Exotic d-wave superconducting state of strongly hole-doped K _xBa_1-xFe₂As₂

We investigate the superconducting phase in the K_xBa _1-xFe₂As₂ 122 compounds from moderate to strong hole-doping regimes. Using the functional renormalization group, we show that, while the system develops a nodeless anisotropic s^± order parameter in the moderately doped regime, gapping out the electron pockets at strong hole doping drives the system into a nodal (cos k_x+cos k _y)(cos k_x-cos k_y) d-wave superconducting state. This is in accordance with recent experimental evidence from measurements on KFe₂As₂ which observe a nodal order parameter in the extreme doping regime. The magnetic instability is strongly suppressed.