Electron-phonon coupling in d-electron solids: A temperature dependent study of rutile TiO2 by first-principles theory and two-photon photoemission

TL;DR

This study combines first-principles theory and two-photon photoemission experiments to investigate how electron-phonon interactions uniquely influence the temperature-dependent electronic structure of rutile TiO2, highlighting differences from sp-band semiconductors.

Contribution

It provides a novel theoretical and experimental analysis of electron-phonon effects on d-orbital derived bands in TiO2, revealing an unusual temperature dependence.

Findings

Unusual temperature dependence of conduction band energies observed in TiO2

First-principles calculations explain phonon-induced interactions between Ti 3d orbitals

Contrast with behavior of sp-orbitals in SiO2 polymorphs

Abstract

Rutile TiO2 is a paradigmatic transition metal oxide with applications in optics, electronics, photocatalysis, etc., that are subject to pervasive electron-phonon interaction. To understand how energies of its electronic bands, and in general semiconductors or metals where the frontier orbitals have a strong d-band character, depend on temperature, we perform a comprehensive theoretical and experimental study of the effects of electron-phonon (e-p) interactions. In a two-photon photoemission (2PP) spectroscopy study we observe an unusual temperature dependence of electronic band energies within the conduction band of reduced rutile TiO2, which is contrary to the well understood sp-band semiconductors and points to a so far unexplained dichotomy in how the e-p interactions affect differently the materials where the frontier orbitals are derived from the sp- and d-orbitals. To develop a broadly applicable model, we employ state-of-the-art first-principles calculations that explain how phonons promote interactions between the Ti-3d orbitals of the conduction band within the octahedral crystal field. The characteristics differences in e-p interactions experienced by the Ti 3d-orbitals of rutile TiO2 crystal lattice are contrasted with the more familiar behavior of the Si 2s-orbitals of stishovite SiO2 polymorph, in which the frontier 2s-orbital experience a similar crystal field with the opposite effect...