Showing papers on "Coherent potential approximation published in 1993"
TL;DR: An efficient technique for calculating surface properties of random alloys based on the coherent-potential approximation within a tight-binding linear-muffin-tin-orbitals basis is presented and it is found that the noble metals segregate strongly towards the surface of their alloys.
Abstract: We present an efficient technique for calculating surface properties of random alloys based on the coherent-potential approximation within a tight-binding linear-muffin-tin-orbitals basis The technique has been applied in the calculation of bulk thermodynamic properties as well as (001) surface energies and work functions for three fcc-based alloys (Cu-Ni, Ag-Pd, and Au-Pt) over the complete concentration range The calculated mixing enthalpies for the Ag-Pd and Au-Pt systems agree with experimental values, and the calculated concentration dependence of the lattice parameters agrees with experiment for all three systems We find that the calculated surface energies and work functions in the unsegregated case exhibit a small positive deviation from a linear concentration dependence Finally, we performed a segregation analysis based on the calculated surface energies by means of a simple thermodynamic model and found in complete agreement with experiment that the noble metals segregate strongly towards the surface of their alloys
151 citations
TL;DR: A multiple-scattering formalism for electromagnetic waves is presented, given in a form similar to the usual Korringa-Kohn-Rostoker form of scalar waves to three-dimensional periodic dielectric structures.
Abstract: In this paper, a multiple-scattering formalism for electromagnetic waves is presented. Its application to the three-dimensional periodic dielectric structures is given in a form similar to the usual Korringa-Kohn-Rostoker form of scalar waves. Using this approach, the band-structure results of touching spheres of diamond structure in a dielectric medium with dielectric constant 12.96 are calculated. The application to disordered systems under the coherent-potential approximation is discussed.
135 citations
TL;DR: A method for calculating the electronic structure of disordered alloys with shortrange order (SRO) which guarantees a positive density of states for all values of the SRO parameter is presented, based on the generalized augmented-space theorem which is valid for alloy with SRO.
Abstract: We present a method for calculating the electronic structure of disordered alloys with shortrange order (SRO) which guarantees a positive density of states for all values of the SRO parameter. The method is based on the generalized augmented-space theorem which is valid for alloys with SRO. This theorem is applied to alloys with SRO in the tight-binding linear-muffin-tin-orbital (TB-LMTO) framework. This is done by using the augmented-space formulation of Mookerjee and the cluster-coherent-potential approximation. As an illustration, the method is applied to a single-band model TB-LMTO Hamiltonian. We find that the SRO can induce substantial changes in the density of states
51 citations
TL;DR: The first implementation of a unified approach to the study of alloys is presented, using the atomic-sphere approximation formulation of the Korringa-Kohn-Rostoker coherent potential approximation method, which allows the treatment of the disordered alloy and of the ordered alloys on an equal footing.
Abstract: We present the first implementation of a unified approach to the study of alloys, using the atomic-sphere approximation formulation of the Korringa-Kohn-Rostoker coherent potential approximation method [P P Singh and A Gonis, Phys Rev B 48, 1989 (1993)], which allows the treatment of the disordered alloy and of the ordered alloys on an equal footing We apply this method to the study of the phase stability of the Ni-Pt alloy and resolve a number of discrepancies that have arisen in recent studies of this system
28 citations
TL;DR: In this article, the authors calculated the fault energy in dilute Cu(Al) alloys and across the composition range of PdAg alloys from first principles using the layer Korringa-Kohn-Rostoker method and treating the compositional disorder within the coherent potential approximation.
Abstract: Stacking fault energies in dilute Cu(Al) alloys and across the composition range of PdAg alloys are calculated from first principles using the layer Korringa-Kohn-Rostoker method and treating the compositional disorder within the coherent potential approximation In Cu(Al), rigid-band behaviour results in a sharp reduction in the fault energy with Al concentration The non-uniform variation of the fault energy in PdAg is understood in terms of the relative band-widths and d-resonance energies of Pd and Ag
28 citations
TL;DR: In this paper, the fast linear muffin-tin orbital coherent potential approximation method allied with density functional theory (LMTO-CPA-DFT) was used to calculate electronic structure and cohesive properties of Ni-Al random alloys on an underlying FCC lattice in all concentration intervals.
