The role of strain in the superconductivity of Nd-doped YBCO films deposited by diode high pressure oxygen sputtering

Superconductivity of Y1Ba2Cu3O7-d c-axis epitaxial films is always depressed when the films thickness is reduced to few unit cells. Here we report a detailed study of the structure, morphology and transport properties of Y(Nd0.05Ba1.95)Cu3O7-d films deposited by high pressure diode oxygen sputtering. Films having thickness between 9 and 250 unit cells deposited on LaAlO3 substrates have been investigated. It is shown that both T-c and J(c) are reduced by decreasing the film thickness. The effect is quite evident even in relatively thick Y(Nd0.05Ba1.95)Cu3O7-d films made of 32 unit cells. Moreover it is shown that the a and b axes of the films approach one to each other with decreasing thickness. A nine unit cell film is almost tetragonal. Application of a simple elasticity theory to account for the structural changes fails to explain the experimental results. Inelastic strain induced by an oxygen disordering in the Cu(1)-O chains may explain the experimental results. (C) 2004 Elsevier B.V. All rights reserved.