Superconducting coherence boosted by outer-layer metallic screening in multilayered cuprates

In multilayered high-T_c cuprates, the inner CuO₂ planes (IPs) are spatially separated from dopant layers and thus remain cleaner than the outer planes (OPs). Here, we investigate five-layer (Cu,C)Ba₂Ca₄Cu₅O_y (Cu1245) and three-layer Ba₂Ca₂Cu₃O₆(F,O)₂ (F0223) using angle-resolved photoemission spectroscopy (ARPES) and uncover an unprecedented situation, in which only the IPs become superconducting while the OPs remain metallic at low temperatures. Our model calculations indicate that over 95% of the OP wavefunction remains confined to OP itself, rendering superconducting proximity negligible. This interlayer decoupling realizes an ideal configuration-a single superconducting CuO₂ layer screened from the dopant layer-induced disorder by heavily overdoped metallic OPs. The clean CuO₂ layer exhibits a large superconducting gap with coherent Bogoliubov peaks extending beyond the antiferromagnetic zone boundary, and a widely extended coherent flat band at the Brillouin zone edge. These findings introduce the "degree of screening" as a physical parameter for probing competition between pseudogap and superconductivity, and for bridging typically disordered real materials with the idealized theoretical models.