arXiv subjects

F. Flores

Publications and source records attributed to F. Flores.

At least 19 recordsLinked to original sources

arXiv2022

A criterion for hypersymmetry on discrete groupoids

Given a Fell bundle $\mathscr C\overset{q}{\to}Ξ$ over the discrete groupoid $Ξ$, we study the symmetry of the associated Hahn algebra $\ell^{\infty,1}(Ξ\!\mid\!\mathscr C)$ in terms of the isotropy subgroups of $Ξ$. We prove that $Ξ$ is symmetric (resp. hypersymmetric) if and only if all of the isotropy subgroups are symmetric (resp. hypersymmetric). We also characterize hypersymmetry using Fell bundles with constant fibers, showing that for discrete groupoids, 'hypersymmetry' equals 'rigid symmetry'.

math.OA↗

arXiv2021

Morphisms of Groupoid Actions and Recurrence

Topological groupoids admit various types of morphisms. We push these notions to the level of continuous groupoid actions to obtain various types of groupoid action morphisms. Some dynamical properties and their relation to these morphisms are studied. Among them are recurrence, various forms of transitivity, minimality, limit, recurrent, periodic and almost periodic points.

math.DS↗

arXiv2010

Nonequilibrium transport in molecular junctions with strong electron-phonon interactions

We present a combined theoretical approach to study the nonequilibrium transport properties of nanoscale systems coupled to metallic electrodes and exhibiting strong electron-phonon interactions. We use the Keldysh Green function formalism to generalize beyond linear theory in the applied voltage an equation of motion method and an interpolative self-energy approximation previously developed in equilibrium. We analyze the specific characteristics of inelastic transport appearing in the intensity versus voltage curves and in the conductance, providing qualitative criteria for the sign of the step-like features in the conductance. Excellent overall agreement between both approaches is found for a wide range of parameters.

cond-mat.mes-hall↗

arXiv2007

Electrical charging during the sharkskin instability of a metallocene melt

Flow instabilities are widely studied because of their economical and theoretical interest, however few results have been published about the polymer electrification during the extrusion. Nevertheless the generation of the electrical charges is characteristic of the interaction between the polymer melt and the die walls. In our study, the capillary extrusion of a metallocene polyethylene (mPE) through a tungsten carbide die is characterized through accurate electrical measurements thanks a Faraday pail. No significant charges are observed since the extrudate surface remains smooth. However, as soon as the sharkskin distortion appears, measurable charges are collected (around 5 10-8 C/m2). Higher level of charges are measured during the spurt or the gross-melt fracture (g.m.f) defects. This work is focused on the electrical charging during the sharkskin instability. The variation of the electrical charges versus the apparent wall shear stress is investigated for different die geometries. This curve exhibits a linear increase, followed by a sudden growth just before the onset of the spurt instability. This abrupt charging corresponds also to the end of the sharkskin instability. It is also well-known that wall slip appears just at the same time, with smaller velocity values than during spurt flow. Our results indicate that electrification could be a signature of the wall slip. We show also that the electrification curves can be shifted according to the time-temperature superposition principle, leading to the conclusion that molecular features of the polymer are also involved in this process.

cond-mat.soft↗

arXiv2007

Modelization of flow electrification in a polymer melt

Flow electrification of polymer melts is an important side effect of polymer processing. The studies dealing with this phenomenon are seldom and most of the scientific work has been focused on flow electrification of aqueous and insulating Newtonian liquids. From that prior art it is well established that the flow electrification in Newtonian liquids is a consequence of the formation of an ionic double layer. Convection of this layer induces the electrification of the liquid at the outlet of the pipe. In those models, the key parameters governing the flow electrification are thus the intrinsic electrical properties of the polymer and the flow characteristics. In this work, we reconsider the assumptions made previously and we propose a new approach to modelise the flow electrification in the particular case of non-Newtonian polymer materials in laminar flow conditions. We establish that, a key parameter for the electrification quantification in the polymer melt is the shape of the velocity profile. Additionally, in some cases, we show that a slip velocity at the polymer/die wall interface must be considered to describe accurately the electrification. As a consequence, we deduce that the slip velocity at the interface can be calculated by measuring the electrification: this work gives an alternative manner to measure the slip velocity during polymer flow.

physics.class-ph↗

arXiv2005

Tuning the conductance of single-walled carbon nanotubes by ion irradiation in the Anderson localization regime

Carbon nanotubes are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge. Defects are known to modify the electrical resistance of carbon nanotubes. They can be present in as-grown carbon nanotubes, but controlling externally their density opens a path towards the tuning of the nanotube electronic characteristics. In this work consecutive Ar+ irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room temperature resistance versus SWNT-length [R(L)] along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a 400 nm SWNT length.

cond-mat.mtrl-sci↗

arXiv2005

Ga-induced atom wire formation and passivation of stepped Si(112)

