Linear Muffin-tin Orbital Calculations Research Articles

Dynamical behavior of x-ray absorption and scattering at theLedge of titanium compounds: Experiment and theory

Detailed experimental results and model caclulations for the evolution of the x-ray fluorescence in the vicinity of the {ital L} absorption edge of titanium in metallic Ti and TiO and insulating TiO{sub 2} and BaTiO{sub 3} are presented. Behavior characteristic of x-ray Raman scattering was found in all samples. We develop a model that describes the valence band emission below the L{sub 3} absorption threshold in terms of the calculated partial density of states and the Kramers-Heisenberg x-ray scattering formalism. Linear muffin-tin orbital band structure calculations were used for the projected density of states in the model. In the region below the L{sub 3} absorption edge very good agreement between experiment and the model was found for all compounds. We propose a mechanism for the energy shift of the emission spectra for TiO{sub 2} and BaTiO{sub 3}. An explanation of the Raman energy loss in terms of the excitation of a valence electron into localized states of {ital d} symmetry in the conduction band is also provided. {copyright} {ital 1999} {ital The American Physical Society}

Band structure and electronic properties of the incommensurate misfit compound

The electronic properties of the incommensurate misfit compound are investigated by different methods. Linear muffin-tin orbital atomic-sphere approximation band-structure calculation shows that this material should be a metal with a Fermi level located near a minimum of the density of states. Experimentally, the electrical resistivity is hopping-like while the magnetic susceptibility is metallic-like with a strong enhancement. We suggest that these paradoxical properties can be reconciled by taking into account the effects of incommensurability and electronic correlations. A comparison with commensurate and with Ti or Cr incommensurate misfit derivatives is discussed.

Three-dimensional electron momentum densities of graphiteand fullerene: a comparison

We report on the three-dimensional electron momentum densities (EMD) of graphite and fullerene obtained by a so-called (,e) experiment, i.e. the coincident detection of an inelastically scattered hard x-ray photon with its recoil electron. A monochromatized flux of 1012 photons s-1 at 150 keV from the high-energy x-ray wiggler beamline of the ESRF was directed onto thin graphite or fullerene targets. Comparison with a pseudopotential and a full-potential linear muffin-tin orbital calculation in the case of graphite is made. Inclusion of electron correlation via the Lam-Platzman correction is discussed. The experimental EMD of C60 shows stronger electron delocalization in the `buckyball' compared to graphite, which is supported by theory.

Electronic and crystal structure of NiTi martensite

All of the first-principles electronic-structure calculations for the martensitic structure of NiTi have used the experimental atomic parameters reported by Michal and Sinclair [Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. B37, 1803 (1981)]. We have used first-principles, full-potential, linear muffin-tin orbital calculations to examine the total energy of all the experimental martensitic structures reported in the literature. We find that another crystal structure, that of Kudoh et al. [Acta Metall. Mater. 33, 2049 (1985)], has the lowest total energy at zero temperature. Ground-state and formation energies were calculated for all of the experimental structures. Total and local densities of states were calculated and compared with each other for the structures of both Kudoh et al. and Michal and Sinclair

Ab initio LMTO calculation of surface localized and resonance states of the SiC(110) zinc-blende surface

The surface localized and resonance states of unrelaxed and relaxed SiC(110) zinc-blende surfaces have been investigated within the local density approximation of density functional theory (DFT), employing a first-principles full-potential self-consistent linear muffin-tin orbital (LMTO) method using a thicker (thirteen layer) slab. Intrinsic surface states appear in the fundamental energy gap for the unrelaxed surface. We have allowed non-bond-length-conserving relaxation of the surface atoms to obtain the minimum energy configuration. The shift in the surface states arising from the relaxation of atoms is small in comparison to other studied heteropolar covalent semiconductors because of the smaller value of the relaxation angle of the SiC(110) surface. New surface states have been predicted in the valence band region. The present atomic geometry is somewhat different from other theoretical results. We observe an inward displacement of the Si cation and an outward displacement of the C anion both parallel and perpendicular to the surface, different from the results of Sabisch et al. No experimental data are available for comparison.

Local aspects of the adsorbate-substrate chemical bond in N/Cu(100) and O/Cu(100)

Both the occupied and unoccupied adsorbate valence states have been probed by x-ray emission and x-ray absorption spectroscopies for N/Cu(100) and the low coverage phase of O/Cu(100). By carrying out experiments using two different measurement geometries, the adsorbate out-of-plane states ${(2p}_{z})$ have been separated from the in-plane states ${(2p}_{\mathrm{xy}}).$ The experimental results are compared to the partial density of states (DOS) obtained from full-potential linear muffin-tin orbital calculations, with good agreement in the case of N/Cu(100). This agreement confirms that it is possible to interpret the experimental data in terms of the adsorbate partial DOS. Our combined experimental and theoretical study also provides a detailed understanding of the local chemical bond for N and O adsorbed on Cu(100).

