Room-Temperature Topological Phase Transition in Quasi-One-Dimensional Material Bi$_4$I$_4$

TL;DR

This study demonstrates a room-temperature topological phase transition in the quasi-one-dimensional material Bi$_4$I$_4$, switching between weak and higher-order topological insulator phases via a structural transition, with implications for novel physics exploration.

Contribution

First experimental evidence of a room-temperature topological phase transition in Bi$_4$I$_4$, revealing a transition from a weak to a higher-order topological insulator through structural changes.

Findings

High-temperature phase is a weak topological insulator with gapless Dirac cones.

Low-temperature phase exhibits gapped surface states and hinge states.

Identifies a rare transition between first-order and second-order topological phases.

Abstract

Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification, and ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In this paper, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material Bi$_4$I$_4$, mediated via a first order structural transition between two distinct stacking orders of the weakly-coupled chains. Using high resolution angle-resolved photoemission spectroscopy on the two natural cleavable surfaces, we identify the high temperature $\beta$ phase to be the first weak topological insulator with gapless Dirac cones on the (100) surface and no Dirac crossing on the (001) surface, while in the low temperature $\alpha$ phase, the topological surface state on the (100) surface opens a gap, consistent with a recent theoretical prediction of a higher-order topological insulator beyond the scope of the established topological materials databases that hosts gapless hinge states. Our results not only identify a rare topological phase transition between first-order and second-order topological insulators but also establish a novel quasi-1D material platform for exploring unprecedented physics.