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

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

2001-03-09

arxiv.org/abs/cond-mat/0103205v1

Physica C 364-365, 450 (2001)

Physics

Condensed Matter

Superconductivity

6 pages. Accepted for publication in Physica C (2001)

Scientific paper

10.1016/S0921-4534(01)00819-X

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.

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