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Showing papers on "Coherent potential approximation published in 2013"


Journal ArticleDOI
TL;DR: In this article, a single-phase high-entropy alloys were investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA).
Abstract: Single-phase high-entropy alloys are investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA). Choosing the paramagnetic face-ce ...

143 citations


Journal ArticleDOI
TL;DR: In this paper, a soft ferromagnetic Heusler phases Fe{}_{2}$NiGe, Fe${}_{1}$CoGe and Fe{1}µ$NiGa are reported to exist in a tetragonal cubic cubic heusler structure with different degrees of disorder.
Abstract: The present work reports on the new soft ferromagnetic Heusler phases Fe${}_{2}$NiGe, Fe${}_{2}$CuGa, and Fe${}_{2}$CuAl, which in previous theoretical studies have been predicted to exist in a tetragonal Heusler structure. Together with the known phases Fe${}_{2}$CoGe and Fe${}_{2}$NiGa these materials have been synthesized and characterized by powder x-ray diffraction, ${}^{57}$Fe M\"ossbauer spectroscopy, superconducting quantum interference device, and energy-dispersive x-ray measurements. In particular M\"ossbauer spectroscopy was used to monitor the degree of local atomic order/disorder and to estimate magnetic moments at the Fe sites from the hyperfine fields. It is shown that in contrast to the previous predictions all the materials except Fe${}_{2}$NiGa basically adopt the inverse cubic Heusler ($X$) structure with differing degrees of disorder. The experimental data are compared with results from ab initio electronic structure calculations on local-density approximation level incorporating the effects of atomic disorder by using the coherent potential approximation. A good agreement between calculated and experimental magnetic moments is found for the cubic inverse Heusler phases. Model calculations on various atomic configurations demonstrate that antisite disorder tends to enhance the stability of the $X$ structure.

85 citations


Journal ArticleDOI
TL;DR: In this paper, a theory of temperature-dependent photoemission is presented, which accurately describes phonon effects in soft and hard x-ray angle-resolved photo-emission.
Abstract: We present a theory of temperature-dependent photoemission which accurately describes phonon effects in soft and hard x-ray angle-resolved photoemission. Our approach is based on a fully relativistic one-step theory of photoemission that quantitatively reproduces the effects of phonon-assisted transitions beyond the usual $\mathbf{k}$-conserving dipole selection rules which lead to the so-called XPS limit in the hard x-ray and/or high temperature regime. Vibrational atomic displacements have been included using the coherent potential approximation in analogy to the treatment of disordered alloys. The applicability of this alloy analogy model is demonstrated by direct comparison to experimental soft x-ray data from W(110) showing very satisfying agreement.

60 citations


Journal ArticleDOI
TL;DR: The thermoelectric and thermodynamic properties of polycrystalline InxCo4Sb12 (0.0 ≤ x ≤ 0.26) skutterudites were investigated and analyzed between 2 and 800 K by means of electrical resistivity, thermopower, thermal conductivity and specific heat measurements as discussed by the authors.
Abstract: The thermoelectric and thermodynamic properties of polycrystalline InxCo4Sb12 (0.0 ≤ x ≤ 0.26) skutterudites were investigated and analysed between 2 and 800 K by means of electrical resistivity, thermopower, thermal conductivity and specific heat measurements. Hall effect, sound velocity and thermal expansion measurements were also made in order to gain insights into the transport and elastic properties of these compounds. The impact of the In filling on the crystal structure as well as the thermal dynamics of the In atoms were tracked down to 4 K using powder neutron diffraction experiments. Analyses of the transport data were compared with the evolution of the electronic band structure with x determined theoretically within the Korringa–Kohn–Rostoker method with the coherent potential approximation. These calculations indicate that In gives rise to a remarkably large p-like density of states located at the conduction band edge. The electrical properties show typical trends of heavily doped semiconductors regardless of the In content. The thermal transport in CoSb3 is strongly influenced by the presence of In in the voids of the crystal structure resulting in a drop in the lattice thermal conductivity values in the whole temperature range. The low value of the Gruneisen parameter suggests that this decrease mainly originates from enhanced mass-fluctuations and point-defect scattering mechanisms. The highest thermoelectric figure of merit ZT ~ 1.0 at 750 K was achieved at the maximum In filling fraction, i.e. for x = 0.26.

