Showing papers on "Electronic structure published in 1978"

Discrete Variational Xα Cluster Calculations. II. Application to the Surface Electronic Structure of MgO

Abstract: Discrete variational (DV) Xα cluster method is extended to ionic crystals with inclusion of the long-ranged Coulomb potential of the ions outside the cluster. As a typical example of partially ionic crystals, the surface and the bulk electronic structures of MgO crystal are calculated. The strong electric field, the reduction in the Madelung potential and the charge redistribution on the surface are found to be important factors for the formation of the surface state. Various features of the DV-Xα cluster method, such as the size and the shape dependence of the cluster, the effect of the exterior potential and so on are investigated.

Exchange Interaction of Manganese 3d5 States with Band Electrons in Cd1−xMnxTe

Abstract: The influence of a system of magnetic-field-aligned paramagnetic ions on the conduction and valence band electron states is studied. A model applicable to large-gap semiconductors is presented, in which the direct influence of a magnetic field on the band electron states is neglected. This assumption allows to calculate the E(k) dependence in the presence of a magnetic field. The calculations are performed in the Kohn-Luttinger model producing a strongly anisotropic and nonparabolic structure. As an illustration of the obtained results measurements of the exciton reflection structure in a magnetic field are presented on the ternary semiconductor CdMnTe. The ratio of the exchange integrals of conduction band electrons and valence band electrons to manganese ions is estimated to α/β = −0.25. Es wird der Einflus eines Systems von im Magnetfeld ausgerichteten paramagnetischen Ionen auf die Leitungs- und Valenzbandelektronenzustande untersucht. Vorgeschlagen wird ein auf breitbandige Halbleiter anwendbares Modell, in dem der direkte Einflus des Magnetfeldes auf die Bandelektronenzustande vernachlassigt wird. Diese Voraussetzung ermoglicht die Berechnung der E(k)-Abhangigkeit bei Vorhandensein eines Magnetfeldes. Die Bcrechnungen werden ausgefuhrt im Kohn-Luttinger-Modell, welches eine stark anisotrope und nichtparobolische Struktur ergibt. Zur Illustration der erhaltenen Ergebnisse werden Messungen der Exzitonen-Reflexions-struktur im Magnetfeld fur CdMnTe-Halbleiter angefuhrt. Das Verhaltnis der Austauschintegrale von Leitungsbandelektronen und Valenzbandelektronen mit Mangan-Ionen wird zu α/β = −0,25 geschatzt.

Theory of the Thermodynamics of Simple Liquid Metals

Abstract: Publisher Summary The chapter presents a discussion on the thermodynamic properties of conducting liquids. The chapter discusses calculating measurable quantities as the compressibility, specific heat, and melting curves of metals from knowledge of the fundamental interactions among the electrons and ions. To understand this problem, it is necessary to consider both the electronic structure and the “bonding” structure or ionic arrangement. These are closely interrelated. For example, any description of the ionic arrangement must take into account the presence of the conduction electrons. Conversely, the electronic structure must in turn be strongly affected by the physical arrangement of the ions. The interplay between these dual aspects of liquid metals is quite complex, which is discussed in this chapter. Liquid metals may be further divided into roughly nonoverlapping classes according to their electronic structure, the chapter defines these classes under normal condition. A central question regarding liquid metals is, of course, why they form simple liquids, or equivalently, why, structurally, they have two body-central forces. The answer, in the case of free-electron metals, lies in the predictions of perturbation theory as applied to an interacting electron gas. The perturbation arises from the metallic ions which carry with them, in an adiabatic way, the fundamental interaction between the electrons and ions—the pseudopotential. The response of the electrons to this perturbation is discussed in the chapter. Once the nature of the interatomic forces is known, the thermodynamics of the liquid metal can be determined by the methods of classical statistical mechanics. Some of the methods for so doing are discussed in this chapter.

