Electronic structure of (Mn1−xPbx)Bi2Te4 : Experimental evidence of topological phase transition

The investigation of methods to control and optimize the physical properties of the intrinsic magnetic topological insulator (TI) MnBi2Te4 is a critical challenge for the development of functional materials for quantum technologies and spintronics. The promising approach is to substitute Mn atoms with Ge, Sn, or Pb atoms in the (Mn1−xAxIV)Bi2Te4 (AIV=Ge,Sn,Pb) solid solutions. This substitution enables manageable tuning of the system's magnetic and electronic properties. In this study, we present a detailed investigation of the electronic structure evolution in (Mn1−xPbx)Bi2Te4 as a function of Pb concentration, using a variety of angle-resolved photoemission spectroscopy based techniques, including photon-energy-dependent studies, spin-resolved, and circular dichroism (CD) measurements. Special emphasis was placed on identifying experimental evidence of the theoretically predicted topological phase transitions (TPTs) in (Mn1−xPbx)Bi2Te4 near x≈50%. The criteria for detecting TPT include the presence or absence of topological surface states (TSS) in the electronic structure, which can be identified by their characteristic helical spin structure in spin-resolved or CD spectra, as well as the closure of the bulk band gap. Our results show that the bulk gap in the MnBi2Te4 -like electronic structure decreases gradually with increasing Pb concentration up to 40%, where it almost closes. From 40% to 60%, the band gap remains unchanged, and above 80%, the PbBi2Te4 -like electronic structure emerges, with the bulk gap reopening. Additionally, the TSS were detected in (Mn1−xPbx)Bi2Te4 samples with Pb concentrations up to at least 30% and beyond 80%, correlating with the regions where the bulk gap is open. However, no TSS were observed at x≈55%, indicating that the system is in topologically distinct phase compared to MnBi2Te4 or PbBi2Te4. At this concentration, the system may be in a semimetallic state or a trivial insulator phase with a very narrow bulk gap. The demonstrated tunability of the electronic structure in (Mn1−xPbx)Bi2Te4 highlights its potential for further exploration in topological and spintronic applications.