Coexistence of Polaronic States and Superconductivity in Iron-Pnictide Compound Ba$_{2}$Ti$_{2}$Fe$_{2}$As$_{4}$O

The electronic structure of iron-pnictide compound superconductor Ba2Ti2Fe2As4O, which has metallic intermediate Ti2O layers, is studied using angle-resolved photoemission spectroscopy. The Ti-related bands show a 'peak-dip-hump' line shape with two branches of dispersion associated with the polaronic states at temperatures below around 120 K. This change in the spectra occurs along with the resistivity anomaly that was not clearly understood in a previous study. Moreover, an energy gap induced by the superconducting proximity effect opens in the polaronic bands at temperatures below T-c (similar to 21 K). Our study provides the spectroscopic evidence that superconductivity coexists with polarons in the same bands near the Fermi level, which provides a suitable platform to study interactions between charge, lattice and spin freedoms in a correlated system.