Small polaron formation in transition metal oxides, like the prototypical material rutile TiO2, remains a puzzle and a challenge to simple theoretical treatment. In our combined experimental and theoretical study, we examine this problem using Raman spectroscopy of photo-excited samples and real-time time-dependent density functional theory (RT-TDDFT), which employs Ehrenfest dynamics to couple the electronic and ionic subsystems. We observe experimentally the unexpected stiffening of the A1g phonon mode under UV illumination and provide a theoretical explanation for this effect. Our analysis also reveals a possible reason for the observed anomalous temperature-dependence of the Hall mobility. Small polaron formation in rutile TiO2 is a strongly non-adiabatic process and is adequately described by Ehrenfest dynamics at time scales of polaron formation.
@article{arxiv.1612.05679,
title = {Polaron-induced phonon localization and stiffening in rutile TiO$_2$},
author = {Grigory Kolesov and Boris A. Kolesov and Efthimios Kaxiras},
journal= {arXiv preprint arXiv:1612.05679},
year = {2017}
}