The $c$-axis dimer and its electronic break-up: the insulator-to-metal transition in Ti$_2$O$_3$

TL;DR

This study investigates the electronic structure of Ti$_2$O$_3$ and reveals how the insulator-metal transition involves the breaking of Ti-Ti dimers and changes in orbital configurations, using advanced spectroscopic techniques.

Contribution

It provides direct spectroscopic evidence of the Ti-Ti dimer's electronic state and details the orbital reconstruction during the insulator-metal transition in Ti$_2$O$_3$.

Findings

Ti-Ti dimers form an $a_{1g}a_{1g}$ singlet in the insulating phase

Spectral weight shifts to lower energies upon heating

Transition involves partial breaking of dimers and increased hopping in the $a$-$b$ plane

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 $e_g^{\pi}$ channel, the opening of which requires the consideration of the multiplet structure of the on-site Coulomb interaction.