Showing papers on "Coherent potential approximation published in 1983"
TL;DR: In this paper, a simple model for the description of this polarization in non-crystalline SiO2 is developed and used for the calculation of the density of states of LO and TO phonons in the framework of a coherent potential approximation for structurally disordered systems.
Abstract: Until now there exists no calculation of the phonon density of states of amorphous SiO2 and related materials, which could interpret the experimentally detected splittings between longitudinal and transverse modes in their optical phonon spectra. This problem is solved by taking into account the influence of the vibrationally induced polarization on the motion of atoms. A simple model for the description of this polarization in non-crystalline SiO2 is developed and used for the calculation of the density of states of LO and TO phonons in the framework of a coherent potential approximation for structurally disordered systems. This new method leads also to the determination of the IR response function and to simple analytical expressions for the microscopic parameters of the model, i.e. the bond angle at the oxygen atom and transverse dynamic effective charges. The values of these parameters are determined from IR transmission spectra of thin SiO2 layers prepared with the help of different methods. The calculated density of states and IR response function are in good agreement with corresponding experimental results.
154 citations
TL;DR: In this paper, the electronic structure of paramagnetic iron is calculated self-consistently within the local spin density functional formalism by applying the coherent potential approximation with the muffin-tin potential model.
Abstract: The electronic structure of paramagnetic iron is calculated self-consistently within the local spin density functional formalism by applying the coherent potential approximation with the muffin-tin potential model. The effective exchange parameters Jn are calculated as the interaction energy between two magnetic moments in the paramagnetic medium obtained. A discussion on the magnetism of iron above the Curie temperature is presented using the Heisenberg model with the calculated Jn.
147 citations
TL;DR: In this paper, the dielectric function of an inhomogeneous system consisting of small particles in a gelatin matrix is investigated using a lattice gas model to describe the topological disorder among the particles.
Abstract: The dielectric function of an inhomogeneous system consisting of small particles in a dielectric host material is investigated using a lattice gas model to describe the topological disorder among the particles By applying the coherent potential approximation, an effective single-particle polarisability is found, which contains the influence of the local electric fields arising from randomly distributed particles In the Maxwell-Garnett theory it is assumed that these fields in a random distribution of dipoles are the same as those calculated for a cubic array of dipoles The disorder is shown to give a substantial red shift of absorption peaks and a comparable blue shift of loss lines Moreover, absorption as well as loss features are very strongly broadened These kinds of shifts and enhanced linewidths have been observed experimentally for many systems A detailed comparison with measured spectra is given for silver particles immersed in a gelatin matrix
61 citations
TL;DR: In this article, the dispersion curves of folded acoustical modes in large period GaAs / Ga 1− x Al x As superlattices were determined by scanning the incident wavelength in a Raman scattering experiment.
32 citations
TL;DR: In this article, the effect of degenerate multi-bands with t2g and eg symmetry on the Curie temperature, the magnetisation curve, the amplitude of local moments and the susceptibility of iron was investigated using the detailed density of states generated by the recursion method for the tight binding model.
Abstract: A previously developed theory of itinerant-electron magnetism at finite temperatures is generalised to include the effect of degenerate multi-bands with t2g and eg symmetry by adopting the static functional integral method and the coherent potential approximation (CPA). Numerical calculations of the Curie temperature, the magnetisation curve, the amplitude of local moments and the susceptibility are made for iron by using the detailed density of states generated by the recursion method for the tight-binding model. Calculated results are shown to account for finite-temperature properties of iron. In particular, the calculation explains the observed very weak temperature dependence in the spin-density asphericity, which cannot be explained by the Stoner theory. The authors' results are compared with those obtained by alternative approaches in which the five sub-bands are taken to be equivalent.
31 citations
TL;DR: In this article, the authors measured the angle-resolved energy distributions of electrons photoemitted from the (100) faces of single crystals of pure Cu and Cu-5 at.% Ni random substitutional alloys, using various photon energies and incident directions.
Abstract: The author have measured the angle-resolved energy distributions of electrons photoemitted from the (100) faces of single crystals of pure Cu and Cu-5 at.% Ni. Cu-23 at.% Ni and Cu-40 at.% Ni random substitutional alloys, using various photon energies and incident directions. They interpret the richly structured data using first-principles calculations of the configurationally averaged photocurrent within the framework of the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA). The quantitative agreement found between theory and experiment shows that the KKR CPA can describe electron states in these alloys in k point by k point detail. They conclude that the electronic structure of Cu-Ni alloys consists, for the most part, of bands whose smearing in energy and wavevector is less than their separation.
