Annealing disorder and photoinduced order of oxygen chains in detwinned YBa2Cu3O6.65 single crystals probed by Raman scattering

Raman scattering in detwinned YBa2Cu3O6.65 Single crystals is studied as a function of photoexcitation and annealing. Copper-oxygen chain-related forb idden Raman bands that are known to strongly bleach with illumination at lo w temperatures, increase their intensity with chain fragmentation induced b y annealing at high temperature. This contrasting behavior proves the conju nction of short Cu-O fragments into longer chains on photoexcitation. We in terpret the Raman modes as due to vibrations at the end of CuO chain fragme nts and Cu-O-Cu monomers, and use their evolution with illumination and ann ealing as anisotropic sensitive markers of oxygen reordering processes. The identification of the ''forbidden'' Raman bands is discussed in the contex t of our results and recent literature in the subject. We also present abso rption measurements performed on GdBa2Cu3Ox thin films with varying oxygen content. These experiments show that the 2.2-eV absorption and the chain-re lated Raman peaks have different dependencies with oxygen content and illum ination, ruling out an explanation that suggests that the Raman intensity r eduction of these modes is due to a photobleaching of intermediate defect s tates. These results highlight the potentialities of Raman scattering for o xygen dynamics studies and demonstrate the presence of photoinduced oxygen orderings in these high-T-c superconductor compounds.