Element Substitution Effect in Transition Metal Oxypnictide Re(O_1-xF_x)TAs (Re=rare earth, T=transition metal)

Different element substitution effects in transition metal oxypnictide Re(O_1-xF_x)TAs, with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, are studied. Similar
to the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145K, which is ascribed to the spin-density-wave instability. Electron doping by F increases T_c to about 50K. While in the case of Gd, Tc is reduced below 10K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO_1-xF_xRuAs and CeO_1-xF_xRuAs compounds. The metallic behaviour is observed down to 4K.