- Universidad Autónoma de Madrid, Departamento de Física Teórica de la Materia Condensada, Graduate StudentImperial College London, Physics, Post-Doc, and 3 moreadd
- Clare Hall College Cambridge (Life member 2005). University of Cambridge (Fellow BBVA 1999). CSIC (Faculty member 198... moreClare Hall College Cambridge (Life member 2005).
University of Cambridge (Fellow BBVA 1999).
CSIC (Faculty member 1988).
University of California-Berkeley (Post doc 1988).
Imperial College-London (Post doc 1986).
UAM-Madrid (PhD Physics 1985).
UAM-Madrid (MSc Physics 1981).
UAM-Madrid & UV-Valencia (BSc Physics 1979).edit - Prof. F. Flores (UAM-Madrid), Prof. J.B. Pendry (IC-London), Prof. M.A. Van Hove (LBL-Berkeley), Prof. G. Somorjai (UC-Berkeley), Prof. D.A. King (U. Cambridge)edit
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ABSTRACT In this work we present a Molecular Beam Epitaxy (MBE) growth method to obtain graphene on noble metals using evaporation of carbon atoms from a carbon solid-source in ultra-high vacuum conditions. We have synthesized graphene... more
ABSTRACT In this work we present a Molecular Beam Epitaxy (MBE) growth method to obtain graphene on noble metals using evaporation of carbon atoms from a carbon solid-source in ultra-high vacuum conditions. We have synthesized graphene (G) on different metal surfaces: from a well studied substrate as platinum, to a substrate where it can only be formed using innovative methods, as is the case of gold. For the characterization of the graphene layers we have used in situ surface science techniques as low energy electron diffraction (LEED), auger electron spectroscopy (AES) and scanning tunneling microscopy (STM).
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NanoSpain, Barcelona, Spain, May 28 - 31, 2019. -- http://www.nanospainconf.org/2019
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Trabajo presentado en la 13th International Conference on the Formation of Semiconductor Interfaces (13th ICFSI), celebrada en Praga del 2 al 8 de julio de 2011.
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Trabajo presentado en la 11th European Vacuum Conference (EVC-11), 8th Iberian Vacuum Meeting (IVM-8) y 6th European Topical Conference on Hard Coatings (ETCHC-6), celebradas en Salamanca del 20 al 24 de septiembre de 2010.
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Poster presentado en la GRAPHEsp2014 (A European Conference/Workshop on the Synthesis, Characterization and Applications of Graphene), celebrada en Lanzarote del 18 al 21 de febrero de 2014.
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Oral presentation given at the NanoSpain Conference held in Madrid (Spain) on March 11-14th 2014.
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Trabajo presentado en el International RILEM Conference on Advances in Construction Materials Through Science and Engineering, celebrado en Hong Kong (China), del 5 al 7 de septiembreHydrogen embrittlement is believed to be one of the... more
Trabajo presentado en el International RILEM Conference on Advances in Construction Materials Through Science and Engineering, celebrado en Hong Kong (China), del 5 al 7 de septiembreHydrogen embrittlement is believed to be one of the main reasons for cracking of metals under stress. High strength steels in these structures often include a ferritic core made of alpha-iron (body centered cubic lattice). Previous work was concerned with the interaction of atomic hydrogen with iron using first principles calculations. We studied the effect of interstitial hydrogen in the iron lattice and the stress induced by the interstitial hydrogen in the host lattice. In this paper we show the dynamical behaviour of hydrogen inside the iron lattice. Using abinitio Molecular Dynamics and by taking statistical averages diffusion coefficients hydrogen diffusion paths are obtained. Depending on temperature, the diffusion path involve going through tetrahedral or octahedral sites. Simulations where a number of hydrogens occasionally meet in one unit cell have been performed to elucidate the effect of interactions between hydrogens. From simulated diffusion path, the diffusion coefficient is calculated from Einstein´s equation. We also show the Fe-Fe interaction weakens. Iron Debye temperature decreases monotonically for increasing concentration of interstitial hydrogen, proving that iron-iron interatomic potential is significantly weakened in the presence of a large number of diffusing hydrogen atoms
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Trabajo presentado en el II International Conference on Computational Modelling of Fracture and Failure of Materials and Structures (CFRAC 2011), celebrada en Barcelona (España), del 6 al 8 de junio de 201
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We report on a new type of carbon extended structure formed by two graphene layers stacked directly on top of each other (stacking AA). This polymorphic form of a graphene bi-layer is meta-stable with a covalent distance between planes of... more
We report on a new type of carbon extended structure formed by two graphene layers stacked directly on top of each other (stacking AA). This polymorphic form of a graphene bi-layer is meta-stable with a covalent distance between planes of 0.156 nm, shorter than the typical van der Waals separation of 0.335 nm, and a 0.267 nm length for the hexagonal two-dimensional unit cell, larger than the typical graphene 0.243 nm. Each carbon makes single bonds to the four nearest carbon neighbours, at 0.154 nm (three in-plane bonds) and 0.156 nm (out-of-plane bond). Depending on the separation between layers, the electronic structure of the bi-layer changes from semi-metal, to metal, to wide-gap semiconductor. We describe the electronic structure of these layers and their behaviour under external anisotropic stresses.
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The COVID-19 pandemic has had worldwide devastating effects on human lives, highlighting the need for tools to predict its development. Dynamics of such public-health threats can often be efficiently analysed through simple models that... more
The COVID-19 pandemic has had worldwide devastating effects on human lives, highlighting the need for tools to predict its development. Dynamics of such public-health threats can often be efficiently analysed through simple models that help to make quantitative timely policy decisions. We benchmark a minimal version of a Susceptible-Infected-Removed model for infectious diseases (SIR) coupled with a simple least-squares Statistical Heuristic Regression (SHR) based on a lognormal distribution. We derived the three free parameters for both models in several cases and tested them against the amount of data needed to bring accuracy in predictions. The SHR model is approximately +/- 2% accurate about 20 days past the second inflexion point in the daily curve of cases, while the SIR model reaches a similar accuracy a fortnight before. All the analyzed cases assert the utility of SHR and SIR approximants as a useful tool to forecast the evolution of the disease. Finally, we have studied si...
