Korringa Kohn Rostoker Research Articles

Band bending at the surface of Bi2Se3 studied from first principles

The band bending (BB) effect on the surface of the second-generation topological insulators implies a serious challenge to design transport devices. The BB is triggered by the effective electric field generated by charged impurities close to the surface and by the inhomogeneous charge distribution of the occupied surface states (SSs). Our self-consistent calculations in the Korringa–Kohn–Rostoker framework showed that in contrast to the bulk bands, the spectrum of the SSs is not bent at the surface. In turn, it is possible to tune the energy level of the Dirac point via the deposited surface dopants. In addition, the electrostatic modifications induced by the charged impurities on the surface induce long range oscillations in the charge density. For dopants located beneath the surface, however, these oscillations become highly suppressed. Our findings are in good agreement with recent experiments, however, our results indicate that the concentration of the surface doping cannot be estimated from the energy shift of the Dirac cone within the scope of the effective continuous model for the protected SSs.

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.

Elastic and Electronic Properties of TiC and TiN Crystals

SummaryA theory for evaluating the temperature variation of second, third and fourth order elastic constants (SOECs, TOECs and FOECs) for face centred cubic crystal structure solids is developed on the basis of Coulomb and Born‐Mayer potential using nearest‐neighbour distance and hardness parameter. The theory thus obtained is first applied to get the higher order elastic constants for TiC and TiN crystals at different temperatures. In addition to elastic properties we have reported electronic properties of these titanium based compounds using Spin Polarised Relativistic Korringa–Kohn–Rostoker (SPR‐KKR) method based on Green Function theory. Energy bands are interpreted in term of density of states. The effect of temperature on higher order elastic constants is discussed in view of the theory proposed. A crossover in energy bands predicts that TiC and TiN are metallic in nature which is in tune with available data.

Magnetic interactions and electronic structure of Ni–Mn–In

The electronic structure and magnetic properties of a magnetic shape memory alloy Ni–Mn–In have been studied using spin polarized fully relativistic Korringa–Kohn–Rostoker (SPRKKR) method. The total energy calculations with different starting magnetic spin configurations show that the ground state of Ni2Mn1.4In0.6 is ferromagnetic. The spin and orbital magnetic moments of Ni2Mn1.4In0.6 and Ni2MnIn are in good agreement with the magnetization measurements. The exchange coupling parameters of the different sublattice interactions exhibit a strong competition between ferromagnetic and antiferromagnetic configurations, due to the substitution of excess Mn atoms at the In site in Ni2Mn1.4In0.6. The Curie temperature of Ni2MnIn, calculated under a mean field approximation, is found to be in relatively good agreement with the experimental values. While Ni2MnIn does not undergo martensite transition, it is shown that a Jahn–Teller splitting of the Ni 3d eg states plays an important role in driving the martensite transformation in Ni2Mn1.4In0.6. We find that both the calculated ultra-violet photoemission spectra and the inverse photoemission spectra are in good agreement with the existing experimental data.

Numerical solution of the relativistic single-site scattering problem for the Coulomb and the Mathieu potential

For a reliable fully-relativistic Korringa–Kohn–Rostoker Green function method, an accurate solution of the underlying single-site scattering problem is necessary. We present an extensive discussion on numerical solutions of the related differential equations by means of standard methods for a direct solution and by means of integral equations. Our implementation is tested and exemplarily demonstrated for a spherically symmetric treatment of a Coulomb potential and for a Mathieu potential to cover the full-potential implementation. For the Coulomb potential we include an analytic discussion of the asymptotic behaviour of irregular scattering solutions close to the origin ().

Probing magnetism and electronic structure of Fe-doped ZnO thin films

Ab-initio calculations using Korringa–Kohn–Rostoker method combined with the coherent potential approximation were performed in order to study the magnetic properties of Fe-doped ZnO thin films with different Fe contents. The extracted parameters are compared with those determined experimentally. Based on total and partial densities of state curves, we demonstrate that there is a competition between p–d exchange and superexchange mechanisms leading to weak ferromagnetic and antiferromagnetic contributions, respectively. The dominant mechanism is found to be antiferromagnetic. However, with increasing Fe content the ferromagnetic contribution increases. In addition, the effect of structural defects on the magnetism of the system is reported. It is shown that both Zn and O vacancies increase ferromagnetism, which is more pronounced in case of Zn.