Transition-metal hydrides: Electronic structure and the heats of formation

Abstract: Calculations of the electronic structure of transition-metal hydrides are applied to the cohesive energy of $3d$ and $4d$ monohydrides, and the single-particle lifetime of states in nonstoichiometric Cu and Pd hydrides. A simple formula is presented which delineates the principal contributions to the cohesive energy of the hydrides: (i) the formation of a metal-hydrogen bonding level derived of states of the pure metal band structure which have $s$ symmetry about the site of the added proton, (ii) a slight increase in binding of the metal $d$ bands due to the added attractive potential, and (iii) the addition of an extra electron to the metal electron sea. The calculations, corrected for Coulomb repulsion at the hydrogen sites, qualitatively reproduce the experimental trends of the heats of formation of the transition-metal hydrides. The single-particle lifetime calculations are in quantitative agreement with Dingle-temperature measurements and they correctly predict the existence of essentially undamped states on the hole sheets of the $\ensuremath{\alpha}$-phase PdH Fermi surface.

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.

K- and L-shell ionization of atoms by relativistic electrons

Abstract: Results of a relativistic calculation of the cross section for the ionization from the K and L shells by high-energy electrons are presented. The calculation use the first-order Born approximation to treat the interaction between the scattered and atomic electrons. Plane waves are used to treat the high-energy scattered electron, while solutions of the Dirac equation in a Hartree-Slater central potential are used to describe the atomic electrons. Results are presented for a set of elements from Z = 18 to 92 and incident energies from 50 keV to 1 GeV. Calculated results are given for the correction due to the density effect arising from the polarizability of the medium. In the calculation of this correction, the medium is treated as composed of free electons.

Optical properties and electronic structure of UO2

Abstract: The near normal incidence reflectivity of UO2 single crystals has been measured in the photon energy range from 0.03 eV to 13 eV. From the reflectivity spectrum the complex dielectric function e (ω) =e1(ω)+ie2(ω) has been derived by means of the Kramers‐Kronig relation. In addition the absorption coefficient was determined from a direct transmission measurement on thin single crystal plates in the weakly absorbing spectral region below the absorption edge. An energy level scheme is proposed which allows a self‐consistent assignment of the structure in e2 with optical transitions between maxima in the density of states. The energy gap found at 2.1±0.1 eV is attributed to a 5f2→5f16deg transition. A crystal field splitting 10Dq=2.8 eV is derived for the 6d conduction states. Good agreement is obtained within this model with XPS measurements and a recent molecular cluster approximation. It disagrees with a previous interpretation of reflectivity data.

Photoemission study of the bulk and surface electronic structure of single crystals of gold

Abstract: Angle-resolved energy distributions for photoelectrons emitted from the (111), (110), and (100) crystal faces of Au are presented for photon energies in the range 7.0-11.6 eV. The variation of peak-energy positions as a function of photon energy and electron emission angle has been studied. We have found very good qualitative agreement between the experimental results and the predictions of the three-step model applied to a relativistic-augmented-plane-wave band structure. To fit the theoretical peak-energy positions to the experimental data, it is necessary to lower bands Nos. 3, 4, and 5 (numbered from the bottom of the valence-band complex) by on the average 0.2 eV and to raise band No. 6 by 0.2 eV along the $\ensuremath{\Gamma}K$ symmetry line. Considerable emission must also be attributed to nondirect processes such as phonon-assisted nondirect transitions, scattering of electrons excited in direct transitions, and surface photoemission. A shoulder due to nondirect processes is always seen at -2.3 eV, whenever the emission due to direct transitions is low. The dispersion of the surface state in the $L$ gap has been measured. A parabolic fit to the data gives ${E}_{0}=0.45\ifmmode\pm\else\textpm\fi{}0.05$ eV and ${m}^{*}=(0.37\ifmmode\pm\else\textpm\fi{}0.05){m}_{0}$. We also report measurements on the local work function for the three low-index crystal faces.

Electronic structure of the noble gas dimer ions. I. Potential energy curves and spectroscopic constants

Abstract: A systematic study of the electronic structure and chemical binding in the dimer ion sequence, Ne2+, Ar2+, Kr+2, and Xe2+, has been carried out using density functional methods. For comparison, ab initio configuration‐interaction calculations were also performed for the Ar2+ ion. These studies include detailed calculations of the pertinent potential energy curves and an analysis of the calculated spectroscopic properties of the bound states of these ions. A regular progression is found in the spectroscopic properties for the ground A 2Σ+1/2u state which leads to some remarkably simple conclusions concerning the nature of the binding and the size of these dimer ions. For the heavier systems, Kr2+ and Xe+2, spin–orbit coupling becomes important, resulting in a strong mixture of the Λ–S coupled Σ and Π states. This mixing affects the strength of the binding in the ground state. A comparison with other ab initio studies and an analysis of the asymptotic behavior at large internuclear separations is given. The...

