Showing papers on "Coherent potential approximation published in 1998"

Harmonic Vibrational Excitations in Disordered Solids and the “Boson Peak”

Abstract: We consider a system of coupled classical harmonic oscillators with spatially fluctuating nearest-neighbor force constants on a simple cubic lattice. The model is solved both by numerical diagonalization and by applying the single-bond coherent potential approximation. The results for the density of states $g(\ensuremath{\omega})$ are in excellent agreement with each other. If the system is near the borderline of stability a low-frequency peak appears in the quantity $g(\ensuremath{\omega})/{\ensuremath{\omega}}^{2}$ as a precursor of the instability. We argue that this peak is the analogon of the ``boson peak,'' observed in structural glasses and other disordered solids. By means of the level distance statistics we show that the peak is not associated with localized states.

Electronic Structure of Disordered Alloys

Abstract: The last decade has seen a rapid progress in understanding the electronic structure of disordered alloys and in this article I shall review the progress made by using the Kohn-Korringa-Rostoker (KKR) based approaches. The KKR approach in conjuction with the coherent-potential approximation (CPA) has emerged as a powerful tool to study the electronic structure of disordered alloys. This method called, KKR- CPA, combined with the local density approximation (LDA) provides a first-principles, parameter-free mean-field theory for treating electronic structure of a disordered alloy. Insofar as the average properties are concerned, the KKR-CPA replaces the disordered system by a suitably chosen translationally invariant effective medium, which is determined self-consistently. To illustrate some of its applications I shall present some recent results for Li-Mg and Nb-Mo alloys. Also our recent attempts to go beyond the KKR-CPA and to include short-range order will be briefly discussed.

Applicability of the coherent-potential approximation in the theory of random alloys

Abstract: The applicability of the coherent-potential approximation (CPA) for the description of electronic properties of completely random alloys is investigated. This is done by calculating the density of states and the total energy for different systems and by c

Effects of isotope disorder on energies and lifetimes of phonons in germanium

Abstract: based on the coherent potential approximation and on large supercells, respectively. For the mass variance of our sample, the two methods lead to rather similar results that agree with the experimental data. This agreement is marginally better for the supercell calculations. Calculations performed for larger mass variances g revealed, however, that the two approaches yield significantly different results when g is larger than 3 310 23 . @S0163-1829~98!02938-5#

Semiquantitative analysis of magnetic phase transitions in the MnFeP1−xAsx series of compounds

Abstract: On the basis of the relation T(σ,H) derived within the framework of the Bean–Rodbell model a semiquantitative analysis of (B,T) magnetic phase diagrams in the hexagonal region of the MnFeP1−xAsx (0.15⩽x⩽0.66) series of compounds was carried out. It was found that the first order transition line in the (B,T) plane is terminated by the isolated critical point for x=0.33 and x=0.5 contents. The positive value of dBC/dT and the saturation of the magnetization was found to be in agreement with the experiment. Thus the Bean–Rodbell model and the molecular field approximation can be applied to describe the discontinuous Ferro–Para magnetoelastic phase transition. Semiquantitative agreement between the theoretical and the experimental (B,T) magnetic phase diagrams was obtained. For the MnFeP0.67As0.33 compound the electronic structure calculations were performed by the Korringa–Kohn–Rostoker with the coherent potential approximation method. The density of states (DOS) curve calculated in the nonmagnetic state sho...

