Showing papers on "Coherent potential approximation published in 1985"
TL;DR: The Korringa-Kohn-Rostoker coherent-potential approximation is extended to transport properties and the structure of the formalism is displayed by applying it to a one-dimensional muffin-tin model alloy.
Abstract: The Korringa-Kohn-Rostoker coherent-potential approximation is a powerful and versatile tool for calculating the single-particle properties of disordered alloys. Here this technique is extended to transport properties. The formalism is based on the one-electron Kubo formula. It yields correct results in the limits of weak scattering or low concentration, i.e., the solution to the Boltzmann equation including vertex corrections. Transport coefficients can also be easily evaluated in the strong-scattering, high-concentration regime where the Boltzmann equation is not valid. The structure of the formalism is displayed by applying it to a one-dimensional muffin-tin model alloy.
255 citations
TL;DR: In this paper, it was shown that the coherent potential approximation becomes asymptotically exact for a wide class of hierarchical model composites made of spherical grains, including thermal conductivity, dielectric constant and magnetic permeability.
Abstract: The effective electrical conductivity of an aggregate, composed of grains of various conductivities, is frequently estimated by the coherent potential approximation, which embodies a local effective medium concept. It is proved rigorously that this approximation is exact for a wide class of hierarchical model composites made of spherical grains: the starting material 0 in the hierarchy is chosen arbitrarily, otherwise, materialj=1, 2, ... consists of equisized spheres, sayj-spheres, of arbitrary conductivities embedded in materialj — 1. The spatial distribution of thej-spheres must satisfy a mild homogeneity condition and their radiusr
j
must, asymptotically, increase faster than exponentially withj. Furthermore, the minimum spacing, 2s
j
, between thej-spheres is such that the ratios
j
/r
j
diverges. On the basis of these and some further ancillary conditions it is established that the coherent potential approximation becomes asymptotically exact for the effective conductivity of materialj→∞. The results extend to other effective parameters of the composites, including the thermal conductivity, dielectric constant and magnetic permeability. In addition, the model composites and the proof of realizability may be generalized to allow non-spherical grains.
162 citations
TL;DR: In this article, a simple model is presented to describe superconductor-semiconductor phase transition for a binary alloy system, consisting of randomly mixed two sites with different site energies and on site attractive interaction between electrons.
Abstract: A simple model is presented to describe superconductor-semiconductor phase transition for a binary alloy system. The model consists of randomly mixed two sites with different site energies and on site attractive interaction between electrons. The site energy difference is treated by the coherent potential approximation. The attractive interaction, which is treated by mean field approximation, stabilizes either the semiconductor phase, or the superconductivity depending on the choice of parameters. Applicability of this model to BaPb 1- x Bi x O 3 is discussed.
37 citations
TL;DR: In this article, the inelastic part of the magnetic neutron scattering cros section of a periodic model for valence fluctuations between two magnetic configurations is calculated using the coherent potential approximation.
15 citations
TL;DR: In this article, the electronic structure of a semi-infinite crystal with a partial coverage by adatoms is studied within the tight-binding formalism, and the effect of surface disorder is treated within the coherent potential approximation.
Abstract: The electronic structure of a semiinfinite crystal with a partial coverage by adatoms is studied within the tight-binding formalism. The effect of the surface is described via the surface Green function method, and the effect of disorder is treated within the coherent potential approximation. The local state densities at/near the surface and the interaction energy of adatoms are calculated as functions of the coverage. The examples representing the case of a gas adsorption on the transition metal surfaces, as well as the adsorption of the transition metal atoms on the transition metal surfaces are investigated numerically.
11 citations
TL;DR: In this paper, the authors investigated the coverage dependence of the work function in the alkali-adsorbed Si(001) system by a model which includes a hopping term between adsorbed atoms as well as a mixing term between the states of the adsorbate and the substrate.
10 citations
TL;DR: In this article, a simple model of α-SiHx is studied; its only adjustable parameter is the hydrogen concentration x. By combining the coherent potential approximation with the potential well analogy explicit results are obtained for quantities such as optical absorption, position of mobility edges, mean free path, density of states at the mobility edge, and pre-exponential factor in the dc conductivity.
Abstract: A simple model of α-SiHx is studied; its only adjustable parameter is the hydrogen concentration x. By combining the coherent potential approximation with the potential well analogy explicit results are obtained for quantities such as optical absorption, position of mobility edges, mean free path, density of states at the mobility edge, and pre-exponential factor in the dc conductivity. The latter is obtained by neglecting the effects of electron-phonon interaction, which is expected to lead to a strongly temperature-dependent mobility. The theoretical results are in reasonable agreement with experimental data.
