Phase diagram and isotopic effect in high-Tc pnictide superconductors

We will talk about the discovery of superconductivity with Tc higher than 40 K in Fe-based superconductors SmFeAsO$_{1-x}$F. Tc higher than McMillan limit of 39 K definitely proves pnictide superconductors high-Tc superconductivity$^{1,2}$. In this talk, we present the transport properties: resistivity, Hall coefficient and transport properties under high magnetic field. These results suggest a quantum phase transition around x=0.14 in SmFeAsO$_{1-x}$F$_{x}$ system. A electronic phase diagram is proposed, and coexistence of superconductivity and spin-density-wave is observed in Sm-1111 and Ba-122 system. We discuss the effect of isotopic effect on T$_{C}$ and T$_{SDW}$ in SmFeAsO$_{1-x}$F$_{x}$ and Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2 }$systems. Our results show that oxygen isotope effect on T$_{C }$and T$_{SDW }$ is very little, while the iron isotope exponent is about 0.35. Surprisingly, the iron isotope exchange shows the same effect on SDW transition as on superconductivity. Our results indicate that electron-phonon interaction plays some role in the superconducting mechanism, but simple electron-phonon coupling mechanism seems to be rather unlikely because a strong magnon-phonon coupling is included$^{3}$. 1. Chen, X. H. et al. \textit{Nature} 453, 761-762 (2008). 2. Liu, R. H. et al. \textit{Phys. Rev. Lett.} 101, 087001 (2008). 3. R. H. Liu et al., Nature 459, 64-67(2009).