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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 ($V>0$) 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., Comment: 35 pages, 11 figures. Appl. Surf. Sci (2022)

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 simulated stochastic individual-based SIR dynamics, which yields a detailed spatial and temporal view of the disease that cannot be given by SIR or SHR methods., Comment: 33 pages, 14 figures, 3 tables

Chemical equilibrium has proven extremely useful to predict the chemical composition of AGB atmospheres. Here we use a recently developed code and an updated thermochemical database, including gaseous and condensed species involving 34 elements, to compute the chemical equilibrium composition of AGB atmospheres of M-, S-, and C-type stars. We include for the first time TixCy clusters, with x = 1-4 and y = 1-4, and selected larger clusters ranging up to Ti13C22, for which thermochemical data is obtained from quantum chemical calculations. We find that in general chemical equilibrium reproduces well the observed abundances of parent molecules in circumstellar envelopes of AGB stars. There are however severe discrepancies, of various orders of magnitude, for some parent molecules: HCN, CS, NH3, and SO2 in M-type stars, H2O and NH3 in S-type stars, and the hydrides H2O, NH3, SiH4, and PH3 in C-type stars. Several molecules not yet observed in AGB atmospheres, like SiC5, SiNH, SiCl, PS, HBO, and the metal-containing molecules MgS, CaS, CaOH, CaCl, CaF, ScO, ZrO, VO, FeS, CoH, and NiS, are good candidates for detection with observatories like ALMA. The first condensates predicted are carbon, TiC, and SiC in C-rich atmospheres and Al2O3 in O-rich outflows. The most probable gas-phase precursors of dust are acetylene, atomic carbon, and/or C3 for carbon dust, SiC2 and Si2C for SiC dust, and atomic Al and AlOH, AlO, and Al2O for Al2O3 dust. In the case of TiC dust, atomic Ti is probably the main supplier of titanium. However, chemical equilibrium predicts that clusters like Ti8C12 and Ti13C22 become the major reservoirs of titanium at the expense of atomic Ti in the region where condensation of TiC is expected to occur, suggesting that the assembly of large TixCy clusters could be related to the formation of the first condensation nuclei of TiC., Comment: Accepted for publication in A&A. Abstract abridged. English edited with respect to previous version

Diffusion barriers for a cluster of three water molecules on Pd(111) have been estimated from ab-initio Density Functional Theory. A model for the diffusion of the trimer based in rotations yields a simple explanation of why the cluster can diffuse faster than a single water molecule by a factor $\approx 10^{2}$. This model is based on the differences between the adsorption geometry for the three monomers forming the cluster. One member interacts strongly with the surface and sits closer to the surface (d) while the other two interact weakly and stay at a larger separation from the surface (u). The trimer rotates nearly freely around the axis determined by the d monomer. Translations of the whole trimer imply breaking the strong interaction of the d monomer with the surface. Alternatively, thermal fluctuations exchange the actual monomer sitting closer to the surface with a lower energetic cost. Rotations around different axis introduce a diffusion mechanism where a strong interaction is kept along the diffusion path between the water molecule defining the axis of rotation and the Pd underneath., Comment: water ; monomer ; trimer ; water clusters ; diffusion ; rotation assisted ; Pd\{111\} ; ab-initio ; density functional theory

We describe a mechanism that explains the formation of hydrocarbons and hydrocarbyls from hydrogenated graphene/graphite; hard C-C bonds are weakened and broken by the synergistic effect of chemisorbed hydrogen and high temperature vibrations. Total energies, optimized structures, and transition states are obtained from Density Functional Theory simulations. These values have been used to determine the Boltzman probability for a thermal fluctuation to overcome the kinetic barriers, yielding the time scale for an event to occur. This mechanism can be used to rationalize the possible routes for the creation of small hydrocarbons and hydrocarbyls from etched graphene/graphite in stellar regions., Comment: 4.5 pages, 4 figures, 2 tables

