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Showing papers on "Coherent potential approximation published in 1996"


Book
31 Dec 1996
TL;DR: In this article, a linear Muffin-Tin Orbital (LMTO) method is proposed for atomic sphere approximation, which is based on Relativistic Theory.
Abstract: Preface. 1. Introduction. 2. Linear Muffin-Tin Orbital (LMTO) Method. 3. Green Function Method. 4. Coherent Potential Approximation (CPA). 5. Selfconsistency within Atomic Sphere Approximation. 6. Relativistic Theory. 7. Bulk Systems, Overlayers and Surfaces. 8. Magnetic Properties. 9. Effective Interatomic Interactions in Alloys. 10. Numerical Implementation. Index.

378 citations


Journal ArticleDOI
TL;DR: In this article, the electrical conductivity tensor of disordered magnetic multilayers is described in terms of ''layer-diagonal'' and ''layer off diagonal'' contributions.
Abstract: Employing the Kubo - Greenwood formula, the electrical conductivity of disordered layered systems is formulated in terms of the (screened) Korringa - Kohn - Rostoker method and the coherent potential approximation. It is shown that the elements of the electrical conductivity tensor can be described in terms of `layer-diagonal' and `layer-off-diagonal' contributions. In order to discuss effects of the underlying electronic structure on the electrical conductivity, but also to point out particular contributions to it, Bloch spectral functions are formulated. A spin-polarized (relativistic) application of the present theory to non-collinear disordered magnetic multilayers allows one to discuss giant magnetoresistance (GMR) on an ab initio level in quite a general context.

62 citations


Journal ArticleDOI
TL;DR: An ab initio determination of the crystallographic phase stability of Fe-Co alloys as a function of concentration is presented, using the coherent potential approximation.
Abstract: We present an ab initio determination of the crystallographic phase stability of Fe-Co alloys as a function of concentration, using the coherent potential approximation. A bcc --> hcp phase tran ...

54 citations


Journal ArticleDOI
TL;DR: The all-electron spin-polarized version of the fully relativistic linear muffin-tin-orbital method in the tight-binding representation is used to describe disorder within the coherent potential approximation.
Abstract: A Green's function method is developed to calculate the electronic and magnetic properties of random alloys containing heavy elements. Based on the local spin density approximation, the all-electron spin-polarized version of the fully relativistic linear muffin-tin-orbital method in the tight-binding representation is used to describe disorder within the coherent potential approximation. The method is first tested on ferromagnetic 3d metals (Fe, Co, Ni) and then applied to study the electronic and magnetic properties of the fcc alloy systems ${\mathrm{Co}}_{50}$${\mathrm{Ni}}_{50}$ and ${\mathrm{Co}}_{50}$${\mathrm{Pt}}_{50}$. \textcopyright{} 1996 The American Physical Society.

32 citations


Journal ArticleDOI
TL;DR: In this article, an ab-initio Green function method for the calculation of the electronic structure of disordered alloys and their surfaces based on the local spin density approximation and the all-electron tight-binding linear muffin-tin orbital method is presented.

10 citations


Journal ArticleDOI
TL;DR: In this article, a reformulation of the Korringa - Kohn - Rostoker (KKR) coherent potential approximation (CPA) is presented, which affords a number of conceptual and practical advantages over conventional formulations of the theory.
Abstract: We present a reformulation of the Korringa - Kohn - Rostoker (KKR) coherent-potential approximation (CPA) which affords a number of conceptual and practical advantages over conventional formulations of the theory. In particular, as presented here the method can facilitate application to systems that cannot be described properly by a muffin-tin approximation to the cell potential and require a full potential treatment. Also, the formalism allows the derivation of the KKR CPA self-consistency condition within both a scattering matrix and a Green function approach, leading to uniquely defined species-resolved charge densities and densities of states. As shown in a companion paper, this formulation also allows the treatment of the so-called charge-transfer effects associated with Wigner - Seitz cells in an alloy which contain net amounts of charge.

10 citations


Journal ArticleDOI
TL;DR: This work extends the existing theory of excitons in such media using a contact potential model for the interaction between electrons and holes to an approximation which interpolates correctly between the limits of weak and strong disorder.
Abstract: Employing a recently developed technique of component weighted two particle Green's functions in the CPA of a binary substitutional alloy $A_cB_{1-c}$ we extend the existing theory of excitons in such media using a contact potential model for the interaction between electrons and holes to an approximation which interpolates correctly between the limits of weak and strong disorder. With our approach we are also able to treat the case where the contact interaction between carriers varies between sites of different types, thus introducing further disorder into the system. Based on this approach we study numerically how the formation of exciton bound states changes as the strengths of the contact potentials associated with either of the two site types are varied through a large range of parameter values.

