Showing papers on "Coherent potential approximation published in 1994"

Theoretical predictions of interface anisotropy in the presence of interdiffusion (invited)

Abstract: The first ab initio electronic structure calculations of the magnetocrystalline anisotropy of superlattices with imperfect interfaces are presented. Specifically the possibility of an interdiffusion between the layers at the interface in Co/Pd and Co/Pt superlattices is considered. The electronic structure calculations use the local spin‐density formalism as implemented with the layer Korringa–Kohn–Rostoker method. Interdiffusion at the interface is modeled in two distinct ways. In the first approach a diffuse interface is represented by ordered arrangement of substitutions, while in the second approach interdiffusion is assumed to produce a substitutionally disordered random alloy on the layers at the interface, which is solved using the coherent potential approximation. The calculated interface anisotropies for superlattices with perfect and imperfect interfaces are, on average, modeled accurately by a simple Neel‐type model. This model always predicts a reduction in magnetic anisotropy resulting from t...

Electronic structures of disordered Ag-Mg alloys.

Abstract: We have calculated the electronic structures of the valence bands in a series of [alpha]-phase (disordered fcc) Ag-Mg alloys over the range 0--30 at. % Mg using the Korringa-Kohn-Rostoker method within the coherent potential approximation (KKR-CPA). We find that the variation of the equilibrium lattice constant with composition is in good agreement with experimental measurements. The bandwidth of the Ag-related [ital d] states decreases with the addition of Mg although the position of the bottom of the band remains roughly fixed in energy with respect to the Fermi level; an observation that is consistent with photoemission measurements. The electronic spectral densities are very sharply peaked at the Fermi energy with widths that are [lt]1% of the Brillouin zone dimension and so the Fermi surfaces of the alloys are well defined throughout the zone. The radius of the neck at the [ital L] point and the belly radii in the [Gamma][ital X] and [Gamma][ital K] direction increases approximately linearly with increasing Mg content. Taking into account some previous work on the origin of the short range order in Ag-rich alloys, we conclude that the local-density approximation KKR-CPA method provides a realistic description of the electronic structure in [alpha]-phase [ital Ag]-Mg alloys.

Electronic structure of metallic alloys using charge-neutral atomic spheres.

Abstract: Based on the idea of charge-neutral atoimc spheres we have calculated the electronic structure of ordered and disordered Cu-Zn, Ni-Pt, and Al-Li alloys using the linear-muffin-tin-orbital (LMTO) method and the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) method in the atomic-sphere approximation (ASA), respectively. The equilibrium lattice constants and the formation energies of ordered alloys obtained with the LMTO-ASA method show that the calculations done with charge-neutral atomic spheres are closer to the experimental results than the conventional equivolume atomic-sphere-type calculations. In the case of disordered alloys, we find that charge-neutral atomic spheres are essential for the stability of these alloys within the KKR-ASA CPA method where the Madelung-type contribution is neglected. Our results clearly indicate that for disordered alloys any future implementation of a full-potential method within the single-site CPA should be carried out with charge-neutral cells rather than the Wigner-Seitz cells.

Theory of transport in inhomogeneous systems and application to magnetic multilayer systems

Abstract: A theory of the electrical conductivity of homogeneous random alloys based on the Korringa—Kohn—Rostoker coherent potential approximation (KKR‐CPA) is generalized to treat an inhomogeneous alloy in which the concentrations of the constituent atoms can vary from site to site. A special case of such a system is an epitaxial multilayer system. We develop the theory for such systems and show how it can be implemented by using the layer Korringa—Kohn—Rostoker technique to calculate the electronic structure. Applications to magnetic multilayers and to the calculation of the giant magnetoresistance are discussed.

A theory for the electrical conductivity of an ordered alloy

Abstract: A method for calculating the two-particle electron Green function (alloy conductivity) based on the cluster expansion for the scattering T-matrix is developed. Taking into account scattering processes on all pairs of atoms, an analytical expression for the conductivity of alloys with short-range and long-range order is obtained. The coherent potential approximation is selected as a zero approximation. It is shown that the change in the electronic spectrum due to ordering leads to an essential change in alloy conductivity.

