A Novel Quasi-One-Dimensional Topological Insulator in Bismuth Iodide $\beta$-Bi$_4$I$_4$

TL;DR

This paper predicts and confirms a new topological insulator phase in quasi-one-dimensional bismuth iodide Bi4I4, expanding the understanding of topological states with experimental evidence of its anisotropic surface electronic structure.

Contribution

The study introduces a new Z2 topological insulator in $eta$-Bi4I4, combining theoretical prediction with experimental ARPES validation of its surface electronic properties.

Findings

Theoretical prediction of a new Z2 topological insulator in $eta$-Bi4I4.

Experimental ARPES measurements confirm the anisotropic surface band crossing.

$eta$-Bi4I4$ exhibits a surface electronic structure consistent with topological insulator behavior.

Abstract

Recent progress in the field of topological states of matter(1,2) has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs)(3-6), followed by closely related ternary compounds(7-16) and predictions of several weak TIs(17-19). However, both the conceptual richness of Z$_2$ classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z$_2$ topological insulator is theoretically predicted and experimentally confirmed in the $\beta$-phase of quasi-one-dimensional bismuth iodide Bi$_4$I$_4$. The electronic structure of $\beta$-Bi$_4$I$_4$, characterized by Z$_2$ invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements on the (001) surface reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.