10 citations


Journal ArticleDOI
TL;DR: In this article, an all-electron fully charge self-consistent Korringa - Kohn - Rostoker coherent potential approximation (KKR-CPA) study of the electronic structures of disordered bcc alloys over the entire composition range is presented.
Abstract: We present an all-electron fully charge-self-consistent Korringa - Kohn - Rostoker coherent potential approximation (KKR-CPA) study of the electronic structures of disordered bcc alloys over the entire composition range. Specific computations are reported for x = 0.0, 0.25, 0.50, 0.75, and 1.0. Extensive comparisons with the predictions of the Nb-based rigid-band model (RBM) and other theoretical results, as well as with the relevant experimental results insofar as they are available are made. The particular issues that we focus on concern the evolution of the Fermi surface (FS), and the changes in the density of states and the superconducting transition temperature of Nb with increasing Mo content. The N- and H-centred FS sheets of Nb are found to shrink essentially rigidly, but the -centred sheets evolve in a highly non-rigid-band manner. The x = 0.25 Mo alloy displays an especially large disorder-induced smearing of the -centred FS sheets. Direct experimental information concerning the FS is available only for the N-centred sheet via positron annihilation and in this regard our results are in accord with the measurements. Concerning superconductivity, we have computed the Hopfield parameters and for the Nb and Mo sites, and used the results to obtain for via the McMillan formula. We find that this simple scheme describes the observed composition dependence of in reasonably well.

10 citations



Journal ArticleDOI
TL;DR: In this paper, the harmonic magnon modes in a Heisenberg ferromagnetic chain in a random weak field were studied and the Lyapunov exponent for the uniform (k = 0) mode was computed using the coherent potential approximation (CPA) in the weak-disorder limit.
Abstract: The harmonic magnon modes in a Heisenberg ferromagnetic chain in a random weak field are studied. The Lyapunov exponent for the uniform ( k = 0) mode is computed using the coherent potential approximation (CPA) in the weak-disorder limit. The CPA results are compared with the numerical and weak-disorder expansions of various random systems. We have found that the inverse localization length and the integrated density of states have anomalous power law behaviour as reported earlier. The CPA also reproduces the dispersion law for the same system, calculated by Pimentel and Stinchcombe using the real space renormalization scaling technique. A brief comment is also made for the uniform weak-field case.

4 citations


Journal ArticleDOI
01 Aug 1996-Pramana
TL;DR: In this paper, the effects of relativistic effects on the electronic structure of ordered and substitutionally disordered Ni-Pt alloys were studied using the atomic-sphere approximation formulation of the KCR CPA.
Abstract: Using the atomic-sphere approximation formulation of the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) method, we have studied the effects of relativity on the electronic structure of ordered and substitutionally disordered Ni-Pt alloys. The inclusion of mass-velocity and Darwin terms are found to be essential for describing the experimentally observed ground-state properties. For the stability of disordered Ni-Pt alloys we find that, in addition to relativity, the minimization of charge-transfer effects are important. We also find that the treatment of ordering tendencies based on the band energy term alone is not sufficiently accurate for alloys with charge-transfer effects. Further analysis, in terms of basis functions, densities of states and non-spherically averaged charge densities, indicate the importance ofs- andd-electrons of Pt for the stability of both ordered and disordered Ni-Pt alloys.

Journal ArticleDOI
TL;DR: In this paper, the influence of spacer randomness on the periods and amplitudes of the oscillations of exchange coupling in magnetic multilayers is studied from first principles and the effect of disorder is treated within the coherent potential approximation.

Journal ArticleDOI
TL;DR: In this article, partial spectral weights of disordered Au-Ag alloys are obtained from the valence band photoemission spectra using synchrotron radiation, taking matrix element effect into account.

Journal ArticleDOI
TL;DR: In this paper, a perpendicular conductance through a single resistive layer is formulated in the coherent potential approximation (CPA) and vertex corrections are evaluated exactly within CPA, and the physical implication of the result is discussed.