In the framework of dielectric theory the static non-local self-energy of an electron near an ultra-thin polarizable layer has been calculated and applied to study binding energies of image-states near free-standing graphene. The corresponding series of eigenvalues and eigenfunctions have been obtained by solving numerically the one-dimensional Schr{\"o}dinger equation. Image-potential-state wave functions accumulate most of their probability outside the slab. We find that a Random Phase Approximation (RPA) for the non-local dielectric function yields a superior description for the potential inside the slab, but a simple Fermi-Thomas theory can be used to get a reasonable quasi-analytical approximation to the full RPA result that can be computed very economically. Binding energies of the image-potential states follow a pattern close to the Rydberg series for a perfect metal with the addition of intermediate states due to the added symmetry of the potential. The formalism only requires a minimal set of free parameters; the slab width and the electronic density. The theoretical calculations are compared to experimental results for work function and image-potential states obtained by two-photon photoemission., Comment: 24 pages; 10 figures. arXiv admin note: text overlap with arXiv:1301.4482

Ab initio density functional theory has been used to analyze flexural modes, elastic constants, and atomic corrugations on single and bi-layer graphene. Frequencies of flexural modes are sensitive to compressive stress; its variation under stress can be related to the anomalous thermal expansion via a simple model based in classical Elasticity Theory [Phys. Rev. B 86, 144103]. Under compression, flexural modes are responsible for a long wavelength rippling with a large amplitude and a marked anharmonic behavior. This is compared with corrugations created by thermal fluctuations and the adsorption of a light impurity (hydrogen). Typical values for the later are in the sub-Angstrom regime, while maximum corrugations associated to bending modes quickly increase up to a few Angstroms under a compressive stress, due to the intrinsic instability of flexural modes

In the framework of the non-local dielectric theory the static non-local self-energy of an electron near an ultra-thin polarizable layer has been calculated and applied to study image-states near free-standing graphene. The corresponding series of eigenvalues and eigenfunctions have been obtained by solving numerically the one-dimensional Schr{\"o}dinger equation. We compare with the Rydgberg's series for a perfect metal and with experimental values measured on graphene layers grown on Ir and Ru surfaces. For free standing films, the appearance of states with binding energies in between the classical series is discussed.

We present a simple analytical approach to study anharmonic effects in single layer, bilayer, and multilayer graphene. The coupling between in plane and out of plane modes leads to negative Gr\"uneisen coefficients and negative thermal expansion. The value of the thermal expansion coefficient depends on the coupling to the substrate. The bending rigidity in bilayer graphene shows a crossover between a long wavelength regime where its value is determined by the in plane elastic properties and a short wavelength regime where its value approaches twice that of a single layer.

Sykes {\it et al.} [Proc. Natl. Acad. Sci. {\bf 102}, 17907 (2005)] have reported how electrons injected from a scanning tunneling microscope modify the diffusion rates of H buried beneath Pd(111). A key point in that experiment is the symmetry between positive and negative voltages for H extraction, which is difficult to explain in view of the large asymmetry in Pd between the electron and hole densities of states. Combining concepts from the theory of ballistic electron microscopy and electron-phonon scattering we show that H diffusion is driven by the $s$-band electrons only, which explains the observed symmetry., Comment: 5 pages and 4 figures

The geometrical and electronic structure of tripotassium doped phenanthrene, \ce{K3C14H10}, have been studied by first-principles density functional theory. The main effect of potassium doping is to inject charge in the narrow phenanthrene conduction band, rendering the system metallic. The Fermi surface for the experimental X-rays unit cell is composed of two sheets with marked one and two dimensional character respectively., Comment: 4 pages, 5 figures. arXiv admin note: substantial text overlap with arXiv:1106.0377

First-principles atomistic molecular dynamics simulation in the micro-canonical and canonical ensembles has been used to study the diffusion of interstitial hydrogen in $\alpha$-iron. Hydrogen to Iron ratios between $\theta=1/16 and 1/2 have been considered by locating interstitial hydrogen atoms at random positions in a $2 \times 2 \times 2$ supercell. We find that the average optimum absorption site and the barrier for diffusion depend on the concentration of interestitials. 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.

Density functional calculations are employed to study the molecular dissociation of hydrogen on graphene, the diffusion of chemisorbed atomic species, and the electronic properties of the resulting hydrogen on graphene system. Our results show that applying stress to the graphene substrate can lower the barrier to dissociation of molecular hydrogen by a factor of six, and change the process from endothermic to exothermic. These values for the barrier and the heat of reaction, unlike the zero stress values, are compatible with the time scales observed in experiments. Diffusion, on the other hand, is not greatly modified by stress. We analyse the electronic structure for configurations relevant to molecular dissociation and adsorption of atomic hydrogen on a graphene single layer. An absolute band gap of 0.5 eV is found for the equilibrium optimum configuration for a narrow range of coverages ($\theta \approx 0.25$). This value is in good agreement with experiment [Elias et al., Science {\bf 323}, 610 (2009)].

