Precise determination of the superconducting gap along the diagonal direction of Bi2Sr2CaCu2O8+y: Evidence for extended s-wave gap symmetry

Abstract

We report high-resolution spectra of the second derivative -d^{2}(ReΣ)/dω^{2} of the real part of electron self-energy Σalong the diagonal direction (where ω= E_{F}-E - Δ_{D} and Δ_{D} is the diagonal superconducting gap) for a nonsuperconducting La_{1.97}Sr_{0.03}CuO_{4} (LSCO) crystal and a superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+y} (BSCCO) crystal with T_{c} = 91 K. The -d^{2}(ReΣ)/dω^{2} spectrum of the nonsuperconducting LSCO shows clear peak features, which match precisely with those in the phonon density of states obtained from neutron scattering. Similarly, if we assign Δ_{D} = 7\pm1 meV for the superconducting BSCCO, the peak features in -d^{2}(ReΣ)/dω^{2} of this compound also match precisely with those in the phonon density of states and the tunneling spectrum. The present results rule out seemingly well accepted d-wave gap symmetry and strongly support an extended s-wave gap symmetry with eight line nodes.