Korringa-Kohn-Rostoker Method Research Articles

Analysis of the optical response of reptile tissues in the visible and UV applying the KKR method.

Structural colors in nature are frequently produced by the ordered arrangement of nanoparticles. Interesting examples include reptiles and birds utilizing lattice-like formation of nanoparticles to produce a variety of colors. A famous example is the panther chameleon which is even able to change its color by actively varying the distance between guanine nanocrystals in its skin. Here, we demonstrate that the application of rigorous electromagnetic methods is important to determine the actual optical response of such biological systems. By applying the Korringa-Kohn-Rostoker (KKR) method we calculate the efficiencies of the reflected diffraction orders that can be viewed from directions other than the specular. Our results reveal that important characteristics of the reflectance spectra, especially within the ultraviolet (UV) and short visible wavelengths region, cannot be predicted by approximate models like the often-applied Maxwell-Garnett approach. Additionally, we show that the KKR method can be employed for the design of multi-layer structures with a desired optical response in the UV regime.

Effect of atomic anti-site disorder on the anisotropic magnetoresistance in Fe50Co50 alloys

In order to understand the anti-site disorder effect on the anisotropic magnetoresistance (AMR) of alloys, alloys were studied in this work using the fully relativistic spin-polarized Korringa–Kohn–Rostoker method. The anti-site disorder was modeled by interchanging Fe and Co atoms and treated by the coherent potential approximation. It is found that the anti-site disorder broadens the spectral function and decreases the conductivity. Our work emphasizes that the absolute variations of resistivity under magnetic moment rotation are less affected by atomic disorder. The annealing procedure improves the AMR by reduction of the total resistivity. At the same time, we also find that the fourth-order term in the angular dependent resistivity becomes weaker when the disorder increases, resulting from increased scattering of the states around the band-crossing.

Equations of state for B2 and bcc Fe1-xSix

Composition, chemical disorder, and magnetism significantly affect the volume and bulk modulus of iron‑silicon (FeSi) alloys at ambient pressure. Here we computed the equations of state for bcc-like (ordered B2 and disordered bcc) Fe-Si alloys up to the inner-core pressure using the first-principles Korringa–Kohn–Rostoker method. Ferromagnetic (FM) and nonmagnetic (NM) states were investigated over a wide compositional range from Fe to FeSi. The present static calculations revealed that magnetism and chemical disorder increased the volume and decreased the bulk modulus also at high pressures. In order to obtain their density and bulk sound velocity under high temperatures relevant to the inner core, we calculated the Helmholtz energy by using the quasi-harmonic approximation with the assumption that the Poisson's ratio of Fe-Si alloys is equivalent to that observed in the inner core. This assumption is valid when the inner core consists of bcc-like Fe-Si. Nevertheless, the obtained density and bulk sound velocity do not match seismological observations unless a temperature gradient inside the inner core is unrealistically large, indicating that the inner core is not a bcc-like Fe-Si alloy.

Effect of Mg Substitution on Structural Stabilization of NaxMnO2 Cathode for Na‐Ion Batteries

As the Li‐ion batteries market encounters numerous obstacles, such as not sufficient production to the demands, there is a need to develop post‐lithium technologies, such as Na‐ion batteries. Na0.67MnO2 cathode material provides high capacity; however, it suffers from phase transitions during the (de)intercalation process. Herein, it is presented that magnesium substitution in Na0.67Mg0.2Mn0.8O2 stabilizes the crystal structure during cycling, which was confirmed by operando X‐ray diffraction measurements as well as the long‐term cycling experiment. It also enhances the electronic conductivity from 9 × 10−6 to 7 × 10−5 S cm−1 and the ionic one from 3 × 10−6 to 4 × 10−5 S cm−1. X‐ray absorption spectroscopy O K‐edge spectra are in line with the enormous exceeding of the theoretical capacity of Na/Na+/NaxMg0.2Mn0.8O2 cell (nonreversible discharge capacity of 310 with 155 mAh g−1 of theoretical capacity). Electronic structure calculations performed by the Korringa–Kohn–Rostoker method in the framework of local spin‐density approximation + U in ordered Na2/3Mg1/3Mn2/3O2 approximant support its semiconducting properties as well as confirm that substitution of Mn with Mg markedly enhances the role of oxygen in constituting electronic states around the Fermi level and its role in the electrochemical process leading to the improved capacity.

