Local Spin Density Functional Calculations Research Articles

Structural investigation ofNp2Co17and analogue compounds under pressure

The structural behavior of Np2Co17 is investigated by means of high-pressure diamond-anvil compression measurements and is compared with that of the isostructural compounds Lu2Co17 and Lu2Ni17. The Th2Ni17-type hexagonal crystal structure is preserved with no measurable discontinuous volume collapses up to the highest achieved pressure, p = 43 GPa. For Np2Co17, fits to the Birch-Murnaghan and Vinet equations of state give values of the isothermal bulk modulus and its pressure derivative of B-0 = 286 GPa and B-0' = 3, revealing that this Np compound is a highly incompressible solid with stiffness comparable to that of superhard covalently bonded materials. For the Lu2T17 (T = Co, Ni) compounds, the measured bulk modulus changes from B-0 = 137 GPa for T = Co to B-0 = 257 GPa for T = Ni. The isothermal equation of state for the studied compounds are in excellent agreement with the results of ab initio fully relativistic, full-potential local spin-density functional calculations. Theoretical estimates of the bulk modulus are given also for Np2Ni17, for which B-0 is predicted to assume values intermediate between those measured for Lu2Ni17 and Np2Co17.

Magnetic Moment Enhancement at Transition Metal and Rare-Earth Metal Surfaces

Magnetism at surface represents an area of great interest in surface physics 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. Here we review the prediction of local spin density functional ab initio electronic structure calculations which have played a key role in the development of this exciting field by providing a clearer understanding of the experimental observations and predicting new systems with desired properties.

A theoretical study of Fe adsorption along Bi-nanolines on the H/Si(0 0 1) surface

We have investigated the energetic stability and equilibrium geometry of the adsorption of transition metal Fe atoms near the self-organized Bi lines on hydrogen passivated Si(0 0 1) surface. Our total energy results show that there is an attractive interaction between Fe adatoms along the Bi-nanolines. For the energetically most stable configuration, the Fe adatoms are seven-fold coordinated, occupying the subsurface interstitial sites aside the Bi-nanolines. With increased coverage, Fe atoms are predicted to form two parallel lines, symmetrically on both sides of the Bi line. Within our local spin-density functional calculations, we find that for the most stable geometries the Fe adatoms exhibit an antiferromagnetic coupling.

On the bonding situation in TlCo2Se2

The electronic structure of TlCo2Se2 has been investigated by means of band structure calculations and x-ray photoelectron andemission spectroscopy (XPS and XES). The formation of the valence band is described inconnection with calculations of partial densities of states of the valence band and CoLα,β x-ray emission () aligned to the binding energy scale. The experimental results are in agreement with thecalculated distribution of Co 3d states, which lie close to the Fermi level. Theeffect of Co–Se bonding is seen as satellite structures in the XPS Co 2p signals.Thermoelectric and Hall effect data as well as resistivity measurements show thatTlCo2Se2 is metallic with electron holes as charge carriers with broadbandmobility characteristics. The observed anisotropy of the resistivity in theab planeand along c remains constant as a function of temperature, i.e. without signs of the magnetic ordering that occurs(TN∼85 K). The results of local spin density functional calculations show ferromagnetic couplingwithin the cobalt plane as a result of direct magnetic interactions between the magneticmoments.

Mn-dopedScN: A dilute ferromagnetic semiconductor with local exchange coupling

We present local spin density functional calculations for Mn-doped $\mathrm{ScN}$ using the linear muffin-tin orbital method in a supercell approach, including structural relaxation. Mn is found to induce a localized state in middle of the band gap of $\mathrm{ScN}$ with ${t}_{2g}$ character which spin splits and leads to a net magnetic moment of about $2--3{\ensuremath{\mu}}_{B}$ per Mn. The defect is a deep acceptor with 0/- transition state at about $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the Fermi level. The magnetic moment increases when in the negative charge state by acquiring more ${e}_{g}$ majority spin electrons and will thus persist even in the presence of the commonly found unintentional background $n$-type doping of $\mathrm{ScN}$. Up to fourth nearest neighbors exchange interactions are found to be ferromagnetic. The experimentally observed high solubility of Mn in $\mathrm{ScN}$, presumably related to their common crystal structure, is favorable for this dilute magnetic semiconductor. Within mean field theory the Curie temperature is estimated to be above $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for 3% Mn. While this may be overly optimistic because of the neglect of disorder, the $n$-type character of $\mathrm{ScN}$ adds further interest to this unusual magnetic semiconductor because the spin-split conduction bands are nondegenerate and thus essentially free from spin-orbit coupling induced spin relaxation.

