Localization by disorder in the infrared conductivity of (formula presented) films

The ab-plane reflectivity of (formula presented) thin films was measured in the (formula presented) range for samples with (formula presented) (formula presented) (formula presented) and (formula presented) (formula presented) as a function of temperature in the normal state. The effective charge density obtained from the integrated spectral weight decreases with increasing x. The variation is consistent with the higher dc resistivity for (formula presented) but is one order of magnitude smaller than what would be expected for (formula presented) In the latter sample, the conductivity is dominated at all temperatures by a large localization peak. Its magnitude increases as the temperature decreases. We relate this peak to the dc resistivity enhancement. A simple localization-by-disorder model accounts for the optical conductivity of the (formula presented) sample.