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We develop a phase-space ab-initio formalism to compute Ballistic Electron Emission Spectroscopy current-voltage I(V)'s in a metal-semiconductor interface. We consider injection of electrons into the conduction band for direct... more
We develop a phase-space ab-initio formalism to compute Ballistic Electron Emission Spectroscopy current-voltage I(V)'s in a metal-semiconductor interface. We consider injection of electrons into the conduction band for direct bias ($V>0$) and injection of holes into the valence band or injection of secondary Auger electrons into the conduction band for reverse bias ($V<0$). Here, an ab-initio description of the semiconductor inversion layer (spanning hundreds of Angstroms) is needed. Such formalism is helpful to get parameter-free best-fit values for the Schottky barrier, a key technological characteristic for metal-semiconductor rectifying interfaces. We have applied the theory to characterize the Au/Ge(001) interface; a double barrier is found for electrons injected into the conduction band -- either directly or created by the Auger process -- while only a single barrier has been identified for holes injected into the valence band.
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We develop a phase-space ab-initio formalism to compute Ballistic Electron Emission Spectroscopy current-voltage I(V)'s in a metal-semiconductor interface. We consider injection of electrons into the conduction band for direct bias... more
We develop a phase-space ab-initio formalism to compute Ballistic Electron Emission Spectroscopy current-voltage I(V)'s in a metal-semiconductor interface. We consider injection of electrons into the conduction band for direct bias ($V>0$) and injection of holes into the valence band or injection of secondary Auger electrons into the conduction band for reverse bias ($V<0$).
Here, an ab-initio description of the semiconductor inversion layer (spanning hundreds of Angstroms) is needed.
Such formalism is helpful to get parameter-free best-fit values for the Schottky barrier, a key technological characteristic for metal-semiconductor rectifying interfaces. We have applied the theory to characterize the Au/Ge(001) interface; a double barrier is found for electrons injected into the conduction band -- either directly or created by the Auger process -- while only a single barrier has been identified for holes injected into the valence band.
Here, an ab-initio description of the semiconductor inversion layer (spanning hundreds of Angstroms) is needed.
Such formalism is helpful to get parameter-free best-fit values for the Schottky barrier, a key technological characteristic for metal-semiconductor rectifying interfaces. We have applied the theory to characterize the Au/Ge(001) interface; a double barrier is found for electrons injected into the conduction band -- either directly or created by the Auger process -- while only a single barrier has been identified for holes injected into the valence band.
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Within the context of a "hydrogen economy", it is paramount to guarantee a stable supply of molecular hydrogen to devices such as fuel cells. At the same time, catalytic conversion of the environmentally harmful methane into... more
Within the context of a "hydrogen economy", it is paramount to guarantee a stable supply of molecular hydrogen to devices such as fuel cells. At the same time, catalytic conversion of the environmentally harmful methane into ethane, with a significantly lower Global Warming Potential, turns into a highly desirable challenge. Herein we propose a first-step novel proof-of-concept mechanism to accomplish both tasks simultaneously. For that purpose we provide transition-state barriers and reaction Helmholtz free energies obtained from first-principles Density Functional Theory by taking account vibrations for 2CH(g) → CH(g) + H(g) to show that molecular hydrogen can be produced by subnanometer Pt and Au nanoparticles from natural gas. Interestingly, the active sites for the reaction are located on different planes on the two nanoparticles, effectively differentiating the working principle of the two metals. The analysis shows that the cycle to reduce CH can be performed on Au ...
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On-surface synthesis is an emerging approach to obtain, in a single step, precisely defined chemical species that cannot be obtained by other synthetic routes. The control of the electronic structure of organic/metal interfaces is crucial... more
On-surface synthesis is an emerging approach to obtain, in a single step, precisely defined chemical species that cannot be obtained by other synthetic routes. The control of the electronic structure of organic/metal interfaces is crucial for defining the performance of many optoelectronic devices. A facile on-surface chemistry route has now been used to synthesize the strong electron-acceptor organic molecule quinoneazine directly on a Cu(110) surface, via thermally activated covalent coupling of para-aminophenol precursors. The mechanism is described using a combination of in situ surface characterization techniques and theoretical methods. Owing to a strong surface-molecule interaction, the quinoneazine molecule accommodates 1.2 electrons at its carbonyl ends, inducing an intramolecular charge redistribution and leading to partial conjugation of the rings, conferring azo-character at the nitrogen sites.
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We have developed a calculation scheme for the elastic electron current in ultra-thin epitaxial heterostructures. Our model uses a Keldysh's non-equilibrium Green's function formalism and a layer-by-layer construction of the... more
We have developed a calculation scheme for the elastic electron current in ultra-thin epitaxial heterostructures. Our model uses a Keldysh's non-equilibrium Green's function formalism and a layer-by-layer construction of the epitaxial film. Such an approach is appropriate to describe the current in a ballistic electron emission microscope (BEEM) where the metal base layer is ultra-thin and generalizes a previous one based on a decimation technique appropriated for thick slabs. This formalism allows a full quantum mechanical description of the transmission across the epitaxial heterostructure interface, including multiple scattering via the Dyson equation, which is deemed a crucial ingredient to describe interfaces of ultra-thin layers properly in the future. We introduce a theoretical formulation needed for ultra-thin layers and we compare with results obtained for thick Au(1 1 1) metal layers. An interesting effect takes place for a width of about ten layers: a BEEM current...
