Lattice instabilities and the effect of copper-oxygen-sheet distortions on superconductivity in dopedLa2CuO4

Synchrotron x-ray and neutron-diffraction measurements demonstrate that ${\mathrm{Nd}}^{3+}$ substitution at the ${\mathrm{La}}^{3+}$ site in both metallic and insulating ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ induces low-temperature structural instabilities which reorient the tilting of the copper-oxygen octahedra. These structural transformations strongly affect superconductivity. In addition, there is a bicritical point involving phases of Bmab, Pccn, and P$4_2$/ncm space-group symmetries in the structural phase diagram of ${\mathrm{La}}_{1.6\mathrm{-}\mathit{x}}$ ${\mathrm{Nd}}_{0.4}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ at x∼0.15. There are also anomalies in the structural transformation temperatures and the superconducting transition temperatures when x∼1/8, providing evidence for an electronic instability at this doping level.