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Showing papers on "Coherent potential approximation published in 2004"


Journal ArticleDOI
TL;DR: In this article, a new class of diluted magnetic semiconductors based on CaO without transition metal elements was proposed and the electronic structure and the magnetic properties of B-, C- or N-doped CaO were calculated by using the Korringa-Kohn-Rostoker method within the local spin density approximation.
Abstract: We propose a new class of diluted magnetic semiconductors based on CaO without transition metal elements. The electronic structure and the magnetic properties of B-, C- or N-doped CaO are calculated by using the Korringa-Kohn-Rostoker method within the local spin density approximation. The substitutional and magnetic disorder is taken into account by the coherent potential approximation. It is found that B, C and N impurities show finite local magnetic moments in CaO at the oxygen-substitutional site. Moreover, these C- and N-doped CaO show the room-temperature ferromagnetism with half-metallic density of states.

130 citations


Journal ArticleDOI
TL;DR: In this article, the Curie temperature of diluted magnetic semiconductors is estimated in the mean field approximation (MFA) using the Korringa-Kohn-Rostoker method.
Abstract: On the basis of ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb, we discuss the origin of ferromagnetism in diluted magnetic semiconductors. The calculations use the Korringa–Kohn–Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. By mapping the total energy results onto a Heisenberg model, the Curie temperature TC is estimated in the mean field approximation (MFA). If impurity bands are formed in the gap, as is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic dependence of TCMFA 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 TCMFA. (Ga, Mn)As is an intermediate case, showing a -like behaviour in the local density approximation (LDA), but a nearly linear c dependence, if the more accurate LDA +U method with U = 4 eV is used.

129 citations


Journal ArticleDOI
TL;DR: In this article, the generalized perturbation method with screened Coulomb interactions is used to obtain effective interactions that yield an accurate and physically transparent description of configurational energetics within the atomic sphere and coherent potential approximations.
Abstract: It is shown that, using the generalized perturbation method (GPM) with screened Coulomb interactions that ensures its consistency with the force theorem, one is able to obtain effective interactions that yield an accurate and physically transparent description of configurational energetics in the framework of the Korringa-Kohn-Rostoker method within the atomic sphere and coherent potential approximations. This is demonstrated with calculations of ordering energies, short-range order parameters, and transition temperatures in the CuZn, CuAu, CuPd, and PtCo systems. Furthermore, we show that the GPM can be used to obtain Heisenberg exchange interaction parameters, which, for instance, capture very well the magnetic configurational energy in bcc Fe.

113 citations


Journal ArticleDOI
TL;DR: In this paper, the electronic structure and magnetic properties of (Zn,Cr)S, Zn, CrSe and Zn CrTe were calculated by using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation (KKR-CPA) within local density approximation.
Abstract: The electronic structure and the magnetic properties of (Zn,Cr)S, (Zn,Cr)Se and (Zn,Cr)Te are calculated by using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation (KKR-CPA) within the local density approximation. Curie temperatures are calculated by the mean field approximation (MFA) and by Monte Carlo simulation. It is found that the MFA strongly overestimates the Curie temperatures for low concentrations due to shortrangeness of interactions in (Zn,Cr)S, (Zn,Cr)Se and (Zn,Cr)Te. The Curie temperatures of (Zn,Cr)Te calculated by Monte Carlo simulation agree very well with recent experimental values.

69 citations


Book
29 Nov 2004
TL;DR: In this article, the screened KKR method for two-dimensional translationally invariant systems and the generalized Madelung problem for three-dimensional systems are presented for spherically symmetric potentials.
Abstract: Preliminary definitions.- Multiple scattering.- Shape functions.- Non-relativistic single-site scattering for spherically symmetric potentials.- Non-relativistic full potential single-site scattering.- Spin-polarized non-relativistic single-site scattering.- Relativistic single-site scattering for spherically symmetric potentials.- Relativistic full potential single-site scattering.- Spin-polarized relativistic single-site scattering for spherically symmetric potentials.- Spin-polarized relativistic full potential single-site scattering.- Scalar-relativistic single-site scattering for spherically symmetric potentials.- Scalar-relativistic full potential single-site scattering.- Phase shifts and resonance energies.- Structure constants.- Green's functions: an in-between summary.- The Screened KKR method for two-dimensional translationally invariant systems.- Charge and magnetization densities.- The Poisson equation and the generalized Madelung problem for two- and three-dimensional translationally invariant systems.- "Near field" corrections.- Practical aspects of full-potential calculations.- Total energies.- The Coherent Potential Approximation.- The embedded cluster method.- Magnetic configurations - rotations of frame.- Related physical properties.

