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

doi:10.1126/science.250.4986.1390

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Science 7 December 1990:
Vol. 250. no. 4986, pp. 1390 - 1394
DOI: 10.1126/science.250.4986.1390

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

M. K. Crawford ¹, W. E. Farneth ¹, E. M. McCarronn III ¹, R. L. Harlow ¹, and A. H. Moudden ²

¹ E. I. du Pont de Nemours Co., Wilmington, DE 19880-0356
² Brookhaven National Laboratory, Upton, Long Island, NY 11794, and Laboratoire Leon Brillouin, CEA-CNRS Saclay 91191, Gif sur Yvette, France

The oxygen isotope effect on the superconducting transitiontemperature ( $agr$ _o) varies as a function of x in La_2-xSr_xCuO₄ andLa_2-xBa_xCuO₄, with the maximum $agr$ _o values ( $agr$ _o $ge$ 0.5) found for xnear 0.12. This unusual x dependence implies that the isotopeeffect is influenced by proximity to the Abma $rarr$ P4₂/ncm structuralphase transition in these systems. Synchrotron x-ray difactionmeasurements reveal little change in lattice parameters or orthorhombicitydue to isotope exchange in strontium-doped materials where $agr$ _o> 0.5, eliminating static structural distortion as a causeof the large isotope effects. The anomalous behavior of $agr$ _o inboth strontium- and barium-doped materials, in combination withthe previously discovered Abma $rarr$ P4₂/ncm structural phase-transitionin La_1.88B_0.12CuO₄, suggests that an electronic contributionto the lattice instability is present and maximizes at $sim$ 1/8 holeper copper atom. These observations indicate a dose connectionbetween hole doping of the Cu-O sheets, tilting instabilitiesof the CuO₆ octahedra, and superconductivity in La₂CuO₄-basedsuperconductors.