Showing papers on "Coherent potential approximation published in 1987"

The Slater–Pauling curve: First principles calculations of the moments of Fe1−cNic and V1−cFec

Abstract: We have performed calculations of the electronic structure of the random substitutional alloys Fe1−cNic and V1−cFec using the spin‐polarized, self‐consistent Korringa–Kohn–Rostoker coherent‐potential approximation (KKR‐CPA) method. This is a first principles method based on spin density functional theory and a local spin density approximation for the exchange and correlation functional. For fcc Fe1−cNic, a range of volumes were considered for 0.25

Neutron Scattering by Ferroelectrics

Abstract: This book presents the recent theoretical and experimental developments of neutron scattering by ferroelectrics. A model description of lattice dynamics of structurally unstable crystals (self consistent phonon approximation, pseudo-spin formalism, coherent potential approximation etc.) is formulated. The effects of nonlinear excitations and lattice defects in neutron scattering are also discussed.

Electronic and transport properties of HgCdTe and HgZnTe

Abstract: Band structures are calculated for HgTe, CdTe, and ZnTe. A near band‐edge approximation to the coherent potential approximation is used to compute the alloy band‐gap variation and electron and hole effective masses in HgZnTe and HgCdTe. Both diagonal and off‐diagonal disorder are included. The alloy‐scattering contributions to the electron and hole mobilities are calculated and found to be unimportant for electrons but significant for hole transport. The consequences for hole mobility are found to be significant if anion disorder is present because of a valence‐band offset.

Tight-binding coherent-potential-approximation study of the electronic states of palladium-noble-metal alloys.

Abstract: We have performed calculations of the electronic states of palladium —noble-metal disordered alloys using an improved version of the tight-binding coherent-potential-approximation (TB-CPA) formalism. Our results for Pd-Ag are in excellent agreement with both experiment and the KorringaKohn-Rostoker (KKR) CPA. For Pd-Au we also obtain good agreement with specific-heat measurements. For Pd-Cu, probably due to charge-transfer effects, we do not obtain satisfactory agreement with experiment at the Cu-rich end. Our calculations indicate that if (a) highly accurate Slater-Koster parameters are used and (b) all the orbitals are included in the CPA conditions (i.e., not using a one-level TB model), the TB-CPA gives results that are equivalent to those of the KRRCPA except in cases where off-diagonal disorder might be important.

Velocity‐field characteristics of III‐V semiconductor alloys: Band structure influences

Abstract: We have calculated the velocity‐field characteristics of semiconductor alloys based on realistic band structures and have obtained the band structures and alloy‐scattering rates from a generalization of the coherent potential approximation method. Although we use proper band structures, we still consider a single electron‐temperature model. The results agree surprisingly well with experiments, and suggest that InP‐based alloys are good candidates for high‐speed devices.

Cluster generalisation of the KKR coherent potential methods for random metallic alloys

Abstract: A generalisation of the single-site KKR-CPA to include effects of clustering is presented here. The formalism combines the usual KKR ideas with the augmented space formalism introduced by the author.

LCAO approach to the coherent potential approximation: electronic structure of substitutionally disordered CuNi alloys

Abstract: An LCAO approach to the density functional first-principles CPA theory of substitutionally disordered alloys is presented. The formal difference from the traditional tight-binding CPA consists of the explicit appearance of the overlap matrix in the CPA equations. More important, the first-principles self-consistent electronic structure theory may be directly implemented into the scheme. The theory is applied to paramagnetic CuNi alloys. Green function calculations involving Brillouin zone integrations are performed with the help of an improved complex energy k-space integration method. Details of the numerical procedures for the evaluation of the density of states (DOS) and the Fermi energy are discussed. Within the LCAO-CPA formalism an explicit formula for the integrated DOS is derived. Numerical results were obtained for the DOS, state-projected DOS and self-energies, Bloch spectral function and the Fermi surface. Good agreement is found with both previous KKR-CPA calculations and experimental data.

