Correlation-induced self-doping in iron-pnictide superconductor Ba$_{2}$Ti$_{2}$Fe$_{2}$As$_{4}$O

The electronic structure of the iron-based superconductor Ba$_{2}$Ti$_{2}$Fe$_{2}$As$_{4}$O ($T_{\mathrm{c}}^{\mathrm{onset}} = $ 23.5 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 3$d$ and Ti 3$d$ orbitals, indicating that the spacer 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 Ti$_{2}$As$_{2}$O 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 3$d$ shells. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.