The geometrical and electronic structure of potassium doped phenanthrene, \ce{K3C14H10}, have been studied by first-principles density functional theory. The main effect of potassium doping is to inject charge in the narrow phenanthrene conduction band, rendering the system metallic. The Fermi surface for the experimental X-rays structure is composed of two sheets with marked one and two dimensional character respectively.

Silicon thin films for coverages ($\theta$) between 0.3 and 3 monolayers have been grown on rutile \ce{TiO2}(110)-(1x1) at room temperature and studied by x-ray and ultra-violet photoelectron spectroscopies, Auger electron spectroscopy, and low energy electron diffraction (LEED). A clear evidence of a strong \ce{Si}/\ce{TiO2} interaction consistent with the high affinity of O for Si has been found. The Ti cations on the substrate are reduced, while the Si film is oxidized, yielding \ce{SiO2} and a mixture of silicon suboxides. Neutral Si atoms are observed at a coverage of 3 monolayers. At the interface region we observe the formation of cross-linking Ti-O-Si bonds. The thin Si overlayer strongly attenuates the $(1 \times 1)$ LEED pattern from the substrate. Finally, thermal annealing results in the improvement of the \ce{SiO2} stoichiometry, but the surface order is not recovered. Using ab-initio density functional theory we have obtained optimum geometrical configurations and corresponding density of states for 1/3 \le \theta \le 1$ monolayers of Si adsorbed on the $1 \times 1$ two-dimensional unit cell.

Using ab-initio density functional theory we study the role of interstitial hydrogen on the energetics of the phase transformation of iron from bcc to hcp along Bain's pathway. The impurity creates an internal stress field that can be released through a tetragonal distortion of the lattice, promoting the bcc (ferromagnetic) $\rightarrow$ fcc (frustrated antiferromagnetic) $\rightarrow$ hcp (ferromagnetic) transition. The transformation between crystal systems is accompanied by a drastic magnetic reorganization and sudden variations of the unit cell volume, that can be one of the reasons for embrittlement and mechanical failure of iron upon hydrogen adsorption.

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 BCT lattice. External stresses of several GPa modify barriers by $\approx$ 10%, and diffusion rates by $\approx$ 30%.

The crystal structure of potassium doped picene with an exact stoichiometry (K3C22H14, K3picene from here onwards) has been theoretically determined within Density Functional Theory allowing complete variational freedom of the crystal structure parameters and the molecular atomic positions. A modified herringbone lattice is obtained in which potassium atoms are intercalated between two paired picene molecules displaying the two possible orientations in the crystal.Along the c-axis, organic molecules alternate with chains formed by three potassium atoms. The electronic structureof the doped material resembles pristine picene, except that now the bottom of the conduction band is occupied by six electrons coming from the ionized K atoms (six per unit cell). Wavefunctions remain based mainly on picene molecular orbitals getting their dispersion from intralayer edge to face CH/pi bonding, while eigenenergies have been modified by the change in the electrostatic potential. The small dispersion along the c-axis is assigned to small H-H overlap. From the calculated electronic density of states we expect metallic behavior for potassium doped picene., Comment: Published version: 8 twocolumn pages, 7 color figures, 2 structural .cif files included

Chemical adsorption of atomic hydrogen on a negatively charged single layer graphene sheet has been analyzed with ab-initio Density Functional Theory calculations. We have simulated both finite clusters and infinite periodic systems to investigate the effect of different ingredients of the theory, e.g. exchange and correlation potentials, basis sets, etc. Hydrogen's electron affinity dominates the energetic balance in the charged systems and the extra electron is predominantly attracted to a region nearby the chemisorbed atom. The main consequences are: (i) the cancellation of the unpaired spin resulting in a singlet ground-state, and (ii) a stronger interaction between hydrogen and the graphene sheet., Comment: 11 pages, 8 figures, to be published in PRB

Graphene layers are stable, hard, and relatively inert. We study how tensile stress affects $\sigma$ and $\pi$ bonds and the resulting change in the chemical activity. Stress affects more strongly $\pi$ bonds that can become chemically active and bind to adsorbed species more strongly. Upon stretch, single C bonds are activated in a geometry mixing $120^{o}$ and $90^{o}$; an intermediate state between $sp^{2}$ and $sp^{3}$ bonding. We use ab-initio density functional theory to study the adsorption of hydrogen on large clusters and 2D periodic models for graphene. The influence of the exchange-correlation functional on the adsorption energy is discussed.

