Persistent Fermi pockets and robust electron pairing in lightly doped CuO2 planes of cuprate superconductors

High temperature superconductivity in cuprate superconductors is generally considered to be generated from doping the Mott insulators. The fundamental nature of the doped parent compounds as well as the microscopic origin of electron pairing remain critical issues in understanding the emergence of superconductivity. Here, using high-resolution spatially-resolved laser angle-resolved photoemission spectroscopy (ARPES), we investigate the intrinsic electronic structures of the CuO2 planes in multilayer cuprates Bi2Sr2Can−1CunO2n+4+δ (n=5~8). The inner CuO2 planes are well shielded from the disorders and provide a rare and ideal platform to probe the intrinsic electronic phase diagram. We observe well-defined Fermi pockets with hole doping levels as low as 0.007, demonstrating an abrupt transition from the parent Mott insulator to a metallic state upon the introduction of an infinitesimal amount of doping. The innermost CuO2 planes (IP0) display gapless Fermi pockets, while the second innermost planes (IP1) exhibit anisotropic superconducting gaps up to ~33 meV, indicative of robust electron pairing coexisting with strong antiferromagnetic order. Our findings provide a revised framework for understanding the doping-driven transitions and pairing mechanisms in cuprate superconductors. The authors find an abrupt transition from the parent Mott insulator to a metallic state upon the introduction of an infinitesimal amount of doping in multilayer cuprate superconductors. The innermost CuO2 planes display gapless Fermi pockets, while the second innermost planes exhibit anisotropic superconducting gaps up to ~33 meV, indicative of robust electron pairing coexisting with strong antiferromagnetic order.