Unconventional superconductors—those that the Bardeen-Cooper-Schrieffer theory can't explain—typically have a single superconducting phase. That’s surprising because their conduction electrons have various theorized ways to couple up, whether through different mediators or in different spin states of the Cooper pairs. Conceivably, they could transition between different sorts of superconducting orders. But so far, only uranium ditelluride and a few other materials have shown such transitions.
Most of those compounds with multiple superconducting phases are heavy-fermion materials. Like other heavy-fermion materials, their strongly correlated electronic behavior arises from the partially filled 4f and 5f orbitals of their rare-earth or actinide ions. Electrons in those orbitals hybridize with conduction electrons to produce quasiparticles of large effective mass—anywhere from 50 to 1000 electron masses. The quasiparticles are too heavy to interact much with the crystal lattice and its phonons, so they can’t form Cooper pairs via the phonon-mediated mechanism of conventional superconductors....
