Evidence of In-Plane Superstructure Formation in Phase-Separated and Staged Single CrystalLa2CuO4+δ

In single crystal ${{\mathrm{La}}_2\mathrm{CuO}}_{4+\delta }$, $\delta \sim 0.015$, c-axis interstitial oxygen ordering (staging) in the oxygen-rich phase of the phase-separated sample was observed through satellite peaks in the neutron scattering at $(0,1,4\pm \sim 0.16)$ which first appear at 250 K. A superconducting $T_c\simeq 31\mathrm{K}$ was achieved by prolonged annealing at 200 K of a staged crystal whose $T_c$ was subsequently enhanced by $\sim 1.5\mathrm{K}$ during a 7 h cooling from 215 to 190 K. This annealing below 215 K initiates a splitting of the staging satellites along the ${\mathbf{a}}^{*}$ direction which signifies a one-dimensional modulation of the in-plane order.