Showing papers on "Coherent potential approximation published in 1982"
TL;DR: In this article, the Korringa-Kohn-Rostoker-coherent potential approximation (KKR-CPA) method for treating electronic states in random alloys has been made charge self-consistent.
Abstract: The Korringa-Kohn-Rostoker-coherent-potential-approximation (KKR-CPA) method for treating electronic states in random alloys has been made charge self-consistent. Results are presented for a Ag0.2Pd0.8 alloy that were obtained using the Hedin-Lundqvist local density exchange-correlation potential.
53 citations
TL;DR: In this article, a self-consistent coherent potential approximation generalised to take the effect of clusters into account is presented. But it does not take into account off-diagonal disorder and short-range order.
Abstract: The authors have developed a self-consistent coherent potential approximation generalised to take the effect of clusters into account. Off-diagonal disorder and short-range order are taken into account. A graphical method married to the recursion technique enables them to work on realistic three-dimensional lattices. Calculations are shown for a binary alloy on a diamond lattice.
45 citations
TL;DR: In this paper, the authors used the fully relativistic version of the Korringa-Kohn-Rostoker coherent potential approximation to study the electronic structure of AucPt1-c random substitutional alloys.
Abstract: The authors have used the fully relativistic version of the Korringa-Kohn-Rostoker coherent potential approximation to study the electronic structure of AucPt1-c random substitutional alloys. They have calculated the averaged density of states, partial density of states, and the Bloch spectral functions at various concentrations covering the full range. Contrary to previous assumptions, the results show clearly that these alloys do not follow the predictions of the rigid band model. Their calculated electronic contribution to the specific heat agrees well with experiment. They predict large relativistic effects in the soft X-ray absorption spectra. They have found well defined Fermi surfaces for all the alloys and have studied their evolution with concentration.
23 citations
TL;DR: In this article, the density of states up to 1 Ryd above the Fermi energy, epsilon F, and the LIII soft X-ray spectra for a number of NicCu1-c alloys using the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) method was calculated.
Abstract: The authors have calculated the density of states up to 1 Ryd above the Fermi energy, epsilon F, and the LIII soft X-ray spectra for a number of NicCu1-c alloys using the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) method. The results reproduce all the prominent features of the experimental absorption spectra in the relevant frequency range. On the basis of the calculations, the authors comment on the variation of the number of d holes on Ni sites within the context of a paramagnetic theory.
16 citations
TL;DR: In this article, the incoherent dynamical structure factor and the diffusion coefficient of the nearest neighbour (NN) jump diffusion model are studied. And the virtual crystal approximation and coherent potential approximation procedures are used to find solutions, and five exact frequency moments of the spectral density are calculated and compared with those given by these theories.
Abstract: The incoherent dynamical structure factor and the diffusion coefficient of the nearest neighbour (NN) jump diffusion model is studied. Diffusion of a specific particle in a static but random arrangement of background particles is considered. The virtual crystal approximation and coherent potential approximation procedures are used to find solutions. First five exact frequency moments of the spectral density are calculated and compared with those given by these theories. In simple cubic and body-centred cubic lattices four first moments are exactly reproduced by the CPA solution.
15 citations
TL;DR: In this paper, the authors applied the tight-binding coherent potential approximation method to a study of the electronic structure of TiFeHx and obtained the densities of states, decomposed per site and also per angular momentum component, as a function of the hydrogen content.
Abstract: We applied the tight-binding coherent potential approximation method to a study of the electronic structure of TiFeHx. The starting point of these calculations was a Slater-Koster fit to the band structure of TiFe at one end of the compositional spectrum and of TiFeH1.0 at the other end. We obtained the densities of states, decomposed per site and also per angular momentum component, as a function of the hydrogen content. These densities of states were used to examine the effect of hydrogenation on both the bonding and antibonding levels as well as on the position of the Fermi level.
14 citations
TL;DR: In this paper, the authors studied the Falicov-Kimball model with a hybridization between localized and band electron states in the alloy analog approximation and showed that in general the Hartree-Fock solution yields an unrealistically steep n ff (Eo) dependence.
13 citations
TL;DR: In this article, the 4f-binding energy relative to the Fermi level is discussed based on the present theory, which is one of the central issues of the recent photoemission spectroscopy studies of Ce and its compounds.
11 citations
TL;DR: Using the Bethe lattice as a simple model of amorphous Si, the effect on the vibrations of strong local defects such as substitutional atoms and broken bonds are calculated by Green function techniques as mentioned in this paper.
