Showing papers on "Coherent potential approximation published in 1973"
241 citations
TL;DR: In this article, low temperature electrical resistivity and Curie temperature of amorphous (Fe 100− x Ni x ) 75 P 15 C 10 alloys (0⩽ x ⩽50) were studied.
61 citations
TL;DR: In this article, the electronic densities of states functions for the random substitutional alloy system Ag-Pd for a number of compositions across the complete range of concentrations are calculated based on the coherent potential approximation.
Abstract: Electronic densities of states functions are calculated for the random substitutional alloy system Ag-Pd for a number of compositions across the complete range of concentrations. The calculations are based on the coherent potential approximation. It was found necessary to extend the simple model Hamiltonian method previously used to describe Cu-Ni alloys because of the stronger dependence of the lattice constant on composition in the Ag-Pd system. It is shown that the results are consistent with published photoemission and optical absorption data, and low temperature specific heat and Pauli spin susceptibility experiments. The possibility of s-d charge transfer in this alloy system is discussed.
59 citations
47 citations
TL;DR: In this paper, a theory for spin waves in a dilute antiferromagnet has been developed which uses the coherent potential approximation (CPA) to treat the randomly varying Ising interactions between nearest Antiferromagnetic neighbours but treats the off-diagonal parts of the Heisenberg interactions in a more approximate fashion.
Abstract: For pt. I see abstr. A78044 of 1972. A theory for spin waves in a dilute antiferromagnet has been developed which uses the coherent potential approximation (CPA) to treat the randomly varying Ising interactions between nearest antiferromagnet neighbours but treats the off-diagonal parts of the Heisenberg interactions in a more approximate fashion that is consistent with the Goldstone theorem. Although similar to the theory that was found to be successful for K(Co, Mn)F3 and (Co, Mn)F2 in paper I, the problem of the magnetic vacancy requires special treatment. A fictitious spin is placed on the Zn atoms but large potentials are introduced there to prevent the spin waves from propagating via these sites. The Green functions, neutron scattering and Raman scattering have been calculated.
42 citations
TL;DR: In this article, a general proof is presented that the coherent potential approximation does not violate analyticity requirements, and a proof is given that coherent potential approximations are not violated analytically.
41 citations
TL;DR: In this paper, the coherent potential approximation (CPA) is applied to spin-disorder scattering in systems described by the s-d model such as moderately doped magnetic semiconductors.
31 citations
TL;DR: In this article, the ensemble-averaged Green function for Anderson's disordered Hamiltonian within the coherent potential approximation is examined and an exact analytical expression for the band-width is derived in the case of a rectangular distribution.
Abstract: We examine calculations of the ensemble-averaged Green function for Anderson's disordered Hamiltonian within the coherent potential approximation and show how the result of this procedure becomes exact if a Lorentzian distribution of site-energy is used. The application of these results to calculations of energy band properties is discussed and an exact analytical expression for the band-width is derived in the case of a rectangular distribution. Finally, we investigate the implications of a localization criterion proposed by Economou and Cohen (1970, 1972) [EC] in this context for various distributions of site-energy and demonstrate the possibility of a mobility edge moving outwards from the unperturbed band edge as randomness increases. The results for square-root and rectangular distributions are shown to be qualitatively similar, resolving a puzzling dissimilarity suggested by Brouers (1971) and Schonhammer (1971).
26 citations
22 citations
TL;DR: In this paper, a continued fraction technique is used to derive an approximation to the density of states of a disordered alloy with a tree of Bethe lattice, which is equivalent to a special case of the theory of Blackman et al.
Abstract: A continued fraction technique is used to derive an approximation to the density of states of a disordered alloy with a tree of Bethe lattice. The approximation in the case of only diagonal disorder provides a new simple derivation of the coherent potential approximation for a certain density of states. For an alloy with both diagonal and off-diagonal disorder the density of states is given in terms of the solution of a certain cubic equation. The self energy in this case is also obtained and used to provide the starting point for a theory of alloys with both diagonal and off-diagonal disorder and a general lattice which is equivalent to a special case of the theory of Blackman et al. Extensive calculations are presented.
22 citations
TL;DR: In this article, an expression for the spin wave stiffness constant D in a ferromagnetic alloy using essentially the random phase approximation was derived for NiFe and NiCo alloys.
Abstract: An expression is derived for the spin wave stiffness constant D in a ferromagnetic alloy using essentially the random phase approximation. On applying the coherent potential approximation a result previously derived by Fukuyama (1973) is obtained. Numerical calculations are made for NiFe and NiCo alloys.
