WEAKLY AND TIGHTLY BOUND POLARONS IN THE INFRARED-SPECTRA OF PEROVSKITES

Infrared reflectivity and/or absorption measurements in chemically dop ed perovskites, including high-T-c superconductors, show polaronic fea tures similar to those observed in the photoinduced spectra of cuprate s. These features, whose intensities depend on doping and temperature, are observed in both n-type and p-type materials. They include extra- phonon infrared-active vibrations (IRAV) and a broad band centered al similar to 1000 cm(-1), which results from overtones of IRAVs and is w ell described in terms of photoexcitation of polarons. Most of the abo ve features survive as extra-Drude peaks in the metallic phase of thes e materials. The strength of the polaron band increases for decreasing temperature, as expected for weakly bound polarons, in compounds whic h are parents of high-T-c superconductors. It decreases, instead, as e xpected for tightly bound polarons, in a Mn-based perovskite and in Cu O, which do not become superconducting upon doping. A comparison with tunneling results shows that the superconducting carriers are strongly coupled to the IRAVs.