Development of Finnis–Sinclair type potentials for Pb, Pb–Bi, and Pb–Ni systems: application to surface segregation

Empirical many-body potentials of a Finnis–Sinclair-type have been constructed for the Pb, Pb–Bi and Pb–Ni systems using both experimental data and physical quantities derived from ab initio linear muffin–tin orbitals calculations. Monte Carlo simulations using these potentials have been employed to compute the solubility limits of Pb–Bi and Pb–Ni alloys, as well as the segregation profiles at the (111) surface of Pb 95Bi 05. The profiles have been found to be oscillatory, with a preferential segregation of Bi occurring at the first surface atom layer, and a slight depletion of Bi in the second layer, in fair agreement with the results of the ab initio linear muffin–tin orbitals calculations. The calculations on Pb–Ni show that within the solubility limit of Ni in Pb, Ni does not segregate to the Pb(111) outermost surface layer, although weak segregation to the second layer is found.

Spectral momentum density of electrons in copper

The spectral-momentum density of electrons in a copper thin film has been directly measured using electron momentum spectroscopy. The measured spectral-momentum density shows two distinct features. The first is a free-electron-like parabola with dispersion spanning 10 eV in energy and 0.65 a.u. in momentum. The other is a weak and extended band located in a narrow range of energies from about 2 to 5 eV below the Fermi level. A spherically averaged linear muffin-tin orbital (LMTO) calculation of copper reproduces these features in both the dispersion pattern and the intensity. After taking into account the elastic and inelastic multiple scattering through a Monte Carlo simulation, the agreement between the calculation and the measurement is good. The measurement and the LMTO calculation are also compared with an available linear-augmented-plane-wave calculation for the energy-integrated electron momentum distribution of the valence band and the agreement is also good.

Electronic structure ofX2ZrSn- andXZrSn-type Heusler alloys withX=Coor Ni

X-ray photoemission spectroscopy (XPS) measurements have been performed on the intermetallic compounds ${\mathrm{Ni}}_{2}\mathrm{ZrSn},$ ${\mathrm{Ni}}_{x}{\mathrm{Zr}}_{1\ensuremath{-}x}\mathrm{Sn},$ ${\mathrm{Co}}_{2}\mathrm{ZrSn},$ and ${\mathrm{Co}}_{x}{\mathrm{Zr}}_{1\ensuremath{-}x}\mathrm{Sn}.$ The XPS valence bands show a small density of states at the Fermi level, and the position of Co and $\mathrm{Ni}d$ states below this energy. The core level XPS spectra are shifted in comparison with the respective spectra of the pure elements suggesting a redistribution of charge between atoms. XPS valence bands have been compared with total density of states distribution curves by the use of spin-polarized linear muffin-tin orbital calculations.

A Linear Muffin-Tin Orbital Calculation of Local Electronic and Magnetic Properties of YFe10Mo2and YFe10Mo2N

Structure and magnetic properties of YFe 10 Mo 2 and YFe 10 Mo 2 N compounds have been studied with neutron diffraction and self-consistent spin-polarized linear muffin-tin orbital (LMTO) band calculation. The diffraction results indicate that the nitrogen atoms are located at 2b sites with nearly 99% occupancy. The electronic structure calculations give excellent results for site-dependent Fe magnetic moments and hyperfine fields. The interstitial-atom effect on the local magnetic moments and hyperfine fields is explained with magneto-volume effect and chemical bonding effect. The contributions of the magneto-volume effect and the chemical bonding effect of the nitrogen to the magnetic moments (µ L o c ), Fermi-contact hyperfine fields ( H F C ) and isomer shifts ( I S ) are reported.

Ab initio LMTO calculation of the electronic structure of an ordered monolayer of Sb on a relaxed GaAs(110) surface

We present electronic surface states as obtained by a comprehensive and systematic study of a monolayer of Sb on a relaxed GaAs(110) zinc-blende surface using the first-principles full-potential self-consistent linear muffin tin orbital (LMTO) method. By considering the well accepted epitaxial continued layer structure (ECLS) model we investigate the surface states in the fundamental band gap along high-symmetry directions of the surface Brillouin zone. For the ordered overlayer Sb/GaAs(110), intrinsic surface states appear in the energy gap region which shift towards the bulk valence and the conduction band region when the relaxations of the surface atoms are considered. A detailed analysis of the surface and resonance states reveals that they are in excellent agreement with the available experimental data and the existing pseudopotential calculations.