TL;DR: In this paper, the dispersion relations for a quasi-particle are found to be anomalous in some region, in the sense that d dek(ω)/dω is negative.
Abstract: A binary alloy is treated by the coherent potential approximation (CPA) and the dispersion relations dek(ω) for a quasi-particle are found to be anomalous in some region, in the sense that d dek(ω)/dω is negative. It is proved that the states in the region of anomalous dispersion correspond to a mobility gap.
TL;DR: In this paper, the phonon dispersion relations of the 00σT branch were measured for Ni95Pt5 and Ni70Pt30 and the dispersion curves break up into two branches.
TL;DR: In this paper, the coherent potential approximation (CPA) was applied to superconducting binary alloys of arbitrary composition, and it was shown that an approximation in which the order parameter Δ does not depend on the space variable leads to a generalized Abrikosov relation, simplifying the CPA equations considerably.
Abstract: With the aid of the Nambu formalism the coherent potential approximation (CPA) is applied to superconducting binary alloys of arbitrary composition. It will be shown that an approximation in which the order parameter Δ does not depend on the space variable leads to a generalized Abrikosov relation, simplifying the CPA equations considerably. This relation yields the BCS equation forTc, however, the density of states is determined by the CPA.
TL;DR: A coherent potential approximation (CPA) theory for disordered molecular solids with interacting bands is proposed in this article, which has a wide range of applications including electronic states coupled via vibronic or spin-orbit couplings, vibrational states with degeneracies in the gas phase or coupled by Fermi resonance, triplet magnetic sublevels coupled via exciton interactions, and phonons in general.
Abstract: A coherent potential approximation (CPA) theory for disordered molecular solids with interacting bands is reported here This theory has a wide range of applications Various examples of interacting bands can be cited, such as electronic states coupled via vibronic or spin‐orbit couplings, vibrational states with degeneracies in the gas phase or coupled by Fermi resonance, triplet magnetic sublevels coupled via exciton interactions, and phonons in general The theory is developed using the self‐consistent condition with a single‐site and single‐band approximation In particular, two approaches are adopted In the first approach, a self‐energy is assigned for each subband In the second approach, a common self‐energy is assumed for all the subbands The two different approaches require different inputs to the theory In one case, the entire dispersion relations of the pure system are called for; in the other, only the partial density‐of‐states functions for each degree of freedom are needed It is also sho
TL;DR: In this paper, the averaged magnon Green function in a random Heisenberg ferromagnet which has two kinds of exchange integrals J A and J B randomly distributed is calculated by the coherent potential approximation.
Abstract: The averaged magnon Green function in a random Heisenberg ferromagnet which has two kinds of exchange integrals J A and J B randomly distributed is calculated by the coherent potential approximation. The density of states of magnon is calculated for various cases, and the correct localized mode is obtained in the low concentration limit. The critical concentration is obtained for two cases, J A >0, J B 0, J B =0. The magnetic properties for the two cases at the critical concentration are different from each other.
TL;DR: In this paper, the spectral density of binary mixtures of C6H6/C6H5D, C6h6/c6h4D2, and C6 h6/ c6h5D was investigated.
Abstract: Calculations, based on a particular model Hamiltonian and the coherent potential approximation (CPA), of the partial densities of states and the k=0 spectral densities are reported for binary mixtures of C6H6/C6H5D, C6H6/C6H4D2, and C6H6/C6D6. The spectral density calculations are compared with the experimental absorptions for the C6H6/C6H5D and C6H6/C6D6 systems. Deviations between theory and experiment for the latter case seem to be more a result of a breakdown in the ideal‐mixed‐crystal assumption than in the CPA model. There is good agreement between‐theory and experiment for the C6H6/C6H5D system. Also, additional experiments to study directly the partial densities of states of binary mixtures are outlined.
TL;DR: In this article, the perturbation term of the one-particle Green's function of an electron in a random lattice is expressed as a sum of terms related with one site, two sites, three sites, and so on.
Abstract: The perturbation term of the one-particle Green's function of an electron in a random lattice is expressed as a sum of terms related with one site, two sites, three sites, and so on. The coherent potential approximation is rederived under the assumption that the sum of the terms related with two or more sites can be ignored if the unperturbed Hamiltonian is chosen in such a way that the sum of the one-site terms is put equal to zero. The next approximation is obtained by assuming that the sum of the terms related with the clusters of sites other than the one sites and the nearest neighbor pairs of sites can be ignored, when the unperturbed Hamiltonian is so chosen that the sum of the terms related with a one site or a nearest nighbor pair of sites is put equal to zero. This approximation is applicable to the random alloy problems where the diagonal as well as off-diagonal randomness exist. If the randomness is restricted only to the diagonal one, the approximation is equivalent to the one proposed by Cyro...
