Electronic states of alkali-metal-dopedC60phases

${\mathrm{Na}}_x{\mathrm{C}}_{60}$, ${\mathrm{K}}_x{\mathrm{C}}_{60}$, and ${\mathrm{Cs}}_x{\mathrm{C}}_{60}$ fullerides in the low-$x$ range were investigated by means of photoemission and yield spectroscopies. The primary aim was the investigation of the lowest unoccupied molecular orbital (LUMO) -derived electronic states as a function of doping. At low doping, double-peak structures in the LUMO-derived spectral weight were consistently found, which are explained in terms of mixed phases present in the specimens. The peak at higher binding energy is attributed to the low-$x$ stable phases, i.e., to $A_1{\mathrm{C}}_{60}$ ($A$ meaning an alkali atom) for K and Cs doping and to $A_2{\mathrm{C}}_{60}$ for Na doping. On the assumption that the fulleride electronic states are only weakly affected by the alkali species, the insulating or metallic nature of all stable phases, which can be obtained upon alkali doping, was established. All phases, with the exception of $A_3{\mathrm{C}}_{60}$, turn out to be insulating in striking contrast with the predictions of band-structure calculations, the separation between the LUMO peak and the Fermi level in the $A_1{\mathrm{C}}_{60}$ and $A_2{\mathrm{C}}_{60}$ phases being the largest. The energy position of the Fermi level relative to the highest occupied molecular orbital-derived peak changes little with doping.