Growth of the optical conductivity in the Cu-O planes

We have studied the development of the optical conductivity as electrons are added to the Cu-O planes in ${\mathrm{Pr}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ by varying x(0≤x≤0.2). In the metallic phases, contributions to the optical conductivity below 3 eV arise from three sources: mobile carriers, mid-infrared excitations, and charge-transfer excitations. The mobile carrier spectral weight grows roughly linearly with x, while the mid-infrared band appears to evolve at low doping via a transfer of spectral weight from the charge-transfer band. Comparing these results with hole doping in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ indicates an electron-hole symmetry that is not anticipated by standard charge-transfer insulator models.