EPR study of the dynamic spin susceptibility in heavily doped ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{\delta }}$

Electron paramagnetic resonance (EPR) studies on single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathrm{\delta }}$ were performed in a wide range of oxygen concentration δ. Intrinsic EPR signals due to the existence of paramagnetic chain fragments could be detected only in a limited δ range (0.7≤δ≤0.9). The linewidth of the signal passes through a minimum near 105 K and broadens exponentially for further decreasing temperatures. This behavior manifests the opening of a pseudogap in the dynamic spin susceptibility of the ${\mathrm{CuO}}_2$ planes. At higher temperatures the linewidth follows a Korringa behavior. The g values reveal axial symmetry with respect to the c axis [${\mathit{g}}_{\mathrm{\parallel }}$=2.28(1); ${\mathit{g}}_{\mathrm{\perp }}$=2.03(1)] and are almost temperature independent. We compare our results with inelastic neutron scattering and nuclear-magnetic-resonance data.