Doping dependence of quasiparticle and Josephson tunneling in high transition temperature superconductors

Tunneling spectroscopies on single crystals of Tl2Ba 2CuO6 (Tl-2201) and Bi2Sr2CaCu 2O8 + delta (Bi-2212) junctions are presented. The superconductor-insulator-normal metal (SIN) tunnel junctions are obtained using the point-contact technique with a Au tip. The tunneling conductances reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with a consistent asymmetry, and weakly decreasing backgrounds. The tunneling density of states (DOS) of Tl-2201 have exhibited the most reproducible data that are consistent with a dx2 -y2 (d-wave) gap symmetry. The analysis of the SIN tunneling data is performed using a model for the d-wave DOS. The gap values for optimally-doped single crystals of Tl-2201 are in the range of 19--25 meV. Tunneling spectra are reported for Bi-2212 over a wide doping range (from underdoped Tc = 70 K to overdoped Tc = 62) using SIN and 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 Tc decreases. This leads to unphysically large values of the strong coupling ratio (∼20). The tunneling spectra are qualitatively similar over the entire doping range even though the gap parameter changes from 15 meV to 65 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 Tl-2201 also displays a weak dip feature at the occupied part of SIN junctions. The results of SIS break junctions from Bi-2212 and generated SIS spectra of Tl-2201 are compared and it is observed that the dip and hump feature are a generic property of high temperature superconductors.