Abstract
We present optical measurements of the tetragonal phase of YBCO. The measurements give strong evidence that the doping for low oxygen content is driven by the clustering of oxygen atoms in the chain plane, giving rise to metallic islands with local orthorhombic structure. By increasing the oxygen content, a percolation transition to macroscopic metallic behaviour takes place at an oxygen content between x=6.1 and x=6.2. The state oxygen order in the chains can be changed by either annealing the sample or by a long-time illumination with visible laser light. From the optical investigation of this persistent photo-conductivity effect, it can be concluded that the additional conductivity due to illumination is most likely due to oxygen order and not to a charge transfer and trapping of electrons at some defects.
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