Showing papers on "Coherent potential approximation published in 1979"
TL;DR: In this paper, the L alpha soft X-ray emission spectra of Cu-Ni alloys using the muffin-tin form of the coherent potential approximation (KKR-CPA) are presented.
Abstract: Calculations of the L alpha soft X-ray emission spectra of Cu-Ni alloys using the muffin-tin form of the coherent potential approximation (KKR-CPA) are presented. The transition matrix elements are determined exactly within the KKR-CPA, and lifetime broadening is included. The results of new experiments on the L alpha spectra are also reported and combined with X-ray photoemission measurements of the core state binding energies, so that the relative positions of the Cu and Ni emission bands may be determined. Good agreement between theory and experiment is found, confirming the KKR-CPA description of the local densities of states in Cu-Ni alloys. Striking non rigid-band behaviour is observed. It is suggested that soft X-ray studies of the type the authors describe should be a useful probe of alloy potential functions.
48 citations
TL;DR: In this article, the periodic Anderson model is replaced by the sum of two single-particle alloy Hamiltonians, the parameters of which have to be determined self-consistently.
Abstract: To describe the electronic properties of mixed valence compounds we study the periodic Anderson model within the frame of the alloy analog approximation. In this approach the model Hamiltonian is replaced by the sum of two single-particle alloy Hamiltonians the parameters of which have to be determined self-consistently. The alloy problem is solved within the coherent potential approximation. In contrast to other treatments of the periodic Anderson model this approximation scheme is exact in both trivially solvable limits of vanishing hybridization and Coulomb repulsion, respectively. For model parameters corresponding to a mixed valence situation only nonmagnetic solutions of the self-consistency equations exist. After discussing the limit of small hybridization analytically we numerically calculate the magnetic susceptibility and the electronic specific heat as a function of temperature for realistic values of the hybridization and Coulomb repulsion. The results are in very good qualitative agreement with experimental data.
26 citations
TL;DR: In this article, the high-field susceptibility, local moments, the asphericity of the magnetic moments, low temperature specific heat coefficient and the coefficient of T 2 dependence of the magnetization are calculated for fcc Fe-Ni alloys in the coherent potential approximation.
Abstract: The high-field susceptibility, local moments, the asphericity of the magnetic moments, the low temperature specific heat coefficient and the coefficient of T 2 dependence of the magnetization are calculated for fcc Fe–Ni alloys in the coherent potential approximation. The difference between the band widths of fcc Fe and Ni is taken into account. The subbands of d -electrons with t 2 g and e g symmetries, which are deduced from the calculated energy bands for Ni and fcc Fe in the interpolation scheme, are used. The calculated results show a good agreement with the experimental results except the Invar region. It is shown that the first-order transition from the ferromagnetic to paramagnetic state occurs at about 30 at% Ni, where the magnetization vanishes suddenly, and the high-field susceptibility is not so large as compared with the observed results.
21 citations
TL;DR: In this paper, the amalgamation type 0-0 excitons in the isotopically mixed crystals of normal and deuterated anthracene are investigated by simultaneous measurement of reflection, fluorescence, and pre-resonant Raman spectra.
Abstract: The amalgamation type 0–0 excitons in the isotopically mixed crystals of normal and deuterated anthracene are investigated by simultaneous measurement of reflection, fluorescence, and pre-resonant Raman spectra. The observed features of the bulk and surface excitons are interpreted in terms of the theoretical band structure for the vibronic excitons calculated on the basis of the dynamical coherent potential approximation. The effect of cluster formation on the dynamical properties of excitons in these mixed crystals is also discussed by the Bethe-Peierls approximation.
18 citations
TL;DR: In this paper, it was shown that the phase boundary of the Potts model is related to the bond percolation problem, and that it behaves as exp(sK)=(ln s)/(s-1/(p-pc/pc) for all 2-D lattices.
