Low-temperature ellipsometry of α′-NaV2O5

The dielectric function of alpha'-NaV2O5 was measured with electric field along the a and b axes in the photon-energy range 0.8-4.5 eV for temperatures down to 4 K. We observe a pronounced decrease of the intensity of the 1-eV peak upon increasing temperature with an activation energy of about 15 meV, indicating that a finite fraction of the rungs becomes occupied with two electrons while others are emptied as temperature increases. No appreciable shifts of peaks were found showing that the change in the valence state of individual V atoms at the phase transition is very small. A remarkable inflection of this temperature dependence at the phase transition at 34 K indicates that charge ordering is associated with the low-temperature phase.