In multilayer high-Tc cuprates with three or more CuO2 layers per unit cell, the inner CuO2 planes (IPs) are spatially separated from the dopant layers and thus remain intrinsically cleaner than the outer planes (OPs). While both interlayer coupling and the presence of clean IPs have been proposed as key factors enhancing superconductivity, their individual roles have been difficult to disentangle because IPs and OPs typically become superconducting simultaneously.
Here, we investigate five-layer (Cu,C)Ba2Ca4Cu5Oy (Cu1245) with Tc = 78 K and three-layer Ba2Ca2Cu3O6(F,O)2 (F0223) with Tc = 100 K using angle-resolved photoemission spectroscopy (ARPES), and uncover a previously inaccessible regime in which only the IPs become superconducting while the OPs remain metallic at low temperatures [1]. Model calculations indicate that more than 95% of the OP wavefunction remains confined to the OP, with minimal hybridization from the superconducting IPs due to a large energy (i.e., doping) difference between OPs and IPs. In particular, we experimentally realize an ideal configuration consisting of a single superconducting CuO2 layer sandwiched between heavily overdoped metallic outer layers, which screen disorder originating from the dopant layers. Strikingly, this clean CuO2 layer exhibits the largest superconducting gap reported among cuprates (Δ0 ~ 60 meV) and coherent Bogoliubov peaks extending beyond the antiferromagnetic zone boundary—long considered the limit beyond which coherence vanishes in heavily underdoped cuprates. Furthermore, a widely extended, coherent flat band emerges at the Brillouin zone edge, indicating suppression of pseudogap-induced damping.
Our results introduce a new physical parameter, the “degree of screening,” to investigate the competition between superconductivity and the pseudogap, potentially shedding new light on the longstanding debate regarding its origin—whether a charge density wave (CDW), a pair density wave (PDW), or other mechanisms. The naturally realized, nearly disorder-free superconducting CuO2 layers provide a model platform for bridging the gap between typically disordered real materials and idealized theoretical models, which generally neglect disorder effects. This work thus opens a route toward addressing one of the central challenges in condensed matter physics: the microscopic mechanism of high-temperature superconductivity.

[1] J. Jeong, et al., arXiv:2507.23260 (2025).