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ABSTRACT Summary The transport and switch of Xe atoms on the Ni-W interface of a scanning tunnelling microscope have been analysed for different geometries as a function of the tip position with respect to the Ni(110) surface. Our results... more
ABSTRACT Summary The transport and switch of Xe atoms on the Ni-W interface of a scanning tunnelling microscope have been analysed for different geometries as a function of the tip position with respect to the Ni(110) surface. Our results show that the control of the different experimental results can only be achieved by a precise control of the position of the tip on the sample. In particular, the tip-sample Xe switch can only be obtained within tip heights of 0.2.
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Ab-initio molecular dynamics (MD) simulations and NMR GIPAW calculations have been used to analyze the coordination and mobility of Li ions in LiTi 2 (PO 4 ) 3 (rhombohedral), LiZr 2 (PO 4 ) 3 (triclinic) and LiZr 2 (PO 4 ) 3... more
Ab-initio molecular dynamics (MD) simulations and NMR GIPAW calculations have been used to analyze the coordination and mobility of Li ions in LiTi 2 (PO 4 ) 3 (rhombohedral), LiZr 2 (PO 4 ) 3 (triclinic) and LiZr 2 (PO 4 ) 3 (rhombohedral) phases. Significant discrepancies between static calculations of 7 Li quadrupolar parameters and experimental values were observed. We demonstrate the dynamical origin of such disagreement by incorporating in the calculations thermal vibrations and local motion of atoms via MD simulations. For LiTi 2 (PO 4 ) 3 , the quadrupolar constant associated with Li ions grows with temperature because the local symmetry of the system decreases, while for the Zr phases, the quadrupolar constant decreases because thermal vibrations reduce the anisotropy of the interaction. Finally, MD yields for both Zr phases Li distributions that compare well with disorder reported by diffraction studies.
Abstract On-surface synthesis is an emerging approach to obtain, in a single step, precisely defined chemical species that cannot be obtained by other synthetic routes. The control of the electronic structure of organic/metal interfaces... more
Abstract On-surface synthesis is an emerging approach to obtain, in a single step, precisely defined chemical species that cannot be obtained by other synthetic routes. The control of the electronic structure of organic/metal interfaces is crucial for defining the performance of many optoelectronic devices. A facile on-surface chemistry route has now been used to synthesize the strong electron-acceptor organic molecule quinoneazine directly on a Cu(110) surface, via thermally activated covalent coupling of para-aminophenol precursors. The mechanism is described using a combination of in-situ surface characterization techniques and theoretical methods. Owing to a strong surface-molecule interaction, the quinoneazine molecule accommodates 1.2 electrons at its carbonyl ends, inducing an intramolecular charge redistribution and leading to partial conjugation of the rings, conferring azo-character at the nitrogen sites.
We have studied the First stages leading to the formation of self-assembled monolayers of S-cysteine molecules adsorbed on a Au(111) surface. Density functional theory (DFT) calculations for the adsorption of individual cysteine molecules... more
We have studied the First stages leading to the formation of self-assembled monolayers of S-cysteine molecules adsorbed on a Au(111) surface. Density functional theory (DFT) calculations for the adsorption of individual cysteine molecules on Au(111)at room temperature show low-energy barriers all over the 2D Au(111) unit cell. As a consequence, cysteine molecules diffuse freely oil the Au(111) Surface and they can be regarded as a 2D molecular gas. The balance between molecule-molecule and molecule-substrate interactions induces molecular condensation and evaporation from the morphological surface structures (steps, reconstruction edges, etc.) as revealed by scanning tunnelling microscopy (STM) images. These processes lead progressively to the formation of a number of stable arrangements, not previously reported, such as single-molecular rows, trimers, and 2D islands. The condensation of these structures is driven by the aggregation of new molecules, stabilized by the formation of electrostatic interactions between adjacent NH3+ and COO- groups, together with adsorption at a slightly more favorable quasi-top site of the herringbone Au reconstruction.
Hydrogen embrittlement is believed to be one of the main reasons for cracking of structures under stress. High strength steels in these structures often include a ferritic core made of alpha-iron (body centered cubic lattice). We compute... more
Hydrogen embrittlement is believed to be one of the main reasons for cracking of structures under stress. High strength steels in these structures often include a ferritic core made of alpha-iron (body centered cubic lattice). We compute the interaction of atomic hydrogen with iron using first principles. The interstitial hydrogen can be placed in two high symmetry positions: octahedral and tetrahedral sites. Our calculations provide diffusion barriers between these sites. These barriers have been analyzed to understand the propagation of hydrogen through the iron lattice. We analyze how these barriers can be modified by the hydrogen concentration. The results show the main site for high and low hydrogen density and they show the diffusion coefficient variation by the hydrogen density.
The performance of a combinatorial simultaneous optimization (SO) algorithm is tested using experimental LEED I(E) data from Cu(100) and Fe 0.57 Al 0.47 (100) surfaces. SO optimizes structures taking advantage of the experimental database... more
The performance of a combinatorial simultaneous optimization (SO) algorithm is tested using experimental LEED I(E) data from Cu(100) and Fe 0.57 Al 0.47 (100) surfaces. SO optimizes structures taking advantage of the experimental database at two levels: (i) commensurate subsets of the database with the number of unknown parameters are chosen to find local solutions using Broyden's method and (ii) these partial structural solutions are used to build a Markov chain over the whole database. This procedure is of global character, the same as simulated annealing or genetic algorithm methods, but displays a very competitive scaling law because after the first iteration candidates are not chosen by a blind/random pick; they are already solutions to the problem with a restricted experimental database.
