Coexistence of trapped and free excess electrons in SrTiO3

X. Hao, Z. Wang, M. Schmid, U. Diebold, C. Franchini

Institut für Angewandte Physik, Technische Universität Wien, 1040 Wien, Austria
Faculty of Physics and Center for Computational Materials Science, University of Vienna, 1090 Wien, Austria
Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, P. R. China

Phys. Rev. B 91 (2015) 085204

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 at iap_tuwien_ac_at).

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