Electronic band structure and bonding in transition metal layered dichalcogenides by atomic orbital methods

Abstract: It is shown that the main features of the electronic structure of transition metal layered dichalcogenides can be calculated in a simple ab initio atomic orbital framework. Examples from Groups IV, V and VI of the transition series are considered.

Electronic structure of the sodium trimer

Abstract: An ab initio configuration-interaction study of the potential energy surface of Na3 is presented. The ground state is predicted to be an obtuse triangle (2 B 2 symmetry), and to be bound by 35·5 kJ/mole relative to Na2(1Σ g +) + Na(2 S). The surface is found to be extremely flat. Two saddle points, an acute triangle (2 A 1 state) and a linear symmetric conformation (2Σ u + state), lie respectively only 2·5 kJ/mole and 12·5 kJ/mole above the minimum. The isotropic hyperfine coupling constant and first ionization potential calculated at the minimum energy geometry are in good agreement with experiment. A simple molecular orbital model involving only the valence s-orbitals provides an adequate qualitative description of the electronic structure. The low-lying empty p and d-orbitals in Na influence the stability of Na3 mainly through contributions to electron correlation and not through orbital hybridization. The dominant features of the surface are discussed in terms of pseudo-rotation and the Jahn-Teller in...

Self-consistent calculations of interface states and electronic structure of the (110) interfaces of Ge-GaAs and AlAs-GaAs

Abstract: Self-consistent pseudopotential techniques, together with a superlattice geometry, are used to investigate the detailed electronic structure of the (110) interfaces of Ge-GaAs and AlAs-GaAs. For Ge-GaAs six types of interface states are found, all lying below the thermal gap. No interface states are found in AlAs-GaAs. For each interface the total charge density, self-consistent potential, projected band structure, and local density of states are presented. The interface states in Ge-GaAs are discussed in detail. We also present results for the conduction- and valence-band discontinuities at these interfaces, discuss superlattice states in AlAs-GaAs, and suggest possible relaxation at the Ge-GaAs (110) interface.

Relativistic effects in ab initio effective core potentials for molecular calculations. Applications to the uranium atom

Abstract: The procedure of deriving ab initio effective core potentials (ECP) to incorporate the Coulomb and exchange effects as well as orthogonality constraints from the inner core electrons is extended to include the dominant relativistic effects on the valence orbitals. An ab initio approach is then described which enables the valence electrons in heavy atoms to be treated in a standard nonrelativistic manner by including the effect of the relativistic core–valence interactions directly into the ECP. The starting point for this procedure is the Pauli Hartree–Fock relativistic treatment of Cowan and Griffin. The pseudo‐orbital transformation and derivation of the l‐dependent effective core potentials are analogous to the nonrelativistic case with certain modifications. Analytic forms for the pseudo‐orbitals and ECP’s are derived for the U atom, and results of valence electron calculations are presented.

(110) surface states of GaAs: Sensitivity of electronic structure to surface structure

Abstract: The (110) surface states of GaAs for three models of surface relaxation are examined by the tight-binding method. Although these relaxations, obtained from analyses of low-energy-electron-diffraction data, involve similar atomic displacements, they give rise to noticeable differences in the relative positions of some surface states when these states are identified by their orbital symmetries. All three relaxations are found to completely remove all surface states from the band gap and to give rise to new surface states. Our calculations indicate the presence of five to six filled and four empty surface states. Results for energies, local densities of state, and the orbital characters of these states are given and compared to experimental data.

Scattering-theoretic approach to the electronic structure of semiconductor surfaces: The (100) surface of tetrahedral semiconductors and Si O 2

