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

Superconducting condensation energy U-0(int) has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that U-0(int) infinity T-c(n) with n = 3-4, which is in sharp contrast to the simple BCS prediction U-0(BCS) = 1/2N(F)Delta(2)(s), with N-F the quasiparticle density of states at the Fermi energy and Delta(s) the superconducting gap. A similar correlation holds if we compute the condensation energy through U-0(cal) = 3 gamma(eff)(n) Delta(2)(s)/4 pi(2)k(B)(2), with gamma(eff)(n) the effective normal state electronic specific heat coefficient. This indicates a general relationship gamma(eff)(n) infinity T-c(m) with m = 1-2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon.