单层VSe $_2$中不同驱动机制的电荷有序态共存
Coexisting charge-ordered states with distinct driving mechanisms in monolayer VSe$_2$
摘要：
将晶体材料细化到二维（2D）为电子相创造了丰富的游乐场，包括电荷，自旋，超导和拓扑秩序。承载电荷密度波（CDW）的散装材料，当减少到薄膜时，显示出CDW增强和可调谐性。然而，仅限于2D的电荷顺序仍然难以捉摸。在这里，我们报告了一个独特的电荷有序状态出现在单层极限的$1T$ -VSe $_2$。系统的扫描隧道显微镜实验表明，双层VSe $_2$在很大程度上保留了体电子结构，托管一个三向CDW。然而，单层VSe $_2$ --一致地跨越不同的衬底--表现出维度交叉，承载具有不同波长和转变温度的两个CDW。电子结构计算表明，虽然一个CDW是块状的，并产生于著名的Peierls机制，另一个是决定性的非常规。观察到的CDW晶格去耦和出现一个平带表明，新的CDW可能会出现从增强的电子-电子相互作用在2D极限。这些发现建立单层-VSe $_2$作为具有不同起源的共存电荷顺序的主机，并且使得能够通过2D材料中的紧急相互作用来定制电子现象。
英文摘要：
Thinning crystalline materials to two dimensions (2D) creates a rich playground for electronic phases, including charge, spin, superconducting, and topological order. Bulk materials hosting charge density waves (CDWs), when reduced to ultrathin films, have shown CDW enhancement and tunability. However, charge order confined to only 2D remains elusive. Here we report a distinct charge ordered state emerging in the monolayer limit of $1T$-VSe$_2$. Systematic scanning tunneling microscopy experiments reveal that bilayer VSe$_2$ largely retains the bulk electronic structure, hosting a tri-directional CDW. However, monolayer VSe$_2$ -- consistently across distinct substrates -- exhibits a dimensional crossover, hosting two CDWs with distinct wavelengths and transition temperatures. Electronic structure calculations reveal that while one CDW is bulk-like and arises from the well-known Peierls mechanism, the other is decidedly unconventional. The observed CDW-lattice decoupling and the emergence of a flat band suggest that the new CDW could arise from enhanced electron-electron interactions in the 2D limit. These findings establish monolayer-VSe$_2$ as a host of coexisting charge orders with distinct origins, and enable the tailoring of electronic phenomena via emergent interactions in 2D materials.
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