First-principles density-functional theory has been used to investigate equilibrium geometries, total energies, and diffusion barriers for H as an interstitial impurity absorbed in alpha-Fe. Internal strains/stresses upon hydrogen... more
First-principles density-functional theory has been used to investigate equilibrium geometries, total energies, and diffusion barriers for H as an interstitial impurity absorbed in alpha-Fe. Internal strains/stresses upon hydrogen absorption are a crucial factor to understand preferred absorption sites and diffusion. For high concentrations, H absorbs near the octahedral site favoring a large tetragonal distortion of the bcc lattice. For low concentration, H absorbs near the tetrahedral site minimizing the elastic energy stored on nearby cells. Diffusion paths depend on the concentration regime too; hydrogen diffuses about ten times faster in the distorted body-centered-tetragonal (bct) lattice. External stresses of several GPa modify barriers by approximate to 10/100, and diffusion rates by approximate to 30/100.
We report on a multiscale approach for the simulation of electrical characteristics of metal disilicide based Schottky-barrier metal oxide semiconductor field-effect transistors (SB-MOSFETs). Atomistic tight-binding method and... more
We report on a multiscale approach for the simulation of electrical characteristics of metal disilicide based Schottky-barrier metal oxide semiconductor field-effect transistors (SB-MOSFETs). Atomistic tight-binding method and nonequilibrium Green's function formalism are combined to calculate the propagation of charge carriers in the metal and the charge distribution at the MSi2(111)/Si(111) and MSi2(111)/Si(100) (with M=Ni, Co, and Fe) contacts. Quantum transmission coefficients at the interfaces are then computed accounting for energy and momentum conservation, and are further used as input parameters for a compact model of SB-MOSFET current-voltage simulations. In the quest for nanodevice performance optimization, this approach allows unveiling the role of different materials in configurations relevant for heterostructure nanowires.
Upon sputtering and annealing in UHV at similar to 1000 K, the rutile TiO2(110) surface undergoes a 1x1 -> 1x2 phase transition. The resulting 1x2 surface is Ti rich, formed by strands of double Ti rows as seen on scanning tunneling... more
Upon sputtering and annealing in UHV at similar to 1000 K, the rutile TiO2(110) surface undergoes a 1x1 -> 1x2 phase transition. The resulting 1x2 surface is Ti rich, formed by strands of double Ti rows as seen on scanning tunneling microscopic images, but its detailed structure and composition have been subject to debate in the literature for years. Recently, Park [Phys. Rev. Lett. 96, 226105 (2006)] have proposed a model where Ti atoms are located on interstitial sites with Ti2O stoichiometry. This model, when it is analyzed using LEED-IV data [Phys. Rev. Lett. 96, 0055502 (2006)], does not yield an agreement between theory and experiment as good as the previous best fit for Onishi and Iwasawa's model for the long-range 1x2 reconstruction. Therefore, the Ti2O3 added row is the preferred one from the point of view low-energy electron diffraction.
We present a technique to efficiently locate the global minimum of a cost function on a complex multi-dimensional parameter space (e.g. a structural reliability R-factor). The method builds a convergent series of structures from... more
We present a technique to efficiently locate the global minimum of a cost function on a complex multi-dimensional parameter space (e.g. a structural reliability R-factor). The method builds a convergent series of structures from combinatorial simultaneous optimization of all the parameters on different subspaces of the experimental data base. Performance has been tested for two model situations: (1) phase retrieval from electron scattering by a single atom and (2) low-energy electron diffraction analysis of simulated I( V) curves for the Ir(1 1 0)-p(2 x 1) missing row. Compared under the same conditions, a reduction in the computation effort, is found w.r.t. previous state-of-the-art methods (e.g., simulated annealing and genetic algorithms). (c) 2006 Elsevier B.V. All rights reserved.
Combining STM, LEED, and density functional theory, we determine the atomic surface structure of rutile TiO2 (110)-(1 x 2): nonstoichiometric Ti2O3 stripes along the [001] direction. LEED patterns are sharp and free of streaks, while STM... more
Combining STM, LEED, and density functional theory, we determine the atomic surface structure of rutile TiO2 (110)-(1 x 2): nonstoichiometric Ti2O3 stripes along the [001] direction. LEED patterns are sharp and free of streaks, while STM images show monatomic steps, wide terraces, and no cross-links. At room temperature, atoms in the Ti2O3 group have large amplitudes of vibration. The long quasi-1D chains display metallic character, show no interaction between them, and cannot couple to bulk or surface states in the gap region, forming good atomic wires.
The surface atomic structure of thin layers of three-dimensional yttrium silicide epitaxially grown on Si(111) 7x7 has been investigated by means of dynamical low-energy electron diffraction analysis. We determine the interlayer distances... more
The surface atomic structure of thin layers of three-dimensional yttrium silicide epitaxially grown on Si(111) 7x7 has been investigated by means of dynamical low-energy electron diffraction analysis. We determine the interlayer distances as well as the lateral and/or vertical relaxations of the atoms in the superficial planes. The epitaxial silicide consists of stacked hexagonal rare-earth planes and graphitelike Si planes with an ordered arrangement of Si vacancies. The ordered net of Si vacancies in the inner planes is responsible for the lateral relaxations of the surrounding Si atoms. The topmost layer does not present a graphitelike structure, forming a buckled Si layer with no vacancies. One of the three Si atoms in the lower plane of this bilayer is closer to the yttrium layer due to the presence of the vacancy in the last Si plane just below. This produces vertical relaxation in the termination layer.
