Probing electronic density of states and magnetic interactions at the rare-earth site in Er${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

By exploiting the $4f$-shell properties of Er in the high-$T_c$ superconductor Er${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, we find by inelastic magnetic neutron scattering that the electrostatic field around the rare-earth site is anomalously large. Above $T_c$ there exists considerable magnetic interaction between the conduction electrons and the rare-earth's $4f$ shell: $N\left(E_F\right)J_{\mathrm{ex}}=0.025\mathrm{}$. This interaction is lost in the superconducting state. The ground-state magnetic moment of Er in the host lattice is ${\mu }_{\mathrm{eff}}=5.3\mathrm{}{\mu }_B$.