Oxygen Isotope Effect and Structural Phase Transitions in La2CuO4-Based Superconductors

Science  07 Dec 1990:
Vol. 250, Issue 4986, pp. 1390-1394
DOI: 10.1126/science.250.4986.1390


The oxygen isotope effect on the superconducting transition temperature (αo) varies as a function of x in La2-xSrxCuO4 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 AbmaP42/ncm structural phase transition in these systems. Synchrotron x-ray difaction 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 AbmaP42/ncm structural phase-transition in La1.88B0.12CuO4, suggests that an electronic contribution to the lattice instability is present and maximizes at ∼1/8 hole per copper atom. These observations indicate a dose connection between hole doping of the Cu-O sheets, tilting instabilities of the CuO6 octahedra, and superconductivity in La2CuO4-based superconductors.