Electronic phase separation in La2−xSrxCuO4+y

We have discovered an electronically driven phase separation in super-oxygenated La2−xSrxCuO4+y, with various Sr contents. The samples have been characterized by muon spin rotation and bulk magnetization measurements. For all Sr levels with x<0.125, excess oxygen produces samples with a superconducting phase with TC near 40 K and a separate magnetic phase, a spin density wave that also orders near 40 K. The observed magnetic region is closely related to the anomalous, 1/8th hole doped, magnetic versions of La2CuO4, and the superconducting region is optimally doped. The two phases are likely to be the only stable ground states in this region of the phase diagram. This simple two-component system may represent the most direct observation of electronic phase separation in cuprate superconductors. Thus La2CuO4, doped with substitutional Sr and interstitial O introduces an interesting new material system for the field of high-temperature superconductivity.