<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mi>−</mml:mi><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mi>y</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>epitaxial thin films<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo>(</mml:mo><mml:mi>x</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn><mml:mn/></mml:math>to 2): Structure, strain, and superconductivity

We have grown (001)-oriented thin films of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{y}$ 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 $y\ensuremath{\sim}4$ 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 $[\ensuremath{\rho}(0\mathrm{K})]:$ higher ${T}_{c}$ for lower $\ensuremath{\rho}(0\mathrm{K}).$ 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 $y&gt;4$ attained by cooling in ozone showed ${T}_{c}$ between 40 and 48 K for $x&lt;0.15.$ For $x&gt;0.30,$ the compositional dependence of the resistivity is explained by both oxygen defects and a structural phase transition at $x=1.8.$

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