We present a novel approach to calculating Low-Energy Electron Diffraction (LEED) intensities for ordered molecular adsorbates. First, the intra-molecular multiple scattering is computed to obtain a non-diagonal molecular T-matrix. This... more
We present a novel approach to calculating Low-Energy Electron Diffraction (LEED) intensities for ordered molecular adsorbates. First, the intra-molecular multiple scattering is computed to obtain a non-diagonal molecular T-matrix. This is then used to represent the entire molecule as a single scattering object in a conventional LEED calculation, where the Layer Doubling technique is applied to assemble the different layers, including the molecular ones. A detailed comparison with conventional layer-type LEED calculations is provided to ascertain the accuracy of this scheme of calculation. Advantages of this scheme for problems involving ordered arrays of molecules adsorbed on surfaces are discussed. (c) 2005 Elsevier B.V. All rights reserved.
We describe a FORTRAN-90 program to compute low-energy electron diffraction I(V) curves. Plane-waves and layer doubling are used to compute the inter-layer multiple-scattering, while the intra-layer multiple-scattering is computed in the... more
We describe a FORTRAN-90 program to compute low-energy electron diffraction I(V) curves. Plane-waves and layer doubling are used to compute the inter-layer multiple-scattering, while the intra-layer multiple-scattering is computed in the standard way expanding the wavefield on a basis of spherical waves. The program is kept as general as possible, in order to allow testing different parts of multiple-scattering calculations. In particular, it can handle non-diagonal t-matrices describing the scattering of non-spherical potentials, anisotropic vibrations, anharmonicity, etc. The program does not use old FORTRAN flavours, and has been written keeping in mind the advantage for parallelism brought forward by FORTRAN-90.
We describe a FORTRAN-90 program that computes scattering t-matrices for a molecule. These can be used in a Low-Energy Electron Diffraction program to solve the molecular structural problem very efficiently. The intramolecular multiple... more
We describe a FORTRAN-90 program that computes scattering t-matrices for a molecule. These can be used in a Low-Energy Electron Diffraction program to solve the molecular structural problem very efficiently. The intramolecular multiple scattering is computed within a Dyson-like approach, using free space Green propagators in a basis of spherical waves. The advantage of this approach is related to exploiting the chemical identity of the molecule, and to the simplicity to translate and rotate these t-matrices without performing a new multiple-scattering calculation for each configuration. FORTRAN-90 routines for rotating the resulting t-matrices using Wigner matrices are also provided.
Ab initio density functional theory has been used to investigate the adsorption of H2O on several close-packed transition and noble metal surfaces. A remarkably common binding mechanism has been identified. On every surface H2O binds... more
Ab initio density functional theory has been used to investigate the adsorption of H2O on several close-packed transition and noble metal surfaces. A remarkably common binding mechanism has been identified. On every surface H2O binds preferentially at an atop adsorption site with the molecular dipole plane nearly parallel to the surface. This binding mode favors interaction of the H2O 1b(1) delocalized molecular orbital with surface wave functions.
We have used a combination of dynamical low-energy-electron diffraction and density functional formalism calculations to find a structural model for two-dimensional (2D) YSi2 layers epitaxially grown on Si(111). Both techniques show that... more
We have used a combination of dynamical low-energy-electron diffraction and density functional formalism calculations to find a structural model for two-dimensional (2D) YSi2 layers epitaxially grown on Si(111). Both techniques show that the geometric structure of the yttrium silicide is quite similar to other 2D rare-earth silicides. The surface termination consists of a relaxed Si-bilayer and underlying Y atoms on T-4 sites [with respect to the Si(111) interface]. The low-energy electron diffraction study shows several occurrences of minima in the R factor. The analysis of diffracted beams measured at non-normal incidence allows us to discriminate the spurious minima.
Ballistic electron emission microscopy currents on Au/Si(111) and Au/Si(100) are analyzed using a Keldysh's formalism. The relative effect of the electron-electron interaction, quasielastic scattering (phonons), and band structure effects... more
Ballistic electron emission microscopy currents on Au/Si(111) and Au/Si(100) are analyzed using a Keldysh's formalism. The relative effect of the electron-electron interaction, quasielastic scattering (phonons), and band structure effects are shown to be relevant to interpret available experiments. The electron-phonon interaction can be introduced in the theory by computing an appropriate self-energy and solving Keldysh's equations self-consistently. The main effect of phonons is to create a more homogeneous current distribution in reciprocal espace, favoring injection on Si(111) over Si(100). Estimates for the electron-electron mean free path on Au are also obtained from this formalism.
The (100), (110), and (111) surfaces of Fe3Si have been studied by quantitative Low-Energy Electron Diffraction and Auger Electron Spectroscopy. Reversible phase transitions between the D0(3) and the CsCl structure develop upon annealing,... more
The (100), (110), and (111) surfaces of Fe3Si have been studied by quantitative Low-Energy Electron Diffraction and Auger Electron Spectroscopy. Reversible phase transitions between the D0(3) and the CsCl structure develop upon annealing, triggered by a substantial reversible surface segregation of Si. On all surfaces, Si termination is preferred either by realizing a topmost Si layer whenever the choice between Si and Fe exists, or by direct Si occupation of nominal Fe sites. Similarities to epitaxial iron silicide lms structural behaviour are identified.
Using a Keldysh Green's function formalism we have theoretically studied ballistic electron emission microscopy BEEM currents through Au/Si(1 0 0) and Au/Si(1 1 1) structures as a function of An thickness. Our analysis shows that for thin... more
Using a Keldysh Green's function formalism we have theoretically studied ballistic electron emission microscopy BEEM currents through Au/Si(1 0 0) and Au/Si(1 1 1) structures as a function of An thickness. Our analysis shows that for thin films. BEEM current is greater for Si(1 0 0) than for Si(1 1 1) substrates, however, in the case of thick films (more than 15 nm) the BEEM current associated with the Si(1 1 1) orientation is greater, in agreement with recent experimental data. This behaviour is due to the change in the symmetry of the k-space current distribution after the electrons propagate through the Au lattice, changing from a sixfold symmetry (which favours the matching with Si(1 0 0) available states) to a threefold one as thickness increases. Moreover. we have analysed how the relative alignment of the metal and the semiconductor affects the final BEEM current. All these results show that taking into account the band structure of the metallic laver is essential for a detailed understanding of the transport process. (C) 2001 Elsevier Science B.V. All rights reserved.
