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Showing papers on "Coherent potential approximation published in 1990"


Journal ArticleDOI
TL;DR: Results of the calculations for the impurity formation energy, and for the bulk moduli, the lattice parameters, and the energy of mixing as a function of concentration in fcc Cu{sub {ital c}Zn{sub 1{minus}{ital c}} alloys show that this generalized density-functional theory will be useful in studying alloy phase stability.
Abstract: We present the details and the derivation of density-functional-based expressions for the total energy and pressure for random substitutional alloys (RSA) using the Korringa-Kohn-Rostoker Green's-function approach in combination with the coherent-potential approximation (CPA) to treat the configurational averaging. This includes algebraic cancellation of various electronic core contributions to the total energy and pressure, as in ordered-solid muffin-tin-potential calculations. Thus, within the CPA, total-energy and pressure calculations for RSA have the same foundation and have been found to have the same accuracy as those obtained in similar calculations for ordered solids. Results of our calculations for the impurity formation energy, and for the bulk moduli, the lattice parameters, and the energy of mixing as a function of concentration in fcc Cu{sub {ital c}}Zn{sub 1{minus}{ital c}} alloys show that this generalized density-functional theory will be useful in studying alloy phase stability.

177 citations


Journal ArticleDOI
TL;DR: The Korringa-Kohn-Rostoker coherent-potential-approximation Green's function for a disordered system of muffin-tin potentials is considered and the resulting expression is shown to possess a perfect derivative form, provided the angular-momentum summations are extended to infinity.
Abstract: We consider the Korringa-Kohn-Rostoker coherent-potential-approximation Green's function for a disordered system of muffin-tin potentials and evaluate its integral over the Wigner-Seitz cell rigorously throughout the complex energy plane. The resulting expression is shown to possess a perfect derivative form, provided the angular-momentum summations are extended to infinity; for a finite angular-momentum cutoff this result is valid to a good approximation. For real energies our expression constitutes a generalization of the Lloyd formula for the density of states. The conventional Lloyd formula contains unphysical singularities that are not only unsatisfactory from a formal viewpoint, but also make its use increasingly cumbersome in multicomponent alloys as the number of atoms in the unit cell becomes larger. Our expression, on the other hand, is free from such singularities, and thus provides a useful basis for evaluating density of states and its energy integral in muffin-tin systems.

170 citations


Journal ArticleDOI
TL;DR: On s'interesse particulierement aux changements, dans la structure electronique, dus au desordre et au remplacement isoelectronique de l'element Pd Gd par Pt 5d, puis par son voisin Au.
Abstract: On s'interesse particulierement aux changements, dans la structure electronique, dus au desordre et au remplacement isoelectronique de l'element Pd Gd par Pt 5d, puis par son voisin Au. On etudie ces controverses concernant la structure electronique de l'alliage Cu 3 Pd desordonne

38 citations


Journal ArticleDOI
TL;DR: In this article, the effect of a random distribution of single-center transition frequencies on the spectrum of a system whose optically active states are Frenkel excitons is addressed from a theoretical perspective.

30 citations


Journal ArticleDOI
TL;DR: In this article, the hyperfine fields in the substitutionally disordered BCC alloy systems FexCr1-x and FexCo 1-x have been studied by means of charge self-consistent Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) electronic structure calculations.
Abstract: The hyperfine fields in the substitutionally disordered BCC alloy systems FexCr1-x and FexCo1-x have been studied by means of charge self-consistent Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) electronic structure calculations. For the various components, the contribution to the Fermi contact hyperfine field due to core polarisation was found to be proportional to the corresponding local spin moment. The contributions of the conduction band, however, did not show such a simple behaviour and turned out to be dominated by the occupation of the s bands for spin up and down. To study the influence of relativistic effects on the hyperfine fields the corresponding matrix elements for the Fermi contact interaction have been calculated nonrelativistically as well as fully relativistically. The importance of contributions to the hyperfine fields coming from non-s electrons have been investigated by performing spin-polarised relativistic linear muffin tin orbital method (SPRLMTO) band structure calculations for hypothetically ordered alloys.

