ORIGIN OF TWEED TEXTURE IN THE SIMULATION OF A CUPRATE SUPERCONDUCTOR

The origin of metastable tweed texture (microstructure) is studied by computer simulation. A two-dimensional model of 99 x 99 unit cells represents a layer with an oxygen deficit of the high-T(c) superconductor YBa2Cu3O7-delta and exhibits the ferroelastic tetragonal-orthorhombic phase transition. The tweed texture is known to be important for flux pinning. In the model, the local ordering of the oxygen atoms produces long-range strain fields, which have been studied computationally by a molecular dynamics technique. The system has a strong tendency to form the tweed texture, as observed experimentally. Well above the structural phase transition temperature, the strain fluctuations show well developed embryos of the tweed texture, whose temperature dependence agrees with theoretical estimates obtained using the independent-site approximation. On quenching to below the transition temperature, the texture first becomes more regular in spacing and coarsens before the system orders macroscopically: the kinetics behaviour is quite different from the traditional model of nucleation and growth.