EPR study of polycrystalline superconductors with YBa2Cu3O7 structure

Electron paramagnetic resonance (EPR) of Gd³⁺, Eu²⁺, and copper ions has been investigated in the high-T_c superconductor with YBa₂Cu₃O_7– structure. It has been established that the system is heterogeneous at 0.150.5 and consists of metallic and dielectric regions. The former arises due to oxygen enrichment while the later due to oxygen deficiency. The integral of exchange interaction between Gd³⁺ localized moments and conduction electrons J_sf=0.016 eV has been determined from the normal state temperature dependence of Gd³⁺ EPR linewidth for metallic regions. T_c depression by gadolinium-localized moments for GdBa₂Cu₃O_7– was estimated to be T_c–2K. Anomalies in linewidth temperature dependence upon transition from the normal to the superconducting state have given information about the value and temperature behavior of the superconductor's energy gap. The model, which gives the opportunity to understand some peculiarities of the EPR signal for YBa₂Cu₃O_7– samples, is proposed in terms of several bottlenecked spinsubsystems: spin-liquid in CuO planes and Cu²⁺-O^– and Cu²⁺-O^2– fragments in CuO chains.