Element substitution effect in transition metal oxypnictide Re(O$_{1-x}$F$_x$)TAs (Re=rare earth, T=transition metal)

Different element substitution effects in transition metal oxypnictide Re(O$_{1-x}$F$_x$)TAs with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, were studied. Similar to the La- or Ce-based systems, we found that the pure NdOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. Electron doping by F increases T$_c$ to about 50 K. While in the case of Gd, the T$_c$ is reduced below 10 K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity was found when La was replaced by Ce in both cases, instead a ferromagnetic ordering transition was likely to form at low temperature in F-doped sample. We also synthesized LaO$_{1-x}$F$_x$RuAs and CeO$_{1-x}$F$_x$RuAs compounds. Metallic behavior was observed down to 4 K.