ELECTRONIC-STRUCTURE OF KXC-60 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 (EF). The experimental result shows that a very narrow peak and a broad distribution of density of states coexist near EF in K3C60. While the narrow peak at EF 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 EF 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.