0428同步年報-2021-全

density of the state within the Sb-BL, topmost quintuple layer (QL) (first QL), and the nearest- neighboring QL (second QL) in Sb 2 Te 3 . Figures 1(e) and 1(f) display the wide range and a magnification of the calculated band structures superimposed on the ARPES band mapping, respectively. The density-functional theory (DFT) calculations identify that the linear Dirac state near E F is derived from the Sb-BL. The free-standing Sb-BL is notably predicted to be a NI with band gap 2.28 eV. The topological Dirac state can thus be expected to appear at the interface between NI Sb-BL and TI Sb 2 Te 3 in the Sb-BL/Sb 2 Te 3 hybrid structure based on a concept of a bulk-boundary correspondence. Surprisingly, their calculations manifest that the topological Dirac state is contributed mainly from the Sb-BL, whereas the band dispersion of the first QL and second QL in Sb 2 Te 3 presents an energy gap. Following the idea of topological band theory, the covered Sb-BL can be effectively regarded as a new surface of the Sb-BL/Sb 2 Te 3 hybrid structure. Sb-BL is hence topologicalized because of the proximity effect between Sb-BL and Sb 2 Te 3 . The interaction between the adjacent NI and TI is complicated. One possibility comes from the energy band hybridization between NI and TI because the type-II band alignment at the interface between NI and TI, that is, the valence-band maximum of a NI, is in the band gap of the TI, which might lead to an anti-crossing between the valence state of NI and the Dirac surface state of the TI, resulting in a new surface state that exists in the adjacent NI. The position of the Dirac point and the shape of the Dirac surface state can consequently be tuned on varying the energy position of the valence band of the NI, for example on altering the doping level. This advantage might be used to manipulate the topological properties of 2D materials. More importantly, the spin spectral weights in the Sb-BL near the Fermi level exhibit clear spin-polarized states. In addition to the topological surface state, a quasi- linear spin-polarized band was found above E F in the first QL. Interestingly, the chirality of its spin texture is opposite that of the Dirac state of Sb-BL. Contrary to the spin- momentum locked spin texture in plane, an additional S z spin band out of plane appears below E F in Sb-BL and the first QL, which is likely due to the strong warping effect in the hexagonal lattice. As a result, the direction of the spin texture of Sb-BL/Sb 2 Te 3 becomes greatly modified via varying the Fermi level. Through the proximity effect induced at the interface, the antimonene/ Sb 2 Te 3 2D NI/3D TI system provides a promising way to form a new TI state and to tune the position of the Dirac point. In summary, the authors provide strong evidence derived from hydrogen etching on Sb 2 Te 3 that large-area and well ordered antimonene presents a 2D topological state. A new topological state formed in the antimonene/Sb 2 Te 3 heterostructure was confirmed with ARPES and calculations with DFT; in particular, the Dirac point was located almost at the Fermi level. The results reveal that Dirac fermions are indeed realized at the interface of a 2D NI and a 3D TI as a result of strong hybridization between antimonene and Sb 2 Te 3 . Due to the proximity effect induced at the interface, their work demonstrates that the antimonene/Sb 2 Te 3 2D NI/3D TI system could provide a promising way to form a new TI state and to modify the direction of the spin texture of Sb-BL/Sb 2 Te 3 via varying the Fermi level. (Reported by Cheng-Maw Cheng) This report features the work of Jung-Chun Andrew Huang 014 ACTIVITY REPORT 2021 Fig. 1 : (a) Band-mapping result of Sb 2 Te 3 taken at photon energy 24 eV. (b) Band-mapping result of antimonene/Sb 2 Te 3 taken at photon energy 24 eV. (c) Enlarged energy scale of the band-mapping result in (b). (d) Calculated band structure of Sb-BL/Sb 2 Te 3 . (e) Overlapping band dispersion extracted from (d) on the band-mapping result of antimonene/Sb 2 Te 3 . (f) Enlargement of energy and momentum scales in (b). [Reproduced from Ref. 1]