Superconductivity-a source of surprises

Superconductivity is ascribed to a tendency of pairwise localization of conduction electrons. As a necessary condition in k space bands with very small as well as large dispersion need to be present at the Fermi level (“flat band/steep band”). The sufficient condition is a strong enough coupling of the flat band states to specific phonons in case of phonon-mediated superconductivity. The electron-phonon coupling parameter λ reveals a peak structure in the phonon q-space. These aspects are discussed for actual examples, e.g., rare earth carbides and carbide halides as well as MgB2.