Crossover from weak to strong pairing in unconventional superconductors

Abstract

Superconductors are classified by their pairing mechanism and the coupling strength, measured as the ratio of the energy gap, $2\ensuremath{\Delta}$, to the critical temperature, ${T}_{\mathrm{c}}$. We present an extensive comparison of the $2\ensuremath{\Delta}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}$ ratios among many single- and multiband superconductors from simple metals to high-${T}_{\mathrm{c}}$ 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 ${T}_{\mathrm{c}}$, 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 ${T}_{\mathrm{c}}$.