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 [Bi2Ca2O4]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 Co 3d states, which are strongly hybridized with O 2p 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 Co 2p3/2->Co 3d (a1g) threshold and the O 1s->O 2p-Co 3d (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 Co 3d to the O 2p states and from the O 2p to the Co 3d 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 O 2p states with suitable symmetry.