Abstract: Using the Bethe lattice as a simple model of amorphous Si, the effect on the vibrations of strong local defects such as substitutional atoms and broken bonds are calculated by Green function techniques. For finite concentrations of such impurities the coherent potential approximation can be adapted and used in the model. Results for amorphous alloys of Si-Ge are given.
11 citations
TL;DR: In this paper, a generalization of the coherent-potential approximation (CPA) is proposed to make it applicable to compositionally modulated alloys, which preserves all the desirable properties of CPA, such as analyticity and ease of computation, and yields results which are much more accurate than those obtained within the original CPA.
Abstract: We propose a generalization of the coherent-potential approximation (CPA) in order to make it applicable to compositionally modulated alloys. This generalization (MODCPA) preserves all the desirable properties of CPA, such as analyticity and ease of computation, and in many cases yields results which are much more accurate than those obtained within the original CPA. Numerical calculations for the cases of one-dimensional and three-dimensional tight-binding model systems reveal that composition modulation may produce increased structure in the density of states with the setting in of energy gaps for certain directions and strengths of modulation. We found that the original CPA is not capable of accounting for such effects of the modulation. On the other hand, modulation effects on the density of states are quite accurately described within the MODCPA. Finally, we also show how the MODCPA can be extended to a cluster theory and how it can be applied to alloys describable by tight-binding Hamiltonians with both diagonal and off-diagonal disorder, and to alloys describable by muffin-tin Hamiltonians.
01 Jan 1982
TL;DR: In this article, the authors compared several single-site methods for calculating total, and component densities of states in alloys describable by muffin-tin Hamiltonians, focusing on the coherent potential approximation (CPA) and the average t-matrix approximation (ATA).
Abstract: We discuss and compare numerically on the basis of a one-dimensional model, several single-site methods for calculating total, and component densities of states in alloys describable by muffin-tin Hamiltonians. We focus on the coherent potential approximation (CPA) and the average t-matrix approximation (ATA) and contrast methods which are based on a knowledge of the ensemble-averaged Green's function with those which are derived from the Lloyd formula for the density of states. We find that the expressions for the total and component densities of states derived from the average Green's function yield physically acceptable results for both CPA and ATA. On this basis we identify a best ATA theory. The expressions based on the Lloyd formula which have been used by various authors for calculating component densities of states in the CPA and total component densities of states in the ATA yield results which are not only incorrect but are often negative.
TL;DR: In this article, the magnetic structure in the ground state of alloys between the ferromagnetic and the antiferromagnetic metals was investigated by using the coherent potential approximation (CPA) combined with the Hartree-Fock (HF) approximation for the Hubbard Hamiltonian.
Abstract: The magnetic structure in the ground state of alloys between the ferromagnetic and the antiferromagnetic metals is investigated by using the coherent potential approximation (CPA) combined with the Hartree-Fock (HF) approximation for the Hubbard Hamiltonian. By bearing in mind Fe-Cr alloys, the successive transitions from the ferromagnetic state to the antiferromagnetic one via the spin glass one with varying the alloy composition are demonstrated by the HF-CPA calculation, which distinguishes Fe atoms with magnetic moments parallel to the bulk magnetization or the sublattice one from those with antiparallel moments. An overall agreement between various magnetic measurements on Fe-Cr and the present calculation is obtained.
TL;DR: In this article, the authors reviewed some of the aspects of the coherent potential approximation (CPA) relevant to the semiconductor problem and illustrated new results for the complex band structure of disordered (Hg1−xCdx)Te, obtained by use of an empirical tight binding scheme and the parameters of Czyzyk and Podgorny for HgTe and CdTe.
Abstract: The success of the coherent potential approximation (CPA) in obtaining semiquantitative theoretical descriptions of the electronic structure and the transport, optical, and magnetic properties of disordered metallic alloys has been recognized for some time. A more limited number of applications has been made to group IV and III–V alloys. This article reviews some of the aspects of the CPA relevant to the semiconductor problem. The theory is illustrated by new results for the complex band structure of disordered (Hg1−xCdx)Te, obtained by use of an empirical tight‐binding scheme and the parameters of Czyzyk and Podgorny for HgTe and CdTe.
TL;DR: In this article, the average t -matrix approximation scheme was used to study the effect of choice of alloy potentials on the band structures of binary alloys, and it was shown that with carefully constructed potentials, the average T-matrix scheme can give as good results as those obtained by the coherent potential approximation scheme.
