Chin. Phys. Lett.  2021, Vol. 38 Issue (9): 097401    DOI: 10.1088/0256-307X/38/9/097401
 CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES |
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
Yu Dong, Yangyang Lv, Zuyu Xu et al  2021 Chin. Phys. Lett. 38 097401
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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.
Received: 01 August 2021      Express Letter Published: 24 August 2021
 PACS: 74.20.Rp (Pairing symmetries (other than s-wave)) 74.25.Fy 74.25.Jb (Electronic structure (photoemission, etc.)) 74.70.-b (Superconducting materials other than cuprates)
Fund: Supported 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 start-up funding from ShanghaiTech University, Innovative Research Team in University (PCSIRT), the Natural Science Foundation of Shanghai Municipality (Grant No. 20ZR1436100), the Science and Technology Commission of Shanghai Municipality (Grant No. YDZX20203100001438), Jiangsu Key Laboratory of Advanced Techniques for Manipulating Electromagnetic Waves, Natural Science Foundation of Jiangsu Province (Grant No. BK20180006), and the Fundamental Research Funds for the Central Universities (Grant No. 020414380117).
 TRENDMD: URL: http://cpl.iphy.ac.cn/10.1088/0256-307X/38/9/097401       OR      http://cpl.iphy.ac.cn/Y2021/V38/I9/097401
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