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

Abstract

Muon spin relaxation ($\mu$SR) 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, A${}_3$C${}_{60}$ 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 $n_s/m^{\ast }$ at $T\to 0$ exhibits a nearly linear scaling with $T_c$. Analogous to the roton-minimum energy scaling with the lambda transition temperature in superfluid ${}^4$He, clear scaling with $T_c$ was also found for the energy of the magnetic resonance mode in cuprates, (Ba,K)Fe${}_2$As${}_2$, CeCoIn${}_5$ and CeCu${}_2$Si${}_2$, as well as the energy of the superconducting coherence peak observed by angle resolved photo emission (ARPES) in the cuprates near ($\pi$,0). Both the superfluid density and the energy of these pair-non-breaking soft-mode excitations determine the superconducting $T_c$ 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 ${\epsilon }_F$ becomes comparable to the spin fluctuation energy scale $\hslash {\omega }_{SF}\sim J$, as the process which leads to pair formation in these correlated electron superconductors.

plenary talk presented at SCES2008 Conference, Buzios, Brazil, August 2008; to be published in the proceedings, Physica B, in press

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