Korringa Kohn Rostoker Research Articles

Design of novel dilute magnetic semiconductors by exhaustive first-principles calculations and scale-bridging simulations

Dilute magnetic semiconductors (DMSs) have attracted growing attention because of their potential advantages in semiconductor spintronics applications. However, as previous research has focused mostly on Mn-doped III–V semiconductors, little is known about other systems. Thus, to discover DMSs that are suitable for practical applications, an exhaustive search for zincblende-type DMSs was performed based on the Korringa–Kohn–Rostoker Green function method combined with the coherent potential approximation (KKR–CPA), and it was found that (Al,Cr)P and (Al,Cr)As maintained ferromagnetism at high temperatures. As the mean-field approximation significantly overestimates the Curie temperature by ignoring the magnetic percolation effect, a Monte Carlo simulation was performed to reproduce the experimental annealing procedure and estimate the Curie temperature using the random phase approximation, which considers the magnetic percolation effect. It was found that the Cr atoms in the AlP and AlAs host semiconductors gather together and form nanoclusters upon annealing based on attractive interactions. Furthermore, upon investigating how the annealing conditions affect the Curie temperature, it was found that the annealing temperature can modulate the density and size of the Cr nanoclusters, thereby controlling the Curie temperature. Moreover, it was deduced that the shapes of the nanoclusters could be changed by altering the dimensions of crystal growth. These results are essential to support materials manufacturing for next-generation semiconductor spintronics.

Co-, Fe-, Ni-doped and co-doped rutile GeO2: insights from ab-initio calculations

Abstract Rutile germanium oxide (rutile GeO2), a semiconductor, can act as a half-metallic compound and is a promising material for spintronic and optoelectronic applications. Calculations were performed using the Korringa–Kohn–Rostoker (KKR) approach and the coherent potential approximation (CPA), which were further combined with two approximations, the local density approximation (LDA) and the self-interaction corrected LDA approximation (LDA-SIC), to study the electronic structure of bulk rutile GeO2 doped and co-doped with three transition-metal impurities: Fe, Co, and Ni. The doping value was set to 10%, while the co-doping level was set to 5% for each impurity. The main findings of this work are: (1) a direct ultrawide bandgap of 4.80 eV is observed and the rutile GeO2 exhibits an N-type semiconducting property. (2) Doped and co-doped GeO2 acquire a magnetic behavior and exhibit half-metallicity. (3) The mechanism responsible for these properties is also studied. (4) The critical temperature can reach 334 K when GeO2 is doped with Fe, while it rises to 398 K when it is co-doped with Fe and Co. (5) The spin polarization can be improved by co-doping. It can be inferred that rutile GeO2 doped or co-doped with (Co, Fe) transition metals can be considered to be potential candidates for spintronic and optoelectronic applications.

First-principles investigation of (Co and N)-doped [formula omitted] for potential use in spintronic applications

The impact of cobalt and nitrogen on the structural, magnetic, and electronic properties of beryllium sulfur (BeS) is investigated in this work using the Korringa Kohn Rostoker (KKR) approach in conjunction with the coherent potential approximation (CPA) and the generalized gradient approximation (GGA). The density of states for Co and N doping indicates a half metallic (HM) behavior with a spin polarization reaching up to 100% at Fermi level. In addition, when increasing the concentration of dopants, the Curie temperature of BeS rises to maximum values of 494.66K and 691.80K for Co and N doped BeS respectively. The stability of the ferromagnetic order (FM) in these compounds is attributed to double-exchange interaction processes. According to our results, these new diluted materials show great potential for spintronic devices.

Tuning Ferromagnetism in a Single Layer of Fe above Room Temperature.

