Showing papers on "Electronic band structure published in 1972"

Electronic structure based on the local atomic environment for tight-binding bands

Abstract: Some new methods are presented for calculating the density of states n(E) and other aspects of electronic structure in a tight-binding band, without use of Bloch's theorem or the band structure E(k). The methods are therefore applicable to calculating the local density of states at surfaces, impurities etc. and relate the electronic structure to the local atomic environment. They depend on developing the Green function as an infinite continued fraction. There is no difficulty in obtaining n(E) in a few minutes computing time correct to the first 50 moments for an s-band and 10 moments for d-bands. The present paper discusses the methods and ideas, with specific applications to follow.

Magnetic properties of rare earth metals

Abstract: 1 Introduction.- 2 Phenomenological Theory of Magnetic Ordering: Importance of Interactions with the Crystal Lattice.- 3 Magnetic Structures of Rare Earth Metals and Alloys.- 4 Bulk Magnetic Properties.- 5 Spin Waves.- 6 Energy Band Structure, Indirect Exchange Interactions and Magnetic Ordering.- 7 Transport Properties.- 8 Hyperfine Interactions.

X-ray photoemission band structure of some transition-metal oxides.

Abstract: The valence-band structures of MnO, CoO, NiO, ${\mathrm{Cu}}_{2}$O, CuO, and Re${\mathrm{O}}_{3}$ have been obtained by x-ray photoemission spectroscopy. It is found that in every case a narrow metal $d$ band lies above the center of the oxygen $2p$ valence band.

Conductivity studies in europium oxide.

Abstract: The electrical and optical properties of EuO are studied in order to help determine conduction mechanisms. A strong relationship between growth parameters and electrical behavior is noted. Free-carrier absorption below the band edge in energy is observed in moderately conducting crystals. By comparing optical and electrical results, an experimental determination is made of carrier-density and scattering-time contributions to resistivity behavior. The carrier-density variations are explained in terms of an earlier proposed model which is developed here. The model consists of a donor-trap level, believed to be caused by an oxygen vacancy, which is above the conduction-band edge at low temperature, but crosses below it (near 50 \ifmmode^\circ\else\textdegree\fi{}K) with increasing temperature. Magnetoresistance and pressure results are presented which strongly support the model.

The Band Structure of Silver and Optical Interband Transitions

Abstract: The band structure of silver has been calculated using the relativistic augmented plane wave method. Relativistic effects must be included if a quantitative interpretation of optical experiments is made. The L′2 → L1 gap is 1 eV smaller in the relativistic calculation than it is in a non-relativistic band structure obtained from the same potential. This reduction of the L-gap is in the present case sufficient to lead to an extra interband edge below the main edge at 3.98 eV. The optical function e2(ω) and the energy distributions of photoemitted electrons have been calculated.

Exciton Band Structure and Properties of a Real Linear Chain in a Molecular Crystal

Abstract: An experimental study is presented that leads to the elucidation of the whole band structure for the triplet exciton state of 1,4‐dibromonaphthalene. We show that the band states are essentially those of a linear chain characterized by nearest neighbor interactions along the c crystallographic axis. Although the crystal is topologically a three‐dimensional network, for the practical purpose of energy transfer and trapping the crystal behaves as a set of linear chains. Heavy doping of DBN‐h6 (host) with up to 18% DBN‐d6 (guest) yielded the expected discrete spectra of a random linear array corresponding to a nearest neighbor interaction of 7.4± 0.1 cm−1 and total bandwidth of 29.6± 0.4 cm−1 for the zero‐zero transition. A symmetric mode at 0+520 cm−1 was shown to have an exciton bandwidth of 15 cm−1, and an unsymmetric vibrational level at 0+250 cm−1 was shown to have a vanishing bandwidth in accordance with expectations from weak coupling theory. The quasiresonance interactions of host and guest were significant and were included in establishing the agreement between experiment and calculation. The total bandwidth is estimated to be 90± 20 cm−1. This type of doping has an effect on line shape even at very low concentration and even in undoped crystals the line shape is asymmetrical, perhaps due to impurities and imperfections that have small zero‐order shifts from the band center. The intensity of transitions to the carbon‐13 isotopic traps is shown to be distributed over the band states in the origin and is clearly visible in the narrow band nontotally symmetric vibrational state. The vibrational analysis of the crystal spectrum (electronic or vibrational states) is shown to be dependent on exciton effects even in the case where no Davydov splitting is observed. Spectra of DBN‐h6 (guest, H) in DBN‐d6 (host, D) were also studied and resonance multiplets were seen and identified in absorption and emission. We identified ··· DHD···, ··· DHHD···, ··· DHHHD···, and probably ··· DHDHD··· in the phosphorescence spectra at 2°K. We present a brief discussion of the effect of dimensionality on the dynamical features of energy transfer and trapping, and we show experimentally, that heavy doping (d6 in h6) enhances the phosphorescence of the crystal about 60‐fold. In addition the existence at 2°K of a Boltzmann distribution over resonance multiplets leads us to propose a long‐range trap‐to‐trap migration as part of the thermalizing process.

