Ab initio study of the double row model of the Si(553)-Au reconstruction

Using x-ray diffraction Ghose et al. [Surf. Sci. {\bf 581} (2005) 199] have recently produced a structural model for the quantum-wire surface Si(553)-Au. This model presents two parallel gold wires located at the step edge. Thus, the structure and the gold coverage are quite different from previous proposals. We present here an ab initio study using density functional theory of the stability, electronic band structure and scanning tunneling microscopy images of this model.Comment: Submitted to Surface Science on December 200

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

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

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

Plasmon tunability in metallodielectric metamaterials

The dielectric properties of metamaterials consisting of periodically arranged metallic nanoparticles of spherical shape are calculated by rigorously solving Maxwell's equations. Effective dielectric functions are obtained by comparing the reflectivity of planar surfaces limiting these materials with Fresnel's formulas for equivalent homogeneous media, showing mixing and splitting of individual-particle modes due to inter-particle interaction. Detailed results for simple cubic and fcc crystals of aluminum spheres in vacuum, silver spheres in vacuum, and silver spheres in a silicon matrix are presented. The filling fraction of the metal f is shown to determine the position of the plasmon modes of these metamaterials. Significant deviations are observed with respect to Maxwell-Garnett effective medium theory for large f, and multiple plasmons are predicted to exist in contrast to Maxwell-Garnett theory.Comment: 6 pages, 4 figure

Estudio teórico de las propiedades electrónicas y estructurales de reconstrucciones superficiales nanoestructuradas inducidas por la deposición de átomos metálicos sobre superficies de silicio

Doctoral Thesis submitted by Sampsa Juhana Riikonen for the degree of Doctor in Physics.[ES]: La presente tesis se centra en el estudio, utilizando cálculos de estructura electrónica dentro de la teoría del funcional de la densidad (DFT), de la estructura atómica y electrónica de varias reconstrucciones que se obtienen después de depositar diversos metales en Si(111) y superficies vecinales de Si(111). Muchas de estas estructuras forman reconstrucciones cuasiunidimensionales donde los átomos de metal se agrupan formando cadenas monoatómicas o con secciones de unos pocos átomos. Por ello han sido propuestas como realizaciones experimentales de un metal unidimensional ideal, y se han buscado manifestaciones del comportamiento conocido como líquido de Tomonaga-Luttinger en sus espectros de fotoemisión, así como indicaciones de transiciones metal-aislante asociadas a distorsiones estructurales del tipo transición de Peierls. Casi todo nuestro estudio se ha centrado en las reconstrucciones inducidas por la deposición de oro Si(557)/Au, Si(553)/Au y Si(111)-(5x2)-Au. En nuestro trabajo hemos conseguido identificar los modelos estructurales energéticamente más estables y hemos estudiado sus propiedades electrónicas que, en general, están en buen acuerdo con lo observado en los experimentos de fotoemisión y de microscopía túnel.Peer reviewe

Magnetism of graphene with defects: Vancancies, substitional metals, and covalent functionalization

Magnetic properties of graphenic nanostructures, relevant for future spintronics applications, depend crucially on doping and on the presence of defects. Here we present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of defects such as: substitutional doping with transition metals, vacancies, chemical functionalization with organic and inorganic molecules, light atoms, and polymers. We have found that such defects can be used to create and control magnetism in graphene-based materials. Our main results can be summarized as follows: (a) Substitutional metallic impurities can be fully understood using a model based on the hybridization between the states of the metal atom, particularly the d electrons, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of the different electronic levels between hybridized graphene-metals which determines all the magnetic properties obtained by the doping; (b) In chemical functionalization, independently of the particular adsorbate, a spin moment of 1.0 Bohr is induced when a molecule chemisorbs on a graphene layer via a single C-C covalent bond. This effect is similar to H adsorption, which saturates one pz orbitals creating an effect on the electronic structure that resembles a single vacancy in a π-tight-binding, however with universal character. The magnetic coupling between adsorbates was also studied and showed a key dependence on the sublattice adsorption site; (c) Monovacancies under isotropic strain display a rich phase diagram of spin solutions with the geometry configuration. Stretching increases the moment in different spin phases and compression reduces or even kills the magnetic moment. The transition to the non-magnetic solutions can be traced to changes in the local structure of graphene that are associated with the global rippling of the layer. All these results provide key information about defects in the magnetism of graphene.Peer Reviewe