A Green's function approach to analysing the effects of random synaptic background activity in a model neural network

Abstract: The effect of randomly distributed synaptic background activity on the states of self-sustained firing in a model neural network with shunting is investigated. Using mean field theory, the steady state of the network is expressed in terms of an ensemble-averaged single-neuron Green's function. This Green's function is shown to satisfy a matrix equation identical in form to that found in the tight-binding-alloy model of excitations on a one-dimensional disordered lattice. The ensemble averaging is then performed using a coherent potential approximation thus allowing the steady-state firing rate of the network to be determined. The firing rate is found to decrease as the mean level of background activity across the network is increased; a uniform background (zero variance) leads to a greater reduction than a randomly distributed one (non-zero variance).

Generalized mean fields and quasi-particle interactions in the Hubbard model

Abstract: The self-consistent theory of the correlation effects in transition metals and their compounds (TMC) and disordered binary alloys has been developed using the Hubbard model and random Hubbard model. In order to obtain the interpolation solution for the quasi-particle spectrum, which is valid for both the atomic and band limits, the novel Irreducible Green’s Function (IGF) method has been used. This method permits to calculate the quasi-particle spectra of many-particle systems with complicated spectra and strong interaction in a very natural and compact way. The essence of the method is deeply related with the notion of the Generalized Mean Fields (GMFs), which determines the elastic-scattering corrections. The inelastic-scattering corrections lead to the damping of the quasi-particles and is the main topic of the present consideration. The calculation of the damping has been done in a self-consistent way for both limits. For the random Hubbard model the weak-coupling case has been considered and the self-energy operator has been calculated using the combination of the IGF method and Coherent Potential Approximation (CPA).

Effective cluster interactions at alloy surfaces and charge self-consistency: Surface segregation in Ni-10 at. % Al and Cu-Ni.

Abstract: First-principles results are presented for the effective cluster interactions at the surface of a random Ni\char21{}10 at. % Al alloy. The derivation is based on an extension of the generalized perturbation method to semi-infinite inhomogeneous binary alloys, using a layer version of the Korringa-Kohn-Rostocker multiple-scattering approach in conjunction with the single-site coherent potential approximation to compute the self-consistent electronic structure of the system. When applied to the bulk, the method yields effective pair interactions that have the full point-group symmetry of the lattice to a very high level of numerical accuracy, despite the fact that intra- and interlayer couplings (scattering-path operators) are treated differently, and which are in perfect agreement with those of a recent three-dimensional treatment. Besides the pair terms, a selected class of triplet and quadruplet interactions are calculated, as well as the point interactions induced by the presence of the surface. The value of the latter in the first lattice plane is strongly exaggerated in our approach, leading to a complete segregation of the minority species to the surface. Using a value corresponding to the difference in the surface energies of the pure components for this term leads to the observed Al concentration of \ensuremath{\approxeq}25% at the surface. Possible reasons for the shortcomings of the theory are analyzed, and test calculations for the well studied Cu-Ni system show that the free energy of the semi-infinite alloy cannot be approximated by the sum over the single-particle band energies, once charge self-consistency is enforced at the surface.

XPS studies of the Zn1-xCoxS electronic structure

Abstract: We have investigated electronic states of diluted magnetic semiconducting Zn1-xCoxS compounds, using X-ray photoelectron spectroscopy (XPS). The amount of Cox substituting Zn in the ZnS lattice was in the range 0

Beyond the Effective Medium: Quasi Modes in Disordered Media

Abstract: For wave propagation in disordered media, the intermediate frequency regime is defined as the frequency range where the wavelength is comparable to the scale of the inhomogeneities. While difficult to treat theoretically, the regime is rich in physical phenomena. The focus of this paper is the transport characteristics of the coherent part of the wave energy in the intermediate frequency range. Through a re-formulation of the well-known coherent potential approximation, a new scheme is developed which is capable of identifying the quasi wave modes as well as yielding their dispersion relations. With no adjustable parameter, this scheme is demonstrated to give an excellent quantitative of the experimental data that showed two branches of the acoustic wave in colloidal suspensions. The new approach is used to predict the quasi modes in a random collection of metallic spheres.

Interband quantum kinetics with static disorder scattering I: Direct CPA solution for a rectangular-pulse-excited semiconductor

Abstract: For a two band model semiconductor alloy with the disorder potential concentrated to the conduction band, the photoexcitation by a long rectangular pulse represents a case soluble in the coherent potential approximation. Explicit analytic expressions for the transient electron distribution are derived using the nonequilibrium Green functions. The evanescent coherent component is gradually superseded by the incoherent distribution whose saturation value is obtained using the Ward identity.

