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We present a theoretical study of the electronic and magnetic properties of iron systems in different environments: pure iron systems @dimer, bcc bulk, ~100! surface, and free-standing iron monolayer#, and lowdimensional iron systems deposited on Ag ~100! surface ~monoatomic linear wires, iron monolayer, planar, and three-dimensional clusters!. Electronic and magnetic properties have been calculated using a recently developed total-energy first-principles method based on density-functional theory with numerical atomic orbitals as a basis set for the description of valence electrons and nonlocal pseudopotentials for the atomic core. The Kohn-Sham equations are solved self-consistently within the generalized gradient approximation for the exchange-correlation potential. Tests on the pseudopotential, the basis set, grid spacing, and k sampling are carefully performed. This technique, which has been proved to be very efficient for large nonmagnetic systems, is applied in this paper to calculate electronic and magnetic properties of different iron nanostructures. The results compare well with previous ab initio all-electron calculations and with experimental data. The method predicts the correct trends in the magnetic moments of Fe systems for a great variety of environments and requires a smaller computational effort than other ab initio methods.

Knowledge of the structure of clusters is essential to predict many of their physical and chemical properties. Using a many-body semiempirical Gupta potential (to perform global minimizations), and first-principles density functional calculations (to confirm the energy ordering of the local minima), we have recently found [Phys. Rev. Lett. 81, 1600 (1998)] that there are many intermediate-size disordered gold nanoclusters with energy near or below the lowest-energy ordered structure. This is especially surprising because we studied ''magic'' cluster sizes, for which very compact-ordered structures exist. Here, we show how the analysis of the local stress can be used to understand the physical origin of this amorphization. We find that the compact ordered structures, which are very stable for pair potentials, are destabilized by the tendency of metallic bonds to contract at the surface, because of the decreased coordination. The amorphization is also favored by the relatively low energy associated to bondlength and coordination disorder in metals. Although these are very general properties of metallic bonding, we find that they are especially important in the case of gold, and we predict some general trends in the tendency of metallic clusters towards amorphous structures. (c) 2000 The American Physical Society.

We discuss the application of the local-spin-density approximation (LSDA) and provide criteria to gauge the reliability of supercell models of $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ and tetrahedral amorphous carbon $(ta\ensuremath{-}\mathrm{C}).$ We identify models of $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ that exhibit a localization on dangling bond consistent with electron-spin-resonance (ESR) experiments and show that a LSDA level description of the electron states is essential to describe these states. We offer an ab initio calculation of a well-isolated floating bond state and show that neither the charge nor spin is well localized. Finally, we suggest the origin of the ESR signal in $ta\ensuremath{-}\mathrm{C}$ is $\ensuremath{\pi}$-bonded pairs at the Fermi level.

Bonding and diffusion of a Ba adatom on a Si(001) surface have been studied using first-principles density-functional calculations. It is found that the favorable bonding site of the adatom is the fourfold site located in the trough between Si dimer rows. The bonding between Ba adatom and the surface is shown to be only slightly ionic in character, with a small charge transfer from Ba to the substrate, and with an important covalent component. The calculated jumping rates show a strongly anisotropic diffusivity of Ba on the surface.

We report ab initio molecular dynamics simulations of ${\mathrm{Ni}}_{2},$ ${\mathrm{Al}}_{2},$ ${\mathrm{Ni}}_{13},$ ${\mathrm{Al}}_{13},$ and ${\mathrm{Ni}}_{12}\mathrm{Al}$ clusters using SIESTA, a fully self-consistent density-functional method that employs linear combinations of atomic orbitals as basis sets, standard norm-conserving pseudopotentials and a generalized-gradient approximation to exchange and correlation. Our results for the pure Ni and Al clusters, which are compared with those obtained by other recent ab initio calculations, are in good agreement with available experimental data. For the binary cluster ${\mathrm{Ni}}_{12}\mathrm{Al}$ our calculations show that a distorted icosahedral configuration with the Al atom at the cluster surface is more stable than that with the Al atom located at the central site, a result that clarifies discrepancies between the results of different semiempirical treatments.

