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CVV Auger-electron (AES) and appearance-potential spectroscopy (APS) are known to yield information on the occupied and the unoccupied part of the local valence density of states (LDOS). AES and APS can be classified as two-particle spectroscopies because of the two additional valence holes or electrons in the final state, respectively. For a system of effectively non-interacting electrons, the two-particle spectrum is simply given by a self-convolution of the one-particle (inverse) photoemission spectrum. For a strongly correlated system, on the other hand, AES and APS provide useful additional information because of the direct interaction between the two final-state holes (electrons) which may give rise to strong correlation
satellites.
This contribution demonstrates that even for a system with low concentration of 3d holes, such as Ni, there are significant correlation effects in the AP line shape. The spin-resolved Ni AP spectrum has been measured for different temperatures up to the Curie point. A quantitative understanding can be achieved within a theoretical approach which (i) starts from the realistic one-particle electronic structure as obtained from an ab-initio (LDA) calculation, which (ii) accounts for the temperature dependence of the LDOS as obtained within a second-order perturbational approach to a (non-orthogonal) Hubbard-type model including 3d, 4s, and 4p orbitals, which (iii) additionally incorporates the direct correlations between the additional final-state electrons (within a ladder approximation), and (iv) which includes the orbitally resolved transition-matrix elements. Each of these points is shown to be necessary for a reasonable understanding of the line shape. |