Kohn Rostoker Research Articles

Electric Field Gradients of Light Impurities in TiO2 Calculated by the Full Potential KKR Green's Function Method

The electric field gradients (EFGs) of B, N, O and Na in TiO2 were calculated by the full potential Korringa–Kohn–Rostoker (KKR) Greens function method in the framework of the density functional theory. The agreement with the experiments was much improved from the previous calculations that were based on the muffin-tin potential model.

Electric Field Gradients of Fluorides Calculated by the Full Potential KKR Green's Function Method

We report on the calculation of electric field gradients (EFGs) of MgF2, MnF2, CoF2, NiF2 and ZnF2. EFGs were calculated by the full potential Korringa–Kohn–Rostoker (KKR) Green's function method in the framework of the density functional theory. While EFG calculation of these fluorides are rather difficult in the muffin-tin-potential model, due to its sensitivity to the muffin-tin radius, no difficulties arise in the present full potential treatments. EFGs calculated by this method well reproduce experimental data.

Spin-orbit Induced Electric Field Gradients in Magnetic Solids

The spin–orbit induced electric field gradient in cubic ferromagnets has been observed experimentally in the past for many systems. Even its dependence on the orientation of the magnetisation with respect to the crystallographic axes could be convincingly demonstrated. A fully relativistic description is presented that is based on the Korringa–Kohn–Rostoker (KKR) method of band structure calculation. Application of this approach to substitutional 5d-transition metals in Fe led to a satisfying agreement with available experimental data. To allow for a more detailed discussion of the results an analytical model has been developed, that treats spin-orbit coupling as a perturbation.

Linear aspects of the Korringa–Kohn–Rostoker formalism

We present the one-dimensional Korringa–Kohn–Rostoker (KKR) method with the aim ofelucidating its linear features, particularly important in optimizing the numericalalgorithms in energy band computations. The conventional KKR equations based onmultiple scattering theory as well as novel forms of the secular matrix with nearly linearenergy dependency of the eigenvalues are presented. The quasi-linear behaviour of theseeigenvalue functions appears after (i) re-normalizing the wavefunctions in such a way that‘irregular’ solutions vanish on the boundary of the ‘muffin-tin’ segments, and (ii)integrating the full Green function over the whole Wigner–Seitz cell. In addition, using theaforementioned approach we derive a one-dimensional analogue of the generalized Lloydformula.The novel KKR approach illustrated in one dimension can be almost directly applied tohigher dimensional cases. This should open prospects for the accurate KKR band structurecomputations of very complex materials.

Structure and electronic states in Cu nanocontacts

We study structure and electronic states of Cu nanocontacts. The Korringa–Kohn–Rostoker (KKR) Green's function method for low-dimensional systems and the density functional theory in the local density approximation are used in our calculations. Atomic relaxations of nanocontacts and electrodes are performed using ab initio based many body potentials constructed by means of the KKR Green's function method and the tight-binding approach. We demonstrate that nanocontacts exhibit stress oscillations during stretching caused by changes in their structure. Performing ab initio calculations of electronic states in nanocontacts in a fully relaxed geometry, we reveal a strong enhancement in the density of electronic states at the Fermi level before breaking.

Effect of Ni and Co impurities on the electronic structure and magnetic properties of BCC iron

A theory of disordered binary alloys AxB1−x (A=Ni,Co;B=Fe; x∼0.06) is used to determine the changes in the electronic structure and magnetic properties of body centered cubic (BCC) iron induced by doping with nickel and cobalt impurities. This approximation is an extension of the cluster-Bethe lattice method, in which we incorporate electronic correlations, itinerant and localized nature of electrons 3d, and both long-range and short-range chemical correlations. The magnetism is described by means of a Hubbard Hamiltonian that in conjunction with Green's functions techniques is used to calculate local densities of electronic states. For it we take an atom in the real lattice and it is joined to a Bethe's lattice with like coordination number. The magnetic moments on sites occupied for A and B atoms are obtained self-consistently. Nickel and cobalt impurities in BCC iron can provide crucial information on the modification of the electronic band structure and magnetic moments from pure Fe. The results obtained are compared with those of both pure Fe and binary alloys of Co–Fe and Ni–Fe, which have been obtained by other authors using methods such as: first-principles electronic structure calculations using the layer Korringa–Kohn–Rostoker (KKR), the full-potential linearized augmented plane wave method, the KKR coherent potential approximation combined with the local-density functional method and by the tight-binding linear-muffin-tin orbitals method, obtained good agree. These results and other that recently we have published indicate to us that our methodology can be a new alternative for calculations of the electronic structure and magnetic properties of impurities and alloys of ferromagnetic transition metals.

