Showing papers by "Arthur J Freeman published in 1984"

Total-energy full-potential linearized augmented-plane-wave method for bulk solids: Electronic and structural properties of tungsten

Abstract: The development of the all-electron full-potential linearized augmented-plane-wave (FLAPW) method for bulk solids is reported. As in the thin-film FLAPW approach, the bulk FLAPW method solves the Kohn-Sham equations for a general charge density and potential. The formalism of Weinert, Wimmer, and Freeman for determining highly accurate total energies of solids within density-functional theory is implemented with all the necessary terms obtained from the FLAPW energy-band calculation. The resulting total-energy FLAPW approach is used to obtain highly accurate total-energy curves for bcc and fcc tungsten from which a number of structural properties (lattice parameters, bulk moduli, etc.) are derived. Calculated total energies have a relative precision of 0.1 mRy; a difference of 34 mRy is found between the (stable) bcc and fcc phases. The use of a simple quadratic form near the equilibrium value of the atomic volume is shown to lead to relatively large errors for the bulk modulus. Finally it is shown that in this all-electron method, all numerical approximations are controlled in that their effects can be minimized. One can therefore conclude that the FLAPW method is very well suited for testing the quality of various implementations of density-functional theory.

Interface magnetism in metals: Ag/Fe(001)

Abstract: A study of the effect of an ordered overlayer of Ag on the magnetism of an Fe(001) surface is presented. In order to understand the changes induced at the Ag/Fe interface, a series of self-consistent spin-polarized local-spin-density calculations were carried out with the use of our allelectron, full-potential, linearized augmented-plane-wave method. While the charge density is found to approach the bulk value within approximately one layer of the interface, the spin density at the interface is found to be strongly perturbed. An enhancement of the Fe magnetic moment at the interface (to 2.52 ${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$/atom) is predicted which, however, is significantly less than that found for the clean Fe(001) surface (2.95 ${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$). The role of the Ag overlayer in delocalizing the Fe surface states responsible for the increased surface magnetization and the effect on the single-particle spectra is described and discussed. The effect of the surface and interface on the contact hyperfine fields is evaluated: The (negative) core-polarization contribution is found to scale with the moment, but the valence contribution changes sign from negative in the interior to positive at the surface. The physical basis and the relationship of these results to the interpretation of M\"ossbauer-effect measurements of the hyperfine field are described.

Prediction of electronic surface states in layered materials: graphite

Abstract: Thin graphite films consisting of up to 25 atomic layers are studied with self-consistent local-density all-electron theory and model-potential calculations. Unoccupied surface states confined in the direction of the $c$ axis are predicted at 3.8 eV above the Fermi energy. They are split off from the bulk free-electron interlayer band. These results provide a convincing interpretation of the surface states found in recent inverse photoemission experiments by Fauster et al.

Origin of the two-peak photoemission and inverse-photoemission spectra in Ce and Ce compounds

Abstract: The origin of the two-peak photoemission and inverse-photoemission spectra in Ce and its pnictide compounds (CeP, CeSb, and CeN) is investigated with ab initio total-energy self-consistent supercell energy-band and molecular-cluster approaches to represent physically the competing mechanisms involved in $d$ or $f$ electron screening (antiscreening in the case of inverse photoemission) of the local $4f$ electron excitations. Good agreement with experiment is obtained in all cases including the "anomalous" case of CeN.

Energetics of Surface Multilayer Relaxation on W (001): Evidence for Short-Range Screening

Abstract: Etude de la relaxation multicouche de la surface de W(001) par l'approche de l'energie totale de fonctionnelle de densite locale de tous les electrons

Bonding of surface states on W(001): All-electron local-density-functional studies

Abstract: Results of self-consistent all-electron full-potential linearized-augmented-plane-wave method local-density-functional calculations are presented for W(001) films consisting of 1, 3, 5, 7, 9, and 8 layers. The theoretical value of the work function for films with five and more layers is found to be within 0.05 eV of the experimental value of 4.63 eV. It is demonstrated that a film of seven layers is sufficiently thick to describe surface states and surface resonance states. An analysis of the dispersion and the change of bonding for the surface (resonance) states of Sigma-bar symmetry suggests that the discrepancies between experimental and theoretical surface energy band structures concerning surface states and surface-resonance states near the M-bar symmetry point are due to displacements of the surface atoms in the high-temperature phase of the W(001) system.