Beyond Atomic Sizes and Hume-Rothery Rules: Understanding and Predicting High-Entropy Alloys

High-entropy alloys constitute a new class of materials that provide an excellent combination of strength, ductility, thermal stability, and oxidation resistance. Although they have attracted extensive attention due to their potential applications, little is known about why these compounds are stable or how to predict which combination of elements will form a single phase. In this article, we present a review of the latest research done on these alloys focusing on the theoretical models devised during the last decade. We discuss semiempirical methods based on the Hume-Rothery rules and stability criteria based on enthalpies of mixing and size mismatch. To provide insights into the electronic and magnetic properties of high-entropy alloys, we show the results of first-principles calculations of the electronic structure of the disordered solid-solution phase based on both Korringa–Kohn–Rostoker coherent potential approximation and large supercell models of example face-centered cubic and body-centered cubic systems. We also discuss in detail a model based on enthalpy considerations that can predict which elemental combinations are most likely to form a single-phase high-entropy alloy. The enthalpies are evaluated via first-principles “high-throughput” density functional theory calculations of the energies of formation of binary compounds, and therefore it requires no experimental or empirically derived input. The model correctly accounts for the specific combinations of metallic elements that are known to form single-phase alloys while rejecting similar combinations that have been tried and shown not to be single phase.

Electronic and Magnetic Properties of Ga1-x M x A (M = Mn and Cr; A = As and N): Ab Initio Study

The both electronic and magnetic properties of Ga 1-x Mn x As, Ga 1-x Mn x N, and Ga 1-x Cr x N have been studied using the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA). The total and local density of state (DOS) of Ga 1-x M x A (M = Mn and Cr; A = As and N) material are established. The total magnetic moment and the gap energies deduced for different values of dilution x in third systems are obtained.

Electronic properties of excess Cr in FeCrxSe alloy

Abstract We have studied the effects of transition-metal Cr in excess on the electronic and magnetic properties of FeCr x Se (x = 0.0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.10) alloys. We have used density functional based method Korringa–Kohn–Rostoker atomic sphere approximation within the coherent potential approximation to calculate the densities of states (DOS), band structures and Fermi surfaces (FS) of these alloys. Our calculations show that these alloys ( FeCr x Se ) are non-magnetic. Our unpolarized calculations show a subtle change in terms of density of states, band structure and Fermi surface with respect to the pure FeSe. We have also estimated bare Sommerfeld constant, electron–phonon coupling constant and the superconducting transition temperature T c for these alloys, which were found to be in good agreement with experiments.

The Korringa–Kohn–Rostoker method with projection potentials: exact result for the density

A well known problem in the Korringa–Kohn–Rostoker (KKR) multiple-scattering method concerns the error in density normalization arising from finite angular momentum expansions used in numerical treatments. It is shown that this problem can be solved if the potential around each atom is understood as a non-local projection potential in angular momentum space and that the density can be calculated exactly without infinite angular momentum sums if the projection acts on a finite subspace of spherical harmonics. This restriction implicates no loss of generality because an arbitrary potential can be approximated by increasing the subspace as closely as desired.

Relative crystal stability of AlxFeNiCrCo high entropy alloys from XRD analysis and formation energy calculation

Electronic structure of AlxFeNiCrCo (x ≤ 3) high-entropy alloys (HEAs) was calculated using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation (KKR-CPA). Total energy minimization was performed for bcc and fcc structures in each alloy composition. The phase stability was investigated from the total energy analysis, which finally allowed to determine the bcc–fcc phase transition for aluminium concentration close to 13 at%. It inspired us to synthesize AlxFeNiCrCo (0 ≤ x ≤ 1.5) using two procedures based on arc melting and sintering to allow for observation of entropy effect on phase formation. The XRD measurements evidently proved an occurence of fcc or bcc structure and their coexistence, depending on Al concentration and temperature. This finding remains in good agreement with theoretical results from free energy analysis, when accounting for KKR-CPA total energy as well as entropy terms. Furthermore, the structure preference, from fcc to bcc HEAs, with increasing Al content was discussed in view of total and atomic-dependent density of states computed in non-magnetic and paramagnetic-like states.

