Abstract
We performed angle-resolved photoemission spectroscopy studies on the triple-layer Bi(2)Sr(2)Ca(2)Cu(3)O(10+δ) over a wide doping range. Although the doping level of the inner CuO(2) plane is extremely low in underdoped samples, the d-wave SC gap is enhanced to the unprecedentedly large value of Δ(0) ~ 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 CuO(2) 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 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.