SUPA, School of Physics and Astronomy , University of St. Andrews , St. Andrews KY16 9SS , United Kingdom.
Suzhou Institute of Nano-Technology and Nanobionics (SINANO), CAS , 398 Ruoshui Road , SEID, SIP, Suzhou 215123 , China.
Nano Lett. 2018 Jul 11;18(7):4493-4499. doi: 10.1021/acs.nanolett.8b01649. Epub 2018 Jun 22.
How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below T = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.
当层状过渡金属二卤化物被减薄到单层极限时，其相互作用的电子态和相如何演变，这是二维材料研究中的一个关键开放性问题。在这里，我们使用角分辨光发射来研究生长在双层石墨烯/SiC 上的单层 VSe 的电子结构。虽然整体电子结构与体 VSe 相似，但我们表明，对于单层，随着温度从 T = 140 ± 5 K 以下降低，在整个费米表面上会出现明显的能隙，同时在低能电子衍射中出现电荷有序超结构。这些观察结果表明，在单层中存在电荷密度波不稳定性，其增强程度远远超过体相。此外，我们对电子结构和 X 射线磁圆二色性的测量都没有显示出铁磁有序的迹象，这与最近的一项实验研究结果以及密度泛函理论的预期相反。因此，我们的研究表明，在单层过渡金属二卤化物中，可以实现相互竞争的相互作用状态和相之间的微妙平衡。
