Polarons in the Infrared Spectra of High-To Materials

Polaronic absorption features, induced by excess charges in both electron and hole doped cuprates, have been measured. The features here reported include additional phonon-like peaks in the far infrared and a broad overtone band centered at ~1000 cm -1 (d band). The peaks in the far infrared are attributed to local modes. The d band is produced by overtone and combination bands of a few of these local modes and is well fitted in a polaron model. Most of these polaronic contributions survive in the metallic phase, where they are superimposed on the Drude term due to free carriers.