Kohn Rostoker Coherent Potential Approximation Research Articles

An investigation of high entropy alloy conductivity using first-principles calculations

The Kubo–Greenwood equation, in combination with the first-principles Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) can be used to calculate the DC residual resistivity of random alloys at T = 0 K. We implemented this method in a multiple scattering theory based ab initio package, MuST, and applied it to the ab initio study of the residual resistivity of the high entropy alloy AlxCoCrFeNi as a function of x. The calculated resistivities are compared with experimental data. We also predict the residual resistivity of refractory high entropy alloy MoNbTaVxW. The calculated resistivity trends are also explained using theoretical arguments.

Hydrogen Adsorption on Ordered and Disordered Pt-Ni Alloys

The bulk properties and chemical reactivity of disordered Pt-Ni alloys in the A1 (fcc) structure are investigated using different methods: Virtual Crystal Approximation (VCA), Korringa–Kohn–Rostoker Coherent Potential Approximation (KKR-CPA), and large explicit supercells generated using Super-Cell Random Approximates (SCRAPs). While VCA predicts lattice constants that closely follow Vegard’s law, the large supercells and KKR-CPA predict lattice constants that are consistently larger than Vegard’s law. KKR-CPA results closely agree with those from the large supercells for the disordered alloys, producing similar projected density of states and magnetic moment across the composition range. For instance, while VCA predicts the disordered alloys to be non-magnetic at a Pt concentration (xPt) ≥ 0.5, KKR-CPA and SCRAPs predict the disordered alloys to remain ferromagnetic to higher Pt concentrations. As xPt decreases, the adsorption of H becomes more exothermic on bulk-terminated (111) surfaces but less exothermic on Pt monolayer-terminated (111) surfaces due largely to strain effects. (111) surfaces cut from the large supercells predict average H adsorption energies on the disordered alloys similar to those on the ordered phases of the same compositions, while VCA predicts H adsorption to be more exothermic.

Effect of Dopant Concentrations on Magnetic States of Ti, Mn, And Ni Doped Iii-V Zincblende Compounds

The effect of Ti, Mn, and Ni dopants concentration on various magnetic states of III-V zincblende compounds (Al1-xTx)M is presented by calculation of multiple scattering theory based on Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA), where T denotes Ti, Mn, and Ni atoms and M represents As and P atoms. The calculations reveal the phase changes in various magnetic states, which are solely controlled by dopant concentrations. The turning points of these behaviors are reported, which are responsible for the change of stable ferromagnetic (FM) state to unstable disordered local moment (DLM) state. The Curie temperature, magnetic moments, and density of states at different concentrations of the dopants are calculated and explain the concurrency with magnetic phase stability.
 Journal of Bangladesh Academy of Sciences, Vol. 42, No. 2, 177-182, 2018

Correlation of electronic structure and magnetic moment in Ga1−xMnxN : First-principles, mean field and high temperature series expansions calculations

Self-consistent ab initio calculations based on density-functional theory and using both full potential linearized augmented plane wave and Korring–Kohn–Rostoker-coherent potential approximation methods, are performed to investigate both electronic and magnetic properties of the Ga1−xMnxN system. Magnetic moments considered to lie along (001) axes are computed. Obtained data from ab initio calculations are used as input for the high temperature series expansions (HTSEs) calculations to compute other magnetic parameters such as the magnetic phase diagram and the critical exponent. The increasing of the dilution x in this system has allowed to verify a series of HTSEs predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from antiferromagnetic to ferromagnetic passing through the spins glace phase.

Effect of Grain Size on Magnetic Properties of ZnO Doped with Nickel Single Impurities

The purpose of our study is to find a suitable material to be used in spintronic applications and to find the relation between the parameters of deposition by spray pyrolysis technic (temperature, concentration of Ni doping) and the ferromagnetic properties (Curie temperature, magnetic moment). The nickel-doped zinc oxide, Zn.1−x.NixO (x = 0.01, 0.03, 0.05), and diluted magnetic semiconductors (DMSs) are synthesized by the spray pyrolysis technic. The results of The X-ray diffraction (XRD) of prepared substrates confirm the incorporation of the dopants into the ZnO lattice structure. The spin-polarized electronic properties has been found and was investigated in detail by using the density-functional theory (DFT), the local density approximation (LDA), and The Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). As result, nickel doping brings up a half-metallic appearance due to the hybridization between the 3d state of Nickel impurities and the oxygen 2p state. The mechanism of the interatomic exchange has been explained as being a p-d double exchange.

