Electronic structure of (formula presented) comparison between substitutional and vacancy doping

The change of the electronic structure across the metal-insulator transition in a Mott-Hubbard system (Formula presented) has been investigated by x-ray absorption and photoemission spectroscopy. The results are compared with (Formula presented) where hole doping is achieved by excess oxygen instead of Sr substitution. It is found that additional doped-hole states are created in the insulator gap for both cases, while the density of states at the Fermi level in (Formula presented) is nearly half of that in (Formula presented) at the same nominal doping. This suggests a strong reduction in the mobility of carriers due to cation vacancies produced by excess oxygen.