Compressive (or expansive) epitaxial strain was induced in (001)-oriented La-214 thin films. For the compressed films of La2-xSrxCuO4+δ and La2-xBaxCuO4+δ on LaSrAlO4 substrates, the superconducting transition temperature (Tc.) reached 44 K and 47 K, respectively, with δ∼ 0. When both films had δ > 0, Tc reached 49 K. These values are higher than those for bulk samples. 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 suppresses formation of the low-temperature tetragonal phase. Both effects result in an increase in the bond length between Cu and the apical oxygen (Oapex), which is responsible for the enhancement of Tc. Furthermore, based on the experimental fact that the lower residual resistivity gives a higher Tc, we speculate that the Tc enhancement is caused by the reduced antiferromagnetic spin fluctuation in the CuO2 planes due to the change in the Cu-Oapex bond length.
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