SCREENING AND CORRELATION-EFFECTS IN CVV AUGER-SPECTRA OF TRANSITION-METALS

We develop a theory of the core-valence-valence Auger spectra from 3d transition metals including core-hole screeing effects. First, we analyse the limiting case when the d electrons fully screen the core hole in the initial state. We employ the variational method including singly excited states to construct the ground state with a completely screened core hole and solve the resulting many-body problem for correlated valence electrons using the self-consistent version of the perturbation theory in powers of U/w (where U is Hubbard on-site interaction and w is the bandwidth) within the T-matrix approximation. The resulting lineshape has a contribution proportional to the one-particle density of valence states and, in addition, also the two-hole-one-electron contributions, but not the two-hole term that had been suggested in the literature. The latter arises only for filled bands. The comparison with experimental data is qualitatively good for early 3d metals, giving a natural explanation of the so-called 'negative-U' behaviour, but deteriorates with increasing atomic number. Next, we present a simple approximation to describe the effect of incomplete relaxation in the initial state. Contributions from the unrelaxed initial state are proportional to a two-hole density of states convolved with a suitable core lineshape. This removes the difficulties with the late transition metals. Our results indicate that the core-hole screening becomes progressively less complete towards the end of the 3d series. The theory then reduces to the standard one, based on two-hole propagators in the low-density approximation.