NMR measurement of the exchange coupling between Cu(2) atoms in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ ($T_c=90\mathrm{}$ K)

The authors report the transverse NMR relaxation of chain [Cu(1)] and plane [Cu(2)] nuclei in a single crystal of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ ($T_c=90\mathrm{}$ K) in the normal state. They demonstrate the existence of a strong indirect spin-spin coupling between Cu(2) nuclei, and show that it can be explained using a model in which the Cu(2) atoms possess electronic magnetic moments which are exchange coupled to one another. In such a model, the strength of the coupling implies an effective electronic exchange coupling $J_{\mathrm{eff}}\approx 1100$ ${\mathrm{cm}}^{-1\mathrm{}}$.