62 citations


Journal ArticleDOI
TL;DR: In this article, the influence of a random field induced by impurities, boundary irregularities etc. on the superfluidity of a quasi-two-dimensional (2D) system of spatially indirect excitons in coupled quantum wells is studied.
Abstract: The influence of a random field induced by impurities, boundary irregularities etc. on the superfluidity of a quasi-two-dimensional (2D) system of spatially indirect excitons in coupled quantum wells is studied. The interaction between excitons is taken into account in the ladder approximation. The random field is allowed to be large compared to the dipole-dipole repulsion between excitons. The coherent potential approximation (CPA) allows us to derive the exciton Green’s function for a wide range of the random field, and the CPA results are used in the weak-scattering limit, which results in the second-order Born approximation. The Green’s function of the collective excitations for the cases of (1) equal electron and hole masses and (2) the “heavy hole” limit are derived analytically. For quasi-two-dimensional excitonic systems, the density of the superfluid component and the Kosterlitz-Thouless temperature of the superfluid phase transition are obtained, and are found to decrease as the random field increases. This puts constraints on the experimental efforts to observe excitonic superfluidity.

53 citations


Journal ArticleDOI
TL;DR: The inclusion of disorder results in a better agreement between calculated and measured spectra for the chlorosomes of green bacteria as compared to the previous report, and it is shown that the circular dichroism spectrum may still show a strong dependence on the cylinder length, even when the exciton wave function is localized in a region smallCompared to the cylinder's size.
Abstract: We study the linear optical properties of helical cylindrical molecular aggregates accounting for the effects of static diagonal disorder. Absorption, linear dichroism, and circular dichroism spectra are presented, calculated using brute force numerical simulations and a modified version of the coherent potential approximation that accounts for finite size effects by using the appropriate open boundary conditions. Excellent agreement between both approaches is found. It is also shown that the inclusion of disorder results in a better agreement between calculated and measured spectra for the chlorosomes of green bacteria as compared to our previous report, where we restricted ourselves to homogeneous cylinders [Didraga, Klugkist, and Knoester, J. Phys. Chem. B 106, 11474 (2002)]. For the excitons that govern the optical response, we also investigate the disorder-induced localization properties. By analyzing an autocorrelation function of the exciton wave function, we find a strongly anisotropic localization behavior, closely following the properties of chiral wave functions which previously have been found for homogenoeus helical cylinders [Didraga and Knoester, J. Chem. Phys. 121, 946 (2004)]. It is shown that the circular dichroism spectrum may still show a strong dependence on the cylinder length, even when the exciton wave function is localized in a region small compared to the cylinder’s size.

53 citations


Journal ArticleDOI
TL;DR: In this article, a theory of the single-particle excitation spectrum is presented on the basis of the projection operator method combined with the many-body coherent-potential approximation (CPA).
Abstract: A theory of the single-particle excitation spectrum is presented on the basis of the projection operator method combined with the many-body coherent-potential approximation (CPA). The theory describes the dynamics of the excitations by means of an energy-dependent Liouville operator accompanied by a coherent potential which is determined by the self-consistent CPA condition. It is shown that the present theory is essentially equivalent to the dynamical CPA and the dynamical mean-field theory. The Hubbard III approximation and the modified perturbation theory are rederived from the theory. A renormalized perturbation scheme for the Green function is developed on the basis of a general formula for the memory function. It interpolates between the weak- and strong-Coulomb interaction limits, and yields the metal-insulator transition for half-filled bands. Numerical calculations have been performed for the Gutzwiller-Hubbard model on a hypercubic lattice in infinite dimensions. The results show that the theory describes quantitatively the quasiparticle weight vs Coulomb interaction curve, yielding a reasonable critical Coulomb interaction for the metal-insulator transition. It produces the overall features of the excitation spectra and the momentum distributions for various Coulomb interaction strengths.

