Exploration of topological electronic structures in bismuth halide series

Quasi-one-dimensional bismuth halides, Bi₄X₄ (X = I, Br), constitute a promising material platform to realize various non-trivial topological phases. Bi₄X₄ can be categorized by its stacking along c axis. Weak topological insulators (TIs) in single-layer stacking, high-order TI in double stacking, and topological phases transitions opportunities in between are on the table. In addition, discovery of gate-tunable Josephson supercurrent as well as existence of electronic instabilities driven by topological surface states (TSS) bring Bi₄X₄ a fertile candidate to study topological many-body physics and emergent quantum physics. In this talk, I will present experimental characterizations for electronic structures among the Bi₄X₄ series in terms of angle-resolved photoemission spectroscopy (ARPES), which is smoking-gun evidence to identify TSS and instabilities in them. State-of-the-art ARPES at synchrotrons enable us to explore (001) and (100) surfaces distinguishably, thus can provide reliable topological characterizations among the series. Taking advantages of it, we discovered a room-temperature topological phase transition between weak TI and high-order TI, an ideal weak TI with spin-polarized electronic instabilities derived from TSS within bulk gap, and gapped TSS due to hinge states in high-order TI. Future efforts to induce topological phase transitions will be also discussed.