Low-energy excitations in NaxCoO2 • γH2O:: Experiments and simulation

We report on resonant inelastic X-ray scattering measurements on Na0.7CoO2 and on the corresponding hydrated compound Na0.35CoO2 center dot yH(2)O at the Co K-edge. The spectra exhibit a single excitation at 10 eV energy transfer which resonates at the energy of the Co K white line. The experimental results are analyzed within the linear muffin-tin orbital approach and compared to the calculated electronic density of states and optical conductivity spectra. The poor agreement between calculations and experiment is attributed to effective screening effects from electrons in the Fermi sea. The 10 eV feature can be tentatively attributed to a charge transfer excited state, or more likely to a damped plasmon mode. (c) 2005 Elsevier B.V. All rights reserved.