Superconductivity at 15.6 K in calcium-doped Tb_1-xCa_xFeAsO: The structure requirement for achieving superconductivity in the hole-doped 1111 phase

National Laboratory for Superconductivity, Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences - P. O. Box 603, Beijing 100190, PRC

Corresponding author: hhwen@aphy.iphy.ac.cn

Received: 17 November 2009
Accepted: 21 December 2009

Abstract

Superconductivity at about 15.6 K was achieved in Tb_1-xCa_xFeAsO by partially substituting Tb³⁺ with Ca²⁺ in the nominal doping region x = 0.40–0.50. A detailed investigation was carried out on a typical sample with doping level of x = 0.44. The upper critical field of this sample was estimated to be 77 T from the magnetic–field–dependent resistivity data. The domination of hole-type charge carriers in the low-temperature region was confirmed by Hall effect measurements. The comparison between the calcium-doped sample Pr_1-xCa_xFeAsO (non-superconductive above 1.8 K) and the strontium-doped sample Pr_1-xSr_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.