Experimental and simulation study of microstructure and magnetic properties of (AlFeMnNi)1−x Nd x

ABSTRACT The high-entropy alloy (Al25Fe30Mn25Ni20)1−x Nd x (all in at.%) (AlFeMnNi)1−x Nd x ) was prepared by vacuum arc melting (AM) and casting method (CM). X-Ray Diffraction (XRD) and Scanning Electron Microscopy, transmission electron microscope and Integrated Energy-Dispersive X-ray spectroscopy were used to study the structure of our system. The (Al25Fe30Mn25Ni20)1−x Nd x crystallises in a Face-centred cube and centred cubic (BCC), depending on the x (x = 0.00, x = 0.01 at.% Nd). The increase of Neodymium (0.01 at.% Nd) content results in the formation of BCC structure. The chemical composition analysis of FCC and BCC crystal structures, their lattice constants demonstrate that the formation of a single BCC solid solution between 760°C and 1480°C. The Korringa–Kohn–Rostoker Method and Green's function combined with coherent potential approximation were used to study the magnetoelectronic properties of (Al25Fe30Mn25Ni20)1−x Nd x . We have obtained the value optimised of lattice parameter 2.9 Å and it is near to that obtained by experimental value using High Score programme and results XRD experimental.

Ab-initio study of tuning the electronic and magnetic properties of [formula omitted] Heusler alloy by Co and Mn compound doping

We report the effects of Mn and Co doping on the electronic properties, magnetic exchange interaction, and Curie temperature of intermetallic Ni2MnGa by Green’s function based Korringa–Kohn–Rostoker method with coherent potential approximation(KKR-CPA). The effect of single and compound doping of Mn and Co at different crystallographic positions on these properties are evaluated by computing the electronic and magnetic structures of Ni2MnGa. The study revealed the possibility of tuning magnetic exchange interaction(Jij) and Curie temperature(TC) upon doping. Moreover, it is noted that doping can stabilize the Jahn–Teller distortion. It is also worth noting that TC responds in a different way with concentration as well as the site of the dopant. This study helps in understanding and realizing the cause for magnetic properties in Ni2MnGa, and experimental peers can also use it for further research on doped Ni2MnGa.

Color optimization of a core–shell nanoparticles layer using machine learning techniques

Neural networks were recently introduced in the field of nanophotonics as an alternative and powerful way to obtain the non-linear mapping between the geometry and composition of arbitrary nanophotonic structures on one hand, and their associated properties and functions on the other. Taking into account the recent advances in the application of the machine learning concept to the design of nanophotonic devices, we employ this tool for the optimization of photonic materials with specific color properties. We train a deep neural network (DNN) to solve the inverse problem, i.e., to obtain the geometrical parameters of the structure that best produce a desired reflected color. The analyzed system is a single layer of core–shell spheres composed of melanin and silica embedded in air, arranged in a hexagonal matrix. The network is trained using a dataset of the three CIE 1976 (L*a*b*) color coordinates obtained from the simulated reflectance spectra of a large set of structures. The direct problem is solved using the Korringa–Kohn–Rostoker method (KKR), widely applied to calculate the optical properties of sphere composites. The color optimization approach used in this work opens up new alternatives for the design of artificial photonic structures with tunable color effects.

Dependence of the Electronic Structure of β‐Si6−zAlzOzN8−z on the (Al,O) Concentration z and on the Temperature

Abstractβ‐Si6−zAlzOzN8−z is a prominent example of systems suitable as hosts for creating materials for light‐emitting diodes (LEDs). In this work, the electronic structure of a series of semiordered and disordered β‐Si6−zAlzOzN8−z systems is investigated by means of ab initio calculations, using the FLAPW and Green function KKR methods. Finite temperature effects are included by averaging over thermodynamic configurations within the alloy analogy model. We found that the dependence of the electronic structure on the (Al,O) concentration z is similar for semiordered and disordered structures. The electronic band gap decreases with increasing z by about 1.5 eV when going from z=0 to z=2. States at the top of the valence band are mostly associated with N atoms whereas the states at the bottom of the conduction band are mostly derived from O atoms. Increasing the temperature leads to a shift of the bottom of the conduction band to lower energies. The amount of this shift increases with increasing z.

