Discovery of switchable weak topological insulator state in quasi-one-dimensional bismuth iodide

TL;DR

This paper reports the experimental discovery of a switchable weak topological insulator state in bismuth iodide, demonstrating quasi-1D Dirac surface states and a phase transition that enables potential spintronic applications.

Contribution

It provides the first experimental evidence of a weak topological insulator in a naturally cleavable crystal with switchable topological phases.

Findings

Observation of quasi-1D Dirac surface states on side surfaces

Detection of topological phase transition near room temperature

Identification of a crystal structure that supports WTI states

Abstract

The major breakthroughs in the understanding of topological materials over the past decade were all triggered by the discovery of the Z$_2$ topological insulator (TI). In three dimensions (3D), the TI is classified as either "strong" or "weak", and experimental confirmations of the strong topological insulator (STI) rapidly followed the theoretical predictions. In contrast, the weak topological insulator has so far eluded experimental verification, since the topological surface states emerge only on particular side surfaces which are typically undetectable in real 3D crystals. Here we provide experimental evidence for the WTI state in a bismuth iodide, $\beta$-Bi4I4. Significantly, the crystal has naturally cleavable top and side planes both stacked via van-der-Waals forces, which have long been desirable for the experimental realization of the WTI state. As a definitive signature of it, we find quasi-1D Dirac TSS at the side-surface (100) while the top-surface (001) is topologically dark. Furthermore, a crystal transition from the $\beta$- to $\alpha$-phase drives a topological phase transition from a nontrivial WTI to the trivial insulator around room temperature. This topological phase, viewed as quantum spin Hall (QSH) insulators stacked three-dimensionally, and excellent functionality with on/off switching will lay a foundation for new technology benefiting from highly directional spin-currents with large density protected against backscattering.