Large-Gap Quantum Spin Hall State and Temperature-Induced Lifshitz Transition in Bi4Br4

Searching for quantum spin Hall insulators with large fully opened energy gap to overcome the thermal disturbance at room temperature has attracted tremendous attention because of the robustness of one-dimensional (1D) spin-momentum locked topological edge states in the practical applications of electronic devices and spintronics. Here, we report the investigation of topological nature of monolayer Bi4Br4 by the techniques of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy. The possible topological nontriviality of 1D edge state integrals within the large energy gap (∼0.2 eV) is revealed by the first-principle calculations. The ARPES measurements at different temperatures show a temperature-induced Lifshitz transition, corresponding to the resistivity anomaly evoked by the chemical potential shift. The connection between the emergency of superconductivity and the Lifshitz transition is discussed.