45 citations


Journal ArticleDOI
TL;DR: In this paper, the dynamical coherent potential approximation (CPA) is introduced, which completely takes account of dynamical spin and charge fluctuations within the single-site approximation, and the effects of orbital degeneracy and the realistic calculation scheme are discussed with an emphasis on Hund's rule coupling.
Abstract: Recent progress in the theory of magnetism and electron correlations is reviewed to clarify the theories developed in the last decade and their mutual relations. A historical development of the theory of magnetism is outlined, and the dynamical coherent potential approximation (CPA) which completely takes account of the dynamical spin and charge fluctuations within the single-site approximation is introduced. Both the dynamical effects on various magnetic properties and the many-body band structure are shown to be explained on the same footing. It is shown that the dynamical CPA is equivalent to the other single-site theories of strongly correlated electrons: the many-body CPA, the dynamical mean-field theory (DMFT), and the projection operator method CPA (PM-CPA). These theories are elucidated with use of a common concept of effective medium or coherent potential. The effects of orbital degeneracy and the realistic calculation scheme are discussed with an emphasis on Hund’s rule coupling. Non-local theor...

43 citations


Journal ArticleDOI
TL;DR: In this paper, the electronic structure and residual resistivity of diluted (Ga, Mn)As magnetic semiconductors are calculated from first principles using the linear muffin-tin orbital method, the coherent potential approximation, and the Kubo-Greenwood linear response theory.
Abstract: The electronic structure and residual resistivity of diluted (Ga, Mn)As magnetic semiconductors are calculated from first principles using the linear muffin-tin orbital method, the coherent potential approximation, and the Kubo-Greenwood linear response theory. Particular attention is paid to the role of native compensating defects such as As antisites and Mn interstitials as well as to different magnetic configurations of the local Mn moments. The order of magnitude of the calculated resistivities compares reasonably well with available experimental data. The concentration variations of the resistivity reflect two basic mechanisms, namely the strength of the impurity scattering and the number of carriers. In agreement with a recent experiment, the calculated resistivities are strongly correlated with the alloy Curie temperatures evaluated in terms of a classical Heisenberg Hamiltonian.

20 citations


Journal ArticleDOI
TL;DR: In this paper, the authors apply the coherent potential approximation to study the eigenstates of a tight-binding Hamiltonian with uncorrelated diagonal disorder and long-range hopping, falling off as a power mu of the intersite distance.
Abstract: The density of states, even for a perfectly ordered tight-binding model, can exhibit a tail-like feature at the top of the band, provided the hopping integral falls off in space slowly enough. We apply the coherent potential approximation to study the eigenstates of a tight-binding Hamiltonian with uncorrelated diagonal disorder and long-range hopping, falling off as a power mu of the intersite distance. For a certain interval of hopping-range exponent mu, we show that the phase-coherence length is infinite for the outermost state of the tail, irrespectively of the strength of disorder. Such an anomalous feature can be explained by the smallness of the phase-space volume for the disorder scattering from this state. As an application of the theory, we mention that ballistic regime can be realized for Frenkel excitons in two-dimensional molecular aggregates, affecting to a large extent the optical response and energy transport.

