Photoemission study of CoO.

Abstract

We have performed resonance photoemission, angle-resolved photoemission, and core-level photoemission studies of single-crystalline CoO. On the one hand, strong correlation effects among the d electrons are observed, as signaled by a strong reduction of Co 3d bandwidths and satellites in both the valence band and the cation core levels. On the other hand, the oxygen states are found to be very bandlike, as indicated by strong dispersions of oxygen states in the valence band and the lack of oxygen satellites. We give estimations of 30 and 1.5 for U/W (Coulomb interaction divided by bandwidth) ratio of Co 3d bands and O 2p bands, respectively. By comparing the experimental and theoretical E versus k relation, we show that the density-functional band calculation works well for the oxygen bands but not for the Co bands. We argue that CoO is not a band insulator, but a charge-transfer insulator. We have also observed the effects of local magnetic order on the electronic structure. Finally, we suggest a guideline on calculating the band structure of CoO: introducing a mechanism that reduces the Co 3d bandwidth by 25% while still retaining the other essential features of the band calculation.