First-principles study of the atomic and electronic structure of the Si(111)-(5x2-Au surface reconstruction

We present a systematic study of the atomic and electronic structure of the Si(111)-(5x2)-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 new 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 new model is the energetically most favorable in the range of low adatom concentrations, while Erwin's "5x2" model becomes favorable for larger adatom concentrations. The crossing between the surface energies of both structures is found close to 1/2 adatoms per 5x2 unit cell, i.e. near the maximum adatom coverage observed in the experiments. Both models, the new structure and Erwin's "5x2" model, seem to provide a good description of many of the available experimental data, particularly of the angle-resolved photoemission measurements

First-Principles Study of Substitutional Metal Impurities in Graphene: Structural, Electronic and Magnetic Properties

We present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of 3d transition metal, noble metal and Zn atoms interacting with carbon monovacancies in graphene. We pay special attention to the electronic and magnetic properties of these substitutional impurities and found that they can be fully understood using a simple model based on the hybridization between the states of the metal atom, particularly the d shell, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of different carbon-metal hybridized electronic levels: (i) bonding states are completely filled for Sc and Ti, and these impurities are non-magnetic; (ii) the non-bonding d shell is partially occupied for V, Cr and Mn and, correspondingly, these impurties present large and localized spin moments; (iii) antibonding states with increasing carbon character are progressively filled for Co, Ni, the noble metals and Zn. The spin moments of these impurities oscillate between 0 and 1 Bohr magnetons and are increasingly delocalized. The substitutional Zn suffers a Jahn-Teller-like distortion from the C3v symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct position at the border between regimes (ii) and (iii) and shows a more complex behavior: while is non-magnetic at the level of GGA calculations, its spin moment can be switched on using GGA+U calculations with moderate values of the U parameter.Comment: 13 figures, 4 tables. Submitted to Phys. Rev. B on September 26th, 200

Transport properties of armchair graphene nanoribbon junctions between graphene electrodes

The transmission properties of armchair graphene nanoribbon junctions between graphene electrodes are investigated by means of first-principles quantum transport calculations. First the dependence of the transmission function on the size of the nanoribbon has been studied. Two regimes are highlighted: for small applied bias transport takes place via tunneling and the length of the ribbon is the key parameter that determines the junction conductance; at higher applied bias resonant transport through HOMO and LUMO starts to play a more determinant role, and the transport properties depend on the details of the geometry (width and length) of the carbon nanoribbon. In the case of the thinnest ribbon it has been verified that a tilted geometry of the central phenyl ring is the most stable configuration. As a consequence of this rotation the conductance decreases due to the misalignment of the $pi$ orbitals between the phenyl ring and the remaining part of the junction. All the computed transmission functions have shown a negligible dependence on different saturations and reconstructions of the edges of the graphene leads, suggesting a general validity of the reported results

Zigzag equilibrium structure in monatomic wires

We have applied first-principles density-functional calculations to the study of the energetics, and the elastic and electronic properties of monatomic wires of Au, Cu, K, and Ca in linear and a planar-zigzag geometries. For Cu and Au wires, the zigzag distortion is favorable even when the linear wire is stretched, but this is not observed for K and Ca wires. In all the cases, the equilibrium structure is an equilateral zigzag (bond angle of 60$^{\rm o}$). Only in the case of Au, the zigzag geometry can also be stabilized for an intermediate bond angle of 131$^{\rm o}$. The relationship between the bond and wire lengths is qualitatively different for the metallic (Au, Cu and, K) and semiconducting (Ca) wires.Comment: 4 pages with 3 postscript figures. To appear in Surf. Science (proceedings of the European Conference on Surface Science, ECOSS-19, Madrid Sept. 2000

Role of the spin-orbit splitting and the dynamical fluctuations in the Si(557)-Au surface

Our it ab initio calculations show that spin-orbit coupling is crucial to understand the electronic structure of the Si(557)-Au surface. The spin-orbit splitting produces the two one-dimensional bands observed in photoemission, which were previously attributed to spin-charge separation in a Luttinger liquid. This spin splitting might have relevance for future device applications. We also show that the apparent Peierls-like transition observed in this surface by scanning tunneling microscopy is a result of the dynamical fluctuations of the step-edge structure, which are quenched as the temperature is decreased

Interplay between electronic and atomic structures in the Si(557)-Au reconstruction from first principles

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.Peer reviewe

Dynamic screening and energy loss of antiprotons colliding with excited Al clusters

We use time-dependent density functional theory to calculate the energy loss of an antiproton colliding with a small Al cluster previously excited. The velocity of the antiproton is such that non-linear effects in the electronic response of the Al cluster are relevant. We obtain that an antiproton penetrating an excited cluster transfers less energy to the cluster than an antiproton penetrating a ground state cluster. We quantify this difference and analyze it in terms of the cluster excitation spectrum.Comment: 23 pages, 4 figures, to be published in Nuclear Instruments and Methods B as a proceeding of the IISC-19 Workshop on Inelastic Ion-Surface Collision

Effect of electron and hole doping on the structure of C, Si, and S nanowires

We use ab initio density functional calculations to study the effect of electron and hole doping on the equilibrium geometry and electronic structure of C, Si, and S monatomic wires. Independent of doping, all these nanowires are found to be metallic. In absence of doping, C wires are straight, whereas Si and S wires display a zigzag structure. Besides two preferred bond angles of 60 deg and 120 deg in Si wires, we find an additional metastable bond angle of 90 deg in S wires. The equilibrium geometry and electronic structure of these nanowires is shown to change drastically upon electron and hole doping.Comment: 5 pages including 5 figure

Universal Magnetic Properties of sp3^3-type Defects in Covalently Functionalized Graphene

Using density-functional calculations, we study the effect of sp$^3$-type defects created by different covalent functionalizations on the electronic and magnetic properties of graphene. We find that the induced magnetic properties are {\it universal}, in the sense that they are largely independent on the particular adsorbates considered. When a weakly-polar single covalent bond is established with the layer, a local spin-moment of 1.0 $\mu_B$ always appears in graphene. This effect is similar to that of H adsorption, which saturates one $p_z$ orbital in the carbon layer. The magnetic couplings between the adsorbates show a strong dependence on the graphene sublattice of chemisorption. Molecules adsorbed at the same sublattice couple ferromagnetically, with an exchange interaction that decays very slowly with distance, while no magnetism is found for adsorbates at opposite sublattices. Similar magnetic properties are obtained if several $p_z$ orbitals are saturated simultaneously by the adsorption of a large molecule. These results might open new routes to engineer the magnetic properties of graphene derivatives by chemical means