Abstract: Some of the properties of the bond diluted s-state Potts model are examined in this paper, and in particular the phase boundary It is shown that as s to 1, the model is simply related to the bond percolation problem By extending the replica arguments of Domany (1978) (which are probably exact), it is shown that near p, the phase boundary behaves as exp(-sK)=(ln s)/(s-1)(p-pc/pc) for all 2-D lattices The authors also construct a coherent potential approximation that is exact for all lattices as s to 1, and for arbitrary s on Bethe lattices
17 citations
TL;DR: In this article, the coherent potential approximation has been applied to the bond diluted Ising model in a transverse field Green function techniques decoupled in a generalised RPA have been used.
Abstract: The coherent potential approximation has been applied to the bond diluted Ising model in a transverse field Green function techniques decoupled in a generalised RPA have been used The static and dynamic properties of this system have been studied at T=O and it is concluded that the form of the CPA equations inhibits the appearance of a second-order phase transition, if the divergence of the static susceptibility is the only criterion locating the transition A discussion of this problem and its relation to other models is also included
10 citations
TL;DR: In this paper, it was shown that the breakdown of analyticity is not found in average Green's functions which are obtained on the basis of the cluster coherent potential approximation for systems with substitutional disorder if the one particle total hamiltonian for a given configuration is expressed as a sum of homomorphic single-cluster hamiltonians.
Abstract: We show that the breakdown of analyticity is not found in average Green's functions which are obtained on the basis of the cluster coherent potential approximation for systems with substitutional disorder if the one particle total hamiltonian for a given configuration is expressed as a sum of homomorphic single-cluster hamiltonians. Disorder can be site-diagonal and/or off-diagonal. In this article the emphasis is laid on off-diagonal cases. We first treat three-dimensional disordered systems with a semielliptic distribution of nearest neighbour transfers { V i j }. We also apply our homomorphic cluster coherent approximation (HCPA) to the bond percolation problem; we obtain the densities of states which show characteristic features found through a computer simulation.
10 citations
TL;DR: In this article, the charge transfer, electrical resistivity, thermoelectric power and Hall mobility of binary alloys are calculated as functions of alloy composition in the coherent potential approximation and the interrelation among the composition dependences of these physical quantities is discussed on the basis of the numerical results and shown to be consistent within a model.
Abstract: The charge transfer, electrical resistivity, thermoelectric power and Hall mobility of binary alloys are calculated as functions of alloy composition in the coherent potential approximation. The inter-relation among the composition dependences of these physical quantities is discussed on the basis of the numerical results and shown to be consistent within a model.
9 citations
TL;DR: In this paper, the paramagnetic excitation spectrum of a substitutional disordered alloy consisting of two different types of magnetic ions is derived in the coherent potential approximation (CPA).
9 citations
TL;DR: In this article, a generalised Hartree-Fock approximation is made for the interaction and the coherent potential approximation for configurational average over the sites of randomly distributed impurity spins, and a reduction of the density of states due to the Kondo effect is found at the Fermi level.
Abstract: Electronic properties of a highly doped Kondo system are discussed. The method of expressing the s-d exchange interaction in terms of the two kinds of quasi-spins, proposed by one of the authors (Kurata (1970)) for the single-impurity Kondo problem, is extended to this system. A generalised Hartree-Fock approximation is made for the interaction and the coherent potential approximation for configurational average over the sites of randomly distributed impurity spins. The density of states of conduction electrons and electrical resistivity are calculated for various concentrations of impurity spins. A reduction of the density of states due to the Kondo effect is found at the Fermi level.
01 Jan 1979
TL;DR: In this paper, the authors review some of the difficulties encountered in the solution of the KKR-CPA equations and the numerical techniques developed to overcome them, and present some numerical techniques to overcome these difficulties.
Abstract: In this lecture, we review some of the difficulties encountered in the solution of the KKR-CPA equations and the numerical techniques developed to overcome them.
TL;DR: In this article, the authors apply the homomorphic cluster coherent potential approximation (HCPA) to systems with site-diagonal and/or offdiagonal randomness, and show numerically that the average Green's function obtained on the basis of the HCPA is analytic off the real axis on the complex energy plane.
