Angle-resolved photoemission study of Nb-doped SrTiO3

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the O 2p valence-band structure of Nb-doped SrTiO3, in which a dilute concentration of electrons are doped into the d(0) band insulator. We have found that ARPES spectra at the valence-band maxima at the M [k=(pi/a,pi/a,0)] and R [k=(pi/a,pi/a, pi/a)] points start from similar to 3.3 eV below the Fermi level (E-F), consistent with the indirect band gap of 3.3 eV and the EF position at the bottom of the conduction band. The peak position of the ARPES spectra were, however, shifted toward higher binding energies by similar to 500 meV from the 3.3 eV threshold. Because the bands at the M and R points have pure O 2p character, we attribute this similar to 500 meV shift to strong coupling of the oxygen p hole with optical phonons, in analogy with the peak shifts observed for d-electron photoemission spectra in various transition-metal oxides.