R. Monreal

Self-consistent LDA calculation in ion neutralization at metal surfaces

Abstract The neutralization of He + scattered off aluminum is calculated via a self-consistent LDA where the metal surface is modeled by an LDA jellium surface, and its structure factor is consistently calculated. This approach includes Auger and plasmon-assisted neutralization channels of He + to the He ground state in front of aluminum. We analyze these neutralization channels, which leads us to a revision of the usual calculations of ion neutralization on surfaces depending on the transferred energy lying below, near, or above the metal plasma frequency. The results of this calculation are compared with those of other methods, namely usual unscreened calculations, calculations which extrapolate bulk results, calculations performed for a step potential surface, and surface calculations in the long-distance limit.

New model for ion neutralization at surfaces

We investigate the neutralization of low energy He+ ions in close collisions with metal surface atoms. In order to describe the neutralization process as completely as possible, we consider Auger neutralization (AN), resonant neutralization (RN) and resonant ionization (RI). Our calculation agrees well with experimental data and shows that in some metals (like Pd) AN is the dominant process, whereas in others (like Al) RN and RI contribute significantly for energies above the threshold for reionization.

Electron emission spectra in Auger processes near metal surfaces

Abstract In this article, the theory for the Auger transition rate of slow ions interacting with free-electron like metal surfaces reported previously is improved and applied to the calculation of the spectrum of the electrons emitted in the Auger neutralization process. The theory shows that the derivative of the spectrum reflects the probability that metal excitations of a given energy are produced in the process. This probability depends crucially on the distance between ion and surface. It is shown that for large and intermediate distances the energy transferred to the metal in the neutralization process is predominantly used to excite surface plasmons bulk plasmons are seen in the spectrum only for short distances. Using a simple model for the metal surface, the derivative of the electron emission spectrum is calculated for three model ions simulating He + , Ne + and Ar + scattered off Al. The calculations can reproduce the experimental result for Ne + and the case of Ar + calls for a better description of the metal surface.

Transient dynamics and waiting time distribution of molecular junctions in the polaronic regime

We develop a theoretical approach to study the transient dynamics and the time-dependent statistics for the Anderson-Holstein model in the regime of strong electron-phonon coupling. For this purpose we adapt a recently introduced diagrammatic approach to the time domain. The generating function for the time-dependent charge transfer probabilities is evaluated numerically by discretizing the Keldysh contour. The method allows us to analyze the system evolution to the steady state after a sudden connection of the dot to the leads, starting from different initial conditions. Simple analytical results are obtained in the regime of very short times. We study in particular the apparent bistable behavior occurring for strong electron-phonon coupling, small bias voltages and a detuned dot level. The results obtained are in remarkable good agreement with numerically exact results obtained by Quantum Monte Carlo methods. We analyze the waiting time distribution and charge transfer probabilities, showing that only a single electron transfer is responsible for the rich structure found in the short times regime. A universal scaling (independent of the model parameters) is found for the relative amplitude of the higher order current cumulants in the short times regime, starting from an initially empty dot. We finally analyze the convergence to the steady state of the differential conductance and of the differential Fano factor at the inelastic threshold, which exhibits a peculiar oscillatory behavior.

Long transient dynamics in the Anderson-Holstein model out of equilibrium

We calculate the time-dependent nonequilibrium current through a single-level quantum dot strongly coupled to a vibrational mode. The nonequilibrium real-time dynamics caused by an instantaneous coupling of the leads to the quantum dot is discussed using an approximate method. The approach, which is specially designed for the strong polaronic regime, is based on the so-called polaron tunneling approximation. Considering different initial dot occupations, we show that a common steady state is reached after times much larger than the typical electron tunneling times due to a polaron blocking effect in the dot charge. A direct comparison is made with numerically exact data, showing good agreement for the time scales accessible by the diagrammatic Monte Carlo simulation method.

Neutralization of slow He2+ on metal surfaces : theory for Auger and cascade electron emission

Abstract Electron emission spectra in the neutralization of slow He 2+ on ajellium surface are calculated. We calculate the Auger transition rates and spectra of He 2+ to He + and He + to He 0 as well as the resonant neutralization rate of He 2+ to He + ∗ . The spectra obtained in this way resemble the surface density of state (SDOS) convolutions at the two different ranges of energy of the two transitions. We also include another mechanism for electron emission: this is the electron cascade process induced in the metal by those Auger electrons that are not able to surmount the surface barrier. We then find a qualitative agreement with experiment. This suggests that information from the SDOS in the spectra is smeared out by electron cascades.

Crystallographic effects in charge exchange processes: He scattering from Ni(110)

Surface channeling experiments with 3 and 5 keV He show intensity maxima in the scattered particle yield for ions and neutrals in major crystallographic directions. The charge fraction He+/He0+He+) exhibits minima in these directions. This effect is related to differences of the trajectories and to different contributions of Auger neutralization and dynamic resonant loss processes. The time constants for the two processes are different, such that the charge fraction changes with the trajectory lengths mainly.

Dressed tunneling approximation for electronic transport through molecular transistors

A theoretical approach for the non-equilibrium transport properties of nanoscale systems coupled to metallic electrodes with strong electron-phonon interactions is presented. It consists in a resummation of the dominant Feynman diagrams from the perturbative expansion in the coupling to the leads. We show that this scheme eliminates the main pathologies found in previous simple analytical approaches for the polaronic regime. The results for the spectral and transport properties are compared with those from several other approaches for a wide range of parameters. The method can be formulated in a simple way to obtain the full counting statistics. Results for the shot and thermal noise are presented.

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 Greens 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 steplike features in the conductance. Excellent overall agreement between both approaches is found for a wide range of parameters.

Channeling effects in He scattering from Ni(110)

The scattering at grazing incidence of He+ ions from a Ni(110) surface has been meausured showing a strong ion yield dependence on the surface orientation. This dependence is explained in terms of the differences in trajectory lengths seen by the ions. We also find that in order to explain the experimentally observed yield it is necessary to take into account the Auger process of capture for He+ and the resonant process of loss for He0. Theoretical calculations of both Auger lifetimes together with the trajectory lengths obtained from Marlowe calculations are in good agreement with the experiment.