Topological Surface States in an Ideal Weak Topological Insulator

The quasi-one-dimensional bismuth halides Bi₄X₄ (X = I, Br) constitute a recently realized material platform that harbors a number of non-trivial topological phases such as weak topological insulator (TI) and higher-order TI. In addition, superconductivity has been observed under pressure, suggesting potentially important roles played by electronic/structural instabilities. Thus, Bi₄X₄ is a promising platform that facilitates non-trivial topology in the presence of electronic/structural instabilities. Therefore, characterizing these compounds’ topological phases as well as finding the origin of the instabilities become essential for both a deeper understanding and future topological engineering. Among the Bi₄X₄ series of materials, we find a specific case of the simplest AA stacking order of the crystal structure at cryogenic temperatures, enabling us to conduct high energy resolution angle-resolved photoemission spectroscopy (ARPES) experiments. In this talk, I will present our ARPES results for identification of its topological character as a weak TI by measurements on both (100) and (001) cleaved surfaces. Furthermore, combined with an effective Hamiltonian, characterization for in-gap van Hove singularities purely derived from topologically non-trivial surface states will be presented.