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X. Hao, Z. Wang, M. Schmid, U. Diebold, C. Franchini:
"Coexistence of trapped and free excess electrons in SrTiO₃";
Physical Review B, 91 (2015), 0852041 - 0852046.

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.