Observation of a Ubiquitous (π, π)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe_2–x Ni _x As₂ Single Crystals

Abstract

In iron-based superconductors, the (0, 𝜋) or (𝜋, 0) nematicity, which describes an electronic anisotropy with a fourfold 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−𝑥Ni𝑥As2 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−𝑦K𝑦Fe2As2, 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.