Coexistence of the spin-density wave and superconductivity in Ba_1-xK_xFe₂As₂

The relation between the spin-density wave (SDW) and superconducting order is a central topic in the current research on the FeAs-based high-T_C superconductors. Conflicting results exist in the LaFeAs(O, F)-class of materials, for which whether the SDW and superconductivity are mutually exclusive or they can coexist has not been settled. Here we show that for the (Ba, K)Fe₂As₂ system, the SDW and superconductivity can coexist in an extended range of compositions. The availability of single crystalline samples and high value of the energy gaps would make the materials a model system to investigate the high-T_C ferropnictide superconductivity.