Tunneling spectra and superconducting gap in Bi2Sr2CaCu2O8+δ and Tl2ba2CuO6-δ

Tunneling spectra are reported for Bi2Sr2CaCu2O8+delta (Bi-2212) over a wide doping range using superconductor-insulator-superconductor (SIS) break junctions. The energy gap inferred from the tunneling data displays a remarkable monotonic dependence on doping, increasing to very large values in the underdoped region even as T-c decreases. This leads to unphysically large values of the strong coupling ratio (similar to 20). The tunneling spectra are qualitatively similar over the entire doping range even though the gap parameter, Delta, changes from 12 meV to 60 meV. Each spectrum exhibits dip and hump features at high bias with characteristic energies that scale with the superconducting gap. Tunneling spectra of near optimally-doped Tl2Ba2CuO6+delta (Tl-2201) also display a weak dip feature in superconductor-insulator-normal metal (SIN) junctions. Generated SIS spectra of Tl-2201 are compared with measured spectra on Bi-2212 and it is concluded that the dip and hump features are generic to high temperature superconductors.