Rostoker Green Function Method Research Articles

First-principles calculations of finite temperature electronic structures and transport properties of Heusler alloy Co2MnSi

In this study, we calculate the temperature-dependent electronic structures and transport properties of the Heusler alloy Co2MnSi on the basis of the Korringa–Kohn–Rostoker Green's function method combined with the coherent potential approximation (CPA). Temperature effects often have a significant influence on the spin-polarization properties of Heusler alloys. To incorporate the contributions of temperature effects, we first consider lattice vibrations and spin fluctuations. Using CPA, we can replace them with random displacements due to local phonons and local magnetic moment disorders, respectively. In the Co2MnSi Heusler alloy, we found that the band structures are smeared by the electron–phonon scattering process and the half-metallic property is eliminated by magnon excitations from the spin-up to spin-down states. Furthermore, we can estimate the electrical resistivity as a function of temperature in the scheme of linear response theory. Including the local phonon disorder, local moment disorder, and Mn–Co antisite disorder in CPA, we can reproduce the temperature-dependent resistivity observed by experiments.

Antiferromagnetic half metallicity in codoped chalcopyrite semiconductors Cu([formula omitted])Se2 (A and B are 3d transition-metal atoms)

Electronic structures and magnetic properties of group I–III–VI2 chalcopyrite-type compounds Cu(Al1−2xAxBx)Se2 are calculated using the Korringa–Kohn–Rostoker Green's function method, where A (Ti, V, Cr, Mn) and B (Fe, Co, Ni) are 3d transition metal atoms, and x is atomic concentration. We found that codoping of Cr–Co and V–Ni pairs at Al site of host CuAlSe2 exhibit antiferromagnetic (AF) half metallicity with low Curie temperature (TC). The AF half metallic property is supported by nullified net magnetic moment and compensated density of states in the minority spin direction. On the other hand, codoping of Cr–Ni, Mn–Co, V–Co, and Ti–Co pairs at Al site of host CuAlSe2 manifest ferrimagnetic half metallicity with a small net magnetization and keeping antiparallel local spin moments. In Mn–Co case TC is close to room temperature. Besides, Cr–Fe, V–Fe, and Ti–Ni codoping cases lead to an instable magnetic ordering and therefore obtain a disordered local moment (spin-glass like) state.

First principles studies of GeTe based dilute magnetic semiconductors

The electronic structure and magnetic properties of GeTe-based dilute magnetic semiconductors (DMS) are investigated by the Korringa–Kohn–Rostoker Green's function method and the projector augmented wave method. Our calculations for the formation energies of transition metal impurities (TM) in GeTe indicate that the solubilities of TM are quite high compared to typical III–V and II–VI based DMS and that the TM doped GeTe has a possibility of room temperature ferromagnetism with high impurity concentrations. The high solubilities originate from the fact that the top of the valence bands of GeTe consists of the Te-5p anti-bonding states which are favorable to acceptor doping. (Ge, Cr)Te system shows strong ferromagnetic interaction by the double exchange mechanism and is a good candidate for DMS with high Curie temperature. Additionally, in the case of (Ge, Mn)Te with the d5 configuration, by introducing the Ge vacancies the p-d exchange interaction is activated and it dominates the antiferromagnetic superexchange, resulting in ferromagnetic exchange interactions between Mn. This explains recent experimental results reasonably. Based on the accurate estimation of the Curie temperatures by Monte Carlo simulation for the classical Heisenberg model with the calculated exchange coupling constants, we discuss the relevance of the TM doped GeTe for semiconductor spintronics.

Electronic picture of spin-polarized tunneling with a Cr tip

We use spin-resolved scanning tunneling spectroscopy with a Cr-covered W-tip to investigate the magnetic switching of single Co islands on Cu ( 1 1 1 ) in polar magnetic fields. The observed hysteretic curve resembles a shape which is well known from so-called butterfly curves of tunneling magnetoresistance measurements. This indicates that not only the Co-island but also the Cr-tip changes its spin orientation in response to the applied magnetic field. For the interpretation of the experimental observations, we perform ab initio calculations by means of the Korringa–Kohn–Rostoker Green's function method. The calculations demonstrate that the Cr-tip is not a perfect antiferromagnet and has an uncompensated magnetic moment which changes its spin orientation continuously due to the applied magnetic field.

The energies and life times of magnons in bulk iron and one-monolayer Fe film

The dynamic magnetic susceptibility is calculated for bulk bcc iron, a supported monolayer Fe/W(1 1 0) and corresponding free standing Fe monolayer. The calculational approach is based on linear response density functional theory formulated within the framework of Korringa–Kohn–Rostoker Green's function method. The calculated susceptibilities are analyzed in terms of dispersion and damping of spin-waves. The results for bulk Fe are in good agreement with experiment and previous theoretical studies. In contrast to the bulk phase, the spin-wave excitations in thin films are well defined in the whole two-dimensional Brillouin zone. It is shown that a non-magnetic substrate changes strongly both the dispersion and damping of the spin-waves.