Condensation of excitons in germanium and silicon

Abstract: The ground state energy of an electron-hole plasma in a Ge type semiconductor is calculated with the help of the 'improved RPA' approximation of Nozieres and Pines (1958). The detailed band structure is taken into account, except for the warping of the valence band. That is, the effects of the anisotropy, of the valence band coupling and of the multivalley structure are discussed, and are found to stabilize the plasma. The numerical results for Ge and Si are in very good agreement with experiments, which provides a check for the validity of the 'improved RPA' approximation.

Single-Electron Energies, Many-Electron Effects, and the Renormalized-Atom Scheme as Applied to Rare-Earth Metals

Abstract: A systematic investigation of certain electronic properties of the rare-earth metals is reported Calculations are performed within the framework of the renormalized-atom method in which Hartree-Fock free-atom solutions, with electronic configurations appropriate to the metal, are initially computed; the wave functions are then renormalized to the Wigner-Seitz sphere and used to construct $l$-dependent Hartree-Fock-Wigner-Seitz crystal potentials The following results are obtained: (i) Recent spectral information together with the free-atom solutions permits us to estimate the change in neutral-atom correlation energy associated with changing the $4f$ electron count; contrary to expectation, we find that correlation effects are more significant in a configuration with one fewer $4f$ and one more $5d$ electron (ii) Band extrema and Fermi levels are placed (iii) The positions of occupied and unoccupied $4f$ levels are estimated in both a one-electron approach and a multielectron method taking screening and relaxation effects into account in a definite way The one-electron approximation for the $4f$ levels fails badly in reproducing the results of recent photoemission experiments, while the multielectron calculations are in surprisingly good accord with experiment The effective Coulomb-interaction energy between two $4f$ electrons at the same site, the familiar $U$, is reduced from the single-particle value of approximately 27 eV to about 7 eV with the inclusion of multielectron effects (iv) Hartree-Fock values for the $4s$- and $5s$-shell exchange splittings are compared with soft-x-ray photoemission studies of the rare-earth fluorides and oxides; the calculated $4s$ splittings are roughly twice as large as experiment while, unexpectedly, the $5s$ results are in almost precise agreement

Ultra-violet photoelectron data on the complete valence shells of molecules recorded using filtered 30.4 nm radiation

Abstract: Ionization energies have been obtained for all the valence orbitals of some simple hydrocarbons and also some of their isoelectronic analogues in which a nitrogen or an oxygen atom replaces CH or CH2 respectively. This required the use of a helium discharge run under conditions such that a high proportion of 30.4 nm radiation was emitted. In order to utilize the full range of the 41 eV photon it was further necessary to use filters, usually polystyrene films about 100 nm thick, selectively to absorb 58.4 nm radiation.Analysis of the results show that in many molecules the summed binding energies of the molecular orbitals built from 2s atomic orbitals may be divided up into atomic contributions which agree in magnitude with those obtained for the same atoms in other molecules, i.e., the atomic contributions are additive as is expected from the application of simple theory. A similar additive behaviour was also found for the orbitals built from 2p and 1s(H) atomic orbitals though theory does not directly indicate this for incompletely filled orbital systems of this type.Certain weak photoelectron bands have been found at high energies which arise from transitions to configurationally mixed ionized states. The experimental results are compared with theoretical predictions.