Tight-binding coherent potential approximation study of ferromagnetic La 2/3 Ba 1/3 MnO 3

Abstract: The local-spin-density band structure of a virtual-crystal model of ferromagnetic La 2/3Ba1/3MnO3, reported earlier by Pickett and Singh@Phys. Rev. B53, 1146 ~1996!#, has been fit to an accurate orthogonal tightbinding ~TB! model. This TB Hamiltonian has spin-independent hopping parameters, and the exchange splitting is confined solely to the Mn on-site parameters « t2g↓2« t2g↑53.44 eV and«eg↓2«eg↑52.86 eV. The crystal-field splitting, as reflected in the Mn d energies, is 1.77 eV for majority spin and 1.19 eV for minority spin. Oxygenps and pp states are allowed distinct site energies, and the derived crystal-field splitting is «pp2«ps 5 1.47 eV. Local Mnd site energy disorder, arising from the random distribution of La 31 nd Ba ions, has been treated within the TB coherent potential approximation. The intrinsic resistivity in the fully polarized phase from this charge disorder is estimated to be 15 mV cm. @S0163-1829 ~98!05020-6#

Electronic topological transitions in Mo-Re random alloys

Abstract: Electronic spectra and hulk properties of Mo-rich bcc Mo-Re random alloys have been studied in the framework of density-functional theory. We show that the Fermi surface topology changes with rheni ...

Van Hove singularity and d -wave pairing in disordered superconductors

Abstract: We apply the coherent potential approximation (CPA) to a simple model for disordered superconductors with d -wave pairing and demonstrate that whilst the effectiveness of an electronic Van Hove singularity to enhance the transition temperature T c is reduced by disorder it is not eliminated. In fact we give a qualitative account of changes in the T c vs. doping curve with increasing disorder and compare our results with experiments on the Y 0.8 Ca 0.2 Ba 2 (Cu 1− y Zn y ) 3 O 7−δ alloys.

Disorder-induced broadening of the density of states for two-dimensional electrons with strong spin-orbit coupling

Abstract: We study theoretically the disorder-induced smearing of the density of states in a two-dimensional electron system taking into account a spin-orbit term in the Hamiltonian of a free electron. We show that the characteristic energy scale for the smearing increases with increasing the spin-orbit coupling. We also demonstrate that in the limit of a strong spin-orbit coupling the diagrams with self-intersections give a parametrically small contribution to the self-energy. As a result, the coherent potential approximation becomes asymptotically exact in this limit. The tail of the density of states has the energy scale which is much smaller than the magnitude of the smearing. We find the shape of the tail using the instanton approach.

Charge-correlation effects in calculations of atomic short-range order in metallic alloys

Abstract: The ``local'' chemical environment that surrounds an atom directly influences its electronic charge density. These atomic charge correlations play an important role in describing the Coulomb and total energies for random substitutional alloys. Although the electronic structure may be well represented by a single-site theory, such as the coherent potential approximation, the electrostatic energy is not as well represented when these charge correlations are ignored. For metals, including the average effect from the charge correlation coming from only the nearest-neighbor shell has been shown to be sufficient to determine accurately the energy of formation. In this paper, we incorporate such charge correlations into the concentration-wave approach for calculating the atomic short-range order in random (substitutional) alloys. We present changes within the formalism, and apply the resulting equations to equiatomic nickel platinum. By including these effects, we obtain significantly better agreement with experimental data. In fact, particular to NiPt, a consequence of the charge correlation is a screening which cancels much of the electrostatic contribution to the energy and thus to the atomic short-range order, resulting in agreement with a picture originally outlined using only ``band-energy'' contributions.

Relativistic and non-relativistic electron transport in disordered alloys I. Theory

Abstract: The electrical conductivity of disordered alloys is calculated using the Korringa-Kohn-Rostoker-coherent-potential approximation alloy theory in conjunction with the local approximation to density functional theory and the Kubo-Greenwood equation. Relativistic and non-relativistic expressions for the conductivity are derived. A technique for evaluation of the Kubo-Greenwood equation for arbitrary crystal symmetry using group-theoretical methods is described. Explicit expressions for scattering and current operators at complex energies below the real axis which occur in the evaluation of the Kubo-Greenwood equation are given.

