The Superconducting Phase Diagram of Ba_1−xK_xFeAs Single Crystals

The discovery of superconductivity in iron pnictides has opened up a new class of high-T_c superconductors. It is quite remarkable that superconductivity is induced in layers of FeAs. These layers are separated by either rare-earth oxides or barium, which when doped act as charge-carrier reservoir. Single crystals of the (122) phase are currently available. We have determined the temperature dependence of the critical field H_c2, for various single crystals of K-doped BaFe₂As₂ using ac-transport measurements. The experiments were done at the pulsed-field facility in Dresden in fields up to 60 T applied parallel and perpendicular to the superconducting planes. The obtained phase diagram serves as an excellent benchmark for theoretical models of the order parameter.