Persistent Topological Surface States and Suppressed Superconducting Spectral Coherence with Excess Fe in iron-based superconductor Fe(Te,Se)

Fe impurities in Fe(Te,Se) have been reported to host possible Majorana zero modes, which were later proposed to be associated with quantum anomalous vortices, offering a promising platform for defect-based topological quantum computation. However, excess Fe can also modify the low-energy electronic structure through carrier doping and impurity-induced decoherence, both of which may critically influence the emergence and robustness of Majorana states. Here we use high-resolution laser-based angle-resolved photoemission spectroscopy to study Fe 1+ y Te 0.6 Se 0.4 crystals with different levels of excess Fe, characterized by their superconducting transition temperatures. With increasing excess Fe, we find the superconducting spectral coherence is progressively suppressed, while the Dirac-type topological surface band remains detectable but becomes progressively broadened. The dxz hole band exhibits pronounced broadening and loss of spectral coherence with increasing excess Fe, in contrast to the comparatively weak response of the d yz band. The absence of substantial systematic band-energy shifts indicates that decoherence, rather than resolvable rigid-band shift, is the dominant spectroscopic consequence of excess Fe. These results show that the topological surface state persists near the Fermi level with increasing excess Fe, while its spectral coherence is progressively degraded. Reducing the excess-Fe concentration is therefore essential for minimizing decoherence and preserving the coherent electronic environment required for robust topological superconductivity.