26 citations


Book
01 Jan 1990
TL;DR: In this article, the Fermi surface has been determined based on the one-particle picture of metal electrons and X-ray spectroscopy, and it has been shown that it is possible to calculate the band structure of a lattice electron.
Abstract: 1. One-Particle Picture of Metal Electrons. Electron gas. Band structure. Many-particle problems. 2. Methods of Band Structure Calculations. Tight binding method. Pseudopotential method. Multiple scattering picture. APW method. KKR method. H-NFE-TB method. Coherent potential approximation. 3. Results of Band Structure Calculations. Simple metals. Transition metals. Compounds and alloys. 4. Experimental Determination of Band Structure. Dynamics of lattice electrons. Experimental determination of the Fermi surface. Photo-electron and X-ray spectroscopy. 5. Further Electronic Properties. Density of states, heat capacity, transition temperature of superconductivity, magnetic susceptibility, and transport quantities. Dielectric function, inelastic electron and photon scattering, and optical reflection. Lattice properties. Electron-phonon-interaction. Electronic properties of localized defects. Other properties. 6. Surfaces, Thin Layers, Quasi-One- and Quasi-Two-Dimensional Conductors. Peculiarities of surfaces and thin films. Ideal surfaces of metals. Systems with low-dimensional structure. Peculiarities of low-dimensional systems. Peierls instability. Electronic excitations in the Peierls system. References. Solid state physics in general. Recent monographs and proceedings. Special literature quoted in the text. Summaries. Index of abbreviations. Natural constants and atomic units. Characteristic quantities of the electron gas. Subject index.

25 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the coherent potential approximation in the dilute alloy limit based on a simple model density of states leads to a very simple rule of the surface segregation in dilute binary transition metal alloys.

18 citations


Journal ArticleDOI
TL;DR: A unified picture of the electronic structure of Si in amorphous and liquid phases is obtained through a well-grounded extension of the coherent-potential approximation to random Bethe lattices with off-diagonal disorder.
Abstract: Bethe lattices of arbitrary number of nearest-neighbor atoms and geometrical configurations are defined for the standard {ital sp}{sup 3}{ital s*} tight-binding Hamiltonian that gives correctly both valence and conduction bands of crystalline Si. Averaged densities of states are obtained through a well-grounded extension of the coherent-potential approximation to random Bethe lattices with off-diagonal disorder. In that way, a unified picture of the electronic structure of Si in amorphous and liquid phases is obtained. Formation energies of dangling and floating bonds are estimated within Chadi's tight-binding scheme.

18 citations


Journal ArticleDOI
TL;DR: In this paper, a formalism for the study of alloy disorder at crystalline interfaces on the phonon boundary scattering cross section is presented. But it is based on t-matrix scattering theory and allows for separation of coherent and incoherent parts of the reflection and transmission channels.
Abstract: The author presents a formalism for the study of the effects of alloy disorder at crystalline interfaces on the phonon boundary scattering cross section. The disordered region is treated within the coherent potential approximation adapted to the interface geometry. The formalism is based on t-matrix scattering theory and allows for separation of the coherent and incoherent parts of the reflection and transmission channels. The theory is developed for a single-phonon-branch model and numerical results are presented for a simple cubic interface.

14 citations


Journal ArticleDOI
TL;DR: Abe et al. as discussed by the authors studied the self-trapping process of photo-created hot excitons which interact strongly with phonons in an effective medium approximation for Green functions.
Abstract: Dynamical self-trapping processes of photo-created hot excitons which interacts strongly with phonons are theoretically studied in an effective-medium approximation for Green functions. The previous formulation [S. Abe: J. Phys. Soc. Jpn. 57 (1988) 4029] of the resonant secondary-emission spectrum in the dynamical coherent potential approximation is extended in such a way that the effect of a finite band width of the phonon branch relevant to self-trapping is taken into account. Emission spectra calculated in this new method indicate that the intensity of hot luminescence from the metastable free state decreases with raising the incident-photon energy from the bottom of the free exciton band, suggesting that a part of hot excitons become self-trapped before they thermalize within the free exciton band states.