Ab initio study of 59Co NMR spectra in Co2FeAl1−xSix Heusler alloys

Ab initio electronic structure calculation of a series of Co2FeAl1−xSix Heusler alloys has been performed, using the Korringa–Kohn–Rostoker-coherent potential approximation method to explain experimental 59Co NMR spectra. Two prominent features are explained semi-quantitatively—a global shift of the 59Co resonance line due to alloying with Al and Si atoms in Co2FeAl1−xSix, and the effect of local disorder in creating distinct satellite lines of 59Co NMR in Co2FeAl. The importance is stressed of the positive contribution to the 59Co hyperfine field from valence electron polarization, which emerges from the half-metallic band structure inherent in Co-based Heusler alloys.

Computational materials design of defect-induced ferrimagnetic MnO

We present a computational materials design for defect-induced ferrimagnetic MnO. The magnetic properties of MnO containing Mn vacancies were investigated using first-principle calculations. For these electronic structure calculations, we employed a pseudo-self-interaction-corrected local density approximation (PSIC-LDA). We used the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) to create a random distribution of atoms at the assigned sites. Having described the magnetic properties with a classic Heisenberg model, we calculated the effective exchange coupling constants by applying the magnetic force theorem to two magnetic sites embedded in the CPA medium. We estimated the Curie temperatures from the calculated exchange interactions. This study found that the Mn vacancies induced ferrimagnetic ground states in MnO, and that the Curie temperature could reach room temperature at Mn vacancy concentrations above 20%. These findings suggest a new route for designing ferrimagnetic materials from anti-ferromagnetic host materials.

Magnetic and electronic properties of Cr- and Mn-doped SnO2: ab initio calculations

The ab initio calculations, based on the Korringa–Kohn–Rostoker (KKR) approximation method combined with the coherent potential approximation (CPA), indicated as KKR–CPA, have been used to study the stability of ferromagnetic and ferrimagnetic states, for systems that are SnO2 doped and co-doped with two transition metals, that is, chromium and manganese. Our results indicate that the ferromagnetic state is more stable than the spin-glass state for the (Sn1−xCrxO2; x = 0.07, 0.09, 0.12 and 0.15)-doped system, while the spin-glass state is more stable than the ferromagnetic state for the (Sn1−xMnxO2; x = 0.02 and 0.05)-doped system. However, the ferromagnetic and/or the ferrimagnetic states are stable for the (Sn0.98−xMn0.02CrxO2; x = 0.05, 0.09 and 0.13)-doped system depending on the Cr concentration. Moreover, we estimated the Curie temperature (Tc) for the Cr-doped tin dioxide (SnO2), and we explained the origin of magnetic behaviour through the total density of states for different doped and co-doped SnO2 systems.

Theoretical Study of TiO2 Doped with Single and Double Impurities

Using first principles calculations within the density-functional theory (DFT) and the local density approximation (LDA), the electronic structure and magnetic properties of rutile TiO2 doped with single and double-impurities Ti(Cr, Mn) O2 has been investigated. The Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) has been used. Chromium doping brings up a half-metallic appearance; it is due to the hybridization between the 3d state of chromium and the oxygen 2p state. The mechanism of the interatomic exchange in this case has been deducted as a double exchange. We also studied Mn doped TiO2, which gave rise to ferromagnetism, due to the p–d exchange. We also study the advantage of codoping TiO2 diluted magnetic oxide to find a suitable material to be used in spintronics.

Search for Resonant-Like Impurity in Ag-Doped CoSb3 Skutterudite: Theoretical and Experimental Study

Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) calculations of Ag-doped CoSb3 point to the presence of either an extra sharp peak of s-symmetry Ag density of states near the valence-band edge when filling the void (2a) or to conventional p-type doping when substituting Sb site (24g). These results suggest a resonant-like impurity level in the former or nearly rigid-band behavior in the latter. To confirm the theoretical predictions, a series of samples with nominal composition Co8Sb24:Agx (x = 0, 0.1, 0.3, 0.4, 0.5) were prepared. Structural and phase composition analyses were carried out by x-ray diffraction, scanning electron microscopy, and scanning thermoelectric microprobe. Investigations of the influence of Ag impurity on the electrical conductivity and Seebeck coefficient were performed over the temperature range from 300 K to 560 K. It was found that doping CoSb3 with Ag leads to an increase of the thermoelectric power factor α2σ in the temperature range from 300 K to 475 K of about an order of magnitude for all doped samples. However, electron probe microanalysis revealed accumulation of Ag mainly in grain boundaries while the presence of Ag in CoSb3 crystallites was not confirmed. This observation corroborates the results of KKR-CPA calculations concerning the formation energy of the AgxCo4Sb12 system, which is much lower than values calculated for AxCo4Sb12 (A = Ca, Ba).

