Showing papers on "Coherent potential approximation published in 2007"

Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys

Abstract: We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within densit ...

Impurities in a Biased Graphene Bilayer

Abstract: erties of the BGB it is important to understand the effects of the unavoidable disorder. In this letter we show that bound states exist for arbitrary weak impurity potentials, and that their properties, such as binding energies and localization lengths, can be externally controlled with a gate bias. Moreover, we obtain the wave-functions of the mid-gap states, from which we derive a simple criterion for when the overlap between wavefunctions becomes important. This overlap results in band gap renormalization and possibly band tails extending into the gap region, as in the case of ordinary heavily doped semiconductors [17], or impurity bands for deep impurities. Unlike ordinary semiconductors, the electronic density of states can be completely controlled via the electric field effect. The impurity interaction problem is studied within the coherent potential approximation (CPA). The Model. The low-energy effective bilayer Hamiltonian has the form [12, 13] (we use units such that ¯

Trends of exchange interactions in dilute magnetic semiconductors

Abstract: We discuss the importance of different exchange mechanisms like double exchange, p–d exchange and anti-ferromagnetic as well as ferromagnetic superexchange in dilute magnetic semiconductors (DMSs). Based on the coherent potential approximation for the electronic structure of the DMSs we show that the different mechanisms exhibit different dependences on the concentration of the magnetic impurities, on the hybridization with the wavefunctions of neighbouring impurities and on the position of the Fermi level in the band gap. However, common to all mechanisms is that, as long as half-metallicity is preserved, they are determined by the hybridization with the orbitals of neighbouring impurities and of the resulting energy gain due to the formation of bonding and anti-bonding hybrids. By calculating the exchange coupling constants Jij(EF) as a function of the position of the Fermi level we obtain a universal trend for the exchange interactions with band filling.

Dilute magnetic semiconductors based on half-heusler alloys

Abstract: We have investigated the electronic structure and magnetism of Mn-doped half-Heusler alloys by using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method within the local density approximation (LDA). Half-Heusler compounds can be attractive for spintronic applications because the crystal structure and lattice constant of these compounds are similar to those of III–V and II–VI compounds, which are often used in present semiconductor technologies. The Curie temperatures of Mn-doped half-Heusler alloys are calculated by the mean field approximation, random phase approximation, and Monte Carlo simulation. Based on our calculation results, we discuss whether or not the half-Heusler-based dilute magnetic semiconductors are useful for realizing semiconductor spintronics.

Theoretical Study on Superconductivity in Boron-Doped Diamond

Abstract: We consider superconductivity in boron (B) doped diamond using a simplified model for the valence band of diamond. We treat the effects of substitutional disorder of B ions by the coherent potential approximation (CPA) and those of the attractive force between holes by the ladder approximation under the assumption of instantaneous interaction with the Debye cutoff. We thereby calculate the quasiparticle life time, the evolution of the single-particle spectra due to doping, and the effect of disorder on the superconducting critical temperature T c . We in particular compare our results with those for supercell calculations to see the role of disorder, which turns out to be of crucial importance to T c .

Carrier-induced ferromagnetism in diluted local-moment systems

Abstract: The electronic and magnetic properties of concentrated and diluted ferromagnetic semiconductors are investigated by using the Kondo lattice model, which describes an interband exchange coupling between itinerant conduction electrons and localized magnetic moments. In our calculations, the electronic problem and the local magnetic problem are solved separately. For the electronic part an interpolating self-energy approach together with a coherent potential approximation (CPA) treatment of a dynamical alloy analogy is used to calculate temperature-dependent quasiparticle densities of states and the electronic self-energy of the diluted local-moment system. For constructing the magnetic phase diagram we use a modified Ruderman-Kittel-Kasuya-Yosida (RKKY) theory by mapping the interband exchange to an effective Heisenberg model. The exchange integrals appear as functionals of the diluted electronic self-energy being therefore temperature- and carrier-concentration-dependent and covering RKKY as well as double exchange behavior. The disorder of the localized moments in the effective Heisenberg model is solved by a generalized locator CPA approach. The main results are (1) extremely low carrier concentrations are sufficient to induce ferromagnetism; (2) the Curie temperature exhibits a strikingly nonmonotonic behavior as a function of carrier concentration with a distinct maximum; (3) ${T}_{C}$ curves break down at critical $n∕x$ due to antiferromagnetic correlations; and (4) the dilution always lowers ${T}_{C}$ but broadens the ferromagnetic region with respect to carrier concentration.

