Revisiting the Electron-Doped SmFeAsO: Enhanced Superconductivity up to 58.6K by Th and F Codoping

In the iron-based high-T_\rm c bulk superconductors, T_\rm c above 50 K was only observed in the electron-doped 1111-type compounds. Here we revisit the electron-doped SmFeAsO polycrystals to make a further investigation for the highest T_\rm c in these materials. To introduce more electron carriers and less crystal lattice distortions, we study the Th and F codoping effects into the Sm-O layers with heavy electron doping. Dozens of Sm_1-xTh_xFeAsO_1-yF_y samples are synthesized through the solid state reaction method, and these samples are carefully characterized by the structural, resistive, and magnetic measurements. We find that the codoping of Th and F clearly enhances the superconducting T_\rm c more than the Th or F single-doped samples, with the highest record T_\rm c up to 58.6 K when x=0.2 and y=0.225. Further element doping causes more impurities and lattice distortions in the samples with a weakened superconductivity.