Evaluation of terminal composition of palladium–silver hydrides in plateau region by electronic structure calculations

Abstract The electronic structures of palladium, palladium–silver alloys (Pd–Ag) and their hydrides were calculated to evaluate the terminal composition in the plateau region in the pressure–composition isotherm. The electronic structures were calculated by the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA), MACHIKANEYAMA 2002 developed by Akai. The Fermi level of palladium at various hydrogen contents moved to the edge of the d-band with increasing the hydrogen content and the Fermi level of PdH 0.6 reached the edge of the d-band. This hydrogen composition, [H]/[Pd] = 0.6, agreed well with the terminal composition in the plateau region. The Fermi level of Pd–Ag was similarly lifted up to the edge of the d-band with increasing the hydrogen content. The terminal compositions in isotherms of Pd–Ag alloys were estimated from the Fermi level position and the density of states at the Fermi level. The estimated compositions agreed with those experimentally obtained. Therefore, the terminal composition in the plateau region in the pressure–composition isotherms of Pd–Ag alloys could be evaluated by using with the KKR-CPA method.

Electronic Structure and Magnetic Properties of Doped GaAs (Zinc Blende) with Double Impurities and Defects

Using ab-initio calculations based on Korringa- Kohn-Rostoker coherent potential approximation (KKR- CPA) method in connection with the local density approx- imation (LDA), we study theoretically the electronic and magnetic properties of a system based on GaAs material. These properties have been studied, with different point de- fects in GaAs, which are Gallium interstitials (Gai), Gal- lium antisites (GaAs), Gallium vacancies (VGa), Arsenic in- terstitials (Asi) and Arsenic antisites (AsGa), Arsenic vacan- cies (VAs), and in-doped or codoped system Ga1−xCoxAs and Ga1−y−zCoyFezAs, respectively. This work presents de- tailed information about total and local density of states at different concentrations of these defects and doped ele- ments; the stability of the ferromagnetic state compared with the spin-glass state has been discussed. The result of elec- tronic properties shows that codoped GaAs material with Fe and Co become ferromagnetic, also doped GaAs with Co or Fe with a defect (hole in Ga) is stable in the ferromagnetic The Curie temperature is estimated from the total energy difference between the Disorder Local Moment (DLM) and the (FM) state by using a mapping on the Heisenberg model in mean field approximation.

Ferromagnetism from Acceptors of Native Point Defects in (Zn, Mn)O Doped Systems

In this paper, we represent a theoretical study of the (Zn, Mn)O doped system with native point defects of ZnO as oxygen interstitials (Oi) and zinc vacancies (VZn). Under these defects, it has been shown that the ground state can be converted from spin glass to ferromagnetic phase, by performing ab initio density functional theory calculations relying on the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method employing the local density approximation (LDA) with the parameterization by Morruzi, Janak, and Williams. The stability of magnetism in the (Zn, Mn)O doped system with different native point defects has been discussed. We find that acceptor-like defects (that is, Oi, OZn, and VZn) may cause the enhancement of the ferromagnetic characteristics in ZnMnO with increasing Tc. Based on the theoretical results, we suggest that the native point defects have a key role with respects to the FM properties. Using the mean field approximation, the Curie temperature in our studied model is estimated.

Materials design of CuIn1−xZn0.5xSn0.5xSe2 for low cost photovoltaic solar cells

Computational material design of a strategy for resource security of chalcopyrite-based photovoltaic solar cells (PVSC) is investigated. The electronic structure of CuIn1−xZn0.5xSn0.5xSe2 (CIZTS) is calculated by using the self-interaction-corrected local density approximation (SIC-LDA) and the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). It is shown that for any degree of substitution for In, CIZTS has a direct band gap and no impurity state is induced in the band gap. We propose a possibility of substitutional co-doping of Zn and Sn for In in CuInSe2 photovoltaic material to reduce the cost of materials for PVSC production.

Electronic structure and magnetism of IV–VI compound based magnetic semiconductors

The electronic structure and the magnetic properties of Mn-doped GeTe are calculated from first-principles. The calculations are based on the local density approximation, and we use the Korringa–Kohn–Rostoker Coherent Potential Approximation (KKR-CPA) method for simulating substitutional and magnetic disorder. It is shown that Mn-3d states appear deep in the valence bands with d5 electron configuration and the system is insulating. By introducing Ge vacancies to dope hole carriers, the ferromagnetism is induced via the p–d exchange mechanism. On the other hand, the electron doping is not efficient to stabilize the ferromagnetic state.

