  In the iron-based high-Tc bulk superconductors, Tc above 50K 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-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 Sm1-x Th-x FeAsO1-y F-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-c more than the Th or F single-doped samples, with the
highest record T-c up to 58.6K when x= 0.2 and y= 0.225. Further element doping
causes more impurities and lattice distortions in the samples with a weakened
superconductivity.