Field emission resonances (FERs) on Cu(100) surface are investigated by means of tunneling regime simulations performed with a plane-wave based transport calculation method. FERs are located near the surface and decay into the vacuum, and their accurate simulation requires a faithful description of vacuum states. This type of simulations is thus not possible using the popular transport methods based on atom-centered localized basis sets and the use of plane waves becomes important. We introduce a procedure to treat self-consistently (SC) the finite bias nonequilibrium problem in tunneling regime. Image potential effects are included in a semiempirical way within the SC calculation. Tunneling through FERs is studied following a practical strategy to approximate the inelastic transmission for states lying in the band gap of the surface. As our approach permits the use of any tip geometry, tip effects on the energy and wave functions of FERs are explored. The method reported here provides an ideal tool for the simulation of FERs aimed at the understanding of experimental STS (scanning tunneling spectroscopy) observations. © 2013 American Physical Society., We acknowledge support from the Basque Departamento de Educación and the UPV/EHU (Grant No. IT-756-13), the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-00), and the ETORTEK program funded by the Basque Departamento de Industria and the Diputación Foral de Gipuzkoa. L.W. Wang is supported by the US DOE/SC/BES under contract No. DE-AC02-05CH11231.

We report on the theoretical study of laser-assisted attosecond photoemission from metals. The full time-dependent quantum approach reveals the role of the resonant interband and nonresonant surface emission processes in formation of final atto-streaking spectra. The present results explain recent experimental data on magnesium and show that the valence band streaking essentially reflects the respective weight of surface and resonant bulk electron ejection. © 2013 American Physical Society., P.M.E. acknowledges partial support from Basque Departamento de Education, Universidades, e Investigation (Grant No. IT-366-07), CONSOLIDER, and the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-00).

Low-temperature scanning tunneling microscopy and density-functional theory (DFT) were used to study the adsorption of water on a Ru(0001) surface covered with half monolayer of oxygen. The oxygen atoms occupy hcp sites in an ordered structure with $(2\ifmmode\times\else\texttimes\fi{}1)$ periodicity. DFT predicts that water is weakly bound to the unmodified surface, 86 meV compared to the $\ensuremath{\sim}200\text{ }\text{meV}$ water-water H bond. Instead, we found that water adsorption causes a shift of half of the oxygen atoms from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms. The energy cost of reconstructing the oxygen overlayer, around 230 meV per displaced oxygen atom, is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. Water forms hydrogen bonds with the fcc O atoms in a $(4\ifmmode\times\else\texttimes\fi{}2)$ superstructure due to alternating orientations of the molecules. Heating to 185 K results in the complete desorption of the water layer, leaving behind the oxygen-honeycomb structure, which is metastable relative to the original $(2\ifmmode\times\else\texttimes\fi{}1)$. This stable structure is not recovered until after heating to temperatures close to 260 K.

We report ab initio calculations of zirconium-coated graphene sheets at several coverages and geometries. We calculate adsorption properties, such as distances and the Zr/graphene binding energies. When increasing the Zr/C coverage ratio, the binding energies show that the Zr atoms have a trend to cluster. The most stable Zr/C coverage corresponds to 0.375, which has no stress between the zirconium and graphene layer. The Zr-graphene binding involves charge transfer to graphene which comes from the $5s$ orbital of Zr and depends on the coverage. We also calculate the diffusion-energy barriers on graphene for the single atom and the ${\text{Zr}}_{3}$ trimer and we see that the ${\text{Zr}}_{3}$ cluster is a faster diffusion unit. We also estimate that the Zr diffusion energy between two substitutional positions is at least 4 eV, which is large enough to fix Zr atoms bound to C vacancies at actual temperatures in high-resolution transmission electronic microscopy experiments.

Magnetic properties of graphenic carbon nanostructures, which are relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni (Nisub) impurities. Nisub defects are nonmagnetic in flat graphene and develop a nonzero-spin moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of Nisub. A similar magnetic-nonmagnetic transition of Nisub can be expected by applying anisotropic strain to a flat graphene layer., E.J.G.S., A.A., and D.S.P. acknowledge support from the Basque Government and UPV/EHU Grant No. IT-366-07, CSIC, the Spanish MEC Grant No. FIS2007-66711-C02-02, and the Basque Government and Diputación Foral de Guipuzcoa through the ETORTEK program. A.G.S.F. acknowledges the support from Brazilian agencies FUNCAP, CAPES/FAPERGS, CNPq Grant No. 306335/2007-7, Rede Nacional de Pesquisa em Nanotubos de Carbono, Rede de Nanobioestruturas, and Instituto do Milenio de Nanotecnologia CNPq/MCT-Brazil.