Dynamical Diffraction Theory by Optical Potential Methods

Abstract: Publisher Summary In the past twenty years, there has been a renewed and steadily increasing interest in dynamical diffraction of X-rays and electrons, which is due partly to the availability of large, perfect crystals and to the development of the electron microscope. New branches have evolved, such as low-energy-electron-diffraction (LEED), channeling of high energy electrons, and positrons or dynamical scattering of Mossbauer quanta. Also, the dynamical theory has made considerable progress. This chapter discusses the conventional form of the dynamical theory. It presents the basic principles in a short, but self-contained way and emphasizes some new methods such as the band structure for complex wave vectors and the t-matrix method. Moreover, it develops at the same time the theory for electron, neutron, and X-ray diffraction by a finite crystal—that is, the matching of the wave fields, the calculations of diffracted intensities, the discussions of Laue and Bragg cases, etc. This chapter discusses the theory for the coherent wave, which is known as the optical potential method in nuclear physics. It explicitly derives the corrections to the potential coefficients due to inelastic waves, thermal motion, and statistically distributed defects, both for electron and X-ray diffraction.

Theory for Superconductivity in d‐Band Metals

Abstract: Superconductivity in transition metals, in transition metal alloys, in A‐15 compounds and in U6X compounds is studied within the framework of the B.C.S. theory. The electron‐phonon coupling constant λ is expressed in terms of measureable normal‐state quantities and an atomic parameter. Then, using the McMillan formula for the superconducting transition temperature Tc, the variation of Tc in the three transition metal series is discussed. Calculated values for Tc are given for several transition metal alloys, U6X compounds and for Sc, Y, Lu, and La. Also, it is shown that in A15 compounds Cooper pair fluctuations may occur even at temperatures several degrees Kelvin above Tc.

The band structures of some transition metal dichalcogenides. II. Group IVA; octahedral coordination

Abstract: For pt. I see ibid., vol. 5, 738 (1972). The band structures of some group IVA dichalcogenides-ZrS2, ZrSe2, HfS2, HfSe2-are calculated using a semiempirical tight binding method. A symmetry analysis of the crystal structure is given together with the selection rules for optical transitions. The experimental properties of the materials are reviewed and certain features in the optical transmission spectra are used to fix the values of three parameters used in the calculation. The joint density of states function is computed and the results of this and the band structure calculations are discussed.

Is there an intimate relation between amorphous and crystalline semiconductors

Abstract: Pressure measurements have often been used to clarify the band structure and the fundamental properties of crystalline semiconductors. An account is given of such measurements on the optical absorption edge and the refractive index of amorphous Ge. Less complete data on Si, GaP and GaAs are also reported. The implication of the results for present models of the amorphous band structure and the optical and transport properties is examined. Finally, the value of the Penn gap in the amorphous phase is estimated from the calculated heat of crystallization.

Reflectivities and Electronic Band Structures of CdTe and HgTe

Abstract: We have measured and calculated the reflectivity spectra of CdTe and HgTe. The measured and calculated reflectivities are compared and prominent features of the reflectivity spectra are identified with critical-point transitions in specific regions of the Brillouin zone. The symmetry and contribution to the reflectivity of important critical points are investigated. Empirical pseudopotential calculations of the band structure and the imaginary part of the frequency-dependent dielectric function, with spin-orbit effects included, are also presented.

Temperature Dependence of Energy Gaps of Some III-V Semiconductors

Abstract: The Brooks-Yu theory is applied to the calculation of the band structure of some III-V semiconductors at finite temperatures. The theory is applied in the context of the empirical pseudopotential method, and utilizes a recent lattice-dynamical calculation of the Debye-Waller factors. The temperature dependence of several band gaps are determined and compared with available experimental data and, in general, good agreement is obtained.

