Polarons and anharmonicity in high Tc cuprates through vibrational spectroscopies

The discovery in 1986 of superconductivity at high critical temperatures attracted the attention of many researchers not only for its potential applications but also because it represented a theoretical challenge to understanding the mechanism that goes beyond the BCS model. Over 35 years of research, both theoretical and experimental, have not been sufficient to reach a comprehensive description of mechanism governing high Tc superconductivity. We summarize here the experimental signature of polarons and of the anharmonic behavior of the strongly polarizable O-2 ion, two of the possible ingredients of superconductivity in HTc materials, evidenced by Raman and by photoinduced infrared spectroscopy.