OPTICAL INVESTIGATIONS OF OXYGEN ORDERING AND PERSISTENT PHOTO-DOPINGIN TETRAGONAL YBCO

We present optical measurements of the tetragonal phase of YBCO. The m easurements give strong evidence that the doping for low oxygen conten t is driven by the clustering of oxygen atoms in the chain plane, givi ng rise to metallic islands with local orthorhombic structure. By incr easing the oxygen content, a percolation transition to macroscopic met allic 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-conduc tivity effect, it can be concluded that the additional conductivity du e to illumination is most likely due to oxygen order and not to a char ge transfer and trapping of electrons at some defects. (C) 1998 Elsevi er Science B.V.