43 citations


Journal ArticleDOI
TL;DR: In this article, the authors derived a coherent potential approximation (CPA) suited for describing the diffusive (hopping) motion of classical particles in a random environment, and the vibrational properties of materials with spatially fluctuating elastic coefficients in topologically disordered materials.
Abstract: Using Gaussian integral transform techniques borrowed from functional-integral field theory and the replica trick we derive a version of the coherent potential approximation (CPA) suited for describing (i) the diffusive (hopping) motion of classical particles in a random environment, and (ii) the vibrational properties of materials with spatially fluctuating elastic coefficients in topologically disordered materials. The effective medium in the present version of the CPA is not a lattice but a homogeneous and isotropic medium, representing an amorphous material on a mesoscopic scale. The transition from a frequency-independent to a frequency-dependent diffusivity (conductivity) is shown to correspond to the boson peak in the vibrational model. The anomalous regimes above the crossover are governed by a complex, frequency-dependent self-energy. The boson peak is shown to be stronger for non-Gaussian disorder than for Gaussian disorder. We demonstrate that the low-frequency nonanalyticity of the off-lattice version of the CPA leads to the correct long-time tails of the velocity autocorrelation function in the hopping problem and to low-frequency Rayleigh scattering in the wave problem. Furthermore we show that the present version of the CPA is capable of treating the percolative aspects of hopping transport adequately.

39 citations


Journal ArticleDOI
TL;DR: In this paper, a pseudopotential calculation of the lattice constants and of the single-crystal elastic constants for polycrystalline alloys with B1-rocksalt structure was performed using density functional perturbation theory within the virtual crystal approximation (VCA) for disordered alloys and the supercell method (SC) for ordered alloys, with the ABINIT program.
Abstract: First-principles pseudopotential calculations of the lattice constants and of the single-crystal elastic constants for Ti x Ta 1 − x N (0 ≤ x ≤ 1) alloys with B1-rocksalt structure were first carried out. These calculations were performed using density functional perturbation theory (DFPT) within the virtual crystal approximation (VCA) for the disordered alloys and the supercell method (SC) for the ordered alloys, with the ABINIT program. For disordered structures, partial comparisons of the lattice constants and of the bulk modulus are provided with calculations that used the coherent potential approximation (CPA) with the exact muffin-tin orbitals (EMTO). For the exchange-correlation potential we used the generalized gradient methods (GGA). The calculated equilibrium lattice parameters by VCA are in good agreement with the stress-free lattice parameters a 0 and exhibit a positive deviation from Vegard's rule corresponding to a positive bowing parameter while the calculated single-crystal stiffness c 12 and c 44 , gradually increase when c 11 decreases from TaN to TiN. In a second stage, we have estimated by homogenization methods the averaged stiffnesses c ij >, the Young modulus and Poisson ratio of polycrystalline Ti x Ta 1 − x N (0 ≤ x ≤ 1) alloys considering a random orientation of crystallites. Finally, comparisons are made with the experimental effective out-of-plane shear elastic modulus C 44 and the effective out-of-plane longitudinal elastic constant C 33 measured by Brillouin light scattering and picosecond ultrasonics, respectively, on thin films elaborated by magnetron sputtering.

36 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed the theory of nonequilibrium coherent potential approximation (NECPA) for analysis of disorder effects in none-ilibrium quantum transport of nanoelectronic devices.
Abstract: Since any realistic electronic device has some degree of disorder, predicting disorder effects in quantum transport is a critical problem. Here we report the theory of nonequilibrium coherent potential approximation (NECPA) for analyzing disorder effects in nonequilibrium quantum transport of nanoelectronic devices. The NECPA is formulated by contour ordered nonequilibrium Green's function (NEGF) where the disorder average is carried out within the coherent potential approximation on the complex-time contour. We have derived a set of new rules that supplement the celebrated Langreth theorem and, as a whole, the generalized Langreth rules allow us to derive NECPA equations for real time Green's functions. The solution of NECPA equations provide the disorder averaged nonequilibrium density matrix as well as other relevant quantities for quantum transport calculations. We establish the excellent accuracy of NECPA by comparing its results to brute force numerical calculations of disordered tight-binding models. Moreover, the connection of NECPA equations which are derived on the complex-time contour, to the nonequilibrium vertex correction theory which is derived on the real-time axis, is made. As an application, we demonstrate that NECPA can be combined with density functional theory to enable analysis of nanoelectronic device physics from atomistic first principles.

