The interplay between electronic correlations and topology is a central topic in the study of quantum materials. In this Letter, we investigate the impact of the orbital-selective Mott phase (OSMP) on the topological properties of ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$ (FTS), an iron chalcogenide superconductor known to host both nontrivial ${\mathrm{Z}}_2$ topology and strong electronic correlations. Using angle-resolved photoemission spectroscopy, we track the evolution of topological surface states across various doping levels and temperatures. We identify a topological phase transition between trivial and nontrivial topology as a function of selenium content, with critical behavior observed between $x=0.04$ and $x=0.09$. Additionally, we find that at elevated temperatures, the coherence of the topological surface state deteriorates due to the emergence of OSMP, despite the topological invariant remaining intact. Our results demonstrate that the nontrivial topology in iron chalcogenide is fragile under strong electronic correlations.

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• Electronic structure
• Chalcogenides
• Angle-resolved photoemission spectroscopy