Showing papers on "Coherent potential approximation published in 2005"

Coulomb Interactions and Ferromagnetism in Pure and Doped Graphene

Abstract: We study the presence of ferromagnetism in the phase diagram of the two-dimensional honeycomb lattice close to half-filling (graphene) as a function of the strength of the Coulomb interaction and doping. We show that exchange interactions between Dirac fermions can stabilize a ferromagnetic phase at low doping when the coupling is sufficiently large. In clean systems the zero-temperature phase diagram shows both first-order and second-order transition lines and two distinct ferromagnetic phases: one phase with only one type of carriers (either electrons or holes) and another with two types of carriers (electrons and holes). Using the coherent potential approximation we argue that disorder further stabilizes the ferromagnetic phase. This work should estimulate Monte Carlo calculations in graphene dealing with the long-range nature of the Coulomb potencial.

Self-interaction correction in multiple scattering theory

Abstract: We propose a simplified version of self-interaction corrected local spin-density (SIC-LSD) approximation, based on multiple scattering theory, which implements self-interaction correction locally, within the KKR method. The multiple scattering aspect of this new SIC-LSD method allows for the description of crystal potentials which vary from site to site in a random fashion and the calculation of physical quantities averaged over ensembles of such potentials using the coherent potential approximation (CPA). This facilitates applications of the SIC to alloys and pseudoalloys which could describe disordered local moment systems, as well as intermediate valences. As a demonstration of the method, we study the well-known $\alpha$-$\gamma$ phase transition in Ce, where we also explain how SIC operates in terms of multiple scattering theory.

Multiple-scattering formalism for correlated systems: A KKR-DMFT approach

Abstract: We present a charge and self-energy self-consistent computational scheme for correlated systems based on the Korringa-Kohn-Rostoker (KKR) multiple scattering theory with the many-body effects described by the means of dynamical mean field theory (DMFT). The corresponding local multiorbital and energy dependent self-energy is included into the set of radial differential equations for the single-site wave functions. The KKR Green's function is written in terms of the multiple scattering path operator, the later one being evaluated using the single-site solution for the $t$-matrix that in turn is determined by the wave functions. An appealing feature of this approach is that it allows to consider local quantum and disorder fluctuations on the same footing. Within the coherent potential approximation (CPA) the correlated atoms are placed into a combined effective medium determined by the DMFT self-consistency condition. Results of corresponding calculations for pure Fe, Ni, and ${\mathrm{Fe}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}$ alloys are presented.

Dilute magnetic semiconductors

Abstract: We describe exchange interactions in dilute magnetic semiconductors (DMS) based on ab initio calculations. The electronic structure of DMS is calculated on the basis of the density functional theory using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). We show that there are two classes of DMS with very different properties. In systems with localized majority d-states deep in the valence band, the ferromagnetism is induced by Zener's p–d exchange interaction. This interaction is weak, but long ranged. For systems with impurity bands in the gap, the ferromagnetism is driven by Zener's double exchange mechanism. This interaction is very strong, but short ranged. Sophisticated Monte Carlo methods show that, for small concentrations, the percolation effect should be included to estimate the Curie temperatures of DMS. In particular, the ferromagnetism is strongly suppressed in double exchange systems due to the absence of percolation for low concentrations.

Core-level shifts in fcc random alloys: A first-principles approach

Abstract: First-principles theoretical calculations of the core-level binding-energy shift (CLS) for eight binary face-centered-cubic (fcc) disordered alloys, CuPd, AgPd, CuNi, NiPd, CuAu, PdAu, CuPt, and Ni ...

First-Principles Materials Design of CuAlO2 Based Dilute Magnetic Semiconducting Oxide

Abstract: The electronic structures and the magnetic properties of dilute magnetic semiconductors (DMSs) based on transparent semiconducting oxide CuAlO2 are calculated by using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation within the local density approximation. We see from the results that we can expect to obtain the half-metallic and high-spin ferromagnetic state is expected to be stable in Mn-, Fe-, Co- and Ni-doped CuAlO2.