26 citations


Journal ArticleDOI
TL;DR: It is shown that the theoretical elastic constants follow a clear trend with the degree of chemical order: namely, C(11 and C(12) decrease, whereas C(44) remains nearly constant with increasing disorder.
Abstract: Ab initio calculations, based on the exact muffin-tin orbitals method are used to determine the elastic properties of Cu-Au alloys with Au/Cu ratio 1/3. The compositional disorder is treated within the coherent potential approximation. The lattice parameters and single-crystal elastic constants are calculated for different partially ordered structures ranging from the fully ordered L1(2) to the random face centered cubic lattice. It is shown that the theoretical elastic constants follow a clear trend with the degree of chemical order: namely, C-11 and C-12 decrease, whereas C-44 remains nearly constant with increasing disorder. The present results are in line with the experimental findings that the impact of the chemical ordering on the fundamental elastic parameters is close to the resolution of the available experimental and theoretical tools.

18 citations


Journal ArticleDOI
TL;DR: In this paper, the authors extended the dual fermion approach to disordered systems using the replica method, which includes intersite scattering via diagrammatic perturbation theory in the dual variables.
Abstract: While the coherent potential approximation (CPA) is the prevalent method for the study of disordered electronic systems, it fails to capture nonlocal correlations and Anderson localization. To incorporate such effects, we extend the dual fermion approach to disordered systems using the replica method. The developed method utilizes the exact mapping to the dual fermion variables, and includes intersite scattering via diagrammatic perturbation theory in the dual variables. The CPA is recovered as a zeroth-order approximation. Results for single- and two-particle quantities show good agreement with a cluster extension of the CPA; moreover, weak localization is captured. As a natural extension of the CPA, our method presents an alternative to existing nonlocal cluster theories for disordered systems, and has potential applications in the study of disordered systems with electronic interactions.

18 citations


Journal ArticleDOI
TL;DR: In this paper, the influence of long-range chemical order on the bulk properties of Ni-Fe alloys was investigated by computing the lattice parameters and the single-and polycrystal elastic moduli for different partially ordered structures in the ferro-and paramagnetic states.
Abstract: Ab initio alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to determine the elastic properties of Ni-Fe alloys with Fe:Ni ratio 1:3. The interplay between magnetic and chemical effects is investigated by computing the lattice parameters and the single-and polycrystal elastic moduli for different partially ordered structures in the ferro-and paramagnetic states. It is found that the influence of long-range chemical order on the bulk properties strongly depends on the magnetic state. The largest magnetic-order-induced changes are obtained for the chemically ordered L1(2) phase. The ferromagnetic L1(2) system possesses similar to 5.4% larger elastic Debye temperature than the paramagnetic L1(2) phase, which in turn has a similar Theta(D) as the chemically disordered face-centered cubic phase in either the ferro-or paramagnetic state. It is concluded that magnetic ordering has a substantially larger impact on the bulk parameters of Ni3Fe than chemical ordering. The calculated trends are explained based on the electronic structure of nonmagnetic, ferromagnetic, and paramagnetic ordered and disordered phases.

16 citations


Journal ArticleDOI
TL;DR: In this article, a Kubo-Greenwood-like equation for the Gilbert damping parameter α is presented that is based on the linear response formalism, and its implementation on the basis of the fully relativistic Korringa-Kohn-Rostoker (KKR) band structure method together with the Coherent Potential Approximation (CPA) alloy theory allows application to a wide range of situations.
Abstract: A Kubo-Greenwood-like equation for the Gilbert damping parameter α is presented that is based on the linear response formalism. Its implementation on the basis of the fully relativistic Korringa-Kohn-Rostoker (KKR) band structure method together with the Coherent Potential Approximation (CPA) alloy theory allows application to a wide range of situations. This is demonstrated by results obtained for the alloy system bcc FexCo1-x as well as for a series of alloys of permalloy with 5d transition metals. To account for the thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. In line with experiment, calculations for Ni correctly describe the rapid change of α when small amounts of Cu are introduced substitutionally.

