Device Physics of Complex Materials, Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands.
High Field Magnet Laboratory (HFML-EMFL), Radboud University, Nijmegen, The Netherlands.
Nature. 2023 Jul;619(7968):46-51. doi: 10.1038/s41586-023-05967-z. Epub 2023 May 24.
In superconductors possessing both time and inversion symmetries, the Zeeman effect of an external magnetic field can break the time-reversal symmetry, forming a conventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state characterized by Cooper pairings with finite momentum. In superconductors lacking (local) inversion symmetry, the Zeeman effect may still act as the underlying mechanism of FFLO states by interacting with spin-orbit coupling (SOC). Specifically, the interplay between the Zeeman effect and Rashba SOC can lead to the formation of more accessible Rashba FFLO states that cover broader regions in the phase diagram. However, when the Zeeman effect is suppressed because of spin locking in the presence of Ising-type SOC, the conventional FFLO scenarios are no longer effective. Instead, an unconventional FFLO state is formed by coupling the orbital effect of magnetic fields with SOC, providing an alternative mechanism in superconductors with broken inversion symmetries. Here we report the discovery of such an orbital FFLO state in the multilayer Ising superconductor 2H-NbSe. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite-momentum Cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of a normal metal, a uniform Ising superconducting phase and a six-fold orbital FFLO state. This study highlights an alternative route to achieving finite-momentum superconductivity and provides a universal mechanism to preparing orbital FFLO states in similar materials with broken inversion symmetries.
在同时具有时间和反演对称性的超导体中，外磁场的塞曼效应可以破坏时间反演对称性，形成具有有限动量的常规富勒-费雷尔-拉金-奥夫钦尼科夫（FFLO）态。在缺乏（局部）反演对称性的超导体中，塞曼效应仍可以通过与自旋轨道耦合（SOC）相互作用而成为 FFLO 态的潜在机制。具体来说，塞曼效应与拉什巴 SOC 的相互作用可以导致更容易形成的拉什巴 FFLO 态，这些态在相图中覆盖更广泛的区域。然而，当由于伊辛型 SOC 的存在而导致自旋锁定从而抑制塞曼效应时，传统的 FFLO 情景不再有效。相反，通过将磁场的轨道效应与 SOC 耦合，形成非常规的 FFLO 态，这为具有反演对称性破缺的超导体提供了一种替代机制。在这里，我们报告了在多层伊辛超导体 2H-NbSe 中发现这种轨道 FFLO 态的情况。传输测量表明，在轨道 FFLO 态中，平移和旋转对称性被破坏，提供了有限动量库珀对的特征标志。我们建立了整个轨道 FFLO 相图，由正常金属、均匀伊辛超导相和六重轨道 FFLO 态组成。这项研究突出了实现有限动量超导的另一种途径，并为在具有反演对称性破缺的类似材料中制备轨道 FFLO 态提供了一种通用机制。
