Polarons and bipolarons in oxides

The physical properties of Fröhlich polarons and bipolarons are reviewed in relation to the recent experimental study of the mid-infrared absorption spectra of high- Tc cuprate superconductors, in particular, of Nd₂CuO_4-y , YBa₂Cu₃O_6+x and La₂CuO_4+x . The spectral region studied is of particular relevance for the open question of the presence of polarons and bipolarons in these materials, since it coincides with the frequency region where polaron and bipolaron absorption is to be expected. In our view, the optical absorption spectra as observed in the doped cuprates are a manifestation of the polaron type relaxed excited final states.