Diagram technique for the Hubbard model. Ladder diagram summation

Abstract: A new diagram technique based on the generalized Wick theorem has been elaborated for the systems with strong electronic correlations. Coulomb repulsion of the electrons of the Hubbard model is considered as the main part of Hamiltonian and is taken into account in a zero order approximation. The hopping matrix elements are considered as a perturbation. One-particle Matsubara-Green function of the model has been investigated and Dyson equation has been obtained. New elements of the theory which are characteristic of this new approach are the local many-particle irreducible Green functions, or Kubo cumulants. They become zero when Coulomb interaction is zero. The main task of this paper is the summing of ladder diagrams which take into account the most essential charge and spin fluctuations of the system. The integral equations, which sum such diagrams, have been established for two different channels. The coherent potential approximation has been used to simplify and solve these equations. On this basis a metal-dielectric phase transition has been investigated.

Short-range order in a steady state of irradiated Cu-Pd alloys: comparison with fluctuations at thermal equilibrium

Abstract: The equilibrium short-range order (SRO) in Cu-Pd alloys is studied theoretically. The evolution of the Fermi surface-related splitting of the (110) diffuse intensity peak with changing temperature is examined. The results are compared with experimental observations for electron-irradiated samples in a steady state, for which the temperature dependence of the splitting was previously found in the composition range from 20 to 28 at.% Pd. The equilibrium state is studied by analysing available experimental and theoretical results and using a recently proposed alpha-expansion theory of SRO which is able to describe the temperature-dependent splitting. It is found that the electronic structure calculations in the framework of the Korringa-Kohn-Rostoker coherent potential approximation overestimate the experimental peak splitting. This discrepancy is attributed to the shift of the intensity peaks with respect to the positions of the corresponding reciprocal-space minima of the effective interatomic interaction towards the (110) and equivalent positions. Combined with an assumption about monotonicity of the temperature behaviour of the splitting, such a shift implies an increase of the splitting with increasing temperature for all compositions considered in this study. The alpha-expansion calculations seem to confirm this conclusion.

Theoretical study of the density of states and magnetic properties of LaCoO 3

Abstract: The density of states and magnetic properties of low-spin, high-spin, and mixing states of ${\mathrm{LaCoO}}_{3}$ have been studied within the unrestricted Hartree-Fock approximation. The real-space recursion method is adopted for computing the electronic structure of the disordered system. The paramagnetic high-spin state is dealt with using the usual binary alloy coherent potential approximation (CPA); an extended trinary alloy CPA approximation is developed to describe the mixing state. In agreement with experiments, our results show that the main features of the quasiparticle spectra in the mixing state are not a sensitive function of the high-spin component, but the spectrum does get broadened due to spin scattering. The increasing of the high-spin component also results in a pileup of the density of states at the Fermi energy which indicates an insulator to metal phase transition. Some limitations of the present approach are also discussed.

Comparison of the electronic states of alloys from the coherent potential approximation and an order-N calculation

Abstract: For historical reasons, the coherent potential approximation was originally proposed for isomorphous models of alloys in which all the atoms of a given species are assumed to have the same charge distribution. Order-N methods for density-functional theory local-density approximation calculations of the electronic structure for clusters of hundreds or thousands of atoms demonstrate that a polymorphous model, in which each atom is different, is more realistic. The predictive ability of isomorphous coherent potential approximations is studied by comparison with order-N calculations.

Sign of the interaction parameter in disordered Fe–Al alloys

Abstract: In the framework of the coherent potential approximation the authors show that the interaction parameter for disordered Fe-Al alloy strongly depends on the concentration. The calculations were provided within linear muffin-tin orbital formalism in the atomic sphere approximation. Calculation data for B2 phase for different concentrations were used to extract the interaction parameter, V(0), for disordered solid solutions and were compared with the parameter treated from the X-ray scattering data. The concentration dependence of V(0) leads to the failure of the application of the regular solid solution model to the study of the phase relations in the Fe-Al system. Parameter V(k{sub s}), which is responsible for ordering tendency, also depends on concentration, and it is in accordance with the experimental data.

