Emerging giant resonant exciton induced by Ta-substitution in anatase TiO$_{2}$: a tunable correlation effect

TL;DR

This study reveals a novel, tunable resonant exciton in anatase TiO$_{2}$ induced by Ta substitution, highlighting the role of d-electron correlations and advanced experimental and computational techniques.

Contribution

It demonstrates the emergence of a resonant exciton in TiO$_{2}$ due to Ta doping, combining experimental high-energy optical conductivity with ab initio calculations.

Findings

Discovery of a resonant exciton in the deep UV region.

Ta substitution activates strong electronic correlations.

The exciton is robust and tunable with doping levels.

Abstract

Titanium dioxide (TiO$_2$) has rich physical properties with potential implications in both fundamental physics and new applications. Up-to-date, the main focus of applied research is to tune its optical properties, which is usually done via doping and/or nano-engineering. However, understanding the role of $d$-electrons in materials and possible functionalization of $d$-electron properties are still major challenges. Herewith, within a combination of an innovative experimental technique, high energy optical conductivity, and of the state-of-the-art {\it ab initio} electronic structure calculations, we report an emerging, novel resonant exciton in the deep ultraviolet region of the optical response. The resonant exciton evolves upon low concentration Ta-substitution in anatase TiO$_{2}$ films. It is surprisingly robust and related to strong electron-electron and electron-hole interactions. The $d$- and $f$- orbitals localization, due to Ta-substitution, plays an unexpected role, activating strong electronic correlations and dominating the optical response under photoexcitation. Our results shed light on a new optical phenomenon in anatase TiO$_{2}$ films and on the possibility of tuning electronic properties by Ta substitution.