Abstract
The question whether excess electrons in 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 . Below this value, the electrons stay delocalized or become large polarons. For oxygen-deficient , small polarons confined to sites are immobile at low temperature but can be thermally activated into a conductive state, which explains the metal-insulator transition observed experimentally.
- Received 8 August 2014
- Revised 26 January 2015
DOI:https://doi.org/10.1103/PhysRevB.91.085204
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