Materials Design of Ferromagnetic Diamond

Abstract: We propose materials design of ferromagnetic diamond without any transition metal elements based on first principles calculations. The electronic structure and the magnetic properties of impurities-doped diamond are calculated by using the Korringa-Kohn-Rostoker method within the local spin density approximation with taking into account disorder using coherent potential approximation. It is found that H atoms which are doped into tetrahedral interstitial sites show finite local magnetic moments. Moreover, the impurities-doped diamond show ferromagnetism with half-metallic density of states.

On heavy carbon doping of MgB2

Abstract: Heavy carbon doping of MgB 2 is studied by first principles electronic structure studies of two types, an ordered supercell (Mg(B 1− x C x ) 2 , x = 0.0833) and also the coherent potential approximation method that incorporates effects of B–C disorder. For the ordered model, the twofold degenerate σ-bands that are the basis of the high temperature superconductivity are split by 60 meV (i.e. 7 meV/% C) and the σ Fermi cylinders contain 0.070 holes/cell, compared to 0.11 for MgB 2 . A virtual crystal treatment tends to overestimate the rate at which σ holes are filled by substitutional carbon. The coherent potential approximation (CPA) calculations give the same rate of band filling as the supercell method. The occupied local density of states of C is almost identical to that of B in the upper 2 eV of the valence bands, but in the range −8 eV to −2 eV, C has a considerably larger density of states. The calculations indicate that the σ Fermi surface cylinders pinch off at the zone center only above the maximum C concentration x ≈ 0.10. These results indicate that Mg(B 1− x C x ) 2 as well as Mg 1− x Al x B 2 is a good system in which to study the evolution of the unusual electron–phonon coupling character and strength as the crucial σ hole states are filled.

Complex inorganic solids : structural, stability, and magnetic properties of alloys

Abstract: Precipitation of Disordered Ni-X Solid Solution Phases in Off-stoichiometric Ordered Ni3X Alloys -An Atomic Scale Study of the Physical Properties of Delta Plutonium and Pu:Al Alloys -Ab Initio Thermodynamics and Structural Studies of Cationic Disordered MgAl2O4 Spinel -Computer Simulation of Molten and Glassy Silica and its Mixtures with Sodium Oxide and Aluminum Oxide -A Computer Model of Carbonitride Precipitation in Steel -Current and Future Applications of CALPHAD Technology -Phase Stability and Ordering in (Ga,Mn)As Alloys -Vacancy Ordering and Non-stoichiometry in TiC1-xox and TiN1-xox -Vacancy-Mediated Phase Transformations: Homogeneous or Heterogeneous? -Calculation of the Phase Diagrams of Alloys with Non Pair Atomic Interactions within the Ring Approximation -Modelling of Phase Separation in Iron-based Ternary Alloys -Short-range Order Parameters in fcc Binary Alloys -Dependence of Ordering Process in Ni-based 1 1/2 0 Alloy on Alloying Elements -Changes of LRO in Anisotropic L l0-Ordered FePd -Ordering Processes Analyzed by Phase Field Method, CVM and PPM -Phase Distribution and Transformation Dynamics Using in-situ Synchrotron Diffraction Methods -Monte Carlo Study of the Precipitation Kinetics of Al3Zr in Al-Zr -Examination of Multi-component Diffusion Between Two Ni-Base Superalloys -Curvature and Basis Function Effects on Electronic and Transport Properties of Carbon Nanotubes -The Behavior of Solid Solutions in Geological Transport Processes: The Quantization of Rock Compositions by Fluid-Rock Interaction -Ab-Initio Study of Diluted Magnetic Semiconductors Variation of Elastic Shear Constants in Transition Metal Alloys -Theoretical Strength, Magnetism and Stability of Metals and Itermetallics -Rejuvenation of Deformation-Damaged Material by Magnetic Annealing - A New Approach to Grain BoundaryEngineering -Coherent-Potential Approximation within the Exact Muffin-tin Theory -Charge Distributions in Metallic Alloys: A Charge Excess Functional Theory Approach -Local Charge Distributions in Metallic Alloys: A Local Field Coherent Potential Approximation Method -On the Development of Alloy Theory -Microscopial Derivation of Ginzburg-Landau-type Functionals for Alloys and their Application to Studies of Antiphase and Interphase Boundaries -Investigation of Structures and Properties of C3P4 Alloy using First-principles Electronic Structure Calculation

