The notion of topological insulators (TIs), characterized by an insulating bulk and conducting topological surface states, can be extended to higher-order topological insulators (HOTIs) hosting gapless modes localized at the boundaries of two or more dimensions lower than the insulating bulk. In this work, by performing high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements with submicron spatial and spin resolution, we systematically investigate the electronic structure and spin texture of quasi-one-dimensional (1D) HOTI candidate Bi₄Br₄. In contrast to the bulk-state-dominant spectra on the (001) surface, we observe gapped surface states on the (100) surface, whose dispersion and spin-polarization agree well with our ab-initio calculations. Moreover, we reveal in-gap states connecting the surface valence and conduction bands, which is a signature of the hinge states inside the (100) surface gap. Our findings provide compelling evidence for the HOTI phase of Bi₄Br₄. The identification of the higher-order topological phase promises applications based on 1D spin-momentum locked current in electronic and spintronic devices.

拓扑绝缘体 （TI） 的概念以绝缘体和导电拓扑表面状态为特征，可以扩展到高阶拓扑绝缘体 （HOTI），这些绝缘体具有位于比绝缘体低两个或多个维度边界的无间隙模式。在这项工作中，通过进行具有亚微米空间和自旋分辨率的高分辨率角度分辨光电子能谱 （ARPES） 测量，我们系统地研究了准一维 （1D） HOTI 候选 Bi₄Br₄ 的电子结构和自旋织构。与 （001） 表面上的体态主导光谱相反，我们在 （100） 表面上观察到间隙表面态，其色散和自旋极化与我们的 ab-initio 计算非常吻合。此外，我们揭示了连接表面价带和导带的间隙内状态，这是 （100） 表面间隙内铰链状态的特征。我们的研究结果为 Bi₄Br₄ 的 HOTI 阶段提供了令人信服的证据。高阶拓扑相的识别有望在电子和自旋电子器件中实现基于 1D 自旋动量锁定电流的应用。