Coherent and Incoherent Excitations of Electron-DopedSrTiO3

Resonant photoemission at the Ti $2p$ and O $1s$ edges on a Nb-doped ${\mathrm{SrTiO}}_3$ thin film revealed that the coherent state (CS) at the Fermi level ($E_F$) had a mainly Ti $3d$ character whereas the incoherent in-gap state (IGS) positioned $\sim 1.5\mathrm{eV}$ below $E_F$ had a mixed character of Ti $3d$ and O $2p$ states. This indicates that the IGS is formed by a spectral-weight transfer from the CS and subsequent spectral-weight redistribution through $d\mathrm{\text{−}}p$ hybridization. We discuss the evolution of the excitation spectrum with $3d$ band filling and rationalize the IGS through a mechanism similar to that proposed by Haldane and Anderson.