Abstract: In order to study effects of choice of alloy potentials on the band structures of binary alloys, calculations of the spectral density and complex energy bands of Nb 1- x Mo x are carried out in the average t -matrix approximation scheme. The alloy potentials are constructed in three ways by using the muffin-tin potentials of pure Nb and Mo. Choice of the alloy potentials remarkably affects the imaginary parts of the energy eigenvalues, while the band structures themselves remain almost unaffected. For alloy systems such as Nb–Mo, it is found that, with carefully constructed potentials, the average t -matrix approximation scheme can give as good results as those obtained by the coherent potential approximation scheme.
TL;DR: In this article, the elastic Green's function for the pinned flux-line lattice in the critical state is defined and evaluated in the coherent potential approximation, and a criterion is derived for the applicability of the dilute-limit approximation.
Abstract: The elastic Green's function for the pinned flux-line lattice in the critical state is defined and evaluated in the coherent potential approximation. The presence of pinning sites contributes an additional restoring force, which must be calculated within a complete theory of the critical state, but which can be measured. Results are given for the dependence of the Green's function on the density of strong flux-line pins, and a criterion is derived for the applicability of the dilute-limit approximation. This criterion is never satisfied near the upper and lower critical fields. Assuming that the pins all interact sufficiently strongly with the flux-line lattice to exceed the threshold criterion, we find that the Green's function decreases (the flux-line lattice becomes more rigid) with increasing pin density.
TL;DR: In this article, the authors measured the vibrational properties of the lowest singlet exciton in anthracene at 10 K. Spectral features due to the 0-0 excitons, vibrons and the multi-particle band edges are interpreted in terms of a classical oscillator model.
Abstract: Thermoreflectance spectra are measured for the vibronic bands of the lowest singlet exciton in anthracene at 10 K. Spectral features due to the 0–0 excitons, vibrons and the multi-particle band edges are interpreted in terms of a classical oscillator model. The characteristic profile of the two-particle absorption band associated with the 390 cm -1 intramolecular vibration is compared with a model calculation by the dynamical coherent potential approximation model. It is concluded that the density of states curve for the exciton has a strong one-dimensional character near the lower band edge, but at higher energies it is of three-dimensional character having a large density of states near the upper edge.
TL;DR: In this article, the results of magnetic measurements and ferromagnetic resonance study on Ni 0.87 Cu 0.13− x Al x and Ni0.77 Cu0.23− xAl x alloys with x ⩽ 10 at 10 % Al are reported.
TL;DR: In this paper, the lattice dynamics of mixed crystals AB1-cCc described by shell models are treated within the framework of a single-site CPA, and the influence of disorder in the core-shell coupling on the density of states and on the one-mode-two-mode behaviour is found to be very important.
Abstract: The lattice dynamics of mixed crystals AB1-cCc described by shell models is treated within the framework of a single-site CPA. Apart from the mass disorder the author allows for disorder in the core-shell coupling and for disorder in the 'breathing'. The influence of disorder in the core-shell coupling on the density of states and on the one-mode-two-mode behaviour is found to be very important.
TL;DR: In this paper, the N6,7 emission spectra from both Au and Pt sites in various AucPt1-c alloys were calculated on the basis of the fully relativistic Korringa-Kohn-Rostoker coherent potential approximation (R KKR CPA).
Abstract: The authors have calculated the N6,7 emission spectra from both Au and Pt sites in various AucPt1-c alloys on the basis of the fully relativistic Korringa-Kohn-Rostoker coherent potential approximation (R KKR CPA) method. They argue that the corresponding experimental study would shed important light on the electronic structure of this interesting system.
TL;DR: In this article, the effect of hydrogenation is renormalized into an effective Si * -Si * bond so that the problem may be reduced to that of a-Si * with random effective bonds.
Abstract: Coherent potential approximation (CPA) is applied to evaluate the total density of states (DOS) and local density of states (LDS) for hydrogenated amorphous silicon (a-Si: H). Three configurations SiH, SiHHSi and SiHSi are considered for the silicon-hydrogen bonding states. Instead of calculating the electronic states of a-Si: H, the effect of hydrogenation is renormalized into an effective Si * –Si * bond so that the problem may be reduced to that of a-Si * with random effective bonds. The calculated Si * -LDS and DOS for three configurations are compared with other theory and experiments.
TL;DR: In this article, a new method of calculation of the coherent potential approximation combined with the Korringa-Kohn-Rostoker method of the band structure calculation is proposed, and the electronic structure of Fe-Al and Pd-Al alloys are calculated by this method.
