Showing papers by "Alex Zunger published in 1978"

Band structure and lattice instability of TiSe2

Abstract: The energy band structure of Ti${\mathrm{Se}}_{2}$, determined in the local-density approach yields a semimetal (band overlap 0.20 \ifmmode\pm\else\textpm\fi{} 0.05 eV) with holes at $\ensuremath{\Gamma}$ and electron pockets only at $L$. The dimensions of the electron pocket indicate the presence of (7-8) \ifmmode\times\else\texttimes\fi{} ${10}^{20}$ carriers/${\mathrm{cm}}^{3}$ in excellent agreement with both transport and angular-resolved photoemission data. The observed charge-density wave is attributed to characteristic "volume" effects, i.e., nesting of parallel electron-hole bands at ${E}_{F}$ separated by the $\ensuremath{\Gamma}\ensuremath{-}L$ zone-boundary wave vector.

First-principles nonlocal-pseudopotential approach in the density-functional formalism: Development and application to atoms

Abstract: We present a method for obtaining first-principles nonlocal atomic pseudopotentials in the density-functional formalism by direct inversion of the pseudopotential eigenvalue problem, where the pseudo-wave-functions are represented as a unitary rotation of the "exact" all-electron wave functions. The usual pseudopotential nonuniqueness of the orbitals is fixed by imposing the physically appealing constraints of maximum similarity to the all-electron wave functions and minimum radial kinetic energy. These potentials are shown to yield very accurate energy eigenvalues, total energy differences, and wave-function moments over a wide range of excited atomic configurations. We have calculated the potentials for 68 transition and nontransition elements of rows 1-5 in the Periodic Table. Their characteristic features, such as classical turning points and minimum potential radii, faithfully reflect the chemical regularities of the Periodic Table. The nonempirical nature of these potentials permits both an analysis of their dominant features in terms of the underlying interelectronic potentials and the systematic improvement of their predictions through inclusion of appropriate correlation terms. As these potentials accurately reproduce both energy eigenvalues and wave functions and can be readily fit to analytic forms with known asymptotic behavior, they can be used directly for studies of many structural and electronic properties of solids (presented in a separate paper).

Self-consistent LCAO calculation of the electronic properties of graphite. I. The regular graphite lattice

Abstract: The electronic properties of the regular graphite lattice are investigated within self-consistent LCAO (linear combination of atomic orbitals) scheme based as a modified extended-H\"uckel approximation. The band structure and interband transition energies agree favorably with previous first-principles calculations. Good agreement with experimental data on the density of valence states, energetic position of the lowest conduction states, equilibrium unit-cell parameters, cohesive energy and vibration force constants, is obtained. The McClure band parameters that were previously adjusted to obtain agreement with Fermi-surface data and the electronic specific heat, are reasonably reproduced. The charge distribution and bonding characteristics of the covalent graphite structure, are discussed. The same calculation scheme is used in part II of this article (following paper) to discuss properties associated with point defects in graphite. The correlation between the electronic properties of the regular and point-defect-containing lattice is studied.

First-principles theoretical study on the electronic properties of the B 32 intermetallic compound LiAl

Abstract: The electronic properties of the ordered LiAl crystal are studied within the self-consistent (non-muffin-tin) numerical-basis-set approach to the local-density formalism. The material appears to be electronically a semimetal with an electron pocket near $X$ (along $\ensuremath{\Delta}$) and a hole pocket at $\ensuremath{\Gamma}$. The band structure and density of states have characteristics similar to that of the tetrahedrally bonded IV-IV semiconductors (LiAl has a ${T}_{d}$ site symmetry); however, the indirect ${\ensuremath{\Gamma}}_{{25}^{\ensuremath{'}}}\ensuremath{-}{X}_{1}$ band gap (which decreases progressively as one goes along the diamond, Si, Ge, and $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ series) becomes negative in LiAl. A study of charge redistribution effects indicates that while the Li-Al bond is an ionically polarized covalent bond, the Al-Al bonds are metalliclike and the Li-Li bonds are essentially nonbonding. Wave-function population analysis indicates that the bottom of the occupied valence band is of predominantly $\mathrm{Li} 2s$ character (hybridized with $\mathrm{Al} 3s$), while at higher energies the $\mathrm{Li} 2s$ character is reduced in favor of the Li and $\mathrm{Al} p$ character, which are dominant around the Fermi energy. The main intrasite-charge-redistribution effects involve pronounced $\mathrm{Li} 2s$ to $\mathrm{Li} 2p$ promotion (with a smaller $s\ensuremath{-}p$ promotion on the Al site) while the intersite (ionic) redistribution effects are found to be small. The observed trends in the measured Knight shifts (relative to the pure constituent metals) as well as the small paramagnetism and its dependence on the Li concentration are discussed in terms of these bonding effects. The abrupt changes in the differential electrical resistivity at \ensuremath{\sim} 100\ifmmode^\circ\else\textdegree\fi{}K is tentatively assigned to a structural instability induced by electron-hole interaction effects.

