Photoemission study of layered transition-metal oxides and oxide-based diluted magnetic semiconductors Original Research Article

By means of photoemission spectroscopy and X-ray absorption spectroscopy, we have investigated the electronic structure of layered transition-metal oxides and oxide-based diluted magnetic semiconductors in which the strong d–d Coulomb interaction and the strong p–d hybridization are competing. Photoemission data of hole-doped Cu–O chains in PrBa2Cu3O7 and PrBa2Cu4O8 show one-dimensional dispersion of the 1/4-filled band. The effect of the electron–lattice coupling manifests in the spectral function of Zn-doped PrBa2Cu4O8. For the hole-doped Co–O triangular lattice in Bi2Sr2Co2O9, photoemission spectral line shape indicates that the carriers in the t2g band are essentially small polarons. The effect of Mn doping in wide-gap semiconductor has been investigated in Zn1−xMnxO where the polaron effect is expected to be important. We argue that the local lattice distortion is playing important roles to give the inhomogeneous electronic states induced by the chemical doping. In Ca2−xSrxRuO4, the interplay between the Jahn–Teller distortion, the orbital state, and the Mott transition is demonstrated by the X-ray absorption measurement. It is suggested that the orbital disorder accompanied by the lattice distortion is important to describe the metal–insulator transition of Ca2−xSrxRuO4. All the results demonstrate that the electronic structures of transition-metal oxides, which are often referred as strongly correlated electron systems, are also affected by the strong electron–lattice coupling (due to the strong p–d hybridization). The collaboration of the strong d–d Coulomb interaction and the strong p–d hybridization gives the enhanced electron–lattice interaction and may induce inhomogeneous electronic states found in some transition-metal oxides.