S. Ugenti

Ab initio calculation of core-valence-valence Auger spectra in closed shell systems

We propose an ab initio method to evaluate the core-valence-valence (CVV) Auger spectrum of systems with filled valence bands. The method is based on the Cini-Sawatzky theory, and aims at estimating the parameters by first-principles calculations in the framework of density-functional theory (DFT). Photoemission energies and the interaction energy for the two holes in the final state are evaluated by performing DFT simulations for the system with varied population of electronic levels. Transition matrix elements are taken from atomic results. The approach takes into account the non-sphericity of the density of states of the emitting atom, spin-orbit interaction in core and valence, and non quadratic terms in the total energy expansion with respect to fractional occupation numbers. It is tested on two benchmark systems, Zn and Cu metals, leading in both cases to L23M45M45 Auger peaks within 2 eV from the experimental ones. Detailed analysis is presented on the relative weight of the various contributions considered in our method, providing the basis for future development. Especially problematic is the evaluation of the hole-hole interaction for systems with broad valence bands: our method underestimates its value in Cu, while we obtain excellent results for this quantity in Zn.

Electronic correlations in graphite and carbon nanotubes from Auger spectroscopy

We have determined the screened on-site Coulomb repulsion in graphite and single wall carbon nanotubes by measuring their Auger spectra and performing a new theoretical analysis based on an extended Cini-Sawatzky approach where only one fit parameter is employed. The experimental lineshape is very well reproduced by the theory and this allows to determine the value of the screened on-site repulsion between

Ab initio calculation of CVV Auger spectra for closed-shell systems

2p

Experimental and theoretical study of electronic correlations in carbon nanotubes and graphite from auger spectroscopy

states, which is found to be 2.1 eV in graphite and 4.6 eV in nanotubes. The latter is robust by varying the nanotube radius from 1 to 2 nm.

On Coster–Kronig line shapes of solids

We propose an ab initio method to evaluate the core-valence-valence (CVV) Auger spectrum of systems with filled valence bands. The method is based on the Cini-Sawatzky theory, and aims at estimating the parameters by first-principles calculations in the framework of density-functional theory (DFT). Photoemission energies and the interaction energy for the two holes in the final state are evaluated by performing DFT simulations for the system with varied population of electronic levels. Transition matrix elements are taken from atomic results. The approach takes into account the non-sphericity of the density of states of the emitting atom, spin-orbit interaction in core and valence, and non quadratic terms in the total energy expansion with respect to fractional occupation numbers. It is tested on two benchmark systems, Zn and Cu metals, leading in both cases to L23M45M45 Auger peaks within 2 eV from the experimental ones. Detailed analysis is presented on the relative weight of the various contributions considered in our method, providing the basis for future development. Especially problematic is the evaluation of the hole-hole interaction for systems with broad valence bands: our method underestimates its value in Cu, while we obtain excellent results for this quantity in Zn.

Spin selectivity by Auger-photoelectron coincidence spectroscopy

We give a quantitative estimate of the screened on-site Coulomb repulsion U in carbon nanotubes and graphite. This is obtained by measuring the Auger spectrum of these materials and performing a theoretical study based on the Greens function method proposed by Cini[Solid. St. Comm. 24, 681 (1977)]. The experimental lineshape is very well reproduced by the theory, where only one fit parameter is employed. This allows to extract the value of the screened on-site repulsion between 2p states, which results U = 4.6 eV for nanotubes and U = 2.1 eV for graphite.

Spin-Dependent On-Site Electron Correlations and Localization in Itinerant Ferromagnets

In this work we set up a model aimed at the calculation of three-hole features like the ones due to core–valence–valence Auger decays following Coster–Kronig transitions. While several experiments made in the 1970s and in the 1990s on the Auger core–valence–valence spectra of transition metals showed the existence of these structures, a theory able to explain and predict them is still missing today. Our model is grounded on the one-step approach, but the use of a valence band fully below the Fermi level allows us to treat our calculations in a three-step approach, so keeping complications to a minimum in this exploratory work. The Hamiltonian of the system is placed in an Anderson-like picture and the spectra are computed by evaluating a three-body Green function. Within our model we arrive at a simple and closed formula covering the whole range between weak and strong correlations. We find that, in general, the satellites cover separated spectral regions with three-hole multiplets, shifted and broadened two-hole features and distorted band-like continua.

Particle-particle response function as a probe for electronic correlations in the p-d Hubbard model

The M3M4,5M4,5 Auger transition from a Cu(111) surface is studied using Angular Resolved Auger-PhotoElectron Coincidence Spectroscopy (AR-APECS). In the experiment two different geometrical configurations of the electron analyzers allow us to sample different emission angles of the ejected electrons leading to different weights of the singlet and triplet contributions in the studied transition. The experimental spectra are modeled within a two-step approach using the Cini theory for the closed band case so as to properly consider the spin-orbit interaction and the hole-hole correlation energy. Ingredients for the theory, like density of states, are obtained fully ab-initio in the framework of density functional theory by performing all-electron calculations. The obtained results confirm the recently discovered selectivity of AR-APECS in the final spin-state.