Electronic structure of random copolymers

Abstract: The electronic structure of random copolymers (RCP) is theoretically investigated by the single-site coherent potential approximation. The results are also compared with those by the band calculation for the corresponding ordered system. In the A1 −x B x binary RCP, a strong reduction in the system band gap (E g(A1−x B x )) is found in the dilute B region when the system has the relation ofE g(A)>E g(B). This dependence is caused by the asymmetric quenching in the density-of-states (DOS) singularity at the band-edge states. The gap-opening mechanism and the asymmetric quenching are discussed by focusing on the role of the spatial dimension on the electronic structure of the random system, and the theoretical treatment is finally applied to the calculation of the joint DOS for the Si-Ge RCP system.

Local approach to the one-band hubbard-model - extension of the coherent-potential approximation

Abstract: We present a simple scheme to extend the earlier single-site coherent-potential-approximation (CPA) theories by utilizing connections of the CPA solution with the exact solution of the Falicov-Kimball model in infinite dimensions, and with the single-impurity Anderson-type models. We study the local spectral density of the model at n =1; in the metallic regime, this exhibits a narrow Abrikosov-Suhl resonance and satellite peaks, which correspond, respectively, to the quasiparticle and Hubbard subband structures. This collective resonance disappears in the split-band (insulating) regime, where the CPA is found to be a good approximation. Comparisons are made with numerical works on finite-sized twodimensional lattices, and good agreement is obtained. The physics of strongly correlated electrons in solids still contains many open questions. In this context, the limit of infinite dimensionality considered by several authors provided great insights. In this limit, due to the freezing of spatial fluctuations, a certain self-consistent mean-field theory becomes exact. ' In this limit, the action becomes purely local, and the bare Green's function of the local dynamics contains information about all other sites which have been integrated out. The momentum conservation is actually trivial in infinite dimensions, and only frequency conservation needs to be ensured in the skeleton expansion. The essential simplification in this limit is that the single-particle (sp) properties can be understood by looking at a single inixed lattice site. This has enabled workers to make an exact mapping of this model to a single-impurity Anderson model with a selfconsistency condition. The earlier coherent-potential-approximation (CPA)like theories reproduce the Hubbard subband structures correctly as U is raised, but are expected to do a bad job in the low-energy region, especially in the metallic state. Indeed, in the CPA, electronlike quasiparticles are unstable for all finite U; this is somewhat counterintuitive, since for small U, we expect a Fermi liquid with Z (1. The drawback of the CPA is that it neglects the propagation of the opposite spin species; it freezes the down-spin configuration while looking at the propagation of an up spin. Strictly speaking, this is not true; it is a good approximation only in the split-band regime, where the local spin-fluctuation time z,f «h /6, the mean hopping time, but is not good at longer time scales. Here, 6 is the bandwidth of noninteracting electrons in our model. In the language of the alloy analogy, the CPA replaces dynamical, annealed disorder by quenched, static disorder. In this paper, we try to rectify the above-mentioned deficiency of the CPA. We make use of our earlier results: (1) the CPA solution for the Hubbard model is identical to the exact solution of the Falicov-Kimball model in infinite dimensions, (2) results identical to (1) above are obtained by looking at the local propagator of a single d impurity hybridized with a conduction electron sea. These connections enable us to look at the local dynamics of a O.-spin electron while the opposite spin species is also allo~ed to hop. In particular, we focus on the impurity density of states (DOS), which is just the full interacting density of states for the lattice, as is required by self-consistency. We study the local spectral density p(co) as a function of the ratio U/b for the particle-hole symmetric case of n =1. We compare our results with those obtained by Tan, Li, and Callaway using Lanczos numerical technique for finite-sized 3 X 3 [twodimensional (2D)] lattices. Our calculation reproduces all the essential features seen in their numerical work; a central "Kondo" peak at small and intermediate U with some weight transferred to the Hubbard subbands, evolving smoothly into the split-band picture in the large-U limit. The CPA results are obtained as the first truncation of our self-consistent scheme, and provide a good agreement with the numerical work in the large-U limit, as expected. The paper is organized as follows. Section II deals with the actual calculation of the DOS, and Sec. III is devoted to a short discussion of our results.

