Interaction Between Dirac Fermion Systems In Bi₂Se₃ Thin Films

Topological insulators(TIs)have different kinds of novel properties because of their topological surface state.It has a wide range of application prospects in the development of new functional devices in the future.Especially,the Z2 TIs open up a new direction in spin-manipulated devices benefits from its dissipation-less spin current.Normally speaking,these new devices require that the planarization and miniaturization of the materials.Although the single-particle band theory has achieved great success in describing the energy band of topological insulators(TIs),whether it is still applicable in the 2D limit has not been well discussed.More importantly,it was found that the electron-electron interaction(EEI)becomes strong and long-ranged,which even dominates the transport properties of ultrathin TI films,implying that the single-particle band theory is not sufficient to describe the EEI in ultrathin TI films.There are two main works in this paper:1.We reconstruct the coupling between up-and down-surface states of Bi2Se3 thin films by means of the Green function with an equivalent interaction.The self-energy is discussed by the random phase approximation and the spectral function of the upsurface state is detected directly by angle resolved photoemission spectroscopy.Our calculated and experimental results demonstrate that the electron-gas-like interaction of the quasi-particles between the up-and down-surface not only results in an opening of bandgap,but also a shift of energy-band of Bi2Se3 TIs as well.Our work paves an avenue to analyze the interaction of quasi-particles with complex dispersions.2.We introduced an annulus stable solution under an arbitrary process angle by the uniaxial symmetry of the Landau-Lifshitz-Gilbert equation(LLG eq.)with a circularly polarized periodic excitation field on long time scale.We checked the stability with the method of numerical simulation.We found the applicable conditions of the traditional small angle approximation by this way.After adding a demagnetizing field in thin film form with the uniaxial symmetry,we explained to some extent the foldover phenomenon in magnetar materials under high power driving.