Ideal Weak Topological Insulator and Protected Helical Saddle Points

• DOI: 10.48550/arxiv.2301.12626
• Publication Date: 2023-01-01
• Authors: Ji Seop Oh; Tianyi Xu; Nikhil Dhale; Sheng Li; Chao Lei; Chiho Yoon; Wenhao Liu; Jianwei Huang; Hanlin Wu; Makoto Hashimoto; Dong-Hui Lu; Chris Jozwiak; Aaron Bostwick; Eli Rotenberg; Chun Ning Lau; Bing Lv; Fan Zhang; R. J. Birgeneau; Ming Yi
• Keywords: Topological insulator; Hamiltonian (control theory); Surface States

The paradigm of classifying three-dimensional (3D) topological insulators into strong and weak ones (STI WTI) opens the door for discovery various phases matter protected by different symmetries defined in dimensions. However, contrast to vast realization STIs, very few materials have been experimentally identified as being close WTI. Even amongst those identified, none exists with surface states (TSS) exposed a global bulk band gap that is stable at all temperatures. Here we report design observation an ideal WTI quasi-one-dimensional (quasi-1D) bismuth halide, Bi$_{4}$I$_{1.2}$Br$_{2.8}$ (BIB). Via angle-resolved photoemission spectroscopy (ARPES), identify BIB hosts TSS on (100)$\prime$ side form two anisotropic $\pi$-offset Dirac cones (DCs) separated momentum while topologically dark (001) top surface. ARPES data fully determine unique side-surface Hamiltonian thereby pairs non-degenerate helical saddle points series four Lifshitz transitions. fact both are 195 meV, combined small velocities, nontrivial spin texture, near-gap chemical potential, qualifies be not only but also fertile ground many-body physics.