Relativistic formulation of the Korringa-Kohn-Rostoker nonlocal coherent-potential approximation

Abstract: The recently introduced Korringa–Kohn–Rostoker nonlocal coherent-potential approximation (KKR-NLCPA) provides a sound basis for systematically including important environmental effects within an ab initio description of disordered systems Here we propose a fully relativistic formulation of the KKR-NLCPA which is designed for the treatment of magnetically-ordered alloys Crucial to its implementation is a reformulation of the basic algorithm and a symmetrization of the fundamental coarse-graining procedure, which we describe in detail As a first application of the approach we study the electronic and magnetic properties of the ferromagnetic FePt system

Engineering the electronic, magnetic and gap-related properties of the quinternary half-metallic Heusler alloys

Abstract: We review the electronic and magnetic properties of the quinternary full Heusler alloys of the type Co$_2$[Cr$_{1-x}$Mn$_x$][Al$_{1-y}$Si$_y$] employing three different approaches : (i) the coherent potential approximation (CPA), (ii) the virtual crystal approximation (VCA), and (iii) supercell calculations (SC). All three methods give similar results and the local environment manifested itself only for small details of the density of states. All alloys under study are shown to be half-metals and their total spin moments follow the so-called Slater-Pauling behavior of the ideal half-metallic systems. We especially concentrate on the properties related to the minority-spin band-gap. We present the possibility to engineer the properties of these alloys by changing the relative concentrations of the low-valent transition metal and $sp$ atoms in a continuous way. Our results show that for realistic applications, ideal are the compounds rich in Si and Cr since they combine large energy gaps (around 0.6 eV), robust half-metallicity with respect to defects (the Fermi level is located near the middle of the gap) and high values of the majority-spin density of states around the Fermi level which are needed for large values of the perfectly spin-polarized current in spintronic devices like spin-valves or magnetic tunnel junctions.

Random Kondo alloys investigated with the coherent potential approximation

Abstract: The interplay between the Kondo effect and disorder is studied. This is done by applying a matrix coherent potential approximation and treating the Kondo interaction on a mean-field level. The resulting equations are shown to agree with those derived by the dynamical mean-field theory. By applying the formalism to a Bethe tree structure with infinite coordination, the effects of diagonal and off-diagonal disorder are studied. Special attention is paid to the behavior of the Kondo and the Fermi-liquid temperature as function of disorder and concentration of the Kondo ions. The nonmonotonous dependence of these quantities is discussed.

First-Principles Study on the Ferromagnetism and Curie Temperature of Mn-Doped AlX and InX (X=N, P, As, and Sb)

Abstract: We investigate the electronic structure and magnetic properties of AlN-, AlP-, AlAs-, AlSb-, InN-, InP-, InAs-, and InSb-based dilute magnetic semiconductors (DMS) with Mn impurities from first-principles. The electronic structure of DMS is calculated by using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA) and the LDA+U method. Describing the magnetic properties by a classical Heisenberg model, effective exchange interactions are calculated by applying magnetic force theorem for two impurities embedded in the CPA medium. With the calculated exchange interactions, T C is estimated by using the mean field approximation, the random phase approximation and the Monte Carlo simulation. It is found that the p–d exchange model [Dietl et al. : Science 287 (2000) 1019] is adequate for a limited class of DMS and insufficient to describe the ferromagnetism in wide gap semiconductor based DMS such as (Ga,Mn)N and the presently inves...

