Lattice-mismatch strain was induced in (001)-oriented thin films of La-214 superconducting oxides. For the compressed films of La2-xSrxCuO4 and La2-xBaxCuO4 on LaSrAlO4 substrates, the superconducting transition temperature (Tc) reached 44 K and 47 K, respectively, which are higher than the values for bulk samples. Moreover, the Tc-x phase diagrams for the films did not show a local minimum at x∼0.125, the so-called “1/8 anomaly”. The compressive strain expands the c-axis via the Poisson effect and seems to suppress formation of the low-temperature tetragonal phase. We speculate that the both effects result an increase in the bond length between Cu and the apical oxygen, which is responsible for the enhancement of Tc. We also found that the lower residual resistivity gives a higher Tc. Based on the experimental results, we suggest that reduction of spin fluctuation is effective to increase Tc in the high-temperature superconductors.
