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ABSTRACT: 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. Comment: 24 pages, 6 figures
12/2010;
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ABSTRACT: We present a theoretical approach to determine the electronic properties of nanoscale systems exhibiting strong electron-electron and electron-phonon interactions and coupled to metallic electrodes. This approach is based on an interpolative ansatz for the electronic self-energy which becomes exact both in the limit of weak and strong coupling to the electrodes. The method provides a generalization of previous interpolative schemes which have been applied to the purely electronic case extensively. As a test case we consider the single level Anderson-Holstein model. The results obtained with the interpolative ansatz are in good agreement with existing data from Numerical Renormalization Group calculations. We also check our results by considering the case of the electrodes represented by a few discrete levels which can be diagonalized exactly. The approximation describes properly the transition from the Kondo regime where electron-electron interactions dominate to the polaronic case characterized by a strong electron-phonon interaction. Comment: 8 pages, 7 figures, accepted for publication in Physical Review B
Physical Review B 11/2008; · 3.69 Impact Factor
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ABSTRACT: A generalized theory of Auger electron transfer processes in the interaction of ions with metal surfaces, including the previously ignored role of d electrons is presented. It is shown that a correct and accurate description of Auger neutralization has to account for the contribution of d electrons, as this is illustrated on the case of He+ ion neutralization on Ag, where the neglect of these leads to a strong overestimation of ion survival probabilities. Crystal lattice site specific rates are calculated and allow for a correct description of crystal azimuthal effects in neutralization.
Physical Review Letters 08/2006; 97(4):047601. · 7.37 Impact Factor
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ABSTRACT: 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. Comment: 5 figures
07/2005;
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ABSTRACT: We develop a theory of the Auger neutralization rate of ions on solid surfaces in which the matrix elements for the transition are calculated by means of a linear combination of atomic orbitals technique. We apply the theory to the calculation of the Auger rate of He+ on unreconstructed Al(111), (100), and (110) surfaces, assuming He+ to approach these surfaces on high symmetry positions and compare them with the results of the jellium model. Although there are substantial differences between the Auger rates calculated with both kinds of approaches, those differences tend to compensate when evaluating the integral along the ion trajectory and, consequently, are of minor influence in some physical magnitudes like the ion survival probability for perpendicular energies larger than 100 eV. We find that many atoms contribute to the Auger process and small effects of lateral corrugation are registered.
Phys. Rev. B. 06/2005; 71(24).
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ABSTRACT: This work is mainly devoted to the description of processes that involve the interaction between an atom and a surface, in which a strong Coulomb repulsion on the atomic site (U) limits the charge exchange to one electron (infinite-U limit). In this limit, the Anderson Hamiltonian for a many-fold (N) of states localized on the atomic site can be represented in terms of auxiliary bosons and physical operators in the mixed boson-electron space can be defined. In this work the Hamiltonian is solved by defining appropriate Green’s functions for physical operators. Then we solved the equations of motion of these Green’s functions, up to a second order in the atom-surface coupling, either for the stationary case or for a real time-dependent problem. We show that our approach reproduces the known exact results in the nondegenerate (N=1) case, and for N≻1 gives excellent agreement with exact calculations and approximations valid for large N (the 1∕N expansion). Finally, the accurate description of dynamical processes is shown by the comparison with the exact results available for a small four-level system. In this case we also compare with results obtained by using the noncrossing approximation and with the usual spinless model calculation.
Phys. Rev. B. 01/2005; 71(3).
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ABSTRACT: Neutralization of He+ ions in grazing incidence scattering on Ag(111) and Ag(110) surfaces is studied. These measurements reveal the existence of an order of magnitude difference in the probability of ion survival on Ag(110) and Ag(111). The effect of changing the direction of ion scattering along the surface azimuthal orientation is investigated. The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulation of scattered ion trajectories are performed and the surviving ion fractions are then calculated using the theoretical Auger neutralization rates, without adjustable parameters. The calculations agree quite well with the experimental data and show that the observed differences in the neutralization probabilities on these surfaces are related to different extensions of the electron density beyond the surface, resulting from different atomic packing. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 07/2004; 241(10):2367 - 2373. · 1.32 Impact Factor
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ABSTRACT: Neutralization of He+ ions in grazing incidence scattering on Ag(111) and Ag(110) surfaces is studied. These measurements reveal the existence of an order of magnitude difference in the probability of ion survival on Ag(110) and Ag(111). The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulation of scattered ion trajectories is performed and the surviving ion fractions are then calculated using the theoretical Auger neutralization rates, without adjustable parameters. The calculations agree quite well with the experimental data and show that the observed differences in the neutralization probabilities on these surfaces are related to different extensions of the electron density beyond the surface, resulting from different atomic packing.