Journal ArticleDOI
TL;DR: In this article, the branch point (BP) energies and resulting valence band offsets for the zincblende phase of InN, GaN, and AlN are calculated from their k-averaged midgap energy.
Abstract: Starting with empirical tight-binding band structures, the branch-point (BP) energies and resulting valence band offsets for the zincblende phase of InN, GaN, and AlN are calculated from their k-averaged midgap energy. Furthermore, the directional dependence of the BPs of GaN and AlN is discussed using the Green's function method of Tersoff. We then show how to obtain the BPs for binary semiconductor alloys within a band-diagonal representation of the coherent potential approximation and apply this method to cubic AlGaN alloys. The resulting band offsets show good agreement to available experimental and theoretical data from the literature. Our results can be used to determine the band alignment in isovalent heterostructures involving pure cubic III-nitrides or AlGaN alloys for arbitrary concentrations.

Journal ArticleDOI
TL;DR: In this paper, the branch point (BP) energies and the valence band offsets (VBOs) for the zincblende phase of InN, GaN, and AlN are calculated from their k-averaged midgap energy.
Abstract: Starting with empirical tight-binding band structures, the branch-point (BP) energies and resulting valence band offsets (VBOs) for the zincblende phase of InN, GaN and AlN are calculated from their k-averaged midgap energy. Furthermore, the directional dependence of the BPs of GaN and AlN is discussed using the Green's function method of Tersoff. We then show how to obtain the BPs for binary semiconductor alloys within a band-diagonal representation of the coherent potential approximation (CPA) and apply this method to cubic AlGaN alloys. The resulting band offsets show good agreement to available experimental and theoretical data from the literature. Our results can be used to determine the band alignment in isovalent heterostructures involving pure cubic III-nitrides or AlGaN alloys for arbitrary concentrations.

Journal ArticleDOI
TL;DR: In this article, the electronic structure and ferromagnetic properties of Zn1−x 1−x mn and Cr 1−mn were investigated by using the Korringa-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA).
Abstract: The electronic structure and ferromagnetic properties of Zn1−x M x O (M = Mn and Cr) have been investigated by using the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA). The half-metal behavior is observed for different doping concentrations. The gap energies are deduced for different dilution x values. The magnetic moment of each atom and the total magnetic moment are computed. A special attention is paid to the discussion of the mechanism of ferromagnetism in these components.

Journal ArticleDOI
TL;DR: In this paper, the impact of chemical disorder can be handled by means of coherent potential approximation (CPA) alloy theory, in the electronic structure calculation as well as in the photoemission analyses.

Journal ArticleDOI
TL;DR: In this paper, two Co2CrAl alloy samples subjected to different heat treatment regimes are studied and the exact distribution of atoms over the sublattices in the samples is determined by X-ray diffraction and neutron diffraction methods.
Abstract: Two Co2CrAl alloy samples subjected to different heat treatment regimes are studied. An exact distribution of atoms over the sublattices in the samples is determined by X-ray diffraction and neutron diffraction methods. These data are used to perform ab initio density of states calculations and to calculate the magnetic moments of the samples in a coherent potential approximation. The calculated magnetic moments are compared to the experimental values. The effect of atomic ordering on the electronic structure near the Fermi level is analyzed using optical methods. The possible causes of the detected temperature dependence of the electrical resistivity, unusual for metallic alloys, are discussed.

Journal ArticleDOI
TL;DR: This work identifies and characterize two different Fermi-liquid regimes and proposes that both regimes are separated by a Lifshitz transition at x=n(c), which could provide a scenario for the non-Fermi liquid properties that were recently observed in Kondo alloy systems.
Abstract: We study the low-energy states of Kondo alloys as a function of the magnetic impurity concentration per site $x$ and the conduction electron average site occupation ${n}_{c}$. Using two complementary approaches, the mean-field coherent potential approximation and the strong-coupling limit, we identify and characterize two different Fermi-liquid regimes. We propose that both regimes are separated by a Lifshitz transition at $x={n}_{c}$. Indeed, we predict a discontinuity of the number of quasiparticles that are enclosed in the Fermi surface. This feature could provide a scenario for the non-Fermi liquid properties that were recently observed in Kondo alloy systems around $x={n}_{c}$.

Journal ArticleDOI
TL;DR: In this article, the generalized coated coherent potential approximation method was used to derive the mathematical formulae for the extended effective medium theory to investigate the optical properties of disordered media composed of core-shell cylinders with the core layer consisting of epsilon-less-than-one material.

