Small polaron formation on the Nb-doped SrTiO3surface

The cubic perovskite strontium titanate SrTiO3 (STO) is one of the most studied, polarizable transition metal oxides. When excess charge is introduced into this material through doping or atomic defects, STO tends to host polarons: quasiparticles formed by excess charge carriers coupled to the crystal phonon field. Their presence alters the properties of this material and is a key for many applications. Considering that polarons form preferentially on or near surfaces, we study small polaron formation at the TiO2 termination of the STO(001) surface via density functional theory calculations. We model several supercell slabs of Nb-doped and undoped STO(001) surfaces with increasing size, also considering the recently observed as-cleaved TiO2-terminated surface hosting Sr adatoms. Our findings suggest that small surface polarons become less stable at low concentrations of Nb doping, in analogy to polarons localized in the bulk. Further, we inspect the stability of different polaron configurations with respect to Nb and Sr impurities and discuss their spectroscopic properties.