Ideal Weak Topological Insulator and Protected Helical Saddle Points

TL;DR

This paper reports the design and experimental observation of an ideal weak topological insulator in a bismuth halide, featuring stable topological surface states and unique helical saddle points within a global bulk band gap.

Contribution

It introduces a new material, Bi4I1.2Br2.8, as an ideal weak topological insulator with stable surface states and detailed topological surface Hamiltonian analysis.

Findings

Identified topological surface states on the side surface of BIB

Discovered two pairs of non-degenerate helical saddle points

Surface states are within a 195 meV bulk band gap

Abstract

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