Collective defect formation in the high-temperature superconductor YBa_2Cu_3O_7 under the influence of adsorbed water molecules

摘要

The mechanisms for defect formation stimulated by the adsorption of water molecules in the surface of YBa_2Cu_3O_7 ceramic are studied, together with the types of defects and their distribution. It is found that a water layer physically bound to the surface reduces the rates of annihilation and capture 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 adsorbed water excites subthreshold formation of 10~(21) cm~(-3) interstitial Ba and Cul atoms and transitions of oxygen from Ol to O5, and vice versa in the volume of crystallites, and the migration of defects and accumulation of Ba atoms in the surface layer, which block diffusive jumps of Ni within the volume of the crystals. These effects are related to the excitation of collective, low-frequency weakly damped motion of heavy holes in the crystal volume when defects are formed on the surface by physically adsorbed H_2O molecules, which is accompanied by Coulomb repulsion of cations from intermediate layers into intersitials and the migration of defects in the field of the collective excitations.