Microscopic phase separation in La2CuO4+x induced by the superconducting transition

The phase separation (PS) effect in superconducting La2CuO4+x (x less than or equal to 0.04) single crystals with low oxygen mobility was studied via mu SR spectroscopy, high-resolution neutron diffraction, and magnetic susceptibility. Despite the fact that all crystals are inside the miscibility gap (0.01<x<0.06), only crystals with a sufficiently large excess oxygen concentration x=0.04 show a macroscopic phase separation according to the neutron-diffraction data. However, in all samples a phase transition to an ordered magnetic state was observed by mu SR spectroscopy concomitantly with the onset of superconductivity. This effect is treated as a microscopic phase separation which is possibly driven by superconductivity. [S0163-1829(98)02341-8].