Showing papers on "Coherent potential approximation published in 2000"

Disorder dependence of the magnetic moment of the half-metallic ferromagnet NiMnSb from first principles

Abstract: Using half-metallic ferromagnets in spin-dependent devices, like spin valves and ferromagnetic tunnel junctions, is expected to increase the device performance. However, using the half-metallic ferromagnet NiMnSb in such devices led to much less than ideal results. One of the possible sources for this behavior is atomic disorder. First-principles calculations of the influence of atomic disorder on the electronic structure of NiMnSb underline the sensitivity of half-metallic properties in NiMnSb to atomic disorder. In this article, we report on the disorder dependence of the total magnetic moment calculated by applying the layer Korringa–Kohn–Rostoker method in conjunction with the coherent potential approximation. We consider the following types of disorder: (1) intermixing of Ni and Mn, (2) partial occupancy of a normally vacant lattice site by Ni and Mn, and (3) partial occupancy of this site by Mn and Sb. In all cases the composition is kept stoichiometric. All three types of disorder decrease the mome...

Scattering of electromagnetic waves by nearly periodic structures

Abstract: We present a method, based on the coherent potential approximation, for the treatment of disorder in photonic crystals. We apply the method to the study of light absorption by a three-dimensional array of plasma spheres distributed randomly in a host dielectric medium. We find that the effect of disorder on the absorbance of a thick slab of the material consisting of many layers of spheres is much less pronounced than in the corresponding case of a single layer of spheres.

Dynamical structure factor in disordered systems

Abstract: We study the spectral width as a function of the external momentum for the dynamical structure factor of a disordered harmonic solid, considered as a toy model for supercooled liquids and glasses. In the contexts of both the single-link coherent potential approximation and a single-defect approximation, two different regimes are clearly identified: if the density of states at zero energy is zero, the usual p^(4) law is recovered for small momentum. On the contrary, if the disorder induces a nonvanishing density of states at zero energy, a linear behavior is obtained. The dynamical structure factor is numerically calculated in lattices as large as 96^(3) and satisfactorily agrees with the analytical computations.

Calculating properties with the polymorphous coherent-potential approximation

Abstract: The formulas for calculating properties of an alloy such as the density of states, the charge density, and the Bloch spectral density function are derived from multiple-scattering theory for the polymorphous coherent-potential approximation (PCPA). The chemical shifts obtained for three alloy systems using the PCPA, the Korringa-Kohn-Rostoker CPA, and the locally self-consistent multiple-scattering method are compared with experiment. A significant improvement in the treatment of Coulomb effects is achieved using the PCPA with only a little more computational effort than for the older isomorphous CPA's. (c) 2000 The American Physical Society.

Model calculations for the vibrational anomalies of a disordered Lennard–Jones solid

Abstract: Using a modified version of the two-site coherent potential approximation (CPA), also known as effective medium approximation (EMA), we can calculate the density of states (DOS) of a disordered rare-gas-solid modelled by a Lennard–Jones potential. As input we use the radial pair distribution of a pertinent molecular-dynamics simulation of Rahman et al. The resulting DOS agrees well with that of the simulation. The DOS exhibits an anomalous enhanced low-frequency contribution (“boson peak”). We investigate and discuss the influence of the quenched structure on this anomaly and on the resulting sound velocity.

Nordheim dependence in the surface resistivity of disordered overlayers

Abstract: A multiple scattering calculation is used to compute the surface resistivity of Cu(100), Al(100), and Al(111) surfaces as a function of coverage by a disordered overlayer of Cu or Al adatoms. The Bloch states of the semi-infinite bulk are described by a layer-Korringa–Kohn–Rostoker calculation combined with the coherent potential approximation to represent the carrier scattering by the disordered overlayer. The diffuse scattering of carriers by the surface disorder results in a Nordheim, or quasiparabolic, dependence of the induced surface resistivity upon the coverage. This result confirms the qualitative behavior observed in a prior calculation using the average t-matrix approximation where the surface was modeled as a random distribution of s-wave scatterers in front of a hard-wall potential representing the surface barrier [D. L. Lessie and E. R. Crosson, J. Appl. Phys. 59, 504 (1986)].

