Absence and existence of ESR in high-Tc superconductors

In high-Tc superconductors having transition temperature lower than 100K, no ESR signal of Cu ions except for impurities has been observed so far. On the other hand, we have succeeded to detect a strong isotropic ESR signal in the materials with three sheets of a Cu-O plane. Above Tc, the integrated ESR intensity increases as the decrease of the temperature. The temperature dependence is stronger than linear suggesting that the center resposible for ESR is localized. The ESR signal suddenly disappears at Tc. The signal is not considered to be due to impurities. Copper ions under the apex oxygen which has trapped a hole are proposed as the origin of this spectrum.