Electronic structure of thePu-based superconductorPuCoGa5and of related actinide-115 compounds

We report a theoretical investigation of the electronic structure of the plutonium-based medium-high-${T}_{c}$ superconductor ${\mathrm{PuCoGa}}_{5}$, on the basis of ab initio local spin-density functional calculations. We furthermore report electronic structure calculations of the related actinide compounds ${\mathrm{PuRhGa}}_{5}$, ${\mathrm{PuIrGa}}_{5}$, ${\mathrm{UCoGa}}_{5}$, ${\mathrm{NpCoGa}}_{5}$, and ${\mathrm{AmCoGa}}_{5}$. ${\mathrm{PuRhGa}}_{5}$ is a superconductor as well, whereas the other materials do not become superconducting. The equilibrium lattice parameters within the tetragonal ${\mathrm{HoCoGa}}_{5}$ crystal structure are well reproduced for ${\mathrm{UCoGa}}_{5}$, ${\mathrm{NpCoGa}}_{5}$, as well as for the three isoelectronic $\mathrm{Pu}\text{\ensuremath{-}}115$ compounds when we assume delocalized $5f$ states. The possibility of a partial $5f$ localization occurring for the $\mathrm{Pu}\text{\ensuremath{-}}115$ compounds is discussed. The electronic structures of the three $\mathrm{Pu}\text{\ensuremath{-}}115$ compounds are computed to be rather similar: in each of the $\mathrm{Pu}\text{\ensuremath{-}}115$ materials the density of states at the Fermi energy is dominated by the $\mathrm{Pu}$ $5f$ contribution. Our total-energy calculations predict antiferromagnetic order to be favorable for all three $\mathrm{Pu}\text{\ensuremath{-}}115$ materials, which is, however, observed experimentally for ${\mathrm{PuIrGa}}_{5}$ only. Within the $\mathrm{Pu}\text{\ensuremath{-}}115$ series some small changes of the bands near the Fermi energy occur, which could be relevant for the superconductivity. A comparison of the ab initio calculated and the experimental properties clearly supports the picture of delocalized $5f$ electrons for ${\mathrm{UCoGa}}_{5}$. The neptunium-based 115 compound is predicted to order antiferromagnetically, which is supported by experiment. Also, the calculated magnetic moment $(0.85\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B})$ compares well with the measured moment $(0.84\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B})$. These findings advocate that the $\mathrm{Np}$ $5f$'s are still to some extent delocalized in ${\mathrm{NpCoGa}}_{5}$. In contrast, for the $\mathrm{Am}\text{\ensuremath{-}}115$ analog the $\mathrm{Am}$ $5f$ electrons can be expected to be localized. We furthermore discuss the theoretical Fermi surfaces and present calculated de Haas-van Alphen quantities for a comparison with future experiments. For ${\mathrm{UCoGa}}_{5}$ we obtain a semi-quantitative agreement with recently reported de Haas-van Alphen experiments. The possible origins of the superconductivity are discussed. Our investigation particularly reinforces the analogy to the heavy-fermion superconductors ${\mathrm{CeCoIn}}_{5}$ and ${\mathrm{CeIrIn}}_{5}$, however, with a stronger coupling strength due to a much stronger $5f$ hybridization.

Effects of intersite Coulomb interactions on ferromagnetism: Application to Fe, Co, and Ni

We reanalyze the condition for metallic ferromagnetism in the framework of the tight-binding approximation and investigate the consequences of the inter-site Coulomb interactions using the Hartree-Fock approximation. We first consider a non-degenerate $s$ band and we show that the inter-site interactions modify the occurrence of ferromagnetism, and we derive a generalized Stoner criterion. We analyze the main effects due to the renormalization of the hopping integrals by the inter-site Coulomb interactions. These effects are strongly dependent on the relative values of the inter-site electron-electron interactions and on the shape of the density of states as illustrated by a study of cubic crystals from which we establish general trends. We then investigate a realistic $spd$ tight-binding model, including intra (Coulomb and exchange) and inter-site charge-charge Coulomb integrals. This model is used to study the electronic structure (band structure, densities of states, magnetic moment) of bulk ferromagnetic 3d transition metals Fe(bcc), Co(hcp and fcc) and Ni(fcc). An excellent agreement with local spin density functional calculations is obtained for the three metals, in particular concerning the relative widths of the majority and minority spin bands. Thus our tight-binding Hartree-Fock model provides a consistent interpretation of this effect.

