226 research outputs found
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
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
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).
Only in the case of Au, the zigzag geometry can also be stabilized for an
intermediate bond angle of 131.
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
Theory of Spin-Dependent Electron Transfer Dynamics at Ar/Co(0001) and Ar/Fe(110) Interfaces
Recent core-hole-clock experiments [Phys. Rev. Lett. , 086801
(2014)] showed that the spin dependence of electron injection times at
Ar/Co(0001) and Ar/Fe(110) interfaces is at variance with the expectations
based on previous calculations for related systems. Here we reconcile theory
and experiment, and demonstrate that the observed dependence is rooted in the
details of the spin-split surface band structures. Our ab initio calculations
back that minority electrons are injected significantly faster than majority
electrons in line with the experimentally reported ultrashort injection times.
The dynamics is particularly sensitive to the size (in reciprocal-space) of the
projected band gaps around for both substrates at the
resonance energies. A simple tunneling model incorporating the spin-dependent
gap sizes further supports these findings.Comment: 5+6 pages, 4+4 figure
Role of k-point sampling in the supercell approach to inelastic electron tunneling spectroscopy simulations of molecular monolayers
Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).While the role of sampling of the electron momentum k in supercell calculations of the elastic electron transmission is well understood, its influence in the case of inelastic electron tunneling (IET) has not yet been systematically explored. Here we compare ab initio IET spectra of molecular monolayers in the commonly used Γ-point approximation to rigorously k-converged results. We study four idealized molecular junctions with either alkanedithiolates or benzenedithiolates, and explore variations due to varying molecular tilt angle, density, as well as chemical identity of the monolayer. We show that the Γ-point approximation is reasonable for a range of systems, but that a rigorous convergence is needed for accurate signal amplitudes. We also describe an approximative scheme which reduces the computational cost of the k-averaged calculation in our implementation.We acknowledge the support of the Basque Departamento de Educacion and the UPV/EHU (Grant No. IT-756-13), the Spanish Ministerio de Economía y Competitividad (MINECO Grants No. MAT2013-46593-C6-2-P and FIS2013-48286-C2-2-P), and the European Union Integrated Project PAMS (Contract No. 610446).Peer Reviewe
Universal Magnetic Properties of sp-type Defects in Covalently Functionalized Graphene
Using density-functional calculations, we study the effect of sp-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 always appears
in graphene. This effect is similar to that of H adsorption, which saturates
one 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 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
Mixed-valency signature in vibrational inelastic electron tunneling spectroscopy
4 páginas, 3 figuras, 1 tabla.-- PACS numbers: 68.37.Ef, 72.10.-d, 72.25.-b, 79.20.RfDensity functional theory simulations of the vibrational inelastic electron tunneling spectroscopy (IETS) of O2 on Ag(110) permits us to solve its unexplained IETS data [ Hahn et al. Phys. Rev. Lett. 85 1914 (2000)]. When semilocal density functional theory is corrected by including static intra-atomic correlations, the IETS simulations are in excellent agreement with the experiment. The unforeseen consequence of our calculations is that when adsorbed along the [001] direction, molecular O2 on Ag(110) is a mixed-valent system. This analysis of IETS unambiguously reveals the paramagnetic nature of O2 on Ag(110).We acknowledge financial support from the Spanish
MICINN (No. FIS2007-066711-CO2-00 and
No. FIS2009-12721-C04-01), and the Basque
Government—UPV/EHU (Grant No. IT-366-07).Peer reviewe
Plasmonic response of metallic nanojunctions driven by single atom motion: Quantum transport revealed in optics
The correlation between transport properties across subnanometric metallic gaps and the optical response of the system is a complex effect that is determined by the fine atomic-scale details of the junction structure. As experimental advances are progressively accessing transport and optical characterization of smaller nanojunctions, a clear connection between the structural, electronic, and optical properties in these nanocavities is needed. Using ab initio calculations, we present here a study of the simultaneous evolution of the structure and the optical response of a plasmonic junction as the particles forming the cavity, two Na380 clusters, approach and retract. Atomic reorganizations are responsible for a large hysteresis of the plasmonic response of the system, which shows a jump-to-contact instability during the approach process and the formation of an atom-sized neck across the junction during retraction. Our calculations demonstrate that, due to the quantization of the conductance in metal nanocontacts, atomic-scale reconfigurations play a crucial role in determining the optical response of the whole system. We observe abrupt changes in the intensities and spectral positions of the dominating plasmon resonances and find a one-to-one correspondence between these jumps and those of the quantized transport as the neck cross-section diminishes. These results reveal an important connection between transport and optics at the atomic scale, which is at the frontier of current optoelectronics and can drive new options in optical engineering of signals driven by the motion and manipulation of single atoms.We acknowledge financial support from Projects FIS2013-41184-P and MAT2013-46593-C6-2-P from MINECO. M.B., P.K., F.M., and D.S.P. also acknowledge support from the ANR-ORGAVOLT project and the Euroregion Aquitaine-Euskadi program. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a Ph.D. grant. P.K. acknowledges financial support
from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union. J.A. also acknowledges support from Grant 70NANB15H321, “PLASMOQUANTUM”, from the US Department of Commerce (NIST).Peer Reviewe
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