We show that two graphene layers stacked directly on top of each other (AA stacking) form strong chemical bonds when the distance between planes is 0.156 nm. Simultaneously, C-C in-plane bonds are considerably weakened from partial double-bond (0.141 nm) to single bond (0.154 nm). This polymorphic form of graphene bilayer is meta-stable w.r.t. the one bound by van der Waals forces at a larger separation (0.335 nm) with an activation energy of 0.16 eV/cell. Similarly to the structure found in hexaprismane, C forms four single bonds in a geometry mixing 90^{0} and 120^{0} angles. Intermediate separations between layers can be stabilized under external anisotropic stresses showing a rich electronic structure changing from semimetal at van der Waals distance, to metal when compressed, to wide gap semiconductor at the meta-stable minimum., Comment: tar gzip latex 4 pages 4 figures

Surface Patterson Functions have been derived by direct inversion of experimental Low-Energy Electron Diffraction I-V spectra measured at multiple incident angles. The direct inversion is computationally simple and can be used to discriminate between different structural models. 1x1 YSi_2 epitaxial layers grown on Si(111) have been used to illustrate the analysis. We introduce a suitable R-factor for the Patterson Function to make the structural discrimination as objective as possible. From six competing models needed to complete the geometrical search, four could easily be discarded, achieving a very significant and useful reduction in the parameter space to be explored by standard dynamical LEED methods. The amount and quality of data needed for this analysis is discussed., Comment: 5 pages, 4 figures

Applying a Keldysh Green`s function method it is shown that hot electrons injected from a STM-tip into a CoSi${}_2$/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 $(2 \times 1)$-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., Comment: 5 pages, RevTex, 4 postscript figures, http://www.icmm.csic.es/Pandres/pedro.htm

We analyze statistical probability distributions of intensities collected by diffraction techniques like Low-Energy Electron Diffraction. A simple theoretical model based in 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 to quantum chaos. A hypothesis on the possible impact of the chaoticity of wavefunctions on correlation factors is tested against the behaviour of the Pendry R-factor and the Root Mean Squared Deviation factor., Comment: LaTex, 8 pages, 13 figures, http://www.icmm.csic.es/Pandres/pedro.htm

Using a Green's function approach, we investigate band structure effects in the BEEM current distribution in reciprocal space. In the elastic limit, this formalism provides a 'parameter free' solution to 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., Comment: 4 pages, 3 postscript figures, revtex

We present a Green's function approach based on a LCAO scheme to compute the elastic propagation of electrons injected from a STM tip into a metallic film. The obtained 2D current distribution 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(111) and Au/Si(100)., Comment: 12 pages, 9 postscript figures, revtex

A systematic transport study of the ballistic electron emission microscopy (BEEM) of Au/Si(100) and Au/Si(111) 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 bandstructure-induced non-forward electron propagation through the Au(111) layer., Comment: 5 pages, Latex-APS, 1 postscript figure, http://www.icmm.csic.es/Pandres/pedro.htm. Phys. Stat. Sol. (b) (to appear), HCIS-10 Conf, Berlin 1997

We address the question of the spatial resolution of ballistic electron emission microscopy (BEEM) of Shottky barriers in Au(111)/Si(100) and Au(111)/Si(111) 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 focusing of the electrons enforces a reinterpretation of existing experimental data., Comment: 7 pages, Latex-APS, 2 postscript figures, http://www.icmm.csic.es/Pandres/pedro.htm, Proc. 10th Conf. on Microscopy of Sem. Mat. (Oxford, April 1997), E. T. Cullis, in print

We analyze 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. 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 $k_{\parallel}$ conservation. 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)., Comment: 17 pages, 5 figures (postscript), Latex, APS, http://www.icmm.csic.es/Pandres/pedro.htm. Appl. Surf. Sci. (in press)

Intensities of LEED and PED 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 wavefunction of a chaotic system. The consequences of this behaviour on surface structural analysis are qualitatively discussed by looking at the behaviour of standard correlation factors., Comment: 5 pages, 4 postscript figures, Latex, APS, http://www.icmm.csic.es/Pandres/pedro.htm