TL;DR: In this article, it was shown that the ensemble-averaged single-particle Green's function for an electron in a disordered $n$-component alloy is described by a simple expression in the coherent-potential approximation.
Abstract: It is shown that the ensemble-averaged single-particle Green's function for an electron in a disordered $n$-component alloy is described by a simple expression in the coherent-potential approximation. Within the appropriate limits, it exhibits dilute alloy, virtual crystal, weak-coupling limit, and atomic behaviors. For a special model band structure of parabolic form, the problem of evaluating the quasiparticle density of states is reduced to that of solving a polynomial of $n+1$ order, which has either $n+1$ real solutions or $n\ensuremath{-}1$ real solutions in addition to two complex-conjugate solutions for a given set of parameters. The types of the density of states \char22{} persistence or amalgamation \char22{} are judged by the types of solutions. The formulation is also useful to prove the sum rules for the numbers of states. In the limit of $n$ infinite, continuous distributions of atomic levels are studied and the criterion for the Anderson transition is estimated.
TL;DR: In this article, a generalization of the coherent potential approximation for the linear random binary alloy was proposed. But the result obtained in the two-site coherent potential approximations was not as good as the one obtained in this paper, since the diagonal and off-diagonal randomness are included.
TL;DR: In this article, the coherent potential approximation (CPA) for a two s-d band model relevant to the investigation of some concentration dependent properties of noble and transition metal based disordered alloys was analyzed using functional derivative techniques.
Abstract: Using the functional derivative techniques, the coherent potential approximation (CPA) is analysed for a two s-d band model relevant to the investigation of some concentration dependent properties of noble and transition metal based disordered alloys. The discussion reveals that when delta , the difference between site potentials may not be regarded as a small parameter, the natural small parameter of the theory is ( gamma 2/ delta wsZ) where gamma is the hybridization constant, ws the band width and Z the number of nearest neighbours. This quantity is the product of the one band small parameter (1/Z) and the small parameter relevant to discuss the two band model split band limit.
TL;DR: In this article, a coherent potential approximation to find the partially average Green's functions and taking some account of the Bloch character of the electron wavefunctions was used to calculate the Knight shift of Li-Mg alloys.
TL;DR: In this paper, a coherent potential approximation for insulating antiferromagnetic alloys is presented, in which the off-diagonal (transfer) parts of the exchange interaction are treated on the same basis as the diagonal parts.
Abstract: A coherent potential approximation for insulating antiferromagnetic alloys is presented, in which the off-diagonal (transfer) parts of the exchange interaction are treated on the same basis as the diagonal parts. The theory uses three parameters which are adjusted to ensure no net scattering on average from a nearest-neighbour pair of atoms when immersed in the coherent crystal. This type of treatment is especially suitable for Heisenberg exchange where the diagonal and off-diagonal terms have the same magnitudes. Results are presented for the one-magnon densities of states at various concentrations for the systems K(Mn, Ni)F3, K(Ni, Zn)F3 and K2(Mn, Ni)F4.
TL;DR: In this article, the authors extended the coherent potential approximation for multiple bands in ferromagnetic substitutional binary alloys and derived the concentration dependence of total magnetic moments and their aspherical contribution in Co-Ni f.c. alloys.
TL;DR: In this paper, the optical spectra for both doubly degenerate and triply degenerate excitons are calculated with the coherent potential approximation, where the lattice vibrations interacting with exciton are treated quasi-classically in terms of the quasi-molecular mode localized around each lattice site.
Abstract: The Jahn-Teller effect on the optical spectra of degenerate exciton is investigated. The lattice vibrations interacting with exciton are treated quasi-classically, in terms of the quasi-molecular mode localized around each lattice site. The optical spectra for both the doubly degenerate and the triply degenerate excitons are calculated with the coherent potential approximation. If the ratio α of the local level splitting \(\sqrt{c^{2}}kT\) to the half of the exciton band width B is larger than unity, the distinct Jahn-Teller splitting of the optical spectra is observable. When α<1 the spectral splitting is washed out due to motional narrowing. A brief discussion on a relevant experiment is given.