Journal ArticleDOI
TL;DR: In this paper, a first-principles approach to magnetic properties of diluted magnetic semiconductors (DMS) is presented that is based on the local spin density approximation (LSDA) as implemented in the framework of the tight-binding linear muffin-tin orbital method, while the effect of randomness is described by the coherent potential approximation.
Abstract: A first-principles approach to magnetic properties of diluted magnetic semiconductors (DMS) is presented that is based on the local spin-density approximation (LSDA) as implemented in the framework of the tight-binding linear muffin-tin orbital method, while the effect of randomness is described by the coherent potential approximation. Application of a real-space Green-function formalism yields the exchange pair interactions between distant magnetic atoms that are needed for quantitative studies of magnetic excitations including the Curie temperatures. We have found that the pair exchange interactions exhibit a strong directional dependence and are exponentially damped with increasing distance between magnetic atoms due to disorder and the half-metallic character of the DMS. As a case study we consider (Ga, Mn) As, (Ga, Mn)N, and (Zn, Cr)Te alloys. The calculations demonstrate that inclusion of disorder and, in particular, realistic distances among magnetic impurities, are needed to obtain critical temperatures which are in good agreement with available experiments.

Journal ArticleDOI
TL;DR: In this paper, the possibility of ferromagnetic ordering is revisited in the band model by using the coherent potential approximation decoupling for the strong on-site Coulomb interaction.
Abstract: The possibility of ferromagnetic ordering is revisited in the band model. The coherent potential approximation decoupling has been used for the strong on-site Coulomb interaction. The driving forces towards the ferromagnetism are the on-site and inter-site molecular fields coming from different Coulomb interactions. Another driving force is the lowering of the kinetic energy with growing magnetic moment coming from the dependence of the hopping integrals on occupation of the neighboring sites involved in hopping. This effect is described by the hopping interaction, Δ t , and by what we call the exchange–hopping interaction, t ex . The exchange–hopping interaction, which is the difference in hopping integrals for different occupation of neighboring lattice sites, acts in a way analogous to the Hund's magnetic exchange interaction. The results are calculated for semi-elliptic density of states (DOS) and for the distorted semi-elliptic DOS with the maximum around the Fermi energy. They show a natural tendency towards the magnetic ordering at the end of the 3D row for the DOS with maximum density around the Fermi energy, when the hopping integrals grow with the occupation of the neighboring lattice sites.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the coherent potential approximation (CPA) can be used together with the potential well analogy (PWA) at low impurity concentration, despite the fact that the CPA does not describe correctly the immediate vicinity of the band edge.
Abstract: The proof is given that the potential-well analogy (PWA) can be used together with the coherent-potential approximation (CPA) at the low impurity concentration despite the fact that the CPA does not describe correctly the immediate vicinity of the band edge. The correspondent criterion of the CPA applicability for three-dimensional systems is presented. It is demonstrated that the CPA is reliable above the mobility edge position predicted by the PWA.

Journal ArticleDOI
TL;DR: In this paper, the electronic structure and magnetic properties of Fe/GaAs/Fe tunnel junction were studied using a first-principles Green's function technique, based on the tight-binding linear muffin-tin orbital method in its atomic spheres approximation, in conjunction with the coherent potential approximation to describe the disorder effects such as the interdiffusion at the interface.

Journal ArticleDOI
TL;DR: In this article, the effects of atomic relaxation on the inelastic incoherent neutron scattering cross sections of disordered Ni$50$Pt$50}$ alloys were studied.
Abstract: Using a combination of density-functional perturbation theory and the itinerant coherent potential approximation, we study the effects of atomic relaxation on the inelastic incoherent neutron scattering cross sections of disordered Ni$_{50}$Pt$_{50}$ alloys. We build on previous work, where empirical force constants were adjusted {\it ad hoc} to agree with experiment. After first relaxing all structural parameters within the local-density approximation for ordered NiPt compounds, density-functional perturbation theory is then used to compute phonon spectra, densities of states, and the force constants. The resulting nearest-neighbor force constants are first compared to those of other ordered structures of different stoichiometry, and then used to generate the inelastic scattering cross sections within the itinerant coherent potential approximation. We find that structural relaxation substantially affects the computed force constants and resulting inelastic cross sections, and that the effect is much more pronounced in random alloys than in ordered alloys.

Journal ArticleDOI
TL;DR: In this paper, the magnetocaloric effect in the Laves phase pseudobinary Ho(Co1−cRhc)2 was studied using a theoretical model, considering both the localized spins of rare earth ions and the itinerant electrons of the transition elements.

