Phase separation inLa2CuO4+δas probed by a paramagnetic surface defect

The phase separation into metallic and insulating regions, occurring in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ at low doping concentrations, is investigated with electron-paramagnetic resonance. A paramagnetic center probing the phase-separation process is found in ceramic or powdered specimens. The intensity of this resonance is shown to be directly related to the surface area of the specimen. This leads to the conclusion that a paramagnetic ${\mathrm{Cu}}^{2+}$ center sited at surfaces or at grain boundaries is present. An additional phase separation is proposed taking place in material regions near these surfaces. © 1996 The American Physical Society.