Electronic topological and structural transition inAuIn2under pressure

Electrical resistivity, thermoelectric power, and high-pressure x-ray-diffraction measurements are carried out to investigate the anomaly observed earlier in fusion data around 3 GPa in the intermetallic compound ${\mathrm{AuIn}}_{2}.$ While the imaging plate high-pressure angle-dispersive data indicate a structural phase transition beyond 8 GPa, the thermoelectric power shows a peak around 2 GPa, indicating the occurrence of an electronically driven isostructural transition. The first-principles linearized muffin-tin orbital calculations reveal that this transition is brought about by interception of the Fermi level by the energy-band maximum. The Lifshitz nature of this transition is responsible for the anomaly in the high-pressure electrical and fusion data.

Interfacial Electronic Charge Transfer and Density of States in Short Period Cu/Cr Multilayers

ABSTRACTNanometer period metallic multilayers are ideal structures to investigate electronic phenomena at interfaces between metal films since interfacial atoms comprise a large atomic fraction of the samples. The multilayers studied were fabricated by magnetron sputtering and consist of bilayers from 1.9 nm to 3.3 nm. X-ray diffraction, cross-section TEM and plan-view TEM show the Cu layers to have a BCC structure Cu in contrast to its equilibrium FCC structure. The electronic structure of the Cu and the Cr layers in several samples of thin Cu/Cr multilayers were studied using x-ray absorption spectroscopy (XAS). Total electron yield was measured and used to study the white lines at the Cu L2 and L3 absorption edges. The white lines at the Cu absorption edges are strongly related to the unoccupied d-orbitals and are used to calculate the amount of charge transfer between the Cr and Cu atoms in interfaces. Analysis of the Cu white lines show a charge transfer of 0.026 electrons/interfacial Cu atom to the interfacial Cr atoms. In the Cu XAS spectra we also observe a van Hove singularity between the L2 and L3 absorption edges as expected from the structural analysis. The absorption spectra are compared to partial density of states obtained from a full-potential linear muffin-tin orbital calculation. The calculations confirm the presence of charge transfer and indicate that it is localized to the first two interfacial layers in both Cu and Cr.

Understanding the isomer shift of Fe impurities in metallic hosts

We have used first-principles linear muffin-tin orbital (LMTO) electronic structure calculations in the atomic sphere approximation (ASA), implemented directly in real space (RS) to investigate theoretically the behavior of the isomer shift of substitutional Fe impurities in metallic hosts. We find that the isomer shift in these systems is dominated by the 4$s$ contribution at the Fe site. Since both the volume occupied by the impurity and the valence of the metallic host vary widely in the systems studied here, the influence of these factors on the isomer shift could be investigated. When RS-LMTO-ASA basis functions are used, a simple picture emerges, which allows us to understand the observed trends of the isomer shift in terms of two quantities: the number of 4$s$ electrons at the impurity site and the probability of finding one of these electrons close to the nucleus. We find that probability follows a nearly universal curve as a function of the volume occupied by the impurity in the host, which leads to more negative isomer shift values as the Fe is compressed into smaller volumes. The number of 4$s$ electrons at the Fe site depends both on the volume occupied by the impurity in the host and on the host valence. Our approach can partially deconvolute these dependences, calling attention to the interesting processes which regulate the isomer shift trends.

Electronic structure of CeNi1−xPdxSn and LaMSn(M=Ni,Cu,Pd)

The electronic structure of CeNi${}_{1\ensuremath{-}x}$Pd${}_{x}$Sn has been studied by photoemission spectroscopy. CeNiSn belongs to the class of Kondo insulating materials. The gap formed at the Fermi level is strongly suppressed by substituting Pd for Ni. The x-ray photoemission spectroscopy (XPS) valence band spectra can be compared with ab initio electronic-structure calculations using the linearized muffin-tin orbital (LMTO) method. We have found a small indirect gap and a low density of states at the Fermi energy for CePdSn. The 3$d$ XPS spectra and LMTO calculations indicate a strong hybridization of the $f$ orbitals with conduction band and the interatomic hybridization which causes the large charge transfer between atoms. We have also observed the correlation between the electronic structure near Fermi energy and the crystallographic properties of the alloyed CeNiSn. We also present the electronic structures of LaNiSn, LaCuSn, and LaPdSn. These compounds are good reference for CeNiSn. At Fermi energy a relatively low density of states is found, for LaCuSn an indirect gap is formed. The metallic samples show a relatively high resistivity at room temperature, the largest for LaCuSn, which demonstrates the influence of the gap on the electric transport properties.

