Fermi surface and superconducting gap in superstructure-free Bi_1.80Pb_0.38Sr_2.01CuO_6−δ

We report an ultrahigh-resolution angle-resolved photoemission spectroscopy on superstructure-free Bi_1.80Pb_0.38Sr_2.01CuO_6−δ with various hole concentrations. The Fermi surface retains a holelike character centered at (Π, Π) from under-to overdoping. The superconducting gap exhibits a d_x²−y²-like anisotropy with a typical gap value of 10-15 meV near (Π, 0). Comparison with Bi₂Sr₂CaCu₂O₈ shows that the size of the superconducting gap and the energy of the hump structure near E_F are well scaled with the maximum T_c(T^max_c). This suggests that the superconducting properties are essentially characterized by T^max_c irrespective of the number of CuO₂ layers or the BiO superstructure.