Investigation of thermoelectric material based on Lu1-xZrxNiSb solid solution. II. Modeling of characteristics

The KKR (AkaiKKR software package) and FLAPW (Elk software package) methods were used to model the structural, thermodynamic, energetic, and electrokinetic characteristics of the Lu1-xZrxNiSb semiconductor solid solution. It is shown that defects of acceptor nature are present in the structure of LuNiSb as a result of vacancies in positions 4a and 4c of Lu and Ni atoms, respectively, which generates two acceptor levels eAVac 4a and eAVac 4c in the band gap εg. When Zr atoms are introduced into the LuNiSb structure by replacing Lu atoms in position 4a, Zr atoms also occupy vacancies in this position, which increases the lattice parameter a(x) and eliminates defects of acceptor nature and corresponding acceptor levels eAVac 4a. When the vacancies are filled, Lu atoms are displaced, which reduces the value of the unit cell parameter and generates defects of donor nature and donor levels eD4a. The Ni atoms return to position 4c, which increases the a(x) value and eliminates defects of acceptor nature and the corresponding acceptor levels eAVac 4c. At the lowest concentration of Zr atoms, the conduction type of Lu1-xZrxNiSb changes from p- to n-type. The simulation results are consistent with experimental studies of Lu1-xZrxNiSb.

Magnetocaloric Effect, Structural, Magnetic and Electronic Properties of High Entropy Alloys AlCoxCr1−xFeNi: First-Principle Calculations and Monte Carlo Simulations

The magnetocaloric effect, electronic structure and magnetic properties of High Entropy Alloys AlCoxCr[Formula: see text]FeNi ([Formula: see text]) were calculated using the Monte Carlo simulation, the Korringa–Kohn–Rostoker method combined with the coherent potential approximation (KKR-CPA), method of linear augmented plane wave (FPLAPW) within generalized gradient approximation (GGA) and modified Becke–Johnson potential (TB-mBJ). Density of states of AlCoxCr[Formula: see text]FeNi are studied and analyzed. The total magnetic moments of magnetic atoms were calculated and compared with theoretical and experimental results. On the other side, the magnetocaloric effect in AlCoxCr[Formula: see text]FeNi was studied by first-principle calculations (FPCs) and Monte Carlo simulations. The magnetic phase transition temperature is estimated. The Curie temperature and exchange integrals were found to decrease with Co substitution. The magnetic entropy changes and relative cooling power are found.

Contrasting role of bismuth doping on the thermoelectric performance of VFeSb half-Heusler

Isoelectronic heavy elemental substitutions are generally preferred for enhancing the phonon scattering in a material without deteriorating its electrical transport. In this work, we demonstrate the efficacy and contrasting role of bismuth, an isoelectronic substitute of antimony, for enhancing the thermoelectric transport in structurally ordered cubic VFeSb half-Heusler. Despite its limited solubility, Bi-substitution at Sb-site, was found to be effective in enhancing the electrical power factor near room temperature. Alongside, synergistic lowering of lattice thermal conductivity was observed due to point defect scattering of phonons by Bi-induced mass and strain fluctuations. The electronic transport properties of nominal compositions and effects of Bi-doping induced disorder were evaluated using the Korringa–Kohn–Rostoker method with a coherent potential approximation (KKR-CPA). A highest thermoelectric figure-of-merit ( zT ) ~ 0.22 at 478 K was attained with improved weighted mobility and quality factor for optimally doped VFeSb 0.98 Bi 0.02 half-Heusler alloy making them prospective compositions for near room temperature thermoelectric applications. • Contrasting role of bismuth as prospective substitute for isovalent antimony in Sb-based half-Heusler. • High-power factor > 3.0 × 10 −3 Wm −1 K −2 at room temperature. • Lattice thermal conductivity reduction by point defect scattering in Bi-substituted VFeSb alloys. • Enhanced weighted mobility, thermoelectric quality factor and zT in Bi-substituted VFeSb alloys.

CHARACTERISTICS OF THERMOMETRIC MATERIAL Lu1-xScxNiSb

CHARACTERISTICS OF THERMOMETRIC MATERIAL Lu1-xScxNiSb

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.

Magnetic Compton profiles of Ni beyond the one-particle picture: Numerically exact and perturbative solvers of dynamical mean-field theory

We calculated the magnetic Compton profiles (MCPs) of Ni using density functional theory supplemented by electronic correlations treated within dynamical mean-field theory (DMFT). We present comparisons between the theoretical and experimental MCPs. The theoretical MCPs were calculated using the KKR method with the perturbative spin-polarized T-matrix fluctuation exchange approximation DMFT solver, as well as with the full potential linear augmented planewave method with the numerically exact continuous-time quantum Monte Carlo DMFT solver. We show that the total magnetic moment decreases with the intra-atomic Coulomb repulsion $U$, which is also reflected in the corresponding MCPs. The total magnetic moment obtained in experimental measurements can be reproduced by intermediate values of $U$. The spectral function reveals that the minority X$_2$ Fermi surface pocket shrinks and gets shallower with respect to the density functional theory calculations.

