NSRRC Activity Report 2022

Physics and Materials Science 017 resolved in Γ–X, whereas the band splitting of RSSs was tiny in Γ–M, suggesting anisotropic behavior of the RSSs. In their DFT calculations, spin-split surface bands (a typical characteristic of RSSs) were observed and showed excellent agreement with the experimental results. An additional Rashba spin orbital coupling (SOC) was included in the DFT calculations; without this Rashba SOC, the RSSs still existed but the band splitting in momentum was somewhat small. This anisotropic behavior of RSSs could be correlated to a warping effect caused by the crystal symmetry in α-Sn. Moreover, an estimated Rashba coefficient α R ≈ 1.5 eV · Å is ~4.5 times larger than that of Au(111). Having RSSs coexisting with TSS1 could provide an efficient and gate- tunable spin-charge conversion, which offers great potential in the application of spintronics devices. In summary, their work reports a comprehensive study on the electronic structure of compressively strained α-Sn/InSb(001). A transition from a topologically trivial to non-trivial phase was observed at the critical thickness of 6 BLs. A pair of RSSs was newly discovered in α-Sn films thicker than 30 BLs; they were regarded as a preformed TSS and this provided insights into the evolution of a TSS in a topological phase transition and the surface electronic structure of a Dirac material. As revealed by their results, α-Sn provides a rich playground for realizing various topological phases of matter, such as topological Dirac semimetal (TDS), TI, etc . The coexistence of RSSs and TSS1, as typified by an efficient and gate-tunable spin-charge conversion, may offer great opportunities for applications in the modern technology of spintronics. (Reported by Cheng-Maw Cheng) This report features the work of Raynien Kwo, Cheng-Maw Cheng and their collaborators published in Phys. Rev. B 105 , 075109 (2022). TLS 21B1 Angle-resolved UPS • High-resolution ARPES • Materials Science, Condensed-matter Physics Reference 1. K. H. M. Chen, K. Y. Lin, S. W. Lien, S. W. Huang, C. K. Cheng, H. Y. Lin, C.-H. Hsu, T.-R. Chang, C.-M. Cheng, M. Hong, J. Kwo, Phys. Rev. B 105 , 075109 (2022). A Simple and Efficient Method for Hydrogen Spillover and Storage Atomic hydrogen spillover and storage was achieved and characterized for single-site catalysts through precise control of the coverage of Ti on graphene. This realization has potential to extend all graphitic materials to store hydrogen. S ince the beginning of the industrial revolution in the 18 th century, coal has been increasingly used as an energy source for the operation of machines, which have made human life convenient. In the 19 th century, the advent of the petrochemical industry accelerated technological development. Although industrial, economical, and technological advances have made human life considerably convenient, they have come at the price of an energy crisis and environmental issues such as global warming. Furthermore, petroleum is expected to be completely exhausted in the future. Therefore, identifying alternative clean energy sources is inevitable. Hydrogen energy is one of the most notable candidates for alternative clean energy because its conversion emits no carbon and produces only water as a by-product, thereby minimizing the progress of global warming. Accordingly, technologies for effective production and storage of hydrogen are warranted. Chung-Lin Wu (National Cheng Kung University) and Chia- Hao Chen (NSRRC), in collaboration with the National Enterprise for nanoScience and nanoTechnology (NEST, Italy), discovered a high-efficiency hydrogen storage method. In early studies, graphene was revealed to be inert when exposed to hydrogen molecules but highly active when exposed to H atoms. 1,2 In their previous works, they demonstrated that single Ti atoms can adsorb on graphene at its energetically favorable hollow sites 3 and studied the structural variation of graphene upon Ti deposition and its hydrogen uptake by conducting scanning tunneling microscopy. 4 Drawing on these previous results, they further