Synthesis and characterization of high quality ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K

ABSTRACTReactive MBE growth was used to synthesize ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K. 2% Cr-doped GaN and 7% Cr-doped AlN were found to have a saturation magnetization moment of 0.42 and 0.6 μB/Cr atom, indicating that ∼14% and ∼20% of the Cr, respectively, are magnetically active. Structural characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) did not find evidence of a ferromagnetic secondary phase. Electrical characterization indicate that the resistivity of the Cr-doped GaN films depends exponentially on temperature as R=Roexp[(To/T)1/2], characteristic of variable range hopping. In contrast, Cr-doped AlN films are highly resistive. Local spin density functional calculations predict that Cr forms a deep level defect in both systems and the t2 level falls approximately at midgap. Our theoretical and experimental results indicate that ferromagnetism in Cr-doped GaN and AlN arises as a result of the double exchange mechanism within the partially filled Cr t2 band.

Generalised Tight-Binding Stoner Model of Magnetism in Rh Clusters and Slabs

A tight-binding model that employs an orthogonal spd basis set and incorporates two-centre intra-atomic Hamiltonian elements as well as local environment dependent on-site atomic energy levels has been shown to have a high degree of transferability. This model is now extended to spin polarised systems by introducing on-site spin splitting that changes with the local magnetic moment, by analogy with density-functional theory. The parametrisation of this generalised Stoner model is determined by a simultaneously fit to the energy levels, average magnetic moments per atom and total energies obtained from local spin-density functional calculations for a number of cluster structures of up to 19 atoms in size, and fcc structures over a range of lattice spacings. We apply our method to rhodium clusters and slabs. For small clusters and ultra-thin films, the calculated magnetic moments and optimised geometric structure are in excellent agreement with first-principles calculations. The electronic structure and magnetic properties of clusters up to 201 atoms are calculated.

Spin-resolved density of states at the surface of NiMnSb

Spin-resolved photoemission measurements from polycrystalline NiMnSb films reveal spin-resolved density of states which are in qualitative agreement with local spin density functional band structure calculations [Phys. Rev. B 51, 10 436 (1995)]. The polarization of electrons close to the Fermi level is however found to be at most 40%, in contrast to the predicted half-metallic behavior. This discrepancy is attributed to lower remnant magnetization of the surface region and/or the presence of other phases.

Surface electronic and magnetic properties of semiconductor FeSi

First-principles local spin-density functional calculations of the electronic and magnetic properties of semiconducting FeSi (1 1 1) surface with both Fe- and Si-terminations have been performed. It is found that the Fe-terminated (1 1 1) surface is strongly ferromagnetic with a magnetic moment of 2.6 μ B/atom on the top Fe monolayer while the Si-terminated (1 1 1) surface is only weakly magnetic or nonmagnetic. These interesting results suggest that a semiconductor-based spintronics device might be developed by using thin FeSi (1 1 1) films connected to a metal lead on the Fe-terminated surface and to a semiconductor substrate on the Si-terminated surface.

Electronic structure and optical spectra of the semimetal ScAs and of the indirect-band-gap semiconductors ScN and GdN

Local (spin) density functional calculations for ScN and GdN are complemented with estimated quasiparticle corrections and calculations of the optical response to evaluate whether these materials are semimetals as suggested by some transport measurements or semiconductors as suggested by optical measurements. The quasiparticle corrections are estimated by assuming that gap corrections are inversely proportional to the dielectric constant and using experimentally known results on the quasiparticle d-band shift in ${\mathrm{Er}}_{x}{\mathrm{Sc}}_{1\ensuremath{-}x}\mathrm{As}.$ Results for the optical response functions and band structures are presented for ScAs, ScN, and GdN. The conclusion is that whereas ScAs is a semimetal, ScN and GdN are both narrow gap (0.9 and 0.7--0.85 eV, respectively) indirect gap $(\ensuremath{\Gamma}\ensuremath{-}X)$ semiconductors, with first direct gap at X at 2.0 and 1.1--1.2 eV, respectively. Due to the strong exchange interaction of $4f$ electrons with the d bands, GdN is predicted to have a magnetic-field-induced redshift of both the indirect and direct absorption edges of about 0.3 eV.

