A weak topological insulator state in quasi-one-dimensional bismuth iodide
A weak topological insulator state in quasi-one-dimensional bismuth iodide
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DOI：
10.1038/s41586-019-0927-7
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发表时间：
2019-02-28
期刊：
影响因子：
64.8
通讯作者：
Kondo, Takeshi
中科院分区：
文献类型：
--
作者：
Noguchi, Ryo;Takahashi, T.;Kondo, Takeshi
在过去的十年里，拓扑材料的重大突破都是由Z（2）型拓扑绝缘体的发现引发的，Z（2）型拓扑绝缘体是一种内部绝缘但表面允许电子流动的材料。在三维空间中，拓扑绝缘体被分为“强”或“弱”1，2，强拓扑绝缘体的实验证实迅速遵循理论预测（3-5）。相比之下，弱拓扑绝缘体（WTI）迄今为止尚未得到实验验证，因为拓扑表面态仅出现在特定的侧表面上，这在真实的三维晶体中通常是不可检测的（6-10）。在这里，我们提供的WTI状态的碘化铋，β-Bi 4 I4的实验证据。值得注意的是，该晶体具有自然可分裂的顶面和侧面-通过货车德瓦尔斯力堆叠-这一直是实验实现WTI态所期望的（11，12）。作为这种状态的一个明确的签名，我们发现一个准一维狄拉克拓扑表面状态的侧面（（100）平面），而顶面（（001）平面）是拓扑黑暗的拓扑表面状态的情况下。我们还发现，从β-相到α-相的晶体转变驱动从非平凡的WTI到正常绝缘体的拓扑相变在大致室温下。弱拓扑相-被视为三维堆叠的量子自旋霍尔绝缘体（13，14）-将为受益于高度定向的密集自旋电流的技术奠定基础，这些自旋电流被保护免受反向散射。
The major breakthroughs in understanding of topological materials over the past decade were all triggered by the discovery of the Z(2)-type topological insulator-a type of material that is insulating in its interior but allows electron flow on its surface. In three dimensions, a topological insulator is classified as either 'strong' or 'weak' 1,2, and experimental confirmations of the strong topological insulator rapidly followed theoretical predictions(3-5). By contrast, the weak topological insulator (WTI) has so far eluded experimental verification, because the topological surface states emerge only on particular side surfaces, which are typically undetectable in real three-dimensional crystals(6-10). Here we provide experimental evidence for the WTI state in a bismuth iodide, beta-Bi4I4. Notably, the crystal has naturally cleavable top and side planes-stacked via van der Waals forces-which have long been desirable for the experimental realization of the WTI state(11,12). As a definitive signature of this state, we find a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. We also find that a crystal transition from the beta-phase to the alpha-phase drives a topological phase transition from a nontrivial WTI to a normal insulator at roughly room temperature. The weak topological phase-viewed as quantum spin Hall insulators stacked three-dimensionally(13,14)-will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering.