Showing papers on "Coherent potential approximation published in 2001"

Anisotropic Lattice Distortions in Random Alloys from First-Principles Theory

Abstract: Within the framework of the exact muffin-tin orbitals (EMTO) theory we have developed a new method to calculate the total energy for random substitutional alloys. The problem of disorder is treated within the coherent potential approximation (CPA), and the total energy is obtained using the full charge density (FCD) technique. The FCD-EMTO-CPA method is suitable for determination of energy changes due to anisotropic lattice distortions in random alloys. In particular, we calculate the elastic constants of the Cu-rich face centered cubic Cu-Zn alloys ( alpha-brass) and optimize the c/a ratio for the hexagonal Zn-rich alloys for both the epsilon and eta phases.

Stabilization of Ferromagnetic States by Electron Doping in Fe-, Co- or Ni-Doped ZnO

Abstract: Detailed guidelines for controlling magnetic states in ZnO-based diluted magnetic semiconductors are given based on ab initio electronic structure calculations within the local spin density approximation using the Korringa-Kohn-Rostoker method. Effects of disorder were taken into account by the coherent potential approximation. It was found that the ferromagnetic state was stabilized by electron doping in the case of Fe-, Co- or Ni-doped ZnO. From the view point of practical applications, it is possible to realize a high-Curie-temperature ferromagnet, because n-type ZnO is easily available.

Anisotropic lattice distortions in random alloys from first-principles theory - art. no. 156401

Abstract: Within the framework of the exact muffin-tin orbitals EMTO) theory we have developed a new method to calculate the total energy for random substitutional alloys. The problem of disorder is treated within the coherent potential approximation (CPA), and the

Material Design of GaN-Based Ferromagnetic Diluted Magnetic Semiconductors

Abstract: Material design of GaN-based ferromagnetic diluted magnetic semiconductors is given based on ab initio calculations within the local spin density approximation. The electronic structure of 3d-transition-metal-atom-doped GaN was calculated by the Korringa-Kohn-Rostoker method combined with the coherent potential approximation. It was found that the ferromagnetic ground states were readily achievable in V-, Cr- or Mn-doped GaN without any additional carrier doping treatments. A simple explanation on the systematic behavior of the magnetic states in GaN-based diluted magnetic semiconductors is also given.

Origin of the boson peak in systems with lattice disorder.

Abstract: The origin of the boson peak in models with force-constant disorder has been established by calculations using the coherent potential approximation The analytical results obtained are supported by precise numerical solutions The boson peak in the disordered system is associated with the lowest van Hove singularity in the spectrum of the reference crystalline system, pushed down in frequency by disorder-induced level-repelling and hybridization effects

Electronic structure and ferromagnetism of transition-metal-impurity-doped zinc oxide

Abstract: The ferromagnetism in ZnO-based diluted magnetic semiconductors (DMSs) is investigated based on the first principles calculations. The electronic structure of a ZnO-based DMS is calculated using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation based on the local density approximation. The stability of the ferromagnetic state compared with that of the spin-glass state is systematically investigated by calculating the total energy difference between them. It is found that the ferromagnetic state is more stable than the spin-glass state in V-, Cr-, Fe-, Co- or Ni-doped ZnO without any additional carrier doping treatments. In the case of the Mn-doped ZnO, the spin-glass state is stable at a carrier concentration of 0%, but the ferromagnetic state is stabilized by the hole doping treatment. Analyzing the calculated density of states, it is proposed that the origin of the stabilization of the ferromagnetism is a double-exchange mechanism.

Systematic and Causal Corrections to the Coherent Potential Approximation

Abstract: The dynamical cluster approximation (DCA) is modified to include disorder. The DCA incorporates nonlocal corrections to local approximations such as the coherent potential approximation (CPA) by mapping the lattice problem with disorder, and in the thermodynamic limit, to a self-consistently embedded finite-sized cluster problem. It satisfies all of the characteristics of a successful cluster approximation. It is causal, preserves the point-group and translational symmetry of the original lattice, recovers the CPA when the cluster size equals one, and becomes exact as ${N}_{c}\ensuremath{\rightarrow}\ensuremath{\infty}.$ We use the DCA to study the Anderson model with binary diagonal disorder. It restores sharp features and band tailing in the density-of-states, which reflect correlations in the local environment of each site. While the DCA does not describe the localization transition, it does describe precursor effects of localization.