Abstract: We report the development of a method for calculating the electronic structure of semiconductor surfaces. It is based on the Koster-Slater idea for treating localized perturbations, which was later extended to describe surfaces. The present method makes use of semiempirical tight-binding Hamiltonians and of a novel way to create free surfaces. A Green's-function scattering-theoretical formulation is employed. The properties of the bulk crystal are built in and preserved. Bound-surface-state energies are determined unambiguously and accurately even for states whose wave functions are very extended. The total and local changes occurring in the density of states due to the surface can be calcualted directly, and therefore very accurately, without having to subtract two large quantities. Some of the structure in the state-density changes is in the form of narrow peaks, which can be identified as resonances or antiresonances. In order to point out advantages of the method and to compare our results with the results of slab calculations, we present applications to the Si and Ge (100) free surfaces. The present method is shown to be very efficient, accurate, and fast. Despite the fact that a tight-binding Hamiltonian of a truly semi-infinite system is treated exactly, the method employs matrices which are much smaller than those arising in slab calculations. Finally, the method is applied to a study of the (100) surfaces of the isoelectronic series Ge-GaAs-ZnSe and to the (100) surfaces of cubic Si${\mathrm{O}}_{2}$.

Electronic structure and proton spin-lattice relaxation in PdH

Abstract: We report a detailed augmented-plane-wave energy-band study and wave-function analysis of stoichiometric PdH which shows that, even though the Fermi surface of PdH is qualitatively similar to that of silver, the simple "proton model" is not valid. Instead, the screening of the proton in PdH is found to be larger than in an isolated H atom due, in part, to the formation of a H-Pd bonding band below the bottom of the $d$-band complex. This result, which is in qualitative agreement with Switendick's earlier calculation, is confirmed by ultraviolet photoemission experiments. A partial density-of-states (DOS) analysis in the energy range spanned by the six valence and conduction bands reveals the quantitative details of the bonding mechanism between the Pd and H constituents. At the Fermi energy, the high $\mathrm{Pd} d$ to $\mathrm{H} s$ DOS ratio \ensuremath{\sim} 10.3 is found to be far higher than expected in silver, despite the fact that the Fermi-surface geometry is similar. The field-induced conduction-electron spin density at the proton site is evaluated from the wave functions at the Fermi energy. The calculated value of the spin-lattice relaxation rate arising from the contact term in the hyperfine interaction is found to be in good agreement with the experimental value of Wiley et al.

Electronic States and Adsorbate-Induced Photoemission Structure on the Pd(111) Surface

Abstract: The electronic structure of the Pd(111) surface is calculated using a recently developed mixed-basis pseudopotential method. Fully self-consistent results, including surface states, work function, local densities of states, and electronic charge densities, are presented. The commonly observed adsorbate-induced changes in $d$-band structure in the photoemission spectra are interpreted in terms of the surface states. An $s\ensuremath{-}p$---like intrinsic surface-state band similar to the one observed on Cu(111) is also predicted.

The theory of the workfunction of caesium suboxides and caesium films

Abstract: The lower workfunction of caesium monolayer films compared with that of bulk caesium is explained as a size effect. By the uncertainty principle, confinement to a monolayer raises the electron kinetic energy and hence the Fermi energy leading to an equivalent fall in workfunction. This effect is substantial for alkali metals because of the low density of electrons per atom. A simple calculation is presented to substantiate this claim. The hypothesis that such a size effect is also important in determining the workfunction of the caesium suboxides is proposed. A simple model of the electronic structure of the caesium suboxides is proposed in which they are viewed as composed of regions approximating caesium metal with other regions, occupied by oxygen ions, which are highly repulsive to conduction electrons.

Molecular orbital description of silver clusters: Electronic structure

Abstract: Electronic properties derived from calculations of Ag clusters up to 39 atoms in size are compared with bulk properties calculated within the same theoretical framework. Density of states profiles (DOS) determined by extended Huckel theory show a broadening with increasing atoms in this range of sizes. The bulk periodic DOS compare favorably with other calculations. Oscillations in electron affinity, binding energy, and ionization potential for open and closed shells of electrons are observed for Ag clusters using the CNDO method.

Tise2 - Semiconductor, Semimetal, or Excitonic Insulator

Abstract: The electronic structure of high-quality $1T\ensuremath{-}\mathrm{Ti}{\mathrm{Se}}_{2}$ has been investigated using angle-resolved synchrotron ultraviolet photoemission. Occupied $\mathrm{Ti} d$ states are clearly observed in small pockets around the zone-edge centers and are found to be roughly degenerate with the highest-lying $\mathrm{Se} p$ states at $\ensuremath{\Gamma}$. The results are relevant to various explanations of the $2{a}_{0}\ifmmode\times\else\texttimes\fi{}2{c}_{0}$ lattice instability in this material, including a possible excitonic electron-hole interaction.