Journal ArticleDOI
TL;DR: In this paper, the effects of various substitutions on the electronic structure of Co in Y(Co 1− x M x ) 2 compounds (M = Al, Si, Fe, Cu) were studied employing the coherent potential approximation as embodied in an allelectron TB-LMTO method.

Journal ArticleDOI
TL;DR: In this paper, the authors present self-consistent calculations of the electronic density of disordered copper-palladium and silver palladium alloys using the polymorphous coherent potential approximation and the Korringa-Kohn-Rostoker coherent-potential approximation.
Abstract: We present self-consistent calculations of the electronic density of states of disordered copper-palladium and silver-palladium alloys using the polymorphous coherent-potential approximation and the Korringa-Kohn-Rostoker coherent-potential approximation. We find that the agreement between the theoretical partial density of states of palladium $d$ bands in copper-rich copper-palladium alloys and experiment is significantly improved when the polymorphous coherent-potential approximation is used. The densities of states of silver-palladium alloys calculated with the two versions of the coherent-potential approximation are identical and agree with experiment. This indicates that the improved treatment of Coulomb effects in the polymorphous coherent-potential approximation is necessary only for alloys such as copper palladium that have considerable charge transfer.

Journal ArticleDOI
TL;DR: In this article, the effects of non-local disorder correlations (alloy short-range order) on the electronic structure of random alloy systems were investigated by mapping the original Anderson disorder problem to that of a selfconsistently embedded cluster.
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 selfconsistently embedded cluster. This cluster problem is then solved using the equations of motion technique. The CPA is recovered for cluster size $N_{c}=1$, and the disorder averaged density-of-states (DOS) is always positive definite. Various new features, compared to those observed in CPA, and related to repeated scattering on pairs of sites, reflecting the effect of SRO are clearly visible in the DOS. It is explicitly shown that the cluster-CPA method always yields positive-definite DOS. 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.

Journal ArticleDOI
TL;DR: In this paper, the magnetization of GaxMn1−xAs was calculated as a function of the temperature for various values of carrier density, supporting the view that the ferromagnetism is induced by the carriers in the bandtail through double exchange-like mechanism.

Journal ArticleDOI
TL;DR: In this paper, a random version of the negative U Hubbard model was used to describe superconductivity and the disorder associated with randomly distributed attractive centers was treated by means of the coherent potential approximation.

Journal ArticleDOI
TL;DR: In this paper, the effect of electron correlations on the matrix elements of the self-energy is investigated with the help of a Gutzwiller-type variational wave function, and it is shown that for strong electron correlations the higher-order dynamical corrections in an renormalized perturbation expansion are screened except in the low-energy regime.
Abstract: A recently proposed combination of a projection operator method with the coherent potential approximation for the computation of the excitation spectra of solids is further extended. In particular, the effect of electron correlations on the matrix elements of the self-energy are investigated. This is done with the help of a Gutzwiller-type variational wave function. Numerical calculations have been performed for a half-filled band of a hypercubic lattice in infinite dimensions. They show that for strong electron correlations the higher-order dynamical corrections in an renormalized perturbation expansion are screened except in the low-energy regime. This provides for a justification of earlier work, where a Hartree-Fock approximation with a cutoff parameter was used for the computation of the static matrix elements.

Journal ArticleDOI
TL;DR: In this article, structural and electron transport properties of Se-and Te-doped CoSb 3 skutterudites in relation to results of electronic structure calculations performed for this system were presented.
Abstract: This work presents structural and electron transport properties of Se- and Te-doped CoSb 3 skutterudites in relation to results of electronic structure calculations performed for this system. The physical quantities were determined from crystal structure studies and measurements of transport coefficients: electrical conductivity, thermal conductivity and Seebeck coefficient as a function of composition and temperature. The electronic structure of CoSb 3 was calculated using the Korringa–Kohn–Rostoker method with the LDA framework. Dispersion curves E( k ) in CoSb 3 computed for experimental crystallographic parameters exhibit a very narrow indirect energy gap at the Fermi level, which however strongly varies along different k-directions. The KKR valence and conduction band curvatures reflect effective masses estimated from experimental data well. The effect of doping CoSb 3 with Se and with Te as well as the dopants’ influence on density of electronic states was investigated by the KKR method combined with the coherent potential approximation (CPA). This method was also used to show an impact of various crystal defects on electronic structure in pure Se- and Te-doped CoSb 3 .

