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New Journal of Physics, 2010
We present a low-temperature scanning tunneling spectroscopy study of the local density of states (LDOS) on the (12110)-surface of the decagonal quasicrystal d-Al-Ni-Co and the (100)-surface of its structurally closely related approximant Y-Al-Ni-Co in the range of ±1 eV around the Fermi energy. The LDOS of both surfaces are dominated by a large parabolic pseudogap centered at the Fermi energy, which can be attributed to the Hume-Rothery stabilization mechanism or to an orbital hybridization. Superimposed on this large pseudogap, a spatially varying fine structure is revealed, whose spatial distribution correlates with the structures of both of the surfaces. This fine structure shows narrow peaks and pseudogaps exhibiting typical energy widths between 50 and 150 meV. The spatial extent of these localized states is of the order of the width of the topographic features, which is about 0.45 nm. Our findings show that specific electronic states are localized on equivalent topographic features regardless of the presence of periodic or quasiperiodic long-range order. We interpret this result as an indication that the 3 2 fine structure in the LDOS is dominated much more by the complex short-range order than by the presence of the quasiperiodic long-range order.
First-principles investigation of the Ni–Fe–Al system
Intermetallics, 2005
By combining ab-initio electron theory and statistical mechanics, the physical properties of the ternary intermetallic system Ni-Fe-Al in the ground state and at finite temperatures were investigated. The Ni-Fe-Al system is not only of high technological interest, but exhibits also rich physics, e.g., a delicate interplay between structure and magnetism over a wide composition range and substantial electronic correlations which is challenging for modern electronic structure methods. The new Stuttgart ab-initio mixed-basis pseudopotential code in the generalized gradient approximation (GGA) was used to determine the energetics in the ground state. Therewith, in combination with the cluster expansion (CE) method a representation of the energy landscape at T =0 over the whole Gibbs triangle was elaborated. At finite temperatures, the cluster variation method (CVM) in tetrahedron approximation was employed in order to calculate the ab-initio ternary phase diagram on the bcc and fcc lattice. Thereby, a miscibility gap in the ternary B2 phase was theoretically verified.
Physical Review B, 1983
Results of self-consistent local-spin-density-functional calculations are reported for the first time for the Ni(110) surface, represented by one-, three-, and five-layer slabs. Calculations for oneand five-layer slabs of Ni(100) are also reported. The behavior of the surface magnetization with varying slab thickness elucidates the nature and origin of the surface magnetic moment. We predict a 13% enhancement of the Ni(110) surface magnetic moment compared to the bulk value. For the Ni(100) surface, we find a smaller surface enhancement about 7%, compared to bulk, which agrees with the results of Jepsen et al. The enhancement of surface magnetic moments on Ni(100) and Ni(110) surfaces is attributed to s-d dehybridization at the surface and to the presence of electrostatic shifts required to maintain layer-by-layer charge neutrality. We find that the total d-electron charge is the same in each layer, which contradicts the sp-to-d charge transfer found by Tersoff and Falicov at transition-metal surfaces. An exchange-split pair of very localized surface states is found on the Ni(110} surface, which is in good agreement with the photoemission measurements of Eberhardt et al. The theoretical exchange splitting, 0.6 eV, is twice as large as that found experimentally. This discrepancy is similar to that found for the bulk Ni bands and is attributed to neglected many-body effects. For Ni(100} it is found that surface states at the Brillouin-zone center are unable to account for the reversal above threshold of the spin polarization of photoemitted electrons, in agreement with other self-consistent calculations. A majority-spin X2 surface resonance on Ni(100} is in good agreement with the experimental surface state of Plummer and Eberhardt but has greater dispersion downward away from the Fermi energy than is found experimentally. We do not find the Xl minority-spin surface-state band observed by Plummer and Eberhardt just below the Fermi energy; instead, we find a flat E& minority-spin surface-state band about 0.5 eV below the Fermi energy. Finally, we find surface core-level chemical shifts to reduced binding energy of 0.39 eV on Ni(100) and 0.45 eV on Ni(110). The polarization of the core states by the valence electrons splits the spin-up and spin-down core states in each layer by about 0.6 eV, thereby permitting, in principle at least, the experimental verification of these surface core-level shifts, since the two spin manifolds are separated.
Electronic structure of ordered NiAl(110) surface
Journal of Electron Spectroscopy and Related Phenomena, 1998
The electronic structure of ordered NiAl(110) surface has been calculated using the recursion method. The density of states and the interatomic energy have been presented. On the basis of these results, the rippled relaxation for the (110) surface of the alloy is discussed. ᭧
Thin Solid Films, 2011
In this study, we perform first principles simulations on both atomically smooth and nanostructured Ni slabs. The latter contains periodically distributed nickel nanoclusters atop a thin metal film gradually growing from adatoms and serving as a promising catalyst. Applying the generalized gradient approximation within the formalism of the density functional theory we compare the atomic and electronic structures of Ni bulk, as well as both perfect and nanostructured (111) surfaces obtained using two different ab initio approaches: (i) the linear combination of atomic orbitals and (ii) the projector augmented plane waves. The most essential inter-atomic forces between the Ni adatoms upon the substrate have been found to be formed via: (i) attractive pair-wise interactions, (ii) repulsive triple-wise interactions within a triangle and (iii) attractive triple-wise interactions within a line between the nearest adatoms. The attractive interactions surmount the repulsive forces, hence resulting in the formation of stable clusters from Ni adatoms. The magnetic moment and the effective charge (within both Mulliken and Bader approaches) of the outer atoms in Ni nanoparticles increase as compared to those for the smooth Ni(111) surface. The calculated electronic charge redistribution in the Ni nanoclusters features them as possible adsorption centers with increasing catalytic activity, e.g., for further synthesis of carbon nanotubes.
