OPTICAL CONDUCTIVITY OF THE HUBBARD-MODEL

We study the response to a static electric field (charge stiffness) an d the frequency-dependent conductivity of the Hubbard model in a reson ant-valence-bond-type paramagnetic phase. This phase is described by m eans of a charge and spin rotational-invariant approach, based on a mi xed fermion-boson representation of the original strongly correlated e lectrons. We found that the Mott transition at half filling is well de scribed by the charge stiffness behavior, and that the values for this quantity off half filling agree reasonably well with numerical result s. Furthermore, for the frequency-dependent conductivity we trace back to the origin of the band that appears inside the Hubbard gap to magn etic pair breaking. This points to a magnetic origin of the midinfrare d band in high-T-c compounds.