Inverse photoemission spectroscopic studies on phase separated La 0.2 Sr 0.8 MnO 3

Abstract We have studied the temperature evolution of the inverse photoemission spectra of phase separated La 0.2 Sr 0.8 MnO 3 . To identify the features in the room temperature experimental spectra, band structure calculations using Korringa–Kohn–Rostoker Green׳s function method were carried out. We find that the features generated by local moment disorder calculations give a better match with the experimental spectrum. In the insulating phase, we observed unusually an increased intensity at around the Fermi level. This puzzling behaviour is attributed to the shift in the chemical potential towards the conduction band and localisation of disorder induced states closer to the conduction band. The present results clearly show the importance of unoccupied electronic states in better understanding of the phase separated systems.

Half-metallic ferromagnetism in TM-doped MgH2 hydride

We show that, in addition to its thermodynamic properties that make it a good candidate for hydrogen storage, the MgH2 hydride exhibits interesting magnetic properties when doped with some transition metals (TM). Using the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation in the framework of first-principle calculations, we study the half-metallic ferromagnetic properties of the MgH2 doped with TM: Co, V, Cr, Ti; Mg0.95TM0.05H2. The ferromagnetic state energy is computed and compared with the disordered local moment state energy. We show, from the electronic structure, that doping MgH2 with TM elements can convert the material to a half-metallic with a high wide impurity band and high magnetic moment. We have found that the corresponding Curie temperature is bigger than the room temperature, which is considered as a relevant parameter for spintronic applications. Moreover, the mechanism of the hybridization and the interaction between the magnetic ions are also investigated showing that the double exchange is the underlying mechanism responsible for the magnetism of such materials .

Computational materials design of LiZnAs-, LiZnP-, and LiZnN-based n-type magnetic semiconductors

The computational materials design of n-type filled tetrahedral compound magnetic semiconductors is proposed on the basis of first-principles calculations within the density functional theory. Using the Korringa–Kohn–Rostoker coherent potential approximation, the electronic structures of electron-doped Li(Zn,Mn)As, Li(Zn,Mn)P, and Li(Zn,Mn)N are calculated. By estimating free energy, the phase diagrams of these systems are predicted. It is shown that these systems are phase-separated and favor spinodal decomposition when the electrons are not doped. By introducing Li interstitials as donor impurities, spinodal decomposition is strongly suppressed and Mn can be doped at a high concentration. Moreover, ferromagnetic interactions between Mn atoms are induced by electron doping. Thus, we can expect electron-mediated ferromagnetism in these systems with a reasonable Curie temperature.

Magnetic dichroism study on Mn1.8Co1.2Ga thin film using a combination of x-ray absorption and photoemission spectroscopy

Using circularly polarised radiation and a combination of bulk-sensitive hard x-ray photoelectron spectroscopy and x-ray-absorption spectroscopy (XAS) we studied the electronic and magnetic structure of epitaxial Mn1.8Co1.2Ga thin films. Spin-resolved Bloch spectral functions, density of states as well as charge and magnetisation densities were investigated by a first-principles analysis of full potential, fully relativistic Korringa–Kohn–Rostoker calculations of the electronic structure. The valence states were experimentally investigated by using linear dichroism in the angular distribution and comparing the results to spin-resolved densities of states. The linear dichroism in the valence band enabled a symmetry analysis of the contributing states. The spectra were in good agreement with the theoretical partial density of states. The element-specific, spin-resolved, unoccupied densities of states for Co and Mn were analysed by using XAS and x-ray magnetic circular dichroism (XMCD) at the L3,2 edges. The spectra were influenced by strong correlation effects. XMCD was used to extract the site-resolved magnetic moments. The experimental values of mMn = 0.7 μB and mCo = 1.05 μB agree very well with the calculated magnetic moments. Magnetic circular dichroism in angle-resolved photoelectron spectroscopy at the Mn and Co 2p core level exhibited a pronounced magnetic dichroism and confirmed the localised character of the Mn d valence states.

