The Stacking Effects In Topological Insulator Bi₂Se₃:a First-principles Study

Topological insulators (TI) as a new kind of quantum state, have very strong spinorbit coupling (SOC) effect. Different from ordinary insulators, a TI has a bulk bandgap whereas its surface has the protected metal state. Because of its many novel anddistinguishing properties in comparison with the conventional materials, TI haveimportant applications in spintronics and quantum computation. Bi2Se3is an importantthree-dimensional strong TI. Similar to graphene, it is a layered van der Waalsmaterial and the stacking sequence of the layers can be varied. Recently, a series ofexperiments have established that in graphene trilayers, tuning the stacking sequencecan lead to significant change of the electronic and optical properties. In addition,tuning of the stacking of the layers will alter the symmetry of the systems, giving riseto novel properties. In order to reveal the stacking effects on Bi2Se3, we studieddifferent stacking sequences of the quintuple layers (QL) in Bi2Se3bulk and film byusing first-principles method. Our research consists of the following two parts:1. The stacking effects in bulk Bi2Se3: We studied the bulk Bi2Se3with ABC, AAAand ABA stacking sequences. It is found that ABC and AAA stackings maintain thecentral symmetry and therefore both lead to similar band structures. The energy gapof AAA stacking is larger than that of ABC stacking. The ABA stacking breaks thecentrosymmetry. As a result, the energy gap becomes indirect much smaller, and theshape of the bands is considerably varied. We determined the topological phase underdifferent stacking sequences based on the occurrence of the band inversion before andafter the application of the SOC. It is found that the charge distribution of the highestoccupied state at Г point, which is concentrated on Se atoms without SOC, is mainlydistributed on Bi atoms when SOC is taken into consideration. This indicates that thestacking sequences do not change the topological phase of Bi2Se3. A salient differencein ABA stacked Bi2Se3is that giant Rashba spin splitting occurs due to the centralasymmetry, which is expected to have important applications in spintronics.2.The stacking effec in film Bi2Se3:.We studied the3QL-Bi2Se3films with ABC,AAA and ABA stackings. It is found that in the band structures of the films withcentrosymmetric ABC and AAA stackings, there are gapped Dirac cones near theFermi level at the Г point. The ABA stacked film with central asymmetry has a zeroindirect gap at the Г point and the highest occupied band has an ‘M’ shape near the Гpoint. The non-centrosymmetry of the ABA stacking and the strong SOC in the filmslead to giant Rashba spin spitting, which can be effectively tuned in large range byapplication of strain along the c-axis.