NMR evidence for a two-step phase separation in Nd1.85Ce0.15CuO4-δ -: art. no. 037002

By Cu NMR we studied the spin and charge structure in Nd2-xCexCuO4-delta. For x=0.15, starting from a superconducting sample, the low temperature magnetic order in the sample reoxygenated under 1 bar oxygen at 900 degreesC reveals a peculiar modulation of the internal field, indicative of a phase characterized by large charge droplets (blob phase). By prolonged reoxygenation at 4 bars the blobs break up and the spin structure changes to that of an ordered antiferromagnet. We conclude that the superconductivity in the n-type systems competes with a genuine type I Mott-insulating state.