Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: (i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification and (ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In this paper, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material ${\mathrm{Bi}}_4{\mathrm{I}}_4$, mediated via a first-order structural transition between two distinct stacking orders of the weakly coupled chains. Using high-resolution angle-resolved photoemission spectroscopy on the two natural cleavable surfaces, we identify the high-temperature $\beta$ phase to be the first weak topological insulator with two gapless Dirac cones on the (100) surface and no Dirac crossing on the (001) surface, while in the low-temperature $\alpha$ phase, the topological surface state on the (100) surface opens a gap, consistent with a recent theoretical prediction of a higher-order topological insulator beyond the scope of the established topological materials databases that hosts gapless hinge states. Our results not only identify a rare topological phase transition between first-order and second-order topological insulators but also establish a novel quasi-1D material platform for exploring unprecedented physics.

中文翻译：

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准一维材料Bi4I4中的室温拓扑相变

准一维 (1D) 材料为表征和调整拓扑相提供了一个优越的平台，原因有两个：(i) 存在多个可切割表面，可以更好地实验识别拓扑分类和 (ii) 对扰动（如应变）的更强响应与更高维的晶体结构相比，用于调整拓扑相。在本文中，我们提供了准一维材料中室温拓扑相变的实验证据${双}_4{\mathrm{我}}_4$，通过弱耦合链的两个不同堆叠顺序之间的一阶结构转换介导。在两个天然可切割表面上使用高分辨率角分辨光电子能谱，我们确定了高温$\beta$ 相成为第一个弱拓扑绝缘体，在 (100) 面上有两个无间隙的狄拉克锥，在 (001) 面上没有狄拉克交叉，而在低温下 $\alpha$阶段，（100）表面上的拓扑表面状态打开了一个间隙，这与最近对高阶拓扑绝缘体的理论预测一致，该理论预测超出了托管无间隙铰链状态的已建立拓扑材料数据库的范围。我们的结果不仅确定了一阶和二阶拓扑绝缘体之间罕见的拓扑相变，而且还建立了一个新的准一维材料平台，用于探索前所未有的物理学。