Showing papers on "Coherent potential approximation published in 2009"

Exchange energies, Curie temperatures and magnons in Heusler compounds

Abstract: Heisenberg exchange parameters for various Heusler compounds with L21 structure were calculated using the Korringa–Kohn–Rostoker method and by employing the magnetic-force theorem to calculate the total energy changes associated with a local rotation of magnetization directions. Random occupation was treated within the coherent potential approximation. Further, the Curie temperatures were calculated in the mean-field approximation and have been found to be in good agreement with the experiment. A procedure for evaluating the spin-stiffness constants for the case of multiple magnetic sublattices is given and the results were compared with measured values. Magnon dispersion curves were obtained by Fourier transforming the calculated exchange parameters.

Thermoelectric Properties and Electronic Structure of Bi- and Ag-Doped Mg2Si1-xGex Compounds

Abstract: Mg2Si1−xGex compounds were prepared from pure elements by melting in tantalum crucibles. The reaction was conducted under an inert gas in a special laboratory setup. Samples for thermoelectric measurements were formed by hot pressing. Structure and phase composition of the obtained materials were investigated by x-ray diffraction (XRD). Morphology and chemical composition were examined by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS), respectively. Thermoelectric properties, i.e., the Seebeck coefficient, the electrical conductivity, and the thermal conductivity, were measured in the temperature range of 500 K to 900 K. The effect of Bi and Ag doping on the thermoelectric performance of Mg-Si-Ge ternary compounds was investigated. The electronic structures of binary compounds were calculated using the Korringa–Kohn–Rostoker (KKR) method. The effects of disorder, including Ge substitution and Bi or Ag doping, were accounted for in the KKR method with coherent potential approximation calculations. The thermoelectric properties of doped Mg2Si1−xGex are discussed with reference to computed density of states as well as the complex energy band structure.

Self-interaction correction in multiple scattering theory: application to transition metal oxides

Abstract: We apply to transition metal monoxides the self-interaction corrected (SIC) local spin density approximation, implemented locally in the multiple scattering theory within the Korringa–Kohn–Rostoker (KKR) band structure method. The calculated electronic structure and in particular magnetic moments and energy gaps are discussed in reference to the earlier SIC results obtained within the linear muffin-tin orbital atomic sphere approximation band structure method, involving transformations between Bloch and Wannier representations, in order to solve the eigenvalue problem and calculate the SIC charge and potential. Since the KKR method can be easily extended to treat disordered alloys, by invoking the coherent potential approximation (CPA), in this paper we compare the CPA approach and supercell calculations to study the electronic structure of NiO with cation vacancies. (Some figures in this article are in colour only in the electronic version)

Jahn–Teller-like origin of the tetragonal distortion in disordered Fe–Pd magnetic shape memory alloys

Abstract: The electronic structure and magnetic properties of disordered FexPd100−x alloys (50

Effect of Nanoparticles on Electron and Thermoelectric Transport

Abstract: Recent experimental results have shown that adding nanoparticles inside a bulk material can enhance the thermoelectric performance by reducing the thermal conductivity and increasing the Seebeck coefficient. In this paper we investigate electron scattering from nanoparticles using different models. We compare the results of the Born approximation to that of the partial-wave method for a single nanoparticle scattering. The partial-wave method is more accurate for particle sizes in the 1 nm to 5 nm range where the point scattering approximation is not valid. The two methods can have different predictions for the thermoelectric properties such as the electrical conductivity and the Seebeck coefficient. To include a random distribution of nanoparticles, we consider an effective medium for the electron scattering using the coherent potential approximation. We compare various theoretical results with the experimental data obtained with ErAs nanoparticles in an InGaAlAs matrix. Reasonably good agreement is found between the measured and theoretical electrical conductivity and Seebeck data in the 300 K to 850 K temperature range.

