${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_y$ epitaxial thin films $(x=0\mathrm{}$ to 2): Structure, strain, and superconductivity

We have grown (001)-oriented thin films of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_y$ with strontium composition $x=0\mathrm{}–2$ 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 $y\sim 4$ 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 $\left[\rho \right(0\mathrm{}\mathrm{K}\left)\right]:$ higher $T_c$ for lower $\rho \left(0\mathrm{}\mathrm{K}\right).\mathrm{}$ The depression of $T_c$ around $x=0.125\mathrm{}$ is smaller than that for the bulk samples, suggesting that the strain suppresses the “1/8 anomaly.” The films with $y>\mathrm{}4\mathrm{}$ attained by cooling in ozone showed $T_c$ between 40 and 48 K for $x<\mathrm{}\mathrm{0.15.}\mathrm{}$ For $x>\mathrm{}0.30,$ the compositional dependence of the resistivity is explained by both oxygen defects and a structural phase transition at $x=\mathrm{1.8.}\mathrm{}$