Electron-spin-resonance studies of the Ag-, Eu-, and Gd-doped Y-Ba-Cu-O high-Tcsuperconductors

The electron-spin resonance (ESR) of pure and Ag-, Eu-, and Gd-doped ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ are measured at temperatures above and below the transition temperature $T_c$. The ESR signals are speculated to be due to electrons in the & levels of the ${\mathrm{Cu}}^{2+}$ ions hybridized with the oxygen 2p electrons in the tetragonal ${\mathrm{CuO}}_4^{2+}$ structure above the quasi Fermi level which lies at the middle of the Mott-Hubbard gap. For the pure orthorhombic structure or properly doped impure structures, the ESR signals disappear even at T>$T_c$, implying a net spin S=0 due to the super exchange force of the ${\mathrm{Cu}}^{2+}$ ions through the intermediate oxygen atoms.