Evidence for the quantum spin Hall effect (QSHE) has been reported in several experimental systems, in the form of edge conductance approaching a quantized value at zero magnetic field. However, another fundamental feature of the QSHE, that of spin-momentum locking in the edge channel, has not been demonstrated. Here, we report that in an applied magnetic field the edge conductance in monolayer ${\mathrm{WTe}}_2$, a candidate QSHE material, is suppressed by the component of the field perpendicular to a particular axis, implying that the spin in the edge states lies along this axis. Surprisingly, the axis is independent of edge orientation, chemical potential, and sample, being fixed relative to the monolayer crystal structure, lying at $(40\pm 2)°$ to the layer normal within the mirror plane. This finding is consistent with a theoretical model in which the bulk bands nearest the Fermi energy have the same parity, leading to simple spin texture with a single, momentum-independent spin axis that is inherited by the edge modes. In addition, the results strengthen the case for spin-momentum locking and, therefore, that monolayer ${\mathrm{WTe}}_2$ is a natural two-dimensional topological insulator exhibiting the QSHE.

中文翻译：

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量子自旋霍尔绝缘体候选单层WTe2中自旋轴的确定

已经在几个实验系统中报道了量子自旋霍尔效应 (QSHE) 的证据，其形式是边缘电导在零磁场下接近量化值。然而，QSHE 的另一个基本特征，即边缘通道中的自旋动量锁定，尚未得到证实。在这里，我们报告了在外加磁场中单层的边缘电导${\mathrm{WTe}}_2$，候选 QSHE 材料，被垂直于特定轴的场分量抑制，这意味着边缘状态中的自旋位于该轴上。令人惊讶的是，轴与边缘取向、化学势和样品无关，相对于单层晶体结构是固定的，位于$(40\pm 2)°$到镜像平面内的层法线。这一发现与理论模型一致，其中最接近费米能量的体带具有相同的奇偶性，导致简单的自旋纹理具有由边缘模式继承的单个、与动量无关的自旋轴。此外，结果加强了自旋动量锁定的情况，因此，单层${\mathrm{WTe}}_2$ 是一个自然的二维拓扑绝缘体，表现出 QSHE。