Abstract
The superconducting feedback resonance in inelastic neutron scattering (INS) has now been found in numerous unconventional superconductors of the cuprate, ferropnictide, and heavy fermion classes. The collective spin excitation appears below Tc at an energy less than the quasiparticle threshold with momentum Q provided the gap changes sign under translation by Q. The resonance has been found in the heavy fermion (HF) superconductors CeCu2Si2, CeCoIn5, and UPd2Al3, and recently in Fe-pnictide Ba1−xKxFe2As2, BaFe2−xCoxAs2, BaFe2−xNixAs2, and FeSe1−xTex compounds and may be a more general phenomenon. Of particular interest is the interaction of the 3d spin exciton with the 4f crystalline electric field (CEF) excitations in rare earth based unconventional superconductors like CeFeAsO1−xFx pnictide and Nd2−xCexCuO4 cuprate where a coupling between 3d spin resonance and 4f CEF excitations leads to intriguing interaction effects observed experimentally by INS.
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