Spin fluctuations in doped antiferromagnets fromLi7NMR relaxation in CuO:Li

${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ NMR relaxation is used to derive information on the low-energy spin excitations induced in antiferromagnetic (AF) CuO by the heterovalent substitution of one percent of ${\mathrm{Cu}}^{2+}$ by ${\mathrm{Li}}^{+}$. The energy gap ${\mathit{E}}_{\mathit{g}}$ between localized and itinerant states, where the extra hole diffuses in the AF matrix, is obtained as ${\mathit{E}}_{\mathit{g}}$≃600 K while the effective correlation time of the hole is ${\mathrm{\tau }}_{\mathit{h}}$≃(1/${\mathrm{\omega }}_{\mathrm{exch}}$)/${\mathit{xe}}_{\mathit{g}}^{\mathrm{-}\mathit{E}}$/T, in agreement with the main conclusions of a previous work in ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Li}}_{\mathit{x}}$O for larger doping [P. Carretta, M. Corti, and A. Rigamonti, Phys. Rev. B 48, 3433 (1993)]. The experimental findings are discussed in comparison with the ones in lightly doped AF ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$, where a drastic changeover in the x dependence of the spin fluctuation properties has been recently found for x≲0.02 [F. C. Chou et al., Phys. Rev. Lett. 71, 2323 (1993)]. Finally, mostly on the basis of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ relaxation rates at different measuring frequencies, it is conjectured that in ${\mathrm{Cu}}_{0.99}$ ${\mathrm{Li}}_{0.01}$O one has a localization of the extra hole, possibly in the ${\mathrm{CuO}}_4$ plaquette [R. J. Gooding, Phys. Rev. Lett. 66, 2266 (1991)], establishing a condition of fast spin fluctuations.