A theoretical model of ballistic-electron-emission microscopy (BEEM) based on linear combination of atomic orbitals Hamiltonians and Keldysh Green's functions is applied to analyze experimental data obtained for CoSi2 /Si(111) contacts.... more
A theoretical model of ballistic-electron-emission microscopy (BEEM) based on linear combination of atomic orbitals Hamiltonians and Keldysh Green's functions is applied to analyze experimental data obtained for CoSi2 /Si(111) contacts. Hot electrons injected from a scanning tunneling microscope tip into the silicide film form a highly focused beam, which even after propagation through films of moderate thickness is narrow enough to allow the observed atomic resolution of interfacial point defects. On (2x1) reconstructed domains a certain fraction of the initial current is injected into localized surface states, leading to the reported contrast in BEEM images, reflecting the topography at the surface. These results confirm that band-structure effects, both in the bulk and at the surface of the metallic overlayer, intricately influence the interface-related information contained in BEEM data. It is found that for a careful analysis of experimental results, a theoretical model going beyond the ballistic hypotesis is required.
Using a decimation technique, and imposing electrostatic self-consistency, we compute the surface electronic structure of various CoSi2(111)/Si(111) phases. The projected band structures and LDOS indicate a richness of surface related... more
Using a decimation technique, and imposing electrostatic self-consistency, we compute the surface electronic structure of various CoSi2(111)/Si(111) phases. The projected band structures and LDOS indicate a richness of surface related features. For the (1 x 1)Co-rich termination excellent agreement with experimental data and a recent DFT investigation is obtained. Strongly localized surface states on the high chain atoms of the (2 x 1) Pandey-chain like reconstruction are identified as causing the experimentally observed surface topography induced contrast in Ballistic Electron Emission Microscopy (BEEM) images of such films. (C) 2000 Published by Elsevier Science B.V.
Using a Green's-function approach that incorporates band-structure effects, and a complementary k-space Monte-Carlo analysis, we show how to get a theoretically consistent determination of the inelastic mean free path lambda(ee)(E) due to... more
Using a Green's-function approach that incorporates band-structure effects, and a complementary k-space Monte-Carlo analysis, we show how to get a theoretically consistent determination of the inelastic mean free path lambda(ee)(E) due to electron-electron interaction from ballistic electron emission spectroscopy. Exploiting experimental data taken at T=77 K on a thin-Au film (<100 Angstrom) deposited on a Si substrate, we find that the energy dependence of lambda(ee)(E) predicted by the standard Fermi-liquid theory provides excellent agreement between theoretical and experimental I(V) spectra. In agreement with theories for real metals, an enhancement of lambda(ee)(E) by a factor of two with respect to its electron-gas value is found.
Using a Green's function approach, we investigate band structure effects in the BEEM current distribution in k(parallel to)-space. In the elastic limit, this formalism provides a "parameter free" solution of the BEEM problem. At low... more
Using a Green's function approach, we investigate band structure effects in the BEEM current distribution in k(parallel to)-space. In the elastic limit, this formalism provides a "parameter free" solution of the BEEM problem. At low temperatures, and for thin metallic layers, the elastic approximation is enough to explain the experimental I(V) curves at low voltages. At higher voltages inelastic effects are approximately taken into account by introducing an effective RPA-electron lifetime, much in similarity with LEED theory. For thick films, however, additional damping mechanisms are required to obtain agreement with experiment.
We analyze statistical probability distributions of intensities collected using diffraction techniques such as low-energy electron diffraction (LEED). A simple theoretical model based an hard-sphere potentials and LEED formalism is... more
We analyze statistical probability distributions of intensities collected using diffraction techniques such as low-energy electron diffraction (LEED). A simple theoretical model based an hard-sphere potentials and LEED formalism is investigated for different values of relevant parameters: energy, angle of incidence, muffin-tin-potential radius, maximum spherical component l(max), number of stacked layers, and full multiple-scattering or kinematic model. Given a complex enough system (e.g., including multiple scattering by at least two Bravais lattices), the computed probability distributions agree rather well with a chi(2)(2) one, characteristic of the Gaussian unitary ensemble universality class associated with quantum chaos. A hypothesis on the possible impact of the chaotic nature of wave functions on correlation factors is tested against the behavior of the Pendry R factor and the root mean square deviation factor. [S0163-1829(99)00804-8].
Applying a Keldysh Green's function method it is shown that hot electrons injected from a scanning tunneling microscope tip into a CoSi2/Si(111) system form a highly focused beam due to the silicide band structure. This explains the... more
Applying a Keldysh Green's function method it is shown that hot electrons injected from a scanning tunneling microscope tip into a CoSi2/Si(111) system form a highly focused beam due to the silicide band structure. This explains the atomic resolution obtained in recent ballistic electron emission microscopy (BEEM) experiments. Localized surface states in the (2 x I) reconstruction are found to be responsible for the also reported anticorrugation of the BEEM current. These results clearly demonstrate the importance of bulk and surface band structure effects for a detailed understanding of BEEM data. [S0031-9007(98)07776-X].