Some Magnetic Properties of Thin Ferromagnetic Films in the Coherent Potential Approximation (S = 1)

Abstract: The coherent potential approximation method taking into account natural disorder connected with different states of spin components is applied to thin magnetic films with spin S = 1. The distribution of spontaneous magnetization as well as the temperature dependence of the critical temperature are obtained for b.c.c. (100) films. The results obtained using Tyablikov decoupling of higher order Green function are improved in relation to the Green function results obtained with the same decoupling procedure by including the line profiles of elementary excitations.

A study of annealed and quenched averaging of the thermodynamic potential in a disordered system: an augmented space approach

Abstract: We use the augmented space method, introduced by one of us, to study the configurational average of the thermodynamic potential of electrons in a disordered system. Invoking the relationship between the scattering diagram summation and the algebraic approaches to cluster generalizations of the coherent potential approximation, we indicate how such an approximation may be carried out on the thermodynamic potential. We also study the difference between annealed and quenched averaging from the scattering diagrammatic approach, to obtain insight into the difference between the two processes.

Impurity Effects on Two-Dimensional Anisotropic Heisenberg Antiferromagnets

Abstract: We study magnetic behavior of a completely random dilute S =1/2 anisotropic Heisenberg antiferromagnet on the two-dimensional square lattice by using the coherent potential approximation. In the anisotropic Heisenberg model, the x y -anisotropy defined by J p ( z ) / J p ( x y ) is enhanced with increasing nonmagnetic impurities, where J p ( z ) and J p ( x y ) are the z and x y components of the coherent exchange constant, respectively. On the other hand, the isotropic Heisenberg model retains the isotropic nature for any impurity concentration. The critical concentration at the percolation threshold is obtained. The Neel temperature and the magnitude of the coherent exchange constant are calculated as a function of the impurity concentration.

The coherent-potential approximation in the tight-binding linearized-muffin-tin-orbital formalism for a single-band model of a solid

Abstract: We have developed a model of a random alloy based on a simplified single-(s-type-)band model of a solid introduced by Andersen to demonstrate the screening of orbitals in the LMTO formalism. To this model we have applied the coherent-potential approximation in the LMTOS put forward by Kudrnovsky et al. The electronic densities of states calculated showed interesting features similar to those found in realistic calculations, for all types of disorder considered, namely purely diagonal, both diagonal and off-diagonal, and purely off-diagonal.

Electronic structure of the BaA1-xBxO3 (A, B = Pb, Bi, Sn, Sb) alloy systems: Implications for superconductivity

Abstract: Coherent potential approximation studies of the electronic structure of the BaA1-xBxO3 (where A and B are different combinations of the elements Pb, Bi, Sn and Sb) alloy systems have been performed with accurate tight-binding hamiltonians. The calculations show how semiconducting BaSnO3 evolves into a metal upon alloying with Sb or Bi. Common characteristics of the densities of states are pointed out. Using the rigid muffin-tin approximation an analysis of the superconducting properties is given.

Cyclotron resonance line shape in a Wigner crystal.

Abstract: The cyclotron resonance absorption spectrum in a Wigner crystal is calculated. Effects of spin-splitting are modelled by substitutional disorder, and calculated in the coherent potential approximation. Due to the increasing strength of the dipole-dipole interaction, the results show a crossover from a double-peak spectrum at small filling factors to a single-peak spectrum at filling factors $\agt 1/6$. Radiation damping and magnetophonon scattering can also influence the cyclotron resonance. The results are in very good agreement with experiments.

Electronic Structure of Planar Defects in Ordered and Disordered High Temperature Intermetallics

Abstract: In this paper self-consistent electronic structure calculations are reported for isolated planar faults in the intermetallic alloy TiAl. The electronic structure calculations for planar faults in ordered stoichiometric TiAl have been performed using the layer Korringa-Kohn-Rostoker method. Calculations for off-stoichometric TiAl, and TiAl alloys with varying compositions of Cr and Mn were performed using the layer Korringa-Kohn-Rostoker method combined with the coherent potential approximation. The most dramatic reductions in fault energies are observed for Mn substitutions. Cr substitutions, however, produce similar changes in fault energies as Ti does. These results are discussed in terms of experimental observations for TiAl.

A full potential Korringa-Kohn-Rostoker Green’s function (fKKR-Gf) method: A total energy calculation of niobium

Abstract: Using only the angular momentum representation a full potential all electron method for the calculation of the electronic structure, solution of Poisson’s equation, and the total energy of ordered and disordered metals and alloys is presented within the multiple scattering theory. The method is applied to niobium and gives excellent agreement with other calculations and experiments.