Abstract

The research on Y-Ba-Cu-O compounds has advanced tremendously since the discovery of high-T_c superconductivity in 1986. First of all, the complete comprehension of the mechanism of superconductivity is of current interest. To date, the phenomenon of high-T_c superconductivity in Y-Ba-Cu-O compounds cannot be understood by a consistent theory. Depending on the oxygen concentration in YBa₂Cu₃O_7-x′ one must distinguish between the metallic (0 ≤ x ≤ 0.5) and the semiconducting (0.6 ≤ x ≤ 1) states. Both phases were prepared a) by magnetron sputtering on SrTiO₃ or sapphire substrates, resulting in thin (1 μm) films and b) by sintering of a polycrystalline mixture of Y₂O₃, BaCO₃, and CuO. The dielectric constant and refractive index were pointed out by Kramers-Kronig analysis. The photocurrent of YBa₂Cu₃O₆ was investigated experimentally and theoretically. It was demonstrated that the classical semiconductor theory holds for this kind of material. The Franz-Keldysh effect was observed in absorption and photocurrent. Beyond the classical features, new and previously not observed optical properties are reported: Superconductivity can be induced and tuned by laser illumination. Furthermore, it is possible to create a gradient of oxygen concentration perpendicular to the surface, causing a gap that depends on the depth of the samples. This leads to tuneable shifts between the photoluminescence and photocurrent maxima. Finally, from the combination of optical and electrical properties, a new model for the calculation of the critical temperature in Y-Ba-Cu-O compounds is presented.

© 1992 Optical Society of America