UC Irvine

We report a study of single-crystal La2CuO4+, a 38-K superconductor produced by high-pressure oxygenation. We employ La139 NMR to probe the behavior of copper spins in the CuO2 planes. At low temperatures we observe two signals, one originating from regions of the crystal rich in oxygen and a second having no excess oxygen (0). Upon warming through 265 5 K, the volume fraction of the crystal poor in oxygen goes to zero. The magnetic shift of the peak intensity of the line originating in the oxygen-rich portion of the crystal does not change in the vicinity of 265 K. These observations provide direct microscopic evidence for phase separation in the crystal. © 1990 The American Physical Society.