13 citations


Journal ArticleDOI
TL;DR: Calculated spectroscopic properties like x-Ray-emission and x-ray photoemission spectroscopy are sensitive with respect to Zn substitution, but too insensitive to resolve differences withrespect to the alloy formation on the different sublattices of Cu.
Abstract: The electronic structure of the Y-Ba-Cu-O system as alloyed by Zn on the Cu(1) and Cu(2) sublattices is calculated for Zn contents up to x=01 Distinct differences for these two cases of alloy formation are found in terms of partial densities of states (DOS's) The Fermi level and the DOS at the Fermi level, however, vary similarly with the Zn concentration This behavior can be explained using a description of localization based on integrated local DOS's In general, calculated spectroscopic properties like x-ray-emission and x-ray photoemission spectroscopy are sensitive with respect to Zn substitution, but too insensitive to resolve differences with respect to the alloy formation on the different sublattices of Cu

Journal ArticleDOI
TL;DR: In this article, the influence of impurity scattering on a heavy fermion system was studied within a mean-field approximation of the Anderson Hamiltonian and using a coherent potential approximation for the impurity scatter.
Abstract: The influence of impurity scattering on a heavy fermion system is studied within a mean-field approximation of the Anderson Hamiltonian and using a coherent potential approximation for the impurity scattering. Two types of scattering mechanisms are investigated: missingf-ions and potential scatterring centers. The dynamical conductivity shows a nearow peak at low frequencies scaling with the effective mass of the heavy quasi-particles together with a peak at finite frequencies due to interband transitions. These features are also found in experimental results on heavy fermion systems.

Journal ArticleDOI
TL;DR: The generalized Takayama-Lin-Liu-Maki model is analyzed with the help of the coherent-potential approximation using the assumption that the order parameter is uniform with one electron per site.
Abstract: The electronic level structure and the dimerization order parameter are studied in doped conjugated polymers when randomly distributed site-type impurities occupy different positions from those in previous investigations. The generalized Takayama-Lin-Liu-Maki model is analyzed with the help of the coherent-potential approximation using the assumption that the order parameter is uniform with one electron per site. The order parameter, the electronic level structure, and its phase diagram are obtained numerically

Journal ArticleDOI
TL;DR: In this paper, an s phase shift semicircular model of a random substitutional binary alloy has been used as a test case for studying the self-consistent Korringa-Kohn-Rostoker cluster coherent-potential approximation (KKR-CCPA).
Abstract: An s phase shift semicircular model of a random substitutional binary alloy has been used as a test case for studying the self-consistent Korringa-Kohn-Rostoker cluster coherent-potential approximation (KKR-CCPA). In addition to being computationally simpler, the model has the advantage that a corresponding model exists in the well known tight-binding framework. A one-to-one correspondence has been shown between the KKR-CCPA equations and the tight-binding CCPA equations. In the tight-binding framework, it has been shown that certain quantities, which were hitherto calculated approximately, can be obtained exactly by the partitioning technique. Results for density of states in the CCPA show rich structure in the impurity band, which arises due to correlated scattering from clusters of atoms.

Journal ArticleDOI
TL;DR: In this paper, the electrical resistivity of the heavy-fermion alloys was calculated through a two-conduction-band slave boson model using the self-consistent coherent potential approximation method.
Abstract: The electrical resistivity of the heavy-fermion alloys is calculated through a two-conduction-band slave boson model using the self-consistent coherent potential approximation method. The results indicate that for very low temperatures the resistivity of the alloy system follows the rho 0+AT2 law and the coefficient A changes from negative values to positive values as the alloy concentration increases. The occurrence of the resistivity maximum at a finite temperature is also obtained on increasing the concentration of heavy-fermion alloys.

Journal ArticleDOI
TL;DR: In this paper, the authors used the realistic tight-binding Hamiltonian to calculate the surface segregation for Cu-Ni alloys, and the model for all reasonable values of parameters predicts the segregation of copper for all alloy compositions.
Abstract: The surface segregation is calculated in the coherent-potential approximation. The influence of the surface potential, d-band fillings and d-level splitting of alloys components on the segregation is examined for a model density of states. The realistic tight-binding Hamiltonian is used to calculate the segregation for Cu-Ni alloys. The model for all reasonable values of parameters predicts the segregation of copper for all alloy compositions.