Abstract: We apply the homomorphic cluster coherent potential approximation (HCPA) to systems with site-diagonal and/or off-diagonal randomness. We show numerically that the average Green's function obtained on the basis of the HCPA is analytic off the real axis on the complex energy plane. We also show that the HCPA reproduces the ordinary single-site CPA and the molecular CPA when systems include only diagonal disorder. The HCPA is advantageous in that it can deal with both site-diagonal and off-diagonal disorder in a unified manner. We also present a numerical result for a disordered chain by taking both kinds of disorder into account. The density of states calculated in the HCPA is in good agreement with the result of a computer simulation.
TL;DR: In this article, the magnetic form factor of Ni-Pt alloy system with 85, 24, 32 and 46 at% Pt was measured by means of the polarized neutron diffraction, with the results that the atomic magnetic moment of Ni atom decreases with increasing Pt concentration and that of Pt atom has parallel magnetic moment.
Abstract: Magnetic form factor of Ni-Pt alloy system with 85, 24, 32 and 46 at% Pt was measured by means of the polarized neutron diffraction The form factors were analyzed with the results that the atomic magnetic moment of Ni atom decreases with increasing Pt concentration and that of Pt atom has parallel magnetic moment Experimental results were explained by a Coherent Potential Approximation In the vicinity of the critical concentration for ferromagnetism, the partial state densities of Ni and Pt atom in the Ni-Pt alloy were calculated to be almost the same with each other, while the partial state densities of Ni and Pd atom in the Ni-Pd alloy turned out to be quite different
TL;DR: In this article, the high-field susceptibility and spin wave spectra in fcc Fe-Ni alloys were calculated numerically in the coherent potential approximation by using the realistic band structures with t 2g and e g symmetries.
TL;DR: In this article, a theoretical model to explain the valence transitions of Sm in Sm 1 − xM 2 S alloys, where M is a transition metal (e.g., Y or La), is presented.
Abstract: We present a theoretical model to explain the valence transitions of Sm in Sm1 − x
M
x
S alloys, where M is a transition metal (e.g., Y or La). The f-level of Sm is described as a zero-width band with a finite Coulomb repulsion U, hybridized with the d band of the alloy, which is treated in the coherent potential approximation (CPA). The results show a change of the number of 4f electrons as a function of the concentration, which explains the valence transition observed in these alloys.
TL;DR: In this article, a theory of surface excitons in mixed molecular crystals is presented by extending the coherent potential approximation developed in the study of an infinite bulk crystal to a suitable one for a semi-infinite crystal.
Abstract: A theory of surface excitons in mixed molecular crystals is presented by extending the coherent potential approximation developed in the study of an infinite bulk crystal to a suitable one for a semi‐infinite crystal. The effect of the semi‐infinite nature of the crystal is taken into account by a localized perturbation method. The surface coherent potential is introduced to calculate the concent ration dependence of the energy of the surface excitons. The results give a qualitative explanation of the recent observation of surface excitons in the mixed crystal system antheracene–perdeuteroanthracene.
TL;DR: In this article, a method of calculating the effect of the random distribution of atoms on the energy-band structure of ternary alloy semiconductors is proposed, where the coherent potential approximation based on the method of linear combination of bond orbitals (LCBO) is used.
Abstract: A method of calculating the effect of the random distribution of atoms on the energy‐band structure of ternary alloy semiconductors is proposed, where the coherent‐potential approximation based on the method of linear combination of bond orbitals (LCBO) is used. In the case of GaAscP1−c, the nonlinear dependence of the energy upon the alloy composition c at various symmetry points within the first Brillouin zone is calculated. It is found that the effect of disorder is an important factor in explaining the bowing effect of band gaps; In particular, the magnitudes of the direct gap Γv15→Γc1, and the indirect gap Γv15→Xc1 agree well with experimental results. The energy‐band structure, including the lifetime broadening of the energy at each Bloch state, is shown for GaAs0.5P0.5. Also, the relation between the LCBO parameters and the effect of disorder on various states is qualitatively discussed. The effect of disorder on the p‐like or s‐like states of the valence band increases with increasing the differen...