Journal ArticleDOI
TL;DR: In this article, first principle atomistic simulations of quantum transport in Si nanochannels under uniaxial tensile strain are presented. But the authors employ a semi-local exchange along with coherent potential approximation to investigate the transport properties of two-terminal Si nanodevices composed of large number of atoms and atomic dopants.
Abstract: We report parameter-free first principle atomistic simulations of quantum transport in Si nanochannels under uniaxial strain. Our model is based on the density functional theory (DFT) analysis within the Keldysh nonequilibrium Green's function (NEGF) formalism. By employing a recently proposed semi-local exchange along with the coherent potential approximation we investigate the transport properties of two-terminal Si nanodevices composed of large number of atoms and atomic dopants. Simulations of the two-terminal device based on the NEGF-DFT are compared quantitatively with the traditional continuum model to establish an important accuracy benchmark. For bulk Si crystals, we calculated the effects of uniaxial strain on band edges and effective masses. For two-terminal Si nanochannels with their channel length of ~ 10 nm, we study the effects of uniaxial strain on the electron transport. With 0.5% uniaxial tensile strain, the conductance along [110] direction is increased by ~ 8% and that along [001] is increased by ~ 2%, which are comparable with the other reported results. This paper qualitatively and quantitatively shows the current capability of first principle atomistic simulations of nanoscale semiconductor devices.

Journal ArticleDOI
TL;DR: In this article, the electronic and magnetic properties of a system based on GaAs material were studied, with different point de-fects in GaAs, which are Gallium interstitials (Gai), Gal- lium antisites (GaAs), Gallium vacancies (VGa), Arsenic in- terstitials(Asi), and AsGaAs antisites(AsGa), and in-doped or codoped system Ga1−xCoxAs and Ga1 −y−zCoyFezAs, respectively.
Abstract: Using ab-initio calculations based on Korringa- Kohn-Rostoker coherent potential approximation (KKR- CPA) method in connection with the local density approx- imation (LDA), we study theoretically the electronic and magnetic properties of a system based on GaAs material. These properties have been studied, with different point de- fects in GaAs, which are Gallium interstitials (Gai), Gal- lium antisites (GaAs), Gallium vacancies (VGa), Arsenic in- terstitials (Asi) and Arsenic antisites (AsGa), Arsenic vacan- cies (VAs), and in-doped or codoped system Ga1−xCoxAs and Ga1−y−zCoyFezAs, respectively. This work presents de- tailed information about total and local density of states at different concentrations of these defects and doped ele- ments; the stability of the ferromagnetic state compared with the spin-glass state has been discussed. The result of elec- tronic properties shows that codoped GaAs material with Fe and Co become ferromagnetic, also doped GaAs with Co or Fe with a defect (hole in Ga) is stable in the ferromagnetic The Curie temperature is estimated from the total energy difference between the Disorder Local Moment (DLM) and the (FM) state by using a mapping on the Heisenberg model in mean field approximation.

Journal ArticleDOI
TL;DR: In this article, the authors consider the analogy between the topological phase transition which occurs on a surface of a non-trivial insulator as a function of spatial coordinate, and the phase transition occurring in the bulk due to the change of internal parameters (such as crystal field and spin-orbit coupling).
Abstract: We consider the analogy between the topological phase transition which occurs on a surface of a non-trivial insulator as a function of spatial coordinate, and the phase transition occurring in the bulk due to the change of internal parameters (such as crystal field and spin–orbit coupling). In both cases the system exhibits a Dirac cone, which is the universal manifestation of a topological phase transition, independent of the type of the driving parameters. In particular, this leads to a simple way of determining the topological class – based solely on the bulk information – even for systems where the translational symmetry is broken by atomic disorder or by strong electron correlations. Here we demonstrate this idea, using as an example the zinc-blende related semiconductors by means of ab-initio fully-relativistic band structure calculations, involving the technique of coherent potential approximation (CPA). (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this article, the influence of the Mg concentration on several important properties of the band structure of ZnMgO alloys in wurtzite structure using ab initio calculations.
Abstract: We analyze the influence of the Mg concentration on several important properties of the band structure of ZnMgO alloys in wurtzite structure using ab initio calculations. For this purpose, the band structure for finite concentrations is defined in terms of the Bloch spectral density, which can be calculated within the coherent potential approximation. We investigate the concentration dependence of the band gap and the crystal-field splitting of the valence bands. The effective electron and hole masses are determined by extending the effective mass model to finite concentrations. We compare our results with experimental results and other calculations.

