Superconductivity at about 15.6 K was achieved in Tb_{ 1-x}Ca_xFeAsO by partially substituting Tb^{ 3+} with Ca^{ 2+} in the nominal doping region x = 0.40 \sim 0.50. A detailed investigation was carried out in a typical sample with doping level of x = 0.44. The upper critical field of this sample was estimated to be 77 Tesla from the magnetic field dependent resistivity data. The domination of hole-like charge carriers in the low-temperature region was confirmed by Hall effect measurements. The comparison between the calcium-doped sample Pr_{ 1-x}Ca_xFeAsO (non-superconductive) and the Strontium-doped sample Pr_{ 1-x}Sr_xFeAsO (superconductive) suggests that a lager ion radius of the doped alkaline-earth element compared with that of the rare-earth element may be a necessary requirement for achieving superconductivity in the hole-doped 1111 phase.
Categories: cond-mat.supr-con