Electronic states and phases of KxC60 from photoemission and X-ray absorption spectroscopy

High-resolution photoemission and soft X-ray absorption spectroscopies have provided valuable information on the electronic structure near the Fermi energy in the superconducting copper oxide compounds, helping to constrain the possible mechanisms of superconductivity. Here we describe the application of these techniques to KxC60, found recently to be superconducting below 19.3 K for x ≈ 3. The photoemission and absorption spectra as a function of x can be fitted by a linear combination of data from just three phases, C60, K3C60 and K6C60, indicating that there is phase separation in our samples. The photoemission spectra clearly show a well defined Fermi edge in the K3C60 phase with a density of states of 5.2×10-3 electrons eV-1 Å-3 and an occupied-band width of 1.2 eV, suggesting that this phase may be a weakly coupled BCS-like (conventional) superconductor. The C1s absorption spectra show large non-rigid-band shifts between the three phases with half and complete filling, in the K3C60 and K6C60 phases respectively, of the conduction band formed from the lowest unoccupied molecular orbital of C60. These observations clearly demonstrate that the conduction band has C 2p character. The non-rigid-band shift coupled with the anomalous occupied-band width implies that there is significant mixing of the electronic states of K and C60 in the superconducting phase.