Journal ArticleDOI
TL;DR: In this article, the authors used an extended Takayama-Lin-Liu-Maki (TLM) model to study the order effects in doped trans-polyacetylene.
Abstract: Disorder effects in doped trans -polyacetylene are studied, using an extended Takayama-Lin-Liu-Maki (TLM) model. Bond- and site-type impurity models are analyzed when the doping is compensated. Electronic states are calculated by the self-consistent Born approximation (SCBA) and also by the coherent potential approximation (CPA). Results do not depend on the type of the impurity. We find that peaks in the electronic density of states become steeper and the energy gap is wider in the CPA than in the SCBA, while change in the order parameter is small. Two critical concentrations, where the gap and the order parameter vanish, come close each other. A phase diagram is given for the electronic structures in the CPA. For the site-type impurities, two impurity bands are found in association with the two signs of impurities. For the bond-type impurities, the total energy recovers the symmetry property for the order parameter.

01 Jul 1990
TL;DR: In this article, the effects of multiple scattering processes that influence the scattering and absorption of electromagnetic radiation by dense foliage is presented as effective dielectric constants which can be used to represent the forest environment as a homogeneous Dielectric slab in electromagnetic scattering models.
Abstract: : The output from a model that calculates the effects of multiple scattering processes that influence the scattering and absorption of electromagnetic radiation by dense foliage is presented. Results are presented as effective dielectric constants which can be used to represent the forest environment as a homogeneous dielectric slab in electromagnetic scattering models. An objective of this report is to present a formalism that allows the reader to incorporate the results into a simulation program. Several models of multiple scattering processes in the foliage media are considered for determining the effective parameters. A brief discussion of the Average T-Matrix Approximation (ATA) and the Coherent Potential Approximation (CPA) are presented. Keywords: Foliage scattering, EM Wave propagation, Effective dielectric, Radar clutter, Dielectric constant.

Journal ArticleDOI
TL;DR: In this paper, the ground state properties of the low-density electron system with the short-range electron-phonon interaction were investigated theoretically in connection with the self-trapping of an electron.
Abstract: The ground-state properties of the low-density electron system with the short-range electron-phonon interaction is investigated theoretically in connection with the self-trapping of an electron. The energy change of the system due to lattice distortions is calculated with the use of Fumi's theorem and the coherent potential approximation. It is shown that the itinerant electrons and the self-trapped electrons coexist in a certain range of the parameters.

Journal ArticleDOI
TL;DR: In this article, a generalisation of the coherent potential approximation and its cluster extensions is developed for systems that have both inherent static disorder as well as potentials that fluctuate randomly with time.
Abstract: Using the ideas of the augmented space formalism introduced some time ago by this author, a generalisation of the coherent potential approximation and its cluster extensions is developed for systems that have both inherent static disorder as well as potentials that fluctuate randomly with time. The formalism reduces to standard results in the adiabatic limit.

Journal ArticleDOI
TL;DR: In this article, the dynamics of an exciton strongly interacting with phonons are studied by calculating the optical absorption and the secondary-emission spectra in a new effective medium approximation which is an extension of the dynamical coherent potential approximation.

Journal ArticleDOI
TL;DR: In this paper, the authors studied self-trapping of a Frenkel exciton in a binary mixed crystal with randomness in the exciton atomic (e i ) and the transfer energies, and the short range exciton-lattice interaction D i.
Abstract: We studied self-trapping of a Frenkel exciton in a binary mixed crystal A x B 1- x with randomness in the exciton atomic (e i ) and the transfer ( t i j ) energies, and the short range exciton-lattice interaction D i . The concentration dependence of the energies of relaxed excitons and the emission energies are calculated for self-trapped ( S i ) states at i =A or B site and free (F) states using the coherent potential approximation. Extrinsic self-trapping and the F-S transition are discussed with phase diagrams obtained for various parameters Δ=e B -e A , D i , the exciton band width T i ( i =A, B) and the concentration x . The results are compared with recent experiments on mixed ionic crystals and II-VI compound semiconductors.

