Superconductivity in (La1-xCex)(O0.9F0.1)FeAs and (La1-xPbx)OFeAs

Here we report the synthesis and superconductivity of (La$_{1-x}$Cex)(O0.9F0.1)FeAs and (La$_{1-x}$Pbx)OFeAs, which were prepared via partially substituting the trivalent La with either Ce in LaO0.9F0.1FeAs or Pb in LaOFeAs, respectively. Influence of the doping level x on the superconducting transition temperatures (Tc) is investigated in correlation with the dopant fraction values in these two sets of superconductors. The accurate existence of the doped Ce or Pb cations inside the typical samples is confirmed by means of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX) measurements. Structural analysis reveals that the increase of Ce content in the samples leads to a systematic shrinkage of the crystal lattice parameters. The observation of superconductivity in (La$_{1-x}$Pbx)OFeAs without F doping indicates a new method for the introduction of charge carriers in this system.