그래핀이 매개된 3차원 위상절연체의 전기적&구조적 특성연구

Abstract

Dirac materials have been widely studied due to their outstanding electronic properties as well as many potential applications for next-generation high speed device and spintronics. As an effort to utilize topologically protected surface state (TSS), we have investigated the electronic properties of graphene-mediated topological insulator (TI) films using synchrotron based angel-resolved photoemission spectroscopy (ARPES) and high-resolution core level spectroscopy (HRCLS) as major experimental tools. To understand the basic driving forces and detailed features of those system, we have also obtained DFT-based calculation of the modified bands. Furthermore, we build up the time resolved ARPES system to study electron dynamics in femto-second time scale.

As a first system, we have studied that the π-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene interfaced topological insulator hybrid material. We make unique Bi2Te2Se (BTS) films covered with a chemical vapor deposition (CVD) graphene and observe two linear Dirac bands from both the CVD graphene and BTS coexisting with their DPs separated by 0.53 eV. We further show that the separation between the two DPs, ΔD-D, can be artificially controlled by the amount of Cs atoms adsorbed on the graphene. Our density functional theory calculation shows the opening of a spin-orbit gap of Δ20 meV in the π -band and a concomitant phase transition from a semi metallic to a quantum spin Hall phase when ΔD-D < 0.20 eV. We thus provide new platform to explore a spin polarizing the π -band of graphene, which can be critical to advance graphene-based spintronics.

As a second system, we have studied the electronic structure of a new type of topological superlattice material, (Bi2)1(Bi2Se3)3, i.e. Bi8Se9. This high-quality thin film has been grown onto epitaxial graphene (EG) formed on SiC(0001). Our in-situ x-ray direction, transmission electron microscopy, and HRCLS reveal that Bi-bilayers are stacked regularly between three QLs in bulk Bi2Se3 with the QL-terminated outermost surface. The ARPES data obtained from both clean and Bi doped Bi8Se9 exhibit the presence of topological surface states (TSSs). Our density functional theory calculations show that the TSSs stem from the inverted Bi and Se states and the topologically nontrivial structures are mainly based on the prototype of three-dimensional TI Bi2Se3. The presence of two-dimensional Bi-bilayers modifies the chemical potential without affecting the overall topological order.

Finally, we have built up the laser based AEPES system and measure the surface state of typical TI, Bi2Te2Se. Using the Circular Dichroic (CD)-ARPES method, we observed the double domain effect from both clean and Cs doped TI as a color difference from opposite direction of TSS. In addition, we measure the time evolution of unoccupied state with pump-probe experiment. This system can be utilized as the tool to study the spin dynamics between two dimensional gas and TSS of TI.