Sohn Egon, Xi Xiaoxiang, He Wen-Yu, Jiang Shengwei, Wang Zefang, Kang Kaifei, Park Ju-Hyun, Berger Helmuth, Forró László, Law Kam Tuen, Shan Jie, Mak Kin Fai
Department of Physics, The Pennsylvania State University, University Park, PA, USA.
Department of Physics and School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA.
Nat Mater. 2018 Jun;17(6):504-508. doi: 10.1038/s41563-018-0061-1. Epub 2018 Apr 30.
Time reversal and spatial inversion are two key symmetries for conventional Bardeen-Cooper-Schrieffer (BCS) superconductivity . Breaking inversion symmetry can lead to mixed-parity Cooper pairing and unconventional superconducting properties. Two-dimensional (2D) NbSe has emerged as a new non-centrosymmetric superconductor with the unique out-of-plane or Ising spin-orbit coupling (SOC). Here we report the observation of an unusual continuous paramagnetic-limited superconductor-normal metal transition in 2D NbSe. Using tunelling spectroscopy under high in-plane magnetic fields, we observe a continuous closing of the superconducting gap at the upper critical field at low temperatures, in stark contrast to the abrupt first-order transition observed in BCS thin-film superconductors. The paramagnetic-limited continuous transition arises from a large spin susceptibility of the superconducting phase due to the Ising SOC. The result is further supported by self-consistent mean-field calculations based on the ab initio band structure of 2D NbSe. Our findings establish 2D NbSe as a promising platform to explore novel spin-dependent superconducting phenomena and device concepts , such as equal-spin Andreev reflection and topological superconductivity.
时间反演和空间反演是传统巴丁 - 库珀 - 施里弗（BCS）超导的两个关键对称性。打破反演对称性会导致混合宇称库珀对和非常规超导特性。二维（2D）NbSe已成为一种新型的非中心对称超导体，具有独特的面外或伊辛自旋轨道耦合（SOC）。在此，我们报告在二维NbSe中观察到一种不寻常的连续顺磁极限超导 - 正常金属转变。利用高面内磁场下的隧道光谱，我们观察到在低温下上临界场处超导能隙的连续闭合，这与BCS薄膜超导体中观察到的一级突变转变形成鲜明对比。顺磁极限连续转变源于由于伊辛自旋轨道耦合导致的超导相的大自旋磁化率。基于二维NbSe的第一性原理能带结构的自洽平均场计算进一步支持了这一结果。我们的发现确立了二维NbSe作为探索新型自旋相关超导现象和器件概念（如等自旋安德列夫反射和拓扑超导）的有前景的平台。
