Spin excitons in the unconventional superconducting and hidden order state of strongly correlated electrons

The formation of collective spin excitons below the single particle continuum is observed in numerous unconventional superconductors. CeCoIn5 is the most well established case for heavy fermion compounds. It is also the first example where the splitting of magnetic excitons by a magnetic field is observed for fields within the tetragonal plane. Contrary to expectations it is revealed as a doublet excitation. We explain the splitting as the result of a strongly anisotropic spin response described within the context of an Anderson lattice type model. Recently it was shown that collective spin excitations also appear within the hidden order phase of non- superconducting CeB6. It is a signature of the itinerant nature of spin response as opposed to the commonly used localized 4f approach in this compound. We show that the salient features of the spin exciton can be explained in an itinerant quasiparticle model supplemented by hidden and antiferromagnetic order.