Electronic structure of KxC₆₀ studied by high-energy resolved photoemission spectroscopy

High-energy resolved low-temperature ultraviolet photoemission spectroscopy has been performed on KxC60 (x=0 - 6) to study the detailed electronic structure near the Fermi level (E_F). The experimental result shows that a very narrow peak and a broad distribution of density of states coexist near E_F in K3C60. While the narrow peak at E_F is correlated to a part of the occupied LUMO (lowest unoccupied molecular orbital) band of C60 as predicted from the band structure calculation, the observed broad distribution of density of states near E_F has no counterpart in the calculation. It was also found that K₄C₆₀ is a semiconductor with an energy gap of more than 0.2 eV. This shows a sharp contrast with the prediction from the band calculation that K4C60 should be a metal with four electrons in the three-fold degenerate LUMO band. Some possible origins to give these discrepancies have been discussed.