Observation of a Ubiquitous ($\pi, \pi$)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe$_{2-x}$Ni$_x$As$_2$ Single Crystals
Yu Dong1,2,3†, Yangyang Lv1,2,3†, Zuyu Xu1,2,3†, M. Abdel-Hafiez4,5, A. N. Vasiliev4,6, Haipeng Zhu7, Junfeng Wang7, Liang Li7, Wanghao Tian1,2,3, Wei Chen1,2,3, Song Bao1,2,3, Jinghui Wang1,2,3,8, Yueshen Wu8, Yulong Huang9, Shiliang Li9, Jie Yuan9, Kui Jin9, Labao Zhang1, Huabing Wang1, Shun-Li Yu1,2,3*, Jinsheng Wen1,2,3*, Jian-Xin Li1,2,3, Jun Li8,1*, and Peiheng Wu1,10
1School of Electronic Science and Engineering & School of Physics, Nanjing University, Nanjing 210093, China 2National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China 3Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China 4National University of Science and Technology (MISiS), Moscow 119049, Russia 5Department of Physics and Astronomy, Box 516, Uppsala University, Uppsala SE-75120, Sweden 6Moscow State University, Moscow 119991, Russia 7Wuhan National High Magnetic Field Center & School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China 8ShanghaiTech Laboratory for Topological Physics & School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China 9Beijing National Laboratory for Condensed Matter Physics & Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 10Synergetic Innovation Center in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China
Abstract：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$_{2-x}$Ni$_x$As$_2$ 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$_{1-y}$K$_y$Fe$_2$As$_2$, 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.
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