Materials Design of Transparent and Half-Metallic Ferromagnets in V- or Cr-Doped ZnS, ZnSe and ZnTe without P- or N-type Doping Treatment

Abstract: We propose materials design to fabricate the transparent and half-metallic ferromagnets in V or Cr-doped ZnS, ZnSe and ZnTe based upon ab initio electronic structure calculation with the local-spin-density-functional approximation. The electronic structure and the magnetic properties of 3d-transition-metal-atom-doped II-VI compound semiconductors of ZnS, ZnSe and ZnTe are systematically calculated using the Korringa-Kohn-Rostoker method with taking into account the substitutional disorder by the coherent potential approximation. It is shown that V or Cr-doped ZnS, ZnSe and ZnTe are ferromagnetic without p- or n-type doping treatment, however, Mn-, Fe-, Co- or Ni-doped ZnS, ZnSe and ZnTe are the spin-glass states.

Thermodynamics of the Cr-Ta-W system by combining the Ab Initio and CALPHAD methods

Abstract: The thermodynamic properties of alloys can be described from the knowledge of the underlying lattice and the atomic numbers of the alloy species by using a first-principles electronic structure approach based on the tight-binding linear muffin-tin orbital method within the coherent potential approximation (TB-LMTO-CPA) and the local density approximation of density functional theory. The generalized perturbation method (GPM) permits direct mapping of the configurational part of the effective one-electron Hamiltonian onto an Ising-like model thus insuring the necessary link between quantum mechanics and statistical thermodynamics. To test the practical application of this approach to multi-component alloys, data assessment has been successfully performed for the Cr-Ta-W alloy system by using the thermodynamic results derived from the TB-LMTO-CPA-GPM and the cluster variation method (CVM) in the tetrahedron approximation for Ta-W alloys as functions of temperature and concentration. These later results predict B2 ordering for the bcc-based Ta-W system with a maximum ordering temperature near 1000 K at 43 at.% Ta. The output thermodynamics were converted to a Redlich-Kister/Bragg-Williams format with an acceptable level of accuracy. The results were then combined with those of the CALPHAD description of the Cr-W and Cr-Ta systems to calculate isothermal sections of the ternary phase diagram of the Cr-Ta-W system.

Electronic structure and magnetic interactions in Mn doped semiconductors

Abstract: The electronic structures of several magnetic semiconductors were calculated using first-principles based electronic structure techniques. Calculations were performed both for periodic supercells and for disordered systems which were treated within the coherent potential approximation. Our results differ from the conventional atomic model for the Mn impurity. We find strong hybridization between the majority Mn d states and the majority valence band. We find that substitutional Mn impurities add states and electrons to the majority valence band but not to the minority. We find relatively strong carrier induced magnetic interactions between impurities. The sign of these interactions may however, change sign as a function of the distance between the impurities.

Two-dimensional folding technique for enhancing Fermi surface signatures in the momentum density: Application to Compton scattering data from an Al-3 at. % Li disordered alloy

Abstract: We present a technique for enhancing Fermi surface (FS) signatures in the two-dimensional (2D) distribution obtained after the 3D momentum density in a crystal is projected along a specific direction in momentum space. These results are useful for investigating fermiology via high-resolution Compton scattering and positron annihilation spectroscopies. We focus on the particular case of the (110) projection in a fcc crystal where the standard approach based on the use of the Lock-Crisp-West (LCW) folding theorem fails to give a clear FS image due to the strong overlap with FS images obtained through projection from higher Brillouin zones. We show how these superposed FS images can be disentangled by using a selected set of reciprocal lattice vectors in the folding process. The applicability of our partial folding scheme is illustrated by considering Compton spectra from an Al--3 at. % Li disordered alloy single crystal. For this purpose, high-resolution Compton profiles along nine directions in the (110) plane were measured. Corresponding highly accurate theoretical profiles in Al--3 at. % Li were computed within the local density approximation (LDA)--based Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) first-principles framework. A good level of overall accord between theory and experiment is obtained, some expected discrepancies reflecting electron correlation effects notwithstanding, and the partial folding scheme is shown to yield a clear FS image in the (110) plane in Al--3 at. % Li.