Ab initio calculations of elastic properties of Ru1−xNixAl superalloys

Abstract: Ab initio total energy calculations based on the exact muffin-tin orbitals method, combined with the coherent potential approximation, have been used to study the thermodynamical and elastic proper ...

Coherent potential approximation for spatially correlated disorder

Abstract: The coherent-potential approximation (CPA) is extended to describe satisfactorily the motion of particles in a random potential which is spatially correlated and smoothly varying. In contrast to existing cluster-CPA methods, the present scheme preserves the simplicity of the conventional CPA but leads to a momentum and frequency-dependent self-energy. Its accuracy is checked by a comparison with the exact moments of the Green's function and with the spectral function from numerical simulations. The scheme is applied to excitonic absorption spectra in different spatial dimensions.

Global-local ansatz and dynamical coherent potential approximation study of off-diagonal exciton-phonon coupling

Abstract: We study the generalized Holstein Hamiltonian in the simultaneous presence of diagonal and off-diagonal exciton-phonon couplings using the global-local Ansatz and the dynamical coherent potential approximation under the Hartree approximation. The former yields detailed exciton-phonon correlations for polaron states in the lowest polaron energy band in addition to the state-of-the-art ground-state wave functions, while the latter reveals polaron dynamic information in the form of extended energy spectra and spectral density functions. Lowest polaron energy bands generated by the two methods are compared.

Effect of cationic disorder on the spin polarization in FeMo double perovskites

Abstract: Ordered Sr2FeMoO6 is expected to have complete spin polarization, however all samples usually present some degree of Fe/Mo disorder which reduces the tunneling magnetoresistance in granular samples. It can be inferred that disorder is detrimental to the half-metallicity. We present an electronic approach of this disorder using a model based on a correlated electron picture with localized Fe-spins and conduction electrons interacting with the local spins via a double-exchange–type mechanism. This succeeds in stabilizing a ferromagnetic ground state in the absence of disorder. Disorder is treated within the dynamical mean-field approach which is equivalent to the coherent potential approximation. We shall show how electron disorder affects the density of states and the ground-state properties with a particular emphasis on the spin polarization.

Theory of Orbital Fluctuation and Electrical Transport in Unconventional Charge Density Wave Phase of Skutterudite PrRu4P12

Abstract: A theory of charge density wave (CDW) ordering and the metal–insulator transition in skutterudite PrRu 4 P 12 is presented. We introduce a simplified model Hamiltonian which involves an interaction of conduction electrons with localized f electrons occupying singlet–triplet crystal field levels, and calculate the solutions of the coherent potential approximation. It is shown that the fluctuation of the f -electron occupancy plays very important roles in the mechanism of unusual transport properties. On the one hand, the fluctuation leads to a severe suppression of the quasi-particle gap in the CDW phase and resultingly produces a characteristic temperature dependence of the resistivity. On the other hand, the fluctuation induces breaking of the particle–hole symmetry, giving a drastic enhancement of the Hall conductivity in the CDW phase. The mechanism of sign reversal of the Hall conductivity is discussed in connection with the properties of the fluctuation. Comparisons of the theoretical results with th...

Phonon spectra of Pd(x)Fe(1-x) alloys with transferable force constants.

Abstract: The transferable force constant model of van de Walle et al (2002 Rev. Mod. Phys. 74 11) has been combined with the itinerant coherent potential approximation to calculate the complete phonon spectra and elastic constants in the magnetic type-II alloy PdxFe1−x across the concentration range. The calculated dispersion curves and elastic constants agree very well with the experiments. We discuss the results in the light of the behavior of inter-atomic force constants between various pairs of chemical species. The results demonstrate that the combination of the transferable force constant model and the ICPA method for configuration averaging serve as an efficient and reliable first-principles-based tool to compute the phonon spectra for disordered alloys at any arbitrary concentration.

