POLARONIC OPTICAL-ABSORPTION IN ELECTRON-DOPED AND HOLE-DOPED CUPRATES

Polaronic features similar to those previously observed in the photoin duced spectra of cuprates have been detected in the reflectivity spect ra of chemically doped parent compounds of high-critical-temperature s uperconductors, both n type and p type. In Nd2CuO4-y these features, w hose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at sim ilar to 1000 cm(-1). The latter band is produced by the overtones of t wo (or three) local modes and is well described in terms of a small-po laron model, with a binding energy of about 500 cm(-1). Most of the ab ove infrared features are shown to survive in the metallic phase of Nd 2-xCexCuO4-y, Bi2Sr2CuO6, and YBa2Cu3O7-y, where they appear as extra- Drude peaks. The occurrence of polarons is attributed to local modes s trongly coupled to carriers, as shown by a comparison with tunneling r esults.