PHASE-SEPARATION IN LA2CUO4+DELTA AS PROBED BY A PARAMAGNETIC SURFACEDEFECT

The phase separation into metallic and insulating regions, occurring i n La2CuO4+delta at low doping concentrations, is investigated with ele ctron-paramagnetic resonance. A paramagnetic center probing the phase- separation process is found in ceramic or powdered specimens. The inte nsity of this resonance is shown to be directly related to the surface area of the specimen. This leads to the conclusion that a paramagneti c Cu2+ center sited at surfaces or at grain boundaries is present. An additional phase separation is proposed taking place in material regio ns near these surfaces.