First-Principles Study on Electronic Structure and Spin State of Rutile (Ti,Co)O2 by Self-Interaction-Corrected Local Density Approximation: Role of Oxygen Vacancy

Abstract: Electronic structure of rutile-TiO2 based dilute magnetic semiconductors (DMS) are investigated within self-interaction-corrected local density approximation (SIC-LDA). These results are compared with those calculated within standard LDA. We employ the Korringa–Kohn–Rostoker method combined with coherent potential approximation. It is found that high-spin state in (Ti,Co)O2 with O vacancy is predicted in the SIC-LDA. This result is in good agreement with the experimental results. As a result, we find that O vacancy in (Ti,Co)O2 is the origin of Co2+ high-spin state and SIC-LDA is indispensable to describe the correct electronic structure and spin state of TiO2-based DMS.

First-principles material design and perspective on semiconductor spintronics materials

Abstract: We investigate the electronic structure and magnetic properties of III–V compound semiconductor based dilute magnetic semiconductors (DMS) from first-principles. The electronic structure of DMS is calculated by using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA). Describing the magnetic properties by a classical Heisenberg model, effective exchange interactions are calculated by applying magnetic force theorem for two impurities embedded in the CPA medium. With the calculated exchange interactions, TC is estimated by using the mean field approximation, the random phase approximation and the Monte Carlo simulation. In the above compounds, the magnetic interactions are well described from double exchange picture. Due to the short-range interactions, high-TC is difficult to achieve in the presently investigated materials. Based on the present results, two strategies towards high-TC are proposed to realize useful DMS materials. One uses spinodal decomposition to realize high blocking temperature in super-paramagnetic blocking phenomena and the other uses co-doping method to realize high concentration doping of magnetic impurities for high-TC.

The magnetism of Fe(1-x)CoxB alloys: First principle calculations

Abstract: Calculations of magnetism are made on transition metal monoboride of Fe ( 1 - x ) Co x B , with the scale 0 ⩽ x ⩽ 1 . In this paper, we calculate the ferromagnetic variation of transition metal monoboride by the method of virtual crystal approximation (VCA), based on density-functional theory (DFT) with generalized gradient approximation (GGA). The variations of ferromagnetism of Fe ( 1 - x ) Co x B alloys by the first-principle spin-polarized calculations are in agreement with experimental results, the prediction of the Stoner model and the Korringa–Kohn–Rostoker (KKR) method with coherent potential approximation (CPA). The spontaneous magnetization decreases with increasing x and vanishes at around x = 0.85 . This complies with earlier findings. The instability at around x = 0.8 is elucidated by electron filling of the Fe 3d orbital both from valence and interstitial electrons.

The phonon spectra and elastic constants of Pd(x)Fe(1-x): an understanding from inter-atomic interactions.

Abstract: Understanding the role of the inter-atomic force constants in lattice dynamics of random binary alloys is a challenging problem. Addressing these inter-atomic interactions accurately is a necessity to obtain an accurate phonon spectrum and to calculate properties from them. Using a combination of ab initio density functional perturbation theory (DFPT) and the itinerant coherent potential approximation (ICPA), an analytic, self-consistent method for performing configuration averaging in random alloys, we model the inter-atomic force constants for Pd0.96Fe0.04 and Pd0.9Fe0.1 alloys based upon the ab initio results and intuitive arguments. The calculated phonon dispersion curves and elastic constants agree very well with the experimental results. Comparison of our results with those obtained in a model potential scheme is also done. The modeling of inter-atomic interactions in random alloys and their roles regarding the phonon-related properties are also discussed in light of these results.

Electronic structure of biopolymers and possible mechanisms of chemical carcinogenesis

Abstract: As an illustration of the applicability of the coherent potential approximation (CPA) method to the treatment of aperiodic biopolymers (DNA and proteins) the results of a calculation with a k-dependent self-energy are presented in the case of the (SN)x and (SN—H)x mixed system. Further we give ab initio SCF LCAO band structures for polyglycine, which were corrected with the aid of the electron polaron model for long-range correlation and with the help of the OÂO method for excitonic effects. Finally, on the basis of the electronic structure of DNA and proteins, different possible mechanisms of chemical carcinogenesis are described.

