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Bi2Se3 is one of the theoretically predicted second generation topological insulators.Its bulk state has an energy gap of about 0.3eV mainly due to the strong spin-orbit coupling (SOC), a value larger than the thermal energy scale at the room temperature (0.026eV).This stoichiometric compound can easily be prepared and has a rather simple surface state with only one single Dirac cone in the two dimensional Brillouin zone.These advantages render physicists an ideal platform to study the physical origins for topological related phenomena and also material scientists a potential material for spintronic devices.To verify the validity of our approach, we have first calculated the electronic structure of Bi2Se3 by using the first-principle method(VASP), among the general good agreement with the published results, the reverse of the order of Bi-pz+ and the Se-pz state has also been observed by including the SOC in our calculations, which is further examined by a more direct representation with local charge density.With the application in mind, we focus further on the effect of the film thickness by modeling it with different numbers of the Quintuple Layers (QLs).It is obvious that these models reflect mainly the change of the van der Waals interaction between the QLs and its influence on the surface states.In this work, we have prepared 5 different models with two to six QLs, respectively.From our calculations, we can clearly see that when the number of QLs reaches 6, the obtained energy gap becomes nearly identical to the experimental value, needless to use more QLs as did in the literatures, which indicates that the van der Waals interaction between 6 QLs has already become comparable to that in the actual system.When the number of QLs becomes less than or equal to 3, the model system becomes metallic.However, if we optimize the model, which leads to the changes of the spaces between the QLs and thus the van der Waals interactions, we obtain, for all the models, the similar values of the energy gap.This result is very meaningful because it implies that the film of Bi2Se3 can be made rather thin while the energy gap can still remain unchanged, since the nature can simply do the same optimization as we did.