9 citations
TL;DR: Theories of the conductivity in weakly disordered systems are extended to be applicable to strongly disordered system in this article, where effects of strong disorder are taken into account within the coherent potential approximation.
Abstract: Theories of the conductivity in weakly disordered systems are extended to be applicable to strongly disordered systems. Effects of the strong disorder are taken into account within the coherent potential approximation. The conductivity of three dimensional systems is discussed.
8 citations
01 Jan 1985
TL;DR: In this article, the authors used the overlap valence shell model to describe the pure compound lattice dynamics, which describes fairly well the dispersion curves and the displacement eigenvectors.
Abstract: From the experimental point of view, the III-V mixed crystals lattice dynamics has been classified into two types: one-mode and two-mode behaviours. The latter one, which occurs the most often, has been accounted for using Coherent Potential Approximation (CPA) models (1). In these treatments, one uses the virtual crystal approximation for short range forces, one treats within the CPA the influence of mass disorder, and eventually the long range electrostatic interactions are handled, at the end, as a perturbation. As a final result one gets two zone-center broadened modes, each of them exhibiting a small longitudinal transverse (LT) splitting. The only compound which, up to now, was currently considered as displaying a one-mode behaviour was GaxIn1−xP. One of us (2) recently reported the observation of two close longitudinal optical modes in this compound and attributed these feature to the weakness of the mass defect induced splitting compared to the LT one. It appears thus worthy to elaborate a model which incorporates from the beginning the long range forces and thus to obtain a general picture of the mixed crystal lattice dynamics. To this purpose we use, to describe the pure compound lattice dynamics the overlap valence shell model (3) which describes fairly well the dispersion curves and the displacement eigenvectors. We perform for the alloy a virtual crystal approximation on the dynamical matrix (including the dipolar interaction scaled on the average lattice parameter) and we treat the diagonal mass disorder within the CPA. The principal difficulty of this calculation is to compute the self-consistent diagonal matrix elements of the Green function which are obtained through integration over the whole Brillouin zone.
5 citations
5 citations
TL;DR: In this paper, the optical spectra of crystalline antracene using a cluster dynamical coherent approximation was calculated using a combination of a one-and a three-dimensional density of states.
TL;DR: In this paper, the lattice dynamics of displacive type phase transitions in solid solutions with a quantum paraelectric and a ferroelectric component is considered and the phonon spectrum of the disordered lattice is obtained and the concentration dependence of the phase transition temperature is calculated.
Abstract: The lattice dynamics of displacive type phase transitions in solid solutions with a quantum paraelectric and a ferroelectric component is considered. Using the self-consistent phonon approximation and the coherent potential approximation the phonon spectrum of the disordered lattice is obtained and the concentration dependence of the phase transition temperature is calculated. Good agreement with experimental results for (Ba, Sr) TiO3 is obtained.
TL;DR: In this article, the coherent potential approximation was generalized for quaternary alloys of the types A x C y B 1− x − y D and A x B 1 − x c y C y D 1− y.
TL;DR: In this article, a Green function formalism is developed for the resonant scattering of acoustic phonons by randomly distributed two-level systems and the randomness is treated by the coherent potential approximation.
Abstract: A Green function formalism is developed for the resonant scattering of acoustic phonons by randomly distributed two-level systems. The randomness is treated by the coherent potential approximation. The theory reproduces the Jacobsen-Stevens dispersion law in the dense limit of the concentration of the two-level system and the results obtained so far by the average t -matrix approximation in the dilute limit. The gradual change of the character of the resonantly scattered phonons as the concentration is varied is investigated through the calculation of various quantities such as the phonon density of states, the neutron scattering cross sections and the sound velocity.
TL;DR: In this paper, a new systematic cluster theory beyond the single-site coherent potential approximation is presented for averaged one-particle Green's functions, developed in augmented space and illustrated for site-diagonal disorder.
Abstract: A new systematic cluster theory beyond the single-site coherent potential approximation is presented for averaged one-particle Green's functions. The formalism is developed in augmented-space and illustrated for site-diagonal disorder. The theoretical feature is to treat overlapping clusters exactly by employing a kind of the matrix continued fraction. The resulting Green's functions are analytic, translationally invariant, and predict proper limiting behaviors at weak or strong scattering and high or low concentrations. Model calculations for a static disordered chain are shown to compare well with the essentially exact calculation.
