Local Spin Density Functional Research Articles

First principles study of the electronic and magnetic structures of U2Ni2SnH2

The electronic and magnetic properties and the chemical bonding in recently evidenced U2Ni2SnH2 are self-consistently calculated within the local spin density-functional (LSDF) theory using the scalar-relativistic augmented spherical wave (ASW) method. Trends of the magnetism are discussed in terms of the changes brought by hydrogen within the pure U2Ni2Sn alloy system from both the volume expansion simulating negative pressure and the bonding between H and lattice constituents U, Ni and Sn pointing to a larger Ni–H bonding versus U–H. The ground state is found to be antiferromagnetic in agreement with experiment. Considering the relativistic effects of spin–orbit coupling an ordered magnetic moment, mU=1 μB is calculated for U(5f), close to the experimental magnitude of mU=0.83 μB.

Band Magnetism in A2T2Sn (A=Ce, U; T=Ni, Pd) from Local Spin Density Functional Calculations

The electronic and magnetic properties of A2T2Sn (A=Ce, U; T=Ni, Pd) intermetallic systems are self-consistently calculated within the local spin density functional (LSDF) theory using the augmented spherical wave (ASW) method. Trends of the magnetism are discussed in terms of the characteristics of the Ce(4f) and U(5f) states as well as the energetic position of the transition metal element d states.

Structural, electronic and magnetic properties of nickel surfaces

The structural, electronic and magnetic properties of the low-index surfaces of Nickel have been investigated via fully self-consistent ab-initio local-spin-density-functional (LSDF) calculations. Our technique is based on ultrasoft pseudopotentials, residuum minimization techniques for the calculation of the electronic ground-state and of the Hellmann–Feynman forces and stresses, and on a conjugate-gradient technique for the optimization of the atomic structure. The calculations were performed for nine-layer symmetric slabs, allowing for the relaxation of the upper three layers. We also present a detailed analysis of electronic surface states.

Local spin density functional investigations of the ternary systems UT2Ge2 (T=Mn,Fe)

The electronic and magnetic structures of the ferromagnetic UMn2Ge2 and the paramagnetic UFe2Ge2 intermetallic systems are investigated ab initio and self-consistently within the Local Spin-Density Functional (LSDF) using the Augmented Spherical Wave (ASW) method. Agreement with experiment for the magnetic ordering as well as for the magnitudes of magnetization are found when spin-orbit coupling effects are included. Further, we discuss the chemical bonding within these systems from the Crystal Orbital Overlap Populations (COOP) which allow to assign a bonding or anti-bonding character for pair interactions.

Ab initio study of the chemical role of carbon within TiAl alloy system: Application to composite materials

Computations within the local spin density functional (LSDF) are carried out for the ternary Ti,Al,C system using the ASW (augmented spherical waves) method to address the chemical role of carbon within TiAl alloy whose electronic structure is firstly investigated. From the preliminary study of the hypothetical compounds formed by insertion and substitution we find carbon to preferably substitute for Al forming Ti-rich phases. The electronic structure of the actually forming carbide Ti2AlC compound is then carried out and a role played by carbon is addressed through chemical bonding results using the so-called COOP (crystal orbital overlap populations) implemented for the first time in an LSDF method.

Man-made materials—an exciting area for hyperfine-interaction investigations

Man-made low-dimensional magnetic systems including surfaces, interfaces and multilayers, have attracted a great amount of attention in the past decade because, as expected, the lowered symmetry and coordination number offer a variety of opportunities for inducing new and exotic phenomena and so hold out the promise of new device applications. Local spin density functional (LSDF) ab initio electronic-structure calculations employing the full-potential-linearized augmented-plane-wave (FLAPW) method have played a key role in the development of this exciting field by not only providing a clearer understanding of the experimental observations but also predicting new systems with desired properties. One of the striking successes of theory in the last decade has been the calculation of hyperfine fields at surfaces and interfaces. Concurrently, several groups have followed the pioneering work of Korecki and Gradmann and have measured hyperfine fields at surfaces and interfaces. In this paper, we review new features of hyperfine-interaction investigations in man-made materials which emphasizes how the close interplay of theoretical determinations and experiment are essential because the hyperfine field is not proportional to the magnetization and so interpretations of experiment are totally dependent on theory.

