Superconductivity and Transport Properties in Th and F Codoped Sm1-xThxFeAsO1-yFy.

A series of Th and F co-doped superconductors Sm1-xThxFeAsO1-yFy 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 Tc increases monotonically with F content, and finally Tc reaches a maximum of about 55K around y = 0.1, and saturates for even more F content (y = 0.15). Similar to the SmFeAsO1-yFy system, the normal state thermopower increases monotonically with the doping level. However the decrease of Tc in the 'overdoped' regime is not observed and possible explanation is discussed. [ABSTRACT FROM AUTHOR]