Energy-scale phenomenology and pairing via resonant spin–charge motion in FeAs, CuO, heavy-fermion and other exotic superconductors

Muon spin relaxation (μSRμSR) studies of the “1111” and “122” FeAs systems have detected static magnetism with variably sized ordered moments in their parent compounds. The phase diagrams of FeAs, CuO, organic BEDT, A3C60A3C60 and heavy-fermion systems indicate competition between static magnetism and superconductivity, associated with first-order phase transitions at quantum phase boundaries. In both FeAs and CuO systems, the superfluid density ns/m*ns/m* at T→0T→0 exhibits a nearly linear scaling with TcTc. Analogous to the roton-minimum energy scaling with the lambda transition temperature in superfluid 4He, clear scaling with TcTc was also found for the energy of the magnetic resonance mode in cuprates, (Ba,K)Fe2As2(Ba,K)Fe2As2, CeCoIn5CeCoIn5 and CeCu2Si2CeCu2Si2, as well as the energy of the superconducting coherence peak observed by angle resolved photo emission (ARPES) in the cuprates near (π,0π,0). Both the superfluid density and the energy of these pair-non-breaking soft-mode excitations determine the superconducting TcTc via phase fluctuations of condensed bosons. Combining these observations and common dispersion relations of spin and charge collective excitations in the cuprates, we propose a resonant spin–charge motion/coupling, “traffic-light resonance,” expected when the charge energy scale εFεF becomes comparable to the spin fluctuation energy scale ℏωSF~JℏωSF~J, as the process which leads to pair formation in these correlated electron superconductors.