Conductivity and Optical Spectra of Pr1-xYxBa2Cu3Oy (x=0, 0.5 and 1.0)

In order to clarify the electronic structure of High-Tc superconducting oxide, we measured the oxygen-nonstoichiometry, conductivity at temperatures, up to 1200 K, and the optical spectra for three kinds of oxides, YBa2Cu3Oy, (Pr0.5Y0.5)Ba2Cu3Oy, and PrBa2Cu3Oy. NO difference was found in oxygen nonstoichiometry. At high temperature, we detected a conductivity minimum (sigma(min)) in the log(10)sigma-log(10)Po(2) plot at constant temperature. From the slope of the Arrhenius plot for sigma(min), the CT gap was 1.37, 1.32 and 1.21 eV for YBa2Cu3Oy, (Pr0.5Y0.5)Ba2Cu3Oy, and PrBa2Cu3Oy respectively. The energy gap (Delta E) between the acceptor level and the top of the valence band was calculated from the slope of the Arrhenius plot for p-type conductivity. The Delta E for the superconducting YBa2Cu3Oy and (Pr0.5Y0.5)Ba2Cu3Oy were zero at 100 K, but that for the nonsuperconducting PrBa2Cu3Oy, was 20 meV at 100 K, even for y=6.93. It was concluded that the conductivity of a high-temperature superconducting oxide was strongly dependent on defect and that the electronic structure could be explained using the p-type semiconducting model which had the acceptor level originating from excess oxygen. Namely, the existence of an energy gap (Delta E) caused a remarkable decrease in the carrier density with decreasing temperature and resulted in the nonsuperconduction of PrBa2Cu3Oy.