SCREENING OF THE MIDDLE CUO2 LAYER IN BI1.6PB0.4SR2CA2CU3O10 DETERMINED FROM CU NMR

Knight-shift measurements of the Cu-63 nuclei in the sandwiching Cu(2) O2 layers and the middle Cu(1)O2 layer in the three-layer bismuth oxid e superconductor have been performed. Spin-lattice-relaxation measurem ents reveal a larger spin-fluctuation enhancement at the Cu(1) sites r elative to the Cu(2) sites. This and a reduced spin susceptibility in the Cu(1)O2 layer indicates that the Cu(2)O2 layers screen the charge transfer from the bismuth bilayer to the Cu(1)O2 layer. Our results sh ow that the Cu(1)O2 layer has distinctly different physical properties than the Cu(2)O2 layer.