UV-Induced Change in the Electronic Structure of SrTiO3 at Low Temperature Probed by Resonant X-ray Emission Spectroscopy

Both Ti $L$ and $K\beta$ resonant x-ray emission spectra (RXES) of SrTiO 3 were measured at low temperature, $T\leq 40$ K. Regardless of the $d^{0}$-ness of a Ti 4+ ion, a $dd$ excitation peak in Ti $L$ RXES is irreversibly created by UV irradiation, directly indicating a mixture of the $d^{1} L $ electronic configuration under reduced symmetry around a Ti 4+ ion, where $ L $ denotes an oxygen hole. This mixture can also be evidenced by the UV-induced change in the relative intensity of two charge-transfer peaks observed in Ti $K\beta$ RXES. A similar change is reported for a cubic-to-tetragonal ferroelectric phase transition in BaTiO 3 . We therefore conclude that the local ferroelectric moment, i.e., displacement of a Ti 4+ ion in an oxygen octahedron, is induced in SrTiO 3 by UV irradiation at low temperature.