Weakly and tightly bound polarons in the infrared spectra of perovskites

Abstract

Infrared reflectivity and/or absorption measurements in chemically doped perovskites, including high-Tc superconductors, show polaronic features similar to those observed in the photoinduced spectra of cuprates. 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 at ~ 1000 cm−1, which results from overtones of IRAVs and is well described in terms of photoexcitation of polarons. Most of the above features survive as extra-Drude peaks in the metallic phase of these materials. The strength of the polaron band increases for decreasing temperature, as expected for weakly bound polarons, in compounds which are parents of high-Tc superconductors. It decreases, instead, as expected for tightly bound polarons, in a Mn-based perovskite and in CuO, which do not become superconducting upon doping. A comparison with tunneling results shows that the superconducting carriers are strongly coupled to the IRAVs.