Evidence of Doping-Dependent Pairing Symmetry in Cuprate Superconductors

Scanning tunneling spectroscopy studies reveal long-range spatial homogeneity and predominantly $d_{x^2{-y}^2}$-pairing spectral characteristics in under- and optimally doped ${\mathrm{YBa}}_2\mathrm{Cu}{}_3\mathrm{O}{}_{7-\delta }$ superconductors, whereas STS on ${\mathrm{YBa}}_2(\mathrm{Cu}{}_{0.9934}\mathrm{Zn}{}_{0.0026}\mathrm{Mg}{}_{0.004}{)}_3\mathrm{O}{}_{6.9}$ 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 $\left(\mathrm{Y}{}_{0.7}\mathrm{Ca}{}_{0.3}\right)\mathrm{Ba}{}_2\mathrm{Cu}{}_3\mathrm{O}{}_{7-\delta }$, $(d_{x^2{-y}^2}+s)$-pairing symmetry is found, suggesting significant changes in the superconducting ground state at a critical doping value.