We present a Green's-function approach based on a linear combination of atomic orbitals scheme to compute the elastic propagation of electrons injected from a scanning tunneling microscope tip into a metallic film. The obtained... more
We present a Green's-function approach based on a linear combination of atomic orbitals scheme to compute the elastic propagation of electrons injected from a scanning tunneling microscope tip into a metallic film. The obtained two-dimensional current distributions in real and reciprocal space furnish a good representation of the elastic component of ballistic electron emission microscopy (BEEM) currents. Since this component accurately approximates the total current in the near-threshold region, this procedure allows-in contrast to prior analyses-to take into account effects of the metal band structure in the modeling of these experiments. The Au band structure, and in particular its gaps appearing in the [111] and [100] directions, provides a good explanation for the previously irreconcilable results of nanometric resolution and similarity of BEEM spectra on both Au/Si(lll) and Au/Si(100). [S0163-1829(98)01243-0].
Intensities of low-energy electron diffraction and photoelectron diffraction are analyzed from a statistical point of view. The probability distribution is compared with a Porter-Thomas law, characteristic of a chaotic quantum system, The... more
Intensities of low-energy electron diffraction and photoelectron diffraction are analyzed from a statistical point of view. The probability distribution is compared with a Porter-Thomas law, characteristic of a chaotic quantum system, The agreement obtained is understood in terms of analogies between simple models and Berry's conjecture for a typical wave function of a chaotic system. The consequences of this behavior on surface structural analysis are qualitatively discussed by looking at the behavior of standard correlation factors. [S0031-9007(97)05221-6].
Using a quantum mechanical approach, we compute the ballistic electron emission microscopy current distribution in reciprocal space to compare experimental and theoretical spectroscopic I(V) curves. In the elastic limit, this formalism is... more
Using a quantum mechanical approach, we compute the ballistic electron emission microscopy current distribution in reciprocal space to compare experimental and theoretical spectroscopic I(V) curves. In the elastic limit, this formalism is a "parameter-free" representation of the problem. At low voltages, low temperatures, and for thin metallic layers, the elastic approximation is enough to explain the experiments (ballistic conditions). At low temperatures, inelastic effects can be taken into account approximately by introducing an effective electron-electron lifetime as an imaginary part in the energy. Ensemble Monte Carlo calculations were also performed to obtain ballistic electron emission microscopy currents in good agreement with the previous approach.
We present a theoretical framework well suited to analyze ballistic electron emission microscopy (BEEM) experiments. At low temperatures and low voltages, near the threshold value of the Schottky barrier, the BEEM current is dominated by... more
We present a theoretical framework well suited to analyze ballistic electron emission microscopy (BEEM) experiments. At low temperatures and low voltages, near the threshold value of the Schottky barrier, the BEEM current is dominated by the elastic component. Using a Keldysh Green's functions method, we analyze the injected distribution of electrons and the subsequent propagation through the metal. Elastic scattering by the lattice results in the formation of focused beams and narrow lines in real space. To obtain the current injected in the semiconductor, we compute the current distribution in reciprocal space and, assuming energy and k(parallel to) conservation, we match states to the projected conduction band minima of the semiconductor. Our results show an important focalization of the injected electron beam and explain the similarity between BEEM currents for Au/Si(111) and Au/Si(100). (C) 1998 Elsevier Science B.V.
A systematic transport study of the ballistic electron emission microscopy (BEEM) of Au/Si(100) and Au/Si(lll) Schottky barriers for different thicknesses of the metal layer and different temperatures is presented. It is shown that the... more
A systematic transport study of the ballistic electron emission microscopy (BEEM) of Au/Si(100) and Au/Si(lll) Schottky barriers for different thicknesses of the metal layer and different temperatures is presented. It is shown that the existing experimental data are compatible with a recently predicted band structure-induced non-forward electron propagation through the Au(lll) layer.
We review the basic ideas of holographic LEED, and the latest progress in the field. We compare several proposed computer reconstruction schemes. Using experimental diffuse LEED data from O/Ni(001) and K/Ni(001) surfaces, we show that the... more
We review the basic ideas of holographic LEED, and the latest progress in the field. We compare several proposed computer reconstruction schemes. Using experimental diffuse LEED data from O/Ni(001) and K/Ni(001) surfaces, we show that the capability now exists for filtering out the effects on the diffraction patterns of possible long-range order amongst the adsorbates, thus making holographic LEED much more analogous to photoelectron holography. Inclusion of a scattered-wave kernel to compensate for the variation of the magnitude of the reference wave at the positions of potential object-wave sources enables the reconstruction of a fully three-dimensional image of substrate atoms in the immediate vicinity of atomic adsorbates from a set of just normal-incidence diffuse LEED patterns.
We address the question of the spatial resolution of ballistic electron emission microscopy (BEEM) of Schottky barriers in Au(111)/Si(100) and Au(lll)/Si(lll) interfaces. A novel combination of Green-function and k-space... more
We address the question of the spatial resolution of ballistic electron emission microscopy (BEEM) of Schottky barriers in Au(111)/Si(100) and Au(lll)/Si(lll) interfaces. A novel combination of Green-function and k-space Ensemble-Monte-Carlo techniques is used to obtain new insights into the spatial and energetic evolution of the STM-tip-induced electrons during their passage through the metallic layer before reaching the metal-semiconductor interface. In particular, it is shown how the effect of band-structure-induced directional focussing of the electrons enforces a reinterpretation of existing experimental data.
We report on the application to measured data of the algorithm for holographic low-energy electron diffraction (LEED), which overcomes the two most important limitations of the technique to date: the ''searchlight'' effect, which tends to... more
We report on the application to measured data of the algorithm for holographic low-energy electron diffraction (LEED), which overcomes the two most important limitations of the technique to date: the ''searchlight'' effect, which tends to highlight only atoms forward scattered by the adsorbates, and the distorting effects on diffuse LEED intensities due to possible long-range order among the adsorbates. The only experimental input required is a set of the most reliably measured diffuse LEED patterns from normally incident electrons. The algorithm is applied to a set of 11 measured diffraction patterns from a K/Ni(001) surface. A fully three-dimensional image is reconstructed from these data by compensating for the anisotropy of the reference wave by an appropriate scattered-wave kernel.