Chemisorption of atoms and molecules controls many interfacial phenomena such as charge transport and catalysis. The question of how the intrinsic properties of the interacting materials define the electronic structure of their interface remains one of the most important, yet intractable problems in surface physics. Through two-photon photoemission spectroscopy we determine a common binding energy of $\ensuremath{\sim}1.8--2.0\text{ }\text{eV}$ with respect to the vacuum for the unoccupied resonance of the $ns$ valence electron of alkali atoms (Li-Cs) chemisorbed at low coverage (less than 0.1 monolayer) on noble metal [Cu(111) and Ag(111)] surfaces. We present a theoretical model based on the semiempirical potentials of the adsorbates and the substrates, their principal mode of interaction through the Coulomb interaction, and the ab initio adsorption structures. Our analysis reveals that atomic size and ionization potential independent interfacial electronic structure is a consequence of the Coulomb interaction among the $ns$ electron, the alkali-atom ionic core, and the induced image charge in the substrate. We expect the same interactions to define the effective electronic potentials for a broad range of molecule/metal interfaces.

We present an ab initio scheme for the calculation of the resonant charge transfer of electrons at surfaces. The electron initially resides in a bound resonance, i.e., appearing below the vacuum level, associated with a core-excited adsorbate. Our treatment is based on first-principles density-functional calculations of this initial situation using finite slabs. These results are combined with bulk calculations of the substrate material to obtain the Hamiltonian of the semi-infinite system in which the electron evolves. Therefore, we include a realistic description of the electronic structure of both subsystems, substrate and adsorbate, and the interaction between them. The surface Green's function is then computed using the transfer matrix method and projected onto a wave packet localized in the adsorbate. The width and energy of the resonance can be obtained from an analysis of the projected Green's function, and the charge transfer time can be estimated. The calculated width is independent of the wave packet used for the projection, at least as far as there are not several overlapping resonances at neighboring energies. Alternatively, one can directly calculate the time evolution of the population of the initial wave packet. Both alternatives are presented and compared. Our first-principles calculations are based on periodic arrangements of adsorbates on the surface. With an appropriate average of the ${\mathbf{k}}_{\ensuremath{\parallel}}$ resolved results, one can extrapolate to the limit of an isolated adsorbate. We discuss several possibilities to do this. As an application, we focus on the case of the $4s$ bound resonance of a core-excited ${\mathrm{Ar}}^{*}(2{p}_{3∕2}^{\ensuremath{-}1}4s)$ adsorbate on Ru(0001), for which there are extensive experimental studies. The calculated values and trends are in good agreement with the experimental observations.

We present a combined theoretical and experimental study of water adsorption on Ru(0001) pre-covered with 0.25 monolayers (ML) of oxygen forming a (2 x 2) structure. Several structures were analyzed by means of Density Functional Theory calculations for which STM simulations were performed and compared with experimental data. Up to 0.25 monolayers the molecules bind to the exposed Ru atoms of the 2 x 2 unit cell via the lone pair orbitals. The molecular plane is almost parallel to the surface with its H atoms pointing towards the chemisorbed O atoms of the 2 x 2 unit cell forming hydrogen bonds. The existence of these additional hydrogen bonds increases the adsorption energy of the water molecule to approximately 616 meV, which is {approx}220 meV more stable than on the clean Ru(0001) surface with a similar configuration. The binding energy shows only a weak dependence on water coverage, with a shallow minimum for a row structure at 0.125 ML. This is consistent with the STM experiments that show a tendency of the molecules to form linear rows at intermediate coverage. Our calculations also suggest the possible formation of water dimers near 0.25 ML.

The quasi-one-dimensional Si(557)-Au reconstruction has attracted a lot of attention in recent years. We study here the interplay between the electronic and structural degrees of freedom in this system. Our calculations are in good agreement with recent experimental data obtained using scanning tunneling microscopy and spectroscopy both at room and low temperatures. Together with the quite successful description of the experimental band structure, these results give further support to the current structural model of the Si(557)-Au surface. We consider in detail the energetics and variation of the band structure as a function of the buckling of the step edge and its implications to explain the observed metal-insulator transition. Finally, we present the results of a first-principles molecular dynamics simulation of several picoseconds performed at room temperature. As expected, we find a strong oscillation of the step-edge atoms. The dynamics associated with other vibrational modes is also observed. Particularly apparent are the oscillations of the height of the restatoms and adatoms and the associated fluctuation of the Si–Au–Si bond angles along the gold chain. This mode, together with step-edge buckling, has a strong influence on the insulating and/or metallic character of the surface., This work was supported by the Basque Departamento de Educación and the UPV/EHU Grant No. 9/UPV 00206.215-13639/2001, the Spanish Ministerio de Educacón y Ciencia Grant No. FIS2004-06490-C3-02, the European Network of Excellence FP6-NoE “NANOQUANTA” Grant No. 500198-2, and the research contracts “Nanomateriales” and “Nanotron” funded by the Basque Departamento de Industria, Comercio y Turismo within the ETORTEK program and the Departamento para la Innovación y la Sociedad del Conocimiento from the Diputación Foral de Guipuzcoa.

