Fragility of Topology under Electronic Correlations in Iron Chalcogenides

The interplay between topology and electronic correlations is one of the central topics in the study of quantum materials. Topological surface states are typically described within an effective single-particle picture in which electronic quasiparticles are treated as independent fermions. However, this description is no longer adequate when strong many-body correlation effects set in—an extreme case being the Mott insulating phase, where electrons become spatially localized and the quasiparticle residue diminishes. Tracking how topological invariants evolve as a function of the quasiparticle residue can therefore yield new insights into the interplay between topology and correlation effects.

Here, we investigate the iron chalcogenide superconductor FeTe_1−xSe_x (FTS), which simultaneously hosts nontrivial Z₂ topology and an orbital-selective Mott phase (OSMP). Using high-resolution laser-based angle-resolved photoemission spectroscopy (ARPES), we tracked the evolution of the topological surface state as a function of selenium content and temperature. We identified a topological phase transition between trivial and nontrivial topology in the lightly Se-doped regime, with the critical behavior occurring between x = 0.04 and x = 0.09, resulting in a trivial topology at the Te end. Furthermore, we found that at elevated temperatures the band inversion—and thus the topological invariant—remains intact, yet the Dirac surface state becomes rapidly incoherent due to the emergence of the OSMP. Our results demonstrate that the nontrivial topology in iron chalcogenides is fragile under strong electronic correlations, highlighting a key distinction between single-particle topology and its practical robustness in correlated systems.

Authors: Younsik Kim*(SNU), Junseo Yoo(SNU), Sehoon Kim(SNU), Sungsoo Hahn, Kiyohisa Tanaka, Li Yu, Minjae Kim* and Changyoung Kim*(SNU)

Publication date: 12 May 2026

DOI: https://doi.org/10.1103/tq1k-g4lb