La-214 thin films under epitaxial strain

Compressive (or expansive) epitaxial strain was induced in (001)-oriented La-214 thin films. For the compressed films of La_2-xSr_xCuO_4+δ and La_2-xBa_xCuO_4+δ on LaSrAlO₄ substrates, the superconducting transition temperature (T_c.) reached 44 K and 47 K, respectively, with δ∼ 0. When both films had δ > 0, T_c reached 49 K. These values are higher than those for bulk samples. The T_c-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 (O_apex), which is responsible for the enhancement of T_c. Furthermore, based on the experimental fact that the lower residual resistivity gives a higher T_c, we speculate that the T_c enhancement is caused by the reduced antiferromagnetic spin fluctuation in the CuO₂ planes due to the change in the Cu-O_apex bond length.