We present an atomic-orbital-based approximate scheme for self-interaction correction (SIC) to the local-density approximation (LDA) of density-functional theory. The method, based on the idea of Filippetti and Spaldin [Phys. Rev. B 67, 125109 (2003)], is implemented in a code using localized numerical atomic-orbital basis sets and is now suitable for both molecules and extended solids. After deriving the fundamental equations as a nonvariational approximation of the self-consistent SIC theory, we present results for a wide range of molecules and insulators. In particular, we investigate the effect of re-scaling the self-interaction correction and we establish a link with the existing atomiclike corrective scheme LDA+U. We find that when no re-scaling is applied, i.e., when we consider the entire atomic correction, the Kohn-Sham highest occupied molecular orbital (HOMO) eigenvalue is a rather good approximation to the experimental ionization potential for molecules. Similarly the HOMO eigenvalues of negatively charged molecules reproduce closely the molecular affinities. In contrast a re-scaling of about 50% is necessary to reproduce insulator band gaps in solids, which otherwise are largely overestimated. The method therefore represents a Kohn-Sham based single-particle theory and offers good prospects for applications where the actual position of the Kohn-Sham eigenvalues is important, such as quantum transport., This work was supported by the Science Foundation of Ireland under Grant No. SFI02/IN1/I175. D.S.P. acknowledges financial support from the University of the Basque Country Grant No. 9/UPV 00206.215-13639/2001, the Spanish Ministerio de Educacion y Ciencia Grant No. FIS2004-06490-C03-00, and the Basque Goverment and Diputacion Foral de Giupuzkoa NANOMATERIALES and NANOTRON grants, ETORTEK program.

We present a systematic study of the atomic and electronic structure of the $\mathrm{Si}(111)\text{\ensuremath{-}}(5\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}\mathrm{Au}$ reconstruction using first-principles electronic structure calculations based on the density functional theory. We analyze the structural models proposed by Marks and Plass [Phys. Rev. Lett. 75, 2172 (1995)], those proposed recently by Erwin [ Phys. Rev. Lett. 91, 206101 (2003)], and a completely different structure that was found during our structural optimizations. We study in detail the energetics and the structural and electronic properties of the different models. For the two most stable models, we also calculate the change in the surface energy as a function of the content of silicon adatoms for a realistic range of concentrations. Our model is the energetically most favorable in the range of low adatom concentrations, while Erwin's ``$5\ifmmode\times\else\texttimes\fi{}2$'' model becomes favorable for larger adatom concentrations. The crossing between the surface energies of both structures is found close to $1∕2\phantom{\rule{0.3em}{0ex}}\text{adatom}\phantom{\rule{0.3em}{0ex}}\text{per}\phantom{\rule{0.3em}{0ex}}5\ifmmode\times\else\texttimes\fi{}2\phantom{\rule{0.3em}{0ex}}\text{unit}\phantom{\rule{0.3em}{0ex}}\text{cell}$, i.e., near the maximum adatom coverage observed in the experiments. Both models, our structure and Erwin's ``$5\ifmmode\times\else\texttimes\fi{}2$'' model, seem to provide a good description of many of the available experimental data, particularly of the angle-resolved photoemission measurements.

