ABSENCE OF CU2+ ELECTRON-SPIN-RESONANCE IN HIGH-TEMPERATURE SUPERCONDUCTORS AND RELATED INSULATORS UP TO 1150 K

One unresolved question in the high-temperature superconductors and their copper oxide parent compounds is the absence of a Cu2+ electron-spin-resonance (ESR) signal even at temperatures well above the Neel temperature. We have extended the measurements up to 1150 K in both single crystals and ceramic pellets of La2CuO4+delta, 0 less than or similar to delta less than or similar to 0.12, and no ESR signal has been observed. Our data are not compatible with the models based only on magnetic fluctuations which predict an ESR linewidth of a few hundred Oe at such high temperatures. Other suggested explanations for the absence of a Cu2+ ESR signal are discussed.