Journal ArticleDOI
TL;DR: The theory of disorder in superconducting alloys was discussed in this article, where the authors use standard Wannier function representation and allow for diagonal and ofi-diagonal disorder.
Abstract: The general theory of disorder in superconducting alloys was discussed We use standard Wannier function representation and allow for diagonal and ofi-diagonal disorder We generalized the coherent potential approximation in the version able to deal with ofi-diagonal disorder in normal systems to treat superconducting state As an illustration we calculate the quasiparticle density of states and gap function of a d-wave superconductor We show inter alia that the rate at which superconductivity disappears depends on the kind of disorder and for ofi-diagonal disorder it depends on the details of its realization

Book ChapterDOI
01 Jan 2004
TL;DR: The coherent potential approximation (CPA) is a useful method for describing the electron correlations as well as the effects of disorder on electrons as discussed by the authors, and it can be used to describe the basic properties of magnetism from metals to insulators, the metalinsulator transition, and the single particle excitations from the Fermi liquid to the insulator.
Abstract: The coherent potential approximation (CPA) is a useful method for describing the electron correlations as well as the effects of disorder on electrons. Among the many-body theories using the CPA, the dynamical CPA, the many-body CPA, and the dynamical mean-field theory are reviewed to clarify how these theories use the CPA concept for the description of the electron correlations. The theories characterized by the momentum independent self-energy are shown to interpolate between the weak and strong Coulomb interaction limits, and therefore describe the basic properties of magnetism from metals to insulators, the metalinsulator transition, and the single particle excitations from the Fermi liquid to the insulator. The relation among various theories are clarified. In particular, it is shown that the dynamical CPA, the many-body CPA, and the dynamical mean-field theory are equivalent to each other, so that the theories of itinerant magnetism and those of the strongly correlated electron systems are unified within the single-site approximation. The nonlocal effects on the selfenergy are also discussed beyond the single-site approximation.

Journal ArticleDOI
TL;DR: In this article, the theory of vibrational excitations of random alloys is presented, which takes into account the scattering of excitations on n-impurity complexes (clusters) and is capable to describe complex structure of their spectra.
Abstract: The theory of vibrational excitations of random alloys is presented, which takes into account the scattering of excitations on n-impurity complexes (clusters) and is capable to describe complex structure of their spectra. The essential feature of the theory consists in two-stage disorder averaging procedure. First, theory equations are averaged and Fourier transformed over the coordinate characterizing the cluster position as a whole. Then they are averaged over inter-impurity distances in the cluster. The derivation of the Green function equations is carried out both in the version similar to average-t-matrix approximation (ATA) and in that similar to coherent potential approximation (CPA). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this paper, an extension of the cluster-Bethe lattice method, in which we incorporate electronic correlations, itinerant and localized nature of electrons 3d, and both long-range and short-range chemical correlations, is used to determine the changes in the electronic structure and magnetic properties of body centered cubic (BCC) iron induced by doping with nickel and cobalt impurities.

Journal ArticleDOI
TL;DR: In this paper, the pressure dependence of the residual resistivity of the doped electron-type and hole-type Kondo insulators (KIs) is calculated within the framework of the slave-boson mean-field theory and the coherent potential approximation.
Abstract: The pressure dependence of the residual resistivity of the doped electron-type and hole-type Kondo insulators (KIs) are calculated within the framework of the slave-boson mean-field theory and the coherent potential approximation. It is shown that as the pressure increases, the resistivity increases and decreases for the dilute doping electron-type and hole-type KIs, respectively. These results are qualitatively in agreement with the experiments.

Journal ArticleDOI
TL;DR: In this article, the effect of local correlation in the phase diagram of binary ferromagnetic alloy was investigated by using the coherent potential approximation and the dynamical mean field approximation, and the Curie temperature varies smoothly as the function of the concentration.