Crossover from weak to strong pairing in unconventional superconductors

TL;DR

This paper compares the pairing mechanisms and coupling strengths in various superconductors, revealing a continuum from weak to strong coupling and suggesting a common unconventional pairing mechanism in Fe-based and cuprate superconductors.

Contribution

It provides a comprehensive analysis of gap ratios across different superconductors, challenging the universality in Fe-based materials and uncovering a universal correlation between gap ratio and Tc.

Findings

Pnictides range from weak to strong coupling, bridging cuprates and simple metals.

A universal correlation between gap ratio and Tc in Fe- and Cu-based superconductors.

Resonance damping in ferropnictides depends on Tc, linking energy separation to superconducting properties.

Abstract

Superconductors are classified by their pairing mechanism and the coupling strength, measured as the ratio of the energy gap to the critical temperature, Tc. We present an extensive comparison of the gap ratios among many single- and multiband superconductors from simple metals to high-Tc cuprates and iron pnictides. Contrary to the recently suggested universality of this ratio in Fe-based superconductors, we find that the coupling in pnictides ranges from weak, near the BCS limit, to strong, as in cuprates, bridging the gap between these two extremes. Moreover, for Fe- and Cu-based materials, our analysis reveals a universal correlation between the gap ratio and Tc, which is not found in conventional superconductors and therefore supports a common unconventional pairing mechanism in both families. An important consequence of this result for ferropnictides is that the separation in energy between the excitonic spin-resonance mode and the particle-hole continuum, which determines the resonance damping, no longer appears independent of Tc.