The theory of Appel and Kohn is used to calculate $T_c$ for a narrow-band superconductor, where the conduction electrons near the Fermi suface have both $d$ and $f$ character. The vertex equations for the symmetrized pair states formed from $d$ and $f$ orbitals are solved in the contact model. The result is used for a parameter study of $T_c$; one pertinent parameter is the pairing interaction due to phonon exchange between $d$ and $f$ electrons. The experimental situation for La is discussed in terms of the $T_c$ results.

• Received 20 November 1972

DOI:https://doi.org/10.1103/PhysRevB.8.1079