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Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state

机译：石墨烯量子自旋霍尔态的对称对称破缺和螺旋边缘传输

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摘要

Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nano-wires and graphene. Recently, a new method has emerged with the recognition that symmetry-protected topological (SPT) phases, which occur in systems with an energy gap to quasiparticle excitations (such as insulators or superconductors), can host robust surface states that remain gapless as long as the relevant global symmetry remains unbroken. The nature of the charge carriers in SPT surface states is intimately tied to the symmetry of the bulk, resulting in one- and two-dimensional electronic systems with novel properties. For example, time reversal symmetry endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, fixing the orientation of their spin relative to their momentum. Weakly breaking this symmetry generates a gap on the surface, resulting in charge carriers with finite effective mass and exotic spin textures. Analogous manipulations have yet to be demonstrated in two-dimensional topological insulators, where the primary example of a SPT phase is the quantum spin Hall state. Here we demonstrate experimentally that charge-neutral monolayer graphene has a quantum spin Hall state when it is subjected to a very large magnetic field angled with respect to the graphene plane. In contrast to time-reversal-symmetric systems, this state is protected by a symmetry of planar spin rotations that emerges as electron spins in a half-filled Landau level are polarized by the large magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferro-magnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with a tunable bandgap and an associated spin texture.
机译：传统上，低尺寸电子系统是通过静电将电子限制在异质结构或本征纳米级材料（如单分子，纳米线和石墨烯）中而获得的。最近，出现了一种新方法，这种方法认识到对称保护的拓扑（SPT）相出现在与准粒子激发能隙相同的系统中（例如绝缘体或超导体），可以保持鲁棒的表面状态，只要保持相关的全局对称性保持不变。 SPT表面态的电荷载流子的性质与主体的对称性紧密相关，从而产生具有新颖特性的一维和二维电子系统。例如，时间反转对称性使三维拓扑绝缘体表面上的无质量电荷载体具有螺旋性，从而固定了其自旋相对于其动量的方向。弱打破这种对称性会在表面上产生间隙，从而导致电荷载流子具有有限的有效质量和奇特的自旋纹理。在二维拓扑绝缘体中尚未证明类似的操纵方式，其中SPT相的主要示例是量子自旋霍尔态。在这里，我们通过实验证明，当电荷中性单层石墨烯受到相对于石墨烯平面成一定角度的很大磁场时，其具有量子自旋霍尔态。与时间反转对称系统相反，这种状态受平面自旋旋转对称性的保护，该对称性是由于大磁场使半填充的朗道能级中的电子自旋极化而出现的。可以通过平衡外加磁场与固有的反铁磁不稳定性之间的关系来调制所产生的螺旋边缘状态的特性，该反铁磁不稳定性往往会自发地破坏自旋旋转对称性。在由此产生的倾斜反铁磁状态中，我们观察到带隙边沿态的传输特征，它们构成了具有可调带隙和相关自旋纹理的新型一维电子系统。

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  • 来源
    《Nature》 |2014年第7484期|528-532|共5页
  • 作者

    A. F. Young; J. D. Sanchez-Yamagishi; B. Hunt; S. H. Choi; K. Watanabe; T. Taniguchi; R. C. Ashoori; P. Jarillo-Herrero;

  • 作者单位

    Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;

    Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;

    Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;

    Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;

    Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan;

    Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan;

    Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;

    Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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