Power-law-like correlation between condensation energy and superconducting transition temperatures in iron pnictide/chalcogenide superconductors: Beyond the BCS understanding

Superconducting condensation energy Uint has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that U with n=3-4, which is in sharp contrast to the simple BCS prediction U0BCS=1/2NFΔs2, with NF the quasiparticle density of states at the Fermi energy and Δs the superconducting gap. A similar correlation holds if we compute the condensation energy through U0cal=3γneffΔs2/4π2kB2, with γneff the effective normal state electronic specific heat coefficient. This indicates a general relationship γnefwith m=1-2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon.