Localization by disorder in the infrared conductivity of Y_1-xPr_xBa₂Cu₃O₇ films

The ab-plane reflectivity of (Y_1-xPr_x)Ba₂Cu₃O₇ thin films was measured in the 30-30 000 cm^-1 range for samples with x=0 (T_c=90 K), x=0.4 (T_c=35 K), and x=0.5 (T_c=19 K) 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 x=0.4, but is one order of magnitude smaller than what would be expected for x=0.5. 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 x=0.5 sample.