Journal ArticleDOI
TL;DR: In this paper, the electronic structure of the ternary carbide system Ti 1 - x Al x C (x = 0.17) is calculated using the coherent potential method of the multiple scattering theory.
Abstract: With the coherent potential method of the multiple scattering theory the electronic structure of the ternary carbide system Ti 1 - x Al x C (x = 0.17) is calculated. The cluster approximation with muffintin crystal potential is used. With the same method the electronic structure of the binary TiC system has been calculated for comparison. It was found that Ti 1 - x Al x C (x = 0.17) can be considered as a semiconductor with a narrow energy gap (2.18 eV). Aluminium doping results in an increase of the valence band width and a decrease of the cohesive energy.

Journal ArticleDOI
TL;DR: In this article, the 3s-multiplet splitting has been used to estimate the local magnetic moment of Fe atoms and to explain the observed concentration dependence of the magnetic moment, the calculations of the electronic structure and the magnetic moments of the Fe atoms were performed in the framework of the tight binding coherent potential approximation.
Abstract: X-ray photoelectron spectra of the valence band and Fe 2p 3 2 and 3s core levels have been measured both for polycrystalline FeCr alloys and amorphous FeB alloys. The 3s-multiplet splitting has been used to estimate the local magnetic moment of Fe atoms. To explain the observed concentration dependence of the magnetic moment, the calculations of the electronic structure and the magnetic moment of the Fe atoms were performed in the framework of the tight-binding coherent potential approximation. Results of the calculations agree with the experimental concentration dependence of the magnetic moment of Fe atoms. This dependence of the atomic magnetic moment on a boron concentration for amorphous FeB alloys obeys the Slater—Pauling rule on going from 15.5 to 19.5 at.% but shows a slight deviation from this linear behaviour at about 15 at.%. For FeCr, alloys the atomic magnetic moment is almost constant for differences of components concentration in agreement with X-ray photoemission spectroscopy measurements, whereas the average magnetic moment of the alloy decreases almost linearly with increasing Cr concentration. The difference in concentration dependence of the atomic magnetic moment is explained in terms of a charge transfer between s, p and d bands which is found to be larger for the amorphous FeB alloys than for the polycrystalline FeCr alloys.

Journal ArticleDOI
TL;DR: In this article, the authors used the Green function method and the chemisorption theory of Einstein and Schrieffer to calculate the energy of O and CO on the multilayer segregated Ni-Cu disordered binary alloy within the many-coupled coherent potential approximation.
Abstract: The Green function method and the chemisorption theory of Einstein and Schrieffer are used to calculate the chemisorption energies of O and CO on the multilayer segregated Ni-Cu disordered binary alloy within the many-coupled self-consistent coherent potential approximation. In general cases, the chemisorption-induced surface segregation can change the surface component and the chemisorption property to varying degrees. When the mutual influence of chemisorption and multilayer segregation is considered, the changes appear slightly mild. The chemisorption energy for O/Ni-Cu (CO/Ni-Cu) depends sensitively on O(CO) coverage θ, and decreases with increasing θ.

Journal ArticleDOI
TL;DR: In this article, excited absorption lines are calculated for weakly disordered tight-binding systems, using the coherent potential approximation (CPA) method. But the scaled lineshapes depend only on the dimension of the system.
Abstract: Exciton absorptison lines are calculated for weakly disordered tight-binding systems, using the coherent-potential approximation (CPA). The scaled lineshapes depend only on the dimension of the system. The CPA results are compared with known exact results for chains, and numerical results for finite two-and three-dimensional systems. Exciton-Absorptionslinien werden fur schwach ungeordnete, stark gebundene Systeme in der Kohaent-Potential-Approximation (CPA) berechnet. Die skalierten Linienformen hangen nur von der Dimension des Systems ab. Die CPA-Ergebnisse werden mit bekannten exakten Resultaten fur Ketten und numerischen Resultaten fur endliche zwei- und dreidimensionale Systeme verglichen.

Journal ArticleDOI
TL;DR: In this paper, the effects of conduction-band impurities in heavy-fermion systems are studied on the basis of the periodic Anderson model in the framework of a slave-boson mean-field theory within the single-site coherent-potential approximation (CPA).
Abstract: The effects of conduction-band (CB) impurities in heavy-fermion systems are studied on the basis of the periodic Anderson model in the framework of a slave-boson mean-field theory within the single-site coherent-potential approximation (CPA). We provide a simplified CPA formalism for the treatment of mixing disorder induced by CB alloying. The density of states for conduction and localized f electrons can be calculated self-consistently in the whole impurity-concentration range. The concentration dependence of the specific-heat coefficient, static magnetic susceptibility and resistivity at low temperatures as well as the Kondo temperature in the alloys is obtained. The results indicate that the experimental observations in upon doping with Au, Ag and Al can be qualitatively explained as the effects of mixing disorder.