Ab initio calculation of the optical and magneto-optical properties of moderately correlated magnetic solids

An approach for the calculation of the optical and magneto-optical properties of solids based on the one-particle Green function is introduced in the framework of the linear muffin-tin orbital (LMTO) method. The approach keeps all advantages of the more accurate Korringa–Kohn–Rostoker (KKR) scheme as the possibility to account for many-body effects in terms of the non-local energy-dependent self-energy but is numerically much more efficient. In particular an incorporation of the single-site self-energy coming from the dynamical mean-field theory (DMFT) is implemented. An application of the approach to bulk Ni and Fe showed rather good agreement with the experimental data, in contrast with the results of standard local spin density approximation (LSDA) computations.

A high-resolution Compton scattering study of hexagonal zinc

The directional Compton profiles (CP) of hcp single crystal of zinc have been measured along the [00·1], [11·0] and [11·1] directions using high energy synchrotron radiation and high-resolution Compton scanning spectrometer at ESRF (Beamline ID15B). The experimental data were compared with corresponding theoretical Korringa–Kohn–Rostoker (KKR) semi-relativistic calculations and previous low-resolution Compton measurements with the use of high-energy γ radiation from Cs isotope. Both the experimental and theoretical directional anisotropies of Compton profiles, shows very small anisotropy of the electron momentum density in hexagonal zinc, at most half of that presented in the literature for cubic systems. No sharp fermiology-related features of momentum density in Zn predicted by theory, especially in the momenta region below 1 a.u., are observed.

Electronic Structure of MCuSn Compounds with M: Ti, Zr and Hf

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electronic structure of MCuSn compounds with M=Ti, Zr and Hf

The electronic properties of the newly synthesised intermetallic compounds MCuSn with M=Ti, Zr, Hf were theoretically investigated using the self-consistent Korringa–Kohn–Rostoker (KKR) method. The calculations confirm the metallic type conductivity as evidenced from electrical resistivity measurements. A pseudo-gap was found at the Fermi level from band structure calculations. It indicates an anisotropic character of the transport properties in all these compounds.

Conceptual improvements of the KKR method

We review some recent conceptual improvements of the Korringa–Kohn–Rostoker (KKR) Green function method for electronic structure calculations. After an introduction into the KKR–Green function method we present an extension of this method into an accurate full-potential scheme, which allows calculation of forces and lattice relaxations. The additional numerical effort compared to the atomic sphere approximation scales only linear with the number of atoms. In addition, we discuss the recently developed screened KKR method which represents a reformulation of the multiple scattering theory with exponentially decreasing structure constants. This method, which has the same accuracy as the standard KKR method, exhibits strong advantages for two-dimensional systems like multilayers or surfaces, since the numerical effort scales linearly with the number of layers. The strength of both methods is illustrated in typical applications.

Crystal structure, magnetic and electronic properties of Co xFe 1− xMnP system

We present new results on the crystal structure, magnetic and electronic properties of the Co x Fe 1− x MnP series of compounds. X-ray diffraction experiments were performed for x=0.525, x=0.65 and x=0.70. A jump both of the lattice parameters and the unit cell volume was evidenced at the metamagnetic phase transition between antiferromagnetic and ferromagnetic states. Moreover the ( P, T) magnetic phase diagrams for x=0.45, x=0.50 and x=0.525 were determined. Mössbauer spectroscopy experiments were performed for several x contents allowing to determine the hyperfine field, the isomer shift and the quadrupole splitting of the iron sites. Spin-polarised electronic structure calculations were performed using the Korringa–Kohn–Rostoker (KKR) method both for CoMnP and FeMnP, in the ferromagnetic state. The calculated local magnetic moments are 0.25 μ B (Co), 2.90 μ B (Mn) and 0.83 μ B (Fe), 2.89 μ B (Mn) in CoMnP and FeMnP, respectively. The electronic structure of the Co x Fe 1− x MnP solid solutions ( x=0.525 and x=0.70) were calculated using the coherent potential approximation (CPA). The calculated values of the magnetic moments and hyperfine fields are compared with the experimental ones.

Perpendicular magnetism in magnetic multilayer systems

The main aspects of the fully relativistic spin-polarized Screened Korringa–Kohn–Rostoker (KKR) method for layered systems (systems with only two-dimensional translational symmetry) are reviewed including, e.g., a discussion of the Dirac–Kohn–Sham Hamiltonian, magnetic configurations, the inhomogeneous coherent potential approximation (CPA) applicable to such systems, and a short description of the (classical) magnetic dipole–dipole interaction.Based on this type of approach “perpendicular magnetism” is then discussed in terms of characteristic features frequently encountered in studies of the magnetic anisotropy energy of magnetic multilayer systems such as oscillations or reorientation transitions. For this purpose layer-resolved band energy contributions and their discrete Fourier transformations with respect to characteristic parameters are discussed for a few prominent systems.

