La2-xSrxCuOy epitaxial thin films (x=0 to 2):: Structure, strain, and superconductivity

We have grown (001)-oriented thin films of La2-xSrxCuOy with strontium composition x=0-2 by reactive coevaporation and characterized them by x-ray-diffraction and resistivity measurements. A systematical change in the c-axis length indicates that single-phase films were obtained for the whole compositional range. The films with the oxygen composition gamma similar to 4 showed superconductivity for x between 0.06 and 0.30. For x=0.15, the superconducting transition temperature (T-c) was maximized to 44 K, due to a strain effect caused by the lattice mismatch between films and substrates. Around this composition, it is found that T-c for the films shows a good correlation with the c-axis length. For x=0.30, T-c for the films strongly depends on the residual resistivity [rho(0 K)]: higher T-c for lower rho(0 K). The depression of T-c around x=0.125 is smaller than that for the bulk samples, suggesting that the strain suppresses the "1/8 anomaly." The films with gamma>4 attained by cooling in ozone showed T-c between 40 and 48 K for x<0.15. For x>0.30, the compositional dependence of the resistivity is explained by both oxygen defects and a structural phase transition at x=1.8.