Resonant photoemission studies of the heavy-fermion superconductors ${\mathrm{CeCu}}_2$${\mathrm{Si}}_2$, ${\mathrm{UBe}}_{13}$, and ${\mathrm{UPt}}_3$

Valence-band photoemission studies, using synchrotron light and employing Fano resonances to enhance $f$-derived features, were made of the three known heavy-fermion superconductors: ${\mathrm{CeCu}}_2$ ${\mathrm{Si}}_2$, ${\mathrm{UBe}}_{13}$, and ${\mathrm{UPt}}_3$. The results for ${\mathrm{CeCu}}_2$ ${\mathrm{Si}}_2$ and ${\mathrm{UBe}}_{13}$ contrast markedly with those reported earlier, reflecting closer control of surface contamination in the present study. We infer from the present study and other considerations that in all three systems there is significant hybridization between the $f$ electrons and the nearest-neighbor ligands, which may be essential to the phenomenon of heavy-fermion superconductivity.