Two-dimensional superstructure in the (001) plane of Bi2[Ca,Sr]3Cu2O8+δ thin films

Thin films of Bi-Ca-Sr-Cu-O showing superconductivity above 100 K have been made by electron beam evaporation with oxygen post-annealing. The major superconducting phase, Bi2[Ca,Sr]3Cu2O8+δ (Tc ∼85 K), is oriented epitaxially with the (100)SrTiO3 substrate. In addition to the one-dimensional incommensurate superstructure along the b axis, which is characteristic of this phase, we have observed by transmission electron microscopy a two-dimensional incommensurate superstructure in the a-b plane with a different periodicity and a different orientation relationship to the subcell.