Persistence of Li induced Kondo moments in the superconducting state of cuprates

Using Li-7 NMR shift data, the anomalous local moment induced by spinless Li impurities persists below T-c in YBa2Cu3O6+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 K) is strongly reduced below T-c. as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment ton 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.