Medium-ranged interactions of transition-metal (3d and 4d) impurity pairs in Al and atomic structures of Al-rich Al–transition-metal alloys

We give systematic ab initio calculations for the interaction energies (from first to eighth neighbors) of impurity pairs X–X (X = Sc–Zn, Y–Cd) in Al and discuss the micromechanism of the structural stability of Al-rich AlX alloys. The calculations are based on the generalized gradient approximation in the density-functional theory and employ the all-electron full-potential Korringa–Kohn–Rostoker Green's function method for point defects, which guarantees the correct embedding of the cluster of impurities and vacancies in an otherwise perfect crystal. We show: (1) the fundamental features of phase diagrams of these alloys, such as ordering and segregation, are understood by the sign of first-neighboring X–X interaction energies; (2) the structural stability of Al-rich AlX alloys such as L1 2 (Al 3Sc and Al 3Y), DO 22 (Al 3V and Al 3Nb), and Mackay icosahedron in AlMn quasicrystal, are understood qualitatively by using the medium-ranged and oscillating X–X interactions, due to the strong sp-d (Al–X) hybridization; (3) the strength and oscillating behavior of the medium-ranged X–X interaction energies are specified very well by the d-electron numbers of X impurities.

Peculiarity of metal–insulator transition due to composition change in the semiconducting TiCo 1− xNi xSb solid solution : I. Electronic structure calculations

The Korringa–Kohn–Rostoker Green's function method within the coherent-potential approximation was applied to calculate the electronic properties of recently synthesised semi-Heusler TiCo 1− x Ni x Sb solid solution. As evidenced parallel from electrical resistivity measurements, the calculations confirm a semiconductor type behaviour for TiCoSb compound and a metallic type one for alloys containing Ni. The electronic conductivity was found to originate from the d-electrons of Ti, Co and Ni, mainly.

Electronic Structure of the Ti1‐xScxNiSn and Zr1‐xScxNiSn Solid Solutions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electronic structure of the Ti 1− xSc xNiSn and Zr 1− xSc xNiSn solid solutions

The electronic properties of the Ti 1− x Sc x NiSn and Zr 1− x Sc x NiSn solid solutions were investigated using the Korringa–Kohn–Rostoker Green's function method within the coherent potential approximation. The calculations confirm the metallic type conductivity of the Sc content compounds as evidenced from electrical resistivity measurements and show that the main source of the electron conductivity originates from the d-electrons of Ti(Zr) and Sc.

Ab Initio approach for atomic relaxation in supported magnetic clusters

We present a newly developed scheme for atomic relaxations of magnetic supported clusters. Our approach is based on the full potential Korringa–Kohn–Rostoker Green's function method and the second moment tight-binding approximation for many-body potentials. We demonstrate that only a few iterations in ab initio calculations are necessary to find an equilibrium structure for supported clusters. As an example, we present our results for small Co clusters on Cu(001). Changes in the electronic and magnetic states of the clusters due to atomic relaxations are revealed.

First-principles calculations for vacancy formation energies in Cu-based alloys

We present the first-principles calculations for the vacancy formation energies in Cu-based alloys. The calculations are based on the generalized-gradient approximation in density functional theory and apply the Korringa–Kohn–Rostoker Green's function method for impurity calculations. The alloy effect is taken into account by the direct configurational averaging method. The calculated results reproduce quantitatively well the experimental vacancy formation energies obtained from the positron annihilation measurement.

First-principles calculations for vacancy formation energies in Cu-based alloys

Abstract We present the first-principles calculations for the vacancy formation energies in Cu-based alloys. The calculations are based on the generalized-gradient approximation in density functional theory and apply the Korringa–Kohn–Rostoker Green's function method for impurity calculations. The alloy effect is taken into account by the direct configurational averaging method. The calculated results reproduce quantitatively well the experimental vacancy formation energies obtained from the positron annihilation measurement.

The magnetic properties in transition metal-doped chalcopyrite semiconductors

We calculated the chemical trends of transition metal-doped chalcopyrite diluted magnetic semiconductors by use of the Korringa–Kohn–Rostoker Green's function method and the coherent potential approximation combined with the local density approximation. The ferromagnetism was stable in V- and Cr-doped chalcopyrite diluted magnetic semiconductors (DMSs). In the cases of Fe and Co doping, however, the spinglass-like state was realized. On the other hand, in the cases of Mn-doped I–III–VI 2 and II–IV–V 2-type DMSs, the ground state was ferromagnetic and spinglass-like states, respectively. We explained these chemical trends by considering the electron configurations and the crystal field effect. Moreover, we evaluated Curie temperatures ( T cs) of Cr-doped chalcopyrite semiconductors and expected that T cs of Cu(Al,Cr)S 2 and Ag(Ga,Cr)S 2 were much higher than room temperature.