We report on a general method for the calculation of the frequency-dependent optical response of clusters based upon time-dependent density functional theory (TDDFT). The implementation is done using explicit propagation in the time domain and a self-consistent program that uses a linear combination of atomic orbitals (LCAO). Our actual calculations employ the SIESTA program, which is designed to be fast and accurate for large clusters. We use the adiabatic local density approximation to account for exchange and correlation effects. Results are presented for the imaginary part of the linear polarizability, $\mathrm{Im}\ensuremath{\alpha}(\ensuremath{\omega}),$ and the dipole strength function, $S(\ensuremath{\omega}),$ of ${\mathrm{C}}_{60}$ and ${\mathrm{Na}}_{8},$ and compared to previous calculations and to experiment. We also develop a method for the calculation of the integrated frequency-dependent second-order nonlinear polarizability for the case of a step function electric field, ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\gamma}}}_{\mathrm{step}}(\ensuremath{\omega}),$ and present results for ${\mathrm{C}}_{60}.$

We performed density-functional theory calculations in the local-density approximation of the structural, electronic, and optical properties of 4-Å-diameter single-walled carbon nanotubes. The calculated relaxed geometries show significant deviations from the ideal rolled graphene sheet configuration. We study the effect of the geometry on the electronic band structure finding the metallic character of the (5,0) nanotube to be a consequence of the high curvature of the nanotube wall. Calculations of the dielectric function and optical absorption of the isolated nanotubes were performed under light polarized parallel and perpendicular to the tube axis. We compare our results to measurements of the optical absorption of zeolite-grown nanotubes and are able to assign the observed maxima to the nanotube chiralities., We acknowledge the Ministerio de Ciencia y Tecnologia (Spain) and the DAAD (Germany) for a Spanish-German Research action (Grant No. HA 1999-0118). D.S.P. acknowledges support from the Spanish MCyT and CSIC under the ‘‘Ramon y Cajal’’ program. P.O. acknowledges support from Fundacion Ramon Areces (Spain), EU Project No. SATURN IST-1999-10593, and Spain-DGI Project No. BMF2000-1312-002-01.

``14-1-11'' phase compounds, based on magnetic Mn ions and typified by ${\mathrm{Ca}}_{14}{\mathrm{MnBi}}_{11}$ and ${\mathrm{Ba}}_{14}{\mathrm{MnBi}}_{11},$ show an unusual magnetic behavior, but the large number (104) of atoms in the primitive cell has precluded any previous full electronic structure study. Using an efficient, local-orbital-based method within the local-spin-density approximation to study the electronic structure, we find a gap between a bonding valence-band complex and an antibonding conduction-band continuum. The bonding bands lack one electron per formula unit of being filled, making them low carrier density p-type metals. The hole resides in the ${\mathrm{MnBi}}_{4}$ tetrahedral unit, and partially compensates for the high-spin ${d}^{5} \mathrm{Mn}$ moment, leaving a net spin near $4{\ensuremath{\mu}}_{B}$ that is consistent with experiment. These manganites are composed of two disjoint but interpenetrating ``jungle gym'' networks of spin-$\frac{4}{2}$ ${\mathrm{MnBi}}_{4}^{9\ensuremath{-}}$ units with ferromagnetic interactions within the same network, and weaker couplings between the networks whose sign and magnitude is sensitive to materials parameters. ${\mathrm{Ca}}_{14}{\mathrm{MnBi}}_{11}$ is calculated to be ferromagnetic as observed, while for ${\mathrm{Ba}}_{14}{\mathrm{MnBi}}_{11}$ (which is antiferromagnetic) the ferromagnetic and antiferromagnetic states are calculated to be essentially degenerate. The band structure of the ferromagnetic states is very close to half metallic.

The Si(557)-Au surface, containing monatomic Au wires parallel to the steps, has been proposed as an experimental realization of an ideal one-dimensional metal. In fact, recent photoemission experiments on this system (Nature 402, 504 (1999)) found two peaks that were interpreted in terms of the spin-charge separation in a Luttinger liquid. Our first-principles density functional calculations reveal two metallic bands associated with Au-Si bonds, instead of the single band expected from the Au 6s states, providing an alternative explanation for the experimental observations., Comment: 5 pages with 3 postscript figures. Submitted to PRL on May 10th 2001

The performance of basis sets made of numerical atomic orbitals is explored in density-functional calculations of solids and molecules. With the aim of optimizing basis quality while maintaining strict localization of the orbitals, as needed for linear-scaling calculations, several schemes have been tried. The best performance is obtained for the basis sets generated according to a new scheme presented here, a flexibilization of previous proposals. Strict localization is maintained while ensuring the continuity of the basis-function derivative at the cutoff radius. The basis sets are tested versus converged plane-wave calculations on a significant variety of systems, including covalent, ionic, and metallic. Satisfactory convergence is obtained for reasonably small basis sizes, with a clear improvement over previous schemes. The transferability of the obtained basis sets is tested in several cases and it is found to be satisfactory as well.