The effect of frenkel defects on the electronic structure of 1T−TiSe2

Abstract: Using a tight binding formalism the wave vector resolved electronic density of states has been calculated in the coherent potential approximation for an 1 T−TiSe2 crystal containing Frenkel defects. Additional structure originating from disorder is discussed. An extra peak in the ΓA direction at 1.5 eV binding energy can be traced back to a resonance caused by titanium vacancies.

Analysis of the quasimetallic reflection band by dynamical coherent potential approximation (DCPA)

Abstract: A theoretical algorithm is presented for numerical calculation of quasimetallic reflection bands of molecular excitons by a unified model which takes account of exciton–intramolecular phonon coupling by the dynamical coherent potential approximation and the polariton effect (the directional dispersion effect) by an anisotropic dielectric tensor. This method is applied to analyze experimental quasimetallic reflection spectra of DCI (1‐1’‐diethyl 2‐2’‐cyanine iodide), TCNQ (tetracyanoquinodimethane), and CTIP [γ‐cyclopropyl‐bis(1,2,3’‐trimethyl‐indolenine‐2‐yl) pentamethinium tetrafluoroborate] crystals. From a comparison of experimental and calculated reflectivity spectra, important exciton parameters, such as the bandwidths and shapes of low dimensional density‐of‐states functions of excitons, have been estimated.

Calculations of the electronic structure of disordered transition metal dihydrides

Abstract: Employing the tight binding coherent potential approximation method, we investigated the electronic structure of the substitutionally disordered transition metal dihydride series Ti1-yNbyHx, Ti1-yVyHx (0 ⩽ y ⩽ 1; 1.5 ⩽ x ⩽ 2.0). These alloys contain disorder on both the metal and hydrogen sublattices. We found that trends in the region of the Fermi level display rigid band-like characteristics for the substitution of niobium or vanadium for titanium, and for the hydrogen disorder as well. The calculations are compared with electronic specific heat measurements and predictions based on nuclear magnetic resonance experiments.

Mn 3d states in photoelectron spectra from Cd1−xMnxTe

Abstract: A parametrized tight-binding model is used to describe the electronic structure of semimagnetic Cd1–xMnxTe. The density of states and the spectral density are calculated by using the coherent potential approximation. The features connected with the presence of the Mn 3d states are identified and the controversial results of angle-resolved and integrated photoemission from Cd1–xMnxTe are then explained. Mit einem parametrisierten tight-binding-Modell wird die Elektronenstruktur von semimagnetischem Cd1–xMnxTe beschrieben. Die Berechnung der Zustands- und Spektraldichte erfolgt mit Hilfe der Methode des koharenten Potentials (CPA). Die mit der Anwesenheit von Mn 3d-Zustanden verbundenen Merkmale werden identifiziert und die einander wiedersprechenden Resultate von winkelaufgelosten und integrierten Photoemissionsspektren von Cd1–xMnxTe erklart.

Electron transfer through film-covered surfaces. Case of monolayer/submonolayer: a coherent potential approximation formalism

Abstract: The problem of electron transfer mediated by the chemisorbed layer with random occupancy on a 2-dimensional lattice is taken up. The anal. employs linear response formalism and CPA for evaluating the current. Two cases referring to (1) the delocalized and (2) the localized electronic states in the chemisorbed layer are treated.

Carrier drift and trapping in one dimensional systems

Abstract: There exist novel classes of materials ranging from conjugated polymers and anisotropic salts to Langmuir-Blodgett films which under certain experimental conditions exhibit one-dimensional excitation dynamics. In this paper we consider the Master equation approach to diffusion drift and trapping in such ‘quasi one-dimensional systems’. Using exact, scaling, and coherent potential approaches we consider a series of phenomena appropriate to the description of such systems. These include the following category of problems:1. the deep trapping problem (exact),2. the bond disorder problem (scaling and coherent medium),3. and finally the combination of1. with2. The relationship to present and planned experiments is discussed.

Magnetic and electronic properties of the alloy system CuxRh1-x

Abstract: The electronic structure of the disordered substitutional alloy system CuxRh1-x has been studied by means of charge self-consistent Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) calculations. The results of these investigations are used to discuss recent photoemission, susceptibility and NMR data on CuxRh1-x in detail.

Electronic structure of disordered CuPd alloys: A two-dimensional positron-annihilation study.

