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

Superconductivity of Y1Ba2Cu3O7-d <f>c</f>-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 <f>Tc</f> and <f>Jc</f> 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 <f>a</f> and <f>b</f> 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. [Copyright &y& Elsevier]