Pseudogap features of intrinsic tunneling in Bi2212 single crystals

The c-axis intrinsic tunneling properties of Bi2212 and HgBr₂-Bi2212 single crystals have been measured in the temperature range 4.2-250 K. The 7-15-unit-cell-high mesa structures on the surfaces of these single crystals were investigated. Clear superconductor-insulator-superconductor like tunneling curves were observed for current applied in the c-axis direction. The dynamic conductance shows both sharp peaks at the superconducting energy gap (SG), followed by dips and wide maxima at larger voltages. The maxima together with depressed conductance at zero voltage persist in the whole temperature range, illustrating the presence of the pseudogap in the quasiparticle excitation spectra, while the SG peaks decrease in voltage and lose their distinctiveness at T → T_c. The intercalation of Bi2212 with HgBr₂ molecules results in a drastic increase of the c-axis lattice constant and resistivity ρ_c. Despite this, the overall shape of the ρ_c(T)-d ependence and the characteristic temperature T* at which an upturn in the ρ_c(T)-dependence sets in upon cooling do not change. This implies that T* is not affected by coupling between the CuO₂ bilayers.