The electronic structure of Mn$_{1-x}$Pb$_x$Bi$_2$Te$_4$: experimental evidence of topological phase transition

This study investigates methods for controlling the physical properties of the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ (MBT) by substituting Mn with Pb in Mn$_{1-x}$Pb$_x$Bi$_2$Te$_4$ (MPBT) solid solutions. This substitution enables tunable magnetic and electronic properties. Using various angle-resolved photoemission spectroscopy (ARPES) techniques, including spin-resolved and circular dichroism (CD) measurements, we analyzed the evolution of the electronic structure across different Pb concentrations, with a focus on topological phase transitions (TPT) near x = 50 %. Key indicators of TPT include the presence or absence of topological surface states (TSS) and bulk band gap closure. The results show a gradual decrease of the bulk band gap in the electronic structure of MPBT up to x = 40 %, where it nearly vanishes, followed by a constant gap value between 40 - 60 %, and its reopening above 80 %, which is accompanied by a transition of the electronic structure of MPBT to a PbBi$_2$Te$_4$-like electronic structure. TSS were observed at x less than 30 % and greater than 80 %, as confirmed by CD and spin-resolved ARPES data, but were absent near x = 55 %, suggesting a distinct topological phase - possibly semi-metallic or a trivial insulator with a narrow gap phase. These findings demonstrate the tunability of the electronic structure of MPBT, making it a promising candidate for topological and spintronic applications.