Electronic structure of strontium titanate

We have employed the techniques of photoelectron spectroscopy (with tunable synchrotron radiation $30\le h\nu \le 100$ eV) and inverse photoemission spectroscopy at $h\nu =9.7\mathrm{}$ eV to study respectively the occupied and unoccupied electronic states of $n$-type ${\mathrm{SrTiO}}_3$ doped with 1 at.% Nb. Doped samples have the advantage that charging effects are avoided and hence a rather accurate determination of peak positions is possible. The total (occupied and empty) experimental density of states agrees with the calculated density of states of Pertosa and Michel-Calendini when their band-gap energy is readjusted. We have no evidence for intrinsic surface states either in the band gap or in the conduction band, as was calculated by Wolfram et al. The admixture of Ti $3d$ states in the O $2p$ valence states is small, as can be concluded from the Ti $3p\to 3d$ resonance behavior.