Exchange Interactions and Curie Temperatures in Dilute Magnetic Semiconductors

Abstract: We discuss the origin of ferromagnetism in dilute magnetic semiconductors based on ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb. We use the Korringa-Kohn- Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. Curie temperatures (T C) are calculated from first-principles by using a mapping on a Heisenberg model in a mean field approximation. It is found that if impurity bands are formed in the gap, as it is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic √c dependence of T C as a function of the Mn concentration c. On the other hand, if the d-states are localized, as in (Ga, Mn)Sb, Zener’s p-d exchange prevails resulting in a linear c-dependence of T C. In order to have more precise estimations of T C, effective exchange coupling constants J ij’s are calculated by using the formula of Liechtenstein et al. It is found that the range of the exchange interaction in (Ga, Mn)N is very short. On the other hand, in (Ga, Mn)As the interaction is weaker but long ranged. Monte Carlo simulations show that the T C values of (Ga, Mn)N are very low since percolation is difficult to achieve for small concentrations and the mean field approximation strongly overestimates T C. Even in (Ga, Mn)As the percolation effect is still important.

Self-consistent theory of scattering at disordered interfaces in layered nanostructures

Abstract: A self-consistent theory of electron scattering at the real disordered interfaces in the layered nanostructures is developed. This theory generalizes, particularly, the well known quantum mechanics results for the electron transmission through and reflection from the perfect potential steps/wells/barriers on the case of the intermixed (alloylike) interfaces. The closed analytical expressions for the probabilities of the specular and diffuse electron transmission and reflection at a single disordered interface are obtained in the self-consistent single-site coherent potential approximation (CPA) and the effective mass approximation for the electronic spectra of different layers. The exact (in the adopted approximations) quantum mechanical transmission amplitude for the electron traveling through two disordered interfaces of a trilayer is also obtained. These results allow studying the interfacial scattering at any angle of an electron incidence at an interface, any materials making up a multilayer (any potential profile and effective masses) and any concentrations of atoms mixed at an interface (particularly, at a nonzero average defect scattering strength). It is shown that the diffuse scattering (caused by the imaginary part of the coherent potential) vanishes at the grazing (with a very small perpendicular to an interface component of velocity) electron incidence at an interface leading to practically specular reflection from an interface (channeling affect). The specular scattering also dominates at close to normal to an interface electron incidence and small interfacial scattering potential fluctuations (from its average value). The diffuse scattering diminishes the specular transmission but may increase or decrease the specular reflection at a disordered interface and permits scattering to the areas (of the parallel to an interface electron momentum component) inaccessible for specular scattering at the perfect interfaces. The obtained specular transmission probability over a potential well of a metallic trilayer exhibits additional (to the conventional resonance states) oscillations caused by the real (average) part of the coherent interfacial potential. The interface roughness associated with the long-range layers' thicknesses fluctuations is accounted for through the semiclassical approximation for the obtained specular transmission probability through a metallic trilayer. For the case of an insulating spacer the obtained tunneling magnetoresistance (TMR) ratio may be expressed (for thick spacer) in the Slonczewski-type form but with the electron polarization and interface factors defined by the electron transmission probabilities (for different spin channels) through a spacer with disordered interfaces. The obtained results are believed to be important for the giant magnetoresistance (GMR) and TMR effects in the real nanostructures with disordered interfaces.

Conductivity and thermopower of heavy fermion systems with disorder

Abstract: We investigate the influence of disorder (impurities) on the electronic (transport) properties of heavy fermion systems, which are described by the periodic Anderson model (PAM). In this paper we consider disorder within the conduction band. The PAM is mapped onto an effective single impurity Anderson model (SIAM) by dynamical mean-field theory (DMFT), and the modified perturbation theory (MPT)—an approximation that is exact up to second order in the Coulomb correlation U and reproduces the atomic limit—is used for the effective SIAM. To include the scattering by the impurities we use the coherent potential approximation (CPA) which is consistent with the DMFT. For various concentrations c of the impurities we calculate the resistivity as well as the thermoelectrical power in a situation describing a Kondo insulator in the pure case ( c = 0 ).