First-principles pseudopotential calculations of magnetic iron.

An ab initio pseudopotential density-functional study of magnetic iron using both the standard local-spin-density approximation and a generalized gradient-expansion approximation is described. The core-valence overlap in this material is found to be important and is treated carefully in the calculation. A mixed-basis approach is employed and the results are comparable with those from all-electron calculations which made use of the same local-spin-density-functional (LSDA) formalism. Results from both all-electron and pseudopotential calculations are found to have significant differences with experimental results. To go beyond LSDA, a recently proposed gradient-expansion exchange and correlation functional is implemented. The calculated results show significantly better agreement with experiment, giving the bcc ferromagnetic phase as the stable structure.

Electronic structure theory of surface, interface and thin-film magnetism

Low dimensional magnetic systems including surfaces, interfaces and thin-films, have attracted a great amount of attention in the past decade because, as expected, the lowered symmetry and coordination number offer a variety of opportunities for inducing new and exotic phenomena and so hold out the promise of new device applications. Local spin density functional (LSDF) ab initio electronic structure calculations played a key role in the development of this exciting field by not only providing a clearer understanding of the experimental observations but also predicting new systems with desired properties. Extensive calculated results reviewed here demonstrate that (1) weakened interatomic hybridization at clean surfaces or interfaces with inert substrates give rise to strong magnetic enhancement and (2) the strong interaction with nonmagnetic transition metals diminishes (entirely in some cases) the ferromagnetism and usually stabilizes the antiferromagnetic configuration. Surprisingly, experimentally observed surface (interface) magnetic anisotropy can be reproduced correctly in the theoretical calculations, although the anisotropy energy is only ∼10 −4−10 −5 eV.

Structural, Electronic, and Magnetic Properties of Thin Films and Superlattices

The importance of interfaces in determining the physical properties of technologically important materials — addressed in the Guest Editors' introduction to the articles in this issue — has stimulated theoretical efforts to determine from first principles a detailed understanding of their chemical, electronic, and mechanical properties. Fortunately, this has now been made possible as a result of the dramatic advances in condensed matter theory made in the last decade, driven in large part by new and sophisticated experiments on high-purity materials that have been well and carefully characterized. Particularly in electronic structure, these advances may be attributable directly to the close collaboration of theoretical and experimental researchers. Indeed, the new found ability to apply fundamental theoretical concepts to real materials (rather than to simple model systems) made possible by using the continued rapid development of computer power, has served to fill the increasingly urgent demand of experimentalists for theoretical interpretation of their data. Also, in some cases, these computational efforts can be used to provide data that would be currently impossible or impractical to obtain experimentally. This development has been an essential element in the phenomenal growth in this area of materials science.More specifically, the advent of accurate self-consistent (local) spin density functional (LSDF) calculations for surfaces, interfaces, and multilayers means that theory is no longer limited to simple, parameter-dependent models. These complex systems are of growing interest because the reduced symmetry, lower coordination number, and availability and role of highly localized surface and interface states offers the possibility of inducing new and exotic phenomena and promotes the possibility of new device applications.

Band-structure description of Mott insulators (NiO, MnO, FeO, CoO)

The unoccupied-states potential correction (USPC) in the local spin-density-functional (LSDF) formalism is proposed as a way to take correlation effects into account approximately. The method is based on using different potentials for the electrons in valence and conduction bands. When applied to antiferromagnetic transition-metal oxides (NiO, MnO, FeO, CoO), it solved the long-standing problem of reproducing the experimental values of the band gaps, which are underestimated in normal LSDF calculations. FeO and CoO become insulators, in contrast to the metallic ground state obtained in the usual LSDF calculations. Moreover, the calculated magnetic properties improved considerably due to the introduction of the USPC. It is shown that band-structure calculations using the USPC provide a correct description of available data from spectral measurements (ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, bremsstrahlung isochromate spectroscopy) for these compounds.