The crystallographic and magnetic properties of an Fe monolayer (ML) grown on 2 ML Au/W(110) substrate are studied with spin-polarized low-energy electron microscopy, density functional theory, and relativistic screened Korringa–Kohn–Rostoker calculations. The single layer of iron atoms possesses hexagonal symmetry and reveals a ferromagnetic order at room temperature. We experimentally demonstrate the possibility of tuning the Curie temperature and the magnitude of magnetization of the Fe monolayer by capping with Au. Taking into account several structural models, the calculation results mostly show ferromagnetic states with enhanced magnetic moments of Fe atoms compared to their bulk value and a further increase in their value after covering with Au. The theoretically calculated Curie temperatures are in fair agreement with those obtained in the experiments. The calculations, furthermore, found evidence for the presence of frustrated isotropic Fe–Fe exchange interactions, and a discussion of the structural effects on the magnetic properties is provided herein.

Full orbital decomposition of Yu-Shiba-Rusinov states based on first principles

We have implemented the Bogoliubov-de Gennes (BdG) equation in a screened Korringa-Kohn- Rostoker (KKR) method for solving, self-consistently, the superconducting state for 3d crystals including substitutional impurities. In this report we extend this theoretical framework to allow for collinear magnetism and apply it to fcc Pb with 3d magnetic impurities. In the presence of magnetic impurities, there is a pair-breaking effect that results in sup-gap Yu-Shiba-Rusinov (YSR) states which we decompose into contributions from the individual orbital character. We determine the spatial extent of these impurity states, showing how the different orbital character affects the details of the YSR states within the superconducting gap. Our work highlights the importance of the first principles based description which captures the quantitative details making direct comparisons possible with experimental findings.

First-principles investigation of Nd(Fe,M)[formula omitted] ([formula omitted] = K–Br) and Nd(Fe,Cr,Co,Ni,Ge,As)[formula omitted]: Possible enhancers of Curie temperature for NdFe[formula omitted] magnetic compounds

We investigate the effects of various dopants (M = K–Br) on the Curie temperature of the magnetic compound NdFe12 through first-principles calculations. Analysis by the Korringa–Kohn–Rostoker method with the coherent potential approximation reveals that doping the Fe sites with optimal concentrations of Ge and As is a promising strategy for increasing the Curie temperature. To search over a wider space, we also perform Bayesian optimization. Out of over 180,000 candidate compositions, co-doped systems with Co, Ge, and As are found to have the highest Curie temperatures.

The effect of Ni doping on the electronic structure and superconductivity in the noncentrosymmetric ThCoC[formula omitted

ThCoC2 is a non-BCS, noncentrosymmetric superconductor with Tc≃2.5 K, in which pairing mechanism was suggested to be either spin fluctuations or the electron–phonon coupling. Earlier experimental work revealed that Tc can be greatly enhanced upon Ni doping in ThCo1−xNixC2, up to 12 K for x=0.4. In this work, using the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR-CPA), the evolution of the electronic structure upon Ni doping was studied and the increase of the density of states at the Fermi level was found. The electron–phonon coupling constant λ was calculated using the rigid muffin tin approximation (RMTA) and the increase in λ(x) was found. This indirectly confirms the electron–phonon coupling scenario as the trend in λ(x) is in qualitative agreement with the experiment when the electron–phonon pairing is assumed.

Monte Carlo calculations of Curie temperatures of Y[formula omitted]Gd[formula omitted](Fe[formula omitted]Co[formula omitted])[formula omitted] pseudobinary system