We present an in-depth analysis of the atomic and electronic structure of the quasi one-dimensional (1D) surface reconstruction of Ga on Si(112) based on Scanning Tunneling Microscopy and Spectroscopy (STM and STS), Rutherford Backscattering Spectrometry (RBS) and Density Functional Theory (DFT) calculations. A new structural model of the Si(112)6 x 1-Ga surface is inferred. It consists of Ga zig-zag chains that are intersected by quasi-periodic vacancy lines or misfit dislocations. The experimentally observed meandering of the vacancy lines is caused by the co-existence of competing 6 x 1 and 5 x 1 unit cells and by the orientational disorder of symmetry breaking Si-Ga dimers inside the vacancy lines. The Ga atoms are fully coordinated, and the surface is chemically passivated. STS data reveal a semiconducting surface and show excellent agreement with calculated Local Density of States (LDOS) and STS curves. The energy gain obtained by fully passivating the surface calls the idea of step-edge decoration as a viable growth method toward 1D metallic structures into question.

cond-mat.other↗

arXiv2005

Kondo Resonance Decoherence by an External Potential

The Kondo problem, for a quantum dot (QD), subjected to an external bias, is analyzed in the limit of infinite Coulomb repulsion by using a consistent equations of motion method based on a slave-boson Hamiltonian. Utilizing a strict perturbative solution in the leads-dot coupling, T, to T^4 and T^6 orders, we calculate the QD spectral density and conductance, as well as the decoherent rate that drive the systemm from the strong to the weak coupling regime. Our results indicate thet the weak coupling regime is reached for voltages larger than a few units of the Kondo temperature.

cond-mat.str-el↗

arXiv2004

Barrier formation at metal/organic interfaces: dipole formation and the Charge Neutrality Level

The barrier formation for metal/organic semiconductor interfaces is analyzed within the Induced Density of Interface States (IDIS) model. Using weak chemisorption theory, we calculate the induced density of states in the organic energy gap and show that it is high enough to control the barrier formation. We calculate the Charge Neutrality Levels of several organic molecules (PTCDA, PTCBI and CBP) and the interface Fermi level for their contact with a Au(111) surface. We find an excellent agreement with the experimental evidence and conclude that the barrier formation is due to the charge transfer between the metal and the states induced in the organic energy gap.

cond-mat.mtrl-sci↗

arXiv2004

Dipole formation at metal/PTCDA interfaces: Role of the Charge Neutrality Level

The formation of a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic dianhydride) interface barrier is analyzed using weak-chemisorption theory. The electronic structure of the uncoupled PTCDA molecule and of the metal surface is calculated. Then, the induced density of interface states is obtained as a function of these two electronic structures and the interaction between both systems. This induced density of states is found to be large enough (even if the metal/PTCDA interaction is weak) for the definition of a Charge Neutrality Level for PTCDA, located 2.45 eV above the highest occupied molecular orbital. We conclude that the metal/PTCDA interface molecular level alignment is due to the electrostatic dipole created by the charge transfer between the two solids.

cond-mat.mtrl-sci↗

arXiv2004

Formation of atom wires on vicinal silicon

The formation of atomic wires via pseudomorphic step-edge decoration on vicinal silicon surfaces has been analyzed for Ga on the Si(112) surface using Scanning Tunneling Microscopy and Density Functional Theory calculations. Based on a chemical potential analysis involving more than thirty candidate structures and considering various fabrication procedures, it is concluded that pseudomorphic growth on stepped Si(112), both under equilibrium and non-equilibrium conditions, must favor formation of Ga zig-zag chains rather than linear atom chains. The surface is non-metallic and presents quasi-one dimensional character in the lowest conduction band.

cond-mat.mtrl-sci↗

arXiv1999

Interpolative method for transport properties of quantum dots in the Kondo regime

We present an interpolative method for describing coherent transport through an interacting quantum dot. The idea of the method is to construct an approximate electron self-energy which becomes exact both in the limits of weak and strong coupling to the leads. The validity of the approximation is first checked for the case of a single (spin-degenerate) dot level. A generalization to the multilevel case is then discussed. We present results both for the density of states and the temperature dependent linear conductance showing the transition from the Kondo to the Coulomb blockade regime.

cond-mat.mes-hall↗

arXiv1999

Transport in multilevel quantum dots: from the Kondo effect to the Coulomb blockade regime

A new theoretical method is introduced to study coherent electron transport in an interacting multilevel quantum dot. The method yields the correct behavior both in the limit of weak and strong coupling to the leads, giving a unified description of Coulomb blockade and the Kondo effect. Results for the density of states and the temperature dependent conductance for a two- level dot are presented. The relevance of these results in connection to recent experiments on the Kondo effect in semiconducting quantum dots is discussed.

cond-mat.mes-hall↗

arXiv1998

Ballistic Electron Emission Microscopy on CoSi${}_2$/Si(111) interfaces: band structure induced atomic-scale resolution and role of localized surface states

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.

cond-mat.mtrl-sci↗

arXiv1998

Quantum mechanical analysis of the elastic propagation of electrons in the Au/Si system: application to Ballistic Electron Emission Microscopy

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).

cond-mat.mtrl-sci↗

arXiv1998

Hot electron transport in Ballistic Electron Emission Spectroscopy: band structure effects and k-space currents

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.

cond-mat.mtrl-sci↗

arXiv1997

A k-space transport analysis of the BEEM spectroscopy of Au/Si Schottky barriers

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.

cond-mat.mtrl-sci↗

arXiv1997

A Transport Analysis of the BEEM Spectroscopy of Au/Si Schottky Barriers

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.

cond-mat.mtrl-sci↗