Ferromagnetism and spinodal decomposition in dilute magnetic nitride semiconductors

Abstract: We propose a materials design for dilute magnetic semiconductors (DMS) based on first-principles calculations by using the Korringa–Kohn–Rostoker coherent potential approximation method. We develop an accurate method for calculations of the Curie temperature (TC) of DMS and show that the mean field approximation completely fails to predict TC for DMS, in particular for wide gap nitride DMS where the exchange interaction is short ranged. The TC calculated for homogeneous DMS by using the present method agrees very well with available experimental values. For more realistic material design, we simulate spinodal nanodecomposition by applying the Monte Carlo method to the Ising model with ab initio chemical pair interactions between magnetic impurities in DMS. It is found that by controlling the dimensionality of the decomposition, various characteristic phases occur in DMS. It is suggested that superparamagnetic blocking phenomena should be important for understanding the magnetism of wide gap DMS.

Magnetic properties of 3d pyrite-type mixed crystals calculated by the full-potential KKR-CPA method.

Abstract: The electronic and magnetic properties of pyrite-type mixed crystals M(1-x)M(x)(')S(2) (M, M(') = Fe,Co,Ni) were investigated by use of the full-potential Korringa-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA). The results well explain the systematic behaviour of these systems as long as they are in a metallic phase. It is also concluded that the full-potential treatment is necessary to describe these anisotropic systems.

Ab initio calculations of the stability of disordered Ti-V-Cr solid solutions and their hydrides

Abstract: The electronic structure of Ti-V-Cr alloys is calculated using the Korringa-Kohn-Rostoker method in the coherent potential approximation. The phase stability of the alloys is investigated as a function of the stoichiometry, the composition, and the hydrogen concentration. The results obtained are in good agreement with the available experimental data and provide an adequate description of the transition from the A2 phase to the A1 phase upon saturation of the alloys with hydrogen. For hydrogen-saturated hydrides with a high chromium concentration, the calculations predict magnetic polarization at chromium atoms with a magnetic moment of up to 2μB.

Magnetic interactions in the MnFe1?xCoxP series of solid?solutions

Abstract: In this report the results of crystal structure investigations, high magnetic field measurements and electronic structure calculations carried out for the MnFe1?xCoxP system are presented. The crystal structure parameters were determined using the x-ray powder diffraction method. On this basis the inter-atomic distances were calculated and the magnetic couplings between magnetic atoms in MnFe1?xCoxP are discussed. Magnetic properties of the series of compounds with x = 0.3, 0.45, 0.5, 0.525, 0.55, 0.65 and 0.7, as determined under strong magnetic field (up to 20?T), are reported. The electronic band structure calculations were performed using the Korringa?Kohn?Rostoker method with the coherent potential approximation (KKR-CPA). The site preference of Co and Fe atoms, located in pyramidal and tetrahedral positions, was analysed and magnetic properties of Co and Fe sublattices were calculated based on total energy computations. The site-decomposed densities of states and the magnetic moment values were calculated in the whole alloy concentration range assuming a ferromagnetic (F) order. For MnFe0.35Co0.65P the KKR-CPA calculations were carried out assuming different types of antiferromagnetic (AF) arrangement in order to elucidate the origin of the AF?F transition. The magnetic interactions between transition metal atoms, as established from the phenomenological analysis of relating magnetic couplings and inter-atomic distances, were discussed based on the evolution of the site-decomposed density of states and the corresponding dependence of local magnetic moments on alloy composition. A satisfying agreement between experimental and calculated values of magnetization and local magnetic moments localized on Mn, Co and Fe sites was found.

Chemical and magnetic impurity effects on electronic properties of semiconductor quantum wires

Abstract: We present a theoretical study of electronic states in magnetic and nonmagnetic semiconductor quantum wires. The effects of chemical and magnetic disorder at paramagnetic temperatures are investigated in single-site coherent potential approximation. It is shown that the nonmagnetic impurity shifts the band of carriers and suppresses the van Hove singularities of the local density of states (LDOS) depending on the value of impurity concentration. The magnetic impurity, however, broadens the band which depends on the strength of exchange coupling, and in the high impurity concentration, the van Hove singularities in the LDOS can completely disappear and the curves become smooth.