01 Jan 1985
TL;DR: In this article, the role of hydrogen in amorphous silicon (a-Si) was investigated using the coherent potential approximation (CPA) method, and first principles calculations of the electronic and transport properties of a-Si:H using the CPA were performed.
Abstract: In previous work1,2, we have obtained an understanding of the role of hydrogen in amorphous silicon (a-Si) performing first principles calculations of the electronic and transport properties of a-Si:H using the coherent potential approximation (CPA).
TL;DR: In this paper, the exponent describing the low frequency spectrum of vibrations for the central force universality class was computed with both the Coherent Potential Approximation and numerical simulations, and the results of both calculations agree surprisingly well.
Abstract: The exponent describing the low frequency spectrum of vibrations for the central force universality class is computed with both the Coherent Potential Approximation and numerical simulations. The results of both calculations agree surprisingly well.
TL;DR: In this paper, a model based on the Hubbard hamitonian where the stochastic fields acting at the sites of the atoms can assume arbitrary directions and values was treated and the coherent potential approximation (CPA)-like method was employed to study the behavior of the magnetically disordered system.
Abstract: We treat a model based on the Hubbard hamitonian where the stochastic fields acting at the sites of the atoms can assume arbitrary directions and values. The averaging procedure introduced by Brandt et al. to study the behaviour of the magnetically disordered system is used for a model with stochastically localized magnetic fields at the atomic sites. The coherent potential approximation (CPA)-like method is employed and the resulting new effects for the density of states, localization function anddc conductivity are demonstrated in two cases.
TL;DR: In this article, a self-consistent set of selfconsistent equations is proposed for the calculation of the disorder and Coulomb contributions to the electron self-energy and the electron-hole interaction in a many-electron system in the presence of a site-diagonal random potential.
Abstract: A new set of self-consistent equations is proposed for the calculation of the disorder and Coulomb contributions to the electron self-energy and the electron-hole interaction in a many-electron system in the presence of a site-diagonal random potential. The treatment of the disorder is along the lines of the coherent-potential approximation. An explicit expression for the Coulomb self-energy is derived within the time-dependent screened Hartree-Fock approximation. Some possible applications of the formalism to the formation of a Coulomb gap in a doped semiconductor and to electron-hole interaction effects in the optical spectra of alloy semiconductor systems are discussed.
TL;DR: In this article, a term describing scattering by randomly distributed impurity atoms is added to the periodic Anderson model to study the properties of a mixed valence compound which contains nonmagnetic impurities.
TL;DR: In this paper, the electron densities of states (DOS) for various concentrations of the Pd 1− y Ag y H x system were used in the evaluation of the electron-phonon coupling constant and the superconducting temperature T c.
Abstract: The tight binding coherent potential approximation is used to calculate the electron densities of states (DOS) for various concentrations of the Pd 1− y Ag y H x system. The DOS are used in the evaluation of the electron-phonon coupling constant λ and the superconducting temperature T c . Our results reproduce the experimentally determined behavior of the superconducting transition temperature's dependency on Ag concentration, and provide a satisfactory explanation of the mechanism of superconductivity in this system.
01 Jan 1985
TL;DR: In this paper, it was shown that the peak of the density of states at the core is split into two peaks separated by a gap, which is a real gap if the resonant states form a periodic lattice, but a pseudogap if they are distributed at random in three dimensions.
Abstract: Successive elastic scatterings on resonant states broaden the core of the corresponding virtual bound levels. If these states have all the same energy and equal coupling strength with the delocalised states, the peak of the density of states at the core is split into two peaks separated by a gap. This is a real gap if the resonant states form a periodic lattice, but a pseudogap if they are distributed at random in three dimensions. This last result is shown using an approximation equivalent to the coherent potential approximation. In one or two dimensions, the CPA appoach leads to inconsistencies coherent with the fact that all states are localised.
TL;DR: In this article, the energy spectrum of the dielectric phase states corresponding to interacting neutral donors and bound excitons is established using the single-site coherent potential approximation, and also the ground state energy of the plasma consisting of the D - impurity band electrons and valence band holes.
TL;DR: In this article, the authors calculate the magnetic susceptibility of the intermediate valence phenomena using the periodic Anderson Hamiltonian (PAH) as the theoretical model, and the calculations were made using the two-particle Green function formalism.