Neutron Diffraction, Electronic Band Structure, and Electrical Resistivity of Mo3−xRuxSb7

Abstract: Neutron diffraction experiments and Korringa−Kohn−Rostoker with coherent potential approximation electronic band structure calculations as well as electrical resistivity measurements have been performed on polycrystalline Mo3−xRuxSb7 samples for 0 ≤ x ≤ 1. Neutron diffraction studies have been undertaken at room temperature and extended down to 4 K to get a better understanding of the crystalline structure modifications as the Ru content increases. Both structural and chemical characterizations have unambiguously revealed a solubility limit of the Ru atoms close to 0.8. Electronic band structure calculations have provided theoretical evidence of a progressive transition from a metalliclike state (x = 0) toward a semiconducting-like character as x = 1 is approached, although the solubility limit of Ru precludes a crossover to a semiconducting behavior. The theoretical prediction has been experimentally confirmed by low-temperature electrical resistivity measurements from 2 up to 350 K.

A study of the disordered dilute magnetic semiconductors Zn1−xCrxSe and Zn1−xFexSe

Abstract: We report on the electronic structure and exchange interactions in the dilute magnetic semiconductors Zn1−xCrxSe and Zn1−xFexSe. The exchange energies and densities of states were obtained using the layer Korringa–Kohn–Rostoker method and the coherent potential approximation. We find that the dominant exchange in all cases is the super-exchange mechanism, and it is found to be antiferromagnetic for Zn1−xFexSe at all concentrations. For Zn1−xCrxSe the exchange interaction changes from ferromagnetic to antiferromagnetic at high concentrations due to the ability of Cr in the antiferromagnetic state to more effectively bond with the host structure. The strength of the super-exchange coupling in the tight binding model is linear in concentration, which we observe for Zn1−xFexSe. However, as in the case of Zn1−xCrxSe, when the chemical and magnetic interactions are strongly coupled this simple scaling is no longer observed.

Theoretical investigation of the residual electrical resistivity concentration dependence of transuranium metal alloys

Abstract: A many-band conductivity model has been derived and applied to analyze the concentration dependence of the electrical residual resistivity (RR) of several actinides based disordered alloys. It was qualitatively shown that equal probabilities of $s\ensuremath{\rightarrow}d$ and $s\ensuremath{\rightarrow}f$ transitions of scattered conductivity electrons lead to the deviations from Nordgeim's rule observed in these alloys. Numerical evaluation of RR of neptunium, plutonium, americium, and curium based alloys was made within the coherent potential approximation (CPA) derived for the many-band conductivity model. Numerical calculations were made using ab initio obtained bcc-Np and fcc-Pu, Am, and Cm densities of states, as the starting point for the iterative CPA procedure. The results of the RR modeling were compared with available experimental data and allow us to conclude that the proposed model offers promising opportunities to investigate RR of similar classes of alloys.

Seebeck Coefficient Calculated by Kubo–Greenwood Formula on the Basis of Density Functional Theory

Abstract: We present a calculation of Seebeck coefficient derived by Kubo–Greenwood formula with the density functional theory. The electronic structure calculation being the Korringa–Kohn–Rsestoker Green's function method with coherent potential approximation and local density functional method (KKR-CPA-LDA). We show the results for transition metals and metal alloys. In particular, the result for palladium-silver alloy shows good agreement with the experimental data. This calculation shows that the role of mean free time associated to each orbital is quite important to obtain an accurate result of the Seebeck coefficient.

