TWINS AND FLUCTUATOR DEFECTS IN YBa_2Cu_3O_7 EPITAXIAL FILMS: MONTE CARLO SIMULATION OF ANNEALING AND DETWINNING

摘要

Monte Carlo simulations were used to model the spatial distributions of oxygen atoms in CuO planes and those of barrier heights for the transition of oxygen to a nearest neighbor lattice site in films close in composition to YBa_2Cu_3O_7 for various annealing conditions in the presence of uniaxial strain P. Three regions can be distinguished, depending on annealing temperature: in region I, equilibrium phase fluctuations occur and metastable defects are missing after annealing ;in regions II and III,quasi-isothermal annealing (rapid relaxation ) results in the formation of metastable point defects(P≠0) and twins(P=0),respectively. A two step activation micromodel is proposed for movement of 90° domain boundaries: the interaction energy rises in the first step of the transition of an atom to a neighboring site and decreases in the second step. For 45° boundaries ,a single-step model is proposed: the interaction energy decreases upon the transition of an atom to a nearest neighbot site.