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We have studied the far infrared reflectance of the mixed phase superconductor Yi.2Ba0.8CuO4_y. The reflectance spectrum shows sharp phonon resonances and an electronic contribution that we attribute to reflection at a mesh-like structure of the conducting phase embedded in nonconducting material; in comparison, the reflectance of single phase superconducting material is mainly determined by the dynamical electronic conductivity. Key word: High T c Superconductivity After the discovery of high temperature superconduc-tivityZ,2 far infrared studies have been used for characterization of properties of the new superconducting materials; studies have been performed for La-Ba-Cu-O ~, La_Sr_Cu_O ~-7 and also for YBa2Cu307 compounds.8-z Recently a transmission study for a mixed phase powder sample of Y. ~Ba^ ~CuO. has been reported. Z4 In this i .L u.6. ~-y paper we present reflection measurements for a mixed phase Y, ~Ba~ ~CuO, sample and compare the l .z u.~ ~-y results with reflection measurements for a single phase YBa2Cu307 sample. We prepared samples from appropriate amounts of Y203 , BaCO 3 and CuO. The powders were carefully 1525 o195-9271/87/12oo-15255o5.oo/o 9 1987 Plenum Publishing Corporation

We report on a study of single-phase (superconducting) YBa2Cu;}07 and (nonconducting) Y2BaCu05 and mixed phases of these compounds in bulk and film samples and on an investigation of ceramic bulk material of the homologous series Tl2Ba2CanCun+i02n+6-We describe preparation and structural characterization by x-ray diffraction analyses and further characterization by electric, magnetic, tunnel junction, and Raman scattering studies, and we present results of far-infrared reflectivity measurements. In a theoretical analysis we relate the phonon structure of reflectivity spectra to geometric structure of a metallic network, with holes filled with nonconducting material. The influence of superconductivity on reflectivity is also investigated.

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We report on a study of the far-infrared reflection for a sintered YBa2Cu30 7 sample that contained a large portion of preferentially oriented crystallites with the a-b plane parallel to the surface and that showed extraordinary high far-infrared reflectivity. From experimental reflection data we determined, by Kramers-Kronig analysis, the dynamical conductivity and extracted the contributions due to free charge carriers and phonons, respectively. We find evidence for an anomalous behavior of the dynamical conductivity at temperatures above T~; the dynamical conductivity increases strongly for temperatures approaching T~ and is strongly frequency dependent. By use of the Mattis-Bardeen theory we obtain an estimate for the superconducting energy gap of 2A/kT~_4.6 (for T< T~).We find that the lowest frequency infrared-active phonon mode is less damped in the superconducting state than in the normal state.