that the observed back-bond resonances are associated primarily with minima in the back-bond surface bands.

Tight-binding calculations of surface states of Si(111)

Semi-empirical tight-binding calculations are reported for Si(111) surfaces. The calculations reproduce quantitatively the results obtained both experimentally and by more elaborate theoretical methods.

A semi-empirical tight-binding calculation of the band structure of MoS 2

Experimental results and the tight-binding method are used together to derive the band structure of MoS 2. the scheme obtained permits a more detailed interpretation of the optical transmission spectrum.