Coexistence of trapped and free excess electrons in SrTiO3

TL;DR

This study uses density functional theory to show that excess electrons in SrTiO3 can be either trapped as small polarons or remain delocalized depending on electron density, explaining its complex electronic behavior.

Contribution

It provides a detailed theoretical analysis demonstrating the coexistence of trapped and free electrons in SrTiO3 based on electron density and oxygen vacancies.

Findings

Electrons form small polarons above 1% density

Electrons are delocalized or form large polarons below 1% density

Temperature activates small polarons into conductive states

Abstract

The question whether excess electrons in SrTiO3 form free or trapped carriers is a crucial aspect for the electronic properties of this important material. This fundamental ambiguity prevents a consistent interpretation of the puzzling experimental situation, where results support one or the other scenario depending on the type of experiment that is conducted. Using density functional theory corrected with an on-site Coulomb interaction U, it is established that excess electrons form small polarons if the electron density is higher than ~ 1%. For densities below this critical value, the electrons stay delocalized or become large polarons. Our modeling of oxygen deficient SrTiO3 provides an alternative picture with respect to previous theoretical studies endowing firm evidence for the observed, temperature-driven metal-insulator transition in samples with homogeneously distributed oxygen vacancies. Small polarons confined to Ti(3+) sites are immobile at low temperature but can be thermally activated into a conductive state.