Electronic states and phases of K_xC₆₀ from photoemission and X-ray absorption spectroscopy

Abstract

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^1–4, helping to constrain the possible mechanisms of superconductivity. Here we describe the application of these techniques to K_xC₆₀, found recently to be superconducting below 19.3 K for x ≈ 3 (refs 5–7). 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 x 10^-3 electrons eV^-1 A^-3 and an occupied-band width of 1.2 eV, suggesting that this phase may be a weakly coupled BCS-like (conventional) superconductor. The Cls 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.