  Scanning tunneling spectroscopy (STS) studies reveal long-range (~100 nm)
spatial homogeneity in optimally and underdoped superconducting
YBa_2Cu_3O_{7-\delta} (YBCO) single crystals and thin films, and macroscopic
spatial modulations in overdoped (Y_{0.7}Ca_{0.3})Ba_2Cu_3O_{7-\delta}
(Ca-YBCO) epitaxial films. In contrast, STS on an optimally doped
YBa_2(Cu_{0.9934}Zn_{0.0026}Mg_{0.004})_3O_{6.9} single crystal exhibits strong
spatial modulations and suppression of superconductivity over a microscopic
scale near the Zn or Mg impurity sites, and the global pairing potential is
also reduced relative to that of optimally doped YBCO, suggesting strong
pair-breaking effects of the non-magnetic impurities. The spectral
characteristics are consistent with d_{x^2-y^2} pairing symmetry for the
optimally and underdoped YBCO, and with (d_{x^2-y^2}+s) for the overdoped
Ca-YBCO. The doping-dependent pairing symmetry suggests interesting changes in
the superconducting ground state, and is consistent with the presence of nodal
quasiparticles for all doping levels. The maximum energy gap \Delta_d is
non-monotonic with the doping level, while the (2\Delta_d/k_BT_c) ratio
increases with decreasing doping. The similarities and contrasts between the
spectra of YBCO and of Bi_2Sr_2CaCu_2O_{8+x} (Bi-2212) are discussed.
