Kohn Rostoker Research Articles

Soft X-ray Fermi surface tomography of palladium and rhodium via momentum microscopy

Fermi surfaces of transition metals, which describe all thermodynamical and transport quantities of solids, often fail to be modeled by one-electron mean-field theory due to strong correlations among the valence electrons. In addition, relativistic spin–orbit coupling pronounced in heavier elements lifts the degeneracy of the energy bands and further modifies the Fermi surface. Palladium and rhodium, two 4d metals attributed to show significant spin–orbit coupling and electron correlations, are ideal for a systematic and fundamental study of the two fundamental physical phenomena and their interplay in the electronic structure. In this study, we explored the Fermi surface of the 4d noble metals palladium and rhodium obtained via high-resolution constant initial state momentum microscopy. The complete 3D-Fermi surfaces of palladium and rhodium were tomographically mapped using soft X-ray photon energies from 34 eV up to 660 eV. To fully capture the orbital angular momentum of states across the Fermi surface, the Fermi surface tomography was performed using p- and s- polarized light. Applicability and limitations of the nearly-free electron final state model in photoemission are discussed using a complex band structure model supported by experimental evidence. The significance of spin–orbit coupling and electron correlations across the Fermi surfaces will be discussed within the context of the photoemission results. State-of-the-art fully relativistic Korringa–Kohn–Rostoker (KKR) calculations within the one-step model of photoemission are used to support the experimental results.

Topological Phase Transitions Driven by Sn Doping in (Mn1−xSnx)Bi2Te4

The antiferromagnetic ordering that MnBi2Te4 shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to its topologically nontrivial nature and a number of fundamental phenomena. At the same time, the possibility to control the electronic and magnetic properties of this system can provide new effective ways for its application in devices. One of the approaches to manipulate MnBi2Te4 properties is the partial substitution of magnetic atoms in the compound with atoms of non-magnetic elements, which inevitably affect the interplay of magnetism and band topology in the system. In this work, we have carried out theoretical modelling of changes in the electronic structure that occur as a result of increasing the concentration of Sn atoms at Mn positions in the (Mn1−xSnx)Bi2Te4 compound both using Korringa–Kohn–Rostoker (KKR) Green’s function method as well as the widespread approach of using supercells with impurity in DFT methods. The calculated band structures were also compared with those experimentally measured by angle-resolved photoelectron spectroscopy (ARPES) for samples with x values of 0, 0.19, 0.36, 0.52 and 0.86. We assume that the complex hybridization of Te-pz and Bi-pz orbitals with Sn and Mn ones leads to a non-linear dependence of band gap on Sn content in Mn positions, which is characterized by a plateau with a zero energy gap at some concentration values, suggesting possible topological phase transitions in the system.

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.

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.

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.

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.

Improving desorption temperature and kinetic properties in MgH2 by vacancy defects: DFT study

The aim of this work is the improvement of the desorption temperature and kinetic properties in MgH2 by magnesium vacancy defects VMg. We use the Korringa – Kohn - Rostoker (KKR) calculation combined with the coherent potential approximation (CPA) in this work. In particular, we find that the formation energy increases with the increasing VMg concentration and, vice versa, for the desorption temperature in MgH2. We also find that the magnesium vacancy defects have an effect on the gravimetric hydrogen capacity by making the magnesium hydride more lightweight. Moreover, the densities of states (DOS) indicate that the stability of MgH2 decreases with the increasing VMg concentration by shifting the Mg and H states to the conduction band (CB). In particular, we observe that it is difficult, after ∼4.8%, to storage the hydrogen into the system without cooling, because the desorption temperature becomes less than 0 °C. We also find that the optimal VMg concentration for the hydrogen vehicles is about 3.7% because its desorption temperature is close to the operating temperature of most modern vehicle engines.

