Anomalous optical absorption in the normal state of overdoped cuprates near the charge-ordering instability

We argue that the hump observed in the optical conductivity at or below a few hundreds of cm(-1), in overdoped cuprates such as the electron-doped Nd2-xCexCuO4-y at x greater than or similar to 0.15 and the hole-doped Bi2Sr2CuO6 and La2-xSrxCuO4, cannot be accounted for within a single-fluid description. We propose instead an interpretation based on the direct excitation of charge collective modes, which become nearly critical in the proximity to a charge-ordering instability. Their critical character entails a peculiar temperature dependence and a pseudoscaling form of the optical spectra, which agree with the experimental data.