c-Axis Dimer and Its Electronic Breakup: The Insulator-to-Metal Transition in Ti_{2}O_{3}

Abstract

We report on our investigation of the electronic structure of Ti_{2}O_{3} using (hard) x-ray photoelectron and soft x-ray absorption spectroscopy. From the distinct satellite structures in the spectra, we have been able to establish unambiguously that the Ti-Ti c-axis dimer in the corundum crystal structure is electronically present and forms an a_{1g}a_{1g} molecular singlet in the low-temperature insulating phase. Upon heating, we observe a considerable spectral weight transfer to lower energies with orbital reconstruction. The insulator-metal transition may be viewed as a transition from a solid of isolated Ti-Ti molecules into a solid of electronically partially broken dimers, where the Ti ions acquire additional hopping in the a-b plane via the e_{g}^{π} channel, the opening of which requires consideration of the multiplet structure of the on-site Coulomb interaction.