Increase in Transition Temperature by Lattice-mismatch Strain in High-temperature Superconductors

Abstract

Lattice-mismatch strain was induced in (001)-oriented thin films of La-214 superconducting oxides. For the compressed films of La_2-xSr_xCuO₄ and La_2-xBa_xCuO₄ on LaSrAlO₄ substrates, the superconducting transition temperature (T_c) reached 44 K and 47 K, respectively, which are higher than the values for bulk samples. Moreover, 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 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 T_c. We also found that the lower residual resistivity gives a higher T_c. Based on the experimental results, we suggest that reduction of spin fluctuation is effective to increase T_c in the high-temperature superconductors.