Model for the boron-doping dependence of the critical temperature of superconducting boron-doped diamond

Abstract: We study the concentration dependence of the superconducting critical temperature Tc in a boron-doped diamond. We evaluate the density of states at Fermi level N0 within the dynamical cluster approximation obtaining higher values than from the coherent potential approximation. We discuss Tc as a function of N0 within the BCS, the McMillan and the Belitz theory. The simplified Belitz theory gives the best agreement with experimental data. Since the density of states follows a simple power law for accessible doping concentrations x, the present theory offers an analytical formula for Tc(x).

Effect of chemical disorder on NiMnSb investigated by appearance potential spectroscopy: an experimental and theoretical study

Abstract: The half-Heusler alloy NiMnSb is one of the local-moment ferromagnets with unique properties for future applications. Band structure calculations exclusively predict majority bands at the Fermi level, thus indicating 100% spin polarization there. For the development of functional materials, chemical disorder has to be considered quantitatively. The magnetization, spin polarization and electronic structure are expected to be sensitive to structural and stoichiometric changes. We studied the influence of chemical disorder on the unoccupied electronic density of states of NiMnSb(0 0 1) by the use of the ab initio coherent potential approximation method and results are discussed along with corresponding measurements.

Model for the boron-doping dependence of the critical temperature of superconducting boron-doped diamond

Abstract: We study the concentration dependence of the superconducting critical temperature Tc in a boron-doped diamond. We evaluate the density of states at Fermi level within the dynamical cluster approximation obtaining higher values than from the coherent potential approximation. We discuss the Tc as a function of density of states within the BCS, the McMillan, and the Belitz theory. The simplified Belitz theory gives the best agreement with experimental data. Since the density of states follows a simple power-law for accessible doping concetrations, the present theory offers an analytical formula for Tc(x).

Evidence of Strong Electron Correlations in Gamma-Iron

Abstract: Single-particle excitation spectra of gamma-Fe in the paramagnetic state have been investigated by means of the first-principles dynamical coherent potential approximation theory which has recently been developed. It is found that the central peak in the density of states consisting of the t2g bands is destroyed by electron correlations, and the Mott-Hubbard type correlated bands appear. The results indicate that the gamma-Fe can behave as correlated electrons at high temperatures.

First-principles dynamical CPA to finite-temperature magnetism of transition metals

Abstract: We present here the first-principles dynamical CPA (coherent potential approximation) combined with the tight-binding LMTO LDA+U method towards quantitative calculations of the electronic structure and magnetism at finite temperatures in transition metals and compounds. The theory takes into account the single-site dynamical charge and spin fluctuations using the functional integral technique as well as an effective medium. Numerical results for Fe, Co, and Ni show that the theory explains quantitatively the high-temperature properties such as the effective Bohr magneton numbers and the excitation spectra in the paramagnetic state, and describes the Curie temperatures semiquantitatively.

Electronic states in disordered alloys: Comparison of methods

Abstract: The history of the development of techniques for calculating the electronic states of disordered systems is reviewed and illustrated with the help of model calculations. The position of the coherent potential approximation in this development is pointed out.

Electronic states in some disordered noble metal—transition metal alloys

Abstract: It is shown that the coherent potential approximation is capable of explaining the densities of states functions as observed in the photoemission experiment for noble metal rich alloys of CuPd, CuNi and AgPd. The analysis reveals the importance of the separation in energy between the d-scattering resonances associated with each of the constituent atomic species in these alloys. In all of these alloys the separation between the d-scattering resonances is sufficient to render both the rigid band and the virtual crystal approximations in capable of explaining the observed photoemission data.

Evidence of Strong Electron Correlations in γ-Iron

Abstract: Single-particle excitation spectra of γ-Fe in the paramagnetic state have been investigated by means of the first-principles dynamical coherent potential approximation theory which has recently been developed. It is found that the central peak in the density of states consisting of the t 2g bands is destroyed by electron correlations, and the Mott–Hubbard type correlated bands appear. The results indicate that the γ-Fe can behave as correlated electrons at high temperatures.