科研之友
微信
新浪微博
Facebook
分享链接
摘要
Two-dimensional (2D) topological insulators (TIs), aka quantum spin Hall (QSH) insulators, have attracted numerous interest in material science and condensed matter physics due to its scientific importance as an unique symmetry protected topological (SPT) quantum state and its potential technological applications ranging from spintronics to topological quantum computation(1,2). This novel electronic state has a bulk gap but can conducts charge and spin current without dissipation via the spin-momentum locked gapless edge state protected by time-reversal symmetry. The prototypical 2D TI was first proposed in graphene(3,4), in which the spin-orbit coupling (SOC) opens a band gap at the Dirac points. However, the rather tiny second-order effective SOC makes the 2D TI state in graphene only appear at an unrealistically low temperature(5,6). So far, the QSH effect is only experimentally verified in HgTe/CdTe7,8 and InAs/GaSb9,10 quantum wells, in stringent conditions, e.g., ultrahigh-quality samples and ultralow temperature, due to their small bulk band gaps (at the order of meV). Therefore new 2D TIs with large bulk gaps which can realize QSH effect easily are still much desired. Extensive effort has been devoted to the search for new QSH insulators with a large SOC gap(11-27). For instance, honeycomb lattice type materials such as silicene, germanene(12) or stanene(13), and chemically modified stanene(16) have been proposed. The element bismuth (Bi) has the largest SOC strength in the periodic table except radioactive elements. Therefore, the above exotic QSH effect can be expected to emerge notably in Bi-based materials. The bilayer Bi film has long been predicted as TI, with an inverted SOC gap of 0.2 eV at G point(11,28,29), which can be described by BHZ model(7). In addition, chemical modification provides excellent control means to improve the key properties of systems with the relevant physics altered at the same time. For example, when the bilayer Bi films are hydrogenated or halogenated from both sides, the stable 2D honeycomb Bi hydride (Bismuthumane) and halide can be obtained, which can be described by modified Kane-Mele model(3,19). The SOC gap at K and K' can reach the recorded 1 eV(18,19), much larger than that of bilayer Bi films(28,29). Besides, chemical group Methyl (-CH3) is used to modified the bilayer Bi film(22). Here we propose cyano (-CN) as another chemical group to tune the bilayer Bi film by passivating every Bi atom with a -CN group from both sides or one side. We find the two BiCN monolayers (regardless of the passivation from both sides or one side) are 2D TIs with a huge SOC gap of approximate 1 eV. The low-energy effective Hamiltonian is developed for the symmetric one (passivation from both sides). Moreover, we investigate monolayer h-BN and MoS2 as candidate substrates, and find the composite systems are van der Waals (vdW) heterostructures.
- 出版日期2016-7-21
- 单位北京理工大学