The close-packed AB2 structures called Laves phases constitute the largest group of intermetallic compounds. In this paper we computationally investigated the pseudo-binary Laves phase system Y1−xGdx(Fe1−yCoy)2 spanning between the YFe2, YCo2, GdFe2, and GdCo2 vertices. While the vast majority of the Y1−xGdx(Fe1−yCoy)2 phase diagram is the ferrimagnetic phase, YCo2 along with a narrow range of concentrations around it is the paramagnetic phase. We presented results obtained by Monte Carlo simulations of the Heisenberg model with parameters derived from first-principles calculations. For calculations we used the Uppsala atomistic spin dynamics (UppASD) code together with the spin-polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) code. From first principles we calculated the magnetic moments and exchange integrals for the considered pseudo-binary system, together with spin-polarized densities of states for boundary compositions. Furthermore, we showed how the compensation point with the effective zero total moment depends on the concentration in the considered ferrimagnetic phases. However, the main result of our study was the determination of the Curie temperature dependence for the system Y1−xGdx(Fe1−yCoy)2. Except for the paramagnetic region around YCo2, the predicted temperatures were in good qualitative and quantitative agreement with experimental results, which confirmed the ability of the method to predict magnetic transition temperatures for systems containing up to three different magnetic elements (Fe, Co, and Gd) simultaneously. For the Y(Fe1−yCoy)2 and Gd(Fe1−yCoy)2 systems our calculations matched the experimentally-confirmed Slater–Pauling-like behavior of TC dependence on the Co concentration. For the Y1−xGdxFe2 system we obtained, also in agreement with the experiment, a linear dependence of TC on the Gd concentration. In addition, on the example of Y0.8Gd0.2Co2 ferrimagnet, we showed the possibility of predicting the non-trivial behavior of the temperature dependence of magnetization, confirmed by comparison with previous measurement results.

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.

Refractive Index versus Coupling Effects for Naked and Surfactant-Coated Au Nanoparticle Films: Implications for Selective Detection of Vapor–Liquid Analytes

From a single dispersion of surfactant-coated Au nanoparticles (NPs), we can assemble colorimetric films of different thickness, organic content, and structure. In this work, we demonstrate how these variables play a fundamental role in determining what sensing mechanism predominates when these films are subjected to liquid-phase ethanol (EtOH), acetone (Ace), and toluene (Tol) analytes. Theoretical calculations, performed with the Korringa–Kohn–Rostoker (KKR) model, agree very well with the experiments if we consider that thinner films with lesser organic/inorganic content (or no organic at all) offer more vacancies for the solvents to partition, leading to effective changes in the permittivity. Instead, thicker films with a more compacted structure suffer minimal changes in the effective permittivity (lesser vacancies), causing color changes dominated by changes in the distance between the Au NPs. These films may be incorporated into a sensor array for improving selectivity toward the detection of vapor/liquid-phase analytes.

Origin of large magnetostriction in palladium cobalt and palladium nickel alloys: Strong pseudo-dipole interactions between palladium–cobalt and palladium–nickel atomic pairs

The origin of the large magnetostriction in palladium cobalt and palladium nickel alloys was investigated. Density functional theory calculations based on the Korringa–Kohn–Rostoker Green function method with the coherent potential approximation revealed that alloying with palladium results in increased magnetization of cobalt and nickel atoms. Also, anomalous magnetization of palladium atoms occurs simultaneously. Employing calculated spin and orbital angular momenta of the atoms, magnetostriction was discussed based on the two-spin model for disordered alloys. Under the assumption that the pseudo-dipole interaction is proportional to the orbital and total angular momenta, the experimental magnetostriction curves can be reproduced. The estimated contributions of each atomic pair to magnetostriction revealed that the large magnetostriction at the palladium-rich side originates from the strong pseudo-dipole interactions between 4d and 3d transition metal atoms, namely, palladium–cobalt and palladium–nickel atomic pairs.

Electron momentum density of hexagonal Zn studied by high-resolution Compton scattering.

High-resolution (0.12 a.u.) electron momentum density projections (Compton profiles) of a hexagonal Zn single crystal have been measured along five high-symmetry directions in reciprocal space. The experiment was performed with the use of 115.6 keV synchrotron radiation on the BL08W station at SPring-8. The quality of the measured Compton profiles is significantly better than that ofprevious medium- and high-resolution data. The experimental data were compared with the corresponding theoretical Korringa-Kohn-Rostoker (KKR) and density functional theory (DFT) calculations. Some minor and major differences between the two theoretical band-structure calculations have been observed. However, the good quality experimental results indicate their better agreement with DFT.