Small-cluster analogs of CO adsorption on Cu/Ru(0001): Total-energy and carbon-metal stretch frequency calculations

Abstract: Total energies as a function of carbon-metal-layer separation and carbon-metal perpendicular stretch frequencies are calculated for small clusters configured to be analogs for the chemisorption system CO on Cu/Ru(0001). The clusters (with 7--10 metal atoms) include analogs of CO/Cu(111) and CO/Ru(0001) with on-top and threefold binding sites. The latter are more stable in both cases. Mixed-metal clusters are also studied (i) with CO bound to Cu in proximity to Ru and (ii) with CO bound to Ru in proximity to Cu. The former, case (i), show enhanced CO binding with respect to CO on pure Cu, and the latter, case (ii), show weakened CO binding with respect to CO on pure Ru. Possible precursor intermediate states are described for commensurate and incommensurate epitaxy. Bonding to Ru sublayers through a Cu adlayer is also described. The results are discussed in conjunction with previous experimental findings, and anticipate future experimental vibrational characterizations.

Electronic structure of the Pt(001) surface with and without an adsorbed gold monolayer

Abstract: Results of all-electron self-consistent semirelativistic local-density-functional linearized-augmented-plane-wave (LAPW) investigations of the clean and Au-covered Pt(001) surface are presented. The charge density within the fourfold hollow sites at the surface was found to be very similar on both surfaces, as expected. The work function of the Au/Pt surface was reduced by 0.43 eV compared to the clean Pt surface. The interface atom 4f/sub 7.2/ core-state level on the Au/Pt surface is shifted by 0.3 eV to reduced binding energy. On the clean Pt surface, the density of states (DOS) on the surface atomic layer shows a large peak at about -1.0 eV due to surface states. This peak persists at -1.0 eV after Au coverage on the Au/Pt surface and is due to a band of interface states localized on the interface Pt atomic layer. Significantly, however, there are no states on the Au/Pt surface which are localized both on the Au and interface-Pt layers. Furthermore, the d-band DOS on the adsorbed Au layer is fully occupied. These results are used to discuss the experimentally observed enhanced reactivity of the Au/Pt surface and lead to the conclusion that the morphology of the experimentally observed surface may be quite different from thatmore » previously thought and modeled here.« less

Electronic structure and superconductivity of fcc Cr

Abstract: Results of self-consistent electronic structure calculations are reported for metastable fcc Cr metal. Unlike the case of bcc Cr which has E/sub F/ at a minimum in the density of states (DOS), the DOS at E/sub F/ in fcc Cr is at a peak making this one of the higher-DOS metals with the fcc structure (e.g., comparable with that of Ni and Pt). A calculated Stoner factor of 0.82 indicates that ferromagnetic ordering is not expected. Calculations of the electron-phonon coupling parameter lambda and superconducting transition temperature T/sub c/ were made using the rigid-ion approximation and strong-coupling theory with various estimates of the (unknown) phonon contribution. We conclude that T/sub c/'sroughly-equal2.5 K are reasonable, although they are substantially smaller than the T/sub c/roughly-equal10 K derived from measurements on Au-Cr-Au sandwiches.

Generation of H−, D− ions on composite surfaces with application to surface/plasma ion source systems

Abstract: We review some salient features of the experimental and theoretical data pertaining to hydrogen negative ion generation on minimum work function composite surfaces consisting of Cs/transition metal substrates. Cesium or hydrogen ion bombardment of a cesium‐activated negatively biased electrode exposed to a cesium–hydrogen discharge results in the release of hydrogen negative ions. These ions originate through desorption of hydrogen particles by incident cesium ions, desorption by incident hydrogen ions, and by backscattering of incident hydrogen. Each process is characterized by a specific energy and angular distribution. The calculation of ion formation in the crystal selvage region is discussed for different approximations to the surface potential. An ab initio, all‐electron, local density functional model for the composite surface electronics is discussed.