Effective masses and valence-band splittings in GaN and AlN

Full-potential (FP) linearized muffin-tin orbital (LMTO) calculations within the local density approximation are used to determine the conduction and valence-band effective mass tensors and the related Rashba-Sheka-Pikus and Kohn-Luttinger Hamiltonian parameters for wurtzite and zinc-blende GaN and AlN. Spin-orbit coupling effects are obtained from atomic-sphere approximation (ASA) LMTO calculations. The $c$-axis uniaxial strain dependence of the crystal field splitting is determined and the combined results are used to interpret experimental data on the strain dependence of exciton splittings in GaN. Corrections beyond the local density approximation on the effective masses and crystal-field splittings as well as the accuracy of the quasicubic model are discussed.

Electron momentum spectroscopy studies on the oxidation of aluminium

Electron momentum spectroscopy has been used to measure the electronic structure of aluminium after exposure to increasing amounts of oxygen. The experimental valence band results showed the structural features of aluminium oxide beginning to dominate the free electron contribution of metallic aluminium as the exposure increased. The interpretation of this is that the aluminium oxide layer increases in thickness as the exposure increases, eventually saturating at a thickness of about 15 Å. After subtracting the intensity of the underlying Al metal contribution from the oxidised states the valence band features of aluminium oxide were resolved even after exposure to only 600 L of oxygen. It is estimated that only about 5 Å of aluminium oxide is formed at this stage. A quantitative comparison was made to a linear muffin-tin orbital (LMTO) calculation for spherically averaged α-Al 2O 3. Best agreement was obtained for the results of the lowest oxygen exposure (600 L). The chemical shifts of the Al 2p core level were also measured in separate experiments for two intermediate oxygen exposures between the aluminium and aluminium oxide cases for further characterisation. These measurements show that electron momentum spectroscopy is able to obtain the electron bands and the momentum densities of very thin disordered surface layers.

Full charge-density scheme with a kinetic-energy correction: Application to ground-state properties of the 4d metals

We present a full charge-density technique to evaluate total energies from the output of self-consistent linear muffin-tin orbitals (LMTO) calculations in the atomic-sphere approximation (ASA). The Coulomb energy is calculated exactly from the complete, nonspherically symmetric charge density defined within nonoverlapping, space-filling Wigner-Seitz cells; the exchange-correlation energy is evaluated by means of the local-density approximation or the generalized gradient approximation applied to the complete charge-density; and the ASA kinetic energy is corrected for the nonspherically symmetric charge density by a gradient expansion. The technique retains most of the simplicity and the computational efficiency of the LMTO-ASA method, and calculations of atomic volumes and elastic constants of the 4d elements show that it has the accuracy of full-potential methods.

Fermi surface ofCeCu2Si2by 2D angular correlation of positron annihilation radiation

We have carried out 2D angular correlation of positron annihilation radiation measurements on oriented single crystals of the heavy-fermion superconductor ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. Measurements have been made at 50 and 4 K which are above and below the Kondo temperature, ${\mathrm{T}}_{\mathrm{K}}$\ensuremath{\sim}15 K, for this compound. The measurements show a small temperature dependence. Experimental results have been compared with linear muffin-tin orbital band-structure calculations for ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ and the isostructural non-f-electron compound, ${\mathrm{LaCu}}_{2}$${\mathrm{Si}}_{2}$. The measured anisotropies agree well with the calculations. In addition, at both temperatures, the measured [001] projected k-space occupation densities reflect clearly the Fermi-surface breaks predicted by the calculations for ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. This implies that local-density approximation calculations provide a good description of the electronic structure in ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. We have also measured the positron lifetime in both superconducting (S-type) and magnetic (A-type) samples of ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. The lifetime and intensity components in the S- and A-type polycrystals are different, suggesting that the nature of the defects that trap positrons is different in the two phases.

Valence-band structure of cubic CdS as determined by angle-resolved photoemission

The valence-band structure of cubic CdS along the \ensuremath{\Gamma}-\ensuremath{\Sigma}-X direction and at all high-symmetry points has been experimentally determined using angle-resolved photoemission and compared to two local density approximation (LDA) calculations as well as to a recent quasiparticle calculation. The Cd 4d level was found to be semibandlike with an energy dispersion of up to 1 eV. The energy difference between the experimental and our calculated linear-muffin-tin orbital (LMTO) LDA energies falls, as expected, along a line of positive gradient. The quasiparticle calculation by Pollmann and co-workers fits the experimental values somewhat better than the LMTO calculation, although a difference of \ensuremath{\sim}1.0 eV was still found to occur for the Cd 4d band. The self-interaction and relaxation-corrected pseudopotential LDA results by the same group give the best fit to within \ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}0.5 eV for nearly all critical energies measured. Comparison with previously reported photoemission results on the wurtzite structure shows that energies at equivalent symmetry points agree within experimental error.