Physical Review Letters 02/2004; 92(1):017601. · 7.37 Impact Factor
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ABSTRACT: Neutralization of He+ ions in grazing incidence scattering on Ag(111) is studied. A small scattered ion fraction is observed. The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulations of scattered ion trajectories are performed and the surviving ion fractions are then calculated using the theoretically estimated Auger neutralization rates. The calculations agree quite well with the experimental data and empirical estimates of the neutralization rates.
Journal of Physics Condensed Matter 02/2003; 15(8):1165. · 2.55 Impact Factor
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ABSTRACT: Charge exchange processes in low energy ion scattering are analyzed and some case study systems (H and He scattered off metals) are discussed in detail. For the He+/Al system, we show that Hagstrum's neutralization model holds if the ion perpendicular energy is smaller than 100 eV. For larger energies, He+ penetrates the metal surface layer and resonant processes become operative; then charge exchange processes have to be analyzed combining the Auger and resonant mechanisms. For the H/Al system, charge exchange can be understood considering only the resonant mechanism. The ion levels and the ion–metal interactions necessary for calculating the charge exchange processes in the He/Al and H/Al systems, are also presented in this chapter.
Advances in Quantum Chemistry.
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256:6-10.
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ABSTRACT: In this article we investigate the effect that hybridization between the 5s- and 4d-orbitals of Ag has in the calculations of Auger neutralization rate and ion survival probability for grazing collisions of He+ with Ag(110) surfaces. We find that it is important to include hybridization to get good agreement with experimental results specially at high incident energies, when the ions get closer to the surface. (c) 2006 Elsevier B.V. All rights reserved.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 256(1):6-10. · 1.21 Impact Factor
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ABSTRACT: We present results of a study of He+ neutralisation on clean and oxygen covered Al(1 1 1). For clean Al(1 1 1) we find a small ion fraction which rapidly increases with increasing ion energy attaining about (0.4 +/- 0.1)% at 4 keV. The overall behaviour is quite different from that observed on previously studied Ag surfaces and is attributed to reionisation processes, a conclusion supported by measurements using He atoms. When O-2 is adsorbed onto Al(1 1 1) at room temperature, we observe a rapid increase in the ion fraction, which attains a broad plateau for exposures higher than about 50 L. Experiments with incident He atoms lead to an ion fraction that is smaller, indicating that in this case a substantial fraction of ions is due to survival of incident ions. We attribute these changes to a decrease of the Auger neutralisation rates on the oxidic surface. (c) 2005 Published by Elsevier B.V.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 232:27-31. · 1.21 Impact Factor
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ABSTRACT: We present a detailed theoretical analysis of the role played by s and d electrons in Auger neutralization processes of He+ at Ag(111) and Ag(110) surfaces. We calculate crystal-lattice-site Auger neutralization rates as a function of the perpendicular distance between ions and surfaces. We find that the rate is very insensitive to the lateral position for large values of the perpendicular distance because the contribution of the delocalized s electrons dominates in this case. In contrast, the contribution of d electrons dominates at short perpendicular distances and the strong spatial localization of these electrons causes a similar strong dependence of the Auger rate with lateral position. We perform molecular dynamic simulations of scattered ion trajectories, which, used together with the Auger neutralization rates, allow us to obtain the theoretical ion fraction that we compare with our measurements. This parameter-free theory is able to reproduce the magnitude of the ion survival probability and its dependence with the azimuthal angle of incidence for both surfaces of Ag, thus showing the important role played by localized electrons in Auger neutralization of He.
Physical Review B. 75(16).
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232:27-31.
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75.
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ABSTRACT: In this work we present an equation of motion approach to analyze the electronic properties of a small system coupled to metallic electrodes in which electron-phonon interaction is present. As a simple model, we consider the single-level Anderson-Holstein Hamiltonian. Two versions of this Hamiltonian, namely, spinless electrons and interacting electrons in the infinite-U limit, are studied in equilibrium and at zero temperature. Differently from previous approaches to the same problem using this method, we do not decouple electronic and vibronic degrees of freedom and keep quantum coherence between the electron at the localized level and the phonons. Also, we improve the method by including corrections that are important for a better description of Kondo physics. We focus in the analysis of the evolution of the resonances with the electron-phonon coupling parameter λ. We find that charge and spin resonances experience very different reduction in their widths, owing to their different physical origins, with the spin resonance being more robust than the charge one to increasing values of λ. This is at variance from the results of simple polaronic approaches to this problem, which treat both kinds of resonances on a similar footing. We expect that our approach is more appropriate to investigate the region of energies near the Fermi level in cases when the Kondo effect is strong.
Phys. Rev. B. 79(11).