TL;DR: In this paper, a recently proposed CPA theory for treating randomly diluted antiferromagnets with uniaxial anisotropy is revised, and it is shown that such a procedure, which ignores dynamical effects occasioned by fluctuations in the local anisotropic field, leads to unphysical results beyond certain critical concentrations at which the theory breaks down.
Abstract: A recently proposed CPA theory (Tahir‐Kheli et al), for treating randomly diluted antiferromagnets with uniaxial anisotropy is revised. It is shown that such a procedure, which ignores dynamical effects occasioned by fluctuations in the local anisotropy field, leads to unphysical results beyond certain critical concentrations at which the theory breaks down. A study of the weakness, the relevance and validity of this theory is presented. The theory is expected to be relevant only when applied to systems with relatively small anisotropy. In fact, owing to its simplicity, within its range of validity, we apply a such CPA to analyse the behaviour of the zero‐field antiferromagnetic resonance in the mixed system of manganese and zinc fluorides as function of zinc concentration. Our results are compared with those from several recent experiments and a very good agreement is found. Moreover, the general features and complexity of an alternative CPA formalism which takes into account dynamical effects of fluctua...
TL;DR: In this article, the electronic density of states of tetrahedrally-bonded amorphous semiconductors was studied and the Weaire-Thorpe model was applied to the random variations of the banding parameter V 1 and the bonding parameter V 2.
Abstract: On the basis of a simple method by which quantitative disorder as well as topological disorder in amorphous solids can be treated systematically, we study the electronic density of states of tetrahedrally-bonded amorphous semiconductors such as amorphous silicon or germanium. We adopt the Weaire-Thorpe model and apply the coherent potential approximation to the random variations of the banding parameter V 1 and the bonding parameter V 2 of this model. It is possible to explain the disappearance of the dip between the two lower peaks of the valence band by introducing fluctuations in V 1 alone. This fact indicates that it is not always necessary to assume the existence of five-membered rings as a cause for the disappearance of the dip. We also discuss the effects of fluctuations in V 2 , missing atoms and broken bonds.
TL;DR: In this article, the effects of quantitative disorder on the valence band in the electronic density of states (DOS) of tetrahedrally-bonded amorphous semiconductors on the basis of a simple Weaire-Thorpe model were studied.
TL;DR: In this article, the amalgamation type 0-0 excitons in the isotopic mixed anthracene crystals were investigated by simultaneous measurements of the reflection and emission spectra.
TL;DR: In this article, a coherent potential approximation (C.P.A) theory has been developed in order to examine the bond dilution problem in an antiferromagnet which contains alternating strong and weak magnetic exchange interactions.
Abstract: A coherent potential approximation (C.P.A.) theory has been developed in order to examine the bond dilution problem in an antiferromagnet which contains alternating strong and weak magnetic exchange interactions. Specifically, we have calculated the concentration dependence of the transition temperature at which the system displays long‐range order. This is done by examining the Green function for the system in the paramagnetic phase. The existence of the strong exchange interaction creates pairs of highly correlated spins in the paramagnetic state. The transition temperature is identified by introducing the weak coupling between these spin pairs and locating the temperature at which the excitation of a pair from the singlet ground state to the lowest excited states becomes soft. When only interpair bonds are broken, a linear concentration vs transition temperature behavior is predicted. This prediction is compared with recent experimental results for the diluted paired antiferromagnet, RbMn1−xMgxCl3.
TL;DR: In this article, the local frequency distribution functions of the surface layers of a semi-infinite simple cubic crystal with randomly adsorbed impurities have been calculated for the (001) surface.
Abstract: The local frequency distribution functions of the surface layers of a semi-infinite simple cubic crystal with randomly adsorbed impurities have been calculated for the (001) surface. A method based on the coherent potential approximation was used. The results are displayed for various concentrations and masses of adsorbed impurities.