Hyperfine Interactions and Magnetism of 3d Transition-Metal-Impurities in II–VI and III–V Compound-Based Diluted Magnetic Semiconductors

Abstract: The electronic structure of II–VI and III–V compound-based diluted magnetic semiconductors is calculated based on the local density approximation (LDA) using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. The magnetism of 3d transition-metal-atom-doped ZnO, ZnS, ZnSe, ZnTe, GaN, GaAs is investigated from first-principles. It is suggested that the double exchange mechanism stabilizes the ferromagnetism in these DMSs. In order to obtain microscopic information on the electronic structure of transition-metal-impurities in semiconductors, the hyperfine field of respective impurities in each host material is calculated. It is found that the agreement with the experimental values is not good, probably because the LDA is not sufficient to describe the core states of transition metals. However, it is suggested that the hyperfine fields clearly reflect the local magnetic moments for 3d impurities.

Ferromagnetic transition in a double-exchange system containing impurities

Abstract: We study the ferromagnetic transition in a three-dimensional double-exchange model containing impurities. The influence of both spin fluctuations and the impurity potential on conduction electrons is described in a coherent potential approximation. In the framework of the thermodynamic approach we construct a Landau functional for the system ``electrons (in disordered environment) + core spins.'' Analyzing the Landau functional we calculate the temperature of the ferromagnetic transition ${T}_{C}$ and paramagnetic susceptibility \ensuremath{\chi}. For ${T}_{C},$ we thus extend the result obtained by Furukawa in the framework of the dynamical mean-field approximation, with which our result coincides in the limit of zero-impurity potential. We find that alloy disorder, able to produce a gap in the density of electron states, can substantially decrease ${T}_{C}$ with respect to the case of no impurities.

First principles calculations of the electronic structure of Fe/sub 1-x/Co/sub x/Pt

Abstract: The L1/sub 0/ alloys CoPt and FePt have strong uniaxial magneto-crystalline anisotropy as a consequence of the layering of Co or Fe and Pt planes. This makes these compounds of interest in permanent magnet applications. In this work we present the results of a study of the magnetic properties of the L1/sub 0/ alloys Fe/sub 1-x/Co/sub x/Pt for the range of compositions from FePt to CoPt. Local spin density calculations using the coherent potential approximation are used to follow trends in the magneto-crystalline anisotropy and magnetic moments across the composition range. Total energy calculations of ordered and disordered alloys are used to develop a pseudo-binary phase diagram of the Fe/Co layer predicting regions of phase separation in this layer at temperatures below 650/spl deg/C.

Many-body CPA for the Holstein double-exchange model

Abstract: A many-body coherent potential approximation (CPA) previously developed for the double exchange (DE) model is extended to include coupling to local quantum phonons. The Holstein-DE model studied (equal to the Holstein model for zero Hund coupling) is considered to be a simple model for the colossal magnetoresistance manganites. We concentrate on effects due to the quantisation of the phonons, such as the formation of polaron subbands. The electronic spectrum and resistivity are investigated for a range of temperature and electron-phonon coupling strengths. Good agreement with experiment is found for the Curie temperature and resistivity with intermediate electron-phonon coupling strength, but phonon quantisation is found not to have a significant effect in this coupling regime.

Formation of a weak ferromagnetic state in Y(Co1-xAlx)2 compounds: a coherent potential approximation study

Abstract: The band structure of substitutionally disordered Y(Co1-xAlx)2 has been calculated for various concentrations in the range 0≤x≤0.25 employing the coherent potential approximation embodied in an all-electron tight-binding linear muffin-tin orbital method. On the basis of the results, we provide a new explanation for the formation of weak ferromagnetic moments in these compounds. The discussion of the non-spin-polarized calculated densities of states is supported by direct evidence of the weak ferromagnetic states for certain lattice constants and Al concentrations. The roles of the disorder and the volume effects are discussed.

