No abstract

In good conductors optical phonons are usually screened, and therefore not observed. However, sharp features due to infrared-active modes in the copper-oxygen planes are observed in the optical conductivity of Pr1.85Ce0.15CuO4 and YBa2Cu3O6.95. Oscillator strengths indicate that the screening of these modes is poor or totally absent. These materials are compared with eta-Mo4O11, 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.

No abstract

The ab plane optical spectrum of underdoped Bi 2 Sr 2 Ca 1Ϫx Y x Cu 2 O 8ϩ␦ has been studied as a function of temperature. The gaplike suppression in scattering rate 1/͑͒ observed below and above T c is found to be related to the shoulder structure in reflectivity R() and the broad peak or downward suppression from Drude-like shape in conductivity 1 (). We conclude that the superconducting gap, which is visible in 1 (), evolves smoothly into the pseudogap. The results obtained are consistent with recent electron tunneling and photoemission measurements. ͓S0163-1829͑99͒05201-7͔

No abstract