Observation of a Ubiquitous (π, π)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe2–x Ni x As2 Single CrystalsSupported by the National Natural Science Foundation of China (Grant Nos. 61771234, 61727805, 11674157, 11674158, 11774152, 11822405, 61521001, 61571219, and 61501222), the National Key Projects for Research and Development of China (Grant Nos. 2016YFA0300401, 2017YFB0503302, 2017YFA0304002, and 2017YFB0503300), the

Abstract

In iron-based superconductors, the (0, π) or (π, 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 (π, π) direction in electron-doped BaFe2 – x Ni x As2 from under-doped to heavily overdoped regimes with x = 0.065–0.18. It shows superconducting gap maxima along the (π, π) direction rotated by 45° from the nematicity along (0, π) or (π, 0) direction observed in the normal state. A similar (π, π)-type nematicity is also observed in the under-doped and optimally doped hole-type Ba1 – y K y Fe2As2, with y = 0.2–0.5. These results suggest that the (π, π) nematic superconducting order is a universal feature that needs to be taken into account in the superconducting pairing mechanism in iron-based superconductors.