Topological phase transition in quasi-one-dimensional bismuth iodide Bi₄I₄

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 reveal the presence of an energy gap on the (100) surface of the low-temperature α-Bi₄I₄, providing spectroscopic evidence for the HOTI phase. Conversely, the high-temperature β-Bi₄I₄ harbors gapless Dirac fermions 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 α-β structural phase transition. Our findings directly evidence a temperature-induced topological phase transition from WTI to HOTI in Bi₄I₄, which paves the way to its potential applications at room temperature.