Initial- versus final-state effects in the narrow-band spectra of heavy-fermion systems

The f-electron spectral density of heavy-fermion systems, as observed by x-ray photoelectron spectroscopy (XPS) and bremsstrahlung isochromat spectroscopy (BIS), is discussed in terms of initial- and final-state effects. The properties of the initial state, described by the periodic Anderson model, are obtained from the adiabatic perturbation theory, with the on-site correlation ${\mathit{U}}_{\mathit{f}\mathit{f}}$ taken as the expansion parameter. In the final state the transient effects following the sudden ionization of the system in XPS or BIS are discussed by using the time-dependent perturbation expansion. The overall spectral shapes, as would be observed experimentally, turn out to be given as a convolution of the adiabatic spectral function, which reflects the many-body interactions in the initial state, and the shape function, which accounts for the nonadiabatic effects following the destruction of the charge neutrality in XPS and BIS. Our treatment allows us to explain the position and the shape of the f-derived peaks in the XPS spectra of actinide intermetallics and to remove the conceptual difficulty in understanding the data acquired independently from thermodynamic (low-energy) and spectroscopic (high-energy) measurements of heavy fermions within a unified framework.