2020同步年報

010 ACTIVITY REPORT 2020 To solve the contradiction existing in transport and ARPES results, Suguru Ito and Iwao Matsuda from the University of Tokyo, Japan, Shu-Jung Tang from National Tsing Hua University, Cheng-Maw Cheng from NSRRC and their teams investigated the evolution of the electronic structure of high-quality Bi nanofilms. With high-resolution ARPES measurements at TLS 21B1 , these authors performed a detailed and systematic study of the electronic structure of Bi nanofilms of varied thickness; they reported the first direct observation of a QSE-induced metal-insulator transition. As shown in Fig. 1 , the visualization of an anomalous level evolution contrasted with tight-binding simulations to highlight an additional mechanism beyond a simple QSE. In addition, an unexpected degeneracy of the top two quantized energy levels detected from ARPES completely breaks a standard quantization rule. This tendency is totally opposite a well known effect of hybridization between top and bot- tom surface states and reconciles a contradiction among previous experiments in an unprecedented manner. An important question is the nature of the central mechanism responsible for the anomalous behaviors. To combine with density function theory (DFT) and the ARPES result, these typical modulations of quantized bulk states can be fully explained only when we consider the deformation of a quantum confinement potential, which is triggered by enhanced effects of Coulomb repulsion with decreasing system size, centering on a size-independent contribution from the surface states. In summary, Ito and Matsuda systematically revisited the strange contradiction among recent studies on the metal-insulator transition in Bi films. They revealed an unexpected mechanism of the transition by high-resolution ARPES combined with DFT calculations. Anomalous evolution and a degeneracy of quantized energy levels indicate that the increased Coulomb repul- sion from the surface states deforms a quantum confinement potential with decreasing thickness. The potential deformation strongly modulates the spatial distributions of quantized wave functions, which leads to an acceleration of the transition beyond the original QSE picture. Their presented study not only solves a serious controversy about a transition discussed for half a century but also introduces a novel size effect that can be universally present in a system with metallic surface states, typically topological materials. 1,2,3 (Reported by Cheng-Maw Cheng) This report features the work of Suguru Ito, Iwao Matsuda and their collaborators published in Science Advances 6 , eaaz5015 (2020). TLS 21B1 U90 − (CGM) ARPES • High-resolution Angle-resolved Photoemission Spectros- copy • Materials Science, Condensed-matter Physics References 1. S. Ito, M. Arita, J. Haruyama, B. Feng, W.-C. Chen, H. Namatame, M. Taniguchi, C.-M. Cheng, G. Bian, S.-J. Tang, T.-C. Chiang, O. Sugino, F. Komori,and I. Matsuda, Sci. Adv. 6 , eaaz5015 (2020). 2. S. Ito, B. Feng, M. Arita, T. Someya, W.-C. Chen, A. Takaya- ma, T. Iimori, H. Namatame, M. Taniguchi, C.-M. Cheng, S.-J. Tang, F. Komori, I. Matsuda, Phys. Rev. B 97 , 155423 (2018). 3. S. Ito, B. Feng, M. Arita, A. Takayama, R.-Y. Liu, T. Someya, W.-C. Chen, T. Iimori, H. Namatame, M. Taniguchi, C.-M. Cheng, S.-J. Tang, F. Komori, K. Kobayashi, T.-C. Chiang, I. Matsuda, Phys. Rev. Lett. 117 , 236402 (2016).