Local-density self-consistent energy-band structure of cubic CdS

Abstract: Self-consistent ab initio studies of the electronic-energy-band structure of cubic CdS are reported within the local-density-functional (LDF) formalism. All electrons are included using our previously reported linear-combination-of-atomic-orbitals method in a numerical basis representation. In the first set of calculations we employ the same lattice constant, exchange (only) potential, and computational parameters as were used by Stukel et al. in their early self-consistent orthogonalized-plane-wave (SCOPW) investigation so that a direct comparison of results can be made and the validity of the SCOPW approach for covalently bonded $4d$ systems can be assessed. In the second set of calculations, the Stukel et al. computational restrictions are relaxed, a more accurate lattice parameter is employed, and the Kohn-Sham exchange and the Singwi et al. correlation potential are used to obtain the local-density formalism solutions to the band problem, including variation of the band structure and related properties with pressure (change of lattice constant). Comparison with optical and x-ray and uv photoemission experiments for excitations of both the $s\ensuremath{-}p$ and metal $d$ bands in the 5-19 eV region indicate very good agreement. The direct gap at $\ensuremath{\Gamma}$ is, however, found to be 0.5 eV (25%) too small, a discrepancy similar to that previously found in nonempirical studies for other heteropolar insulators (e.g., Ne and LiF). This is traced to the neglect of the different orbital relaxation at the ${\ensuremath{\Gamma}}_{25}$ and ${\ensuremath{\Gamma}}_{1}$ band edges and to the noncancellation of the self-interaction terms characteristic of the local-density potential. Simple atomic total-energy models for these effects are shown to bring this gap into good agreement with experiment. It is concluded that a first-principles (parameter-free) exchange and correlation LDF model describes very well the main electronic-structure features of the system.

Density-Functional Pseudopotential Approach to Crystal Phase Stability and Electronic Structure

Abstract: We show that a first-principles nonlocal pseudopotential theory, based on an all-electron density-functional formalism, provides an essentially exact topological separation of both the octet ${A}^{N}{B}^{8\ensuremath{-}N}$ structures and the suboctet ${A}^{N}{B}^{P\ensuremath{-}N}(3l~Pl~6)$ structures for 77 and 56 non-transition-metal compounds, respectively. These pseudopotentials which also yield accurate descriptions of atomic and bulk solid electronic structure have been computed for 68 transition and nontransition elements.

Self-consistent LCAO calculation of the electronic properties of graphite. II. Point vacancy in the two-dimensional crystal

Abstract: The electronic properties of a point vacancy in the two-dimensional graphite crystal are investigated within the small-periodic-cluster approach using a self-consistent all-valence-electron LCAO (linear combination of atomic orbitals) scheme previously employed for the calculations of the band structure and optical spectra of the regular lattice (part I). Eight crystal bands, 54-96 $\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}$ points in the Brillouin zone, selected according to the "mean value theorem" and ${2}^{2}$-${5}^{2}$ primitive unit cells around the defect site are allowed to interact. A doubly degenerate singly occupied $\ensuremath{\sigma}$ defect level is shown to appear in the $\ensuremath{\sigma}\ensuremath{-}{\ensuremath{\sigma}}^{*}$ band gap, 3.5 eV above the $\ensuremath{\sigma}$ band edge, with a wave function that is about 80% localized on the three nearest-neighbor atoms. The density of electronic states, charge distribution and Poisson electrostatic potential of the defect structure are computed and used to discuss the characteristic feature of the defect in connection with Coulson's "defect molecule" model and with current models of electron trapping mechanisms used to interpret the experimental data on Hall coefficient, resistivity and diamagnetic susceptibility of damaged graphite. Both symmetric and Jahn-Teller lattice distortions are introduced around the defect site, the results being used to interpret the experimentally observed decrease in lattice constant, the observed optical absorption and the vibronic parameters of the Jahn-Teller effect. Symmetric lattice relaxations are shown to have a moderate effect on the lattice energy and on the position of the defect level, these changes being mainly due to the response of the $\ensuremath{\pi}$ subsystem to accumulation of excess $\ensuremath{\pi}$ charge on the surrounding bonds, while Jahn-Teller distortions are shown to have a small effect on the system due to the relative rigidity of the $\ensuremath{\sigma}$ skeleton. The energy of vacancy formation as well as the energy of atom displacement and vacancy migration are directly computed from the change in total lattice energy, the results being in good agreement with experiment. The importance of introducing charge self-consistency in treating the charge redistribution in the system as well as the significance of allowing more distant atoms to interact with the vacancy electrons, is emphasized.

Structurally induced semimetal-to-semiconductor transition in 1T-TiSe2

Abstract: We show that observed changes in the nature of the conducting state of $1T$-Ti${\mathrm{Se}}_{2}$ in going from the normal semimetallic state to the charge-density-wave semiconducting state can be successfully modeled by variation of a single structural parameter, $z$, which modulates the Ti-Se bond length. These ab initio band-structure results lead to a number of interesting experimental consequences.

On the first principles Hartree—Fock and local density pseudopotentials

Abstract: Angular momentum projected Hartree—Fock (HF) and local density functional (LDF) nonlocal atomic pseudopotentials are derived from first principles in