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 with an on-site Coulomb interaction U, we show that excess electrons form small polarons if the density of electronic carriers is higher than ~1020 cm-3. Below this value, the electrons stay delocalized or become large polarons. For oxygen-deficient SrTiO3, small polarons confined to Ti3+ sites are immobile at low temperature but can be thermally activated into a conductive state, which explains the metal-insulator transition observed experimentally.
Corresponding author: Cesare Franchini. Reprints also available from Ulrike Diebold (diebold).
Users with online access to Physical Review B can load the article from the publisher.