Abstract: The authors calculate the magnetic susceptibility of the intermediate valence phenomena. They have used the periodic Anderson Hamiltonian (PAH) as the theoretical model, and the calculations were made using the two-particle Green function formalism. The PAH is decoupled using the alloy analogue approximation (AAA). The average one-particle Green functions are calculated using the coherent potential approximation (CPA), and with these functions they calculate the density of states. These results are compared with a pseudo-virtual-crystal approximation model. The magnetic susceptibility is expressed in terms of a two-particle Green function. This function, with the above model, can be reduced to two one-particle Green functions, and these can be averaged using the two-particle extension of the CPA formalism. The AAA breaks the spherical symmetry of the Hamiltonian, giving different results for the longitudinal and transverse terms. From symmetry considerations the authors find that the total susceptibility is the weighted average of the two terms. The susceptibility can be interpreted in terms of the Pauli, Van-Vleck-and Curie-Weiss-like terms. An extra term arises from the random character of the model. The results are compared with the experimental results.
TL;DR: An analytical and statistically homogeneous cluster theory for elementary excitations in disordered systems is developed on the basis of a graphical method in augmented space as discussed by the authors, which preserves the original lattice symmetry and including all overlapping cluster scattering effects.
Abstract: An analytical and statistically homogeneous cluster theory for elementary excitations in disordered systems is developed on the basis of a graphical method in augmented space. An effective Hamiltonian diagram, preserving the original lattice symmetry and including all overlapping cluster scattering effects, is constructed. The expression for the averaged one-particle Green's function, which corresponds to an interpolation formula connecting between weak and strong scattering and exhibits a momentum-dependent self-energy, is obtained from that diagram by introducing a Bethe lattice approximation. Calculations of the density of states and the spectral function for simple cubic lattices are presented, and show a better fit to exact numerical results than the single-site coherent potential approximation.
TL;DR: In this paper, the results of x-ray photoemission spectroscopy (XPS) and electrical resistivity measurements on amorphous metal alloys of the form (Mo 1−xRux)80Si20 are reported.
Abstract: The results of x-ray photoemission spectroscopy (XPS) and electrical resistivity measurements on amorphous metal alloys of the form (Mo1−xRux)80Si20 are reported The XPS valence band spectra are dominated by the Mo-4d subbands and the electronic structure of the amorphous alloys can best be described in the virtual crystal limit of the coherent potential approximation (CPA) formalism The density of electronic states at the Fermi level sharply decreases in the alloys as the Ru/Mo ratio is increased Associated with this drop is an increase in electrical resistivity and decrease in the superconducting transition temperatures of the alloys These relationships are discussed in the context of current theories of the electrical resistivity and superconductivity in amorphous alloys
TL;DR: In this paper, the electronic density of clusters composed from Ti and Cu was calculated in the local coherent potential approximation (LCPA) using real space representations and the muffin-tin approximation calculations were carried out for different composition rates and for different structures simulating the possible short range order of the amorphous alloys.
Abstract: The electronic density of states of clusters composed from Ti and Cu was calculated in the local coherent potential approximation (LCPA). Using real space representations and the muffin-tin approximation calculations were carried out for different composition rates and for different structures simulating the possible short range order of the amorphous alloys. By comparing the numerical results with experimental data one can collect arguments for the geometrical structure of these alloys.
TL;DR: In this paper, the effects of alloy fluctuations on surface states and to assess the sensitivity of these effects to the localization of the states are presented for the local electronic surface densities of states of semi-infinite 1D chains.
TL;DR: In this article, a coherent potential approximation for the electrical conductivity of disordered binary alloys is formulated on the basis of the quantum kinetic equations, where the configuration average is denoted by a projection operator.
Abstract: The coherent potential approximation for the electrical conductivity of disordered binary alloys is formulated on the basis of the quantum kinetic equations. The configuration average is denoted by a projection operator. The qualitative properties in the coherent potential approximation for the average density operator ( rho (z)) are examined and compared with those of its exact counterpart. Finally, for the case of a general one-band model of a random binary alloy, the DC electrical conductivity tensor is calculated. This new formulation constitutes a generalisation of a method used for the study of equilibrium properties of random binary alloys without additional assumptions, in contrast to Velicky's work where an additional decoupling approximation is assumed in generalising the CPA idea for the equilibrium properties to that of the transport properties. Furthermore, the new formulation has already been shown to be capable of accommodating the average t-matrix approximation (ATA) scheme.