Optical conductivity of the trellis-lattice t-J model:: Charge fluctuations in NaV2O5

Optical conductivity of the trellis lattice t-J model at quarter filling is calculated by an exact-diagonalization technique on small clusters, whereby the valence state of V ions of NaV2O5 is considered. We show that the experimental features at similar to 1 eV. including peak positions, presence of shoulders, and anisotropic spectral weight, can be reproduced in a reasonable range of parameter values, only by assuming that the system is in the charge disproportionated ground state. Possible reconciliation with experimental data suggesting the presence of uniform ladders at T > T-c is discussed.