Disorder-Induced Universalily of Hole-doped 122-Type Pnictide Superconductors Phase Diagrams*

Unconventional superconductivity in iron-based pnictides is induced by the suppression of a universal spin density wave ground state (SDW) in which the magnetic moments are AFM aligned in the ab plane. Several hole-doped family members were shown to exhibit reentrance, at different doping, to a C₄ magnetic phase with spin reorientation to the out-of-plane direction. The discovery of this phase near superconductivity provided insights into the competing mechanisms underlying their complex phase diagrams but the conditions under which it is stabilized are not well understood. Here, we report the results of a comprehensive study of hole-doped materials as a function of ionic size and dopant concentration. We find that the C₄ phase is confined to regions in which the As-Fe-As bond angle is close to that of a perfect tetrahedron and that the T_N's of the SDW and C₄ phases are linked by a scaling equation consistent with a common origin as itinerant nesting instabilities. Furthermore, we show that all the SDW and C₄ transition temperatures collapse onto a universal phase diagram when the hole concentrations are scaled to the C₄ dome which represent the amount of disorder.