Photo-induced changes in charge-ordered state of Ti₄O₇

We have investigated photo-induced effects on the charge-ordered state of Ti₄O₇ with pump-probe spectroscopy. Reflectivity of the probe light changes after the pulsed pump excitation, and then recovers. The photo-induced effects are observed only when the pump power exceeds a threshold value, indicative of cooperative nature of the formation process, and the recovery rate shows thermally activated behaviour. We propose that the photo-induced state is a metastable charge localized state where charge disorder is induced by a photon-assisted charge transfer process from Ti₂⁶⁺ dimers to the neighbouring Ti⁴⁺ions. Moreover, it is found that subsequent cw laser irradiation converts the photo-induced state into the charge-ordered state. We interpret this result in terms of formation of Ti₂⁶⁺ dimers via an inverse charge transfer process assisted by the cw optical excitation.