The relation between c-axis lattice parameter and superconducting transition temperature of NdBa 2Cu 3O y thin films

Abstract

c-axis-oriented NdBa 2Cu 3O y thin films on (100) SrTiO 3 substrates were prepared by an off-axis rf sputtering method in different conditions. The superconducting property was measured by a standard DC four-probe technique, and the c-axis lattice parameter was determined from X-ray diffraction pattern. Under the optimum depositing temperature (740°C< T s<780°C) and pressure (1.2–1.8×10 −1 mbar, P Ar/ P O 2 =4–2), thin films with T c0=90 K in a deviation of 0.5 K were prepared reproducibly. A parabolic relation of T c0 to the c-axis lattice parameter was observed, indicating an optimal oxygen content for the best superconductivity. The oxygen out-diffusion was investigated by in situ high-temperature X-ray diffraction in high-vacuum (∼10 −4 mbar). The result suggested that oxygen out-diffusion occurred below 770 K and resulted in an expansion of the c lattice parameter. The expansion coefficients of the c-axis for NdBa 2Cu 3O y and the a-axis for SrTiO 3 were determined from the result of the in situ high-temperature X-ray diffraction.