Thickness effect on the structure and superconductivity of Nd1.2Ba1.8Cu3O7+x epitaxial films

We have studied the effect of thickness on the structure and superconductivity of Nd1.2Ba1.8Cu3O7+x (NdBCO) epitaxial films deposited on SrTiO3 (100) substrates. High quality NdBCO films were deposited by high oxygen pressure sputtering. The samples have been carefully characterized by transport measurements and X-ray diffraction using conventional and synchrotron radiation sources. Two structural transformations are observed. The first one is a crossover from the orthorhombic and twinned structure, typical of thick samples, to a pseudotetragonal phase at thickness lower than 60 unit cells. The pseudotetragonal phase exhibits peculiar X-ray diffuse scattering around the Bragg reflections, associated to nanostructured twinned domains. The second one appears at thickness lower than 20 unit cells and is assigned to a pseudotetragonal-tetragonal transition. The transport properties are not modified at the first crossover, while a substantial decrease of T-c and of the number of holes is observed at the pseudotetragonal-tetragonal transformation. Our results suggest that the structural rearrangement and the transport properties are explained by the ordering of Cu(I)O chains. (c) 2007 Elsevier B.V. All rights reserved.