标题： 剪切抗性拓扑结构在准一维范德华材料Bi$_4$Br$_4$ 显示英文标题

标题： Shear-resistant topology in quasi one-dimensional van der Waals material Bi$_4$Br$_4$

摘要： Bi$_4$Br$_4$是一种典型的准一维（1D）材料，其中共价键结合的溴化铋链以平行、并排和层状的方式排列，在它们之间存在范德华（vdW）间隙。 到目前为止，该化合物已报道了两种不同的结构，$\alpha$-Bi$_4$Br$_4$和$\beta$-Bi$_4$Br$_4$，在两种结构中，相邻链均沿平面方向移动了$\mathbf{b}/2$，即半个晶胞矢量，但它们的垂直堆叠方式不同。 虽然已知不同的层排列会导致不同的电子性质，但原子链之间可能的面内位移的影响仍然是一个开放问题。 在此，利用扫描隧道显微镜和谱学（STM/STS），我们报告一种新的Bi$_4$Br$_4$(001)结构，在平面内相邻链之间存在$\mathbf{b}/3$的位移以及AB层堆叠。 我们确定剪切应变是这种新结构的起因，由于链间结合较弱，剪切应变可以很容易导致相邻原子链的位移。 对于观察到的$b/3$结构，（残留）原子链位移对应于面内剪切应变为$\gamma\approx7.5\%$。 STS显示出体绝缘能隙和表面台阶处的金属边缘态，表明这种新结构也是一种高阶拓扑绝缘体，与$\alpha$-Bi$_4$Br$_4$一致，符合密度泛函理论（DFT）计算的结果。 显示英文摘要

摘要： Bi$_4$Br$_4$ is a prototypical quasi one-dimensional (1D) material in which covalently bonded bismuth bromide chains are arranged in parallel, side-by-side and layer-by-layer, with van der Waals (vdW) gaps in between. So far, two different structures have been reported for this compound, $\alpha$-Bi$_4$Br$_4$ and $\beta$-Bi$_4$Br$_4$ , in both of which neighboring chains are shifted by $\mathbf{b}/2$, i.e., half a unit cell vector in the plane, but which differ in their vertical stacking. While the different layer arrangements are known to result in distinct electronic properties, the effect of possible in-plane shifts between the atomic chains remains an open question. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we report a new Bi$_4$Br$_4$(001) structure, with a shift of $\mathbf{b}/3$ between neighboring chains in the plane and AB layer stacking. We determine shear strain to be the origin of this new structure, which can readily result in shifts of neighboring atomic chains because of the weak inter-chain bonding. For the observed $b/3$ structure, the (residual) atomic chain shift corresponds to an in-plane shear strain of $\gamma\approx7.5\%$. STS reveals a bulk insulating gap and metallic edge states at surface steps, indicating that the new structure is also a higher-order topological insulator, just like $\alpha$-Bi$_4$Br$_4$, in agreement with density functional theory (DFT) calculations.