First-principles calculation of transition-metal Seebeck coefficients

The Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA; KKR-CPA) and linear response theory is applied to first-principles calculation of the Seebeck coefficients of pure metals. The main objective is to develop a practical first-principles scheme that can calculate the conductivities and Seebeck coefficients of metallic systems at finite temperature. Thus, it is necessary to include the effects of electron-phonon scattering, which plays a crucial role at finite temperature, particularly for ordered-structure systems where the conductivity diverges at T=0K. The approach combines three components: linear response theory in the framework of the KKR method; phonon calculations; and an alloy analogy applied to the local static phonons using the KKR-CPA. The calculated Cu resistivity and Seebeck coefficients for various transition-metal elements at finite temperature show reasonably good overall agreement with experiment. The present approach provides a framework applicable to a wide range of materials, including pure metals, compounds, doped semiconductors, ordered and disordered alloys, opening up the possibilities of computational design of useful thermoelectric materials.

First-principles calculations of electronic, magnetic and optical properties of HoN doped with TM (Ti, V, Cr, Mn, Co and Ni)

Using the first-principles calculations within the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA), the structural, optical and magnetic properties of rare-earth nitride Ho[Formula: see text]TM[Formula: see text]N doped with transition metal (TM) atoms (Ti, V, Cr, Mn, Co and Ni) are investigated as a function the generalized gradient approximation and self-interaction correction (GGA–SIC) approximation. The optical properties are studied in detail by using ab-initio calculations. Using GGA–SIC we have showed that the bandgap value is in good agreement with the experimental value. Using GGA–SIC approximation for HoN, we have obtained a bandgap of 0.9 eV. Some of the dilute magnetic semiconductors (DMS) like Ho[Formula: see text]TM[Formula: see text]N under study exhibit a half-metallic behavior, which makes them suitable for spintronic applications. Moreover, the optical absorption spectra confirm the ferromagnetic stability based on the charge state of magnetic impurities.

Ab initio calculations of the magnetic properties of TM (Ti, V)-doped zinc-blende ZnO

In order to promote suitable material to be used in spintronics devices, this study purposes to evaluate the magnetic properties of the titanium and vanadium-doped zinc-blende ZnO from first-principles. The calculations of these properties are based on the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA), using the local density approximation (LDA). We have calculated and discussed the density of states (DOSs) in the energy phase diagrams for different concentration values, of the dopants. We have also investigated the magnetic and half-metallic properties of this doped compound. Additionally, we showed the mechanism of the exchange coupling interaction. Finally, we estimated and studied the Curie temperature for different concentrations.

Fully-relativistic full-potential multiple scattering theory: A pathology-free scheme

The Green function plays an essential role in the Korringa–Kohn–Rostoker(KKR) multiple scattering method. In practice, it is constructed from the regular and irregular solutions of the local Kohn–Sham equation and robust methods exist for spherical potentials. However, when applied to a non-spherical potential, numerical errors from the irregular solutions give rise to pathological behaviors of the charge density at small radius. Here we present a full-potential implementation of the fully-relativistic KKR method to perform ab initio self-consistent calculation by directly solving the Dirac differential equations using the generalized variable phase (sine and cosine matrices) formalism Liu et al. (2016). The pathology around the origin is completely eliminated by carrying out the energy integration of the single-site Green function along the real axis. By using an efficient pole-searching technique to identify the zeros of the well-behaved Jost matrices, we demonstrated that this scheme is numerically stable and computationally efficient, with speed comparable to the conventional contour energy integration method, while free of the pathology problem of the charge density. As an application, this method is utilized to investigate the crystal structures of polonium and their bulk properties, which is challenging for a conventional real-energy scheme. The noble metals are also calculated, both as a test of our method and to study the relativistic effects.

