Polaronic optical absorption in electron-doped and hole-doped cuprates

Polaronic features similar to those previously observed in the photoinduced spectra of cuprates have been detected in the reflectivity spectra of chemically doped parent compounds of high-critical-temperature superconductors, both n type and p type. In Nd2CuO4-y these features, whose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at similar to 1000 cm(-1). The latter band is produced by the overtones of two (or three) local modes and is well described in terms of a small-polaron model, with a binding energy of about 500 cm(-1). Most of the above infrared features are shown to survive in the metallic phase of Nd2-xCexCuO4-y, Bi2Sr2CuO6, and YBa2Cu3O7-y, where they appear as extra-Drude peaks. The occurrence of polarons is attributed to local modes strongly coupled to carriers, as shown by a comparison with tunneling results.