Superconducting electronic state in optimally doped YBa2 Cu3 O7-δ observed with laser-excited angle-resolved photoemission spectroscopy

Low-energy electronic structure of optimally doped YBa2 Cu3 O7-δ is investigated using laser-excited angle-resolved photoemission spectroscopy. The surface state and the CuO chain band that usually overlap the CuO2 plane derived bands are not detected, thus enabling a clear observation of the bulk superconducting state. The observed bilayer splitting of the Fermi surface is ∼0.08-1 along (0,0) - (π,π) direction, significantly larger than Bi2 Sr2 CaCu2 O8+δ. The kink structure of the band dispersion reflecting the renormalization effect at ∼60 meV shows up similarly as in other hole-doped cuprates. The momentum dependence of the superconducting gap shows d x2 - y2 -wave-like amplitude but exhibits a nonzero minimum of ∼12 meV along the (0,0) - (π,π) direction. Possible origins of such an unexpected "nodeless" gap behavior are discussed.