L. Duo

Off-Site Contributions to Electron Correlation; An Extension to the Hubbard Model Studied by Auger Spectroscopy

A new theory of the lineshape of CVV Auger processes has been developed which extends the Hubbard Hamiltonian to include off-site correlation effects. The new theory shows that the neglect of off-site correlations explains the failure of the standard Cini-Sawatzky model of CVV processes to yield simultaneous agreement with the experimental kinetic energy and profile of the N6,7O4,5O4,5 Auger spectra of Au which correspond to the U > W, quasi-atomic, case. The theory predicts that the contribution from off-site correlations will be less important in the U < W, bandlike region, and this is shown to be consistent with the results of high-resolution measurements of the KVV spectrum of Si.

Experimental and theoretical study of the PL2,3VV Auger lineshape of GaP(110)

Abstract A joint experimental and theoretical investigation of the PL 2.3 VV Auger transition on GaP(110) is presented. The theoretical data, calculated within a pure band-like picture, are compared to the experimental spectra. The angular momentum character of the five features constituting the lineshape is identified. Strong contributions to the experimental data from many-body effects may be ruled out due to the good overall agreement with the theoretical lineshape therefore supporting a single particle description for this Auger feature. We discuss effects such as Coulomb correlation, initial state core hole screening, self-energy corrections, that may possibly contribute to the relative energy shifts observed in one of the pp components of the compared spectra.

The Dielectric Constant of Pd-Ni Alloys

In the last few years a lot of work has been devoted to the deduction of the dielectric constant of materials exploiting the electron energy loss spectroscopy in the reflection mode (REELS). The transition metals palladium, nickel and their alloys represent a particular class of solids in which bulk plasmons and interband transitions are strongly superimposed to their surface counterparts, generating very complex energy loss spectra. In order to deduce information on the dielectric constant e of such materials, an adequate spectral deconvolution is required in order to obtain their pure bulk contribution which is proportional to the optical loss function Im(−1/e). The main assumption made in deducing the dielectric constant by REELS measurements is that the momentum transfer q of incoming electron is negligible; this means to work in the optical limit [e(ω,q) → e(ω, 0)]. This condition may be accomplished by using suitable primary energies of about some keV. In this frame, a procedure is presented in this work which exploits the simultaneous analysis of two REEL spectra taken at different primary energies (0.7 and 2.0 keV) and/or incidence angles (40 and 80 degrees). The sensitivity of surface effects to these two last parameters makes our deconvolution technique a good tool for the removal of their corresponding contribution to the spectra. The analysis of three compositions of PdxNi1-x system (x = 0.2, 0.4 and 0.8) has shown no particular trend in the energetic positions of the observed structure as a function of the composition, whilst an increase of the low energy loss function intensity is found for increasing Pd content. The dielectric constants for the three compositions are also deduced by a conventional Kramers-Kronig analysis and compared with those of pure Pd and Ni as reported in literature.

The virtual bound states of Fe in AuFe studied by photoemission

By using synchrotron radiation photoemission with photon energies near to the Cooper minimum of Au the authors have detected virtual bound states (VBS) in Au90Fe10. A VBS peaking 0.7 eV below the Fermi energy is attributed to Fe minority states. Less intense features are observed at about 2.3 eV, 4-5.5 eV and 7.3 eV below the Fermi energy, close to the edges of the Au 5d bands, which they suggest are due to the Fe majority states broadened by strong hybridization with the Au valence band. These observations agree with the Friedel-Anderson model which predicts nondegenerate virtual levels with the majority spin population completely occupied and with the minority states spanning the Fermi level.

SATELLITE STRUCTURES OF THE NICKEL L3 VV AUGER SPECTRUM IN NICKEL SILICIDES - A SYNCHROTRON RADIATION INVESTIGATION

We report the first synchrotron radiation investigation of the satellite structures of the Ni L3M4, 5M4, 5 Auger spectrum in the Ni compounds Ni2Si and NiSi2. The line shape of the satellites is extracted for both compounds for a set of increasing ionizing energies; the procedure used to generate the difference spectra is described. A comparision of the two sets of satellites with each other and with results reported in the literature for the case of pure Ni allows us to identify the changes in the line shape due to the different chemical environment. Possible mechanisms leading to the formation of these structures are discussed.

P L2, 3 VV AUGER-SPECTROSCOPY AT THE GAP(110)/YB INTERFACE - ANALYSIS OF THE CHEMICAL-BOND

We report a P L2, 3 VV Auger line-shape study of the early stages of the GaP(110)/Yb interface formation. Strong similarities with the evolution displayed by the Si L2, 3 VV Auger profiles measured from the Si(111)/Yb interface and nearly-d0 metal silicides (Ca-silicides, Gd-silicides) is pointed out and exploited to determine the character of the P 3p-derived valence states in the Yb phosphide-like reaction products. A comparison, at different Yb coverages, with the self-folded partial density of states calculated for stoichiometric YbP is moreover performed. On this basis, the chemical bond is mainly described in terms of the formation of P 3p-Yb 5d hybrids.

UNOCCUPIED 3D-DERIVED STATES IN NI SILICIDES VIA NI L2,3 X-RAY ABSORPTION-SPECTROSCOPY

Abstract We report Ni L2.3 X-ray absorption spectra as measured from bulk ingots of Ni2Si and NiSi2. A similar amount of unoccupied 3d character if found in the electronic structure of both silicides regardless of the differences occuring in the coordination local to the Ni sites. This is discussed in terms of the results of calculations of the partial density of states of Ni silicides and contrasted with recently reported ultraviolet inverse photoemission data which suggest complete depletion of the unoccupied Ni 3d-related states upon moving from Ni2Si to NiSi2.