Polaron-induced phonon localization and stiffening in rutile TiO$_2$

TL;DR

This study combines experimental Raman spectroscopy and RT-TDDFT simulations to reveal how small polarons cause phonon stiffening and influence charge mobility in rutile TiO$_2$, advancing understanding of polaron effects in transition metal oxides.

Contribution

It provides the first combined experimental and theoretical analysis of polaron-induced phonon stiffening in rutile TiO$_2$, using Ehrenfest dynamics to explain non-adiabatic polaron formation.

Findings

Observed phonon stiffening under UV illumination.

Explained anomalous temperature dependence of Hall mobility.

Demonstrated the effectiveness of RT-TDDFT with Ehrenfest dynamics for polaron studies.

Abstract

Small polaron formation in transition metal oxides, like the prototypical material rutile TiO$_2$, 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 $A_{1g}$ 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 TiO$_2$ is a strongly non-adiabatic process and is adequately described by Ehrenfest dynamics at time scales of polaron formation.