Self-consistent cluster CPA methods and the nested CPA theory

Abstract: The coherent potential approximation (CPA), is a useful tool to treat systems with disorder. Cluster theories have been proposed to go beyond the translation invariant single-site CPA approximation and include some short-range correlations. In this framework one can also treat simultaneously diagonal disorder (in the site-diagonal elements of the Hamiltonian) and non-diagonal disorder (in the bond energies). It proves difficult to obtain reasonable results, free of non-analyticities, for lattices of dimension higher than one ( D >1). We show electronic structure results obtained for a Hubbard model, treated in mean-field approximation, on a square lattice and a simple cubic lattice, with the simultaneous inclusion of diagonal and non-diagonal disorder. We compare the results obtained using three different methods to treat the problem: a self-consistent 2-site cluster CPA method, the Blackman–Esterling–Berk single-site like extension of the CPA and a nested CPA approach.

Electronic structure and physical properties of solids : the uses of the LMTO method : lectures of a workshop held at Mont Saint Odile, France, October 2-5, 1998

Abstract: Ground and Excited-State Formalisms.- Developing the MTO Formalism.- From ASA Towards the Full Potential.- A Full-Potential LMTO Method Based on Smooth Hankel Functions.- Full-Potential LMTO Total Energy and Force Calculations.- Excited States Calculated by Means of the Linear Mu.n-Tin Orbital Method.- Magnetic Properties.- Fully Relativistic Band Structure Calculations for Magnetic Solids - Formalism and Application.- First Principles Theory of Magneto-Crystalline Anisotropy.- On the Implementation of the Self-Interaction Corrected Local Spin Density Approximation for d- and f-Electron Systems.- Ab Initio Theory of the Interlayer Exchange Coupling.- Disordered Alloys.- Disordered Alloys and Their Surfaces: The Coherent Potential Approximation.- Locally Self-Consistent Green's Function Method and Its Application in the Theory of Random Alloys.- Large-Scale Real-Space Calculations.- Sparse Direct Methods: An Introduction.- Real-Space Tight-Binding LMTO Approach to Magnetic Anisotropy: Application to Nickel Films on Copper.- Combining Real Space and Tight Binding Methods for Studying Large Metallic Systems.

Topology-dependent modelling of microwave scattering from melting snowflakes

Abstract: A new method for calculating the electromagnetic properties of melting snowflakes is presented. This takes account of the topological changes that accompany the melting process, and is based on the coherent potential approximation which is used in modelling composite materials. Sample results for the effective permittivity in partially melted flakes are contrasted with those from previous approaches, as are sample extinction and absorption cross-sections.

XANES Calculation of Chalcogenide Spinel CdIn 2 S 4

Abstract: The shape of the x-ray K absorption spectrum of sulfur in the normal spinel CdIn2S4 is calculated using the FEFF7 program. Local densities of free electron states of S, Cd, and In are calculated in the theory of multiple scattering in the local coherent potential approximation. A comparison of the obtained results with the experimental x-ray SK spectrum demonstrates good agreement between them.

CPA susceptibility of the double exchange model

Abstract: We use a many-body coherent potential approximation (CPA) to study the paramagnetic state of the double exchange model with finite local spins S. Our CPA treats dynamical effects due to the quantum nature of the local spins in a local approximation, becomes exact in the atomic limit, and reduces to Kubo's one-electron CPA in the empty band limit and to dynamical mean field theory (DMFT) in the classical S→∞ limit. We solve the CPA equations of motion for all S in the strong Hund coupling limit, determining all quantities self-consistently. We show that the CPA does not give a ferromagnetic transition for any S; this is due to the local spin probability distribution function P(Sz) being inaccurately obtained. We bypass this difficulty by extrapolating P(Sz) from the exact (within DMFT) S=∞ result, and obtain a magnetic transition for all S with the expected Curie laws at band filling n=0, 1 .

