  Following the discovery of superconductivity in an iron-based arsenide
LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc
was pushed up surprisingly to above 40 K by either applying pressure[2] or
replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to
55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was
found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands
for the lanthanide elements) at an apparently optimal doping level. However,
the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice
mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation
of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+
in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice
mismatch, hence induces the high temperature superconductivity.
