Two-particle spectra of heavy-fermion systems

Appearance-potential spectra of heavy-fermion systems are presented and analyzed in the frame of the Gunnarsson-Schönhammer approach for the Anderson-impurity model. This procedure has been used successfully to describe a wide variety of one-particle spectroscopies of 4f systems. Here we present a generalization of the Gunnarsson-Schönhammer approach for two-particle response functions, measured in appearance-potential spectroscopy. Calculations and experiments are presented for ${\mathrm{CeNi}}_5$ and ${\mathrm{CePd}}_3$. It appears that the first term in the 1/N expansion already gives strikingly good agremeent with the experimental data and the Gunnarsson-Schönhammer approach describes two-particle response functions as reliably as those of one-particle spectra.