FEATURES OF SIMULATION OF CHARACTERISTICS OF THERMOMETRIC MATERIAL Lu1-xZrxNiSb

The results of modeling the thermometric characteristics of the semiconductor solid solution Lu1-xZrxNiSb, which is a promising thermometric material for the manufacture of sensitive elements of thermoelectric and electro resistive thermocouples, are presented. Modeling of the electronic structure of Lu1-xZrxNiSb was performed by the Korringa-Kohn-Rostoker (KKR) method in the approximation of coherent potential and local density and by the full-potential method of linearized plane waves (FLAPW). KKR simulations were performed using the AkaiKKR software package in the local density approximation for the exchangecorrelation potential with parameterization Moruzzi, Janak, Williams in the semi-relativistic one taking into account the spin-orbit interaction. The implementation of the method in the Elk software package was used to perform FLAPW calculations. To check the limits of the existence of the thermometric material Lu1-xZrxNiSb, both methods were used to calculate the change in the values of the period of the unit cell a(x) in the range x=0–1.0. It is shown that there is an agreement between the change in the values of a(x) Lu1-xZrxNiSb calculated by the FLAPW method and the results of experimental studies. The obtained result indicates higher accuracy of modeling of structural parameters Lu1-xZrxNiSb by the FLAPW method in comparison with the KKR method. To study the possibility of obtaining thermometric material Lu1-xZrxNiSb and to establish the limits of its existence in the form of a continuous solid solution, modeling of thermodynamic characteristics in the approximation of harmonic oscillations of atoms within the theory of DFT density functional for a hypothetical solid solution Lu1-xZrxNiSb, x=0–1.0. The change in the values of the enthalpy of mixing ΔH and the total energy E Lu1-xZrxNiSb, x=0–1.0, allows us to state that the thermometric material exists in the form of a solid substitution solution in the concentration range x=0–0.20, stratification occurs (spinoidal phase decay) and thermometric material does not exist. To understand the mechanisms of electrical conductivity of the thermometric material Lu1-xZrxNiSb, the methods of entry of impurity Zr atoms into the matrix of the basic semiconductor p-LuNiSb and their occupation of different crystallographic positions, as well as the presence of vacancies in them, were investigated. For this purpose, its electronic structure was modeled for different variants of the spatial arrangement of atoms and the presence of vacancies in crystallographic positions. It is shown that the most acceptable results of experimental studies are the model of the electronic structure of p-LuNiSb, which assumes the presence of vacancies in the crystallographic positions of 4a Lu atoms (~0.005) and 4c Ni atoms (~0.04). In this model of the spatial arrangement of atoms and the presence of vacancies at positions 4a and 4c, the LuNiSb compound is a semiconductor of the hole-type conductivity, in which the Fermi level eF is located near the level of the valence band eV. The kinetic characteristics of the semiconductor thermometric material Lu1-xZrxNiSb, in particular, the temperature dependences of the resistivity ρ(T,x) and the thermopower coefficient α(T,x) are modeled. It is established that at the lowest concentrations of impurity atoms Zr the Fermi level eF Lu1-xZrxNiSb passes from the bandgap to the conduction band eС. This is indicated by the negative values of the thermopower coefficient α(T,x) and the metallic conductivity type Lu1-xZrxNiSb. This changes the type of main current carriers from holes to electrons.

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.

Strong spin-orbit torque effect on magnetic defects due to topological surface state electrons in Bi2Te3

We investigate the spin-orbit torque exerted on the magnetic moments of the transition-metal impurities Cr, Mn, Fe and Co, embedded in the surface of the topological insulator Bi$_{2}$Te$ _{3} $, in response to an electric field and a consequent electrical current flow in the surface. The multiple scattering problem of electrons off impurity atoms is solved by first-principles calculations within the full-potential relativistic Korringa-Kohn-Rostoker (KKR) Green function method, while the spin-orbit torque calculations are carried out by combining the KKR method with the semiclassical Boltzmann transport equation. We analyze the correlation of the spin-orbit torque to the spin accumulation and spin flux in the defects. We compare the torque on different magnetic impurities and unveil the effect of resonant scattering. In addition, we calculate the resistivity and the Joule heat as a function of the torque in these systems. We predict that the Mn/Bi$_{2}$Te$_{3}$ is optimal among the studied systems.

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.