Angle-resolved photoemission and first-principles studies of topological thin films.

摘要

Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface. The exotic electronic properties of topological materials are of great interest for spin-related electronics and quantum computation. In this thesis research, the combination of angleresolved photoemission spectroscopy (ARPES) and first principles calculation is used to examine the electronic properties of topological thin films and 2D electronic systems with large spin-orbit splitting. The topological thin films are prepared in situ by molecular beam epitaxy (MBE) method and characterized by experimental tools such as reflection high-energy electron diffraction (RHEED) and low energy electron diffraction (LEED). The systems investigated in this thesis include topological Sb, Bi2Te3, Be2Se 3 thin films, Bi films, and Bi/Ag surface alloy.;Topological Sb films have been successfully fabricated on Si(111) substrates. By examining the connection pattern between surface states and the quantum well bulk states, our photoemission spectra show clearly the topological order of the Sb films. When topological films become ultrathin, the quantum tunneling effect breaks the degeneracy at the Dirac point of the topological surface bands, resulting in a gap. Our ARPES mapping of the surface band structure of a 4-BL Sb film reveals no energy gap at the Dirac point. This lack of tunneling gap can be explained by a strong interfacial bonding between the film and the substrate.;The topological order of topological materials is a robust quantity, but the topological surface states themselves can be highly sensitive to the boundary conditions. Specifically, the surface states of Bi2Se3 and Bi2Te3 form a single Dirac cone at the zone center. Our first-principles calculations based on a slab geometry show that, upon hydrogen termination of either face of the slab, the Dirac cone associated with this face is replaced by three Dirac cones centered at the time-reversal-invariant M¯ points at the zone boundary. The critical behavior of the TI film near the quantum critical point is also studied theoretically. When the strength of the spin-orbit coupling (SOC) is tuned across the critical point, the topological surface states, while protected by symmetry in the bulk limit, can be missing completely in topological films even at large film thicknesses.;We have observed, using angle-resolved photoemission, a structural phase transformation of Bi films deposited on Si(111)-(7x7). Films with thicknesses 20 to ~100 A, upon annealing, first order into a metastable pseudocubic (PC) phase and then transform into a stable rhombohedral (RH) phase with very different topologies for the quantum well subband structures. The PC phase shows a surface band with a maximum near the Fermi level at G , whereas the RH phase shows a Dirac-like subband around M¯ along K¯ -- M¯ -- K¯ . The formation of the metastable phase over a wide thickness range can be attributed to a surface nucleation mechanism.;Finally, we have studied the electronic structure of the Bi/Ag surface alloy, a system possessing a huge Rashba splitting in its surface bands. The Bi/Ag surface alloy is prepared by depositing Bi onto ultrathin Ag films followed by annealing. The electronic structure of the system is measured using circular angle resolved photoemission spectroscopy (CARPES). The results reveal two interesting phenomena: the hybridization of spin polarized surface states with Ag bulk quantum well states and the umklapp scattering by the perturbed surface potential. In addition, our CARPES spectra show clearly a unique dichroism pattern which is closely related to the spin texture of this 2D strongly spin-orbit coupled electron system.