E. C. Goldberg

Charge-transfer processes in atom-surface collisions at low energies: a Green's function approach

Abstract A Greens functions analysis of the charge transfer probability is performed within the one particle Anderson-Newns model and used to calculate the probabilities of the different charge-states of a scattered or sputtered atom by finite and semi-infinite linear chains. This formalism has the advantage that it allows one to handle simultaneously continuous band states, localized surface states and core-like states of the substrate without a significative increase in the complexity of the numerical calculation. It also allows one to incorporate, at least within a semi-classical level, the effects on the ionization probability of variations in the velocity of the projectile along the trajectory. As expected, these become important at low velocities, where velocity changes largely affect the phase interferences taking place within the double transit of the projectile through the collision region in the scattering process.

Relevant effects of localized atomic interactions and surface density of states on charge transfer in ion-surface collisions

Through a time-dependent quantum-mechanical calculation of the H + scattering by a highly oriented pyrolitic graphite (HOPG) surface, we are able to satisfactorily reproduce the interesting features we observed in ion scattering experiments in H + /HOPG system. We found that the combined effects of the semimetal character of HOPG together with the localized nature of the carbon atom states primarily determine the angular dependence and the magnitude of the ion fractions for large outgoing angles. The spin fluctuation effects (not considered in the present calculation) are discussed as one of the the main causes of the disagreement between the spinless theory results and the experiments for small exit angles.

Quantum-mechanical interference in charge exchange between hydrogen and graphene-like surfaces

The neutral to negative charge fluctuation of a hydrogen atom in front of a graphene surface is calculated by using the Anderson model within an infinite intra atomic Coulomb repulsion approximation. We perform an ab initio calculation of the Anderson hybridization function that allows investigation of the effect of quantum-mechanical interference related to the Berry phase inherent to the graphene band structure. We find that consideration of the interaction of hydrogen on top of many C atoms leads to a marked asymmetry of the imaginary part of the hybridization function with respect to the Fermi level. Consequently, Fano factors larger than one and strongly dependent on the energy around the Fermi level are predicted. Moreover, the suppression of the hybridization for energies above the Fermi level can explain the unexpected large negative ion formation measured in the scattering of protons by graphite-like surfaces.

Spin fluctuation effects on the conductance through a single Pd atom contact

A controversy about the conductance through single atoms still exists. There are many experiments where values lower than the quantum unity G(0) = 2e(2)/h have been found associated to Kondo regimes with high Kondo temperatures. Specifically in the Pd single atom contact, conductance values close to G(0)/2 at room temperature have been reported. In this work we propose a theoretical analysis of a break junction of Pd where the charge fluctuation in the single atom contact is limited to the most probable one: [Formula: see text]. The projected density of states and the characteristics of the electron transport are calculated by using a realistic description of the interacting system. A Kondo regime is found where the conductance values and their dependence on temperature are in good agreement with the experimental trends observed in the conduction of single molecule transistors based on transition metal coordination complexes.

A new proposal to solve the time-dependent infinite-U Anderson model applied to ion–surface scattering processes

Abstract The interaction between delocalized conduction band states and localized states on an atom site is well described by an Anderson Hamiltonian. The time dependence of this Hamiltonian comes from the trajectory dependence of the parameters in processes where there are atoms moving near a surface (scattering and emission processes). Interesting many-body effects are expected due to the large Coulomb repulsion in the atom site. In this work the time-dependent Keldysh Green functions defined in a mixed space of fermions and bosons with the constraint of a total number Q =1, are calculated by using the equation of motion method. The dynamical equations obtained in this form allow to calculate the charge state of the moving atom, and in the stationary limit they reproduce a spectral density of states with properties in an excellent agreement with exact results.

Ion fractions in 6 keV Ne+, Ar+ and Na+ scattering from a GaAs(110) surface

Abstract We used Ion Scattering Spectrometry (ISS) with Time-of-Flight (TOF) analysis to study the neutralization of 6 keV Ne+, Ar+ and Na+ backscattering from a flat and ordered GaAs(110) surface. At low incident angles an important contribution to the total (neutral + ion) backscattering spectra comes from quasi single collisions with As and Ga first layer surface atoms. We observed that the ion fraction in this contribution is strongly dependent on both the incident projectile and the target atom, suggesting that the violent collision plays an important role in the neutralization process. In order to interpret these dependencies we performed a calculation that discriminates the interaction of the projectile with the extended and localized states of the solid.

He+ scattering from Ni, Al, and their alloy. Model calculation of the charge exchange

Abstract The changes in the He+ neutralization probability when scattered from pure Al and Ni targets and from a NiAl alloy are analyzed. The neutralization probability is calculated using a time-dependent quantum formalism which allows incorporation of several electronic bands of localized and extended nature in the description of the target band structure. The parameters of the interaction Hamiltonian are obtained from an ab-initio model based on a pair-bonds superposition. We find that for He+ scattering from Al atoms in NiAl the neutralization is less probable than for He+ scattering from pure Al, while for the scattering from Ni atoms there is no change in the neutralization probability when going from pure Ni to the alloy target. These results are in agreement with the experimental trends which show a matrix-dependent signal only for the scattering from Al. The marked difference in the neutralization behavior of He+ can be explained by taking into account the differences in the electronic structure between pure Al and NiAl alloy, and the presence of the corelike Al-2p state which promotes the charge exchange process at low primary He+ energies.

Ionic Hamiltonians for transition metal atoms: effective exchange coupling and Kondo temperature

An ionic Hamiltonian for describing the interaction between a metal and a d-shell transition metal atom having an orbital singlet state is introduced and its properties analyzed using the Schrieffer-Wolf transformation (exchange coupling) and the poor mans scaling method (Kondo temperature). We find that the effective exchange coupling between the metal and the atom has an antiferromagnetic or a ferromagnetic interaction depending on the kind of atomic fluctuations, either [Formula: see text] or [Formula: see text], associated with the metal-atom coupling. We present a general scheme for all those processes and calculate, for the antiferromagnetic interaction, the corresponding Kondo-temperature.

Localized description of surface energy gap effects in the resonant charge exchange between atoms and surfaces.

The resonant charge exchange between atoms and surfaces is described by considering a localized atomistic view of the solid within the Anderson model. The presence of a surface energy gap is treated within a simplified tight-binding model of the solid, and a proper calculation of the Hamiltonian terms based on a LCAO expansion of the solid eigenstates is performed. It is found that interference terms jointly with a surface projected gap maximum at the Γ point and the Fermi level inside it, lead to hybridization widths negligible around the Fermi level. This result can explain experimental observations related to long-lived adsorbate states and anomalous neutral fractions of low energy ions in alkali/Cu(111) systems.

Scattering of low‐energy He+ from solid surfaces: Ga and Ca

We analize the role played by localized states in the solid in the neutralization (ionization) of He+ (He) projectiles scattered from metallic surfaces, using a time‐dependent quantum formalism and interaction hamiltonian parameters calculated from an ab‐initio pair‐atom interaction model.