Effect of strain on the structure and critical temperature in superconducting Nd-doped YBa2Cu3O7-delta

A detailed study of the correlation between the superconducting critical temperature and the strain in as-grown, high-quality, c-axis epitaxial, Nd-doped YBa2Cu3O7-delta (YBCO) films is reported. Samples with thicknesses ranging from 8 to 250 unit cells have been deposited by high-pressure oxygen sputtering on LaAlO3 (100) single-crystal substrates. The a, b, and c axis of the films have been accurately determined by x-ray diffraction from the measurements of the (005) and the (038)-(308) reflections. Below a certain thickness, a crossover from orthorhombic to tetragonal structure is observed, together with a decrease of the critical temperature. These results cannot be explained by the elastic deformation of the film, but point to inelastic strain induced by the large mismatch with the substrate related to an oxygen reorganization in the Cu(1)-O plane.