Band calculations using broadband Green’s functions and the KKR method with applications to magneto-optics and photonic crystals

In this paper, we develop a method of simulations of using the broadband Green’s function (BBGF) in the Korringa Kohn Rostoker (KKR) method. The merit of the method is broadband such that once the initial setup is completed, the calculation at every frequency is calculated rapidly. The broadband Green’s function consists of a frequency-independent spatial summation and a spectral summation that has most of the factors frequency independent. Both summations have fast convergence. Analytical expressions of broadband coefficients of cylindrical waves are derived from the BBGF. The broadband cylindrical waves coefficients are then combined with the KKR method to calculate the determinant equation for a wide range of frequencies. Numerical results of the band diagrams and band surface currents are illustrated for magneto-optics and photonic crystals. The CPU time requirements, including setup, using MATLAB on a standard laptop, for computing the determinant at 1000 frequencies at a Bloch vector are 0.411 s and 1.206 s, respectively, for the cases of topological crystal of small scatterer relative to cell size and the photonic crystal of large scatterer comparable to cell size.

Multiple-scattering approach for multi-spin chiral magnetic interactions: application to the one- and two-dimensional Rashba electron gas

Various multi-spin magnetic exchange interactions (MEI) of chiral nature have been recently unveiled. Owing to their potential impact on the realisation of twisted spin-textures, their future implication in spintronics or quantum computing is very promising. Here, I address the long-range behavior of multi-spin MEI on the basis of a multiple-scattering formalism implementable in Green functions based methods such as the Korringa–Kohn–Rostoker (KKR) Green function framework. I consider the impact of spin–orbit coupling (SOC) as described in the one- (1D) and two-dimensional (2D) Rashba model, from which the analytical forms of the four- and six-spin interactions are extracted and compared to the well known bilinear isotropic, anisotropic and Dzyaloshinskii–Moriya interactions (DMI). Similarly to the DMI between two sites i and j, there is a four-spin chiral vector perpendicular to the bond connecting the two sites. The oscillatory behavior of the MEI and their decay as function of interatomic distances are analysed and quantified for the Rashba surfaces states characterizing Au surfaces. The interplay of beating effects and strength of SOC gives rise to a wide parameter space where chiral MEI are more prominent than the isotropic ones. The multi-spin interactions for a plaquette of N magnetic moments decay like simplifying to for equidistant atoms, where d is the dimension of the mediating electrons, qF the Fermi wave vector, L the perimeter of the plaquette while P is the product of interatomic distances. This recovers the behavior of the bilinear MEI, , and shows that increasing the perimeter of the plaquette weakens the MEI. More important, the power-law pertaining to the distance-dependent 1D MEI is insensitive to the number of atoms in the plaquette in contrast to the linear dependence associated with the 2D MEI. Furthermore, the N-dependence of qF offers the possibility of tuning the interactions amplitude by engineering the electronic occupation.

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.

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.

Impurity-dependent gyrotropic motion, deflection and pinning of current-driven ultrasmall skyrmions in PdFe/Ir(111) surface

Resting on multi-scale modelling simulations, we explore dynamical aspects characterizing magnetic skyrmions driven by spin-transfer-torque towards repulsive and pinning 3d and 4d single atomic defects embedded in a Pd layer deposited on the Fe/Ir(111) surface. The latter is known to host sub-10 nm skyrmions which are of great interest in information technology. The Landau–Lifshitz–Gilbert equation is parametrized with magnetic exchange interactions extracted from the ab-initio all-electron full potential Korringa–Kohn–Rostoker Green function method, where spin–orbit coupling is added self-consistently. Depending on the nature of the defect and the magnitude of the applied magnetic field, the skyrmion deforms by either shrinking or increasing in size, experiencing thereby elliptical distortions. After applying a magnetic field of 10 T, ultrasmall skyrmions are driven along a straight line towards the various defects which permits a simple analysis of the impact of the impurities. Independently from the nature of the skyrmion-defect complex interaction, being repulsive or pinning, a gyrotropic motion is observed. A repulsive force leads to a skyrmion trajectory similar to the one induced by an attractive one. We unveil that the circular motion is clockwise around pinning impurities but counter clockwise around the repulsive ones, which can be used to identify the interaction nature of the defects by observing the skyrmions trajectories. Moreover, and as expected, the skyrmion always escapes the repulsive defects in contrast to the pinning defects, which require a minimal depinning current to observe impurity avoidance. This unveils the richness of the motion regimes of skyrmions. We discuss the results of the simulations in terms of the Thiele equation, which provides a reasonable qualitative description of the observed phenomena. Finally, we show an example of a double track made of pinning impurities, where the engineering of their mutual distance allows to control the skyrmion motion with enhanced velocity.

