Bi_(2)Te_(3)(111)和Al_(2)O_(3)(0001)衬底对Bi(111)双原子层的电子结构及拓扑性质的影响

Effects of Bi_(2)Te_(3)(111)and Al_(2)O_(3)(0001)substrates on electronic and topological properties of Bi(111)bilayer

晶体铋沿(111)面方向的双原子层及薄膜具有新奇的拓扑性质.在实验生长或者实际应用中,其必然与衬底接触.本文采用紧束缚近似方法与第一性原理计算研究了Bi双原子层及其与Bi_(2)Te_(3)和Al_(2)O_(3)衬底形成的异质结的电子结构.计算结果表明, Bi双层是具有0.2 eV的半导体.当其与具有拓扑表面态的Bi_(2)Te_(3)形成异质结时,两者电子态之间有很强的杂化,不利于Bi(111)双层拓扑电子态的观测.将其放在绝缘体Al_(2)O_(3)(0001)时,导带与价带与衬底电子态杂化较小,并且展现出巨大的Rashba自旋劈裂.这是由于衬底诱导Bi(111)双原子层中心反演对称性破缺和自旋-轨道耦合共同作用的结果.进一步采用紧束缚近似计算得到的结果发现,衬底Al_(2)O_(3)(0001)对Bi(111)双层的作用等效于一个约为0.5—0.6 V/A(1A=0.1 nm)的外电场.此外, Bi(111)双原子层与衬底Bi_(2)Te_(3)电子态之间的强杂化会导致其发生拓扑相变,即由二维拓扑绝缘体转变为平庸的绝缘体.本文为人们在Bi(111)双层的生长和将其进行实际应用时如何选择合适的衬底并进行电子性质的调控提供了指导作用.

The bilayer and thin films of Bi(111)have demonstrated novel topological properties.Here,we investigate the electronic structures of Bi/Bi_(2)Te_(3)(111)and Bi/Al_(2)O_(3)(0001)by combining first-principles and tight-binding approximation calculations.Our results show that the Bi(111)bilayer is a semiconductor with a gap of about 0.2 eV.Its electronic states are strongly disturbed by the interaction with Bi_(2)Te_(3)(111)thin films,no matter whether the substrate has a band gap or Dirac surface state.Moreover,it is hard to see Rashba-type band splittings in such systems.In contrast,it demonstrates clean and giant Rashba-type splittings as strongly hybridized with insulating Al_(2)O_(3)(0001),which is due to the broken inversion symmetry induced by interfacing and the strong atomic spin-orbit coupling in Bi.Our tight-binding approximation analyses further reveal that the effect of substrate Al_(2)O_(3)(0001)on the band structure of the Bi(111)bilayer is equivalent to the action of external electric field in a range between 0.5 and 0.6 V/Å.Moreover,we find that the strong hybridization between Bi(111)bilayer and the electronic state of the substrate Bi_(2)Te_(3)(111)can lead to a topological phase transition,i.e.the change from a two-dimensional topological insulator into a mediocre insulator.Our study thus provides an insight into the interface-engineering of electronic states of Bi(111)bilayer.