Strain-controlled critical temperature in REBa₂Cu₃O_y-coated conductors

Recently, we succeeded in detwinning REBa₂Cu₃O₇ (RE123, RE = rare-earth elements)-coated conductors by annealing under an external uniaxial strain. Using the untwinned RE123 tapes, the uniaxial-strain dependencies of the critical temperature T_c along the a and b crystal axes were investigated over a wide strain region from compression to tension. We found that the strain dependencies of T_c for the a and b axes obey a power law but exhibit opposite slopes. In particular, the maximum value of T_c is obtained when the CuO₂ plane becomes a square, and its lattice constant is close to 0.385 nm. It is suggested that a tetragonal structure with a 0.385 nm is the optimum condition for a high critical temperature in high-T_c cuprates.