Fig. 1: Schematics of the crystal and band structure of BFO/STO(111) thin film.

a Crystal structure of BFO/STO(111) thin film with R3c symmetry. The blue arrows on the Bi³⁺- and Fe³⁺- atoms denote the displacements from the ideal cubic perovskite which arise the electric polarization (P) along the [111]_pc or [0001]_h direction (pc and h denote pseudocubic and hexagonal notation, respectively) (Q_FE). The FeO₆ octahedra have an antiphase rotation around the polar axis which couples with the magnetic ordering (Q_AFD). b Schematics of the band structure and optical excitation. c Optical absorption spectrum of BFO/STO thin film in the ground state and the one observed just after the pump pulse irradiation (t = 0.2 ps). The in-gap absorption observed in the current transient optical spectroscopy just after the CT excitation can be assigned as the electron excitation in the Fe²⁺-like excited state to the unoccupied Fe 3d bands from the theoretical calculation shown later. At the same time, it corresponds to a self-trapped CT exciton due to the strong CT instability in the scenario suggested by Pisarev et al.^21,22.