Origin of Magnetism from Native Point Defects in ZnO

Based on the first-principle calculations by using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of different point defects in ZnO, which are Zinc interstitials (Zni), Zinc antisites (ZnO), Oxygen interstitials (Oi) and Oxygen antisites (OZn) defects in ZnO. The supercell calculations were also performed using the full potential local-orbital (FPLO) band structure scheme. This work presents detailed information about total and local density of states at some concentrations of these defects; the stability of the ferromagnetic state compared with the spin-glass state is investigated by comparing calculating their total energy. The results show on one hand that Zni and ZnO produce a shallow donor bellow the bottom of the conduction band (CB), while Oi and OZn produces the shallow acceptors above the top of the valence band (VB), and moment magnetic; on other hand that the ferromagnetic state is more stable than the spin-glass in Oxygen interstitials (Oi) and vice versa for oxygen antisites (OZn) of native point defects in ZnO. The other native point defects (Zni, ZnO, VO, and VZn) have a zero magnetic moment. The results show that the Curie temperature increases with the concentration of interstitial oxygen.

First-principles materials design of CuInSe2-based high-efficiency photovoltaic solar cells

Based on ab initio electronic structure calculations by self-interaction-corrected local-density-approximation (SIC-LDA) with the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA), we propose a materials design for high efficiency photovoltaic solar cells (PVSCs). It is shown that (i) the concentration dependence of the mixing energy of CuIn1−xGaxSe2 shows upward convexity, thus this system favors phase separation. Due to the type II band alignment between CuInSe2 and CuGaSe2, efficient electron–hole separation is realized in decomposed phase of this system. (ii) CuIn1−xZn0.5xSn0.5xSe2 has a direct band gap and no impurity state appears in the gap. Therefore, cost reduction is possible by using Zn and Sn instead of In. (iii) n-type CuAl1−xSnxS2 and p-type Cu1−xVCuxAlS2 have negative activation energy for doped impurities and are expected to be low-resistive transparent conducting sulfides, which should be useful for CuInSe2-based PVSCs.

Design of half-metallic ferrimagnets: Doped MnX (X=Te, Se, S)

Half-metallic ferrimagnetism of transition-metal-doped MnX (transition-metal=V, Cr and X=Te, Se, S) is examined in the framework of first-principles Korringa–Kohn–Rostoker-coherent potential approximation calculation. Such unusual ferrimagnetism becomes possible due to the coexistence of ferromagnetic coupling among transition-metal impurities (here, Cr and V) and antiferro-magnetic coupling among Mn ions. In particular, NaCl-type MnSe and MnS compounds exhibit clear half-metallic ferrimagnetism even when the doping concentrations are as low as a few percents. On the other hand, NiAs-type MnTe compounds exhibit pseudo half-metallic ferrimagnetism. For a metastable zinc-blende structure, half-metallic ferrimagnetism is realized in all cases except in a low doping region.

First-principles material design and perspective on semiconductor spintronics materials

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.

Magnetic Properties of Ni Alloys for Superconducting Wire Substrates: A First-Principles Study

Ni-based alloys, especially Ni–W alloys, are widely used as substrates for high-temperature superconducting (HTSC) wires. Ni–W alloys have many useful properties, including, strength, flexibility, durability, and the ability to texture thin films grown on them. Ni metal has an fcc structure, and is well known to exhibit band magnetism on account of the splitting of its Ni 3d bands, which produce a non-integer magnetic moment per Ni atom (magnetic moment of Pure Ni: 0.6 µB/atom). However, the magnetic moment in a Ni-based alloy substrate interferes with the superconducting behavior of the film. Consequently, suitable alloys with reduced ferromagnetism are required. In this paper, we report first-principles calculations of the magnetic moments of a series of Ni-based alloys by the Korringa–Kohn–Rostoker coherent-potential approximation (KKR-CPA) method within local-density approximation (LDA). Using the KKR-CPA method, it is possible to treat any given Ni alloy composition. Our calculation results reproduce the experimental data well, showing the concentration dependences of the magnetic moments of various Ni alloys. The series of calculations of Ni 3d metal, Ni 4d metal and Ni 5d metal alloys reveal early 4d and early 5d metals (i.e., Nb, Mo, Ta, and W) successfully lower the magnetic moments. For these elements, the magnetic moments of the alloys become zero at around 10 at. % owing to the strong hybridization between the Ni 3d band and the Nb/Mo 4d or Ta/W 5d band around the Fermi level.