Conductivity of lattice-mismatched epitaxial BaTiO₃ films exhibiting abrupt step-like temperature dependence

BaTiO₃/(La,Sr)₂CuO₄ multilayered films are epitaxially grown on [001] SrTiO₃ single crystals, X-ray diffractometry shows that the BaTiO₃ layer is a mixture of a- and c-axis-oriented regions. The current-voltage characteristics of the films are measured between RT and 150°C, and the temperature (T) dependence of the conductivity σ of the films is deduced. The as-grown films show mostly activation-type T-dependence of σ but exhibit an abrupt change typically once of twenty times. When the films are post-annealed, the films constantly exhibit anomalous abrupt changes in the T-dependence of σ.