Abstract
We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the valence-band structure of Nb-doped , in which a dilute concentration of electrons are doped into the band insulator. We have found that ARPES spectra at the valence-band maxima at the and points start from below the Fermi level , consistent with the indirect band gap of 3.3 eV and the position at the bottom of the conduction band. The peak position of the ARPES spectra were, however, shifted toward higher binding energies by from the 3.3 eV threshold. Because the bands at the and points have pure character, we attribute this shift to strong coupling of the oxygen hole with optical phonons, in analogy with the peak shifts observed for -electron photoemission spectra in various transition-metal oxides.
- Received 15 December 2008
DOI:https://doi.org/10.1103/PhysRevB.79.113103
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