The $c$-axis dimer and its electronic break-up: 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 observed 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 $e_g^{\ddot{I}}€$ channel, the opening of which requires the consideration of the multiplet structure of the on-site Coulomb interaction.

7 pages, 4 figures