Chae Jimin, Kang Seoung-Hun, Park Sang Han, Park Hanbum, Jeong Kwangsik, Kim Tae Hyeon, Hong Seok-Bo, Kim Keun Su, Kwon Young-Kyun, Kim Jeong Won, Cho Mann-Ho
Department of Physics , Yonsei University , Seoul 03722 , Korea.
Korea Institute for Advanced Study , Hoegiro 85 , Seoul 02455 , Korea.
ACS Nano. 2019 Apr 23;13(4):3931-3939. doi: 10.1021/acsnano.8b07012. Epub 2019 Apr 10.
Topological insulator (TI), a band insulator with topologically protected edge states, is one of the most interesting materials in the field of condensed matter. Bismuth selenide (BiSe) is the most spotlighted three-dimensional TI material; it has a Dirac cone at each top and bottom surface and a relatively wide bandgap. For application, suppression of the bulk effect is crucial, but in ultrathin TI materials, with thicknesses less than 3 QL, the finite size effect works on the linear dispersion of the surface states, so that the surface band has a finite bandgap because of the hybridization between the top and bottom surface states and Rashba splitting, resulting from the structure inversion asymmetry. Here, we studied the gapless top surface Dirac state of strained 3 QL BiSe/graphene heterostructures. A strain caused by the graphene layer reduces the bandgap of surface states, and the band bending resulting from the charge transfer at the BiSe-graphene interface induces localization of surface states to each top and bottom layer to suppress the overlap of the two surface states. In addition, we verified the independent transport channel of the top surface Dirac state in BiSe/graphene heterostructures by measuring the magneto-conductance. Our findings suggest that the strain and the proximity effect in TI/non-TI heterostructures may be feasible ways to engineer the topological surface states beyond the physical and topological thickness limit.
拓扑绝缘体（TI）是一种具有拓扑保护边缘态的能带绝缘体，是凝聚态物质领域中最有趣的材料之一。硒化铋（BiSe）是最受关注的三维TI材料；它在每个顶面和底面都有一个狄拉克锥，并且具有相对较宽的带隙。对于应用而言，抑制体效应至关重要，但在厚度小于3个量子层（QL）的超薄TI材料中，有限尺寸效应作用于表面态的线性色散，使得由于顶面和底面态之间的杂化以及结构反演不对称导致的Rashba分裂，表面能带具有有限的带隙。在此，我们研究了应变的3 QL BiSe/石墨烯异质结构的无隙顶面狄拉克态。由石墨烯层引起的应变减小了表面态的带隙，并且BiSe - 石墨烯界面处电荷转移导致的能带弯曲将表面态局域到每个顶面和底面层，以抑制两个表面态的重叠。此外，我们通过测量磁电导验证了BiSe/石墨烯异质结构中顶面狄拉克态的独立输运通道。我们的研究结果表明，TI/非TI异质结构中的应变和近邻效应可能是在超出物理和拓扑厚度极限的情况下调控拓扑表面态的可行方法。