In-donor complexes in Si and Ge: structure and electric field gradients

The electronic and geometrical structure of neutral InD (D=P, As, Sb) acceptor–donor pairs in silicon and germanium is studied using two complementary ab-initio methods, i.e. the all-electron Korringa–Kohn–Rostoker (KKR) Green's function method and the pseudopotential ab-initio molecular dynamics method. Furthermore the electric field gradients are predicted at the In sites, which can be measured by PAC experiments using the 117Cd/117In probe. The results for the InD pairs are compared with the geometrical structures and in particular with the electric field gradients of the isoelectronic [CdD]− complexes in silicon and germanium. The lattice relaxations calculated by both theoretical methods for the InD complexes agree quite well and are similar to the relaxations for the [CdD]− complexes, which have been studied extensively in PAC experiments using the 111In/111Cd probe. In contrast to this the calculated In electric field gradients are, however, considerably larger than the Cd ones.

A note on a wavefunction formulation of the Korringa - Kohn - Rostoker method for calculating the electronic structure of layered systems

With the formal similarity between three-dimensional layered systems and one-dimensional chains of potential wells in mind, we discuss the wavefunction formulation of the Korringa - Kohn - Rostoker (KKR) formalism in one spatial dimension. We show that screening allows the construction of wavefunctions for such interesting non-periodic chains as two semi-infinite bulks separated by a spacer segment. We also discover a powerful analogue of the Friedel sum usually associated with scattering of electrons by point defects.

Magnetism of free and supported vanadium clusters

Magnetic properties of free and supported Vanadium clusters of up to four atoms have been calculated self-consistently using the density functional theory. For the free clusters we have used the self-consistent field - linear combination of atomic orbitals - molecular orbital theory with a Gaussian basis for the atom. The geometries, together with the preferred spin multiplicities, were optimized by using the method of steepest descent. For supported clusters on Cu(001) and Ag(001), we have used the self-consistent Korringa - Kohn - Rostoker (KKR)-Green's function approach. Both free and supported clusters are found to be magnetic, although the magnetic moments depend strongly on the cluster size. While free clusters have ferromagnetic ground states, the supported V clusters in general prefer antiferromagnetic configurations. The role of inter-atomic distances, coordination, and surface morphology on the magnetic properties of V clusters are discussed.

Theoretical Study of Substitution of Si for Ge in Mn5Ge3

The intermetallic compounds of manganese present a lot of crystallographic and magnetic structures. The ternary compounds Mn 5 (Ge1 _x Six )3 exhibit a complex and rich magnetic phase diagram. Thus this system has been experimentally studied in order to characterize the nature of chemical bondings and magnetic interactions between manganese and metalloid atoms [1]. Mn 5 Ge3 is a two sublattices ferromagnet and Mn 5 Si3 — a complex non-collinear antiferromagnetic material. Both the compounds adopt hexagonal packing with space group P6 3/mcm. We have undertaken band structure calculations of this system with the use of augmented spherical waves (ASW) [2] and Korringa—Kohn—Rostoker (KKR) [3] methods. The disordered systems were described by KKR method with coherent potential approximation (CPA).

Spin Density in Real and Momentum Space in Multi-Atom Alloys by KKR-CPA Method

The application of the spin-polarized version of multiple scattering the- ory for obtaining electron charge and spin densities in both real and momen- tum spaces of concentrated, multi-atom disordered alloys is presented. This method is based on the Korringa—Kohn—Rostoker (KKR) band structure ap- proach and coherent potential approximation (CPA) method. The effective one-electron potential is constructed within local spin density approxima- tion. The magnetic neutron form factors are in real space of our main inter- est. With the recent developments of new synchrotron photon sources, the Compton profile becomes the most interesting target in momentum space. In the most of examples, spin momentum density and its specific structure due � to Fermi surface will be shown. To get accurate enough description in mo- mentum space and quantity like Compton profile, the determination of the Fermi surface must be done with high precision. In this context we show how to apply generalized Lloyd formula for accurate determination of the Fermi level. Also we show how to use efficiently complex energy integration method for the computation of matrix elements, G(τ, τ) or G(p, p), of the KKR—CPA Green . function. Results for the iron—silicon ferromagnetic binary alloys and half-metallic ferromagnetic Heusler alloys are presented. PACS numbers: 71.15.Cr, 71.18.+y, 78.90.+t

Reformulation of the Korringa - Kohn - Rostoker coherent potential approximation for the treatment of space-filling cell potentials and charge-transfer effects

We present a reformulation of the Korringa - Kohn - Rostoker (KKR) coherent-potential approximation (CPA) which affords a number of conceptual and practical advantages over conventional formulations of the theory. In particular, as presented here the method can facilitate application to systems that cannot be described properly by a muffin-tin approximation to the cell potential and require a full potential treatment. Also, the formalism allows the derivation of the KKR CPA self-consistency condition within both a scattering matrix and a Green function approach, leading to uniquely defined species-resolved charge densities and densities of states. As shown in a companion paper, this formulation also allows the treatment of the so-called charge-transfer effects associated with Wigner - Seitz cells in an alloy which contain net amounts of charge.

Fermi surface pressure dependence in potassium

The pressure dependence of the Fermi surface of K has been calculated using the Korringa–Kohn–Rostoker (KKR) method and local exchange potentials adjusted to simulate the effect of a nonlocal operator. Agreement with the experimental data is obtained. Some effective masses have also been calculated.