Transition temperature of d-f-band superconductors

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.