B-doped diamond: Superconductivity without Fermi surface

Abstract: We consider superconductivity in boron (B) doped diamond using a simplified model for the valence band of diamond. We treat the substitutional disorder of B ions by the coherent potential approximation and the attractive force between holes by the ladder approximation under the assumption of instantaneous interaction. We thereby calculate the quasiparticle life time, the evolution of the single-particle spectra due to doping, and the effect of disorder on the critical temperature Tc. We in particular compare our results with those for supercell calculations to see the role of disorder, which turns out to be of crucial importance.

The Hubbard model on the kagome lattice

Abstract: In itinerant magnetically frustrated materials it is important to elucidate the effect of the electronic correlation on the band structure and how it evolves as a function of the electronic concentration n. In this work we address this problem by considering the Hubbard model on a kagome lattice with a single orbital per site, treating the local Coulomb repulsion U within the coherent potential approximation. A metal–insulator transition occurs in the half-filled band (n = 1) at Uc = 3.635t. A nearly flat band, characteristic of frustrated lattices, is present at the top of the band structure. For concentrations n>4/3, the chemical potential may be located in this very narrow band, and one obtains a significant enhancement of the γ coefficient of the specific heat depending on the value of U. This effect can be at the origin of the heavy fermion behaviour observed in LiV2O4.

Effect of diffusion and alloying on the magnetic and transport properties of Fe/V/Fe trilayers

Abstract: The magnetic and transport properties of the $\mathrm{Fe}∕\mathrm{V}∕\mathrm{Fe}(001)$ trilayers were studied using the self-consistent Green's function technique based on the tight-binding linear muffin-tin orbital method in the atomic-sphere approximation. The coherent potential approximation was used to describe the effects of interdiffusion and alloying at the interfaces on the properties of the semi-infinite bcc $\mathrm{Fe}(001)∕m\mathrm{Fe}∕n\mathrm{V}∕m\mathrm{Fe}∕\mathrm{Fe}(001)$ trilayers. The electric conductance was calculated using the Kubo-Landauer formalism, in the current-perpendicular-to-plane geometry. It is shown that a dipole moment is created at the $\mathrm{Fe}∕\mathrm{V}$ interface due to the charge transfer from vanadium to iron, and a small induced magnetic moment is present in the first vanadium layer and is antiparallel to that of iron. The interlayer exchange coupling shows rapid oscillations for small spacer thicknesses, and the interdiffusion and alloying at the interface stabilize the ferromagnetic coupling. Moreover, the interdiffusion reduces the vanadium-induced magnetic moment and increases the iron magnetic moment at the interface. The giant magnetoresistance (GMR) ratio presents damped oscillations as a function of the vanadium spacer thickness. The interdiffusion and the presence of Mn impurities at the interface reduce considerably the GMR ratio and produce results that are in agreement with experimental data.

Viscoelastic modeling of rocks saturated with heavy oil

Abstract: While properties of bulk heavy oil can be approximated by an appropriate viscoelastic model, only a few attempts to model properties of rocks saturated with heavy oil have been reported (Eastwood, 1993; Leurer and Dvorkin, 2006). Rock physics for heavy oil is different from rock physics for conventional fluids because its viscoelastic rheology makes Gassmann theory and all its extensions inapplicable in principle. In this paper, we estimate the properties of such rock-heavy oil mixtures by considering (1) a system of layers of solid and a viscoelastic medium and (2) by computing Hashin-Shtrikman bounds for this system. These two methods essentially give approximate bounds for the frequencyand temperature-dependent properties of these rocks. We also propose how to compute a realistic estimate of these properties that would lie between these bounds. This proposed estimate is based on one particular equivalent-medium approach known as coherent potential approximation (CPA) (Berryman, 1980). In a more general form, this approximation can be used for approximate fluid substitution for heavy oil.

