Electronic structure and correlation in β-Ti3O5 and λ-Ti3O5 studied by hard x-ray photoelectron spectroscopy

We have conducted hard x-ray photoelectron spectroscopy investigations of the electronic structure changes and electron correlation phenomena which take place upon the photoinduced reversible phase transition between beta- and lambda-Ti3O. From valence band spectra of beta- and lambda-Ti3O5, we have identified the bipolaron caused by the sigma-type bonding of d(xy) orbitals in beta-Ti3O5 and the pi stacking between the d(xy) orbitals between different Ti sites in lambda-Ti3O5, previously predicted by ab initio calculations. This indicates that the single electron band picture is valid for the description of photoinduced phase transitions. On the other hand, the Ti 2p and Ti 1s core level spectra exhibit nonlocal screening satellite features, which are typical spectroscopic signs of strong electron correlation in the coherent Ti t(2g) states. The most striking result we obtain is that correlation in the valence band also manifests to reduce the plasmon energy, which results in an enhancement of the valence electron mass by a factor of 2.7.