Abstract: Two-dimensional--angular-correlation experiments using posi- tron-annihilation spectroscopy were performed on a series of disordered Cu-rich CuPd-alloy single crystals. The results are compared with theoretical calculations based on the Korringa-Kohn-Rostoker coherent-potential approximation. Our experiments confirm the theoretically predicted flattening of the alloy Fermi surface near (110) with increasing Pd concentration. The momentum densities and the two-dimensional--angular-correlation spectra around zero momentum exhibit a characteristic signature of the electronic states near the valence-band edge in the alloy.

Electronic density of states in pseudo-binary compounds

Abstract: A new method to calculate the electronic density of states in pseudo-binary intermetallic compounds of (A1-xBx)mCn type is developed by combining the coherent potential approximation and the recursion method. Numerical examples are given for the density of states of the tight-binding s-band model with a simple atomic structure. An application to the tight-binding d-band model for Y(Fe-Co)2 and Y(Mn-Co)2 compounds with the cubic Laves structure is also given.

Material Constants of Composites. II. An Improvement over the Coherent Potential Approximation for Crystals with Site-Diagonal Disorder†

Abstract: It is shown that the coherent potential approximation violates a fundamental requirement, called additivity in a previous work. A new approximation for the calculation of the self-energy of crystals with site-diagonal disorder is proposed, which interpolates between the two dilute limits of the average t-matrix approximation and fulfils the requirement of additivity. Es wird gezeigt, das die „coherent potential”-Naherung eine fundamentale Forderung verletzt, die in einer vorhergehenden Arbeit Additivitat genannt wurde. Es wird eine neue Naherung fur die Berechnung der Selbstenergie ungeordneter Kristalle vorgeschlagen, die eine Interpolation zwischen den beiden Grenzfallen der „average t-matrix”-Naherung darstellt und die Forderung nach Additivitat erfullt.

Positron-annihilation study of the electronic structure of Cu91Ge

Abstract: A novel effect of alloying on the electron momentum density in a disordered ..cap alpha..-Cu/sub 91/Ge/sub 9/ alloy is reported. The effect, which is the appearance of a bump in the derivative spectra at low momenta, is predicted by theory employing the coherent-potential approximation and is observed in two-dimensional angular correlation experiments on the alloy. It reflects the presence of an s-p impurity band far below the Fermi level. Accurate experimental values for the Fermi-surface radii k/sub 100/ and k/sub 110/ and for the associated disorder-induced smearings in CuGe are also reported for the first time.

A simple cluster improvement to the coherent potential approximation

Abstract: An extremely simple way of including cluster corrections in CPA calculations is presented. The procedure is tested on the one-dimensional binary alloy density of states and gives an excellent agreement with Monte-Carlo calculations. Une methode simple est presente pour introduire les corrections d'amas aux calculs CPA. La methode est applique aux alliages binaires unidimensionnelles et un excellent accord avec les calculs Monte-Carlo est trouve.

Physical properties of amorphous silicon alloy films: Comparison with electronic structure calculations

Abstract: Optical bandgap measurements for two amorphous alloy structures a-(SiC)x Ge1−xHy and a-Six Sn1−xHy have been performed for a wide range of germanium, tin and hydrogen concentrations. The experimental measurements are in excellent agreement with the calculated optical bandgap and electronic density of states. The model uses the coherent potential approximation with diagonal disorder while the off-diagonal disorder is treated within the virtual crystal approximation. Photoluminescence measurements for (SiC)x Ge1−x Hy have identified phonon side bands attributed to carbon.

Electronic structure of the disordered‐local‐moment state of face‐centered‐cubic iron

Abstract: We calculate the electronic structure of fcc iron in the disordered‐local‐moment state using the self‐consistent Korringa–Kohn–Rostoker coherent‐potential approximation (KKR‐CPA). As the volume is reduced the moments decrease and, as many previous calculations have shown, there is also a change from ferromagnetic to antiferromagnetic correlations between local moments. In this paper, we show that there is a corresponding change from split to common band behavior of the electronic states and suggest that this is important in understanding the changing nature of magnetic interactions.