TL;DR: In this paper, a coherent potential approximation for a distribution of transfer integrals (off-diagonal disorder) in a single band on a dilute close-packed lattice with first and second neighbour interactions is developed.
Abstract: A coherent potential approximation is developed for a distribution of transfer integrals (off-diagonal disorder) in a single band on a dilute close-packed lattice with first and second neighbour interactions. The moments of the density of states are evaluated and compared to exact results up to order six. It is suggested that this model provides a reasonable approximation to the structurally disordered material with the same distribution of transfer integrals arising via the pair distribution function. Results are computed for distributions based on experimental observation in liquid Fe and on those computed by the Percus-Yevick method for a liquid of hard spheres.
TL;DR: In this paper, a self-consistent solution for the single-particle Green's function in random binary alloys within the coherent potential approximation is considered, which leads to a first-principles approximate definition of the matrix elements of the singleparticle Hamiltonian even in the tight-binding case, obtaining a physical ground for such an approach.
Abstract: The possibility of obtaining a self-consistent solution for the single-particle Green's function in random binary alloys within the coherent potential approximation is considered. It is shown that the present approach leads to a first-principles approximate definition of the matrix elements of the single-particle Hamiltonian even in the tight-binding case, obtaining a physical ground for such an approach.
TL;DR: In this paper, the magnetic moments of Co atom in Fe-Co-Ni alloys have been determined over whole concentration range of the fcc structure by polarized neutron diffuse scattering experiment.
Abstract: The magnetic moments of Co atom in Fe–Co–Ni alloys have been determined over whole concentration range of the fcc structure by polarized neutron diffuse scattering experiment. The results support qualitatively the theoretical calculations under the coherent potential approximation. The magnetic moments of Fe atoms have also been obtained in the alloys by means of Mossbauer effect. The results lie in between the theoretical values obtained either by the coherent potential or by the rigid band approximation.
TL;DR: In this article, the thermal properties of the XY model for spin-1/2 on a linear chain, in the presence of random fields HiSiz, are evaluated using the fermion representation of the Hamiltonian and the coherent potential approximation (CPA).
Abstract: The thermal properties of the XY model for spin-1/2 on a linear chain, in the presence of random fields hiSiz, are evaluated using the fermion representation of the Hamiltonian and the coherent potential approximation (CPA). The specific heat and the thermal conductivity are obtained from the appropriate Green functions. The results are discussed for a model of praseodymium ethyl sulphate (PrES) where the hyperfine interaction is considered to give rise to random fields. This interaction is found to give only a small reduction in the spin specific heat, but to be very effective reducing the spin thermal conductivity. Other mechanisms must be invoked to give a full account of the experimental results.
TL;DR: In this paper, the authors considered a disordered Heisenberg ferromagnet with spin values equal to 1/2 but two distinct exchange integrals are distributed randomly among bonds.
Abstract: We consider a bond model of a disordered Heisenberg ferromagnet in which spin values are equal to 1/2 but two distinct exchange integrals are distributed randomly among bonds. The coherent potential approximation in the form of Oguchi, Ono and Ishikawa is extended so as to include the pair effects of the bond clustering by adapting the treatment of the alloy problem by Schwartz and Ehrenreich. The self‐consistency condition which determines the coherent exchange integral is derived. The density of states of magnons is calculated from the configurationally averaged Green function. The applications for the one‐dimensional case and the two‐dimensional case are discussed.
TL;DR: In this article, an expression of the exchange stiffness constant of spin waves in disordered ferromagnetic alloys is obtained in the combined approximation of the random phase approximation and the coherent potential approximation.
Abstract: An expression of the exchange stiffness constant of spin waves in disordered ferromagnetic alloys is obtained in the combined approximation of the random phase approximation and the coherent potential approximation. Both the diagonal randomness and the off-diagonal randomness are taken into account. By using the realistic band structures, the exchange stiffness constants in fcc Fe-Ni alloys are calculated numerically. The difference between the observed and calculated values of the exchange stiffness constants in fcc Fe-Ni alloys is attributed to the inter-atomic exchange interaction between d-electrons.