Spin-filter effect in metallic nanowires

We present ab initio calculations for metallic nanowires with a diameter of few atoms. The electronic structure is calculated using the screened Korringa Kohn Rostoker Green's function method, while electronic transport properties are obtained using a Green's function formulation of the Landauer formalism. We focus on the effect of scattering due to transition metal impurities on the conductance of a Cu wire. For a single defect, our results show a reduction of the transmission for energies at the impurity d state and due to the spin-polarization conductance is different for the two-spin directions causing a spin-filter effect. For a defect pair, quantum interference effects lead to a complicated energy dependence of the conductance.

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

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.

First-principles calculations for point-defect energies in metals and phase diagrams of binary alloys

We discuss the present status of the first-principles electronic-structure calculations for defect energies in metals. The calculations apply density functional theory in the generalized-gradient approximation of Perdew and Wang, together with a full-potential version of Korringa–Kohn–Rostoker Green's function method, developed by the Jülich group. It is shown that: (1) the present calculations reproduce very well the experimental results for vacancy formation energies in metals, as well as the bulk properties such as equilibrium lattice parameters and bulk moduli of metals; and (2) the type of the phase diagram of a binary A–B alloy can be characterized by the interaction energies between a pair of impurity B (A) atoms in the host metal A (B). The observed temperature dependence of the solid solubility limit of Rh in Pd is also reproduced very well by the free-energy calculations based on the cluster variation method with the pair- (up to the eighth neighbor) and many-body (up to a tetrahedron of first-nearest neighbors) interaction energies, all of which are determined by the present first-principles calculations.

Effect of lattice relaxations on the hyperfine fields of heavy impurities in Fe

We present first-principles calculations of hyperfine fields of heavy impurities in BCC Fe. In particular, the effect of lattice relaxations on the calculated hyperfine fields are studied. The calculations are based on a full-potential Korringa–Kohn–Rostoker Green's function method for defects and employ the local spin–density approximation for the exchange and correlation effects. The non-spherical parts of the potential and the charge density are treated correctly, while the forces are calculated by an ionic version of the Hellmann–Feynman theorem. Lattice statics methods are used to describe the longer ranged relaxations. The calculated hyperfine fields of 5sp and 6sp elements are compared with the available experimental data and it is shown that the inclusion of lattice relaxations in the calculation improves the agreement with experiments.

First-principles calculations for vacancy formation energies in Ni and Fe: non-local effect beyond the LSDA and magnetism

The first-principles calculations are presented for the vacancy formation energies ( E V F) in the nonmagnetic and ferromagnetic states (NM and FM) of Ni (FCC) and Fe (BCC). The calculations are based on density-functional theory (DFT) and the full-potential Korringa–Kohn–Rostoker Green's function method for impurities. The non-local effect (NLE) beyond the local-spin-density approximation for DFT is taken into account within the generalized-gradient approximation of Perdew and Wang (PW91-GGA). The magnetism, being the difference of FM- E V F with NM- E V F, is large (∼0.6 eV) for Fe, but small (∼0.1 eV) for Ni, while the NLE is large (∼0.6 eV) for both transition metals. We found that the measured E V F at high temperatures, corresponding to E V F in the paramagnetic state, is close to the calculated NM- E V F.

First-principles calculations for vacancy formation energies in Cu and Al; non-local effect beyond the LSDA and lattice distortion

We show ab initio calculations for vacancy formation energies in Cu and Al. The calculations are based on density-functional theory and the full-potential Korringa–Kohn–Rostoker Green's function method for impurities. The non-local effect beyond the local-spin-density approximation (LSDA) for density-functional theory is taken into account within the generalized-gradient approximation (GGA) of Perdew and Wang. The lattice relaxation around a vacancy is also investigated using calculated Hellmann–Feynman forces exerted on atoms in the vicinity of a vacancy. We show that the GGA calculations reproduce very well the experimental values of vacancy formation energies and bulk properties of Cu and Al, as they correct the deficiency of LSDA results (underestimation of equilibrium lattice parameters, overestimation of bulk moduli, and vacancy formation energies). It is also shown that the GGA calculations reduce the LSDA results for the lattice relaxation energy for a vacancy in Cu.

Ab initio calculation of forces and lattice relaxations in metallic alloys

Abstract We present first-principles calculations of forces and lattice relaxations in bcc Fe. In particular, relaxations around 3d, 4d and 5d transition metal impurities are calculated. The calculations are based on a full-potential Korringa-Kohn- Rostoker Green's function method for defects and employ the local spin density approximation for the exchange and correlation effects. The non- spherical parts of the potential and the charge density are treated correctly, while the forces are calculated by an ionic version of the Hellmann-Feynman theorem. Lattice statics methods are used to describe the longer ranged relaxations. The results are compared with lattice parameter measurements for the volume changes. Because of the correct treatment of the sharp shape of the Wigner-Seitz cell, the angular momentum expansion coefficients of the cell potential have discontinuities in the first derivative, which cause some complications when solving the radial equations. An effective method to get around these discontinuities is introduced.