Abstract: The fast linear muffin-tin orbital coherent potential approximation method allied with density functional theory (LMTO-CPA-DFT) is used to calculate electronic structure and cohesive properties of Ni-Al random alloys on an underlying FCC lattice in all concentration intervals. Binding curves obtained in the calculations are used in the Debye-Gruneisen analysis to determine thermal properties and temperature-dependent Connolly-Williams cluster interactions. The calculated lattice constants, bulk moduli, enthalpies and free energies of formation are in good agreement with experiment. The globally and locally relaxed cluster interactions are compared with the results obtained from the total energy-band calculations for elements and ordered phases.
19 citations
TL;DR: It is found that a change of the photoionization matrix element with the binding energy in the valence band can cause a substantial discrepancy between the density of states and thephotoemission spectrum for [ital d]-band metals and alloys even in the x-ray photoemission spectroscopy regime.
Abstract: It is found that a change of the photoionization matrix element with the binding energy in the valence band can cause a substantial discrepancy between the density of states and the photoemission spectrum for [ital d]-band metals and alloys even in the x-ray photoemission spectroscopy regime. This is the main cause of the controversial discrepancy between photoemission spectra and theoretical density of states predicted in the Korringa-Kohn-Rostoker coherent-potential approximation for Cu-Pd alloys, rather than the local lattice relaxation around Pd atomic sites.
16 citations
TL;DR: In this paper, an effective one-band model Heff has been proposed, which is based on the three-band Hubbard hamiltonian H and projecting out of the Hilbert space a group of high-energy states, but the hopping terms depend on occupation numbers.
Abstract: Starting from the three-band Hubbard hamiltonian H and projecting out of the Hilbert space a group of high-energy states, we obtain an effective one-band model Heff. Heff has a form similar to the Hubbard model, but the hopping terms depend on occupation numbers. We give analytical, accurate enough expressions relating the parameters of Heff to those of H. Treating Heff in an approximation similar to Hubbard III the problem is reduced to an alloy with diagonal and off-diagonal disorder. Using the criterion of the closing of the gap in this effective alloy for the metal-insulator transition, we obtain a metal-insulator phase diagram for the undoped system as a function of the parameters of H. The system is insulating for realistic parameters.
11 citations
TL;DR: In this paper, the effect of a concentration c of impurities on the uncorrelated electronic states in the coherent potential approximation (CPA) was investigated, and the results indicated that the sensitivity of the Neel temperature of the antiferromagnetic (AF) layered copper perovskites (typically La2CuO4) to diamagnetic impurities such as Zn is reportedly much larger than in the K2NiF4 family.
Abstract: To investigate why the sensitivity of the Neel temperature TN of the antiferromagnetic (AF) layered copper perovskites (typically La2CuO4) to diamagnetic impurities such as Zn is reportedly much larger than in the AF members of the K2NiF4 family, we first treat the effect of a concentration c of impurities on the uncorrelated electronic states in the coherent potential approximation (CPA). Then we consider the Heisenberg hamiltonian as the large correlation limit of the Hubbard hamiltonian for a single band of impurity-modified electronic states. The correlation effects are treated variationally. The model is solved explicity by using a rectangular density of states, and we obtain the c-dependent exchange J, staggered moment Sq, spin wave velocity and transverse susceptibility at zero temperature. We take into consideration several recently proposed formulae for TN in the clean limit, and include the impurity effects by exploiting the results obtained, in order to test their predictions against the experimental TN(c) data for La2Cu1−cZncO4. Our results suggest that, to explain the difference between the K2NiF4 and the La2 CuO4 families, one should consider both the sign and the magnitude of the difference I≡ϵB−ϵA between impurity (B) and host (A) ionic potentials. The slowly decreasing trend of TN(c) in the K2NiF4 family is reproduced if I is negative and sizeable, or positive but very small, while the quick decrease typical of the copper perovskites requires a positive and rather large I. For reasonable values of the interaction parameters, among the several models we compare, only the model of Chakravarty, Halperin and Nelson is able to semi-quantitatively reproduce the non-linear behaviour of TN(c) reported for La2Cu1−cZncO4, provided the spin stiffness is assumed to scale with c as appropriate to Fermi liquids.
