Collective defect formation in the high-temperature superconductor YBa2Cu3O7 under the influence of adsorbed water molecules

The mechanisms for defect formation stimulated by the adsorption of water m olecules in the surface of YBa2Cu3O7 ceramic are studied, together with the types of defects and their distributions. It is found that a water layer p hysically bound to the surface reduces the rates of annihilation and captur e of positrons, changes the amount of barium and copper on the surface by a factor of two, and inhibits diffusive jumps of nickel atoms. A layer of ad sorbed water excites subthreshold formation of 10(21) cm(-3) interstitial B a and Cu1 atoms and transitions of oxygen from O1 to O5, and vice versa in the volume of crystallites, and the migration of defects and accumulation o f Ba atoms in the surface layer, which block diffusive jumps of Ni within t he volume of the crystals. These effects are related to the excitation of c ollective, low-frequency weakly damped motion of heavy holes in the crystal volume when defects are formed on the surface by physically adsorbed H2O m olecules, which is accompanied by Coulomb repulsion of cations from interme diate layers into interstitials and the migration of defects in the field o f the collective excitations. (C) 1999 American Institute of Physics. [S106 3-7761(99)01608-X].