Quasi-One-Dimensional Higher-Order Topological Insulators*

Quasi-1D β-Bi₄X₄ (X=Br, I) are prototype weak topological insulators (TI). For the α phases, recent high-throughput database screening suggests that Bi₄Br₄ is a rare higher-order TI (HOTI) whereas Bi₄I₄ has trivial symmetry indicators. We show that in fact the two α phases are both pristine HOTIs yet with distinct hinge state patterns. The location of inversion center dictates Bi₄Br₄ (Bi₄I₄) to feature opposite (the same) dimerizations of a surface or intrinsic (bulk or extrinsic) origin at two side surfaces. Our results imply the possible existence of many topological materials beyond the scope of symmetry indicators and establish a new TI paradigm in a unique material platform. [arXiv:2005.14710]