Electronic structure and magnetism of interfaces : sandwiches and modulated structures

Abstract: It is now recognized that one of the most important developments in the last decade lies in the preparation of synthetic structures on the submicron level. The excitement surrounding this development lies in the promise of producing materials with desired properties to specification, of permitting new scientific phenomena to be investigated and novel device applications to be made on artificial materials not found in nature - in other words, materials engineering on the microstructure scale. The range and variety of new phenomena that can be studied pose an exciting challenge. Examples on the microstructure domain include thin film junctions (sandwiches) of dissimilar metals which are either magnetic or superconducting etc., and modulated structures consisting of alternating layers of materials A and B, i.e. ABABA --. In this paper, results of all-electron self-consistent spin-polarized band structure studies on a AgFe sandwich, and on CuNi and PdAu coherent modulated structures are presented. For CuNi, modulations along [111] and [100] for different wave lengths and supercells up to 12 layers predict Ni moments to be reduced vs. f.c.c. Ni, but no "dead" layers even in the most dilute system. Adjacent Cu layers acquire small magnetic moments, which may be measured by NMR. Increased exchange enhancements are found in PdAu and discussed in relation to the strongly exchange enhanced susceptibility observed in PdAu sandwiches by Brodsky and Freeman. For AgFe we find that the Ag serves to greatly decrease the enhanced magnetism (to 2.95 µB) found for the free Fe surface.

Electronic structure and magnetism of surfaces, interfaces and modulated structures (superlattices)

Abstract: Recent developments in the study of surfaces and interfaces of metals and of artificial materials such as bimetallic sandwiches and modulated structures are described. Key questions include the effects on magnetism of reduced dimensionality and the possibility of magnetically “dead” layers. These developments have stimulated an intensified theoretical effort to investigate and describe the electronic and magnetic structure of surfaces and interfaces. One notable success has been the development of a highly accurate full-potential all-electron method (the FLAPW method) for solving the local spin density equations self-consistently for a single slab geometry. We describe here this advanced state of ab initio calculations in determining the magnetic properties of transition metal surfaces such as those of the ferromagnetic metals Ni(001) and Fe(001) and the Ni/Cu(001) interface. For both clean Fe and Ni(001) we find an enhancement of the magnetic moments in the surface layer. The magnetism of surface and interface Ni layers on Cu(001) (no “dead” layers are found) is described and compared to the clean Ni(001) results. Finally, the role ofμSR experiments in answering some of the questions raised in these studies will be discussed.

Theory of hyperfine anomalies in muonic atoms

Abstract: Negative muon spin precession experiments by Yamazaki et al. have found giant hyperfine anomalies in muonic atoms ranging from a few percent up to 36%. In order to understand their results, we present Breit interaction calculations based on atomic self-consistent unrestricted Dirac-Fock solutions which explicitly include all electrons and the negative muon. The Breit interaction results (including the relativistic correction for the bound muong-factor), vary from near zero forμ−O/N to −5% forμ−Pd/Rh; this latter is much larger than the calculated muonic or nuclear Bohr-Weisskopf anomalies and much smaller than the 36% measured value. Forμ}Ni/Co we find a calculated range of results (depending on assumed electronic configurations) of −2.3 to −2.7% in excellent agreement with recent measurements of the Yamazaki group. This excellent agreement inμ}Ni/Co provides strong support for the earlier suggestions that the discrepancy in the case ofμ−Pd/Rh is due to experimental factors.

Local Density Approach to Surfaces and Adsorbed Layers

Abstract: We show that the local density problem for the thin film geometry can be solved with high accuracy by employing our all-electron full-potential linearized augmented-plane-wave (FLAPW) method. This is achieved by removing all shape approximations in the charge density and the potential and by using a highly flexible variational basis set. Its utility even for treating molecules on surfaces is demonstrated for the severe test case of the (nearly) free O2 molecule where the FLAPW method is found to be at least as accurate as state-of-the art localized-basis molecular methods. The high accuracy and precision of the FLAPW approach for metallic surfaces is shown by our finding that for the interpretation of surface sensitive photoemission measurements of the Al-2p core levels both a surface induced core level shift of 120 meV to reduced binding energies and a 38 meV crystal field splitting are important. Based on all-electron local density calculations we explain the lowering of the work function of a transition metal surface [W(001)] upon deposition of a Cs over-layer by a multiple dipole formation. Finally, we demonstrate for a graphite monolayer that local density total energies give excellent descriptions of equilibrium geometries and discuss the overestimation of local-density cohesive energies due to an incomplete treatment of correlation effects in the free atom.