Dependence of magnetism of VAu4 alloy on the state of chemical order : A first principles study

Abstract: The strong dependence of the magnetic properties of the alloy VAu4 upon the degree of chemical order has been a subject of intense experimental studies and controversial theoretical interpretations. In the framework of density functional theory using the coherent potential approximation embodied in the Korringa-Kohn-Rostoker method, we perform first principles calculations of VAu4 varying the degree of atomic chemical order from a disordered fcc alloy to the fully ordered MoNi4-type structure. In contrast to the conventional point of view, partially also based on earlier first principles studies of the ordered structure, our results suggest a localized character of the vanadium moments rather than being weakly itinerant. Moreover, we find that in the fully ordered alloy an antiferromagnetic state is more stable than the ferromagnetic. This finding leads to a significant revision of the earlier descriptions of magnetism in VAu4, which were based either on itinerant or local moment pictures. Investigating fcc Au-V alloys richer in vanadium, we also study the role of local environment effects on the stabilization of the magnetic moments at the V atoms and advocate a ferrimagnetic character of the experimentally observed state with a small spontaneous magnetization.

Metal–insulator-like transition in the energy spectrum of a one-dimensional proton (ionic) conductor

Abstract: We propose a modification of the well-known coherent potential approximation (CPA) for fermionic systems with short-range interactions. We calculate the spectra of a one-dimensional proton (ionic) conductor described by the fermionic lattice model within this approach. Depending on correlation values we observe either quasi-metallic or dielectric behaviour of the system at half-filling. Away from half filling, the system is always in the quasi-metallic state.

First Principles Calculations of Thermodynamic Quantities and Phase Diagrams of High Temperature BCC Ta-W and Mo-Ta Alloys

Abstract: The thermodynamic properties of high temperature metals and alloys are studied using the statistical moment method, going beyond the quasi-harmonic approximations. Including the power moments of the atomic displacements up to the fourth order, the Helmholtz free energies and the related thermodynamic quantities are derived explicitly in closed analytic forms. The configurational entropy term is taken into account by using the tetrahedron cluster approximation of the cluster variation method (CVM). The energetics of the binary (Ta-W and Mo-Ta) alloys are treated within the framework of the first-principles TB-LMTO (tight-binding linear muffin tin orbital) method coupled to CPA (coherent potential approximation) and GPM (generalized perturbation method). The equilibrium phase diagrams are calculated for the refractory Ta-W and Mo-Ta bcc alloys.

Kondo coupling and Curie temperature of Ga1-xMnxAs in the coherent potential approximation

Abstract: The ferromagnetism of Ga 1 - x Mn x As is studied in the coherent potential approximation (CPA). In this work, we used the exact Hilbert transformation of the face-centered cubic (fcc) density of states (DOS), which is different from the usual semi-circle DOS employed in previous works. Using Weiss molecular theory, we obtained a nonlinear relation of Curie temperature with respect to Kondo coupling. Our calculated T C agrees very well with measured values.

Calculated transport properties of half-metallic diluted antiferromagnetic semiconductors

Abstract: The dc conductivity at T = 0 K of half-metallic diluted antiferromagnetic semiconductors and their super lattice is calculated by the use of the Kubo–Greenwood formula applied to the Korringa–Kohn–Rostoker (KKR) Green's function method and the coherent potential approximation in the framework of the local density approximation (LDA) of the density functional method (KKR-CPA-LDA). The calculated dc conductivity of superstructures composed of diluted antiferromagnetic semiconductors shows significant dependence on the magnetic structure of the superstructures. The mechanisms responsible for such behaviour are discussed on the basis of the first-principles calculation.

Theoretical Aspects of Superconductivity in Boron-Doped Diamond

Abstract: discuss the electronic states of B-doped diamond by placing particular emphasis on the effects of disorder on the electronic states induced by the B impurities. We then consider the superconductivity of B-doped diamond by calculating the impurity states of B atoms and superconducting critical temperature by the coherent potential approximation. We show that the effects of disorder play a crucial role in the electronic structure and superconducting critical temperature of B-doped diamond.