10 citations
TL;DR: The generalized perturbation method in the atomic-sphere approximation (ASA) for calculating the effective cluster interactions is described, which is the next step towards developing a first-principles method that can be easily applied to describe substitutionally disordered alloys based on simple lattice structures as well as complex lattICE structures with low symmetry.
Abstract: We describe the generalized perturbation method in the atomic-sphere approximation (ASA) for calculating the effective cluster interactions. Based on our development of Korringa-Kohn-Rostoker coherent-potential approximation in the ASA [Singh [ital et] [ital al]., Phys. Rev. B 44, 8578 (1991)], the present approach is the next step towards developing a first-principles method that can be easily applied to describe substitutionally disordered alloys based on simple lattice structures as well as complex lattice structures with low symmetry. To test the accuracy of the ASA results, we have calculated the effective pair interactions (EPI) up to fourth-nearest neighbors for the substitutionally disordered Pd[sub 0.5]V[sub 0.5] and Pd[sub 0.75]Rh[sub 0.25] alloys. Our calculated EPI's are in good agreement with the respective muffin-tin results.
10 citations
TL;DR: The electronic structure and properties of two different types of random Ag overlayers on a nonrandom Pd(001) substrate have been studied and the layer-resolved local densities of states and the work functions were obtained.
Abstract: The electronic structure and properties of two different types of random Ag overlayers on a nonrandom Pd(001) substrate have been studied. One overlayer is an Ag-Pd alloy and the other is Ag plus vacancies. Calculations were performed by means of a self-consistent, surface Green's-function technique based on the tight-binding linear-muffin-tin-orbital theory. The disorder was included via the coherent-potential approximation generated to inhomogeneous systems. The layer-resolved local densities of states and the work functions were obtained
TL;DR: For the first time, the Harris functional was applied in the framework of coherent potential approximation (CPA) in this article, where the lattice parameters, bulk moduli, coefficients of thermal expansion and mixing energy for disordered alloys were investigated.
TL;DR: A generalization of the Korringa-Kohn-Rostoker cluster coherent-potential approximation for systems with short-range order (SRO) is presented, using the generalized augmentedspace formalism of Gray and Kaplan.
Abstract: We present a generalization of the Korringa-Kohn-Rostoker cluster coherent-potential approximation for systems with short-range order (SRO). For this purpose we have used the generalized augmentedspace formalism of Gray and Kaplan, in which one can deal with independent (corresponding to purely random systems) as well as dependent (corresponding to systems with SRO) random variables. The expression for the configuration-averaged Green's function in this case is essentially an expansion about the Green's function for a purely random system, and contains an infinite number of terms. For simplicity, we truncate the series after the second-order correction term
TL;DR: Based on all-electron self-consistent fully relativistic Korringa-Kohn-Rostoker coherent potential approximation results, concentration- and lattice-parameter-dependent effective pair interactions up to the fifth nearest neighbors are calculated in terms of the fully relatIVistic embedded cluster method for the Au-Pd system.
Abstract: Based on all-electron self-consistent fully relativistic Korringa-Kohn-Rostoker coherent potential approximation results, concentration- and lattice-parameter-dependent effective pair interactions up to the fifth nearest neighbors are calculated in terms of the fully relativistic embedded cluster method for the Au-Pd system. The obtained effective pair interactions are then used to evaluate a simplified (V 1 , V 2 ) phase diagram, ordering energies, and related quantities
TL;DR: An expression for the configuration-averaged Green's function is derived from which one can determine various electronic properties of the alloys, in particular, the charge densities, which are needed for charge self-consistent calculations.
Abstract: We present a method for the determination of the electronic structure of random alloys within the self-consistent cluster coherent-potential approximation in the tight-binding linear-muffin-tin-orbital method. This formalism combines the simplicity of the tight-binding linear-muffin-tin-orbital scheme with the cluster coherent-potential approximation to determine the effective medium self-consistently. This approximation is analytic and guarantees non-negative and single-valued density of states at all energies. We have derived an expression for the configuration-averaged Green's function from which one can determine various electronic properties of the alloys, in particular, the charge densities, which are needed for charge self-consistent calculations
TL;DR: In this article, an estimate of the influence of topological disorder on the electronic structure was made analytically to obtain the exact solution of the coherent potential approximation (CPA) equations with a Lorenzian-like shape describing the density of states of pure metals.
