Persistence of Li Induced Kondo Moments in the Superconducting State of Cuprates

Using ${}^7\mathrm{Li}$ NMR shift data, the anomalous local moment induced by spinless Li impurities persists below $T_c$ in $\mathrm{YBa}{}_2\mathrm{Cu}{}_3\mathrm{O}{}_{6+y}$. In the underdoped regime, the moments retain their Curie law below $T_c$. In contrast, near optimal doping, the large Kondo screening observed above $T_c$ $(T_K=135\mathrm{K})$ is strongly reduced below $T_c$ as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment (on first near neighbor Cu) is not drastically modified below $T_c$, which allows a comparison with STM determination of the local density of states. Our results constrain theoretical models of the impurity electronic properties.