  Iron chalcogenide superconductors are multi-band systems with strong electron
correlations. Here we use angle-resolved photoemission spectroscopy to study
band dependent correlation effects in single-layer FeSe/Nb:BaTiO3/KTaO3, a new
iron chalcogenide superconductor with non-degenerate electron pockets and
interface-enhanced superconductivity. The non-degeneracy of the electron bands
helps to resolve the temperature dependent evolution of different bands. With
increasing temperature, the single layer FeSe undergoes a band-selective
localization, in which the coherent spectral weight of one electron band is
completely depleted while that of the other one remains finite. In addition,
the spectral weight of the incoherent background is enhanced with increasing
temperature, indicating a coherent-incoherent crossover. Signatures of
polaronic behavior are observed, suggesting electron-boson interactions. These
phenomena help to construct a more complete picture of electron correlations in
the FeSe family.