Journal ArticleDOI
TL;DR: Experimental band topography is compared to the calculations using the methods of Korringa-Kohn-Rostoker with the coherent potential approximation (CPA) and the linearized augmented plane wave with local orbitals (LAPW+LO) method, which indicate that at least one of the hole pockets has a two-dimensional character.
Abstract: The electronic structure of superconducting Fe1:03Te0:94S0:06 has been studied by angle-resolved photoemission spectroscopy (ARPES). Experimental band topography is compared to the calculations using the methods of Korringa‐Kohn‐Rostoker (KKR) with the coherent potential approximation (CPA) and the linearized augmented plane wave with local orbitals (LAPWC LO) method. The region of the0 point exhibits two hole pockets and a quasiparticle peak close to the chemical potential ( ) with undetectable dispersion. This flat band with mainly d z 2 orbital character is most likely formed by the top of the outer hole pocket or is evidence of a third hole band. It may cover up to 3% of the Brillouin zone volume and should give rise to a Van Hove singularity. Studies performed for various photon energies indicate that at least one of the hole pockets has a two-dimensional character. The apparently nondispersing peak at is clearly visible for 40 eV and higher photon energies, due to an effect of the photoionization cross-section rather than band dimensionality. Orbital characters calculated by LAPWC LO for stoichiometric FeTe do not reveal the flat dz2 band but are in agreement with the experiment for the other dispersions around0 in Fe1:03Te0:94S0:06. (Some figures may appear in colour only in the online journal)

Journal ArticleDOI
TL;DR: This work implements the vertex corrections in a Korringa-Kohn-Rostoker multiple scattering scheme and investigates a system of an iron-cobalt alloy layer embedded in copper, finding that vertex corrections play an important role for this system.
Abstract: The theoretical description of modern nanoelectronic devices requires a quantum mechanical treatment and often involves disorder, e.g. from alloys. Therefore, the ab initio theory of transport using non-equilibrium Green’s functions is extended to the case of disorder described by the coherent potential approximation. This requires the calculation of non-equilibrium vertex corrections. We implement the vertex corrections in a Korringa–Kohn–Rostoker multiple scattering scheme. In order to verify our implementation and to demonstrate the accuracy and applicability we investigate a system of an iron-cobalt alloy layer embedded in copper. The results obtained with the coherent potential approximation are compared to supercell calculations. It turns out that vertex corrections play an important role for this system.

Journal ArticleDOI
TL;DR: In this article, it has been established that the solution energy of all these impurities decreases upon the transition into the paramagnetic state, which is most clearly pronounced at temperatures close to TC.
Abstract: The method based on the densityfunctional theo ry has been used to study the solubility of 3p (Al, Si) and 4p (Ga, Ge) elements in ferromagnetic and paramagnetic states of bcc iron. To simulate the paramagnetic state, two different approaches have been employed, which were implemented using the SIESTA and LSGF packages. It has been established that the solution energy of all these impurities decreases upon the transition into the paramagnetic state. The solution energies obtained by averaging over the ensemble of unpolarized magnetic configurations agree well with the values obtained in the coherent� potential approximation. At the same time, the allowance for the magnetic polarization in the vicinity of an impurity leads to a decrease in the solution energy, which is most clearly pronounced at temperatures close to TC. The temperature dependence of the solution energies of the impurities in the paramagnetic state is discussed.

Journal ArticleDOI
01 Jun 2013-EPL
TL;DR: In this article, first-principles calculations of the magnetocrystalline anisotropy energy (MAE) of the L10-like FexPt1?x samples studied experimentally by Barmak and co-workers were performed.
Abstract: We perform first-principles calculations of the magnetocrystalline anisotropy energy (MAE) of the L10-like FexPt1?x samples studied experimentally by Barmak and co-workers (see J. Appl. Phys., 98 (2005) 033904). The variation of composition and long-range chemical order in the samples was studied in terms of the coherent potential approximation. In accordance with experimental observations, we find that, in the presence of long-range chemical disorder, Fe-rich samples exhibit a larger MAE than stoichiometric FePt. By considering the site-and species-resolved contributions to the MAE, we infer that the MAE is primarily a function of the degree of completeness of the nominal Fe layers in the L10 FePt structure.

