Orthorhombic-tetragonal transition in twin-free (110) YBa2Cu3O7 films

Twin-free (110) YBa2Cu3O7 films have been grown by pulsed laser deposition on vicinal (110) STO substrates making use of a double PrBa2Cu3O7 template layer. Such samples offer a unique opportunity to analyze the physics of the tetragonal-orthorhombic transition and of strain relaxation, and to investigate the fine details of the film-substrate alignment. Mapping of relevant regions of reciprocal space has been performed by x-ray diffraction, and a clear interpretation of the complex experimental data has been obtained after attentive analysis. The presence of a strained layer characterized by an angle gamma>90 degrees between the a and b axes has been observed. The details of the tetragonal-orthorhombic transition have been investigated by repeating reciprocal-space mapping (a) after oxygen depletion of a measured sample, and (b) after restoring the former oxygen content. The mechanisms of detwinning, strain relaxation, and epitaxial alignment taking place during the tetragonal-orthorhombic transition are discussed in detail.