Optical Properties of a Quantum Well of A1−x Bx Alloy Semiconductor in the Coherent Potential Approximation

Abstract: We have theoretically studied optical properties of a quantum well (QW) in which the well region is constructed from a binary alloy semiconductor A1−xBx in the coherent potential approximation (CPA). A tight binding model is used for a single particle (electron, hole, Frenkel exciton) in the well composed by a rectangular array of Nxx Nyx Nz sites. The effect of the diagonal randomness is reduced to the coherent potential σ( E), which is assumed to be the same for all sites, and is selfconsistently determined with the average Green’s function. For a slab (∞, ∞, Nz) and wire (∞, Ny, Nz), the density of states ( E) is composed of Nz (or Nyx Nz) subbands, each shows the two (one)-dimensional van-Hove singularity. When x (or 1−x) is small, a B (A) impurity-band always appears at the lower (higher) energy side of the lowest (highest) host-band. The change of ( E) and the absorption spectrum by changing the well-width and the dimensionality is discussed in detail.

Ab Initio Theory of Perpendicular Transport in Metallic Magnetic Multilayers

Abstract: The current-perpendicular-to-plane (CPP) magnetoconductance of a sample sandwiched by two ideal non-magnetic leads is described at an ab initio level. The socalled ‘active’ part of the system is a trilayer consisting of two magnetic slabs of finite thickness separated by a non-magnetic spacer. We use a transmission matrix formulation of the conductance based on surface Green functions as formulated by means of the tight-binding linear muffin-tin orbital method. An equivalent and computationally more efficient formulation of the problem based on reflection matrices is also presented. The formalism is extended to the case of lateral supercells with random arrangements of atoms which in turn allows to deal with ballistic and diffusive transport on equal footing. Applications refer to fcc-based Co/Cu/Co(001) trilayers.

Interplay of Disorder and Correlations

Abstract: I address here the question of the mutual interplay of strong correlations and disorder in the system. I consider random version of the Hubbard model. Diagonal randomness is introduced {\it via} random on-site energies and treated by the coherent potential approximation. Strong, short ranged, electron - electron interactions are described by the slave boson technique and found to induce additional disorder in the system. As an example I calculate the density of states of the random interacting binary alloy and compare it with that for non-interacting system.

Tests of the Polymorphous Coherent Potential Approximation

Abstract: The coherent potential approximation (CPA) is a powerful mathematical technique for approximating the electronic structure of substitutional solid solution alloys. Most applications of the CPA to date have assumed an isomorphous model of the alloy in which all of the A atoms are assumed to be the same, as are all of the B atoms. The derivation of self-consistent potentials for the alloys within the framework of the CPA and the isomorphous model leads inevitably to the conclusion that the Madelung potential at each site must be zero. The approximate theory resulting from this derivation is called the KKR-CPA. The polymorphous CPA (PCPA) makes use of supercells that contain many atoms, and the Madelung potentials at all of the sites are calculated exactly. PCPA calculations produce a polymorphous alloy model in which every atom in the supercell is different. Tests will be shown that demonstrate the advantages of the PCPA over the KKR-CPA in explaining experiments that depend critically on the charge transfer in an alloy.

Theories for Many-Electron Systems with Strong Electron—Phonon and Interelectron Coulombic Interactions

Abstract: The main objective of solid state theory is to clarify the microscopic origins of various macroscopic properties of solids, such as electric, magnetic and optical properties, which change from one crystal to the other. The difference of these macroscopic properties between crystals mainly originates from the difference of the microscopic nature of electrons in each crystal. In particular, it mainly comes from the outer electrons, which are in the outermost orbitals of the atoms or molecules constituting each crystal. Consequently, one of the most important problems in solid state theory is to find the relation between the microscopic nature of the outermost electrons and the macroscopic properties of solids.

Interstitial boron in tungsten : modeling of lattice tetragonality

Abstract: In the framework of linear density functional theory we study conditions of formation of W-B solid solutions. On the basis of the coherent potential approximation, we consider ordering tendencies, study electronic structure and provide total energy calculations. We present the energetics and electronic structure changes of a homogeneous axis deformation (Bain transformation) in W-B solid solutions. Our modeling shows that an increase of the atomic fraction of boron leads to a decrease of the number of states at the Fermi level N ( E F ), thus decreasing the metallic character in the system. At the same time for boron concentrations higher than 0.15 we find once more an increase of N ( E F ). We demonstrate the behavior of the total density of states (DOS) in the vicinity of the Fermi energy with (001) displacive transformation and see drastic changes in the DOS for tetragonality c/a changes from 1 to 1.4.