c-Axis Dimer and Its Electronic Breakup: The Insulator-to-Metal Transition in Ti2O3

Abstract

We report on our investigation of the electronic structure ofTi2O3using (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-Tic-axis dimer in the corundum crystal structure is electronically present and forms ana1ga1gmolecular 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 thea−bplane via theegπchannel, the opening of which requires consideration of the multiplet structure of the on-site Coulomb interaction.

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Plain Language Summary

As a transition metal oxide heats up, it can suddenly change from an insulator to a conductor. This fascinating phenomenon is accompanied by changes in the crystal structure and, often, also in the magnetic structure. Band-structure calculations, the standard theory for describing the electronic structure of solid-state materials, fail to describe this behavior. In particular, band theory has great difficulties explaining how these materials become insulators at low temperature. It is clear that interactions among electrons must be taken into account; however, these types of problems that involve many interacting particles can quickly become unsolvable. It is therefore important in these calculations to have good starting points and smart approximations. We have experimentally identified the key elements of the electronic structure of such a system so that a valid and accurate theoretical model can be constructed.

We used x-ray spectroscopies to study the electronic structure of the transition metal oxideTi2O3. From distinct features in our spectra, we found that the insulating phase can be viewed as a solid consisting of electronically isolated Ti-Ti diatomic molecules (dimers)—contradicting results from band-structure calculations. We also see that as temperature increases, dimers partially break up, which causes the transition toward a metallic state. This is associated with a reconstruction of the electron orbitals that opens up an extra channel for transferring electrons, along with a dramatic decrease in the effective Coulomb interaction.

These experiments reveal an amazing amount of detail about all of the relevant ingredients in a metal-insulator transition. These findings may therefore serve as an important benchmark for future theoretical studies.