Magnetic Properties of Transition Metal-Doped CdSe

In this paper, the electronic, structural, and magnetic properties of transition metal (TM) (V, Cr, Mn, Fe, Co, Ni, Cu)-doped CdSe are investigated using first-principles calculations within the density functional theory (DFT) and the local density approximation (LDA) in connection with the Korringa–Kohn–Rostoker coherent potential approximation (KKR–CPA) implemented in the AKAI MACHIKANEAMA code. The 3d level locations of majority spin of those magnetic ions are studied. The spin–spin interaction at low concentration between magnetic impurities leads to ferromagnetic behavior. While the increase of TM concentration beyond 10 % leads to antiferromagnetic interactions between the magnetic ions, which suppress the ferromagnetism. Magnetic stability and Curie temperature are also calculated for different TM doping. And finally, two TMs are chosen to make a codoping in CdSe semiconductor material.

BROADBAND CALCULATIONS OF BAND DIAGRAMS IN PERIODIC STRUCTURES USING THE BROADBAND GREEN'S FUNCTION WITH LOW WAVENUMBER EXTRACTION (BBGFL)

We apply the method of the Broadband Green's Functions with Low wavenumber extraction (BBGFL) to calculate band diagrams in periodic structures. We consider 2D impenetrable objects placed in a 2D periodic lattice. The low wavenumber extraction is applied to the 2D periodic Green's function for the lattice which is used to formulate the surface integral equation. The low wavenumber extraction accelerates the convergence of the Floquet modes expansion. Using the BBGFL to the surface integral equation and the Method of Moments gives a linear eigenvalue equation that gives the broadband (multi-band) solutions simultaneously for a given point in the first Brillouin zone. The method only requires the calculation of the periodic Green's function at a single low wavenumber. Numerical results are illustrated for the 2D hexagonal lattice to show the computational efficiency and accuracy of the method. Because of the acceleration of convergence, an eigenvalue problem with dimensions 49 plane wave Floquet modes are sufficient to give the multi-band solutions that are in excellent agreement with results of the Korringa Kohn Rostoker (KKR) method. The multiband solutions for the band problem and the complementary band problem are also discussed.

Electronic origin of the step-like character of the discharge curve for NaxCoO2-y cathode

This paper shows electronic approach to an explanation of the step-like character of the discharge/charge curve of Na / Na +/ Na x CoO 2-y battery. On a basis of comprehensive experimental studies of physicochemical properties of Na x CoO 2-y cathode material (XRD, electrical conductivity, thermoelectric power, electronic specific heat) supported by calculations performed using the Korringa–Kohn–Rostoker method with the coherent potential approximation to account for chemical disorder, it has been shown that the observed step-like character of the discharge curve reflects the variation of the chemical potential of electrons (Fermi level) in the density of states of Na x CoO 2-y, which is anomalously perturbed by a presence of the oxygen vacancy defects and sodium ordering.

KKR–CPA study of electronic structure and relative stability of Mg2X ([formula omitted] = Si, Ge, Sn) thermoelectrics containing point defects

Different types of chemical disorder like vacancies, interstitials and anti-site defects are investigated in binary Mg2X (X=Si, Ge or Sn) and pseudo-ternary Mg2Si1-xSnx thermoelectric materials from the charge self-consistent electronic structure calculations, using the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR–CPA). This approach allows to directly compare computational results for possible types of crystal imperfections (treated as random), since the symmetry of the unit cell is kept unchanged in all considered cases. The influence of the aforementioned point defects on electronic structure and relative crystal stability is investigated from computed densities of states (DOS) and formation energy (Eform) analysis, respectively. It is found that the anti-site disorder in stoichiometric Mg2X compounds is highly unfavourable due to constitution of huge and non-bonding impurity peaks at the Fermi level (EF), the conclusion which remains in line with electronegativity and bonding criteria. On the other hand, the small amount of constituent elements (Si or Mg) located in the interstitial void (within ideal stoichiometry) results in strong modification of electronic states on both sides of the energy gap, yielding in principle either p-type (Mg) or n-type (Si) doping. In off-stoichiometric Mg2-xX1-y, the vacancy defects may lead to either p-type (Mg-site) or n-type (X-site) doping, due to EF shifting into the valence or conduction band edge, respectively. However, the total energy analysis indicates that the site preference of such defects’ onset tends to change with increasing atomic number of X. Si vacancy defects energetically favourable in Mg2-xSi1-y (whatever the considered chemical potential limits) are no more valid in Mg2-xSn1-y, where Mg vacancy defects are expected to be preferred. Mg2-xGe1-y exhibits intermediate behaviours.

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.