Calculated magnetic properties of co-doped CdTe(V, P): First-principles calculations

In order to shed light on the magnetic properties of the host compound CdTe co-doped with the vanadium and the phosphorus elements. This work aims to contribute to the research for development and innovation to find new materials used in spintronic devices. For this purpose, we apply the first-principles calculations. We have investigated and discussed the half-metallic ferromagnetic behavior and the polarization as function of the concentrations of the diamagnetic compound phosphorus. The Korringa–Kohn–Rostoker (KKR) method is combined with the coherent potential approximation, within the local density approximation. Moreover, analyzing the calculated density of states, the mechanism of the exchange coupling interaction has been investigated. Furthermore, the stability of the system is studied by calculating the total energies. Besides, we have estimated and compared the evolution of the Curie temperature in connection with the room temperature. The theoretical Curie temperature is estimated by using the mean field approximation. Finally, as a function of the concentrations of phosphorus we have shown how the crystal field and the exchange splittings vary.

Electronic, Magnetic and Optical Properties of TM (Ti, V, Cr, Mn and Co)-Doped Rare-Earth Nitride GdN: First-Principles Theory

The half-metallic ferromagnetic behavior of rare-earth nitride Gd0.95 TM0.05N (TM = Ti, V, Cr, Mn and Co), based on diluted magnetic semiconductors (DMSs), is investigated using the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA) within a framework of density functional theory (DFT). The energy difference between the ferromagnetic and disorder local moment states has been evaluated. The exchange interactions obtained from first-principles calculations resulted in ferromagnetic states with Curie temperatures within the ambient conditions. Moreover, the optical absorption spectra obtained by ab initio calculations confirm the ferromagnetic stability based on the charge state of magnetic impurities.

Influence of Cu Doping on the Electronic Structure and Magnetic Properties of the Mn2VAl Heusler Compound

Detailed investigations on the electronic and magnetic properties of the Heusler compounds Mn2−xCuxVAl with L21 structure have been performed. Polycrystalline samples have been examined by X‐ray diffraction and magnetization measurements have been performed. The degrees of the B2 and L21 atomic ordering obtained from the intensity ratios of the X‐ray patterns have been determined. The Curie temperatures decrease with Cu content, ranging between 771 K (x = 0) and 580 K (x = 0.5). Additionally, electronic band structure calculations using the Korringa–Kohn–Rostoker (KKR) Green's function method have been performed. The substitutional disorder was accounted by the means of the Coherent Potential Approximation (CPA). The site occupation considered in the calculations has been correlated with those obtained by the X‐ray experiments. Our study is giving an insight on the evolution of the half‐metallic fully compensated ferrimagnet (HMFi) character with disorder and Cu doping showing restrictions in obtaining of a HMFi by doping with 3d metals.

Effect of Tuning the Structure on the Optical and Magnetic Properties by Various Transition Metal Doping in ZnO/TM (TM = Fe, FeCo, Cr, and Mn) Thin Films

We have investigated in this work the physical, optical, electronic, and the magnetic behavior (Curie temperature, magnetic moment) of Zn.1-x.MxO (M = Fe 5%, Co 1%, Cr 5%, and Mn 5%), diluted magnetic semiconductors (DMSs). The samples were deposited on glass substrate by the spray pyrolysis technique, and the results of the x-ray diffraction (XRD) of the prepared substrates was used to prove the incorporation of the dopants into the ZnO lattice host; the ferromagnetic and the antiferromagnetic state competitions and their effects on the physical, magnetic, and optical properties, were investigated. The electronic structure and magnetic properties of transition metal (TM) defects, were investigated in detail, by using the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA). As a result, doping by TM impurities induce the ferromagnetism with different competitions between the ferromagnetic and antiferromagnetic states, which affects the physical properties of the TM (Fe, Fe/Co, Cr, and Mn)-doped ZnO, with a Curie temperature closer to room-temperature ferromagnetic.

Electronic and Magnetic Properties of SnFe2O4 Spinel Ferrites

In this work, the electronic and magnetic properties of SnFe2O4 spinel ferrite with various cation distributions (mixed, normal and inverse spinel phases) were studied. The calculations were performed by Korringa–Kohn–Rostoker (KKR)-coherent potential approximation (CPA) method with generalized gradient (GGA) approximation for the exchange and correlation functional. Our spin-polarized calculations give a half-metallic character for SnFe2O4 inverse spinel phase and near half-metallic character for SnFe2O4 mixed spinel phase and metal character for SnFe2O4 normal spinel phase. These results, which contribute to understanding the effect of the cation distribution in SnFe2O4 ferrite with spinel structure in the existence of gap states occupied by majority and minority spin electrons, the spin magnetic moments, the magnetization and the half-metallic behaviour.

