Evidence of doping-dependent pairing symmetry in cuprate superconductors

Scanning tunneling spectroscopy studies reveal long-range spatial homogeneity and predominantly d(x2)-(y2)-pairing spectral characteristics in under- and optimally doped YBa2Cu3O7-delta superconductors, whereas STS on YBa2(Cu0.9934Zn0.0026Mg0.004)(3)O-6.9 exhibits microscopic spatial modulations and strong scattering near the Zn or Mg impurity sites, together with global suppression of the pairing potential. In contrast, in overdoped (Y0.7Ca0.3)Ba2Cu3O7-delta, (d(x2)-(y2) + s)-pairing symmetry is found, suggesting significant changes in the superconducting ground state at a critical doping value.