Ab initio theoretical studies of atomic and electronic structures of III-nitride (110) surfaces
Brazilian Journal of Physics, 1999
We present a systematic theoretical study of several III-nitride 110 surfaces based on accurate, parameter-free, self-consistent total energy and force calculations using the density functional theory, the generalized gradient approximationGGA for the exchange-correlation term, and the Full Potential Linear Augmented Plane Wave FPLAPW approach associated with the slab supercell model. We studied AlN, BN, GaN and InN and analyzed the theoretical trends for the equilibrium atomic structures and surface band structures. We used supercells built up of 7 atomic layers and a vacuum region equivalent of 5 atomic layers. For the Ga and In species, the 3d and 4d electrons were treated properly as valence electrons. In connection with the atomic structures, we are concerned with the LEED parameters 1?, 1x, 2? , d12?, d12x and ! for the 110 surface. We analyzed the changes in the bond-lengths and in the bond-angles at the anion and cation sites. We conclude that similarly to the III-arsenide 110 and III-phosphide 110 surfaces, the III-nitride 110 surfaces relax such that the cation-surface atom moves inward and the N-surface atom moves outward. The large Coulomb energy of the III-nitrides as compared with the other III-V compounds re ects in the smaller value of the tilt angle ! and in the small value of 1?. T o our knowledge, this is the rst time the FPLAPW method is used for such a systematic study of III-nitrides, and we compare our results with recent results obtained with other approaches as reported in the literature.
Electronic Structures and Magnetic Properties of NiAl and Ni3Al
Quantum Matter, 2016
The adsorption characteristics of transition metal (TM) adatoms on olympicene, a graphene-like molecule consisting of five rings of which four are benzene-like rings, are analyzed by means of firstprinciples calculations. The electronic structures and magnetic properties of 3d transition metals on olympicene were investigated in the framework of generalized-gradient approximation using relativistic density functional theory. Our calculations were based on full-potential local orbital (FPLO) method. Stable structure and total energies were obtained. Relaxed TM@Olympicene molecules, unlike the pure organic olympicene, were partially buckled. Binding energy calculations showed high stability for Ni@Olympicene whereas Mn did not bind to olympicene surface. Electronic charge transfers were consistent with Pauling electronegativities. The related HOMO-LUMO gaps decreased with respect to pure olympicene. Our calculated total and TM spin magnetic moments indicated that TM@Olympicene were strongly magnetized. Orbital polarization correction (OPC) were also used in order to get an upper estimation for orbital magnetic moment of all 3d atoms. Fe@Olympicene shows highest orbital moment of 0.18 m B. The Atoms in Molecules (AIM) approach was used to analyze the nature of TMeC bonds in TM@Olympicene complexes. The results show the TMeC bonds have a degree of covalency in their interactions.
Calculation of electronic properties of boundaries in Ni3Al
Acta Materialia, 2001
The effects of boron doping on both the Ni-d DOS and the cohesive properties of Ni 3 Al have been studied through the use of supercell models having the local composition of Ni 3 Al with and without B at grain boundaries. The calculations of the density of states are compared with experimental results obtained from electron energy loss near edge structures. The results suggest an increase in the intensity of the Ni-L 2,3 white line when there is Ni enrichment; they also suggest that this increase disappears when B is added to the Ni-enriched model boundary, in agreement with experiment. The relationship between these results and cohesive enhancement as observed in B-doped Ni 3 Al is discussed.
Spin-polarized band-structure calculations for Ni
Physical Review B, 1979
The electronic structure of nickel as a function of the lattice constant has been studied by the self-consistent spin-polarized augmented-plane-wave method. The results confirm previous findings by Wang and Callaway regarding the different forms of the local-exchange approximation. The present calculations have incorporated the mass-velocity and Darwin relativistic effects and lead to an ordering of the energy levels at L which is consistent with photoemission measurements of Eastman et al. The computed changes in Fermi surface and magneton number with pressure were in reasonable agreement with corresponding measurements of the de Haasvan Alphen effect and magnetization 'Reference 5. Slater-Koster interpolation from 20-point mesh. 'Tetrahedron interpolation from 89-point mesh. Callaway and Wang (Ref. 4). We have assumed that there is a misprint in this paper. 'Tetrahedron interpolation from equivalent of 240-point mesh. fReference 28. &Data obtained by R. W. Stark as reported in Ref. 5. "Reference 27. 'Goy and Grimes, see Ref. 29.
Magnetism at the Ni(001) surface: A high-precision, all-electron local-spin-density-functional study
Physical Review B, 1984
We present the results of high-precision, all-electron, self-consistent local-spin-density-functional calculations on a seven-layer Ni(001) film using the full-potential linearized-augmented-plane-wave method. It is found that the surface atoms have a magnetic moment which is enhanced by almost 20% compared with the bulklike atoms in the interior of the film. There is no indication of a Friedel-type oscillation in the layer-by-layer magnetic moments. Although the negative core-contact spin densities for the surface atoms are enhanced in magnitude by 20%, the contribution from the (4s-derived) valence electrons changes sign and becomes slightly positive in the surface layer. This causes a net decrease in magnitude of the total contact spin density by 20%. In agreement with photoemission experiments we find the majority-spin M3 surface state to be occupied, contrary to the early results of Wang and Freeman for a nine-layer film and to recently presented results obtained by Jepsen et al. on a five-layer film. The work function is found to be 5.37 eV, in good agreement with the experimental value of 5.22+0.04 eV. For the core levels of the surface atoms we obtain a shift between 0.3 and 0.5 eV towards reduced binding energies which is explained in terms of d-band narrowing and layer-by-layer charge neutrality.