Superconductivity in metals with incomplete inner shells

Through screening by outer (s) electrons the plasma oscillations of inner (d) bands are transformed into an acoustic phonon branch if certain conditions are fulfilled. Similar to the lattice-phonon branch this provides an interaction with free electrons which leads to superconductivity. Dependence of the transition temperature T c on the number of d electrons follows Matthias' rule. Suggestions for increasing T c are discussed.