FIRST-PRINCIPLES PHASE DIAGRAM OF THE Ce-Th SYSTEM

Abstract: Actinide physics has seen a remarkable focus the last decade or so due to the combination of improved experimental diamond-anvil-cell techniques and the development of fast computers and more advanced theory. All f-electron systems are expected to have multiphase phase diagrams due to the sensitivity of the f-electron band to external influences such as pressure and temperature. For instance, compression of an f-electron metal generally causes the occupation of f-states to change due to the shift of these bands relative to others. This can in some cases, as in the Ce-Th system, cause the crystal to adopt a lower symmetry structure at elevated pressures. Here we study the phase stabilities of Ce, Th, and the Ce-Th system as a function of compression. Theoretically, both Ce and Th metals are rather well described within the DFT, although a proper treatment of the Ce-Th alloys has not yet been presented. In the present paper we revisit this problem by applying the modern theory of random alloys based on the coherent potential approximation (CPA).

The two-particle propagator and magnetic susceptibility in the Hubbard model

Abstract: We treat the two-particle Green function in the Hubbard model using the recently developed ?-CPA, a hybrid treatment that applies the coherent potential approximation?(CPA) up to a time ? related to the inverse of the bandwidth, after which the system is averaged using the virtual crystal approximation. This model, with suitable approximations, does predict magnetism for a modified Stoner criterion. The evaluation of the two-particle propagator in the ?-CPA requires the solution of the pure CPA, within whose formalism the vertex correction and the weighted Green functions are obtained. The dynamical susceptibility, including the vertex correction and the weighted scattering by the residual interaction, is calculated and shows a spin wave spectrum in the ferromagnetic regime.

Applicability of the coherent potential approximation for transmission through disordered interfaces

Abstract: We present a study of ballistic conductance through substitutionally disordered Fe–Cr(100) interfaces The effects of interface disorder on the conductance are investigated using (a) a supercell approach (SA) where disorder is approximated by randomly generated configurations within large two-dimensional supercells and (b) the layer adapted coherent potential approximation (CPA) The SA gives simultaneously the specular and diffuse part of the transmission The CPA, on the other hand, accounts only for specular transmission We observe excellent agreement between the CPA result and the specular part of the transmission obtained via the SA However, the CPA, in the absence of vertex corrections does not account for the diffuse transmission and therefore cannot be used to describe phenomena such as current perpendicular to the plane giant magnetoresistance (CPP GMR)

Ab initio studies of the energy characteristics and magnetic properties of point defects in GaAs

Abstract: The formation energies of intrinsic point defects and solution energies of transition metal impurities in gallium arsenide are determined on the basis of ab initio calculations using the method of a locally self-consistent Green’s function, which is a generalization of the coherent potential approximation. Based on the calculated energies, the conclusion is made that the AsGa antisite defect is the most common intrinsic defect in GaAs. Calculations showed that transition metal impurities, except for Ni, preferentially occupy gallium sites substitutionally. The magnetic moments of impurity atoms are calculated as a function of the chemical environment. It is shown that, in compensated GaAs, Mn atoms tend to form clusters.

Is High TC Possible in (Ga,Mn)N?: Monte Carlo Simulation vs. Mean Field Approximation

Abstract: The magnetic properties of diluted magnetic semiconductors (DMSs) are calculated from first-principles by mapping the ab initio results on a classical Heisenberg model. By using the Korringa–Kohn–Rostoker coherent potential approximation method within the local density approximation, the electronic structure of (Ga,Mn)N and (Ga,Mn)As is calculated. Effective exchange coupling constants J ij s are calculated by using the formula of Liechtenstein et al. (A.~I. Liechtenstein, M. I. Katsnelson, V. P. Antropov, and V. A. Gubanov, 1987, J.~Magn. Magn. Mater. Vol. 67, p. 65). It is found that the range of the exchange interaction in (Ga,Mn)N is very short due to the exponential decay of the impurity wave function in the gap. On the other hand, in (Ga,Mn)As, the interaction is weaker but long ranged because the extended valence hole states mediate the ferromagnetic interaction. Monte Carlo simulations show that the TC values of (Ga,Mn)N are very low since percolation is difficult to achieve for small concentrations and the mean field approximation strongly overestimates TC. Even in (Ga,Mn)As the percolation effect is still important.