Journal ArticleDOI
TL;DR: In this article, it was shown that the dependence of the coherent exchange couplings on the impurity concentration is the same as that of antiferromagnets in two-dimensional doped Ising-like Heisenberg ferromagnetes.
Abstract: Using the 2-site CPA, we show that in the two-dimensional doped Ising-like Heisenberg ferromagnets the dependence of the coherent exchange couplings on the impurity concentration is the same as that of antiferromagnets. The coherent magnetic fields and magnetic susceptibilities are obtained within the framework of the 1-site CPA. The magnetic susceptibilities obtained from the CPA are compared with the perturbative results in the low impurity concentration range.

Journal ArticleDOI
TL;DR: In this article, a two sublattice model is proposed to describe the magnetic behavior of pseudo-ternary Heusler compounds (X1−cX′c)2YZ.

Book ChapterDOI
01 Jan 1996
TL;DR: In this paper, the effect of the finiteness of quasi-particle lifetimes on the deviation from Vegard's rule in terms of the Electronic Topological Transition (ETT) is discussed.
Abstract: In this paper, we discuss about the variations of the equilibrium lattice parameter versus the alloy concentration in AgPd fcc solid solutions. We interpret the experimentally observed deviations from Vegard’s rule in terms of the Electronic Topological Transition (ETT) related to the d band filling which occurs about the equiatomic concentration. Through simple Rigid Band Model (RBM) calculations, we show how the finiteness of quasi-particle lifetimes affects these deviations, namely reducing their amplitude. RBM calculations are also used to compare versus the experiment the predictions of ab initio Local Density Approximation, Korringa-Kohn-Rostoker Coherent Potential Approximation (LDA-KKR-CPA) calculations. On this basis we shall discuss the apparent underestimation of the relevant quasi-particle lifetime in the CPA theory.

Book ChapterDOI
01 Jan 1996
TL;DR: In this article, the authors used the locally self-consistent multiple scattering method (LSMS) to calculate the electronic structure of solids with no long-range order using an order-N method, which has been shown to be completely converged for calculations within the density functional theory local density approximation (DFT-LDA).
Abstract: We have been calculating the electronic structure of solids with no long-range order using an order-N method.1 The origin of the term order-N is that the computer time required to carry out conventional calculations on N atoms increases as N3, but computational physicists have recently developed methods for which the increase in time is only linear in N. We call the order-N method that we use the locally self-consistent multiple scattering method (LSMS),2 and it has been shown to be a completely converged method for calculations within the density functional theory local density approximation (DFT-LDA).3

Journal ArticleDOI
TL;DR: In this paper, the effects of conduction-band impurities in heavy-fermion (HF) systems are investigated by introducing mixing disorder into the n-CB periodic Anderson model under the slave-boson mean-field theory.

Journal ArticleDOI
TL;DR: In this article, the Monte Carlo (MC) method is used for the analysis of IR absorption bands for non-uniform adsorbate layers, and the relation between the exact Monte Carlo and the conventional coherent potential approximation is discussed.

Proceedings ArticleDOI
26 Mar 1996
TL;DR: In this paper, the energy current density operator for a combined electron and vibrating ions of the lattice system with the electron-lattice interaction term included is derived for isotropically disordered mixed crystals.
Abstract: In disordered mixed crystals the effect of alloy scattering is known to be of significant importance in comparison with that of lattice scattering in the consideration of electrical and thermal transport properties. In this work we consider the thermoelectric power in such isotropically disordered mixed crystals. We derive the energy current density operator for a combined electron and vibrating ions of the lattice system with the electron-lattice interaction term included. In this preliminary report, the relative importance of electrons and lattice vibrations to the thermoelectric power is discussed. The coherent potential approximation may be used to obtain the thermoelectric power of the disordered mixed crystal as function of the concentration. Our operator formulation is in coordinate space which besides containing previous formulations for metals and alloys as special cases, admits of the coherent-potential approximation methods to be applied as well.