Large polaron absorption in high-Tc cuprates

Infrared evidence for the existence of polaronic carriers in superconducting cuprates is reviewed. Absorption features like shake-off bands in the mid-infrared and extra-phonon peaks in the far-infrared can be easily resolved and studied in the insulating phase at low doping, where they are well described by the large polaron model. Recent data are in close agreement with the two-step model of Devreese and coworkers. As the carrier density increases, polaron-polaron interactions have to be taken into account. A renormalized polaronic conductivity proposed by Tempere and Devreese well accounts for the experimental observations in the electron-doped cuprate NCCO.,In hole-doped LSCO, however, when approaching the metallic phase the absorption spectra are dominated by huge peaks at extremely low frequencies, which indicate a cross-over to a new regime. Indeed, those exotic spectral features are most probably related to the formation of dynamic charge stripes in the Cu-O planes.