Transport and optical properties of diluted magnetic semiconductors

Abstract: We investigated the transport and optical properties of diluted magnetic semiconductors theoretically by using a simple model where carriers move in a single band. In this model the carrier feels a nonmagnetic potential at a magnetic impurity site, and its spin interacts with the localized spins of the magnetic impurities through exchange interactions. The electronic states of a carrier were calculated by using the coherent potential approximation (CPA). The magnetism was investigated by minimizing the free-energy and the electrical conductivity was calculated by using the Kubo formula. We examined the results in several typical cases which correspond to (Ga1−xMn x )As with x = 0.05.

Coherent Potential Approximation Within the Exact Muffin-Tin Orbitals Theory

Abstract: An implementation of the coherent potential approximation (CPA) is carried out within the frameworks of the exact muffin-tin orbitals (EMTO) theory. During the self-consistent iterations the Poisson equation is solved using the spherical cell approximation, and the charge transfer between alloy components is treated within the screened impurity model. The total energy is calculated using the full charge density (FCD) technique. The FCD-EMTO-CPA method is suitable for accurate determination of the electronic structure and total energy of completely random alloys with a substi-tutional disorder on any kind of underlying crystal lattice. The accuracy of the method is demonstrated through test calculations performed on face centered cubic (fcc), body centered cubic (bcc), and hexagonal close packed (hcp) Cu-Zn binary alloys.

Ground-state properties and molecular theory of Curie temperature in the coherent potential approximation of diluted magnetic semiconductors

Abstract: Using spin-1/2 description of valence holes and Kondo coupling between local spins and carriers, GaAs-based III-V diluted magnetic semiconductors (DMS)are studied in the coherent potential approximation(CPA). Our calculated relation of ground-state energy and impurity magnetization shows that ferromagnetism is always favorable at low temperatures. For very weak Kondo coupling, the density of states (DOS) of the host semiconductor is not modified much. Impurity band can be generated at the host band bottom only when Kondo coupling is strong enough. Using Weiss molecular theory, we predict a linear relation of Curie temperature with respect to Kondo coupling and doping concentration $x$ if the hole density is proportional to $x$.

Limitation of Mean Field Approximation for Carrier States and Ferromagnetism in Diluted Magnetic Semiconductors

Abstract: In spite of a great deal of recent experimental and theoretical activities, there has not yet been a consensus on the fundamental mechanism leading to the ferromagnetism of Ga1 xMnxAs. 1) In diluted magnetic semiconductors (DMSs), there are two types of disorder: substitutional disorder and the thermal fluctuation of localized spins. Neglecting disorder effects, the mean-field Zener model (the phenomenological model) predicts the possibility of a high Curie temperature for some materials. Properly taking the disorder effects into consideration, however, is indispensable in the calculation of the Curie temperature TC of DMSs, as clarified in this study. To show the importance of the disorder effects, we calculate TC by applying three approximations to a simple model: the coherent potential approximation (CPA), the virtual crystal approximation (VCA), and the mean-field approximation (MFA). In the present model, Ga1 xMnxAs is regarded as a semiconducting alloy in which a mole fraction x of Ga ions (symbol: A) in GaAs are replaced at random by Mn ions (symbol:M). A single carrier (a p hole) moving in Ga1 xMnxAs is affected by the local potential EA or EM I Sm, depending on whether it is on a Ga site or a Mn site. Here, EA and EM denote the spinindependent potentials on the Ga ion and Mn ion, respectively; I Sm represents the p–d exchange interaction between the carrier (p hole) and the localized spin Sm (d spin) of Mn located on the m site. Setting EA 1⁄4 0, we employ the model density of states (DOS) of semicircular form:

Cluster coherent potential approximation for the electronic structure of disordered alloys

Abstract: We extend the single-site coherent potential approximation (CPA) to include the effects of non-local disorder correlations (alloy short-range order) on the electronic structure of random alloy systems. This is achieved by mapping the original Anderson disorder problem to that of a self-consistently embedded cluster. This cluster problem is then solved using the equations of motion technique. The CPA is recovered for cluster size Nc = 1. Various new features, compared to those observed in CPA, and related to repeated scattering on pairs of sites, reflecting the effect of short-range order are clearly visible in the density of states. It is explicitly shown that the cluster-CPA method always yields a positive-definite density of states. Anderson localization effects have been investigated within this approach. In general, we find that Anderson localization sets in before band splitting occurs, and that increasing partial order drives a continuous transition from an Anderson insulator to an incoherent metal.

