NSRRC Activity Report 2022

Physics and Materials Science 015 CaCu 3 M 4 O 12 indeed provides a unique opportunity to explore Kondo phenomena in transition metal compounds, where one may achieve lower Kondo temperatures by suitably varying the M constituent.” (Reported by Ashish Chainani) This report features the work of Liu Hao Tjeng, Atsushi Hariki and their collaborators published in Phys. Rev. X 12 , 011017 (2022). TPS 45A1 Submicron Soft X-ray Spectroscopy SP 12U1 HAXPES/Photoemission • Hard/Soft PES, XAS • Condensed-matter Physics References 1. G. R. Stewart, Rev. Mod. Phys. 56 , 755 (1984). 2. S. Kondo, D. C. Johnston, C. A. Swenson, F. Borsa, A. V. Mahajan, L. L. Miller, T. Gu, A. I. Goldman, M. B. Maple, D. A. Gajewski, E. J. Freeman, N. R. Dilley, R. P. Dickey, J. Mer- rin, K. Kojima, G. M. Luke, Y. J. Uemura, O. Chmaissem, J. D. Jorgensen, Phys. Rev. Lett. 78 , 3729 (1997). 3. W. Kobayashi, I. Terasaki, J.-i. Takeya, I. Tsukada, Y. Ando, J. Phys. Soc. Jpn. 73 , 2373 (2004). 4. H. Xiang, X. Liu, E. Zhao, J. Meng, Z. Wu, Phys. Rev. B 76 , 155103 (2007). 5. D. Takegami , C.-Y. Kuo, K. Kasebayashi, J.-G. Kim, C. F. Chang, C. E. Liu, C. N. Wu, D. Kasinathan, S. G. Altendorf, K. Hoefer, F. Meneghin, A. Marino, Y. F. Liao, K. D. Tsuei, C. T. Chen, K.-T. Ko, A. Günther, S. G. Ebbinghaus , J. W. Seo, D. H. Lee, G. Ryu, A. C. Komarek, S. Sugano, Y. Shi- makawa, A. Tanaka, T. Mizokawa, J. Kuneš, L. H. Tjeng , A. Hariki, Phys. Rev. X 12 , 011017 (2022). High-Speed and Energy-Efficient Electronics The coexistence of a topological surface state and a Rashba surface state in a Dirac semimetal, α-Sn, may significantly enhance the potential for spintronic applications. T he diverse topological phases of matter have been studied in detail, not only out of scientific interest, but also for their potential applications. Indicated by their special Dirac cones in the band structure, novel quantum oscillations are frequently observed in systems hosting Dirac fermions. Applications utilizing these Dirac states are now being extensively investigated in many fields, including thermoelectric devices, photonic devices, spin-based field-effect transistors, and memories, etc . A group IV element with a diamond structure, α-Sn is one of the materials that provides an ideal platform for realizing abundant topological phases based on its non-trivial band topology. Moreover, a large spin-to-charge conversion at room temperature has been demonstrated via spin pumping, while an efficient current-induced magnetization switching was achieved in α-Sn/magnetic metal heterostructures, thus making α-Sn attractive for spintronic applications. In the exploration of topological phase transitions, bandgap engineering via quantum confinement effects or strain modulation has been a commonly used approach in addition to chemical doping. Band evolution in a few layers of thin films attracts great attention since the confinement effect has a significant influence on the bandgap. A thickness-dependent topological phase transition often occurs when the system undergoes a band inversion during the bandgap engineering. A single-layer α-Sn, especially in the (111) orientation, is of great interest because of its honeycomb-like structure in analogy to graphene and its large quantum-spin-Hall gap. As the film becomes thicker, a two-dimensional to three-dimensional (3D) band transition takes place and the unstrained α-Sn becomes a zero-gap semiconductor. Besides the interesting phase transition found in α-Sn(111), the band evolution in the (001) orientation is also worth studying but has yet to be fully explored in a wide thickness range. To deepen understanding of the evolution of topological transition in α-Sn(001) for increasing thickness, Raynien Kwo (National Tsing Hua University), Cheng-Maw Cheng (NSRRC) and their teams studied the electronic structure of in-plane compressively strained α-Sn(001) on InSb(001) substrates with varying thicknesses ranging from a few bilayers (BL) to 370 BLs. Comprehensive angle-resolved photoemission spectroscopy (ARPES) experiments at TLS 21B1 were carried out on high-quality α-Sn thin films prepared using molecular beam epitaxy (MBE). A critical thickness of 5–6 BLs for the transition between topologically trivial and non-trivial phases was experimentally determined for the first time in undoped α-Sn(001) thin films. As the film thickness exceeded 30 BLs, additional Rashba-like surface states (RSS) were newly identified and could be associated with the preformed topological surface states (TSS) in the phase transition to a 3D topological insulator (TI). Moreover, they compared with the density functional theory (DFT) calculations and found the 3D Dirac nodes at k z ≈ 0.0276