Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697, USA.
Department of Physics, Fudan University, Shanghai 200433, China.
Sci Adv. 2017 Mar 31;3(3):e1602579. doi: 10.1126/sciadv.1602579. eCollection 2017 Mar.
Superconductivity that spontaneously breaks time-reversal symmetry (TRS) has been found, so far, only in a handful of three-dimensional (3D) crystals with bulk inversion symmetry. We report an observation of spontaneous TRS breaking in a 2D superconducting system without inversion symmetry: the epitaxial bilayer films of bismuth and nickel. The evidence comes from the onset of the polar Kerr effect at the superconducting transition in the absence of an external magnetic field, detected by the ultrasensitive loop-less fiber-optic Sagnac interferometer. Because of strong spin-orbit interaction and lack of inversion symmetry in a Bi/Ni bilayer, superconducting pairing cannot be classified as singlet or triplet. We propose a theoretical model where magnetic fluctuations in Ni induce the superconducting pairing of the [Formula: see text] orbital symmetry between the electrons in Bi. In this model, the order parameter spontaneously breaks the TRS and has a nonzero phase winding number around the Fermi surface, thus making it a rare example of a 2D topological superconductor.
迄今为止，自发打破时间反演对称性（TRS）的超导性仅在具有体反转对称的少数几种三维（3D）晶体中被发现。我们报告了在没有反转对称的二维超导系统中观察到自发 TRS 破缺的现象：铋和镍的外延双层膜。这一证据来自超灵敏无环光纤萨格纳克干涉仪在超导转变过程中检测到的外加磁场不存在时的极克尔效应的出现。由于双层 Bi/Ni 中存在强自旋轨道相互作用和没有反转对称，超导配对不能被分类为单重态或三重态。我们提出了一个理论模型，其中 Ni 中的磁涨落诱导 Bi 中电子的 [Formula: see text] 轨道对称性的超导配对。在这个模型中，序参量自发地打破了 TRS，并且在费米面周围有非零的相位缠绕数，因此它成为 2D 拓扑超导体的罕见例子。