Dynamics of a single electron in the disordered Holstein model

Abstract: We study, at zero temperature, the dynamics of a single electron in a Holstein model augmented by site-diagonal, binary-alloy{endash}type disorder. The average over the phonon vacuum and the alloy configurations is performed within a generalized dynamic coherent potential approximation. We present numerical results for a Bethe lattice with infinite coordination number. In particular, we investigate, in the intermediate electron-phonon coupling regime, the spectral and diffusion properties in the vicinity of the high-energy edge of the lowest polaronic subband. To characterize the diffusion properties, we define a spectrally resolved delocalization time, which is, for a given energy, the characteristic time scale on which the electron leaves a given site. We find the delocalization times substantially enhanced for states with a large phonon content, i.e., in the absence (presence) of alloy-type disorder at the high-energy edge(s) of the polaronic subband (minisubbands). According to their delocalization times, we discriminate between {open_quotes}fast{close_quotes} quasiparticlelike and {open_quotes}sluggish{close_quotes} defectlike polaron states and qualitatively address the issue of trapping of an electronic carrier.

Simulations of decohesion and slip of the Σ3⟨111⟩ grain boundary in tungsten with non-empirically derived interatomic potentials: the influence of boron interstitials

Abstract: Monte Carlo atomistic simulations of the properties of Σ3111 grain boundaries in W are carried out. We demonstrate the influence of boron additive on the resistance of the grain boundary with respect to different shifts. The interatomic potentials used in these simulations are obtained from ab initio total-energy calculations. These calculations are performed in the framework of density functional theory in the coherent potential approximation. A recursion procedure for extracting A-B-type interatomic potentials is suggested.

First-principles theory of the temperature and compositional dependence of atomic short-range order in disordered Cu-Pd alloys

Abstract: We combine the first-principles, Korringa-Kohn-Rostoker coherent potential approximation based calculations of compositional fluctuations with a statistical mechanical ring approximation to study the temperature ~T! and composition ~c! dependence of the atomic short-range order ~SRO! in disordered, face-centred cubic, Cu-Pd alloys. The fourfold splitting of SRO peaks around the equivalent X(0,1,0) points in reciprocal space is obtained in a wide T-c region. Such splitting is shown to be an ‘‘energy’’ effect caused by the absolute minima of the Fourier transform of the effective atomic interactions and related previously to the existence of nested sheets of the disordered alloy’s Fermi surface. However, we find that the T dependence of the SRO peak position is mostly an ‘‘entropy’’ effect. Both the calculated T and c dependences of the SRO peaks position are in good correspondence with the experimental data. The real-space effective atomic interactions and SRO parameters indicate the tendency for longer-period structures with increasing Pd concentration, as observed.

Light scattering from disordered overlayers of metallic nanoparticles

Abstract: We develop a theory for light scattering from a disordered layer of metal nanoparticles resting on a sample. Averaging over different disorder realizations is done by a coherent potential approximation. The calculational scheme takes into account effects of retardation, multipole excitations, and interactions with the sample. We apply the theory to a system similar to the one studied experimentally by Stuart and Hall [Phys. Rev. Lett. 80, 5663 (1998)] who used a layered $\mathrm{Si}/{\mathrm{SiO}}_{2}/\mathrm{Si}$ sample. The calculated results agree rather well with the experimental ones. In particular we find conspicuous maxima in the scattering intensity at long wavelengths (much longer than those corresponding to plasmon resonances in the particles). We show that these maxima have their origin in interference phenomena in the layered sample.

Relativistic calculation of magnetic linear response functions using the Korringa–Kohn–Rostoker Green's function method

Abstract: The relativistic KKR (Korringa–Kohn–Rostoker) Green's function method of band-structure calculation supplies an extremely flexible basis for calculating magnetic linear response functions of solids. An important feature of this approach is that it accounts properly for the influence of all relativistic effects. A brief introduction to this formalism is presented, together with some recent extensions to it. In particular, the inclusion of the orbital magnetization density induced by an external magnetic field allows a direct comparison with experiment for many different properties. This is demonstrated for the induced magnetic form factor, the magnetic susceptibility and the Knight shift of transition metals. A further appealing feature of the KKR formalism is that it is applicable in principle to any complex system. This property is exemplified by applications for the disordered alloy systems AgxPd1−x and AgxPt1−x that are treated with the help of the coherent potential approximation alloy theory.

