Superconductivity and Transport Properties in Th and F Codoped Sm_1-xTh_xFeAsO_1-yF_y

A series of Th and F co-doped superconductors Sm_1-xTh_xFeAsO_1-yF_y are synthesized and the variation of superconductivity with the doping level is investigated. At the fixed Th doping level x = 0.1, the superconducting transition temperature T_c increases monotonically with F content, and finally T_c reaches a maximum of about 55K around y = 0.1, and saturates for even more F content (y = 0.15). Similar to the SmFeAsO_1-yF_y system, the normal state thermopower increases monotonically with the doping level. However the decrease of T_c in the `overdoped' regime is not observed and possible explanation is discussed.