Spatial homogeneity and doping dependence of quasiparticle tunneling spectra in cuprate superconductors

Scanning tunneling spectroscopy (STS) studies reveal long-range (similar to 100 nm) spatial homogeneity in optimally and underdoped superconducting YB a2Cu3O7-delta (YBCO) single crystals and thin films, and macroscopic spatia l modulations in overdoped (Y0.7Ca0.3)BaCu3O7-delta (Ca-YBCO) epitaxial fil ms. In contrast, STS on an optimally doped YBa2(Cu0.9934Zn0.0026Mg0.004)(3) O-6.9 Single crystal exhibits strong spatial modulations and suppression of superconductivity over a microscopic scale near the Zn or Mg impurity site s, and the global pairing potential is also reduced relative to that of opt imally doped YBCO, suggesting strong pair-breaking effects of the non-magne tic impurities. The spectral characteristics are consistent with d(x2-y2) p airing symmetry for the optimally and underdoped YBCO, and with (d(x2-y2) s) for the overdoped Ca-YBCO. The doping-dependent pairing symmetry sugges ts interesting changes in the superconducting ground state, and is consiste nt with the presence of nodal quasiparticles for all doping levels. The max imum energy gap Delta (d) is non-monotonic with the doping level, while the (2 Delta (d)/k(B)T(c)) ratio increases with decreasing doping. The similar ities and contrasts between the spectra of YBCO and of Bi2Sr2CaCu2O8+x are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.