Synthesis and characteristics of Mg doped ZnO thin films: Experimental and ab-initio study

Structural and physical properties of Mg doped zinc oxide (MZO) thin films have been investigated. The samples have been synthesized using a simple spray pyrolysis technique. The hexagonal Wurtzite structured ZnO thin films were confirmed by X-ray diffraction (XRD). The morphology of the thin films was characterized by atomic force microscopy (AFM). The optical properties were studied in details by using ultraviolet–visible (UV–Vis) spectrophotometry and Hall Effect measurements for electrical characteristic. The results detected by AFM indicate in a qualitative and quantitative manner how the surface properties (topography, roughness) vary with Mg concentration. The optical characterization showed that the transmittance of MZO thin films increased from 75 to about 90% and optical band gap energy from 3.20 to 3.43 (eV). The electrical properties study showed the lowest resistivity value was 3.45×10−2 (Ω·cm), which was obtained for the film doped with ZnO:Mg (2at.%). In order to confirm our results on optical properties, the band structure of Wurtzite Zn1−xMgxO was calculated by ab initio calculations using the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA).

Ab Initio Study of Electronic and Magnetic Properties of Ga1-x Co x N (Doped) and Ga1-x-y Co x Cr y N (Co-doped)

Using the Korring–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA) we study the electronic structure and magnetic properties of the doped and co-doped of Ga1-xCoxN and Ga1-x-yCoxCryN materials with single and double impurities, respectively. The total and local density of state (DOS) in two systems are obtained We investigate the effect of the different values of dilution in Ga1-xCoxN and Ga1-x-yCoxCryN. On the other hand, the magnetic moment and the gap energies are deduced for different values of concentrations.

Excess of manganese in an iron-based superconductor: a first-principles study of its electronic, magnetic and superconducting properties

Alloying and doping have always been an effective way of improving the properties of materials—be it electronic, magnetic or superconducting. In this direction, we present a first-principles investigation of the electronic, magnetic and superconducting properties of the systems (where x = 0.0, 0.01, 0.02 and 0.03). The calculations were performed using the Korringa–Kohn–Rostoker (KKR) atomic sphere approximation (ASA). In order to treat the disorder, we employed the coherent-potential approximation (CPA) into the KKR method. We performed both unpolarized and spin-polarized calculations and found that these systems are close to being magnetic. The results have been analyzed in terms of the density of states (DOS), electronic band structures, Fermi surfaces and the superconducting transition temperature Tc. We find that there are no significant changes in the shape of the Fermi surfaces in unpolarized calculations with respect to pure FeSe, but in spin-polarized calculations, the excess of the transition-metal Mn atoms substantially modifies the Fermi surface compared with the parent FeSe and compounds. We have also estimated the value of electron–phonon mass enhancement factor λ which is the sum of the electron–phonon coupling constant as well as the electron–electron interaction. Using λ, we have calculated the superconducting transition temperature Tc for the Mn doped superconductor. The calculated Tc are in good agreement with the experimental results. Additionally, Hopfield parameters are also calculated from the charge self-consistent potential using the KKR–ASA–CPA method for understanding the trend of Tc and DOS at Fermi energy.

Fully relativistic multiple scattering calculations for general potentials

The formal basis for fully relativistic Korringa-Kohn-Rostoker (KKR) or multiple scattering calculations for the electronic Green function in case of a general potential is discussed. Simple criteria are given to identify situations that require to distinguish between right and left hand side solutions to the Dirac equation when setting up the electronic Green function. In addition various technical aspects of an implementation of the relativistic KKR for general local and non-local potentials will be discussed.