Effects of Ta on the magnetic structure of permalloy

Abstract: In permalloy (Py) based heterostructures Ta has deleterious effects on the magnetic properties resulting in magnetic dead layers. Using the Korringa–Kohn–Rostoker coherent potential approximation method, it is shown that the average moment of Py (Ni0.8Fe0.2) is rapidly quenched by Ta substitutional additions. Locally self-consistent multiple-scattering method calculations for large supercell models of Py0.9Ta0.1 show that Ta additions also result in substantial fluctuations in the size of the local moments. The configuration dependent reductions in the local moments are larger for Ni than for Fe sites and are the largest for Ni sites closest to Ta.

Ordering and segregation in X Pt ( X =V, Cu, and Au) random alloys

Abstract: We examine the phase stability and the ordering tendencies of some Pt-based fcc random alloys using the generalized perturbation method (GPM) implemented in the linear muffin-tin orbitals (LMTO) basis. The reference medium for the GPM is chosen as the completely disordered state of the alloy and its electronic structure is described in the coherent potential approximation (CPA). Ordering tendencies and phase stability are examined via effective pair interactions and their lattice Fourier transforms. Relativistic effects on the ground state cohesive properties and the ordering tendencies are determined by carrying out nonrelativistic and fully relativistic (in some cases also scalar-relativistic) LMTO-CPA calculations. In all cases considered, namely $X\mathrm{Pt}$ with $X=\mathrm{V},$ Cu, and Au, the correct ordering tendency is obtained. The ordering tendency is found to be somewhat overestimated in the GPM. Relativistic effects are found to be most prominent in AuPt, where the nonrelativistic LMTO-CPA-GPM description shows a tendency towards ${L1}_{1}$ ordering and the correct result, i.e., phase segregation, is obtained only in the fully relativistic description. The sensitivity of the ordering tendency to factors such as lattice relaxation and volume per atom is examined briefly. Finally, the effect on the phase stability of adding a third component, such as V or Au to CuPt alloy, is studied by extending the formalism to the case of a ternary alloy.

Coherent potential approach to exchange-induced band splitting in diluted magnetic semiconductors under a saturating magnetic field

Abstract: Applying the coherent potential approximation (CPA) to a model which describes magnetic and chemical disorder in an AII1-xMnxBVI-type diluted magnetic semiconductor (DMS) with a strong applied magnetic field, we calculate the band-edge energy shift as a function of the Mn concentration (x) The effects of the band offset and exchange interaction on the extended carrier state in a DMS are studied systematically The apparent enhancement of the p-d exchange interaction which is observed in Cd1-xMnxS is explained well as a multiple-scattering effect which is significant for small x in disordered systems The dominant origin is not a large attractive chemical potential at the Mn site, but a large exchange energy relative to the bandwidth

CPA density of states and conductivity in a double-exchange system containing impurities

Abstract: We study density of states and conductivity of the doped double-exchange system, treating interaction of charge carriers both with the localized spins and with the impurities in the coherent potential approximation. It is shown that under appropriate conditions there is a gap between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phase. This can explain metal-insulator transition frequently observed in manganites and magnetic semiconductors. Activated conductivity in the insulator phase is numerically calculated.

Magnetic anisotropy of disordered Mn-Pt layered surface compounds on Pt(111)

Abstract: The magnetic moments and the anisotropy energy of surface compounds formed by MnxPt(12x) (x P@0,1# )o n aP t~111! substrate are calculated by means of the fully relativistic spin-polarized screened Korringa-Kohn-Rostoker method. The binary compositions are modeled within the coherent potential approximation. Different Mn concentrations at each layer have been considered in order to understand the influence on the magnetic properties of both the chemical environment of the Mn atoms and the segregation of Pt from the bulk. Special emphasis is devoted to the study of the layered sequences Pt/MnxPt(12x) .