Electronic structure and charge transfer processes in a Bi-Ca misfit cobaltate

We report polarization-dependent on-resonance and off-resonance photoemission and x-ray absorption measurements to probe the character and the symmetry of the occupied and empty valence-band (VB) states of the four-layered misfit cobaltate [Bi2 Ca2 O4] RS [CoO2] 1.67. These experiments bring clear evidence that the one-electron removal VB photoemission spectra, between the Fermi energy (EF) and ∼4.5 eV, contain Co3d states, which are strongly hybridized with O2p states in the binding energy region of ∼1.8-∼4.5 eV below EF. The states at the Fermi energy have a1g out-of-plane symmetry. In addition, by tuning the incident photon energy across the Co2 p3/2 →Co3d (a1g) threshold and the O1s→O2p-Co3d (a1g) hybridized threshold, both found at ∼2 eV below the absorption edge maximum, it is possible to observe a transition from a Raman regime to an Auger regime. From this transition the charge transfer dynamics from the Co3d to the O2p states and from the O2p to the Co3d states have been estimated to be ∼3 and ∼3.3 fs, respectively. This combined information suggests an electron hopping process between neighboring Co a1g states across EF and mediated by O2p states with suitable symmetry.