Journal ArticleDOI
G. H. Schadler1
TL;DR: In this article, the relativistic Korringa-Kohn-Rostocher Green's function (RKKR-GF) Coherent Potential Approximation (CPA) method was used for electronic structure calculations for stoichiometric and substoichiometric UCx.

Journal ArticleDOI
TL;DR: In this article, the electronic structure of the FCC phase of sputter-deposited Fe-Cu-Ag ternary alloy films and their X-ray photoelectron spectra using the tight-binding linear muffin-tin orbital method were discussed.
Abstract: The authors discuss the electronic structure of the FCC phase of sputter-deposited Fe-Cu-Ag ternary alloy films and determine theoretically their X-ray photoelectron spectra using the tight-binding linear muffin-tin orbital method and the coherent potential approximation. The calculated results are in good agreement with the experimental data.

Journal ArticleDOI
TL;DR: In this paper, a theory to study the electronic density of states (DOS) of trigonal and chain Se1−xTex was presented, and the energy bands of pure Se and Te were calculated by making use of the information of the optical gaps.
Abstract: We present a theory to study the electronic density of states (DOS) of trigonal and chain Se1−xTex. By making use of the information of the optical gaps, we calculate the energy bands of pure Se and Te both in the trigonal and chain form by the tight-binding method. Using this result, we calculate the DOS of trigonal and chain Se1−xTex in the coherent potential approximation for three and one dimension respectively. We also evaluate the optical gap as functions of the Te concentration x, which we compare with experimental data. In view of all these results, we ascertain that some physical properties of a Se1−xTex mixture is different according as the system is a three-dimensional crystal or a one-dimensional chain.

Journal ArticleDOI
TL;DR: Le formalisme utilise permet de calculer la fonction de Green dans la representation mixte pour un superreseau ayant un general de monocouches.
Abstract: Le formalisme utilise permet de calculer la fonction de Green dans la representation mixte pour un superreseau ayant un general de monocouches. Les couches desordonnees sont decrites dans l'approximation du potentiel coherent. Deux sortes d'arrangement sont etudies. Les resultats montrent la sensibilite du spectre de phonons vis-a-vis de la coherence a longue distance de la composition chimique des superreseaux et de la perfection de leur preparation

Journal ArticleDOI
TL;DR: In this paper, a simple treatment of the X-ray photoemission spectra based on the electronic density of states (DOS) and the atomic photoionization cross-sections of alloy constituents, is presented.
Abstract: The electronic structures of fcc and bcc phases of sputter-deposited Fe-Cu and Fe-Ag alloy films have been studied using the tight-binding linear muffin-tin orbital method in conjunction with the coherent potential approximation. A simple treatment of the X-ray photoemission spectra based on the electronic density of states (DOS) and the atomic photoionization cross-sections of alloy constituents, is presented. The calculated XPS results are in good agreement with the recent experimental data on these alloys.

Journal ArticleDOI
TL;DR: Les resultats experimentaux sont interpretes via des simulations relativistes de l'approximation du potentiel coherent de Korringa-Kohn-Kostoker des bandes d'energie complexe, des densites spectrales et des densite d'etat.
Abstract: Mesures realisees par emission photoelectronique sur la surface (100) de cet alliage a l'aide des excitations He I, Ne I, He II et Ne II. Les resultats experimentaux sont interpretes via des simulations relativistes de l'approximation du potentiel coherent de Korringa-Kohn-Kostoker des bandes d'energie complexe, des densites spectrales et des densites d'etat

Journal ArticleDOI
TL;DR: In this paper, a formulation of the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) for the treatment of substitutionally disordered alloys within the KKR atomic sphere approximation (ASA) was presented.

Journal ArticleDOI
TL;DR: In this article, the electronic structure of an amorphous Ge-Sn alloy is modelled by an extension of the coherent potential approximation (CPA), which takes into account the chemical order and the absence of Sn dangling bonds found experimentally.