Abstract: A new method of calculation of the coherent potential approximation combined with the Korringa-Kohn-Rostoker method of the band structure calculation is proposed. The electronic structure of Fe–Al and Pd–Al alloys are calculated by this method. The concentration dependence of the electronic specific heat coefficient, the (high field) susceptibility and the nuclear spin-lattice relaxation time of the alloys is discussed in the light of the calculation.
TL;DR: In this article, the role of disorder in impurity bands in mixed organic crystals is examined in terms of the Anderson model, and a coherent potential approximation is used to calculate averaged Green's functions; the L (E ) criterion is employed to study the transition in the nature of the eigenstates.
01 Jan 1982
TL;DR: In this paper, the authors introduced the coherent medium approximation (CMA) to obtain an approximate but easily tractable coherent medium, which is a generalized use of the coherent potential approximation in the master equation.
Abstract: We have presented the coherent medium approach to hopping conduction problems where the motion of carriers obey the usual random walk equation and discussed the existence of the coherent medium which is defined through an average of the random walk propagator. We have introduced the coherent medium approximation (CMA) to obtain an approximate but easily tractable coherent medium. The CMA is a generalized use of the coherent potential approximation (CPA) in the master equation. The CPA is one of the most fruitful methods in treating random systems and is widely used in electron 19 and phonon problems. 18 We have applied the CMA to various cases of hopping conduction in one- and three-dimensions and compared the results with experiment. A simple comparison of the CMA with other methods has also been given.
TL;DR: In this article, the spin wave stiffness constant in NiCu alloys is calculated in the RPA and cluster coherent potential approximation, and it is shown that the stiffness constant decreases when the concentration of Cu increases.
Abstract: The spin wave stiffness constant in NiCu alloys is calculated in the RPA and cluster coherent potential approximation. It is shown that the stiffness constant D decreases when the concentration of Cu increases.
Die Spinwellensteifigkeitskonstante wird in der RPA und Cluster-CPA berechnet. Die Steifigkeitskostante D nimmt ab, wenn die Cu-Konzentration wachst.
TL;DR: In this article, a two-site dynamical coherent potential approximation for exciton-phonon interaction models corresponding to a homomorphic partition of the hamiltonian is described.
TL;DR: In this article, the concentration dependence of the reststrahl absorption in various mixed crystals exhibiting one, two and mixed mode behavior was investigated using the coherent potential approximation (cpa).
Abstract: The concentration dependence of the reststrahl absorption in various mixed crystals exhibiting one, two and mixed mode behaviour is investigated using the coherent potential approximation (cpa). The phonon Green’s function, the impurity mode frequencies and the strength of absorption are calculated in the Einstein model from the generalizedcpa proposed by Tripathi and Behera which takes into account both mass and force constant changes. The introduction of a phenomenological concentration dependence of the force constant change parameter is shown to provide a satisfactory explanation of the concentration dependence of the experimental data for the twenty mixed crystal systems analysed. It is conjectured that the nearest neighbour force constant of an impurity atom substituted at a host site is very much different from that of a perfect crystal consisting of these impurity atoms and that both these play an important role in determining the one, two and mixed mode behaviour of the mixed crystals.
TL;DR: In this paper, the lattice dynamics of PdDx and PdHx were analyzed with reference to the coherent neutron scattering experiments of Rowe et al. and incoherent neutron scattering lineshape reported by Rahman et al (1976) Disorder in the octahedral interstitial sublattice was treated within a coherent potential approximation format suitable for treating both on-site and off-diagonal randomness.
Abstract: Lattice dynamics of PdDx and PdHx are analysed with reference to the coherent neutron scattering experiments of Rowe et al (1974) and incoherent neutron scattering lineshape reported by Rahman et al (1976) Disorder in the octahedral interstitial sublattice is treated within a coherent potential approximation format suitable for treating both 'on-site' and 'off-diagonal' randomness The authors' results lead to a substantial readjustment (upwards) in the accepted values of the D-D potential Moreover, an adjustment (downwards) is also indicated in the published value of the Pd-H potential
TL;DR: In this article, a self-consistent cluster coherent potential approximation of the electron density of states of a random, binary alloy on abcc lattice is presented. But the method is not suitable for the case of binary lattice.
Abstract: This paper describes and analyses a self-consistent cluster coherent potential approximation calculation of the electron density of states of a random, binary alloy on abcc lattice.