Light scattering on the high-temperature superconductors

The high temperature superconductors have been examined by the technique of Raman scattering in several limits: the insulating phase, the normal and superconducting state of the superconducting phase, and an optically induced metastable phase. In all cases, the analysis and proposed phenomenological models involved either an examination of the inelastic background scattering or the phonon excitation spectrum. Specifically, the character, temperature dependence, critical temperature dependence and the copper-oxygen covalency dependence of the inelastic background scattering has been studied in all three phases. Analysis of the superconducting phase reveals a marginal Fermi-liquid like character of the electronic polarizability, and a decidedly non-traditional shift of the scattering intensity of the electronic excitations at low temperature. On removing oxygen, the system passes through a metal-insulator transition and the inelastic background becomes dominantly magnetic in origin. Examinations of the open-quote allowed close-quote Raman active phonons in the superconducting phase expose a strong coupling of two modes to the back-ground electronic excitation spectrum, and a dramatic renormalization of these modes below Tc. Further, two sharply resonant Raman open-quote forbidden close-quote modes can be bleached out of the spectrum at low temperature with a sufficiently higher laser dosage. A transition from this optically induced metastable state to the normal state occurs on warming the crystal back to room temperature. On reducing the oxygen concentration, the coupling strength of the two asymmetric phonons diminishes rapidly, the renormalization effects vanish, and the compound no longer exhibits metastability