The electronic structures of uranium borides from local spin density functional calculations

The electronic structures of the binary uranium borides UB2, UB4, ‘UB6’, UB12 and MB6 (M=Th, Np) are calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The analyses of the properties of hybridization and of chemical bonding allow us to propose trends for explaining the stability of the different compounds.

Chemical bonding and magnetism in the germanides HT-UCo2Ge2, UGe3 and U3Co4Ge7 from local spin density functional calculations

The electronic and magnetic structures of HT-UCo2Ge2 (high temperature form), UGe3 and of the new intermetallic system U3Co4Ge7 are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) which was recently combined with the ASW method. From this we address the modifications of both chemical bonding characteristics and of magnetic polarisations of uranium and cobalt in HT-UCo2Ge2 and UGe3, these two compounds being considered as the building blocks of U3Co4Ge7.

Chemical bonding and magnetism in the ternary germanides UT2Ge2 (T = 3d transition metal) from local spin density functional calculations

The electronic and magnetic structures of UT2Ge2 (T = Mn, Fe, Co, Ni, Cu) germanides are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) which was recently combined with the ASW method. From this we propose a mechanism for the evolution of bonding within the series. The magnetic properties and orders of all compounds are discussed from spin-only as well as from spin-orbit coupling calculations.

Theoretical Study of CO and NO Vibrational Frequencies in Cu−Water Clusters and Implications for Cu-Exchanged Zeolites

Local spin density functional theory calculations of vibrational frequencies were performed for small Cu-containing complexes in an effort to assess models of exchanged Cu ion sites in zeolites and to help interpret infrared spectroscopy results. Model complexes consisted of Cun+ (n = 0−2) ions with varying coordination to water ligands and to more realistic fragments of zeolites. Calculated CO and NO vibrational frequencies for Cu-bound mono- and dicarbonyl and mono- and dinitrosyl species lie in ranges consistent with experimentally observed frequencies and confirm earlier assignments. Our results show a clear linear correlation between bond length and frequency for both carbonyl and nitrosyl complexes. The (nominal) oxidation state of Cu in these complexes is the most important factor in determining CO and NO frequencies and bond lengths, with the local coordination of Cu and the presence of explicit countercharges producing secondary effects.

Mass spectrometry and density functional studies of neutral and anionic tin clusters

The anionic tin clusters Sn x −, x = 2–7 have been generated and studied using laser ablation Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Gas phase ion-molecule chemistry has revealed that some clusters are reactive towards 2-propanol and hydrogen sulflde and all the clusters are unreactive towards methanol, ethanol, nitrous oxide and methane. Energy minimised structures, obtained from non-relativistic local spin density functional calculations including post-SCF non-local energy corrections, were doublets and favoured cyclic or closed structures for x ≥ 3. Inspection of the highest occupied molecular orbitals for the anions revealed significant delocalisation of the additional electron, leading to cluster anions which resemble a sphere, with the charge distributed evenly over all the atoms. Adiabatic electron affinities and atomisation energies determined from energy minimised neutral and anion calculations compare well with literature values.

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.

Theoretical investigation of substitutional Fe impurities in Gd

We applied the real space muffin-tin-orbital (RS-LMTO-ASA) scheme to study the electronic structure around substitutional Fe impurities in an h.c.p. Gd host. In these self-consistent local spin density functional calculations, the 4f electrons of Gd were treated as a half-filled core state, with all spins aligned. We performed calculations for the substitutional Fe impurity in both the ferromagnetic and antiferromagnetic configurations relative to the host, and obtained values for the magnetic moment, the hyperfine field and the isomer shift at the impurity site. The theoretical results are compared with experimental values found in the literature. This analysis shows that the moment of the Fe impurity orders antiferromagnetically with respect to that of the Gd host.

Geometric and electronic structures of the MoC 4 cluster

Local spin density functional calculations are performed to study the geometric and electronic structures of the MoC 4 cluster. A number of possible isomeric structures are examined. The most stable structures is found to be a linear chain, where Mo is at one end of the chain. The electronic structures of this chain in the neutral, cationic and anionic charge states are presented.