Influence of substrates on epitaxial thin films of high-temperature superconductors

For growth of high-quality epitaxial thin films of high-temperature superconductors (HTSCs), it is essential to choose appropriate substrates. For a thin film of La_2-xM_xCuO₄ (M=Ba, Sr), the superconducting transition temperature (T_c) is modified by the strain due to the lattice mismatch between the film and the substrate. For the compressively strained (001) films of La_2-xSr_xCuO₄ and La_2-xBa_xCuO₄ on LaSrAlO₄ substrates, T_c reached 44 and 47 K, respectively, which are higher than the values for bulk samples. Moreover, the T_c-x phase diagrams for the compressed films did not show a local minimum at x∼0.125, the so-called "1/8 anomal". Based on the experimental results, we suggest that the reduction of antiferromagnetic spin fluctuation increases T_c in HTSCs. For thin films of Bi₂Sr₂CaCu₂O₈, we foun d that substrates with a small lattice mismatch are very effective in attaining a smooth and almost precipitate-free surface.