Journal ArticleDOI
TL;DR: In this paper, the effects of substituting transition metals (Co, Rh, Ni and Pd) on the Ru site with respect to the electrical conductivity and Seebeck coefficient for the binary semiconducting intermetallic compound RuGa2 have been investigated above room temperature.
Abstract: The effects of substituting transition metals (TM: Co, Rh, Ni and Pd) on the Ru site with respect to the electrical conductivity and Seebeck coefficient for the binary semiconducting intermetallic compound RuGa2 have been investigated above room temperature. Only Rh substituted RuGa2 exhibited a higher electrical conductivity compared with undoped RuGa2. The sign of the Seebeck coefficient at 373K for all doped samples is negative and their magnitudes exhibit rather large values of 150 < «S373K« < 350μVK11, indicating that Co, Rh, Ni and Pd work as n-type dopants. However, the sign changes from negative to positive at high temperature. This implies that the effective carrier doping is insufficient to obtain a high efficiency n-type material. To investigate the effect of TM substitution on the electronic density of states, the Korringa-Kohn-Rostoker Green’s function method under a coherent potential approximation has been employed. The calculation indicates that the dopant d-states at the conduction band affect the transport properties. [doi:10.2320/matertrans.E-M2013804]

Journal ArticleDOI
TL;DR: Differences in the attenuation rates predicted by the two approaches, which cannot be attributed to numerical errors, are found for problems involving multiple wave directions and large disorder.
Abstract: A method is proposed for determining the modal spectra of waves supported by arrays, which are composed of multiple rows of scatterers randomly disordered around an underlying periodic configuration. The method is applied to the canonical problem of arrays of identical small acoustically soft circular cylinders and disorder in the location of the rows. Two different approaches are adopted to calculate the modes: (i) forming an ensemble average of the modes from individual realizations (loosely: extract information, then average); and (ii) extracting the modes from the ensemble average wave field (loosely: average, then extract information). Differences in the attenuation rates predicted by the two approaches, which cannot be attributed to numerical errors, are found for problems involving multiple wave directions and large disorder. A form of the coherent potential approximation (CPA) is also devised. Comparisons of the CPA to the results given by random sampling show that it gives high accuracy.

Journal ArticleDOI
TL;DR: In this article, the gap energy is calculated using the Korringa-Kohn-Rostoker method combined with a coherent potential approximation and using optical measurements for CoFe2O4 and MnFe 2O4.
Abstract: CoFe2O4 and MnFe2O4 ferrite nanoparticles were prepared by the co-precipitation method. The structural evolutions of the nanophase have been studied. The refinement result showed that the type of cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is a partially inverse spinel. The morphology of the samples has been determined using transmission electron microscopy and the magnetic properties of the samples are given. The gap energy is calculated using the Korringa–Kohn–Rostoker method combined with a coherent potential approximation and using optical measurements for CoFe2O4 and MnFe2O4. This study has been extended theoretically to the mixed spinel ferrites, FeCoMnxFe1−xO4. Indeed, the saturation magnetization, the critical temperature, exchange interactions and Curie constant for the cation distribution , have been calculated. The high temperature series expansion combined with the Pade approximant are used to determine the critical temperature and critical exponent (γ) associated with the magnetic susceptibility of mixed spinels.

Journal ArticleDOI
ChuanZhen Zhao1, Rong Zhang1, Bin Liu1, Ming Li1, Xiangqian Xiu1, Zili Xie1, Youdou Zheng1 
TL;DR: In this article, a model for the band-gap energy of InxGayAl1−x−yN is developed based on modification of the simplified coherent potential approximation, and the parameters of the model are obtained by fitting the experimental bandgap energies of their ternary alloys.
Abstract: Based on modification of the simplified coherent potential approximation, a model for the band-gap energy of InxGayAl1−x−yN is developed. The parameters of the model are obtained by fitting the experimental band-gap energy of their ternary alloys. It is found that the results agree with the experimental values better than those reported by others, and that the band-gap reduction of InxGayAl1−x−yN with increasing In or Ga content is mainly due to enhanced intraband coupling within the conduction band, and separately within the valence band.