Phonon screening in high-temperature superconductors

In good conductors optical phonons are usually screened, and therefore not observed. However, sharp features due to infrared-active modes in the coppe r-oxygen planes are observed in the optical conductivity of Pr1.85Ce0.15CuO 4 and YBa2Cu3O6.95. Oscillator strengths indicate that the screening of the se modes is poor or totally absent. These materials are compared with eta-M o4O11, in which lattice modes appear suddenly below the charge-density wave transition. It is proposed that poor screening in the cuprates originates from fluctuating charge inhomogeneities in the copper-oxygen planes.