Proximity-Induced Nodal Metal in an Extremely Underdoped CuO2 Plane in Triple-Layer Cuprates

ARPES studies have established that the high - T _c cuprates with single and double CuO ₂ layers evolve from the Mott insulator to the pseudogap state with a Fermi arc, on which the superconducting (SC) gap opens. In four- to six-layer cuprates, on the other hand, small hole Fermi pockets are formed in the innermost CuO ₂ planes, indicating antiferromagnetism. Here, we performed ARPES studies on the triple-layer Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _10+δ over a wide doping range, and found that, although the doping level of the inner CuO ₂ plane was extremely low in underdoped samples, the d -wave SC gap was enhanced to the unprecedentedly large value of Δ ₀ ～100 meV at the antinode and persisted well above T _c without the appearance of a Fermi arc, indicating a robust ″nodal metal″. We attribute the nodal metallic behavior to the unique local environment of the inner clean CuO ₂ plane in the triple-layer cuprates, sandwiched by nearly optimally-doped two outer CuO ₂ planes and hence subject to strong proximity effect from both sides. In the nodal metal, quasiparticle peaks showed electron-hole symmetry, suggesting d -wave pairing fluctuations. Thus the proximity effect on the innermost CuO ₂ plane is the strongest in the triple-layer cuprates, which explains why the T _c reaches the maximum at the layer number of three in every multi-layer cuprate family.