Oxygen Isotope Effect and Structural Phase Transitions in La 2 CuO 4 -Based Superconductors

Abstract

The oxygen isotope effect on the superconducting transition temperature (α_o) varies as a function of x in La_2-xSr_xCuO₄ and La_2-xBa_xCuO₄, with the maximum α_o values (α_o ≥ 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 difaction measurements reveal little change in lattice parameters or orthorhombicity due to isotope exchange in strontium-doped materials where α_o > 0.5, eliminating static structural distortion as a cause of the large isotope effects. The anomalous behavior of α_o in both strontium- and barium-doped materials, in combination with the previously discovered Abma → P4₂/ncm structural phase-transition in La_1.88B_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 dose connection between hole doping of the Cu-O sheets, tilting instabilities of the CuO₆ octahedra, and superconductivity in La₂CuO₄-based superconductors.