Polaronic optical absorption in semiconducting and superconducting oxides

The infrared and Raman spectra of High-Tc superconductors (HCTS) in the normal phase exhibit vibrational modes (IRAV) which are superimposed to a Drude background from high-mobility carriers. In the far infrared the IRAV appear as well-resolved peaks, shifted in energy with respect to the normal modes of the lattice. In the midinfrared they are grouped into a broad polaron band peaked at about 0.1 eV. At corresponding energies, in the semiconducting phase one observes bands that are unambiguously identified as polaronic. Moreover, the IRAV show remarkable correspondences with the peaks in the Eliashberg function as determined in the same systems by tunneling measurements. These results point towards the existence of two types of carriers in HCTS, one at least being polaronic in nature.