Local spin density functional approach to the theory of exchange interactions in ferromagnetic metals and alloys

Rigorous expressions for the exchange parameters of classical Heisenberg model applied to crystals are obtained using a local spin density functional (LSDF) approach and KKR-Green functions formalism. The spin wave stiffness constant and Curie temperature ( T c) of ferromagnetic metals are obtained without any model assumptions as to the character of exchange interactions. The concentration dependence of T c for binary ferromagnetic alloys is investigated in the framework of the single-site CPA-theory. The corresponding calculations are carried out for simple metals Fe, Ni and disordered NiPd alloys.

Generalized exchange local-spin-density-functional theory: Introduction and theory.

A local-spin-density-functional (LSD) scheme is developed using the normalization conditions for an electron gas around a nucleus of charge Z. The correlation factor for electrons of parallel spin, the Fermi hole, is decomposed into single-particle correlation factors. This leads to an orbital-dependent exchange-only density with no adjustable parameters such as the \ensuremath{\alpha} of the X\ensuremath{\alpha} theory; the parameters ${B}_{1}$, ${B}_{2}$, and ${\ensuremath{\alpha}}^{\mathrm{lim}}$ are constant for all atoms once the shape of the Fermi hole is chosen. These parameters are rigorously calculated without assuming an approximate shape for the Fermi-hole correlation factor. The exchange density using this unspecified Fermi-hole correlation factor reduces exactly to the homogeneous free-electron-gas exchange density at the high electron-density limit. The theoretical relationship of this generalized exchange (GX) scheme to the X\ensuremath{\alpha} and free-electron-gas exchange densities is discussed, and it is found that the X\ensuremath{\alpha} scheme needs a classical approximation to justify the use of a variable \ensuremath{\alpha}. These schemes are compared numerically using the total energies and eigenvalues for the atoms helium to krypton, and it is found that the LSD GX scheme gives the most reliable results.

LSDF-approach to the theory of exchange interactions in magnetic metals

In the framework of the KKR-Green's function formalism and the local spin-density functional (LSDF) approach the rigorous expressions applicable for band-structure calculations of effective exchange interactions parameters and spin-wave stiffness constant are presented. The Curie temperature for ferromagnetic metals are also calculated making use of the mean-field approximation.

Local spin density functional analysis of exchange and correlation effects on some first row diatomic molecules

We have constructed an extended Hartree–Fock computational method and program which is very similar to standard quantum chemical program packages for molecular electronic structure calculations but allows exchange and/or correlation to be treated by a local approximation as an option. The local spin density functionals accounting for exchange and/or correlation can be included in the self-consistent field (SCF) iterations to examine their effect or added in the final estimation of the total energy to gain efficiency at an often marginal loss in accuracy. We report calculations for H2, HF, N2, F2, and OH− in modest basis sets to examine the effect of local approximations for exchange and/or correlation on the bond strength, bond length, and vibrational frequencies of small covalently bonded molecules. The results show a systematic pattern which can be understood in terms of the dependence of the Hartree–Fock exchange effect on the spatial extent of the canonical orbitals.

THE LOCAL SPIN-DENSITY-FUNCTIONAL METHOD : APPLICATION TO THE METAL-INSULATOR TRANSITION IN DOPED SEMICONDUCTORS

The principles of the Local Spin-Density-Functional (LSDF) method, its usefulness to study interacting electron systems, especially those involving local magnetic moments, and its various applications are outlined. Experimental evidences for magnetic moments localized on charged impurities in doped semiconductors are briefly reviewed. The insulator-to-metal transition and the vanishing of local spin polarization, which occur as the impurity concentration increases, are described using the LSDF formalism.