Electronic Structure Study of Topological Insulators and Chalcogenides by Angle Resolved Photoemission Spectroscopy

Abstract

We investigated a new class of emerging materials such as topological insulator (TI) and transition metal dichalcogenide (TMDC) materials with strong application potential in spintronics or quantum electronic devices. We were conducted to investigate the valence transition and the electronic structures of those materials by using the x-ray spectroscopy tools such as x-ray photoemission (XPS) and angle-resolved photoemission spectroscopy (APRES). Then we shows the advantage of x-ray spectroscopy to investigate directly the electronic structures of the surface-sensitive materials. TIs have been a new attentive class of quantum matter in which a gapless surface state is located in a large bulk band gap. However, It is not easy to experimentally approach to identify a TI because TIs are not described by a local order parameters involved with a spontaneously broken symmetry but rather by topological order of quantum entanglement. Bi1−xSbx alloy belongs to this class of materials as a strong TI (ν0 = 1) which firstly challenges experimentally by angle- and spin-resolved photoemission spectroscopy. This Bi1−xSbx alloy, however, has a degree of bulk disorder and a small band gap, resulting in discrepancy of surface band topology that are not clearly settled. In this thesis, we firstly identified the detailed nature of the surface band topology of Bi0.9Sb0.1 which is in the topological phase. Although it have been much studied in various ways, Bi0.9Sb0.1 has been controversial near specific symmetry point like as M ̄ point because of its complex surface topology and bulk disorder with a small band gap. Therefore, we reexamined the nature of surface topology from Γ ̄ to M ̄ of 1st Brillouin zone. To investigate not well-defined surface topology near M ̄ , we conducted the ARPES measurement using different photon energy which leads to distinguish only surface states to be robust against the change of the photon energy. In addition to the nature of the surface band topology, we interpreted the implication of the anisotropic electronic structure by the change of the Fermi velocity. Secondly, we looked into the electronic reconstruction of IrTe2, the CdI2-type’s TMDC, across the q1/5 = (1/5 0 1/5) charge order phase transition using the ARPES and the unfolding scheme of ab-initio calculation. The unfolding scheme which can be able to resolve the complicated folded band features leads to provide us information on the electronic reconstruction involving the transition. Also, we identified the Dirac-cone like band by the enhancement of contribution of the surface states in the ARPES spectra as a possibility opening a test field for a topological phase transition. As an extended thematic study, we investigated the topological surface states of Bi2X3 (X = Se, Te) with doping magnetic impurity. Bi2X3, (X = Se, Te) as the second generation of 3D TI have identified the surface states composed of a single Dirac cone with robust metallicity. Breaking time reversal symmetry (TRS) is one way to destroy this robust surface metallicity because gapless surface states are protected by TRS. Gd doped Bi2X3, (X = Se, Te) are the 3D TI systems with magnetic dopants of Gd which lead to break the TRS and create a gap on the surface states. The rare-earth (RE) dopants (Gd) will create a gap without electric doping effect differing from divalent dopants. Hence, we identified the surface state band gap formation at the Dirac point by magnetic dopants (Gd). In summary, we have investigated the electronic structures of the TI and TMDC materials by using ARPES. We found that the overall of surface band topology and the anisotropic electronic structure describe the topological insulating character and that the surface state band gap formation at the Dirac point with broken TRS by magnetic dopants is going to realize the insulating massive Dirac fermion state. In addition, the electronic investigation of the dimerized IrTe2 by means of ARPES and the unfolding band calculation provide us information on the electronic reconstruction involving the charge order phase transition. The spectroscopic approaches using ARPES directly enable us to complete from the comprehensive Fermi surface feature to the specific electronic band structure to unveil controversial features.