28 citations
TL;DR: In this article, the equations of strong coupling superconductivity in disordered transition metal alloys have been derived by means of irreducible Green's functions and on the basis of the alloy version of the Barisic-Labbe-Friedel model for electron-ion interaction.
Abstract: The equations of strong coupling superconductivity in disordered transition metal alloys have been derived by means of “irreducible” Green's functions and on the basis of the alloy version of the Barisic-Labbe-Friedel model for electron-ion interaction. The configurational averaging has been performed by means of the coherent potential approximation. Making some approximations, we have obtained the formulas for the transition temperatureT
c and the electron-phonon coupling constant λ. These depend on the alloy component and total densities of states, the phonon Green's function, and the parameters of the model.
25 citations
TL;DR: In this article, the electronic structure of paramagnetic NicPt1-c random substitutional alloys using the relativistic generalisation of the Korringa-Kohn-Rostoker coherent potential approximation (R KKR CPA) and a first-principles but not self-consistent crystal potential was studied.
Abstract: The authors have studied the electronic structure of paramagnetic NicPt1-c random substitutional alloys using the relativistic generalisation of the Korringa-Kohn-Rostoker coherent potential approximation (R KKR CPA) and a first-principles but not self-consistent crystal potential. Spectral functions as well as fully and partially averaged densities of states were calculated. They find a complex set of well defined bands and a Fermi surface which changes subtly with concentration. The results agree with the scant photoemission measurements on these alloys and suggest a number of new effects which could be observed in further experiments. However, they are unable to account for the very broad peak in the linear coefficient of the specific heat near the critical concentration for the occurrence of ferromagnetism. They suggest that this peak is due to spin disorder scattering.
21 citations
TL;DR: In this paper, the diffusion constant of a particle or an excitation in randomly fluctuating media is studied theoretically and explicit calculation of D has been performed by using the dynamical coherent potential approximation and by computer experiments.
Abstract: Diffusion of a particle or an excitation in randomly fluctuating media is studied theoretically. The model is a tight-binding Hamiltonian whose diagonal or off-diagonal elements are modulated by mutually-independent random noises. The non-white modulation models are investigated precisely. The diffusion constant ( D ) depends on the noise-parameters such as the amplitude ( Δ ) and the correlation time (γ -1 ) and also depends on the static band width ( W ). Explicit calculation of D has been performed by using the dynamical coherent potential approximation and by computer experiments. In the case that diagonal elements are modulated by discrete-jump noises, D as a function of γ has a minimum in the vicinity of \(\hbar\gamma{=}\varDelta\) if Δ ≫ W . On the contrary, D (γ) is a monotonically-increasing function of γ if Δ ≪ W . In the off-diagonal fluctuation case, the diffusion constant exhibits a feature qualitatively different from the diagonal fluctuation model.
18 citations
TL;DR: In this paper, the influence of disorder scattering on the zone-centre band structure and electron mobility in II-VI semiconductor alloys was analyzed using coherent potential approximation and a k.p alloy Hamiltonian.
Abstract: The coherent potential approximation and a k.p alloy Hamiltonian are used to provide a detailed analysis of the influence of disorder scattering on the zone-centre band structure and the electron mobility in II-VI semiconductor alloys. The weak-scattering approximation to the electron mobility is shown to apply to the common II-VI alloys. The effect of alloy disorder on the zone-centre band structure of a zero-gap alloy is examined. The total electron mobility in the mixed-crystal system Hg1-xCdxTe is evaluated, and the relative strength of disorder scattering in comparison with other scattering mechanisms is assessed, for a range of impurity concentration and alloy composition.
16 citations
TL;DR: In this paper, the authors present a first-principles description of the electronic structure in metallic spin-glass alloys, which consists of a Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) calculation.
Abstract: The authors present a first-principles description of the electronic structure in metallic spin-glass alloys. The method is illustrated by explicit calculations for the AgcMn1-c system. In this case it consists of a Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) calculation for a three-component alloy. Namely, the lattice sites are taken to be occupied by Ag atoms with probability c, Mn atoms with local moments pointing 'up', Mn up arrow , with probability 1/2(1-c) and Mn atoms with the opposite orientation, Mn down arrow , with the same probability 1/2(1-c). They find that the Mn impurity band is exchange split, as in spin-polarised band theory with aligned spins, and the resulting gain in energy supports the moments. In this sense their theory is a generalisation of the Anderson model of impurity moment formation to finite concentrations. They study the density of states, Bloch spectral functions and the Fermi surface.
TL;DR: In this paper, the authors present physical reasons for the coherent potential approximation (CPA) calculation method's abillity to predict little band gap bowing or alloy scattering, while at the same time predicting large deviation from virtual crystal behavior in other energy ranges.
