单层Bi_2Se_3拓扑绝缘体薄膜电子结构与光学性质的理论研究

Theoretical study of electronic structure and optical properties of monolayer topological insulator Bi_2Se_3 thin film

文章利用第一性原理计算研究了厚度为1QL(quintuple layer)的Bi_2Se_3单层薄膜的成键特点、电子结构和光学性质。电荷密度差分分析表明,1QL薄膜内Bi与Se原子层间除表现为强度不同的共价键成分外,还有电荷转移引起的弱的离子键成分。光学性质计算表明,薄膜的光学性质表现出不同于体相的特征。介电函数虚部在低能区和高能区分别出现2个多峰结构,而且相对于体相,2个多峰结构分别发生明显的蓝移和红移现象。低能区的蓝移与近期的实验结果是一致的,而高能区的红移是首次预测的。结合电子结构计算,把蓝移和红移归因于表面电子态之间以及表面与体电子态之间耦合作用所致。此外,相对于体相,在介电函数实部和电子能量损失谱中也观察到峰位移动,而且反射系数的多峰结构的能量分布范围比较广,表明薄膜具有较低的光学透明度。

The bonding characters,electronic structure and optical properties of Bi2Se3thin film with thickness of one quintuple layer(QL)are studied by using the first principles calculation.The charge density difference results show that the Bi and Se atoms exhibit different strength of covalent bonds,and weak ionic bond components are induced by charge transfer.The optical properties of the thin film exhibit different characteristics from those of the bulk.The imaginary dielectric function has two multiple-peak structures in low energy region and high energy region,respectively.Moreover,compared with the bulk,the two multiple-peak structures have obvious blue shift and red shift phenomena.The blue shift in low energy region is consistent with the recent experimental results while the red shift in high energy region is firstly predicted.Combined with electronic structure calculation,blue shift and red shift are attributed to the coupling interaction of the surface electronic states and the coupling interaction between the surface and the bulk electronic states.In addition,the peak shifts are also observed in the real dielectric function and the electron energy loss spectrum with respect to the bulk.The energy distribution of multiple-peak structures in reflection coefficient is relatively wide,indicating that the film has a lower optical transparency.