Quasi-particle lifetimes effects on deviations from Vegard’s rule in Ag-Pd disordered alloys

Abstract: The deviations from linearity in the concentration dependence of lattice spacings in AgcPd1-c have been recently related to the topology changes shown by the Fermi Surface, or Electronic Topological Transitions (ETTs), of this alloy. Using an extension of the ETTs theory to finite-temperature and impurity scattering case, and in terms of a simple Rigid Band Model, we study such deviations and interpret them as the results of the filling of the hole pocket centered at theX point of the Brillouin Zone, remarking the role of the finiteness of the quasi-particle lifetime of the relevant states. We also show howab initio calculations based on the Coherent Potential Approximation underestimate these lifetimes.

Acoustic and elastic waves in random media – CPA

Abstract: We study the propagation of acoustic and elastic waves in random composites consisting of spherical inclusions in a homogeneous elastic host (fluid or solid), using various extensions of the Coherent Potential Approximation (CPA) method, well-known from the electronic problem. We calculate the phase velocity, the scattering mean free path and the transport velocity. The results are compared with experimental data and accurate computational results.

Investigation of Hole Distribution in Y1-xPrxBa2Cu3O7

Abstract: The electronic structure of Y1-xPrxBa2Cu3O7 is described by a tight-binding Hamiltonian in which the Pr-O hybridization and the 2pσ-2pπ Coulomb repulsion are taken into account. The dependence of planar hole-density, localized hole-density and chemical potential of holes on Pr concentration is calculated by a coherent potential approximation. The result shows that the insensitivity of hole-density on the CuO3 chains with the Pr-doping is due to the 2pσ-2pπ repulsion. Some other experimental facts can also be well interpreted.

Theoretical study of magnetic phase transitions in invar alloy

Abstract: The electronic structure and magnetic phase transitions in substitutional disordered binary alloys Fe 1-x Ni x with fcc structure exhibiting invar behavior are studied using the Korringa-Kohn-Rostoker method in combination with the coherent-potential approximation. The difference in energy of the paramagnetic (PM) and ferromagnetic (FM) states of the Fe 1-x Ni x system is calculated for concentrations close to the critical value x = 0.35, corresponding to the invar alloy. The calculations are carried out by the fixed-spin-moment method within the atomic-sphere and muffin-tin-sphere approximations to the charge density. It is found that the difference between the total energies of the PM and FM states is very small, which is in accordance with previous calculations of the invar effect in Fe 65 Ni 35 . The calculated magnetic moment, equilibrium volume, and bulk modulus as a junction of concentration correlate well with experimental data.

Effect of 4d transition metals on the electronic structure and magnetic properties of

Abstract: The electronic structure and magnetic properties of bcc random alloys with Fe and 4d transition metals were studied using the first-principles Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method. The average magnetic moments of the alloys agreed well with the experimental results. The local magnetic moment at the Fe site was enhanced in all alloys.

Pseudo-quaternary boro-carbides: a two sub-lattice model

Abstract: We consider recently studied boro-carbides RE(Ni1−xMx)2B2C, with emphasis in RE=Y and Ho Ho compounds exhibit in some cases spiral magnetic order We extended a recent previous work on Heusler compounds to study the specific heat of Y boro-carbides doped with Co and to calculate the coupling between Ho moments in neighbouring planes The effect of impurity disorder is included in the electronic structure via the Coherent Potential Approximation (CPA) and in the absence of experimental data for the pseudoquaternary specific heat, we restricted ourselves to the dilute limit and determine the impurity potential via Friedel's sum rule The electron–phonon coupling (λ(x)) is extracted from the measured superconducting transition temperature, using the Allen–Dines formula The existent specific heat data for Y compounds is used to fix the parameters of the two sub-lattice model Finally we study the change, with transition metal concentration, of the angle between moments of the spiral magnetic order