OXYGEN ISOTOPE EFFECT AND STRUCTURAL PHASE-TRANSITIONS IN LA2CUO4-BASED SUPERCONDUCTORS

The oxygen isotope effect on the superconducting transition temperature (αo) varies as a function of x in La2-xSr xCuO4 and La2-xBaxCuO4, with the maximum αo values (αo≥0.5) found for x near 0.12. This unusual x dependence implies that the isotope effect is influenced by proximity to the Abma → P42/ncm structural phase transition in these systems. Synchrotron x-ray diffraction measurements reveal little change in lattice parameters or orthorhombicity due to isotope exchange in strontium-doped materials where αo>0.5, eliminating static structural distortion as a cause of the large isotope effects. The anomalous behavior of αo in both strontium- and barium-doped materials, in combination with the previously discovered Abma → P4 2/ncm structural phase-transition in La1.88Ba 0.12CuO4, suggests that an electronic contribution to the lattice instability is present and maximizes at ∼1/8 hole per copper atom. These observations indicate a close connection between hole doping of the Cu-O sheets, tilting instabilities of the CuO6 octahedra, and superconductivity in La2CuO4-based superconductors.