Abstract
Layered cobalt oxide Bi2Sr2CoO6+δ has attracted attention because of its isostructure to the cuprate high Tc superconductor Bi2Sr2CuO6+δ (Bi2201). However, the replacement of Cu by Co completely suppresses superconductivity and the crystal becomes an insulator. In addition, with various oxygen contents, Bi2Sr2CoO6+δ exhibits a series of surprisingly rich magnetic properties of ferromagnetism and anti ferromagnetism. Because of these reasons, we have carried out an angle-resolved photoemission (ARPES) measurement to study the electronic band structure of Bi2Sr2CoO6+δ with Tpeak~280 K and ~150 K. We are mostly interested in the valence band maximum (bands nearest to EF) in Bi2Sr2CoO6+δ which is closely related to the electronic and magnetic properties in Bi2Sr2CoO6+δ and expect to measure a "k" _"∥" -dependent dispersion. The ARPES results reveal that the bands nearest to EF, which are originated from the Co-O planes, are nondispersive showing localized characters, in contrast to bands dispersing across EF in Bi2Sr2CuO6+δ. By investigating the photon energy dependence, we observe that near the Brillouin zone center the effective mass of the peak at EB~6.0 eV, which is from the BiO planes, varies with photon energy, and estimate the inner potential V0 at about 13.5 eV in Bi2Sr2CoO6+δ with the Tpeak~280 K sample. Furthermore, the periodicity of the variation is twice as large as we anticipated by the reciprocal lattice constant "2π" /"c" . It indicates that the band structure along the kz direction is strongly influenced by the bilayer structure in Bi2Sr2CoO6+δ. In the in-plane measurement, we discover that the effective mass of the state at EB~6.0 eV in the direction along Γ→M→Γ" is smaller than along Γ→X→Γ'. By concerning the bonding directions, it indicates that although the whole Bi-O bonding is disordered due to large O vacancies, it forms a more continuous, regular order along Γ→M→Γ" and make electrons move more easily.