Abstract: Physical reasons are presented for the coherent potential approximation (CPA) calculation method’s abillity to predict little band gap bowing or alloy scattering, while at the same time predicting large deviation from virtual crystal behavior in other energy ranges. Accurate treatment of structural properties requires that calculations be doubly self‐consistent: a density functional method plus CPA is needed. Experimental values of bond energies are greater than those predicted by simple theories. Weakened HgTe bonds in alloys help to explain the loss of Hg when the materials undergo surface treatments.
TL;DR: In this article, a simplified model was proposed to describe a lattice of intermediate valence ions fluctuating between two magnetic configurations, on which it is possible to add electrons either in localized states of the ions or in a band of extended states.
TL;DR: In this paper, the densities of disordered and ordered ferromagnetic Fe3Pt alloys are calculated in the tight-binding d-band model with intra-atomic Coulomb interaction by making use of the coherent potential approximation for disordered alloys and the recursion method for ordered alloys.
Abstract: Densities of states of disordered and ordered ferromagnetic Fe3Pt alloys are calculated in the tight-binding d-band model with intra-atomic Coulomb interaction by making use of the coherent potential approximation for disordered alloys and the recursion method for ordered alloys. Magnetic properties such as the magnetisation, local magnetic moments low-temperature specific heat coefficient and spontaneous volume striction are calculated and compared with experiment. It is found that both the ordered and disordered Fe3Pt alloys are strong ferromagnets and that the ground-state properties can be explained by the Hartree-Fock approximation.
TL;DR: In this article, the electronic densities of states in PtMn and PtCr alloys were calculated in the cluster coherent potential approximation, and numerical calculations were made for the concentration near ordering L12 type structure.
Abstract: The electronic densities of states in PtMn and PtCr alloys are calculated in the cluster coherent potential approximation. Numerical calculations are made for the concentration near ordering L12 type structure. Changes in the values and the positions of the peaks in the densities of states are observed.
TL;DR: In this paper, the decoupling procedure involved in the configurational average of the Kittler-Falicov method for alloys is discussed, and a relationship between this method and the coherent potential approximation is established.
Abstract: The decoupling procedure involved in the configurational average of the Kittler-Falicov method for alloys is discussed. As a result of this analysis, a relationship between this method and the coherent potential approximation is established.
TL;DR: In this article, the successive transitions from the ferromagnetic state to the antiferromagnetic one via the spin glass one with decreasing the Fe concentration in disordered Fe-Cr alloys are demonstrated with use of the coherent potential approximation combined with the Hartree-Fock approximation for the Hubbard Hamiltonian.
TL;DR: In this paper, the Ginzburg-Landau functional with two biquadratically coupled order parameters is derived for magnetic, superconducting narrow-band alloys in a random lattice model, where Coulomb repulsion and contact-type attraction of the itinerant electrons are taken into account.
Abstract: Magnetic, superconducting narrow-band alloys are studied within a random lattice model. Coulomb repulsion and contact-type attraction of the itinerant electrons, and the indirect exchange between local magnetic moments are taken into account. Averaging the free energy expansion in coherent potential approximation, the Ginzburg-Landau functional is derived with two biquadratically coupled order parameters. Coherent orderings and concentration-dependent critical temperatures are determined. The coefficients of theq2-terms, i.e. the stiffness and diffusion constants, involve vertex corrections due to impurity scattering. The coexistence criterion resulting from the 4th-order couplings cannot be fulfilled, referring to a first order transition between ferromagnetism and superconductivity. Tetracritical behaviour is found by including the direct exchanges between the localized spins.
TL;DR: In this paper, the density expansion of the complex optical potential is studied with path integrals, in terms of exact medium propagators and scattering matrices in the medium, and a source dependent generalization of the path integral is used to derive a functional equation for the optical potential.
Abstract: Multiple scattering of a wave in a system of scatterers with random, uncorrelated positions is studied with path integrals. The Edwards–Gulyaev expression for the position averaged Green’s function is used to find the density expansion of the complex optical potential. The expansion is in terms of exact medium propagators and scattering matrices in the medium. The first term is the coherent potential approximation. A source dependent generalization of the path integral is used to derive a functional equation for the optical potential. This leads to a hierarchy for correlation functions that involves exact medium propagators and scattering matrices. The simplest truncations yield new integral equations that are generalizations of the coherent potential approximation and are compatible with the density expansion.
TL;DR: In this paper, the vibrational density of amorphous silicon was obtained by extending the Coherent Potential Approximation to the bonds in a Bethe lattice model. But the results were not applied to treat other impurities in similar amorphized systems.