TL;DR: In this article, the static and dynamical properties of excitons in a lattice with a random distribution of polarizable atoms are studied using Green's function techniques using the ladder diagram approximation.
Abstract: The static and dynamical properties of excitons in a lattice with a random distribution of polarizable atoms are studied using Green’s function techniques. Exciton transport is related to the configurationally averaged particle–hole Green’s function which is calculated using the ladder diagram approximation. Degenerate four wave mixing (D4WM) using resonant pump beams and an off resonant probe is shown to provide a direct probe for exciton transport. A disorder induced extra resonance is predicted whose width is proportional to the exciton diffusion coefficient. Numerical calculations are presented for the diffusion coefficient and the D4WM signal as the excitation energy is tuned across the exciton band.
Journal Article•
Journal Article•
TL;DR: In this article, the absorption spectrum of the mixture of molecules adsorbed on a Cu(100) surface as a function of isotope composition is calculated by the Monte-Carlo method.
Abstract: The absorption spectrum of the {sup 12}C{sup 16}O-{sup 17}C{sup 18}O isotope mixture of molecules adsorbed on a Cu(100) surface as a function of isotope composition is calculated by the Monte-Carlo method. The results are compared with the experimental and theoretical data obtained within the framework of the coherent potential approximation. The Monte-Carlo method is shown to provide better agreement with experiment than the coherent-potential approximation. 10 refs., 3 figs., 1 tab.
TL;DR: In this article, the surface segregation in CoNi, IrPt and RhPt is calculated within the coherent potential approximation, assuming that the metals form substitutional alloys and only diagonal disorder is taken into account.
TL;DR: It is found that substantial increase in the density of states at the Coulomb gap occurs only when the ratio between the hopping integral t and the gap width B exceeds a critical value.
Abstract: In this paper, we use the coherent potential approximation to study the role quantum hopping plays on the Coulomb gap of insulating strongly correlated d=2 and d=3 dimensional systems. We find that substantial increase in the density of states at the Coulomb gap occurs only when the ratio between the hopping integral t and the gap width B exceeds a critical value. We estimate that the hopping integral corresponding to the experimental condition n${\mathit{n}}_{\mathit{c}}$/3 satisfies t/B0.05. For such values of t/B, quantum hopping brings about little change in the single particle density of states. The classical Coulomb gap therefore remains intact in both three and two dimensional systems, in contrast to a previous claim that the gap disappears for d=2 systems. The implication of these results on experiments on doped semiconductors is discussed.
TL;DR: In this paper, a method of calculating the total energy for disordered alloys is presented and a formula for the potential in the self-consistent Korringa-Kohn-Rostoker coherentpotential approximation is also given by using the muffin-tin approximation after the variation of the overall energy is made.
Abstract: A method of calculating the total energy for disordered alloys is presented. A formula for the potential in the self-consistent Korringa-Kohn-Rostoker coherent-potential approximation is also given by using the muffin-tin approximation after the variation of the total energy is made. This method was applied to Cu-Pd alloys. The numerical results indicate that the disordered state is stable over the whole concentration range of Pd. This agrees with experiments.
TL;DR: Using the all-electron fully relativistic Karringa-Kohn-Rostoker coherent-potential-approximation method self-consistent total-energy calculations were performed for the system Au/Pd, an interesting quantitative comparison for the alloy total energies, spectral densities, and properties related to the Fermi energy or FermI surface can be presented.
Abstract: Using the all-electron fully relativistic Karringa-Kohn-Rostoker coherent-potential-approximation method self-consistent total-energy calculations were performed for the system Au/Pd. The calculated equilibrium lattice constants are in fairly good agreement with the experimental data. In particular, microscopic reasons for the breaks in the variation of the lattice constants with concentration can be derived from the results. Since the calculations were carried out self-consistently for a maximum angular momentum quantum number of 2 as well as 3, an interesting quantitative comparison for the alloy total energies, spectral densities, and properties related to the Fermi energy or Fermi surface can be presented.
TL;DR: The augmented-space formalism (ASF) is used to discuss the configuration averaging of random observables that one encounters in random systems, viz. the one-electron Green function or other related properties.
