NaV2O5:An Exotic Exciton System

We show that the phase transition which sodium vanadate undergoes at T-c = 34 K is driven by a charge ordering. The relevant effective Hamiltonian is of the Frenkel exciton type, with a very large bandwidth to molecular energy ratio. This causes strong non-Heitler-London effects and a temperature dependent gap that vanishes at T-c. In addition to the phase transition, the model qualitatively explains the observed absorption spectrum and the anomaly in the static dielectric constant. Within our model, the observed spin-gap opening at T-c results from exciton-spinon coupling.