Abstract: The vibrational density of states of amorphous Silicon, when there is a finite concentration of dangling bonds or impurities, such as hydrogen, is obtained by extending the Coherent Potential Approximation to the bonds in a Bethe lattice model. The results are encouraging and it is concluded that this theory can be applied to treat other impurities in similar amorphous systems.
TL;DR: In this paper, an unusual semiconductor alloy, Hg l−x Cd x Te, a breakdown of the Virtual Crystal Approximation (VGA) in that alloy was reported.
Abstract: This paper involves an unusual semiconductor alloy, Hg l−x Cd x Te, a breakdown of the Virtual Crystal Approximation (VGA) in that alloy (the first such breakdown in a semiconductor alloy), the success of the Coherent Potential Approximation (CPA), the “first order” explanation of these results in terms of energies of atomic valence levels, and the qualitative explanation of the unusual properties of Hg l−x Cd x Te in terms of the results reported above. The genesis of the work was the use of photoemission spectroscopy. Cd and Hg core shifts are detected as a function of alloy composition and related to bonding concepts developed here.
TL;DR: In this article, the dynamics of alpha solutions of Be in a V matrix for various concentrations were investigated and the concentration dependence of the LO spectrum width and LO energy position, as well as the deformation of the lattice vibrations spectrum were compared with calculations in terms of coherent potential approximation.
Abstract: Cold neutron inelastic scattering is used to investigate the dynamics of alpha solutions of Be in a V matrix for various concentrations. The increase of Be concentration leads to a broadening of the local oscillation (LO) spectrum and to a shift of its centre. The concentration dependence of the LO spectrum width and LO energy position, as well as the deformation of the lattice vibrations spectrum are compared with calculations in terms of the coherent potential approximation.
TL;DR: In this article, an experimental method which allows us to survey alterations in the X-ray photoelectron spectra under -to -transition is suggested, and the densities of states of valence electrons calculated in the multi-band coherent potential approximation (CPA) reflect general features of XPS measurements.
TL;DR: In this article, measurements of resonant first order Raman scattering on optical and disorder activated acoustical phonons in Aluminium rich Ga1−mAlxAs samples in the vicinity of the direct energy gap are reported.
Abstract: Measurements of resonant first order Raman scattering on optical and disorder activated acoustical phonons in Aluminium rich Ga1−mAlxAs samples in the vicinity of the direct energy gap are reported. A calculation of the infrared dielectric response of Ga1−xAlxAs using a shell-model and the coherent potential approximation accounts for the display of the optically active vibrations.
TL;DR: The continuous time random walk (CTRW) and the homomorphic cluster coherent potential approximation (HCCPA) are known to be useful approximate methods for the analysis of transport between localised states in ordinary disordered systems.
TL;DR: In this article, the phonon density of states is evaluated for the mixed system PdyAg1-xDx using a coherent potential approximation, which enables the simultaneous presence of substitutional and interstitial disorder.
Abstract: The phonon density of states is evaluated for the mixed system PdyAg1-xDx A coherent potential approximation described earlier is used which enables the authors to treat the simultaneous presence of substitutional and interstitial disorder Their results indicate that the Ag-D force constants are approximately the same as those for Pd-D
01 Jan 1983
TL;DR: In this paper, a system consisting of two impurities embedded in a periodic tight-binding Hamiltonian (TBH) is examined, and the average DOS and the electrical conductivity of the system are investigated.
Abstract: In this chapter we examine first a system consisting of two “impurities” embedded in a periodic tight-binding Hamiltonian (TBH). This prepares the way for the approximate treatment of a disordered system containing a nonzero concentration of impurities. We examine in particular the average DOS, which depends on the average Green’s function , and transport coefficients like the electrical conductivity, which depends on an average of the form , where A is a known operator. Finally we discuss the important question of disorder-induced localization.
TL;DR: In this paper, the authors extended the Hubbard model to include the effect of random atomic potentials and transfer integrals of electron and showed that the band gap due to the correlation effect in an insular phase vanishes in the presence of a certain value of the randomness.
Abstract: The Hubbard model is extended to include the effect of random atomic potentials and transfer integrals of electron. The density of states of quasi-particles is calculated with the use of Hubbard's third approximation for the electron correlation problem and the coherent potential approximation for the random problem (Shiba's method). The results suggest that the band gap due to the correlation effect in an insular phase vanishes in the presence of a certain value of the randomness. However, this does not necessarily imply an insulator-to-metal transition which would be determined by the relative position of the mobility edge and the Fermi level.