BIS VERSUS OTHER SPECTROSCOPIES IN NARROW BAND SOLIDS

Bremsstrahlung isochromat spectroscopy (BIS) is the most direct method allowing us to probe the unoccupied states in solids. When applied to broad bands, the BIS spectra are commonly interpreted within a single particle framework. However, when the correlation of the outermost states becomes so strong that the Koopmans' approximation looses gradually its validity the final states can no longer be interpreted in terms of initial density of states. The situation is particularly interesting in the Actinides and Lanthanides wheres the f-states can be very weakly hybridized with the band states. The many-body nature of the hybridized f-states is reflected by the duality of their excitations : at low energy they form a spectrum showing similarities with an extremely narrow metallic band whereas at high energy they have a localized character reminiscent of purely atomic final states. The equivalence between photoemission and BIS is demonstrated by the mirror property between corresponding electron and hole configurations. However, threshold spectroscopies like X-ray absorption or electron energy loss applied to correlated systems are markedly different from BIS since the deep hole modifies locally the density of final states above EF