Study of Energetic and Magnetic Properties of FexNi1 – x Monolayer Film on Nonmagnetic Metallic Substrates

Within the variational spin density functional method, the description of the temperature and ferromagnetic ordering effects on the adsorption of atoms of transition metals placed on paramagnetic substrates at formation of a monolayer film of FexNi1 – x alloy depending on the alloy component concentration and the coating parameter θ has been accomplished for the first time. The influence of different orientations of substrate surface planes is studied. The adsorption energy of a close-packed plane is minimal for all the systems presented. It is shown that a monolayer film without mixing is formed on the Au and W substrates at large values of the coating parameter and iron concentration, while silver atoms are forced out to the surface of the Ag substrate. The comparison of values of the adsorption energy calculated at θ = 1 and T = 0 K is carried out within the variational and the first-principles approaches. The calculation of magnetic moments and parameters of exchange interaction of the FexNi1 – x bulk alloy using the Korringa–Kohn–Rostoker method is accomplished at the equilibrium values of lattice constant obtained within the first-principles calculations. The exchange interaction is of a ferromagnetic nature, it increases with the increase in nickel concentration in the nearest polytypic neighbors $$J_{1}^{{{\text{Ni}} - {\text{Fe}}}}$$ and decreases in the nearest equitype neighbors $$J_{1}^{{{\text{Ni}} - {\text{Ni}}}}$$. The exchange interaction between the second neighbors occurs only in equitype atoms; it is of a ferromagnetic nature in iron atoms and is weakly antiferromagnetic in nickel atoms.

Theoretical approaches to study the optical response of the red-legged honeycreeper's plumage (Cyanerpes cyaneus).

In this paper, we investigate the unusual color effect exhibited by the plumage of the heads of Cyanerpes cyaneus males, whose color turns from green to turquoise as the angle between the illumination and observation directions is increased. This singular color effect is characteristic of species that have quasi-ordered nanostructures of short-range order within the feather barbs. However, among species of the same family and even within feather patches of the same individual, one can find barbs with different characteristics, both macroscopic (curvature, shape, cross-sectional area) and in their internal microstructure. We apply the Korringa-Kohn-Rostoker method with the averaging technique to model the reflectance spectra for different angles of incidence and explain the dependence of the observed color with the incidence-collection angle. To investigate the influence of the disorder in the optical response of the spongy matrix, we apply the integral method for a two-dimensional cylinder system that simulates the distribution of air cavities within the $ \beta $β-keratin medium. The experimental reflectance was interpreted as the result of multiple reflections in the internal interfaces generated by large air voids present within the spongy matrix. The application of rigorous methods to the study of natural photonic structures is of fundamental relevance for the design of efficient bioinspired artificial materials.

Ab initio investigation on Half-Heusler CoTiSi

We report band structural calculations on half-Heusler alloy CoTiSi using Korringa–Kohn–Rostoker method applied for various formalism of potential. Murnaghan equation of state is used to minimize ground state total energy and then equilibrium lattice parameter is estimated. From density of state (DOS), it is seen that the Fermi energy is not lying in the gap and a close inspection of DOSs at the Fermi level reveals the absence of gap around the Fermi level suggesting the absence of half-metallic nature and the same is also confirmed by electronic band structure diagram.