Observation of a Ubiquitous (\pi, \pi)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe_2-xNi_xAs_2 Single Crystals

In iron-based superconductors, the (0, \pi) or (\pi, 0) nematicity, which describes an electronic anisotropy with a four-fold symmetry breaking, is well established and believed to be important for understanding the superconducting mechanism. However, how exactly such a nematic order observed in the normal state can be related to the superconducting pairing is still elusive. Here, by performing angular-dependent in-plane magnetoresistivity using ultra-thin flakes in the steep superconducting transition region, we unveil a nematic superconducting order along the (\pi, \pi) direction in electron-doped BaFe_2-xNi_xAs_2 from under-doped to heavily overdoped regimes with x=0.065–0.18. It shows superconducting gap maxima along the (\pi, \pi) direction rotated by 45^\circ from the nematicity along (0, \pi) or (\pi, 0) direction observed in the normal state. A similar (\pi, \pi)-type nematicity is also observed in the under-doped and optimally doped hole-type Ba_1-yK_yFe_2As_2, with y = 0.2–0.5. These results suggest that the (\pi, \pi) nematic superconducting order is a universal feature that needs to be taken into account in the superconducting pairing mechanism in iron-based superconductors.