Topological Phase Transition in Quasi-One-Dimensional Bismuth Iodide Bi4I4

TL;DR

This study uses micro-ARPES to investigate the topological phase transition in Bi4I4, revealing a temperature-induced change from a weak topological insulator to a higher-order topological insulator with potential room-temperature applications.

Contribution

It provides the first spectroscopic evidence of a temperature-driven topological phase transition in quasi-1D Bi4I4, demonstrating tunable topological phases with detailed electronic structure analysis.

Findings

Bi4I4 exhibits a HOTI phase with an energy gap on the (100) surface at low temperature.

High-temperature Bi4I4 shows a WTI phase with gapless Dirac surface states.

Temperature induces a hysteresis in the surface gap, indicating a topological phase transition.

Abstract

The exploration of topological quantum materials and topological phase transitions is at the forefront of modern condensed matter physics. Quasi-one-dimensional (quasi-1D) bismuth iodide Bi4I4 exhibits versatile topological phases of matter including weak topological insulator (WTI) and higher-order topological insulator (HOTI) phases with high tunability in response to external parameters. In this work, performing laser-based angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we comprehensively investigate the fine electronic structure and topological phase transition of Bi4I4. Our examination of the low-temperature {\alpha}-phase reveals the presence of an energy gap on the (100) surface, providing spectroscopic evidence for the HOTI phase. Conversely, the high-temperature {\beta}-Bi4I4 harbors a gapless Dirac fermion on the (100) surface alongside gapped states on the (001) surface, thereby establishing a WTI phase. By tracking the temperature evolution of the (100) surface states, we unveil a thermal hysteresis of the surface gap in line with the {\alpha}-{\beta} structural phase transition. Our findings elucidate the topological properties of Bi4I4 and directly evidence a temperature-induced topological phase transition from WTI to HOTI, which paves the way to potential applications based on the room-temperature topological phase transition in the quasi-1D topological quantum material.