  The films of FeSe on substrates have attracted attention because of their
unusually high-temperature (Tc) superconducting properties whose origins
continue to be debated. To disentangle the competing effects of the substrate
and interlayer and intralayer processes, we present here results of density
functional theory (DFT)-based analysis of the electronic structure of
unsupported FeSe films consisting of 1 to 5 layers (1L-5L). Furthermore, by
solving the Bardeen-Schrieffer-Cooper (BCS) equation with spin-wave exchange
attraction derived from the Hubbard model, we find the superconducting critical
temperature Tc for 1L-5L and bulk FeSe systems in reasonable agreement with
experimental data. Our results point to the importance of correlation effects
in superconducting properties of single- and multi-layer FeSe films,
independently of the role of substrate.