Electron momentum density in Ni75Cu25 and Ni75Co25 disordered alloys: a high-resolution Compton-scattering study

Abstract: We report high-resolution Compton profiles (CPs) of Ni75Cu25 and Ni75Co25 disordered, ferromagnetic alloys measured along [100], [110] and [111] crystallographic directions. The directional CP anisotropies, profile derivatives and reciprocal form factors B(z) are analysed in detail and compared with first-principles KKR-CPA (Korringa–Kohn–Rostoker coherent potential approximation) calculations. Good overall agreement is found between the experimental and theoretical anisotropies. It follows from experiment that the majority-spin Fermi surface of the Ni75Cu25 and Ni75Co25 alloys occurs at higher momentum than the theory predicts. Also, a splitting in the secondary maxima in experimental B(z) functions of the alloys is observed, which is not predicted by the KKR-CPA calculations.

Anomalous diffusion of light in TiO2-powder layers near the absorption edge

Abstract: The transfer of visible and near-IR radiation in layers of polydisperse TiO2 (rutile) particles has been studied by measuring diffusion transmission and coherent backscattering in order to determine the optical parameters of the samples under investigation. An approach based on the coherent potential approximation is applied. It has been shown that, in the short-wavelength region of the visible range, anomalous diffusion of light occurs due to the effect of interference at mesoscopic scales on the transport characteristics of the scattering medium.

Phase Stability and Ordering in (Ga,Mn)As Alloys

Abstract: We have studied the phase stability and the possible ordering in ferromagnetic semiconducting alloys (Ga,Mn)As on an ab initio level. The main conclusions of our study can be summarized as follows: (i) The alloys are thermodynamically unstable with respect to segregation into related compounds or alloys with extremal chemical composition. (ii) The As-antisites have a stabilizing effect and make the incorporation of Mn atoms energetically favorable. (iii) At a critical temperature of about 800 K domains of two types, namely, with an enhanced and with a lowered concentration of impurities, may appear. The expected form of these domains is a cube with side length of approximately 120–130 A, arranged to form a simple cubic lattice. (iv) At short distances, the Warren-Cowley parameters indicate mutual attraction of Mn↑-Mn↑ and Mn↓-As paris, and repulsion of all other pairs of impurities.

High-temperature superconductivity and band antiferromagnetism

Abstract: We analyze the competition between high-temperature superconductivity (SC) and antiferromagnetism (AF) using the extended Hubbard model containing following matrix elements in the Hamiltonian: Hund's on-site field FH, single-site Coulomb repulsion U = ( i , i | 1 / r | i , i ) , two-site charge–charge, exchange, pair exchange and assisted hopping interactions V = ( i , j | 1 / r | i , j ) , J = ( i , j | 1 / r | j , i ) , J ′ = ( i , i | 1 / r | j , j ) , Δ t = ( i , i | 1 / r | j , i ) . In our model, we introduce the possibility of AF ordering by dividing the crystal lattice into two interpenetrating sub-lattices α , β . We use Hartree–Fock (H–F) approximation for all interactions except the strong on-site Coulomb repulsion. The self-energies Σ γ σ ( e ) ( γ = α , β ) are calculated within the coherent potential approximation (CPA). To obtain the SC transition temperature TC and the Neel's temperature TN we solve the coupled equations of motion for the Green's functions. We present numerical results. They show that the AF at half-filling destroys the superconductivity of the s0-wave symmetry. Increase of the Coulomb repulsion in the CPA causes increasing dumping of s0-wave SC, shifts it away from the half-filling point, enhances SC temperature for sub-lattice magnetic moments; 0