Universal relation between magnetic resonance and superconducting gap in unconventional superconductors

Unconventional superconductors such as the high-transition temperature (Tc) cuprates, heavy-fermion systems and iron arsenide-based compounds exhibit antiferromagnetic fluctuations that are dominated by a resonance, a collective spinone excitation mode in the superconducting state. Here we demonstrate the existence of a universal linear relation, Er ∝ 2Δ , between the magnetic resonance energy (Er) and the superconducting pairing gap (Δ), spanning two orders of magnitude in energy. This relation is valid for materials that range from being close to the Mott-insulating limit to being on the border of itinerant magnetism. Since the common excitonic picture of the resonance has not led to such universality, our observation suggests a much deeper connection between antiferromagnetic fluctuations and unconventional superconductivity.