Electronic Energy-Band Structure of Sn S 2 and Sn Se 2

Abstract: The local-empirical-pseudopotential method is used to calculate the electronic band structure of Sn${\mathrm{S}}_{2}$ and Sn${\mathrm{Se}}_{2}$. The pseudopotential form factors for the constituent elements Sn, S, and Se are determined from previous pseudopotential calculations for other crystals. Slight adjustments were made to give the correct fundamental gaps. A group-theoretical study of the symmetry properties of these crystals is included. The imaginary part of the dielectric function, ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$, is calculated for Sn${\mathrm{S}}_{2}$. Some comparison is made between the theory and the existing experimental data.

INDO and MINDO/2 Crystal Orbital Study of Polyacetylene, Polyethylene, and Polyglycine

Abstract: Approximate valence electron self‐consistent field crystal orbital calculations using INDO and MINDO/2 parameters are described for polyacetylene, polyethylene, and polyglycine in a planar and α‐helical conformation. The electronic energy band structure and electronic charge distribution are discussed and compared with previous theoretical calculations and available experimental results.

Self-Consistent Energy Bands in Vanadium at Normal and Reduced Lattice Spacings

Abstract: Self-consistent energy-band calculations for vanadium were performed by the augmented-plane-wave (APW) method, for different values of the statistical exchange parameter $\ensuremath{\alpha}$, at normal and reduced lattice constants. Comparisons have been made, with Fermi-surface, soft-x-ray, photoemission, and electronic specific-heat experiments, and reasonable agreement was found.

Electronic Band Structure of Niobium Nitride

Abstract: The augmented-plane-wave method is applied to calculate the electronic band structure of niobium nitride (NbN). The results are qualitatively similar to those obtained from previous energy-band calculations for the $3d$ transition-metal monoxides which form with the same rocksalt structure. The Fermi level for NbN falls in the lower portion of the ${t}_{2g}$ manifold of the niobium $4d$ bands so that the conduction electrons in NbN are predominantly $4d$-like in character. These results are compared with some of the previous band-structure models that have been proposed to explain the electronic properties of NbN and other closely related refractory hard metals with the rocksalt structure.

Energy Bands in Boron Nitride and Graphite

Abstract: The tight-binding approximation has been applied for calculation of the band structure of planar hexagonal crystal layers of graphite and boron nitride. The theoretical width of the valence band of 15.9 eV in graphite and the distance between the centers of $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ band of 5.3 eV as obtained in the present calculation are in good agreement with Chalkin's experimental work yielding 15 and 5 eV, respectively. For boron nitride, the width of the energy gap and the width of the valence band were calculated to be 4.9 and 6.8 eV, respectively.

Valence band structures and core-electron energy levels in the monochalcogenides of gallium. Photoelectron spectroscopic study

Abstract: X-ray induced photoemission studies on single crystals of GaS, GaSe and GaTe reveal the details of the occupancy of the valence-bands in these solids. As expected from their electronic and crystallographic (layer) structures, GaS and GaSe have remarkably similar valence-band profiles: peaks occur (for GaS) at 4, 9 and 14.5 eV and (for GaSe) at 3.5, 8 and 14 eV below the respective valence-band edges. U.-v. photoemission in ultrahigh vacuum was used to locate the precise positions of the Fermi levels and valence band edges. These occur, for GaS, respectively at 5.8 and 6.5 eV and, for GaSe, at 5.2 and 5.4 eV below the vacuum level. Agreement between the experimentally determined peak shapes and positions and the values predicted theoretically on the basis of a semi-empirical tight-binding approach is good. No theoretically estimated band structure appears to be available for comparison with the observed GaTe band profile.Core-level binding energies have been evaluated for levels extending from ca. 40 to 1010 eV. There is evidence that the currently accepted values for the core-level binding energies in elemental tellurium are in error.