  The electronic structure of the intercalated iron-based superconductor
Ba2Ti2Fe2As4O (Tc - 21.5 K) has been investigated by using angle-resolved
photoemission spectroscopy and combined local density approximation and
dynamical mean field theory calculations. The electronic states near the Fermi
level are dominated by both the Fe 3d and Ti 3d orbitals, indicating that the
spacing layers separating different FeAs layers are also metallic. By counting
the enclosed volumes of the Fermi surface sheets, we observe a large
self-doping effect, i.e. 0.25 electrons per unit cell are transferred from the
FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state.
This exotic behavior is successfully reproduced by our dynamical mean field
calculations, in which the self-doping effect is attributed to the electronic
correlations in the Fe 3d shell. Our work provides an alternative route of
effective doping without element substitution for iron-based superconductors.
