Phase separation, antiferromagnetism, and superconductivity in superconducting-superoxygenated La(2-x)Sr(x)CuO(4 + delta)

摘要

La(sub 2-x)Sr(sub x)CuO(sub 4+(delta)) with x = 0.01, 0.025, 0.050, 0.10 and 0.16 and excess oxygen (delta) incorporated by high-pressure O(sub 2) anneals. These compounds were examined using time-of-flight neutron diffraction data. Various models were fit by Rietveld least-squares refinement, with the maximum amount of (delta) being only of the order of 10 standard deviations. (delta) is largest for x near 0, is zero for x = 0.10 and is intermediate for x = 0.16. Only the sample with x = 0.01 is found to phase separate distinctly into a nearly stoichiometric phase with (delta) (approx) 0 and an oxygen-rich superconducting phase as the temperature is lowered. Coincidence of phase separation and Neel temperature strongly suggests that the phase separation is driven by free energy provided by long-range antiferromagnetic ordering in the nearly stoichiometric, weakly Sr-doped La(sub 2-x)Sr(sub x)CuO(sub 4). The excess oxygen stoichiometry shows that at low values of x, hole doping is provided primarily by the excess oxygen, and is enhanced substantially by phase separation. At larger values of x, excess oxygen is no longer incorporated, and hole doping is provided by the substitution of Sr(sup +2) for La(sup +3).