Schottky-barrier heights can be modified by passivation of semiconductor surfaces and the resulting change in the metal-semiconductor interaction. We present theoretical calculations for two different semiconductors: Si and GaAs... more
Schottky-barrier heights can be modified by passivation of semiconductor surfaces and the resulting change in the metal-semiconductor interaction. We present theoretical calculations for two different semiconductors: Si and GaAs passivated with Sb. Passivation of GaAs(110) surfaces introduces ohmic contacts, while Si(111) presents smaller changes in its barrier height. These results are similar to the ones previously found for the same semiconductor surfaces passivated with H.
We present a theory for the scanning tunneling microscope (STM) current based on a Keldysh Green function formalism. In our formalism, we solve self-consistently an ab initio linear combination of atomic orbitals Hamiltonian within a... more
We present a theory for the scanning tunneling microscope (STM) current based on a Keldysh Green function formalism. In our formalism, we solve self-consistently an ab initio linear combination of atomic orbitals Hamiltonian within a local density formalism. Total energy calculations for xenon deposited on metal surfaces are performed to obtain the equilibrium position, and the Green functions needed to compute the current are obtained at the same time. Structural and nonstructural effects that can influence the correct interpretation of experimental STM results are studied. We find good agreement between our calculations and experimental images taken under highly controlled conditions, and we conclude that STM images should be analyzed by comparing iteratively the theory and the experiment, much in the same way as it is usually done for other surface sensitive techniques Like low-energy electron diffraction, photoelectron diffraction, surface-extended x-ray-absorption fine structure spectroscopy, etc.
We use first-order renormalized forward scattering coupled to a simulated annealing optimization algorithm to get the unknown structural parameters of a surface over a wide range of different possibilities. The method is very fast and... more
We use first-order renormalized forward scattering coupled to a simulated annealing optimization algorithm to get the unknown structural parameters of a surface over a wide range of different possibilities. The method is very fast and provides in an automated manner a quasi-global solution ready to be further refined using other techniques.
Ballistic electron emission microscopy (BEEM) currents for Au-Si interfaces are analyzed using a Keldysh method that takes into account the elastic propagation of electrons through the metallic layer. Our analysis shows that electrons... more
Ballistic electron emission microscopy (BEEM) currents for Au-Si interfaces are analyzed using a Keldysh method that takes into account the elastic propagation of electrons through the metallic layer. Our analysis shows that electrons being transmitted through the metal are focused along specific directions. This is easily understood in terms of the metallic Fermi surface characteristics. Using these ideas, we can explain both the behavior of the different Au/Si interfaces and the high lateral resolution found for BEEM experiments.
We study the sensitivity of LEED to stacking faults in the early stages of epitaxial growth. By theoretical simulation of stacking faults in different locations, in particular for Cu/Cu(111), Co/Cu(111), and p(4 x 4)Pb/Cu/Cu(111), we show... more
We study the sensitivity of LEED to stacking faults in the early stages of epitaxial growth. By theoretical simulation of stacking faults in different locations, in particular for Cu/Cu(111), Co/Cu(111), and p(4 x 4)Pb/Cu/Cu(111), we show that quantitative LEED can detect stacking faults down to three or four layers deep into the substrate. This is also true in the presence of a surfactant layer. The modification of the intensity spectra by stacking faults is large enough to be identified by visual comparison with defect-free surfaces. In the cases where different stacking sequences coexist on the surface, a minimum coverage of 30/100-50/100 imperfect stacking sequences is needed in order to be clearly detected in the LEED I-V curves. Therefore, our results provide guidance to conduct experiments where early detection of stacking faults is important.
We study the origin of the nanometric resolution seen in BEEM experiments. The problem is separated in three steps: (1) tunnelling from the STM tip to the metal, (2) elastic propagation in the metallic layer, and (3) overcoming the... more
We study the origin of the nanometric resolution seen in BEEM experiments. The problem is separated in three steps: (1) tunnelling from the STM tip to the metal, (2) elastic propagation in the metallic layer, and (3) overcoming the Schottky barrier in the metal-semiconductor interface. For the first step we apply an LCAO model and a Green's functions formalism due to Keldysh. The second step is investigated extending our formulation for step 1. To compute the necessary Green's functions we use an asymptotic approximation for thick layers where metallic band structure effects are included, and a decimation technique for thin layers. The third step is analyzed by looking at the matching of the incoming electrons and the available states in the semiconductor. For thick metallic layers (approximate to 100 - 150 Angstrom), the elastic interaction between propagating electrons and the metallic periodic lattice results in a few focalized beams of nanometric size, providing a simple explanation for the observed resolution For thin metallic layers (approximate to 20 - 50 Angstrom), we study the propagation layer by layer and we analyze the distribution of current as a function of the depth in the metal.
A beam is directed against the surface to form an interference pattern. The intensity distribution of the pattern is detected before calculating numerical image intensity data corresponding to the intensity distribution.The data are... more
A beam is directed against the surface to form an interference pattern. The intensity distribution of the pattern is detected before calculating numerical image intensity data corresponding to the intensity distribution.The data are reconstructed by a fast Fourier transform to form an image e.g. hologram indicating the location of a scatterer atom on the surface relative to an associated emitter atom on the surface. The pattern is a diffuse low energy electron diffraction pattern.ADVANTAGE - Conveys structure as experiment is being performed, utilising of source of electrons readily available in most laboratories. 14pp Dwg.No.1/3