Proximity-induced nodal metal in an extremely underdoped CuO₂ plane in triple-layer cuprates

We performed angle-resolved photoemission spectroscopy studies on the triple-layer Bi₂Sr₂Ca₂Cu₃O_10+δ over a wide doping range. Although the doping level of the inner CuO₂ plane is extremely low in underdoped samples, the d-wave SC gap is enhanced to the unprecedentedly large value of Δ₀ ~ 80-100 meV at the antinode. This gap persists well above T_c without a Fermi arc, indicating a "nodal metal". We attribute the nodal metallic behavior to the unique local environment of the inner clean CuO₂ plane, sandwiched by nearly optimally-doped two outer planes and hence subject to strong proximity effect from both sides. In the nodal metal, quasiparticle peaks show electron-hole symmetry, suggesting d-wave pairing fluctuations. Thus the proximity effect on the innermost CuO₂ 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.