Abstract: We have used the augmented-space formalism (ASF) to discuss the configuration averaging of random observables that one encounters in random systems, viz. the one-electron Green function or other related properties. The Hamiltonian of the constituents is obtained within the first-principles tight-binding linearized-muffin-tin-orbitals scheme. A self-consistent approach for the cluster coherent-potential function has been developed for substitutional binary alloys. Configuration averaging for the single-site coherent-potential approximation (CPA) and its cluster generalization (CCPA) is discussed using the ASF
TL;DR: In this paper, it was shown that the equations of the Coherent Potential Approximation (C.P.A) can be solved in real space using the recursion method.
Abstract: We show that the equations of the Coherent Potential Approximation (C.P.A.) can be solved in real space using the recursion method. The Green's function, from which the density of states is calculated, is then represented by a continued fraction, and the method provides a quick and precise solution. A first application is presented
TL;DR: In this article, a semiclassical WKB treatment of the density of states spectrum of tight-binding electrons moving in a disordered two-dimensional lattice in the presence of a transverse magnetic field is presented.
Abstract: A semiclassical WKB treatment of the density of states spectrum of tight-binding electrons moving in a disordered two dimensional lattice in the presence of a transverse magnetic field is presented. The disorder is accounted for in the coherent potential approximation and analytical results are derived. For both ordered and disordered systems the line position of magnetic subbands as well as the cluster lineshape of the density of states agree quite well with exact numerical results.
TL;DR: In this paper, the authors studied the metal-insulator transition of BaBi1-xPbxO3 and Ba 1-xKxBiO3 within a Holstein molecular crystal model.
TL;DR: A consistent approach for applying the coherent-potential approximation to the various representations of the linear muffin-tin orbital method and the results for the ensemble-averaged Green functions in the tight-binding representation yield quantities that are consistent with the traditional applications of the single-site CPA.
Abstract: We describe a consistent approach for applying the coherent-potential approximation (CPA) to the various representations of the linear muffin-tin orbital method. Unlike the previous works of Kudrnovsky [ital et] [ital al]. [Phys. Rev. B 35, 2487 (1987); 41, 7515 (1990)], our results for the ensemble-averaged Green functions in the tight-binding representation yield [ital E]- and [bold r]-dependent quantities that are consistent with the traditional applications of the single-site CPA. To illustrate the reliability and the usefulness of our approach we compare the nonspherically averaged charge densities, calculated in real space, of ordered NiPt in [ital L]1[sub 0] structure and the substitutionally disordered Ni[sub 0.5]Pt[sub 0.5] on a face-centered-cubic lattice.
TL;DR: The noise properties of semiconductor alloy-based resonant-tunneling double-barrier quantum-well structures are calculated within a nonequilibrium Green's-function formulation using the coherent-potential approximation including vertex corrections.
Abstract: The noise properties of semiconductor alloy-based resonant-tunneling double-barrier quantum-well structures are calculated within a nonequilibrium Green's-function formulation. Alloy scattering is treated in the coherent-potential approximation including vertex corrections. Different noise characteristics are found depending on applied bias, scattering strength, and the transparency of the barriers. A characteristic frequency for the noise current power spectrum S(ω) is given by the inverse response time
TL;DR: By combining the Coherent Potential Approximation with the spin polarized relativistic Korringa-Kohn-Rostoker method of band structure calculation, this article showed that it is possible to describe the electronic structure of disordered magnetic alloys in a fully relativist way.
Abstract: By combining the Coherent Potential Approximation with the spin polarized relativistic Korringa-Kohn-Rostoker method of band structure calculation it got possible to describe the electronic structure of disordered magnetic alloys in a fully relativistic way. This new development allowed us to extend our theoretical description of the magnetic X-ray dichroism (MXD), which is based on the spin polarized relativistic Green's function formalism, to deal not only with pure and impurity systems but also with this interesting class of materials. We demonstrate this by calculations of K- and L-edge MXD spectra in the alloy systems FexCo1−x, CoxPt1−x and FexPt1−x, which are found in a very satisfying agreement with corresponding experimental data. In addition the relationship of the integrated MXD-spectra with the calculated spin magnetic moments is discussed in short.