By partially substituting the tri-valence element La with di-valence element Sr in LaOFeAs, we introduced holes into the system. For the first time, we successfully synthesized the hole-doped new superconductors (La1−xSrx)OFeAs. The maximum superconducting transition temperature at about 25 K was observed at a doping level of x = 0.13. It is evidenced by Hall effect measurements that the conduction in this type of material is dominated by hole-like charge carriers, rather than electron-like ones. Together with the data of the electron-doped system La(O1−xFx)FeAs, a generic phase diagram is depicted and is revealed to be similar to that of the cuprate superconductors.

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We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.