  In iron-based superconductor Fe(Se,Te), a flat band-like feature near the
Fermi level was observed around the Brillouin zone center in the
superconducting state. It is under debate whether this is the evidence on the
presence of the BCS-BEC crossover in the superconductor. High-resolution
laser-based angle-resolved photoemission measurements are carried out on high
quality single crystals of FeSe0.45Te0.55 superconductor to address the issue.
By employing different polarization geometries, we have resolved and isolated
the dyz band and the topological surface band, making it possible to study
their superconducting behaviors separately. The dyz band alone does not form a
flat band-like feature in the superconducting state and the measured dispersion
can be well described by the BCS picture. We find that the flat band-like
feature is formed from the combination of the dyz band and the topological
surface state band in the superconducting state. These results reveal the
origin of the flat band-like feature and rule out the presence of BCS-BEC
crossover in Fe(Se,Te) superconductor.
