Charge density waves appear in numerous condensed matter platforms ranging from high-temperature superconductors to quantum Hall systems. Despite such ubiquity, there has been a lack of direct experimental study of boundary states that can uniquely stem from the charge order. Here we directly visualize the bulk and boundary phenomenology of the charge density wave in a topological material, Ta₂Se₈I, using scanning tunnelling microscopy. At a monolayer step edge, we demonstrate the presence of an in-gap boundary mode persisting up to the charge ordering temperature with modulations along the edge that match the charge density wave wavevector along the edge. Furthermore, these results manifesting the presence of an edge state challenge the existing axion insulator interpretation of the charge-ordered phase in this compound.

电荷密度波出现在从高温超导体到量子霍尔系统的众多凝聚态物质平台中。尽管如此普遍，但仍缺乏对独特地源于电荷顺序的边界态的直接实验研究。在这里，我们使用扫描隧道显微镜直接可视化拓扑材料 Ta ₂ Se ₈ I 中电荷密度波的体积和边界现象。在单层阶梯边缘，我们证明了间隙内边界模式的存在，该模式持续到电荷排序温度，并且沿边缘的调制与沿边缘的电荷密度波波矢量相匹配。此外，这些结果表明边缘态的存在，挑战了该化合物中电荷有序相的现有轴子绝缘体解释。