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 $U_0^{int}\propto T_c^n$ with $n$ = 3 to 4, which is in sharp contrast to the simple BCS prediction $U_0^{BCS}=1/2N_F\Delta_s^2$ with $N_F$ the quasiparticle density of states at the Fermi energy, $\Delta_s$ the superconducting gap. A similar correlation holds if we compute the condensation energy through $U_0^{cal}=3\gamma_n^{eff}\Delta_s^2/4\pi^2k_B^2$ with $\gamma_n^{eff}$ the effective normal state electronic specific heat coefficient. This indicates a general relationship $\gamma_n^{eff} \propto T_c^m$ with $m$ = 1 to 2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon. 
Jie Xing, Sheng Li, Bin Zeng, Gang Mu, Bing Shen, J. Schneeloch, R. D.   Zhong, T. S. Liu, G. D. Gu, and Hai-Hu Wen
cond-mat.supr-con