11Reflection with UV laser on / off Absorption also observed in the reflection.Not simple Drude responsePhoto-induced mid-IR absorption band
12Model for photo-enhancement of e based on polarons K. Nasu, Phys. Rev. B 67, (2003).Strong e-ph couplingSoft mode (Ti)Breathing mode (O)t2geg(3d)1“Large polarons” and “small polarons”Large polarons (extended over many sites): conduction, metastableSmall polarons (at one site) : no conduction, stable
13Analogy with strongly correlated “dirty metal” oxides Conduction due to hopping and/or tunnelingChemically- doped Mott insulators.“Incoherent peak” in optical spectras(w)wCoherent peak (usual Drude peak)Incoherent peak (mid-IR peak)w0 ≠0Alternative view : binding energy of polaronsMicroscopic models needed.
14Summary IR absorption in SrTiO3 under photo-excitations above Eg Two broad absorption bands (mIR-visible)Incoherent carrier dynamics (“generalized Drude response”) and/or real trapping states.Microscopic origin ? large / small polarons ?Similar to those observed for n- and p-type SrTiO3 (IR, photoemission, and XAS)