| In the BCS theory the glue for Cooper pairing is provided by phonons. But there is no reason why other bosonic low-energy excitations could not replace phonons. It has been known for a long time that in particular crystalline electric field (CEF) excitations of the incomplete 4f or 5f shell can lead to Cooper pair formation. There is by now strong theoretical and experimental evidence that in UPd2Al3 superconductivity, i.e., Cooper-pair formation is due to such excitations. Also in some of the filled skutterudites these excitations play together with phonons an important role. |
|