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

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Abstract
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.

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